Negative Zr Anomalies Research Articles

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

Dating fluid infiltration and deformation in the subducted ultramafic oceanic lithosphere by perovskite geochronology

Carbonated ultramafic rocks (i.e., ophicarbonates) form by peridotite serpentinization and carbonation at the seafloor. Their metamorphic evolution is linked to that of serpentinites, hence dating deformation and metamorphism in ophicarbonates will add constraints to the P-T-t history of the subducted ultramafic lithosphere. A potential, yet underexplored, suitable mineral for geochronology in ophicarbonates is perovskite (CaTiO3). We present trace element and geochronological data (LA-ICPMS and ID-TIMS) from perovskite in ophicarbonate rocks and calcite veins from two metamorphic terranes, Lanzo Massif and Val Malenco (Alps). Metamorphic perovskite in ophicarbonates is characterized by enrichment in heavy REE, and strong negative anomalies in Zr and Hf, as well as Sr and Pb, which are attributed to coexistence of a Zr-rich phase and mobilization of these elements by fluids, respectively. Other trace elements broadly reflect the composition of the protolith and allow the distinction of perovskite from mafic and ultramafic rocks.In the Lanzo Massif, the 49.6 ± 1.0 Ma age of perovskite in shear bands constrains H2-mediated carbonate reduction and abiotic methane and graphite formation at near peak metamorphic conditions during deformation. Perovskite from Val Malenco is found in calcite veins cutting across serpentinite shear foliation and its age of 48.9 ± 0.5 Ma coincides with a major thrusting and extension event that is distinct in age from the initial nappe stacking. The Val Malenco perovskite age can be reproduced by LA-ICPMS and ID-TIMS and is proposed as a suitable secondary reference material for metamorphic, low U perovskite. These results show that metamorphic perovskite geochronology yields geologically meaningful UPb ages and is a viable tool to date deformation and fluid flow in the subducted oceanic lithosphere.

Origin of Ni-Cu-(PGE + Au) sulfides in late-Archean komatiitic suite of rocks in the Shankaraghatta belt, Western Dharwar Craton (India)

The Shankaraghatta ultramafic–mafic suite of rocks (12 × 0.3 km) within the late ArcheanShimoga greenstone belt (Western Dharwar Craton, southern India) is constituted of sulfide-bearing serpentinized dunite flanked by metabasaltic schist rocks, which are probable analogues of the adcumulate-mesocumulate sheet flows in a komatiitic sequence, flanked by komatiitic basalts. Owing to limited field exposures in this study, the bulk concentrations of Ni, Cr, Ti, Fe, and Mg were used to classify the rocks into different volcanogenic facies observed in a komatiitic flow. The mineralized mesocumulate (serpentinized dunites) flanking its S-poor adcumulate counterpart host 5–8 modal % disseminated sulfide globules formed by an assemblage of pentlandite-millerite-pyrite-chalcopyrite. Millerite is the dominant sulfide mineral, whereas coarse euhedral pyrite grains are abundant in the serpentinites. The sulfide-bearing serpentinites have higher platinum-group element (PGE) abundances than those lacking visible sulfides (∑PGEsulfide-bearing samples = 440–1848 ppb; ∑PGEsulfide-lacking samples = 4.8–69 ppb) and show a strong PPGE (Pd, Pt, Rh) enriched trend with distinct negative Pt-anomalies in the primitive-mantle normalized PGE diagram. The mineralized serpentinites have higher Ni and Cu concentrations (0.5–2.2 wt% and 29–201 ppm, respectively) compared to their unmineralized counterparts (0.1–0.6 wt% and 22–72 ppm, respectively). Major element data indicate a komatiitic composition for the serpentinites, whereas the schists are komatiitic basalts. Chondrite-normalized REE plots for the serpentinites and schists show LREE-enriched patterns (La/SmNserp = 1.8–3.5; La/SmNschist = 2.5), with almost flat to slightly depleted HREEs (Gd/YbNserp = 1.1–1.7; Gd/YbNschist = 1.8). The rocks show prominent negative anomalies of Zr and Hf, which along with the subchondritic Al2O3/TiO2 ratios for serpentinites (~9.9), indicate derivation of the komatiitic magma from a plume that originated from a deep mantle source (>400 km) with the mantle residue retaining a majorite garnet phase. The komatiites and komatiitic basalts from which the serpentinites and schists were derived have a common mantle source. They are connected by fractional crystallization as understood from collinear and coherent patterns in bivariate plots between major oxides such as MgO, and alteration resistant immobile trace elements such as Nb, that are plotted with other major and trace elements, as well as from the coherent patterns observed in the chondrite-normalized REE and multi-element spider diagrams. The serpentinites and schists show signatures of crustal contamination as defined by negative anomalies of Nb, Ta, and Ti,high La/Sm ratios, along with higher concentrations of trace elements like La and Th, and lower Nb/Th ratios(Nb/Thavgserp = 6.6; Nb/Thavgschist = 3.7). AFC (Assimilation Fractional Crystallization) models indicate that the schists underwent a greater extent of crustal contamination (50% AFC) relative to the serpentinites (12% AFC). Sulfide saturation was achieved late, after emplacement of the komatiitic magma and crystallization of the olivine adcumulates, when the melt was slightly more differentiated to form the mesocumulates. The sulfides are later overprinted by low-temperature alteration processes, as evident from the significant enrichments in serpentinites for Au (AuNavg: 2.1), As (AsNavg: 20), Bi (BiNavg: 8.2), and Te (TeNavg: 1.5), which occurred perhaps during the replacement ofpentlandites by millerites.

