Growth of subcontinental lithosphere: evidence from repeated

Lithos 48 Ž1999. 287–316
Growth of subcontinental lithosphere: evidence from repeated
dike injections in the Balmuccia lherzolite massif, Italian Alps
Samuel B. Mukasa
a
a,)
, John W. Shervais
b
Department of Geological Sciences, UniÕersity of Michigan, 2534 C.C. Little Bldg., Ann Arbor, MI 48109-1063, USA
b
Department of Geological Sciences, UniÕersity of South Carolina, Columbia, SC 29028, USA
Abstract
The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust
251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of
lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal
emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor
websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein
suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always
older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein
suite. Phlogopite Žphl. veinlets that formed during interaction with the adjacent crust are the youngest event. There are at
least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a
polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from
aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where
plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional
and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite.
Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of
compositions defined by ocean–island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic
compositions similar to those of Cr-diopside suite lherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic
compositions similar to mid-ocean ridge basalt ŽMORB. asthenosphere. Lead isotopic compositions for all of the samples
fall in the present-day MORB field on the 208 Pbr204 Pb vs. 206 Pbr204 Pb diagram but are displaced above this field on the
207
Pbr204 Pb vs. 206 Pbr204 Pb diagram. There is overlap in the data between the Cr-diopside suite and the Al-augite and
hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other
samples. In Pb–Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in
magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during
extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt ŽOIB.-like
lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous
isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts,
)
Corresponding author. Fax: q1-313-763-4690; E-mail: [email protected]
0024-4937r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 4 - 4 9 3 7 Ž 9 9 . 0 0 0 3 3 - X
288
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of
emplacement, assimilation of crustal materials. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Balmuccia lherzolite massif; Dike rocks; Subcontinental lithosphere
1. Introduction
Understanding the origin and evolution of continental lithosphere is a fundamental goal of solid
earth geophysics, which seeks to characterize the
material properties and physical state of the Earth.
This goal is important because continental lithosphere records the bulk of Earth history and because
a rigid lithosphere is central to plate tectonics. Although the lithosphere is defined by its physical
properties, it must consist of real rocks with distinct
petrologic origins and geochemical characteristics.
Field-based petrologic and geochemical studies are
paramount to our goal of understanding lithosphere
evolution because they provide the ultimate ‘‘ground
truth’’ for broader scale geophysical experiments
that can only infer regional scale structures and
average physical properties.
In this contribution, we review the petrologic and
geochemical characteristics of the Balmuccia lherzolite massif, a fragment of young, stabilized subcontinental lithosphere, and present new geochemical and
isotopic data which constrain the origin of its peridotite and dike rock lithologies. We also present data
on mafic lithologies adjacent to the massif which
represent melts ponded at the crust–mantle interface
subsequent to emplacement of the Balmuccia mantle
diapir.
The Balmuccia lherzolite massif is an alpine peridotite emplaced into granulite facies metabasites of
the Ivrea–Verbano zone after the Late Paleozoic
Hercynian orogeny ŽShervais and Mukasa, 1991..
The Balmuccia massif is characterized by fresh,
unserpentinized lherzolite and dunite, and by a wide
range in dike rock lithologies, including pyroxenites,
websterites, and gabbronorites ŽLensch, 1971; Rivalenti et al., 1975; Ernst, 1978; Shervais, 1979a;
Shervais and Mukasa, 1991.. The range in lithologies found within this massif is comparable with that
observed in xenolith suites from alkali basalts Že.g.,
Wilshire and Shervais, 1975; Wilshire et al., 1988..
The Balmuccia massif offers some advantages over
xenolith studies, however, because structural rela-
tionships which formed in the mantle before emplacement are preserved, and crosscutting relationships between dikes of different generations and
composition are readily observed. This, combined
with the lack of significant low-temperature alteration throughout most of the massif make Balmuccia
an ideal location to study petrologic and geochemical
relations within rocks of the upper mantle.
The Balmuccia massif does not represent old,
Archean-type lithosphere, but rather young stabilized
lithosphere, as defined by Nelson Ž1991.. Similar
lithosphere is thought to underlie extensionally stabilized orogens such as the Hercynian of Europe and
the Great Basin of western North America ŽMenxies
and Dupuy, 1991; Nelson, 1991; Shervais and
Mukasa, 1991..
2. Geologic setting
The Ivrea–Verbano zone represents a cross-section through the lowermost continental crust of the
South Alpine plate exposed by its subsequent collision with Europe during the Alpine orogeny
ŽMehnert, 1975; Fountain, 1976.. The Ivrea zone
consists of metamorphosed supracrustal rocks Župper
amphibolite to granulite facies paragneisses, calcsilicates, marbles, and charnockites., intruded by
lens-shaped bodies of mafic gabbro granulites which
are 8–10 km thick and ) 50 km long Žthe so-called
‘Basic Formation’.. Metamorphic grade increases
from SE to NW across the zone, suggesting a progression downward into the crust ŽSchmid and Wood,
1976; Zingg, 1980, 1983; Sills, 1984.. The eastern
boundary of the Ivrea–Verbano zone is the Pogallo
Line, a mylonitic shear zone which separates upper
amphibolite facies kinzigites of the Ivrea zone from
lower amphibolite to greenschist facies gneisses and
schists of the Strona–Ceneri Zone ŽBoriani, 1970;
Hodges and Fountain, 1984.. The western boundary
of the Ivrea–Verbano zone is the Insubric Line, a
major tectonic dislocation that separates Paleozoic
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
age rocks of the South Alpine plate from rocks of the
North Alpine plate ŽEurope. metamorphosed during
the Alpine orogeny ŽGausser, 1968; Boriani and
Sacchi, 1974..
The Balmuccia lherzolite is one of several large,
kilometer-scale peridotite massifs found within the
Ivrea–Verbano zone ŽErnst, 1978; Shervais, 1979a,b;
Sinigoi et al., 1980, 1983; Rivalenti et al., 1981;
Shervais and Mukasa, 1991.. The major peridotite
bodies ŽFinero, Balmuccia, and Baldissero. are located along the western margin of the Ivrea zone,
within mafic granulites of the ‘Basic Formation’
ŽLensch, 1971; Ernst, 1978.. The Basic Formation
comprises gabbro or gabbronorite granulite Ž"garnet,
amphibole Žamph.., clinopyroxenite, dunite, websterite, and Žalong its eastern margin. potassic
diorites, all complexly intercalated with layers of
paragneiss, marble, and calc-silicate. Petrologic and
chemical studies of the Basic Formation carried out
by Rivalenti and his coworkers ŽRivalenti et al.,
1975, 1981, 1984. suggest that it represents a single
layered intrusive complex related to subduction zone
magmatism along the southern margin of the South
Alpine mini-plate Že.g., Hamilton, 1981.. The presence of paragneiss intercalations and compositional
breaks between some units of the layered series all
suggest that multiple magma chambers were involved ŽAlberti and Longo-Salvador, 1978; Bigioggero et al., 1978; Shervais, 1979c..
3. Field relations
The Balmuccia massif is an elongate lens, 4.5 km
long = 0.5 km wide = 1.1 q km high, located just
east of the Insubric Line, between Val Sesia and Val
Mastellone ŽFig. 1.. The western contact of the
massif is a mylonite zone several meters wide, which
splays off of the Insubric line near the north end of
the massif. Dikes of psuedotachylite ranging in
thickness from a few centimeters to several tens of
meters are common along this boundary and in the
adjacent rocks. Many of these psuedotachylite dikes
are characterized by columnar jointing and resemble
mafic igneous dikes, but their compositions mimic
their host rocks, showing that they formed in situ
ŽShervais, 1979c.. The eastern margin of the massif
is in sharp igneous contact with a series of layered
289
websterites, dunites, pyroxene pegmatoids, and minor gabbronorite referred to as the Contact Series
ŽShervais, 1979b,c.. The Contact Series intrudes the
adjacent gabbro granulites and represent a thin
Ž- 100 m. carapace of magma which ponded at the
crust–mantle boundary subsequent to, or coincident
with, emplacement of the mantle peridotite at crustal
levels ŽShervais and Mukasa, 1991..
The peridotite displays a prominent mineral foliation defined by flattened olivine and spinel grains,
and by thin Ž1 cm. discontinuous layers of pyroxene
and spinel. This foliation trends NNE in the central
and southern portions of the massif, but in the northern part of the massif, and along the eastern and
southeastern margins, the foliation trends NNW ŽFig.
1.. The NNW-trending foliation is parallel to foliations and layering in the adjacent gabbro granulites,
and probably formed during emplacement of the
massif into the lower crust ŽShervais, 1979c..
Decimeter-size pods of albite granite found within
pyroxene pegmatoids of the Contact Series contain
clear, euhedral zircons up to 2 mm long. One of
these zircons has a concordant U–Pb age of 251 " 2
Ma ŽWright and Shervais, 1980.. This Late PermianrEarly Triassic age is somewhat younger than
ages obtained from zircon in diorite near the top of
the Basic Formation Ž285 Ma; Pin, 1986. and from
monazite in paragneiss Ž270–275 Ma; Koppel,
1974;
¨
Koppel
and Grunenfelder,
1979.. This age corre¨
¨
sponds to a post-Hercynian phase of extensional
orogeny, and suggests a gap between peak metamorphism in the Ivrea zone and emplacement of the
peridotite massif ŽShervais and Mukasa, 1991..
4. Petrologic relations
4.1. Peridotite wall rocks
Peridotite of the Balmuccia massif consists of two
main lithologies: lherzolite and dunite; harzburgite is
rare. These lithologies form the pre-existing wall
rock into which dikes and veins of the Cr-diopside
suite, Al-augite suite, the late gabbro suite, and the
hydrous vein suite were intruded. Lherzolite is by far
the dominant lithology, comprising ) 85% of the
massif. Dunite is the next most abundant lithology
290
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Fig. 1. Geologic map of the Balmuccia massif, from Shervais Ž1979c..
