Journal of Petrology Advance Access originally published online on April 9, 2008
Journal of Petrology 2008 49(5):937-970; doi:10.1093/petrology/egn012
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Petrology and U–Pb Zircon Geochronology of Amphibole-rich Cumulates with Sanukitic Affinity from Husky Ridge (Northern Victoria Land, Antarctica): Insights into the Role of Amphibole in the Petrogenesis of Subduction-related Magmas
1CNR–Istituto Di Geoscienze E Georisorse, U.O. Di Pavia, Via Ferrata 1, I27100 Pavia, Italy
2Dipartimento Di Scienze Della Terra, Università Di Pavia, Via Ferrata 1, I27100 Pavia, Italy
RECEIVED MAY 14, 2007; ACCEPTED FEBRUARY 18, 2008
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONA microanalytical trace element and geochronological study was carried out on mafic amphibole-rich cumulates (quartz diorites) cropping out in northern Victoria Land (Antarctica). Associated tonalites and basement rocks were also investigated. Rock textures and major and trace element mineral compositions reveal the presence in quartz diorites of two mineral assemblages: (1) clinopyroxene-I + brown amphibole ± dark mica; (2) clinopyroxene-II + green amphibole + plagioclase + quartz. Both mineral assemblages contain mafic phases with elevated Mg-number, but their trace element signatures differ significantly. In situ U–Pb zircon geochronology was carried out to support petrogenetic and geological interpretations. Quartz diorites were emplaced in the mid-crust probably at 516 ± 3 Ma. Parental melts of quartz diorites were computed by applying solid/liquid partition coefficients. The melt in equilibrium with the first mineral assemblage (melt-I) is extremely depleted in heavy rare earth elements (HREE), Y, Ti, Zr and Hf (at about 0·2 times normal mid-ocean ridge basalt) and enriched in B, Th, U, the large ion lithophile elements and light REE (LREE). It shares many similarities with sanukitic melts (e.g. Setouchi andesites), which originated by equilibration of subduction-derived sediment melts with a refractory mantle. The melt in equilibrium with the second mineral assemblage (melt-II) is characterized by a steep LREE enrichment (LaN/YbN up to 39), a U-shaped HREE pattern and low Ti, which is depleted relative to HREE. The trace element signature of melt-II can be acquired through amphibole crystallization starting from a sanukitic melt similar to melt-I, probably in a deeper magma chamber. Our results allow us to constrain that melts from the subducted slab were produced on a regional scale, in accordance with literature data, below a large sector of the east Gondwana margin during the mid-Cambrian. Implications for the role of amphibole in petrogenesis of subduction-related magmas are also discussed.
KEY WORDS: amphibole; sanukite; high-Mg andesites; Ross Orogeny; Antarctica
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INTRODUCTION |
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Most studies on primary melt composition in modern and fossil active margins are carried out on volcanic rocks, which represent only a portion of the magmatism produced during a collisional event, as melts may stall at different levels within the crust. In addition, the low-pressure mineral assemblage found in volcanic rocks may not represent that of mid- to high-pressure conditions, which mostly controls the evolution of the magma suite (Davidson et al., 2007
). Petrological and geochemical information from volcanic rocks alone is thus not fully instructive about the petrogenetic processes and their variation during convergent tectonics and magmatism. The importance of mafic and ultramafic intrusive rocks in yielding additional constraints on petrogenetic processes at the origin of subduction-related magmatism has been recently shown, for instance, in the well-exposed sections of the arc lower crust of Kohistan (Pakistan, Garrido et al., 2006; Jagoutz et al., 2006) and Talkeetna (Alaska, Greene et al., 2006). Mafic and ultramafic intrusive rocks exposed along orogenic belts may thus be important sources of information on hidden crustal processes that occurred during the subduction event. These rocks locally preserve relict mineral phases or complex zoning that is the record of variations in magma composition (e.g. Blundy & Shimizu, 1991; Tiepolo & Tribuzio, 2005). Petrological results, when coupled with age patterns from in situ U–Pb analyses, may provide the timing of the different magmatic events and important constraints for the evolution of an orogen (Giacomini et al., 2007).
Increasing attention has been recently paid to amphibole-rich mafic intrusive rocks. Many arc lavas seem to be residual after amphibole crystallization but rarely they carry gabbroic crustal xenoliths in which amphibole is a dominant mineral phase. It is supposed that a cryptic amphibole crystallization at mid-crustal depths occurs in subduction-related contexts (Davidson et al., 2007). The presence of large volumes of amphibole cumulates at mid-crustal levels has also implications for the H2O budget in arc crust because of the capability of amphibole to act as a ‘sponge’ (Davidson et al., 2007). Amphibole crystallization has been also proposed, together with minor garnet, to account for the steep light rare earth element (LREE) enrichment of adakites (Macpherson et al., 2006; Rodriguez et al., 2007).
Amphibole-rich intrusive rocks are exhumed along many orogenic belts. Examples are, for instance, the hornblendites and amphibole-rich gabbros of the Adamello batholith and the Bregaglia intrusion in the Italian Alps (Tiepolo et al., 2002; Tiepolo & Tribuzio, 2005), the quartz melagabbros from the Glenelg River Complex in the Delamerian Orogen of Australia (Kemp, 2003, 2004) and the high-Mg diorites in Kyushu, Japan (Kamei et al., 2004). The involvement of high-Mg andesite (HMA) melts with adakitic, boninitic or sanukitic affinity was proposed for the origin of these amphibole-rich mafic intrusive rocks. Adakites are HMA with high LaN/YbN ratios (>10) and high Sr contents (>400 ppm) formed by partial melting of the subducted slab in eclogite facies (Drummond & Defant, 1990). Boninites are HMA extremely depleted in high field strength elements (HFSE) and heavy rare earth elements [e.g. HREE at about 0·01 normal mid-ocean ridge basalt (N-MORB)] characterized by large ion lithophile element (LILE) enrichment. Their origin is commonly attributed to high-temperature partial melting of a refractory mantle wedge (Crawford et al., 1989); in particular, the LILE enrichment is related to the addition of a H2O-rich component from the dehydration of the subducted slab (e.g. Pearce et al. 1992). Sanukitoids are Archaean high-Mg diorites with modern analogues in the Setouchi Volcanic Belt HMA. They share similarities with the low-SiO2 adakites and are characterized by strong LILE and LREE enrichments, low HREE contents, high Cr and Ni, and Mg-number >> 0·62 (Martin et al., 2005). The petrogenesis of Setouchi andesites is related to the equilibrium reaction of a mantle peridotite with silicic liquids derived from the partial melting of the subducted sediments (Shimoda et al., 1998).
At the Husky Ridge locality, in the northern Victoria Land sector of the Ross Orogen (Antarctica), amphibole-rich mafic cumulates are exposed. The knowledge of the geochemical signature of mantle-derived melts along this section of Gondwana during the Ross Orogeny is fragmentary and poorly constrained in time. The nearby mafic–ultramafic layered sequence from Niagara Icefall has been recently shown to have formed from boninite-type melts (Tribuzio et al., 2007). Conversely, most available literature on subduction-related magmatic products deals with dioritic to granitoid rocks that record extensive crustal contamination (Dallai et al., 2003; Rocchi et al., 2004).