Geochemistry of ultramafic and mafic rocks from the northern Central Asian Orogenic Belt (Tuva, Central Asia) - constraints on lower and middle arc crust formation linked to late Proterozoic intra-oceanic subduction

The Agardagh Tes-Chem complex (ATCC) in Tuva, Central Asia (50.5°N, 95°E) exposes a rare mafic to ultramafic crust-mantle fragment that developed within a late Neoproterozoic (~570 Ma) intra-oceanic island arc system that was accreted to the Tuva-Mongolian microcontinent during the formation of the Central Asian Orogenic Belt.Residual mantle rocks (harzburgites and dunites) are highly refractory with high Cr# (0.59–0.83) and intermediate Mg# (0.46–0.52) in spinel and experienced high degrees of total melt extraction (up to 25%). In ultramafic cumulate rocks (wehrlites and pyroxenites), Cr# and Mg# in spinel are distinctly lower (0.22–0.45 and 0.34–0.37), and rare earth element (REE) abundances of clinopyroxenes range between 0.1 and 1-times chondritic with chondrite normalized La/Yb of ~0.04. Trace element abundances in wehrlite clinopyroxene are lower than in pyroxenite clinopyroxene, whereas all have positive Sr but pronounced negative Zr anomalies. Calculated equilibrium melts of wehrlites and clinopyroxenites suggest strongly depleted hydrous high-MgO high-SiO2 primary magmas (picrites to boninites) from which these rocks have evolved.Incompatible trace element concentrations in mafic (i.e. gabbroic to dioritic) rocks vary strongly with REE contents between ~0.7 and 20-times chondritic, and chondrite normalized La/Yb between 0.25 and 0.98. Initial εNd varies between +4.8 and +7.1. These features, along with the broad textural and modal variations indicate that the mafic rocks developed from a single intrusive body that underwent substantial crustal assimilation. Cr and Ni contents in the mafic rocks are low and indicate their formation from already differentiated magmas.The evolution of the ATCC prior to amalgamation is explained by a two stage magmatic process. During the initial stages of melting low degree magmas evolved from a refractory mantle wedge fluxed by hydrous fluids and sediments or sediment derived melts to form enriched upper crustal shallow level intrusive and extrusive rocks. Later stage melting from a less hydrous and more depleted but SiO2-enriched source produced the parental picrites/boninites from which the lower and middle arc crustal rocks formed. These magmas differentiated within at least two distinct magma reservoirs at lowermost crustal levels, where cumulate wehrlites formed mostly independently from cumulate pyroxenites. This is evident from distinctly different trace-element compositions of these two rock types. The wehrlite derivative magmas subsequently intruded to mid-crustal levels where they crystallized the mafic rocks within a single intrusion, but accompanied by assimilation of preexisting enriched arc crust.