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
and occurs throughout the massif, but is most common in the southern half. Mineral compositions in
both rock types show that both lherzolite and dunite
are related to dike rocks of the Cr-diopside suite.
4.1.1. Lherzolite
Spinel lherzolite is the dominant rock type of the
Balmuccia massif; harzburgite is rare and gradational
into lherzolite ŽFig. 2a.. Lherzolite typically contains
60–70% olivine, 20–25% opx, and 12% clinopyroxene Žcpx. by volume. The lherzolites and harzburgites are characterized by porphyroclastic textures
throughout the central and southern portions of the
massif, with porphyroclasts of olivine and orthopyroxene Žopx. up to 1.5 cm across in a equigranularfoliate matrix Ž0.5–2.0 mm grain size. of olivine,
pyroxene, and spinel ŽShervais and Mukasa, 1991..
Textures in the northern part of the massif are ‘secondary protogranular’ ŽMercier and Nicolas, 1975.,
with relatively strain-free porphyroclasts, common
spinel inclusions, and Žin some samples. a continuous range in grain size from 0.25 mm to 2.5 cm.
Along the eastern and southeastern margins of the
massif the porphyroclastic texture has been overprinted by a fine-grained equigranular-foliate texture
with rare relic porphyroclasts ŽShervais and Mukasa,
1991..
Mineral compositions in the lherzolites and
harzburgites are uniformly magnesian, with olivine
Fo 89 – 92 ŽFo 88 – 89 adjacent to some Al-augite suite
dikes., opx En 88.5 – 90.5 , and cpx En 45Wo 49 . Lherzolite pyroxenes are rich in Cr, Al, and Na, but low in
Ti. Spinel is variable in composition, with cr-number
s w100 = CrrŽCr q Al.x of 10–25 in normal lherzolites, and 3–12 in lherzolite adjacent to Al-augite
suite pyroxenites.
4.1.2. Dunite
Dunite is common in the southern and central
parts of the massif, where it may comprise as much
as 10% of the outcrop. Contacts between dunite and
adjacent lherzolite may be sharp or gradational, and
some tabular dunites with sharp contacts resemble
intrusive dikes. Dunite is commonly found between
or adjacent to some Cr-diopside websterites. These
dunites may represent melt extraction zones related
to the adjacent websterite dikes, but the volume of
material in the dikes exceeds that which could be
291
extracted from the wall rock locally. This implies
that melt extraction probably occurred in response to
magma flow through the dike conduit by a mechanism similar to the zone-refining model proposed by
Quick Ž1981a,b.. Mineral compositions in the dunites
are more refractory than in the lherzolites: olivine
ranges from Fo 90 to Fo 91 , and Cr-spinel has cr-numbers of 30–45.
4.2. Dike rocks
Dike rocks of the Balmuccia massif can be divided into four distinct suites Žfrom oldest to
youngest.: the Cr-diopside suite, the Al-augite suite
Ž‘‘ariegites’’., the late gabbro suite, and the hydrous
vein suite Žhornblendite.. Phlogopite Žphl. veins
found along the eastern margin of the massif Žadjacent to the Contact Series. show crustal isotopic
signatures and are discussed separately below. Relative age relationships between and within these suites
are based on crosscutting relations and on relative
degrees of deformation in dikes that are discordant to
foliation.
4.2.1. Cr-diopside suite
The Cr-diopside suite comprises clinopyroxenites,
websterites Ž"olivine., and bronzitites ŽFig. 2a.. At
least three generations of dikes can be demonstrated,
based on crosscutting relationships: Ž1. thin Ž- 10
cm. layers parallel to foliation; Ž2. thick Ž10–150
cm. layers subparallel to foliation ŽFig. 2a., which
crosscut the thin layers at low angles Ž- 158.; Ž3.
thick Ž10–60 cm. dikes which crosscut thick layers
at moderate to high angles ŽFig. 2b.. Additional
generations of Cr-diopside websterite can be postulated based on the occurrence of partial melting
residues such as spinel streaks, ‘ghost layers’ of
pyroxene and spinel-rich lherzolite in dunite, and
Mg-rich bronzitites which are crosscut by later Crdiopside-rich veins ŽShervais, 1979b..
Layers which are parallel to foliation originated as
dikes and were rotated into the foliation, as demonstrated by low-angle intersections between thick and
thin layers and along-strike bifurcation of some thick
layers ŽShervais, 1979b,c; Nicolas and Jackson, 1982;
Sinigoi et al., 1983.. Modal mineralogy of the layers
varies from nearly pure Cr-diopside to nearly pure
enstatite or bronzite; olivine is a common accessory
292
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Fig. 2. Field photographs of dike rocks in the Balmuccia lherzolite massif. ŽA. Gradational contact between dunite Žthe light band just to the
right of the thickest dike in the photo. and lherzolite Žnubbly surface covering the right side of the photo.; both lithologies cut by
foliation-parallel dikes of Cr-diopside websterite and clinopyroxenite up to 35 cm thick. ŽB. Discordant Cr-diopside websterite dike
crosscuts older, foliation-parallel Cr-diopside websterites; top of photo to left, foliation dips ; 458 to west. ŽC. Al-augite suite dike of
spinel-rich clinopyroxenite Žgray; nearly vertical. cuts lherzolite with folded, dismembered Cr-diopside dike; note flattening of spinel
parallel to foliation. ŽD. Zoned Al-augite dike, with margins of spinel-rich clinopyroxenite and a center of thick spinel-poor websterite. ŽE.
Gabbronorite dike of late gabbro suite intruded parallel to foliation; top of photo to left, foliation approximately vertical. ŽF. Gabbro dike of
late gabbro suite intruded perpendicular to foliation; note lack of deformation.
but spinel, amph, and phl are rare. Mineral compositions in the Cr-diopside suite websterites are similar
to those in the lherzolites, with olivine Fo 90 – 91 ,
Cr-spinels with cr-numbers 5–22, and pyroxenes
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
293
which cover the same range in MgrFe, Al, Cr, Ti,
and Na as lherzolite pyroxenes ŽShervais and Mukasa,
1991..
4.2.2. Al-augite suite
Dike rocks of the Al-augite suite are characterized
by gray, Cr-poor, Al-rich cpx and green hercynite
spinel ŽShervais, 1979a,b; Sinigoi et al., 1983; Shervais and Mukasa, 1991.. Kaersutitic amph is a ubiquitous accessory phase in all dikes of this suite, and
plagioclase ŽAn 50 . is an accessory phase in the more
differentiated members of the suite. Al-augite suite
dikes crosscut dikes of the Cr-diopside suite and are
always less deformed.
The oldest Žmost deformed. members of the Alaugite suite are coarse-grained Ž2–20 mm. spinel-rich
pyroxenites that are directly comparable with Alaugite suite xenoliths in alkali basalts ŽFig. 2c..
Al-augite suite spinel pyroxenite dikes range in
thickness from 1 to 20 cm, with spinel generally
concentrated near the center of the dike. They are
commonly folded, with fold axes subparallel to foliation in the adjacent wall rock. Mineral compositions
are relatively magnesian, with olivine Fo 88 – 89.5 and
mg-numbers in pyroxene and amph 87–90.
Websterites and rare bronzitites of the Al-augite
suite are more fractionated than the spinel-rich pyroxenites, and are characterized by finer grain size
Ž1–2 mm., less abundant modal spinel, and more
abundant modal opx. Plagioclase ŽAn 45 – 50 . is a common accessory phase in the websterites, but olivine
is absent. The websterites form dikes up to 1 m thick
which are commonly zoned, with thin Ž- 3 cm.
selvages of spinel-rich pyroxenite adjacent to the
wall rock and thick Ž10–70 cm. cores of spinel-poor
websterite ŽFig. 2d.. Pyroxenes in the websterites
and bronzitites are enriched in Fe Žmg-numberss
82–87., Al, Ti, and Na relative to the more primitive
spinel-rich pyroxenites ŽShervais and Mukasa, 1991..
In zoned dikes, pyroxenes in the margins are more
magnesian than those in the dike interiors ŽFig. 3..
Lherzolites adjacent to pyroxenite dikes of the
Al-augite suite are enriched in FeO, TiO 2 , and alkalis relative to ‘‘normal’’ lherzolite far from any
intrusions. Anhydrous silicate mineral phases reflect
this enrichment with lower mg-numbers and higher
minor element contents; spinel is enriched in Al 2 O 3 ,
resulting in higher spinel mg-numbers close to the
Fig. 3. Electron microprobe traverse of mineral compositions in
Al-augite suite dikes and in peridotite wall rock adjacent to those
dikes. Samples are 2 cm in diameter drill cores taken across over a
meter of outcrop. Horizontal axis is distance in centimeters from
arbitrary reference point near first drill core. Vertical scales are
ratios cr-number s100=CrrwCrqAlx in spinel Žtop. and mgnumber s100=MgrwMgqFex in opx Žmiddle. and cpx Žbottom..
Note drops in both ratios as dikes are approached, and note drop
in pyroxene mg-number within the thicker, zoned Al-augite websterite dike.
dikes as a result of the coupled MgAl–FeCr exchange ŽFig. 3.. Lherzolite adjacent to pyroxenite
dikes may be modally enriched in amph; this amph
is characterized by low mg-numbers Žsimilar to values in the adjacent dikes. and high TiO 2 that increases with decreasing mg-numbers.
294
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
4.2.3. Late gabbro suite
The late gabbro suite forms dikes 5 to 100 cm
thick which are relatively undeformed, even when
they crosscut the foliation at high angles ŽFig. 2e,f..
The general lack of deformation implies that they
were emplaced late in the kinematic history of the
massif, just before its emplacement into the crust.
Gabbros and gabbronorites of this group vary from
plagioclase-rich, with pyroxenite reaction zones 2 cm
thick adjacent to the peridotite wall rock ŽFig. 2f., to
relatively mafic-rich with 45–55% pyroxene ŽFig.