An in situ geochemical and geochronological study was therefore carried out on the mafic amphibole-rich cumulates and associated rocks cropping out at the Husky Ridge locality. These rocks represent an important record to: (1) constrain the composition and affinity of the mantle melts developed during the Ross Orogeny in this sector of Gondwana; (2) unravel the petrogenetic processes at the origin of the amphibole-rich intrusive rocks; (3) explore the role of amphibole during arc magma evolution; that is, its capability to give residual melts with steep REE patterns, typical of adakitic melts.
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GEOLOGICAL SETTING AND FIELD RELATIONS |
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The Ross–Delamerian Orogen is broadly the result of the subduction of the Palaeo-Pacific oceanic plate beneath the continental margin of Gondwana during the Early Palaeozoic (e.g. Cooper & Tulloch, 1992
; Münker & Crawford, 2000; Boger & Miller, 2004; Foster et al., 2005). It is at present represented by a belt extending from southeastern Australia to the margin of the East Antarctic craton (e.g. Borg & DePaolo, 1991; Stump, 1995; Rocchi et al., 1998). The Transantarctic Mountains extend across the entire Antarctic continent and represent the uplifted basement of the Ross Orogen (Stump, 1995). The oldest magmatic products with calc-alkaline affinity found in southern Victoria Land indicate that subduction began at 
540 Ma (Allibone & Wysoczanski, 2002).
In northern Victoria Land (NVL; Fig. 1), three major fault-bounded tectonostratigraphic terranes are identified from west to east: the Wilson, Bowers and Robertson Bay terranes. The origin of these terranes is debated because they are interpreted either as units accreted during the Ross Orogeny (e.g. Tessensohn & Henjes-Kunst, 2005) or as an arc–back-arc–trench system developed during SW-dipping subduction (Federico et al., 2006). There is general consensus, however, in considering the Wilson terrane as representative of the active continental margin of Gondwana at the onset of the subduction. The magmatic products along this continental margin during the Palaeozoic subduction are known as the Granite Harbour Intrusive series (Gunn & Warren, 1962). In particular, geological and geochronological evidence indicates different phases of emplacement of magma intrusion. In NVL, magmatic products related to the early stages of subduction (e.g. Early Cambrian) are limited to sporadic deformed granitoids with an age between 544 ± 4 Ma and 530 ± 15 Ma (Black & Sheraton, 1990; Rocchi et al., 2004); in addition, rare mafic to ultramafic enclaves dated at 521 ± 4 Ma were found in the Teall Nunatak intrusion (Giacomini et al., 2007). The majority of magmatic products in NVL have ages that cluster around 500 Ma and are mostly granitoids (from granites and granodiorites through tonalites to minor gabbro-diorites) with calcalkaline affinity and variable K2O enrichment (Di Vincenzo et al., 1997; Rocchi et al., 1998; Dallai et al., 2003). Peraluminous leucogranites and basic melts with shoshonitic affinity were intruded during a late orogenic phase, which probably post-dated the subduction event, at about 480 Ma (Rocchi et al., 1998; Di Vincenzo & Rocchi, 1999). The rare mafic and ultramafic intrusive rocks in NVL are mostly scattered along the suture zone between the Wilson and Bowers terranes (Fig. 1). In particular, three main gabbroic sequences preserve their original intrusive features; namely, the Niagara Icefall, Husky Ridge and Tiger Gabbro sequences. A petrological, geochemical and geochronological study has been recently carried out on the Niagara Icefall mafic–ultramafic intrusion and revealed its boninitic affinity and crystallization age at about 514 Ma (Tribuzio et al., 2007). The petrogenetic affinity and age of the other two gabbroic sequences is poorly defined.
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The Husky Ridge (73°20'S, 166°20'E) is located in the middle of the Meander Glacier, c. 10 km north of Mt Murchison. The exposed surface of the outcrop is c. 2 km in length and about 200 m in width. It mainly consists of amphibolite-facies migmatitic gneisses, locally sillimanite bearing, equilibrated at pressure and temperature conditions of about 5 kbar and 640°C (Castelli et al., 2003
). In the eastern sector of the ridge, a body of about 100 m x 50 m of undeformed to weakly deformed amphibole-rich mafic intrusive rocks was found. The host rocks are quartz- and biotite-rich gneisses containing either garnet + white mica or clinopyroxene + amphibole. The occurrence of amphibole-rich mafic intrusive rocks as an undeformed body within amphibolite-facies foliated rocks was similarly observed in the Bregaglia intrusion of the Central Alps (Diethelm, 1985; Tiepolo et al., 2002). The amphibole-rich mafic intrusive rocks consist of large amphibole crystals and minor fine-grained plagioclase-bearing matrix. Layering related to variations in hornblende to plagioclase modal ratios is locally observed (Fig. 2). Plagioclase-rich veinlets (up to 10 cm in width) with subordinate amphibole, quartz and biotite and no fine-grained matrix are locally present. In the easternmost sector of the ridge (c. 300 m from the amphibole-rich mafic intrusive rocks), the gneisses contain an intrusive body made of a foliated hornblende- and biotite-bearing tonalite locally displaying modal layering. No contact relations between the amphibole-rich mafic intrusive rocks and tonalites are observed.
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PETROGRAPHY |
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Quartz diorites
According to estimated mineral proportions (Table 1), the Husky Ridge amphibole-rich intrusive rocks are quartz meladiorites to quartz diorites. The large amphibole grains (up to 2 cm) are subhedral with brown cores and green rims (up to 2 mm wide; Fig. 3). The boundary between the two amphiboles is relatively sharp. The green amphibole rim shows intergrowth with plagioclase. Clinopyroxene and minor altered biotite are included in the brown amphibole cores, where no plagioclase occurs. A thin aureole made of green amphibole rims the clinopyroxene that forms inclusions in the brown amphibole. The fine-grained matrix consists of clinopyroxene, green amphibole, plagioclase and quartz (Fig. 3). Textural relations in the matrix indicate early crystallization of clinopyroxene relative to that of plagioclase and green amphibole. In particular, green amphibole is locally observed to replace clinopyroxene. Plagioclase shows widespread low-temperature alteration. Mineral proportions in the matrix vary from sample to sample, but the mafic/felsic mineral ratio is nearly constant at 1:1. In particular, clinopyroxene within matrix varies from accessory to a major constituent, and quartz (up to 10 vol. %) is modally subordinate to plagioclase. Accessory matrix minerals are titanite, oxide phases, apatite and zircon. Five amphibole-rich mafic rocks, considered to represent the modal variations occurring in this rock-type, were selected for the present study.
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Tonalite and host basement rocks
One tonalite and one basement sample were selected for the geochemical and geochronological study, to define the role of the continental crust in the petrogenesis of amphibole-rich intrusive rocks. The selected tonalite has a hypidiomorphic and foliated texture given by the preferential orientation of major minerals (i.e. plagioclase, quartz, amphibole and biotite). Plagioclase and biotite are euhedral to subhedral and are modally predominant over anhedral quartz and amphibole, respectively (Table 1). Accessory minerals in the tonalite are oxide phases, apatite, zircon, allanite and titanite. The selected basement sample has a gneissic texture and mostly consists of quartz, plagioclase and biotite with a well-defined orientation. Accessory garnet and white mica occur in quartz–feldspathic layers. White mica is also locally associated with biotite. Other accessory minerals are oxide phases, apatite, zircon and allanite.