Petrochemical constrains on the origin and tectonic setting of mafic to intermediate dykes from Tikar plain, Central Cameroon Shear Zone

The Tikar plain is located on the Cameroon Central Shear Zone. It is also part of the North Equatorial Pan-African Belt. It is formed of granitoids intruded in places by mafic and intermediate dykes. The mafic dykes are essentially banded gabbros composed of plagioclases, pyroxenes, amphiboles, biotites and opaques. Their textures range from porphyroblastic to porphyritic. The intermediate dykes are monzonites and monzodiorites and are characterized, respectively, by cataclastic and mylonitic textures. The minerals identified are amphiboles, potassium feldspar, pyroxenes, epidotes, sphenes and opaques. Seritization reaction is mostly present on the mafic and intermediate dykes, while chloritization is much more pronounced on the intermediate dykes. The Tikar plain dykes are high-k calc-alkaline to shoshonitic. They are characterized by low to moderate SiO2 content (42.08–61.96 wt%), low to high TiO2 (0.47–2 wt%) and low Ni (1.48–99.18 ppm) contents. The mafic dykes show fractional trends with negative anomalies of Zr, U and P and positive Rb, Ba, Ta, Pb and Sr in multi-element diagrams, while the intermediate dykes present negative anomalies of Nb, Ta, Zr, Sr P and Ti and relative positive anomalies of Rb, Ba and Pb. The rare-earth elements (REE) patterns show positive Eu anomalies for the mafic dykes and negative anomalies for the intermediate dykes. The REE spectrum of all the dykes shows enrichment in LREE with relatively flat HREE, which can indicate arc magmatism. In the Zr–Ti/100–Sr/2 diagram, the mafic dykes plot in the island arc tholeiite and calc-alkaline basalt fields. The Th, Nb and LREE concentrations indicate that the subducted lithosphere with crustal component contributed to generation of the intermediate dykes of the Tikar plain. The geochemical characteristics of the mafic to intermediate dykes suggest their derivation from a various degree of partial melting in the garnet spinel facies, probably between depths of 80 and 100 km. The collision between the Central African Fold Belt and the northern edge of the Congo craton resulting in crustal thickening, sub-crustal lithospheric delamination and upwelling of the asthenosphere may have been the principal process in the generation of the intermediate dykes in the Tikar plain. The magma for the mafic and intermediate dyke would have migrated through the faults network of the Central Cameroon Shear Zone before crystallizing in the granito-gneissic basement rocks.

Mixing of heterogeneous, high-MgO, plume-derived magmas at the base of the crust in the Central Iapetus Magmatic Province (Ma 610-550): Origin of parental magmas to a global LIP event