2e.. Opx is ubiquitous, making these units ‘‘gabbronorites’’ by lithology, and kaersutitic amph is a
common accessory phase. Grain size in the late
gabbros varies from coarse Žup to 1 cm. to fine Ž1–2
mm. in the more recrystallized dikes. Late gabbro
dikes oriented subparallel to foliation in the lherzolite may have plagioclase-rich lenses that are flattened parallel to foliation ŽFig. 2e..
Most of the late gabbros are characterized by
plagioclase An 45 – 50 and by pyroxenes with relatively
low mg-numbers compared to the Al-augite pyroxenites Ž75 to 80., but some dikes have mg-numbers that
are quite high Ž88–91.. In all cases, the pyroxenes
have high Na, Ti, and Al, and low Cr ŽShervais and
Mukasa, 1991.. These gabbros were formerly considered part of the Al-Augite suite of dikes Že.g.,
Shervais and Mukasa, 1991., but their isotopic compositions are distinct from dikes of the Al-Augite
suite, requiring a different source for the parent
magmas.
4.2.4. Hydrous Õein suite
Hornblendite veins Žoften referred to as ‘‘lherzites’’ in the older literature. are rare in the Balmuccia massif; only five have been found ŽShervais and
Mukasa, 1991.. The hornblendite veins range in
thickness from about 1 mm up to 2 cm, and consist
almost entirely of kaersutite, with accessory pyroxene, hercynite spinel, and ilmenite. The veins are
medium- to coarse-grained Ž5–25 mm. and are undeformed. The coarse grain size and lack of deformation suggest that the hornblendite veins were intruded relatively late in the kinematics history of the
massif, but their age relative to the late gabbro suite
of dikes is not known. Pyroxenes in the hornblendite
veins have mg-numbers Ž87–89. similar to those in
primitive Al-augite suite spinel pyroxenite dikes, but
are richer in TiO 2 . The kaersutites have high mgnumbers Ž85–87.5., TiO 2 Ž3.5–4.3 wt.%., and K 2 O
Ž0.7–1.0 wt.%..
Lherzolite adjacent to the hornblendite veins is
modally enriched in amph within a few centimeters
of the vein ŽShervais, 1985, 1987.. The modal abundance of amph and its grain size decrease rapidly
with increasing distance from the veins. Textural
relations and mass balance calculations show that
amph within the lherzolite is being formed in part by
the resorbtion of pre-existing pyroxene ŽShervais,
1985, 1987; Cooke, 1992.. The compositional
changes show that K 2 O in the melt phase percolating
through the lherzolite is rapidly used up by the
amph-forming reactions, such that amph formed far
from the vein is lower in K 2 O and TiO 2 , and higher
in Na 2 O and MgO, than amph formed close to the
vein ŽFig. 4.. The effects of this amph metasomatism
do not extend more than 4 cm from a 1-cm vein.
4.3. Phlogopite Õeins
Mica is extremely rare in the Balmuccia massif: it
is found only along the SE contact area adjacent to
the Contact Series, as scarce tiny flakes disseminated
in lherzolite and as phl veinlets ŽGaruti and Sinigoi,
1978.. Phl veinlets range in thickness from 1mm up
to 4 mm, form an irregular network in lherzolite and
at least one veinlet crosscuts an Al-augite suite pyroxenite. The composition of the phl in these veinlets
is similar to that reported by Garuti and Sinigoi
Ž1978.. The restriction of disseminated phl and phl
veinlets to the eastern-most margin of the massif,
within 2 m of the Contact Series, suggests that the
mica found here is a late feature related to metasomatic exchange between the Balmuccia lherzolite
and fluids emanating from the Contact Series parent
magma.
4.4. Contact series
Rocks of the Contact Series include dunite,
harzburgite, websterite, gabbronorite, and pegmatoidal clinopyroxenite ŽShervais, 1979b,c.. These
rocks form a thin layered intrusion Ž100 m thick.
between the peridotite massif and gabbronorite granulites of the Lower Layered Series to the east ŽFig.
1.. The Contact Series has a sharp, primary igneous
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Fig. 4. Electron microprobe traverse of mineral compositions in hornblendite vein and in peridotite wall rock adjacent to the vein. Horizontal axis is distance in millimeters from
one margin of the hornblendite vein. Vertical scales are wt.% TiO 2 , Na 2 O, and K 2 O, and the molar ratio KrwKqNax. Note drop in TiO 2 and K 2 O away from the vein and the
increase in Na 2 O.
295
296
Table 1
Whole rock analyses by XRF. Major elements in wt.% oxide, trace elements in parts per million. Fe 2 OU3 and FeOU are total iron as either Fe 2 O 3 or FeO
Sample no. 75B-162
75B-175B 75B-175C 75B-175A 75B-203
76B-405A 78B-707
75B-194
75B-47
76B299CW 76B-405B 78B-700
Average
Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside Cr-diopside
websterite cpxite
cpxite
cpxite
cpxite
websterite websterite websterite websterite bronzitite bronzitite bronzitite WEB
websterite
SiO 2
TiO 2
Al 2 O 3
Fe 2 OU3
FeOU
MnO
MgO
CaO
Na 2 O
K 2O
P2 O5
50.2
0.17
3.67
5.56
5.00
0.11
25.75
13.53
0.10
0.02
0.02
99.13
50.9
0.16
3.08
5.96
5.36
0.14
25.09
13.19
0.42
0.02
0.00
98.96
Cr
Ni
Sc
V
Rb
Sr
Zr
4480
1310
52
147
nd
nd
19
5900
689
41
95
nd
49
nd
Mga
90.17
89.29
51.5
0.18
3.42
4.89
4.40
0.12
22.07
16.07
0.73
0.02
0.00
99.00
6320
691
47
111
8
57
9
89.94
50.8
0.18
3.44
5.05
4.54
0.12
23.58
15.35
0.42
0.02
0.00
98.96
6220
776
39
103
28
49
8
90.25
50.8
0.19
3.89
5.28
4.75
0.13
22.66
15.51
0.27
0.02
0.07
98.82
6250
961
58
147
21
20
11
89.48
50.5
0.20
3.75
6.36
5.72
0.13
22.17
15.98
0.01
0.02
0.07
99.19
2340
730
53
144
9
72
3
87.35
50.1
0.24
4.88
5.60
5.04
0.12
21.86
16.12
0.15
0.02
0.02
99.11
3820
1200
63
184
nd
32
nd
88.55
48.7
0.10
1.93
6.40
5.76
0.11
30.03
11.49
0.13
0.02
0.05
98.96
4090
1360
27
83
4
50
10
90.29
52.7
0.34
5.73
8.46
7.61
0.16
24.61
7.12
0.11
0.02
0.02
99.27
52.5
0.22
5.01
7.84
7.05
0.15
29.82
3.65
0.01
0.03
0.00
99.23
2210
813
31
173
nd
21
23
3280
749
22
141
nd
16
16
85.22
88.29
52.7
0.27
5.48
9.58
8.62
0.17
28.36
2.48
0.01
0.02
0.00
99.07
2720
753
17
150
4
21
5
85.43
52.3
0.44
4.70
14.82
13.34
0.24
24.84
1.58
0.01
0.03
0.03
98.99
1700
462
nd
222
nd
32
7
76.86
51.04
0.20
4.03
6.45
5.81
0.13
25.09
11.86
0.21
0.02
0.02
99.06
4330
912
41
134
7
35
9
88.57
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Suite
76B-158A 76B-299R 76B-300R 76B-300C 76B-301C 76B-301R 76B-302C 76B-302R 76B307WR 76B307ER 76B-307C 76B-522A
Suite
Al-augite
Sp-Pxite
Al-augite
Sp-Pxite
44.3
1.05
13.51
8.84
7.95
0.15
18.22
12.85
0.48
0.04
0.01
99.45
38.4
0.40
20.74
8.46
7.61
0.13
22.69
8.41
0.00
0.05
0.00
99.28
533
553
52
371
nd
45
46
838
1310
48
338
nd
28
14
80.33
84.16
SiO 2
TiO 2
Al 2 O 3
Fe 2 OU3
FeOU
MnO
MgO
CaO
Na 2 O
K 2O
P2 O5
Cr
Ni
Sc
V
Rb
Sr
Zr
Mga
Al-augite
Sp-Pxite
43.3
0.61
13.79
9.09
8.18
0.15
17.58
13.35
1.03
0.02
0.03
98.95
838
530
40
296
33
54
8
79.30
Al-augite
Sp-Pxite
51.2
0.30
6.48
7.65
6.88
0.15
19.89
12.37
0.71
0.04
0.01
98.80
739
505
73
259
11
23
23
83.74
Al-augite
websterite
51.3
0.29
6.43
6.99
6.29
0.13
20.12
13.84
0.44
0.03
0.03
99.60
739
478
70
258
8
15
23
85.08
Al-augite
websterite
Al-augite
Sp-Pxite
49.2
0.35
8.65
8.00
7.20
0.15
19.14
13.51
0.36
0.04
0.09
99.49
49.0
0.37
9.57
7.34
6.60
0.14
19.43
13.55
0.00
0.04
0.05
99.49
1160
586
69
281
nd
18
15
1140
531
72
286
nd
27
nd
82.58
83.99
Al-augite
websterite
49.2
0.37
8.56
7.43
6.69
0.15
17.92
14.86
0.71
0.05
0.10
99.35
837
433
79
284
10
66
8
82.70
Al-augite
Sp-Pxite
45.3
0.41
12.68
7.89
7.10
0.14
17.39
14.97
0.44
0.04
0.09
99.35
310
490
51
325
nd
59
14
81.37
Al-augite
Sp-Pxite
44.5
0.44
12.72
7.29
6.56
0.14
17.65
15.34
1.16
0.02
0.00
99.26
185
532
67
307
34
38
8
82.75
Al-augite
Sp-Pxite
Al-augite
websterite
Al-augite
websterite
45.2
0.47
12.29
7.70
6.93
0.15
17.37
15.05
1.44
0.03
0.00
99.70
45.1
0.44
12.04
7.59
6.83
0.14
19.22
13.99
0.75
0.04
0.01
99.32
44.5
1.06
14.62
9.57
8.61
0.16
15.87
12.75
0.66
0.05
0.06
99.30
366
477
71
314
nd
36
nd
305
550
70
297
nd
13
22
909
609
82
245
nd
182
51
81.72
83.38
76.67
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Sample no. 75B-44
297
298
Table 1 Žcontinued.