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MINERAL COMPOSITION |
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Analytical procedures
Major element mineral chemistry of clinopyroxene, plagioclase and amphibole from the quartz diorites and the tonalite was determined at CNR–Istituto di Geoscienze e Georisorse, U.O. di Padova, using a Cameca SX 50 electron microprobe and at the Dipartimento di Scienze della Terra Università di Milano using a Jeol Super Probe electron microprobe. Operating conditions were set at 15 kV accelerating voltage and 15 nA beam current on the sample. Trace element mineral composition of amphibole, clinopyroxene, plagioclase and titanite was determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at the CNR–Istituto di Geoscienze e Georisorse, U.O. di Pavia. The instrument couples a Nd:YAG laser working at 266 nm with a quadrupole ICP mass spectrometer type DRCe from Perkin Elmer. The laser was operated at 10 Hz with a pulse energy on the sample of 0.01–0.03 mJ. The spot size was set to 30 µm. Data reduction was performed with the ‘Glitter’ software package (van Achtenbergh et al., 2001
) using NIST SRM 612 as the external standard and 44Ca as the internal one. Precision and accuracy, assessed during each analytical run on the BCR-2 USGS reference glass, are up to better than 6% relative.
Quartz diorites
Clinopyroxene
Clinopyroxene included into brown amphibole shows relatively high Mg/(Mg + Fe) ratios (Mg-number = 0·78–0·84) and low contents of Ti (<0·01 a.p.f.u.) and Al (0·02–0·07 a.p.f.u.) (Table 2; Fig. 4). The clinopyroxene in the matrix has slightly lower Mg-number. Trace element determination on clinopyroxene included in brown amphibole (Table 3) reveals a bell-shaped chondrite-normalized REE pattern with the maximum at middle REE (MREE) at about four times C1 (Fig. 5). The LaN/SmN ratio ranges between 0·4 and 0·5 and no Eu anomaly is observed. Cr contents in the clinopyroxene included in brown amphibole range between 1130 and 2000 ppm. Its incompatible element pattern shows depletion in Ba, Nb, Pb and Ti relative to the neighbouring elements; conversely, a marked enrichment in Li, Th and U relative to other elements is observed.
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Clinopyroxene in the matrix is characterized by a weak LREE enrichment (LaN/SmN = 0·9–2·2) over HREE, which are almost flat or slightly convex upward at 1·7–6·0 times C1. No Eu anomaly is observed (Fig. 5). Cr is between 107 and 557 ppm. The incompatible element pattern generally parallels that of clinopyroxene included in brown amphibole. Slightly lower contents of Th and higher contents of Pb are observed. A more marked negative Ti anomaly is also detected.
Brown amphibole
Brown amphibole has a relatively homogeneous major and trace element composition (Tables 4 and 6). It is characterized by Ti contents up to 0· 18 a.p.f.u. and Mg-number of 0·70 (Fig. 6). AlTot is up to 1· 87 a.p.f.u., and the alkali content (Na + K) ranges from 0·37 to 0·57 a.p.f.u. The chondrite-normalized REE pattern varies from flat to slightly convex downward (LaN/SmN = 0·4–0·7), with MREE at about 20 times C1 (Fig. 5). No significant Eu anomaly is observed. The highest Cr and Ni contents (up to 1010 and 442 ppm, respectively) are from brown amphibole in matrix-poor samples. The incompatible element pattern reveals a strong depletion of Rb relative to Ba, which is at the same level as LREE (Fig. 5). Nb and Ta are slightly depleted relative to LREE, whereas both Pb and Zr–Hf are significantly depleted relative to the neighbouring HREE. Th contents are comparable with those of LREE, whereas U is significantly enriched (U/Th = 0·95–2·3). Li and B contents are variable, with values up to 18 ppm and 11· 4 ppm, respectively.
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Green amphibole
The green amphiboles in the matrix and the green amphibole rimming brown amphibole have similar major and trace element compositions (Tables 5 and 6). Compared with brown amphibole, green amphibole shows lower AlTot, Ti and (Na + K)A contents, coupled with slightly higher Mg-number (up to 0·74; Fig. 6). The lowest AlTot and Ti contents of green amphibole are 1· 07 and 0·03 a.p.f.u., respectively. The chondrite-normalized REE pattern of green amphibole is characterized by LREE enrichment over HREE (LaN/SmN = 1· 0–2·5). LREE concentrations in green amphibole are about twice those in brown amphibole (Fig. 5). The incompatible trace element pattern reveals lower Sr, Pb and U contents than in brown amphiboles and significantly higher concentrations of Nb and Ta. Cr contents are comparable with or slightly lower than those in brown amphibole (374–807 ppm), whereas Ni values are always slightly lower (196–300 ppm). Li and B contents are similar to those observed in brown amphibole.
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Trace element profile across amphibole
A core-to-rim trace element profile was performed across one well-preserved amphibole grain from sample TT334 to constrain the chemical transition between brown and green amphibole (Fig. 7). A total of 39 analyses were carried out with a spot size of 40 µm and at intervals of about 40–50 µm (details are given in Electronic Appendix 1, which is available for downloading at http://www.petrology.oxfordjournals.ac.uk).
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Transition metals (e.g. Cr, Mg, Zn, Ni, Co) and some LILE (Ba and Rb) show minor variations within and between the two types of amphibole. The REE concentrations are different but homogeneous within the green and brown amphibole domains. The transition between these two domains is relatively narrow (about 200 µm) and intermediate concentrations of REE are observed. The green amphibole is depleted in HREE relative to the brown amphibole by a factor of about two. Conversely, the LREE content in the green amphibole is twice that in the brown amphibole. Ti, Nb and Ta contents differ the most (by a factor of about five) between the two types of amphibole. Ti confirms the presence of a narrow (about 200 µm) interface region with values intermediate to those of brown and green amphiboles. Ti contents in brown and green amphiboles are almost constant. Zr and Nb increase slightly from the core to the rim of brown amphiboles. Values are almost constant across the interface and then quickly increase, becoming nearly constant for the whole width of the green amphibole. Th, U and LILE (Li and B) concentrations are highly scattered, and no significant variations can be detected across the amphibole grain. Pb contents in green amphibole are depleted with respect to those in brown amphibole, but are almost constant within the two amphibole types. To sum up, the element distribution across the amphibole grain reveals that Mg, Cr and Ba show no systematic variations between the two types of amphibole, whereas the concentrations of elements such as REE, Ti and Nb differ significantly between brown and green amphibole. In particular, the latter elements show marked, sudden variations within a zone of a few hundred microns. It should be noted that none of the analysed elements show constant, steady variations in concentrations from the brown core to the green rim.
Plagioclase and titanite
The An content of plagioclase ranges from 35 to 46 mol % (Tables 7 and 8). The chondrite-normalized element pattern is characterized by a marked fractionation between LREE and MREE (La is about 10 times C1, and Sm less than 0·5 times C1; Fig. 8). Slightly lower REE values are observed in plagioclase from samples with low An contents. If plagioclase of quartz diorites from the Husky Ridge is compared with that formed from MOR-type melts, more than one order of magnitude of LREE enrichment is observed (Fig. 8). The concentrations of Sr and Ba in analysed plagioclase show a narrow range of variation, with values of 665–744 ppm and 118–131 ppm, respectively. Ti is between 11· 4 and 21·3 ppm; Li is below 0·35 ppm, and B ranges between 4·5 and 9·5 ppm.
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The trace element composition of three titanite crystals from sample TT329 was determined (Table 8). The REE pattern has a marked enrichment in LREE (up to 3000–7200 times C1) over HREE (at about 400–650 times C1) and a weak negative Eu anomaly. One of the crystals does not show the negative Eu anomaly, and this feature is coupled with the highest LREE, Th and U contents. The incompatible element pattern reveals U and Th values comparable with those of LREE. Zr–Hf and Nb–Ta are slightly depleted relative to the neighbouring elements. Sr and Pb contents are less than 10 times C1 chondrite.
Tonalite
Amphibole from the tonalite sample (Tables 6 and 9) has lower Ti (0·13–0·14 a.p.f.u.) and higher AlTot (1· 89–2·14 a.p.f.u.) than amphibole from quartz diorites (Fig. 6). Its low Mg-number (0·50–0·53) is noteworthy. The REE pattern (Fig. 5) differs from that of brown and green amphiboles in the quartz diorites. It is characterized by a strong depletion of LREE (LaN/SmN = 0·17–0·21) relative to MREE and HREE, which are about 60 times C1. Eu shows a marked negative anomaly relative to the neighbouring REE. The incompatible element pattern reveals higher Rb and significantly lower Th and U contents than the two types of amphibole in quartz diorites. Zr, Hf, Nb, Ta and LREE contents are almost comparable with those of green amphibole in quartz diorites. Cr contents are lower than in quartz diorites, with values in the 113–280 ppm range. Li is below 7·5 ppm, and B ranges between 1· 6 and 3·98 ppm.
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Plagioclase from the tonalite sample has An contents of 42–48 mol %. The REE pattern reveals generally higher values than in plagioclase from quartz diorites and a less marked LREE/MREE fractionation (Fig. 8). Sr concentration is 641 ppm, whereas that of Ba and Pb is 61 and 12·5 ppm, respectively. Li is <0·2 ppm and B is at 3·81 ppm.
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U–PB ZIRCON GEOCHRONOLOGY AND TRACE ELEMENT COMPOSITION |
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Analytical methods
Zircons for U–Pb geochronology were separated from three quartz diorites, the tonalite and the Bt-gneiss using standard magnetic techniques and heavy liquids. Because of the paucity of zircons within quartz diorites, the three samples were combined (TT328, TT331 and TT327). Grains without fractures and as free from inclusions as possible were hand picked, embedded in epoxy resin and polished to
µm using diamond paste. Prior to age determination all zircon grains were investigated by cathodoluminescence (CL) to highlight zoning and possible inherited cores. In situ U–Pb geochronology was carried out by excimer laser ablation (ELA)-ICPMS at CNR–Istituto di Geoscienze e Georisorse (IGG), Unità di Pavia. The laser ablation instrument couples an ArF excimer laser microprobe at 193 nm (Geolas200Q-Microlas) with a ThermoFinnigan Element I sector field high-resolution ICPMS system. The analytical method is basically as described by Tiepolo (2003). Instrumental and laser-induced U/Pb fractionations were corrected using the 1065 Ma 91500 zircon (Widenbeck et al., 1995) as an external standard. The same integration intervals and spot size were used on both the external standard and unknowns. During each analytical run reference zircon 02123 (295 Ma; Ketchum et al., 2001) was analysed together with unknowns for quality control: its mean concordia age was 297 ± 10 Ma (see Electronic Appendix 2). The spot size was set to 20 or 10 µm and laser fluence to 12 J/cm2. Data reduction was carried out using the ‘Glitter’ software package (van Achterbergh et al., 2001). During each analytical run the reproducibility on the standards was propagated to all determinations according to the equation of Horstwood et al. (2003). After this operation, analyses are considered accurate within quoted errors. Concordia ages were determined and concordia plots were constructed using the Isoplot/EX 3.0 software (Ludwig, 2000). All errors on concordia ages in the text are given as 2
. The trace element composition of zircon was determined according to the method described above, but using 29Si as the internal standard.
Quartz diorites
Fifty-eight zircon grains were selected. They are mostly euhedral and with prismatic habit (Fig. 9). Most crystals show well-developed oscillatory zoning typical of growth under magmatic conditions (e.g. Vavra et al., 1996). A few zircons characterized by dark homogeneous cores and brighter rims with faint oscillatory zoning (e.g. crystal 34) were also found.
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A total of 83 analyses were carried out and results are mostly concordant (Table 10); only few data reveal significant Pb loss. Ages obtained on domains with oscillatory zoning show a continuous spread from 527 ± 14 Ma to 473 ± 13 Ma (Fig. 10). The older ages were obtained in the inner portion of the grains, whereas the younger ones mostly pertain to the outer domains. One inner portion with low CL emission gave a concordant Pan-African age of 666 ± 19 Ma, which probably represents inheritance from the host basement. A few homogeneous zircons with low CL emission and faint oscillatory zoning yielded ages of 450 to 268 Ma, which do not represent magmatic events but resetting of the U–Pb system. Similar young ages observed by Bomparola et al. (2007
) in the Lower Peak granites of NVL were attributed to fluid circulation and pervasive deformation. Intraplate reactivation processes, with fluid-assisted deformation leading to zircon resetting at about 450 Ma, have also been observed in NVL by Di Vincenzo et al. (2007).
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Zircon trace element composition was determined on selected grains to constrain the origin of the age spread and better define the age of intrusion (Fig. 11 and Table 12). However, results show that, independently of the age, all grains have a similar chondrite-normalized REE patterns, with a steady decrease from HREE (about 2000 times C1) to LREE and a positive Ce anomaly.
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The core to rim continuity in crystal habit, the CL zoning patterns and the homogeneous chemical composition indicate a single-phase zircon growth. Because of the significant Pb loss and U–Pb system perturbation experienced by some of the selected zircon grains, the age of intrusion of quartz diorites is likely to approach the upper limit of the observed age spread. By considering all the analyses within error with the oldest age determination, a mean concordia age of 516 ± 3 Ma is obtained (21 analyses, MSWD = 0·0038; probability of concordance 0·95). This is retained as the best estimate for the intrusion age of the quartz diorites.
The younger apparent ages, mainly obtained from zircon rims, are probably related to minor and variable Pb loss, which may have caused the shift of data along the concordia. The uncertainty of the technique, unfortunately, means that it is not possible to appreciate if a U–Pb discordance occurs in these data. The late orogenic granitoid intrusions at 480 Ma that are widespread in NVL (e.g. Rocchi et al., 1998) or even younger processes (see, e.g. Di Vincenzo et al., 2007) may be responsible for the perturbation of the U–Pb zircon system. In particular, K–Ar ages on biotite from migmatitic gneisses of the nearby Husky Bluff yielded 484 ± 6 and 489 ± 6 Ma, interpreted as regional cooling ages, postdating the tectonic juxtaposition of the Wilson and Bowers terranes (Vita-Scaillet & Lombardo, 2003). External zircon domains recording a late opening of the U–Pb system at about 490 ± 3 Ma were also found in gabbronorites from the neighbouring mafic–ultramafic complex of the Niagara Icefall complex intruded at 514 ± 2 Ma (Tribuzio et al., 2007).
Tonalite
Twenty-nine zircons were selected from the tonalite. They are structurally composite, with inherited cores surrounded by multiple overgrowths. Inner cores show rounded surfaces frequently cutting oscillatory and sector zoning (Fig. 9). A thin non-cathodoluminescent zone, generally attributed to segregated impurities, usually separates the domains. Up to three overgrowths were identified around the inner cores. In crystal 5, where the various overgrowths are well represented, a first domain around the inner core has rounded boundaries and low CL emission. The second overgrowth, with a thickness of up to 50 µm, has a prismatic habit and well-developed oscillatory zoning. The third overgrowth is about 10 µm thick and follows the prismatic habit of the adjacent domain, but with poorly developed oscillatory zoning. This thin outer overgrowth is common to many other zircon crystals.