76B-522B
75B-145
75B-43
76B-351A
78B-711
76B-349A
76B333-3
78B703A3
76B333-1
76B333-2
Suite
Late
gabbro
gabbro
Late
gabbro
gabbro
Late
gabbro
gabbro
Late
gabbro
gabbro
Late
gabbro
gabbro
Late
gabbro
gabbro
Hydrous
HB vein
Hydrous
HB vein
Wall rock
lherz
Wall rock
lherz
SiO 2
TiO 2
Al 2 O 3
Fe 2 OU3
FeOU
MnO
MgO
CaO
Na 2 O
K 2O
P2 O5
47.1
0.76
16.66
8.53
7.68
0.14
13.43
10.55
2.14
0.05
0.05
99.41
46.5
0.85
16.54
7.29
6.56
0.12
13.68
11.45
2.55
0.06
0.03
99.07
46.5
0.93
15.30
9.48
8.53
0.15
13.82
11.04
1.86
0.04
0.01
99.13
46.1
0.87
15.10
6.79
6.11
0.12
16.04
11.87
1.96
0.04
0.01
98.90
44.9
1.01
15.72
9.84
8.85
0.16
13.84
11.86
2.08
0.03
0.03
99.47
50.2
0.40
21.79
1.81
1.63
0.04
6.61
13.24
4.60
0.14
0.02
98.85
41.8
2.97
9.02
7.65
6.88
0.09
26.58
7.75
2.00
0.41
0.01
98.28
44.3
0.32
3.12
9.66
8.69
0.14
39.32
1.95
0.02
0.02
0.00
99.28
40.6
0.46
3.30
9.58
8.62
0.14
40.45
3.19
0.17
0.04
0.00
98.37
Cr
Ni
Sc
V
Rb
Sr
Zr
584
386
57
174
nd
483
24
521
628
33
182
nd
491
68
824
722
42
198
nd
252
60
590
401
42
237
nd
310
73
14
301
22
117
16
1200
nd
3310
2080
50
271
nd
306
22
Mga
75.73
78.81
82.40
73.59
87.86
87.32
674
418
40
212
39
228
70
74.28
34.5
2.79
10.74
7.80
7.02
0.11
27.58
11.37
2.13
0.59
0.00
97.61
1770
2140
52
518
22
304
8
87.51
2090
2100
1
67
0
25
0
88.97
2080
2190
0
71
0
37
6
89.32
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Sample no.
76B333-4
76B333-5
76B333-6
78B703A1
78B703A2
78B703A4
78B703A5
78B703A6
78B703A7
Suite
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
Wall rock
lherz
44.1
0.35
3.21
9.40
8.46
0.14
38.93
2.31
0.44
0.03
0.00
99.38
42.4
0.20
2.48
9.83
8.85
0.14
41.21
1.52
0.39
0.04
0.00
98.65
44.8
0.21
3.01
9.23
8.31
0.13
38.36
2.88
0.14
0.02
0.00
99.23
41.1
0.15
2.16
10.23
9.20
0.13
43.97
0.90
0.00
0.02
0.00
99.25
44.2
0.36
3.43
9.11
8.20
0.13
39.30
2.25
0.09
0.02
0.00
99.36
44.5
0.33
3.38
9.34
8.40
0.14
38.82
2.36
0.00
0.02
0.00
99.32
44.2
0.20
2.72
9.24
8.31
0.14
39.35
2.39
0.00
0.02
0.00
98.72
44.2
0.12
2.06
9.78
8.80
0.14
41.03
1.48
0.00
0.02
0.00
99.30
42.4
0.09
1.71
9.86
8.87
0.14
42.27
1.73
0.00
0.02
0.00
98.66
SiO 2
TiO 2
Al 2 O 3
Fe 2 OU3
FeOU
MnO
MgO
CaO
Na 2 O
K 2O
P2 O5
Cr
Ni
Sc
V
Rb
Sr
Zr
Mga
2520
2060
0
56
0
25
6
89.14
2070
2230
0
52
0
25
0
89.25
2310
2030
0
64
0
23
23
89.17
3180
2620
0
61
0
0
17
89.49
2400
2150
1
118
0
16
19
89.53
2040
2100
12
106
8
35
17
89.17
2420
2040
0
91
0
25
8
89.40
2380
2200
0
64
0
0
12
89.26
1990
2320
11
54
0
11
0
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Sample no.
89.47
299
300
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
contact with the peridotite massif and a less distinct
intrusive contact with the adjacent granulites. Northeast of the massif, the Contact Series has been
repeated by faulting ŽFig. 1.. A layer of metapelitic
granulite Žstronalite. which lies just NE of the Contact Series here probably represents a septum of wall
rock to the gabbronorite intrusives.
Pyroxenites are the dominant lithology of the
Contact Series. Both websterites Ž1–3 mm grain
size. and clinopyroxenite pegmatoids Žcpx up to 15
cm. are common. The websterites grade into gabbronorites with increasing feldspar. The pyroxenites
and gabbronorites consist of pyroxene Žmg-numbers
68–84., amph, plagioclase ŽAn 50 ., garnet, Ti-magnetite, and ilmenite. Garnet forms as exsolution
lamellae in cpx and interstitially.
Peridotites in the Contact Series are dunites or
harzburgites that form lens-shaped pods concordant
with layering in the surrounding pyroxenites and
gabbros. Equigranular-mosaic Žadcumulate. textures,
Fo 82 – 87 olivine, and Cr-spinel with low mg-numbers
Ž52–68. and low cr-numbers Ž10–25. characterize
the peridotites. Hornblendite and amph pyroxenite
veins 1–4 cm thick are common within the peridotites, as are zoned pyroxenitergabbro dikes. The
zoned dikes have pyroxenite selvages Ž1 to 2 cm
thick. against the olivine-rich wall rock, with thick
Žup to 80 cm. cores of anorthositic gabbronorite.
reflects variations in the modal abundance of accumulated minerals in the dikes. Chrome concentrations co-vary with CaO, reflecting relative proportions of Cr-diopside and enstatite in the dikes, and
the high concentration of Cr in cpx relative to opx.
TiO 2 does not co-vary with CaO and Cr, however,
despite its affinity for cpx over opx ŽFig. 5.. In fact,
TiO 2 appears to be highest in the bronzitite dikes,
reflecting in part the common occurrence of pargasite and Ti-phl in these dikes. This suggests that
these bronzitite dikes crystallized from more evolved
magmas than the more common cpx-rich dikes, and
that they contain more residual liquid.
5.1.2. Al-augite suite
Dike rocks of the Al-augite suite exhibit regular
variations in major and trace element concentrations
that reflect in part fractionation of the parent magmas and in part variations in the modal abundance of
accumulated minerals in the dikes. Pyroxenite dikes
are high in MgO, CaO, and Cr, and low in Al 2 O 3 ,
and TiO 2 compared to the late gabbros, whereas FeO
displays a similar range in both ŽFig. 5.. An orthopyroxenite of the Al-augite suite is rich in MgO Žreflecting the high MgO content of opx.. These variations are reflected in the mg-numbers of the dikes,
which range from 82 to 86 in the more primitive
spinel clinopyroxenites to 76 to 83 in the more
evolved spinel-poor websterites.
5. Analytical results
5.1. Whole-rock chemistry
Thirty-three samples of dike rock were analyzed
for major and selected trace elements by X-ray fluorescence ŽXRF. spectrometry ŽAppendix A.. The
results are presented in Table 1. Samples from zoned
Al-augite dikes have the same sample number followed by a letter indicating where in the dike the
sample was taken ŽR s rim, WR s west rim, ER s
east rim, C s core.. In general the rims are spinel
pyroxenites and the cores are spinel-poor websterite.
Subsamples from other zoned dikes labeled A and B
are noted in the descriptions.
5.1.1. Cr-diopside suite
Pyroxenites and websterites of the Cr-diopside
suite exhibit a range in compositions that largely
5.1.3. Late gabbro suite
The late gabbro suite is characterized by gabbronorites with mg-numbers Ž74 to 88. that range
from higher than spinel pyroxenites of the Al-augite
suite Žmg-numbers 82–86. to values similar to the
more evolved spinel-poor websterites. These gabbros, which were intruded late in the kinematic
history of the massif, crystallized at relatively lower
pressures, where plagioclase saturation occurs close
to the liquidus. They are higher in FeOU , TiO 2 ,
Al 2 O 3 , and alkalis than Al-augite suite pyroxenites,
and lower in CaO, Cr, and Sc ŽFig. 5..
5.1.4. Hydrous Õeins
Because of their rarity and small size, we only
have whole rock geochemical data for two hornblendite veins from within the massif. The veins are
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
301
Fig. 5. Mg-number variation diagrams for dike rocks of the Balmuccia massif and for some lherzolite wall rocks. Symbols: Žv . Cr-diopside
suite; Ž`. Al-augite suite; Žj. late gabbro suite; Žl. hornblendites; Žq. lherzolite and dunite.
characterized by high mg-numbers Ž87. and TiO 2 ,
and by lower CaO, Al 2 O 3 , and Cr than correspond-
ing dikes of the Al-augite suite ŽFig. 5.. Bulk rock
analyses of serial slabs of wall rock adjacent to
302
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Fig. 6. Whole rock geochemistry of two hornblendite veins and their adjacent wall rock. Sample divided into series of slabs 1 to 2 cm thick parallel to the veins, and analyzed by
XRF spectrometry. Horizontal axis is the distance from center of each hornblendite vein in centimeters; serial slab compositions plotted at geometric midpoint of slab.