Forty-seven age determinations were carried out on the various zircon domains from the tonalite, and 30 of them yielded concordant results (Table 11). Thirty-six analyses were carried out at a spot size of 20 µm and 11 at 10 µm spot size. This allowed us to analyse the outermost thin overgrowths, but at the expense of precision. The inner zircon domains yielded Neoproterozoic, Palaeoproterozoic and Archaean ages with concordia U–Pb ages clustering at 2600, 2300, 2250, 1080 and 890 Ma (Fig. 12). One inner domain gave an Early Cambrian age of 545 ± 18 Ma. The thin outermost rims (seven analyses) yielded younger ages, with a mean concordia age of 489 ± 8 Ma (2).
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The trace element composition of zircons was determined on representative age domains in selected grains (Fig. 11 and Table 12). The REE pattern of the outermost rim at 489 Ma shows marked LREE depletion (La = 0·3 times C1 chondrite) and HREE enrichment (1200 times C1 chondrite), with strong HREE fractionation (YbN/GdN >> 1) and positive Ce anomaly. The REE pattern of the Early Cambrian domain parallels that of the thin external overgrowth at 489 Ma but with slightly higher HREE values. The domain at 890 Ma shows a REE pattern characterized by nearly flat HREE at about 100 times C1 chondrite and a marked negative Eu anomaly, suggesting equilibration with garnet (Rubatto, 2002
) and plagioclase. Palaeoproterozoic domains have REE patterns similar to those of the younger domains but at higher concentrations (HREE at about 100 times C1 chondrite) and with marked negative Eu anomalies. The definition of the age of emplacement of the tonalite body is not straightforward. The outermost thin zircon domains, showing oscillatory zoning and REE pattern typical of growth under magmatic conditions, probably crystallized from the tonalitic melt. However, their age of 489 ± 8 Ma may not represent the age of the tonalite intrusion. Similarly to quartz diorites, younger tectonic reactivation processes may have caused the resetting of the U–Pb system in zircon. Although current data do not allow us to define the emplacement age of the tonalite body, the large crustal contribution from the host basement (see next section) in the petrogenesis of the tonalitic melt is noteworthy for the present study.
The host basement (biotite gneiss)
Twenty-seven zircons were selected from the Bt-gneiss. They are prismatic, and under CL they show inherited cores and multiple overgrowths similar to those observed in the tonalite sample. Inner cores frequently display relics of oscillatory zoning (Fig. 9) that are truncated by rounded boundaries, suggesting resorption. The outermost overgrowths have low luminescence.
A total of 33 analyses were carried out on this sample, and only 15 analyses gave concordant ages (Table 11, Fig. 13). Discordant ages are scattered and do not yield additional geochronological information. The densest age clusters are at about 600 and 750 Ma. These age peaks agree with two of the major Pan-African events responsible for the accretion of Gondwana (e.g. Meert, 2003). There is a minor peak at about 1050 Ma, and four concordia ages from inner cores are between 2600 and 2300 Ma. One low luminescence overgrowth in zircon 3 yields a Cambrian age at 536 ± 18 Ma. Most of the age clusters of the Bt-gneiss resemble those of the tonalite. Zircons from the Bt-gneiss show chondrite-normalized REE patterns typical of growth under magmatic conditions, with marked negative Eu anomaly (Fig. 11 and Table 12). No evidence of equilibration with garnet was observed.
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EVIDENCE FOR OPEN-SYSTEM EVOLUTION OF QUARTZ DIORITES |
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The texture and mineral chemistry of quartz diorites suggest the presence of two distinct mineral assemblages not in chemical equilibrium: (1) clinopyroxene included in brown amphibole (hereafter clinopyroxene-I) + brown amphibole + dark mica; (2) clinopyroxene in the matrix (hereafter clinopyroxene-II) + green amphibole + plagioclase + quartz + accessory phases. In particular, the minerals from the second assemblage show marked LREE enrichment over HREE and Ti depletion.
The two mineral assemblages may be interpreted as the product of crystallization of a single melt through simple closed-system fractional crystallization. This hypothesis, however, contrasts with both rock texture (Fig. 3) and mineral trace element composition, and in particular with the chemical profile across amphibole (Fig. 7). The transition from brown to green amphibole, occurring in about 200 µm, is sharp and sudden. The narrow interface precludes the continuous growth of amphibole from a melt undergoing differentiation through fractional crystallization in a closed system or through another processes implying a steady change of melt composition, such as assimilation and fractional crystallization (AFC). In either case, zoning would be more gradual. The matrix assemblage may alternatively be interpreted as the product of crystallization of a trapped melt. This process may lead to extreme trace element enrichments in the crystallizing products (Cawthorn, 1996). However, crystallization of a trapped melt is also unlikely, as this contrasts with the high modal proportion of matrix minerals (20–70 vol. %) in the studied samples. The above considerations lead us to suppose that the two mineral assemblages were derived from two distinct melt pulses. Quartz diorites thus formed in an open system in which a crystal mush consisting of brown amphibole (± clinopyroxene± biotite) interacted with a compositionally different melt.
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PARENTAL MELTS OF QUARTZ DIORITES |
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Because of the composite and cumulus nature of the quartz diorites, the bulk rock would give an average composition of the two mineral assemblages and prevent accurate definition of the chemical composition of the parental melts. We therefore adopted a different approach; that is, equilibrium melts with the two distinct mineral assemblages were computed from mineral compositions by applying solid/liquid partition coefficients. Melts in equilibrium with clinopyroxene and amphibole from the two mineral assemblages were calculated with the solid/liquid partition coefficients reported in Electronic Appendix 3. Crystal chemical differences between the two clinopyroxene and amphibole generations are almost negligible and do not justify the adoption of different sets of solid/liquid partition coefficients. The presence of quartz in the second mineral assemblage, however, suggests that the parental melt was more SiO2-rich than that in equilibrium with clinopyroxene-I and brown amphibole. Slightly higher Dsolid/liquid values are therefore expected for elements with high charge/ionic radius (Z/r) ratios (e.g. Tiepolo et al., 2007
). None the less, because of the uncertainty in SiO2 differences and the lack of a large self-consistent set of DS/L values at higher SiO2 contents, we adopted the same set of values for both systems. The trace element composition of the melt in equilibrium with the second mineral assemblage may thus be slightly overestimated for element with high Z/r ratios such as REE and HFSE; however, no inter-element fractionation is expected.
Melt-I: clinopyroxene-I and brown amphibole
The melt composition calculated from the average composition of clinopyroxene-I (Fig. 14; Table 13) is characterized by LREE enrichment over HREE (LaN = 2·7; LaN/YbN = 12·8), which are nearly flat (DyN/YbN = 1· 0) at about 0·2 times N-MORB. A slight positive Sr anomaly is observed, whereas Th, U and Pb are strongly enriched (>100 times) relative to N-MORB. Li is about 20 times N-MORB.
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The average composition of the melt in equilibrium with brown amphibole (Table 13) shows an incompatible trace element pattern characterized by a LREE enrichment over HREE comparable with that observed in the melt from clinopyroxene-I (LaN/YbN = 11) but with slightly higher contents; for instance, HREE are nearly flat at about 0·5 times N-MORB (Fig. 14). A weak negative Sr anomaly is observed; Nb and Ta are about two times N-MORB and thus depleted relative to both LREE and LILE. The NbN/TaN ratio approaches unity. Zr and Hf contents are similar to HREE concentrations (about 0·4 times N-MORB). U is more than 100 times N-MORB. Th, Pb and Rb (about 20–30 times N-MORB) are enriched relative to LREE, whereas Ba is less than 10 times N-MORB. Li and B are about 10 and 600 times N-MORB.
Most of trace elements show a similar fractionation in the liquid computed from clinopyroxene-I and brown amphibole, but a difference of about two times in the absolute values is observed. These differences are probably related to uncertainties in the consistency of the DS/L between amphibole and clinopyroxene. Therefore, in the discussion below, the average composition between the melt computed from clinopyroxene-I and from brown amphibole (hereafter Melt-I) will be considered. This approach allows us to overcome the uncertainties related to the choice of DS/L and to obtain information on the concentration of elements (such as Ba, Rb and Ta) that are below detection limits in clinopyroxene.
Melt-II: clinopyroxene-II and green amphibole
The mean incompatible element pattern of melt in equilibrium with clinopyroxene-II (Fig. 14; Table 13) is characterized by a strong enrichment of LREE over HREE, which are convex-downward (DyN/YbN = 0·79) at about 0·2 times N-MORB. LREE contents are around 10 times N-MORB and the LaN/YbN ratio is approximately three times higher than in melt-I (around 38). No negative Sr and Eu anomalies are observed. The low Ti contents (less than 0·1 times N-MORB) produce a marked negative anomaly in the incompatible trace element pattern. Nb is depleted relative to LREE and shows normalized values similar to those of melt-I. Li, B, Th, Pb and U are enriched relative to all other elements.
The melt in equilibrium with green amphibole (Fig. 14; Table 13) has a N-MORB normalized incompatible element pattern that parallels that of many elements that are computed from clinopyroxene-II. HREE contents are slightly higher but with the same convex-down pattern. The LREE enrichment and strong Ti depletion resemble those of melts calculated from clinopyroxene-II. The melt in equilibrium with green amphibole shows a marked negative Sr anomaly, compatible with the later crystallization of green amphibole and plagioclase relative to clinopyroxene-II. Nb–Ta contents are higher than in the melt computed from clinopyroxene-II whereas lower Zr–Hf values are observed. The NbN/TaN ratio is below unity.
The two melts computed from the matrix assemblage share many peculiar similarities (e.g. extreme LREE enrichment, Ti and HREE depletion) but also differences that contrast with crystallization of clinopyroxene-II and green amphibole from the same liquid. The higher Cr and Ni contents in green amphibole contrast with the textural evidence of its late crystallization relative to clinopyroxene-II. This evidence and the optical continuity between the two amphibole types indicate that green amphibole may be a reaction product partially replacing brown amphibole, and may thus not represent a liquidus phase. The textural evidence for the early crystallization of clinopyroxene-II among the matrix minerals led us to consider the sole melt in equilibrium with clinopyroxene II (hereafter melt-II) to represent the parental melt of the matrix mineral assemblage. The melt computed from clinopyroxene-II does not record the crystallization of plagioclase, thus suggesting that it is not affected by the late precipitation of accessory phases (e.g. titanite, apatite and zircon).
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MELT-I: HMA WITH SANUKITIC AFFINITY |
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Melt-I has clinopyroxene as a liquidus phase, followed by amphibole. Plagioclase is not a liquidus phase. The relatively high Cr contents (up to 2000 ppm) and Mg-number (0·84) of clinopyroxene-I are consistent with a mantle origin. This is also consistent with the brown amphibole compositions (Cr = 1100 ppm, Ni = 410 ppm and Mg-number = 0·74). According to the KdCpx/liquidFe/Mg exchange coefficients for pressure conditions of 0·5–1· 0 GPa (Gudfinnsson & Presnall, 2000
), the molar Mg-number of melt-I is inferred to be up to 0·62. Although melt-I records a fractional crystallization process of mafic minerals, its incompatible trace element pattern is considered to retain significant information about the geochemical signature of the primary mantle melt.
Clinopyroxene-I chemistry suggests crystallization from a melt with relatively low Al2O3, Na2O and TiO2 and relatively high MgO and Cr2O3. Calculated equilibrium melts have low HREE, Y, Zr, Hf and Ti and exceptionally high B, Pb, Th and U contents relative to N-MORB, indicating that melt-I is not a common arc magma, of either oceanic or continental setting. The major element composition of clinopyroxene-I resembles that of clinopyroxenes in mafic intrusive rocks crystallized from HMA (Fig. 4). In particular, close similarities are observed to the clinopyroxenes from Cambrian quartz melagabbros with boninitic affinity from the Glenelg River Complex (SE Australia; Kemp, 2003, 2004) and to clinopyroxene from the Early Cretaceous high-Mg diorites with sanukitic affinity from Kyushu (SW Japan; Kamei et al., 2004). Chemical analogies are also found with the clinopyroxenes from the evolved intrusive products (gabbronorites) of the adjacent Niagara Icefall mafic–ultramafic complex with boninite affinity (Tribuzio et al., 2007).
Although the low HREE, Y, Zr, Hf and Ti contents suggest a boninite affinity, Melt-I cannot be considered a typical boninite (e.g. Taylor et al., 1994), because of the absence of orthopyroxene as liquidus phase, the slightly higher HREE contents and the LREE, Th, Nb enrichment relative to HREE (Fig. 15). HMA with boninite affinity but variably enriched in LREE and LILE have been reported elsewhere in the Delamerian Orogen: in the Heathcoat Greenstone Belt of Central Victoria (Australia; Crawford & Cameron, 1985) and in the Heat Creek Bed formation in New Zealand (Münker & Cooper, 1999). However, in both cases orthopyroxene is still a major mafic mineral. Striking chemical similarities are observed between melt-I and the HMA sanukitoids from the Setouchi Volcanic Belt (Tatsumi & Ishizaka, 1982; Shimoda et al., 1998; Fig. 15). A peculiar feature of the sanukitoids with respect to boninites is the rare occurrence of orthopyroxene and the presence of mafic minerals such as biotite ± amphibole ± clinopyroxene (Martin et al., 2005). We therefore propose a close relationship between melt-I from the Husky Ridge and sanukitic melts.
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The petrogenesis of melt-I is suggested to involve a refractory mantle that accounts for the low HREE, Zr, Hf and Ti contents and an agent rich in H2O, B, Th, U, LREE and Nb. Marine terrigenous sediments, which are recycled at convergent margins, are a realistic source for the addition of H2O, Th, U, LREE and possibly Nb and B into the mantle (Plank & Langmuir, 1998
; Plank, 2005). Direct partial melting of the sedimentary cover of the slab, or sediment transfer into the hot zone of the mantle wedge through buoyancy or subduction erosion and consequent melting (Goss & Kay, 2006), may induce the percolation of a sediment-derived melt into the mantle wedge. The partial melting of a highly refractory mantle wedge reacted and equilibrated with a sediment-derived hydrous melt may thus account for the trace element composition of melt-I. This process is basically that proposed to account for the petrogenesis of the sanukitic HMA of the Setouchi Volcanic Belt (e.g. Shimoda et al., 1998). |
MELT-II: EVOLVED SANUKITOID BY AMPHIBOLE CRYSTALLIZATION |
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The lower Cr2O3 contents and Mg-number of clinopyroxene-II than clinopyroxene-I, the higher Si/Al ratio of green amphibole than brown amphibole, the presence of accessory quartz and the Na-rich composition of plagioclase (An up to 35 mol %) suggest that melt-II is more evolved than melt-I. The high LILE contents and the marked negative Nb–Ta anomaly are evidence for a subduction component. Similarly to melt-I, however, the trace element signature is different from that of common arc lavas. Among subduction-related melts, marked LREE/HREE fractionation and low HREE contents are typical of adakites (LaN/YbN > 10; Yb < 1· 8 ppm; Defant & Drummond, 1990
). In addition, the composition of green amphibole indicates that melt-II possesses a NbN/TaN ratio lower than unity, which is also a compositional feature of adakitic melts (e.g. Foley et al., 2002). The lack of olivine and the presence of clinopyroxene as an early crystallizing mineral are also consistent with an adakitic origin. However, the low HREE contents, the negative Ti anomaly and the U-shaped HREE pattern contrast with a typical adakite (Fig. 14). In addition, the Sr content of melt-II (about 210 ppm) is lower than that of typical adakites (Sr > 400 ppm; Defant & Drummond, 1990).