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
303
Table 2
Clinopyroxene and amphibole trace element data by SIMS for three of the rock suites in the Balmiccia massif. Abbreviations for the
material analyzed are as follows: amph, amphibole; cpx, clinopyroxene
Sample and
material analyzed
Rock type
La
Ce
Nd
Cr-diopside lherzolite suite
90B-722-1 cpx Ža. Lherzolite
90B-722-1 cpx Žb.
0.14
0.09
0.48
0.37
1.47
1.24
Spinel pyroxenite suite
75B-158 cpx Ža.
Pyroxenite
75B-158 cpx Žb.
75B-158 cpx Žc.
76B-307R cpx
Pyroxenite
90B-719 cpx Ža.
Pyroxenite
90B-719 cpx Žb.
0.86
1.43
1.31
0.80
0.65
0.67
4.62
7.15
6.69
3.17
2.75
2.70
7.61
8.85
8.67
3.67
3.54
3.81
Late gabbro suite
75B-142 amph
75B-142 cpx
76B-522B cpx
Sr
Sm
6.9 1.02
7.9 0.98
36.7
49.0
47.1
40.9
38.4
39.7
3.55
4.33
3.95
1.90
1.84
1.96
Zr
Ti
7.2
5.4
Dy
Y
Er
Yb
V
Cr
1772 0.42 2.41 13.7 1.55 1.85 281.1 5632
1684 0.41 2.30 12.8 1.48 1.61 290.5 5245
486.2
521.4
477.1
472.7
478.6
491.7
421.8
189.6
181.3
232.3
148.8
175.3
Gabbronorite 3.21 14.67 16.10 596.3 6.44 102.7 32 506 3.26 9.42 58.9 5.75 6.50 806.1
2.35 11.56 12.59 81.6 5.23 118.5 8045 2.46 6.47 34.0 3.37 3.63 329.9
Gabbronorite 1.67 9.02 11.90 81.5 5.30 102.4 10 111 2.05 7.79 40.7 4.31 4.58 437.2
982.0
521.7
773.7
hornblendite veins show that the lherzolite wall rock
is enriched in Al 2 O 3 , CaO, TiO 2 , Na 2 O, and K 2 O
33.1
52.3
35.3
22.8
14.3
12.2
Eu
6694
7857
6804
3738
3430
3384
1.43
1.59
1.48
0.74
0.68
0.72
4.69
5.27
4.92
2.76
2.81
2.87
17.6
24.0
19.2
17.0
16.9
16.5
2.63
2.78
2.78
1.80
1.84
1.54
2.69
3.02
2.87
1.90
1.76
1.68
relative to lherzolite far from the veins ŽFig. 6.. The
overall effect on the major oxides does not seem to
Fig. 7. Trace element patterns for cpx and amph normalized to the PRIMA values of Sun and McDonough Ž1989.. The diagram includes
patterns for cpx from one sample of Cr-diopside suite lherzolite, two samples of Al-augite suite pyroxenites, and three samples of the late
gabbro suite. It also includes one pattern for amph from a late gabbro suite feldspathic websterite. Most of the samples have been analyzed
in duplicates to assess reproducibility. The Cr-diopside websterite compositional range shown with the stippled pattern is from Rivalenti et
al. Ž1995.. See text for discussion.
304
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
persist for more than 4 cm away from these veins,
which are less than 2 cm thick.
5.2. Trace element concentrations in cpx and amph
Concentrations of the rare earth elements ŽREE.
La, Ce, Nd, Sm, Eu, Dy, Er, and Yb, and the trace
elements Sr, Zr, Y, Ti, V and Cr have been determined by secondary ionization mass spectrometry
ŽSIMS. on cpx and amph and normalized to the
primitive mantle ŽPRIMA. values of Sun and McDonough Ž1989.. The data are listed in Table 2 and
are presented graphically in Fig. 7. The Cr-diopside
Fig. 8. Sets of diagrams with bulk-rock REE and trace element abundance patterns determined by INAA for two hornblendite veins
Ž76B-333 and 78B-703A. and serial slabs of the wall rock around them. The legends for diagrams Žb. and Žd. also apply to diagrams Ža. and
Žc., respectively. The numerical values marking each symbol are distances in centimeters on either side of each vein. See text for discussion.
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
suite sample analyzed in duplicates as a measure of
reproducibility Ž90B-722-1. is a typical lherzolite, far
from any of the dikes in the massif. Its cpx has
strong light REE depletions ŽLa at ; 0.1 to 0.2
times PRIMA. and flat heavy REE at ; 3 times
PRIMA. Moreover, it has the lowest Sr, Zr and Ti
concentrations measured, but has more pronounced
positive anomalies of Y and V compared to the other
samples. The stippled fields on the diagram in Fig. 4,
based on SIMS cpx data by Rivalenti et al. Ž1995.,
show that websterite dikes of the same Cr-diopside
suite are generally more enriched in the light REE
than the host lherzolite Žassuming that we analyzed a
representative sample of the host rock.. These dikes
also have slightly lower heavy REE, similar Zr and
Ti, and higher Cr and Sr compared to the host
lherzolite.
Two samples of the Al-augite suite Ž75B-158 and
90B-719. have been analyzed in triplicate and duplicate, respectively, to document reproducibility, and
one other sample from this group Ž76B-307R. has
been analyzed just once ŽFig. 7.. Cpx from the three
samples displays light REE concentrations that are
either slightly higher than or are comparable to those
in cpx from the Cr-diopside suite dikes. However,
they have higher heavy REE concentrations overall
compared to the cpx from the Cr-diopside suite
websterites, but overlap with the cpx from the host
lherzolite sample. Also, cpx from all three samples
305
of the Al-augite suite have Sr, Zr, Y, Ti, V and Cr
concentrations that are higher than those in cpx from
all the rocks of the Cr-diopside suite.
Two samples of the late gabbro suite have been
analyzed — both amph and cpx from sample 75B142 and cpx from sample 76B-522B ŽFig. 7.. Cpx
from this suite has the highest overall concentrations
of REE, and additionally exhibits the only positive
Eu anomaly observed for all rock suites. Sr, Zr, Y,
and Ti in the late gabbro suite cpx are also the
highest of all those measured. V and Cr in cpx are
intermediate between the low and high values for
cpx in the Cr-diopside suite lherzolite and Al-augite
suite dikes. Amph from gabbronorite sample 75B-142
has the highest overall concentrations for all of the
elements except Zr and Cr.
5.3. Trace elements in bulk hornblendite Õeins and
adjacent serialized host lherzolite domains
Two hornblendite veins of nearly pure kaersutite
Ž76B-333 and 78B-703. and serialized adjacent host
lherzolite domains have been analyzed for bulk rock
REE and Ba, Th, U, Ta, Hf, Zn, Sc, Cr, Co, and Ni
concentrations to assess the character and degree of
ionic exchange between the host rock and the fluid
phase or magma that formed the veins ŽFig. 8 and
Table 3.. Both veins have convex though not identical REE patterns with the middle REE reaching 5
Table 3
Trace element data Žby INAA. for two hornblendite veins and their serialized host lherzolite slabs. Concentrations are in parts per million
Sample
From
vein Žcm.
76B-333-1
76B-333-2
76B-333-3
76B-333-4
76B-333-5
76B-333-6
76B-333-7
78B-703A-1
78B-703A-2
78B-703A-3
78B-703A-4
78B-703A-5
78B-703A-6
78B-703A-7
y2.1
y1.0
Hbl vein
q1.25
q2.75
q4.25
) 4.25
y3.1
y1.5
Hbl vein
q1.5
q3.0
q4.5
q6.1
Ba Th
U
Ta
La
0.16
0.10
0.1 0.14 0.47
0.1
0.02 0.08
0.06
18
0.12
0.21
0.08
0.06
100
0.41
0.09
0.09
0.06
15
0.07
15
Ce
Nd Hf
0.68
0.76
2.40 5
0.24
1.10
0.81
0.21
0.22
0.45
0.19
0.17
0.24
0.32
0.08
0.21
0.34
Sm
0.24
0.35
1.88
0.26
0.15
0.31
0.30
0.08
0.25
1.79
0.22
0.16 0.18
0.17 0.10
0.09
Eu
Tb
0.08
0.13
0.78
0.10
0.06
0.11
0.12
0.03
0.08
0.73
0.08
0.07
0.04
0.03
0.06
0.08
0.47
0.05
0.07
0.08
0.10
0.02
Yb
0.31
0.37
1.64
0.33
0.24
0.34
0.47
0.06
0.21
0.44 0.84
0.05 0.17
0.21
0.14
0.21
Lu
Zn Sc
Cr
Co
Ni
0.05
0.05
0.22
0.05
0.03
0.06
0.07
0.01
0.04
0.14
0.04
0.03
0.02
0.03
60
53
43
54
45
43
53
61
61
84
51
50
48
43
2122
2221
3376
2601
2183
2340
2462
3012
2362
1908
2040
2552
2385
2232
113
106.2
88.3
112.1
115.9
108.6
102
132
110.9
88.7
109.4
107.2
115.6
116
2041
1986
2012
2043
2141
1966
1877
2552
2124
1970
2067
1996
2132
2202
10.8
12.7
31.5
11.4
9.5
13.7
16.4
6.3
13.3
34.4
12.7
13.9
10.5
10.8
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
306
times PRIMA and the light REE considerably less
abundant than the heavy REE. In addition, they have
on average at least five times the concentration for
every REE analyzed Žexcept La and Ce. compared to
the host lherzolite slabs. This difference in concentrations is also generally true for the rest of the
highly incompatible trace elements Že.g., Ba and Th.,
but not for the most compatible elements Že.g., Cr,
Co and Ni.. Most striking of all for sample 76B-333
is that slabs of host lherzolite closest to the hornblendite vein do not have the highest trace element
concentrations. Rather slabs at 4.25 and ) 4.25 cm
from the contact have the higher values compared to
slabs at 1, 2.1 and 2.75 cm away from the contact.
Moreover, the trace element distribution is not symmetrical, in the sense that slabs that are equal distant
from but on opposite sides of the vein do not have
the same trace element concentrations.
In contrast, for sample 78B-703A, slabs that are
closest on either side of the hornblendite vein have
the highest trace element concentrations. Asymmetry
in trace element distributions is observed for this
sample as well in the slabs at about 3 cm on either
side of the vein. Though not every element was
analyzed in all of the samples, the limited available
data indicate that Ba and Th are enriched in both the
hornblendite veins and adjacent host lherzolite. While
there are differences between slabs, overall most
have not deviated much in pattern and concentration
from the ‘‘average lherzolite from Balmuccia’’ values in ŽHartmann and Wedepohl, 1993..
5.4. Isotopic compositions
Mineral separates from 16 samples have been
analyzed for the isotopic compositions of Sr and Nd,
Table 4
Rb–Sr, Sm–Nd and U–Pb isotopic and concentration data. Analytical errors for 87 Srr86 Sr and 143 Ndr144 Nd are based on the 2 s in-run
statistics. Errors for the Pb isotopic ratios are 0.1%, based on replicate analyses of NIST standard SRM-981. Abbreviations for the material
analyzed are as follows: amph, amphibole; cpx, clinopyroxene. The ´ values were calculated using the 143 Ndr144 Nd value for chondritic
uniform reservoir ŽCHUR. at 250 Ma of 0.512316
Sample and
material analyzed
Rock type
Rb Žppm.
Sr Žppm.
87
Rbr
86
Sr
Ž 87 Srr
86
Sr. t 250
Sm Žppm.
Nd Žppm.
147
Smr
Cr-diopside lherzolite and websterite suite
90B-715 cpx
Websterite 0.001
90B-716 cpx
Websterite 0.002
90B-718 cpx
Lherzolite
0.003
90B-721 cpx
Websterite 0.005
90B-722-1 cpx
Lherzolite
90B-723-4 cpx
Lherzolite
25.9
36.2
13.6
23.6
5.7
5.2
0.00013
0.00013
0.00055
0.00058
0.70334 " 1
0.70374 " 1
0.70349 " 1
0.70324 " 1
0.70345 " 1
0.70303 " 6
0.1
1.1
1.3
0.7
0.9
0.8
4.1
2.7
1.7
1.1
1.1
0.15800
0.30594
0.24614
0.46044
0.47080
Spinel pyroxenite suite
75B-158 cpx
Pyroxenite
76B-300C cpx
Websterite
76B-300R cpx
Pyroxenite
76B-302C cpx
Websterite
76B-302R cpx
Pyroxenite
76B-307C cpx
Websterite
76B-307R cpx
Pyroxenite
90B-719 cpx
Pyroxenite
0.133
0.042
0.051
0.024
0.041
0.031
0.054
0.001
34.5
26.5
28.5
25.6
28.9
29.9
29.4
16.8
0.0028
0.00098
0.00114
0.00045
0.00091
0.00053
0.00122
0.00025
0.70270 " 1
0.70320 " 1
0.70305 " 1
0.70301 " 1
0.70324 " 1
0.70302 " 2
0.70295 " 1
0.70311 " 2
3.3
2.1
0.9
1.1
1.5
1.8
1.6
1.1
8.0
4.9
2.0
2.5
3.7
4.3
3.7
2.3
0.25167
0.26064
0.26715
0.26557
0.25396
0.25320
0.26223
0.29095
Gabbronorite 1.489
Gabbronorite 0.042
282
75.4
0.00925
0.00031
0.70214 " 1
0.70216 " 1
4.9
4.1
13.1
10.2
0.22556
0.24578
0.70289 " 1
4.8
15.8
0.18316
Late gabbro suite
75B-142 amph
76B-522B cpx
Hornblendite suite
90B-725 amph
Hornblendite
348
144
Nd
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
and most for the concentrations of Rb, Sr, Sm, and
Nd as well ŽTable 4 and Fig. 9.. Thirteen of the
samples have also been analyzed for their Pb isotopic compositions, and when possible for their U
and Pb concentrations. Details for the analytical
procedures are provided in Appendix A. It is assumed in presenting these results that appreciable
diffusive exchange between minerals in the lherzolite
and dikes was possible until the massif cooled
through the blocking temperature for zircon of
; 9008C ŽMezger, 1990. at 250 Ma ŽWright and
Shervais, 1980.. Hence, ´ Nd values and ‘‘initial’’ Sr
isotopic ratios have been calculated for 250 Ma.
ŽZindler and Hart, 1986., local granulite and kinzigite in the crustal basement ŽPin and Sills, 1986;
Voshage et al., 1988; Lu et al., 1997., and our earlier
determinations ŽShervais and Mukasa, 1991. on six
Cr-diopside suite websterite dikes, two late-stage
gabbronorite dikes, two hornblendite veins in lherzolite, one phl vein in lherzolite, and two Contact
Series rocks from outside the massif Ža pyroxene
pegmatoid and a hornblendite vein in cumulate
dunite.. The five new Cr-diopside samples are remarkably similar in their Nd and Sr isotopic compositions to our earlier six determinations for this suite,
and together define a cluster that fall within the field
for ocean island basalts ŽOIB. at the time of emplacement of the massif at 250 Ma ŽFig. 9.. This
range exceeds the analytical uncertainties for each
point and is clearly unlike MORB.
However, the cluster would be even smaller were
it not for samples 75B-194 and 90B-716 which have
the lowest ´ Nd values. Sample 75B-194 is a 3-cm
5.4.1. Nd and Sr isotopic compositions
The new Nd and Sr isotopic data represent five
samples from the Cr-diopside suite, eight from the
Al-augite suite, two from the late gabbro suite, and
one from the hydrous vein suite. These are compared
in Fig. 9 to the mid-ocean ridge basalt ŽMORB. field
Ž 143 Ndr
144
Nd. t 250
Ž ´ Nd . t250
U Žppm.
0.51255 " 2
0.51264 " 2
0.51260 " 2
0.51307 " 2
0.51276 " 2
q 4.6
q6.4
q5.6
0.51260 " 2
0.51264 " 2
0.51267 " 2
0.51266" 1
0.51262 " 1
0.51262 " 1
0.51265 " 1
0.51259 " 1
q5.6
q6.3
q7.0
q6.8
q6.0
q5.9
q6.5
q5.4
0.005
0.51284 " 1
0.51283 " 2
q10.2
q10.0
0.51270 " 2
q 7.6
Pb Žppm.
m
0.30
0.01
0.004
0.01
307
206
Pbr
204
Pb
207
Pbr
204
Pb
208
Pbr
18.512
18.597
15.660
15.654
38.396
38.549
0.30
0.13
0.13
0.9
5.5
18.422
18.564
18.534
15.592
15.623
15.572
38.240
38.375
38.276
2.2
0.004
0.003
0.004
0.005
0.005
0.009
0.1
0.3
0.4
0.3
0.2
0.4
0.2
0.3
0.6
0.7
1.0
0.7
1.5
1.7
18.548
18.338
18.345
18.363
18.367
18.260
18.431
18.357
15.594
15.618
15.609
15.612
15.595
15.594
15.598
15.612
38.557
38.150
38.224
38.247
38.215
38.046
38.134
38.293
0.006
0.007
0.25
0.05
1.4
9.2
18.145
18.308
15.586
15.564
37.951
37.952
18.517
15.592
38.306
q8.6
0.24
204
Pb
308
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
Fig. 9. Sr–Nd: Ža. ´ Nd vs. initial 87 Srr86 Sr diagram for cpx, amph, and phl mineral separates from the various dike suites of the Balmuccia
massif. Assuming that diffusive exchange between minerals was possible until emplacement of the peridotite body into the lower crust, the
isotopic compositions have been corrected for decay using an age of 250 Ma ŽWright and Shervais, 1980.. Data for the Contact Series rocks
and 10 of the other 27 points plotted are from Shervais and Mukasa Ž1991.. Granulite and kinzigite data, which overlap the range plotted
Žhere shown with an arrow., are from Pin and Sills Ž1986., Voshage et al. Ž1988., and Lu et al. Ž1997.. The MORB field is from Zindler and
Hart Ž1986. and references therein.
single crystal from an unrecrystallized websterite
dike with 87 Srr86 Sr s 0.70351 and 143 Ndr144 Nd s
0.51242 Ž ´ Nd s q2.2. while 90B-716 comes from
the less deformed group of the Cr-diopside suite
websterites and has 87 Srr86 Sr s 0.70374 and
143
Ndr144 Nd s 0.51253 Ž ´ Nd s q4.2.. These values may reflect the primary compositions of at least
some of the Cr-diopside websterite dikes before recrystallization and re-equilibration with the host lherzolites Ž87 Srr86 Sr s 0.70339–0.7055, ´ Nd s q6.5 to
q7.2, determined on whole-rock powders; Voshage
et al., 1988..
All of the cpx separates from the Al-augite suite,
except sample 75B-158, overlap completely with the
field for the volumetrically dominant Cr-diopside
suite. This suggests either that melting of lithospheric materials similar to the Cr-diopside suite
generated the Al-augite suite magmas or that these
dikes may have had a different source area but have
largely equilibrated with the Cr-diopside suite via
subsolidus diffusion.