The steep REE pattern is a relatively uncommon geochemical signature that can be used to constrain the petrogenesis of melt-II. The LREE/HREE fractionation in adakites is commonly attributed to the partial melting of the subducted slab under eclogite-facies conditions, leaving garnet in the solid residue (Drummond & Defant, 1990; Martin et al., 2005, and references therein). However, it has been shown that other processes are capable of producing melts with steep REE patterns; that is, partial melting of the lower crust (Petford & Atherton, 1996; Xu et al., 2002; Chung et al., 2003; Garrison & Davidson, 2003; Stevenson et al., 2005) and melt differentiation through crystallization of garnet-bearing assemblages at mantle or crustal levels (Macpherson et al., 2006; Rodriguez et al., 2007). In the following sections, we explore the role that the crust and fractional crystallization may have played in producing the geochemical signature of melt-II.
The crustal contribution
The presence of inherited ‘old’ zircons (at 666 ± 19 Ma) in quartz diorites and the relatively high abundance of trace elements with crustal affinity such as Rb, LREE and Th in melt-II indicate the involvement of a crustal component in their petrogenesis. However, a pure crustal origin for melt II is unlikely. Crustal melts from the lower basaltic crust have generally lower Mg-number (close to 0·30; Rapp & Watson, 1995) than that inferred for melt-II (about 0·52–0·56). Therefore, Melt-II may have originated by hybridization of melt-I with upper crustal acid liquids as suggested for the similar Mg-rich rocks with boninitic affinity from the Glenelg River Complex in the Delamerian Orogen (SE Australia; Kemp, 2004). At Glenelg River petrographic evidence, such as intermingling with two-mica leucogranite and the local presence of ocellar textures, showed that the parental melt of the quartz melagabbros was variably hybridized by felsic magmas of crustal origin. At the Husky Ridge, no peraluminous leucogranite is exposed and no ocellar textures, alkali feldspar and white mica occur in quartz diorites.
Despite the lack of relations between quartz diorites and tonalites in the exposed crustal section, the occurrence of the tonalitic body a few tens of meters distant from the quartz diorites does not allow us to exclude an a priori a role of the tonalitic melt as hybridizing agent. However, the mineral assemblage of the tonalite and that of the matrix assemblage in the quartz diorites are different. Clinopyroxene, one of the major matrix minerals in the quartz diorites, is absent in the tonalites and the opposite holds for biotite. Green amphibole from the tonalite is significantly richer in Fe and HREE (Mg-number = 0·5; Y 88 ppm) than green amphibole from quartz diorites (Mg-number = 0·7; Y 13 ppm). In addition, opposite LREE fractionation trends characterize the two amphiboles. The abundance of inherited zircons from the host basement in the tonalite and their near-total absence in the quartz diorites is further evidence against a direct link between the tonalite and melt II.
A petrogenetic process controlled by crustal contamination for the origin of the geochemical signature of melt-II is also unlikely. Because the mean continental crust is about two times higher in HREE and Zr than melt-II (Rudnick & Fountain, 1995), a substantial crustal contribution is unable to yield melts with the observed low HREE concentrations. Furthermore, to produce a melt with high LaN/YbN ratios (up to 38), a mineral phase such as garnet, capable of strongly fractionating LREE from HREE, should remain in the solid residue. Nevertheless, the concave-upward HREE pattern of melt-II contrasts with residual garnet, which would produce melts with high Dy/Yb ratios.
Differentiation through amphibole crystallization
The LREE enrichment and Ti depletion of melt-II may have been acquired through differentiation by fractional crystallization at crustal levels. The low HREE, the relatively high Mg-number, and the high Th and U contents common to both melt-II and melt-I indicate a genetic relationship. The hypothesis that melt-II is a differentiated product of melt-I is thus explored. In particular, because melt-II lacks the garnet signature, a process dominated by amphibole crystallization is hypothesized.
A fractional crystallization process driven by amphibole was simulated starting from melt-I and using two different sets of DAmph/L (Electronic Appendix 3): (1) those previously adopted for melt computation (
); (2) a set reporting higher compatibility for HREE [
; the highest among those reported by Tiepolo et al. (2007)]. As discussed above, the more evolved nature of melt-II with respect to melt-I may justify the choice of slightly higher DS/L for the elements with high Z/r ratios. Furthermore, because a fractional crystallization process dominated by amphibole separation causes a SiO2 enrichment in the residual melt, the DAmph/L values are expected to increase during the differentiation process. Results for TiN/YbN and the LaN/YbN ratio are reported in Fig. 16. A mean residual melt fraction around 30% accounts for the trace element variation between melt-I and melt-II using , whereas a residual melt fraction of about 40% is obtained with the set of partition coefficients. Amphibole crystallization is therefore able to produce the three times increase of the LaN/YbN ratio in the residual melt and the negative Ti anomaly in the incompatible element pattern. The same modelling holds for the origin of the U-shaped pattern of melt-II.
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; Electronic Appendix 2). 
, target values (melt-II). Elements are normalized to N-MORB (GERM: The significantly lower NbN/TaN ratio of the melt in equilibrium with green amphibole relative to that of the melt in equilibrium with brown amphibole also supports amphibole crystallization for the origin of melt-II. Among the major rock-forming minerals, only amphibole was demonstrated to prefer Nb with respect to Ta (Tiepolo et al., 2000b
). According to equation (2) of Tiepolo et al. (2000b) and the amphibole composition in Amph-gabbros, the 
ratio is around 1·7. The crystallization of either brown or green amphibole may lead to a decrease of the Nb/Ta ratio in the residual melt. A model of the NbN/TaN evolution in residual melt after brown amphibole fractional crystallization indicates that, starting from melt-I, the Nb/Ta ratio of melt-II can be accounted with a residual melt fraction of about 60% (Fig. 16). This residual melt fraction is slightly lower than that inferred from LaN/YbN and TiN/YbN modelling. However, because of the uncertainties related to DS/L values and those intrinsic into the Nb/Ta modelling (about 7–20%; Tiepolo et al., 2000b), we maintain that the results are essentially similar.