The two new analyses of cpx from the late gabbro
dike suite Ž75B-142 and 76B-522B. are strongly
depleted in the heavy isotopes of Sr and Nd relative
to the other samples studied here, with 87 Srr86 Sr s
0.70217–0.70218 and 143 Ndr144 Nd s 0.51280–
0.51281 Ž ´ Nd s q9.4–q 9.6. ŽFig. 9 and Table 4.,
which is indistinguishable from the earlier two determinations for this suite by Shervais and Mukasa
Ž1991.. These isotopic compositions are similar to
MORB and are distinct from the OIB-like compositions that characterize other lithologies of the massif.
One new kaersutite from hornblendite suite sample 90B-725 has been analyzed for comparison with
the two earlier determinations by Shervais and
Mukasa Ž1991.. Two of the three samples are very
similar in their Nd and Sr isotopic compositions to
the Cr-diopside suite and the majority of the samples
from the Al-augite suite. The third one has a
MORB-like Sr isotopic composition though a somewhat lower Nd value ŽFig. 9.. We have included on
the diagram in Fig. 9 the field for the isotopically
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
309
Fig. 10. Ža. 207 Pbr204 Pb vs. 206 Pbr204 Pb and Žb. 208 Pbr204 Pb vs. 206 Pbr204 Pb variation diagrams for cpx and amph mineral separates
from the dike suites of the Balmuccia massif. The symbols used are the same as in Fig. 9. Note that there are still no Pb data for the rocks of
the Contact Series. The present-day MORB field and Northern Hemisphere reference line ŽNHRL. from Hart Ž1984. and Zindler and Hart
Ž1986. are included for comparison.
310
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
enriched pegmatoidal pyroxenite, hornblendite and
glimmerite Ž) 90% phl. first reported by Shervais
and Mukasa Ž1991., now all recognized to be associated with the intrusive Contact Series along the
southeastern margin of the massif.
5.4.2. Pb isotopic compositions
The Pb isotopic data are presented in Table 4 and
diagrams in Fig. 10. Their acquisition from Balmuccia mineral separates has been a challenge because
of the exceedingly low concentrations of Pb Ž; 50–
400 ppb.. For most samples, however, the blank was
- 10% of the total Pb analyzed. One measure of the
robustness of the Pb data is the good reproducibility
obtained on cpx separates from the margins and
interiors of the compositionally zoned Al-augite suite
dikes Žspinel pyroxenite at the margins and websterite in the interiors for samples 76B-300, 76B-302,
and 76B-307.. U concentrations in the cpx are - 12
ppb which in the average sample sizes used for
isotope dilution Ž; 6 mg. yielded ; 70 pg of U or
less. This value not being much larger than the total
U blanks of 10–40 pg Žwith most around 20 pg., we
have decided not to make any decay corrections on
the Pb ratios, except for one demonstration data
point which shows the effect to be small. This data
point is shown as a black square with a cross which
with the decay correction shifts to the position of the
gray square with a cross on the 207 Pbr204 Pb vs.
206
Pbr204 Pb diagram ŽFig. 10a.. The effect on the
208
Pbr204 Pb vs. 206 Pbr204 Pb diagram ŽFig. 10b.
seems to be more substantial. However, the ThrU
value used in the decay correction was assumed from
generalizations about mantle compositions.
Fig. 10 shows the Pb isotopic compositions to be
more diverse in the Cr-diopside suite than in either
the Al-augite suite or the late gabbro suite. Within
the Cr-diopside suite, the websterite dikes are more
heterogeneous than the host lherzolite samples and
include samples with the most radiogenic Pb compositions. Similar to the relationships on the Nd–Sr
diagram in Fig. 9, the field for the Al-augite suite
overlaps almost completely with the Cr-diopside
field, but is itself much smaller and toward the less
radiogenic end of the cluster. The least radiogenic
values of all belong to an amph from sample 75B-142
and cpx from sample 76B-522B, two gabbronorites
of the late gabbro suite ŽTable 4 and Fig. 10.. This
character is consistent with the MORB-like Nd and
Sr isotopic signatures the samples exhibit in Fig. 9.
Note that all of the samples fall above the present-day
MORB field on the 207 Pbr204 Pb vs. 206 Pbr204 Pb
diagram ŽFig. 10a. but that they fall within it on the
208
Pbr204 Pb vs. 206 Pbr204 Pb diagram ŽFig. 10b..
The significance of this might be a time-integrated
record of high UrTh in the lithospheric mantle
represented by the Balmuccia peridotites and in the
sublithospheric source materials for the dikes and
veins.
6. Discussion
6.1. Relationship between tectonism and deformation
The Balmuccia massif represents a small fragment
of subcontinental lithospheric mantle which was emplaced into lower continental crust during an episode
of Late Permian extensional tectonics ŽShervais and
Mukasa, 1991.. Before its emplacement at crustal
levels, this lithospheric mantle underwent a long
history of partial melting, magmatic intrusion, and
plastic flow deformation. The evidence for these
events is preserved as dikes of the four main intrusive series described here: the Cr-diopside dike suite,
the Al-augite dike suite, the late gabbro dike suite,
and the hydrous vein suite Žhornblendites.. These
dikes record the evolution of the subcontinental
lithospheric mantle of southern Europe before the
Late Paleozoic Hercynian orogeny, and provide us
with insights into the processes involved in the
chemical fractionation of the Earth’s mantle.
6.2. Dike suites in the subcontinental lithospheric
mantle
6.2.1. Cr-diopside suite
Field and chemical evidence shows that websterites of the Cr-diopside suite represent a series of
magmatic events that affected the upper mantle over
an extended period of time. Chemical variations
within dikes of the Cr-diopside suite reflect crystal
accumulation from a magnesian parent magma.
Pyroxenite and websterite compositions can be effectively modeled as mixtures of Cr-diopside and
enstatite, with only traces of residual liquid Žnow
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
represented modally by interstitial amph and phl..
The predominance of Mg-rich mineral compositions
in these dikes implies that the parent magmas underwent limited evolution by crystal fractionation within
the mantle. However, these dikes may have been
affected by melt extraction events after their formation.
Many Cr-diopside suite dikes are clearly intrusive
into their surrounding rocks. In particular, some
younger Cr-diopside suite dikes crosscut older, concordant dikes at high angles and offset them dilationally Že.g., Fig. 2b.. The evidence for at least three
generations of Cr-diopside suite dikes, and the occurrence of older Cr-diopside suite layers that have been
affected by later melting events, shows that this suite
does not represent a single magmatic episode.
Isotopically, websterites of the Cr-diopside suite
resemble OIB. Similar isotopic compositions were
found in whole-rock samples of lherzolite by Voshage
et al. Ž1988.. The mantle source region of the Balmuccia massif formed part of the subcontinental
lithospheric mantle before its emplacement, and it
was underlain by MORB-type asthenosphere Žshown
by the intrusion of MORB dikes late in its history..
Evidence from xenoliths ŽFrey and Green, 1974;
Frey and Prinz, 1978; Downes, 1987; Zindler and
Jagoutz, 1988; Menzies, 1989., alpine massifs
ŽLoubet and Allegre, 1982; Menzies and Halliday,
1988; Fabries et al., 1991; Menxies and Dupuy,
1991; Reisberg et al., 1991., mafic lavas ŽWeaver
and Tarney, 1981; Fitton et al., 1988; Leat et al.,
1988; Hart et al., 1989., and geophysical constraints
ŽJordan, 1978, 1988. suggests that many sections of
subcontinental lithospheric mantle formed by the
reenrichment of depleted, ‘‘infertile’’ peridotie similar to MORB-source asthenosphere. This implies that
subcontinental lithospheric mantle is chemically and
isotopically zoned either vertically or laterally Že.g.,
Menzies, 1989, 1990; Menxies and Dupuy, 1991..
6.2.2. Al-augite suite
We suggest that the Al-augite suite formed by
polybaric fractionation of an alkaline, aluminous parent magma yielding the spinel pyroxenites and websterites that crosscut the older Cr-diopside suite dikes.
Formation of the Al-augite dike series by within-dike
crystal fractionation is demonstrated by mineral
chemical and whole-rock chemical trends of FeOU ,
311
TiO 2 , Al 2 O 3 , and alkali enrichment, and by field
relations which show within-dike zonations from
spinel pyroxenite to websterite ŽShervais, 1979b;
Sinigoi et al., 1983.. In zoned dikes, more primitive
mineral and whole-rock compositions are found in
the dike margins Žadjacent to the wall rock. whereas
the more evolved mineral and whole rock compositions are found in the cores ŽFig. 3.. These relationships imply that fractionation proceeded by nucleation of crystals on the dike walls, and by reaction
between the evolving magma and its magnesian wall
rock.
Isotopically, rocks of the Al-augite dike series
have OIB-like compositions that overlap dikes of the
Cr-diopside suite. This implies that, despite the differences between these two suites in parent magma
composition and evolution, they were derived from
an isotopically similar source region in the lower
subcontinental lithospheric mantle.
6.2.3. Late gabbro suite
The late gabbros crystallized at lower pressures
than the Al-augite suite pyroxenites, where plagioclase saturation occurs near the liquidus. They commonly have reacted margins of pyroxenite that separate the feldspar-rich core from the olivine-rich wall
rocks. Mineral chemical and whole-rock chemical
trends suggest that the parent magma of the late
gabbro dikes was lower in TiO 2 and higher in Al 2 O 3
and Na 2 O than the older Al-augite dike series. In
addition, the late gabbro dikes have isotopic composition similar to MORB, and must have been derived
from the convecting asthenosphere below the lithospheric source of the lherzolite massif. Thus,
emplacement of these dikes is probably related to
extensional thinning of the lithosphere and the concomitant rise of asthenosphere mantle during extension ŽShervais, 1979a; Shervais and Mukasa, 1991..
Cpx in the late gabbro suite is also predominantly
Al-augite, which might suggest a genetic link between this suite and the spinel-rich pyroxenites of
the Al-augite suite Žsensu stricto.. The MORB-like
isotopic compositions of cpx from the late gabbros,
however, show that these rocks and Al-augite suite
pyroxenite and websterites were derived from at
least two isotopically distinct source regions within
the mantle, and do not represent differentiation of a
single consanguineous magma suite.