The early crystallization of clinopyroxene among matrix minerals (i.e. the amphibole replacement by clinopyroxene as a liquidus phase) is not fully consistent with an evolution of melt-II by continuous amphibole crystallization. There are, however, different possible explanations for the early crystallization of clinopyroxene in melt-II. The crystallization of large amounts of amphibole requires a melt capable of supplying the necessary H2O for the whole process. The stability of amphibole is also controlled by dehydrogenation [i.e. the incorporation of less H2O than the stoichiometry (e.g. Tiepolo et al., 1999)], which may occur when high-charge cations such as Ti or Fe3+ are available to balance the charge deficit. Because of the relatively high compatibility of Ti in amphibole, however, Ti is rapidly depleted as the melt evolves. A low H2O content coupled with low Ti concentrations may thus inhibit amphibole crystallization and enhance that of clinopyroxene. Alternatively, heating during decompression crystallization (Blundy et al., 2006; Davidson et al., 2007) may shift the system away from the restricted thermal stability field of amphipbole and enhance the crystallization of clinopyroxene.
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THE PETROGENETIC MODEL: A SUMMARY |
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Textural and chemical evidence prevents the quartz diorites of the Husky Ridge from being considered as a closed system in terms of fractional crystallization and two different melt injections must be invoked in the petrogenetic process. The first melt injection (melt-I) has a sanukitic affinity, is rich in H2O and lacks plagioclase as an early liquidus phase. It formed by partial melting of a refractory mantle equilibrated with silicic melts derived from partial melting of the sediment cover of the subducted slab, similar to the Setouchi andesites (Shimoda et al., 1998
). Melt-I stalls, cools and crosses the amphibole-in phase boundary at mid-crustal levels (i.e. 5 kbar; Castelli et al., 2003). The growth and accumulation of large brown amphibole at the expense of the early crystallizing clinopyroxene is thus enhanced. The crystal mush dominated by brown amphibole is subsequently injected by a residual melt (melt-II) with plagioclase at the liquidus and characterized by strong LREE enrichment and Ti depletion. As a consequence of this interaction, the pre-existing brown amphibole is variably replaced by a new generation of amphibole (green amphibole) with the trace element signature typical of melt-II. The peculiar trace element signature of melt-II is acquired through amphibole crystallization starting from a sanukitic melt similar to melt-I, which probably occurred in a deeper magma chambers. Heating of the magma during decompression crystallization (e.g. Blundy et al., 2006) and/or the unavailability of enough H2O and Ti for brown amphibole stability may have caused the replacement at the liquidus of amphibole with clinopyroxene. |
TECTONIC IMPLICATIONS |
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The intrusion of the Husky Ridge quartz diorites occurred at mid-crustal levels during the Mid-Cambrian (
515 Ma). The U–Pb zircon ages from the Bt-gneiss, showing peaks at the major Pan-African events of accretion of Gondwana, confirm that the basement rocks belong to the Gondwana active margin (i.e. the Wilson Terrane), in agreement with Castelli et al. (2003).
The sanukitic intrusion of the Husky Ridge has no equal in the Ross orogenic belt in Antarctica. Major compositional differences are observed with respect to the mafic rocks of the Terra Nova Intrusive Complex and Teall Nunatak intrusion, ranging in age from 510 to 480 Ma (Di Vincenzo & Rocchi, 1999; Dallai et al., 2003; Rocchi et al., 2004; Giacomini et al., 2007). Geochemical affinities are observed with the nearby mid-Cambrian mafic–ultramafic Niagara Icefall Complex of boninitic affinity (Tribuzio et al., 2007), as a refractory mantle is required to account for the low HREE, Y, Zr and Hf in both complexes. The absence of orthopyroxene as a liquidus phase, the abundant amphibole at the liquidus, and the enrichment in LILE and LREE suggest that the parental melts of the Husky Ridge intrusion were not typical boninites. Heterogeneous migration and release of the sediment-derived melts from the subducted slab in the highly refractory mantle may be at the centre of these differences. Both the Husky Ridge and Niagara Icefall intrusive sequences require a high heat flux to promote the second-stage melting of the refractory mantle. In particular, the Husky Ridge intrusion shows that the heat flux also induced partial melting in the sediment cover of the subducted slab.
The partial melting of the subducted slab during the Ross–Delamerian Orogeny has been already proposed. Small intrusive bodies with adakitic affinity ranging in age from 516 ± 10 to 531 ± 10 Ma were reported in south Victoria Land (Allibone & Wysoczanski, 2002). The involvement of melts from the sediment cover of the slab was indicated by the study of Ol-bearing cumulates from the Teall Nunatak Intrusion (NVL) dated at 521 ± 2 Ma (Giacomini et al., 2007). Furthermore, the presence of intrusive products recording melts from the subducted slab was reported in the Delamerian Orogen (e.g. the Glenelg River Complex, SE Australia; Kemp, 2004). On the basis of present data, we propose that the slab melting process was nearly synchronous across Victoria Land. Slab roll-back was proposed as a possible explanation for the abrupt termination at the end of the Cambrian of convergent deformation throughout the Ross–Delamerian Orogen, followed by buoyant uplift, exhumation and post-collisional magmatism (e.g. Foden et al., 2006, and references therein). The influx of new asthenosphere as a result of slab roll-back is also a suitable mechanism to supply heat at the regional scale required for the partial melting of both the subducted slab sediments and the subcontinental mantle previously depleted by melt extraction.
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CONCLUDING REMARKS ON THE ROLE OF AMPHIBOLE IN ARC MAGMAS |
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The Husky Ridge quartz diorites are evidence for the occurrence at mid-crustal levels of cumulus rocks in which amphibole may be up to 90% in volume and for the open-system evolution of subduction-related magmas within the crust. The products of a poorly evolved melt and those of a residual liquid after amphibole crystallization coexist at chemical disequilibrium at the thin-section scale. This work shows that the absence of amphibole among the liquidus phases of a residual melt (e.g. melt-II) does not preclude that it has crystallized at deeper crustal levels (see Davidson et al., 2007). Useful geochemical markers to detect the cryptic amphibole crystallization are: (1) the U-shaped pattern of HREE; (2) the NbN/TaN ratios below unity; (3) the negative Ti anomaly with respect to HREE.
According to the petrogenetic process proposed for melt-II, amphibole fractionation alone may account for the marked LREE enrichment and HREE depletion (i.e. adakitic signature) observed in some arc lavas and the involvement of garnet is not required. In particular, high proportions of amphibole (50%) are capable of shifting the LaN/YbN ratio towards significantly high values (up to 39), which are three times those of the parental melt.
The quartz diorites from the Husky Ridge are an example of how amphibole-rich mafic bodies may act as reservoir for the H2O storage in the crust. Based on the data reported by Tiepolo et al. (2000a), a mean exchange coefficient for H2O between pargasitic amphibole and melt can be assumed to be close to 0·3. According to this value and the amphibole modal proportions (up to 90 vol. %), quartz diorites from the Husky Ridge can subtract up to 27% of H2O from the parental mantle melt. In addition, considering that primitive arc magma may have H2O content up to 5% (Wallace, 2005), quartz diorites may be a crustal reservoir for up to 1· 4% of H2O.
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SUPPLEMENTARY DATA |
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Supplementary data for this paper are available at Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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Stefania Fiameni provided very generous support in zircon preparation and characterization by CL and LA-ICPMS. Alberto Zanetti is gratefully acknowledged for the stimulating discussion. Luigi Dallai shared with us the emotion of the Husky Ridge sampling during the 20th Italian Antarctic expedition and also sacrificed his camera for science. Claudio Ghezzo is gratefully acknowledged for having introduced us into the Antarctic world. The manuscript benefited by the constructive reviews of T. Kemp, A. F. Cooper and P. T. Leat. The editor John Gamble is also gratefully acknowledged. The PNRA is thanked for having funded the purchase of the LA-ICPMS system.
*Corresponding author. E-mail: tiepolo{at}crystal.unipv.it
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