312
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
6.2.4. Hydrous Õein suite
Hornblendites of the hydrous vein suite are enigmatic. Amphiboles in the veins are compositionally
distinct from accessory amph in the older Al-augite
suite dikes and thus cannot represent residual fluids
filter-pressed from pyroxenites, as has been proposed
for hornblendite veins in the Lherz massif ŽWilshire
et al., 1980.. Chemically similar accessory amph are
found in the late gabbro dikes; however, the vein
amph have OIB-like isotopic signatures similar to
Cr-diopside websterites and the older Al-augite suite
dikes, and distinct from the MORB-type signatures
of the younger gabbro dikes. These data suggest that
the hornblendite veins represent a separate intrusion
of water-rich magma, derived by small fractions of
partial melting of the lower subcontinental lithospheric mantle ŽShervais and Mukasa, 1991..
Ionic exchange between the hydrous vein melts
and the lherzolite wall rocks occurred over distances
of just a few centimeters. While the asymmetrical
distribution of trace elements in the serialized lherzolite domains around the hornblendite veins might
suggest some chromatographic effects Že.g., Navon
and Stopler, 1987., the evidence is more compelling
in the Balmuccia massif for two other possibilities.
One is that the trace element heterogeneity could be
accounted for by the decimeter-scale mineralogical
variations in the host rocks. The other is that dikes
and veins remobilized by melting, and there is ample
evidence of this, have left ghost domains in the
lherzolite with elevated trace element concentrations.
tism, with the addition of abundant kaersutite immediately adjacent to the vein, and progressively less
amph further away. Interstitial amph in lherzolite
near the vein is rich in K 2 O and has a high
K 2 OrNa 2 O ratio; K 2 O concentration and K 2 Or
Na 2 O drop rapidly at greater distances from the vein
ŽFig. 4.. Similar variations are seen in whole rock
compositions ŽFig. 6..
Both situations apparently involve the percolation
of magma into the wall rock from the intrusions and
reaction between this magma and the pre-existing
phases. In both cases, the extent of metasomatism
does not extend more than a few decimeters into the
lherzolite wall rock. This distance is controlled in
part by the volume of magma or metasomatic fluid
available: metasomatized zones are thinnest next to
cm-scale hornblendite veins, and thicker next to
decimeter scale dikes. When this magma penetrates
the wall rock, some components Že.g., K 2 O. are
quickly consumed by reactions between the magma
and pre-existing phases, leaving a residual melt which
is depleted in these components relative to the parent
magma. These data show that pervasive metasomatism of lithospheric mantle is not possible without
large volumes of magma or metasomatic fluids. It is
important to remember, however, that this fluid is
added to the lithosphere most effectively as dikes
and veins which alter the bulk composition of the
lithosphere Žon the scale of partial melting. even if
metasomatism of the lherzolite wall rock is limited.
6.4. Crust–mantle interactions during emplacement
6.3. Metasomatic effects of dike and Õein intrusion
on wall rock
Lherzolites adjacent to Al-augite suite dikes and
hydrous veins exhibit systematic changes in their
whole-rock chemistry, mineral chemistry, and modal
mineralogy, which reflect metasomatic exchange between the lherzolite and the intrusive magmas. These
changes differ in their nature and extent depending
on the size and composition of the intruding magma.
Lherzolite adjacent to Al-augite suite dikes is enriched in Al, Fe, and Ti, as shown by lower mgnumbers in pyroxenes adjacent to Al-augite dikes,
and by spinels with lower Cr2 O 3 and higher Al 2 O 3
near the dikes ŽFig. 3.. Lherzolite adjacent to hornblendite veins undergoes pervasive modal metasoma-
The last magmas to accompany the massif during
its rise to crustal levels ponded at the crust–mantle
interface to form the layered Contact Series. These
melts crystallized at lower pressures than the older
dike rocks within the massif, and experienced more
extensive fractional crystallization. During crystallization, the Contact Series magma assimilated portions of the adjacent gabbro granulites, and absorbed
fluids containing an isotopically enriched component. These interactions with crustal material resulted
in the addition of a crustal isotopic signature to the
Contact Series that obscured their original magmatic
affinity. Fluids from the Contact Series penetrated
the adjacent lherzolite body Ždriven by chemical
potential gradients between the lherzolite and the
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
magma. to form phl veins and disseminated phl;
other fluids penetrated cumulate dunites within the
Contact Series and Lower Layered Series to form
hornblendite veins. All of these hydrous vein minerals have crustal isotopic signatures, in contrast to the
hornblendite veins within the massif.
7. Conclusions
The Balmuccia lherzolite massif preserves clearly
the evolutionary development of subcontinental
lithospheric mantle. The pre-emplacement history of
this massif involved repeated episodes of partial
melting and magma intrusion, accompanied by continuous plastic deformation. Judging from the intense
degree of deformation exhibited by the oldest dikes,
it appears that many of these dikes may have been
injected into the peridotite before the massif became
stabilized at the base of the subcontinental lithospheric mantle. The occurrence of minerals with
OIB-like Nd and Sr isotopic compositions shows that
melts with these geochemical characteristics are not
always direct products of plume magmatism originating at deeper levels in the mantle. These results
suggest that such melts could be generated in the
subcontinental mantle lithosphere. Though not constrained by our results, it is likely that the subcontinental lithospheric mantle acquired the isotopic signature from the diapiric rise of OIB-asthenosphere
plumes. Later emplacement of this subcontinental
mantle lithosphere at crustal levels involves extensional orogenesis, accompanied by renewed melting
and magma intrusion. The provenance of these melts
changes with time, from lithosphere derived to asthenosphere derived, in response to thinning of the
lithosphere and uplift of the underlying asthenosphere.
The generation of basaltic melts from mantle
source regions that have undergone similar complex
histories clearly involves a variety of processes whose
effects will be difficult to discern. The remelting of
older dike rocks in particular offers the possibility of
tapping chemically and isotopically distinct source
regions that may coexist on a scale of meters. If
these dikesrveins include hydrous phases rich in
light rare earth elements ŽLREE. and RbrSr, later
melting events can create a cryptic enrichment in the
313
heavy isotopes of Sr and Nd which can no longer be
correlated with the older modal metasomatic event.
The complexities observed over short distances
within the Balmuccia massif also suggest caution in
the interpretation of xenolith chemical and petrologic
data, as we cannot observe the structural context
where they originated.
8. Uncited reference
Wernicke, 1985
Acknowledgements
This manuscript benefited greatly from reviews
by Tanya Furman and Bill McDonough, and from
discussions with our colleagues, H.G. Wilshire,
J.E. Nielsen and A.V. Andronikov. We also wish
to thank A. Koh and S.R. Zeff for assistance
with mineral separations, mass spectrometry and
manuscript preparation. This work was supported by
the National Science Foundation grants to S.B.M.
and J.W.S.
Appendix A. Analytical methods
A.1. Sample preparations
Whole-rock powders were prepared from samples
free of any visible surface weathering using standard
procedures of jaw-crushing pre-cleaned rock chips.
Mineral separates were also prepared from unweathered samples using nylon sieves for sizing, a magnetic separator, and hand picking under a binocular
microscope. Prior to dissolution for the isotopic analyses, the mineral separates were acid washed in
warm, distilled 2.5 N HCl for 15 min, and warm
distilled 5% HF also for 15 min, with an H 2 O rinse
after each of these steps.
A.2. XRF and instrumental neutron actiÕation analysis (INAA)
XRF analyses were performed with a Philips PW1400 spectrometer at the University of South Carolina. Samples were prepared as fluxed glass disks
using a Li tetraborate flux Ž5r1 ratio., and all data
were reduced using the fundamental parameters algorithm of Rousseau Ž1989.. Concentrations for the
314
S.B. Mukasa, J.W. SherÕaisr Lithos 48 (1999) 287–316
trace elements Ba, Th, U, Ta, Hf, Zn, Sc, Cr, Co,
and Ni as well as all the REE were determined by
INAA using reactor and counting facilities at the
University of Michigan. Estimated uncertainties for
the XRF are "2% or better and those for INAA fall
between "1 ŽLa. and "3% ŽLu..
A.3. Secondary ionization mass spectrometry
Mineral trace element compositions were determined with a Cameca IMS-3f ion microprobe at the
Woods Hole Oceanographic Institution. The analytical procedures for this instrument are described by
Shimizu and LeRoex Ž1986. and references therein.
A.4. Thermal ionization mass spectrometry (TIMS)
After standard dissolution and column procedures
described by Mukasa et al. Ž1987, 1991., each sample was dried to a solid, treated with a drop of 14 N
HNO 3 , re-dried and then loaded on appropriate filaments Žsingle rhenium for Pb, Sr, and Rb, and triple
tantalum–rhenium–tantalum for Nd and Sm.. Lead
was loaded with a silica gel-phosphoric acid solution, Sr with tantalum tetrachloride, and Rb, Sm and
Nd with a 10% nitric acid solution. The samples
were run on VG Sector thermal ionization mass
spectrometers at the University of Michigan. Lead
isotopic compositions are corrected for fractionation
using a factor of 0.12 " 0.02% per atomic mass unit,
based on replicate analyses of NIST Standard NBS981. Nd and Sr ratios were normalized to 146 Ndr
144
Nd s 0.721900 and 86 Srr88 Sr s 0.119400, respectively. Measurements for the NIST Standard
SRM-987 give 87 Srr86 Sr s 0.710245 " 10, and for
the La Jolla Nd Standard values of 143 Ndr144 Nd s
0.511842 " 10. Total blanks averaged 0.04 ng for
Pb, 0.02 ng for U, 0.02 ng for Nd, 0.02 ng for Sm,
0.07 ng for Rb and 0.1 ng for Sr.
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