High-MgO (>12 wt%) magmas represent some of the most primary and high temperature melts from mantle plumes. The compositional diversity of high-MgO magmas gained by fractional melting within inhomogeneous mantle sources, is often overprinted by wall rock assimilation, magma mixing and fractional crystallisation within magma chambers at various depths within the crust. The deepest intrusions, at the base of the crust, are the first stop for magmas as they leave the mantle and such localities offer vital insights into the crustal processes that modify high-MgO melts. The Seiland Igneous Province (SIP) represents a rare exposure of a deep crustal magma conduit system that transported large volumes of mantle-derived melts through the lower continental crust. In this work, trace element compositions of clinopyroxenes in dunite, wehrlite and olivine clinopyroxenite samples from the Reinfjord intrusion were measured using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The melts calculated to be in equilibrium with the clinopyroxenes represent derivatives of mantle-derived magmas and have steep rare earth element (REE) profiles with La/YbPM of 8.4–14., Sm/YbPM of 3.5–4.8, and negative anomalies in Nd, Zr and Hf. Assimilation, recharge and fractional crystallisation modelling shows that the ultramafic cumulates in the Reinfjord intrusion formed through concurrent fractional crystallisation and repetitive recharge (and mixing) of new primitive magmas from the mantle with very limited input from crustal sources. The recharge and mixing model is strongly supported by field and petrographic data. Two end-member ultramafic magmas are identified; the first end-member melt composition (MELTCPX0) is similar to the melt compositions that are in equilibrium with the most trace element depleted clinopyroxenes from Reinfjord. The second end-member melt composition (MELTPIC0) is similar to a number of picrite dykes found throughout the SIP, which have less steep LREE/HREE and lack strong negative anomalies in Zr and Hf. The REEBOX PRO melting model (Brown and Lesher, 2016) was used to forward model adiabatic decompression melting of lithologically homogeneous and heterogeneous sources containing anhydrous/hydrous peridotite ± pyroxenite. It was found that MELTPIC0 formed by melting of a peridotite source with up to 10% pyroxenite component and mantle potential temperatures of up to 1450 °C. MELTCPX0, however, formed by melting of a carbonate- metasomatised peridotite with potential temperatures of up to 1630 °C. These results indicate a compositionally and thermally inhomogeneous mantle plume beneath the SIP.

Two extensional events in the early evolution of the Yangtze Block, South China: Geochemical and isotopic evidence from two sets of Paleoproterozoic alkali porphyry in the Northern Kongling terrane

A set of granite porphyry (grnt‐porphyry) and sub‐rhyolite porphyry (rhy‐porphyry) with specific blue quartz phenocrysts was disintegrated in the northern Kongling terrane, China. Geochemical and isotopic analyses were performed on samples from Kongziqiao (KZQ) and Baizhuping (BZP). In terms of major elements, both are rich in silicon and alkalis, but depleted in calcium and magnesium. Their trace element contents are also similar, exhibiting negative anomalies for large‐ion lithophile elements (LILEs) such as K, Rb and Sr but positive anomalies for high‐field‐strength elements (HFSEs) such as Hf, Ce and Lu. Other elements such as Nb, Ti, P, Sm, Ho, Y and Yb show negative anomalies; however, the KZQ grnt‐porphyry is characterized by positive Ba and negative Zr anomalies; whereas the BZP sub rhy‐porphyry has negative Ba, U and Ta anomalies. The overall geochemistry is characteristic of A‐type granite formed in an extensional environment (A2‐type). Yield ages of the KZQ grnt‐porphyry are (2,239 Ma) and that of the BZP (1,895 Ma) sub rhy‐porphyry are Paleoproterozoic. Analysis of Hf isotopes shows that both have negative ε Hf (t) values, but the εHf (t) value of the KZQ grnt‐porphyry is slightly more positive (minimum −12.4; mean − 8.8) and its depleted mantle model age (TDM1 = 2.85–3.06 Ga, mean 2.92 Ga) is slightly older than the BZP sub rhy‐porphyry (minimum ε Hf (t) = −17.1, mean − 14.9; TDM1 = 2.79–2.92 Ga, mean 2.85 Ga). This suggests that they are derived from partial melting of different generations of ancient crustal material, which implies significant stratification in the Archean crust. The ε Nd (t) values of two samples of each rock type also indicate that both were derived from Ca. 2.9 Ga ancient crust (εNd (t) KZQ = −5.58, −4.74; εNd (t)BZP = −8.55, −8.22). The petrogenesis of both rock types is related to deep crustal extensional collapse caused by post‐orogenic extension before 2.2 Ga and after 1.9 Ga. The formation of two porphyries represents the changes in tectonic system of the Yangtze Block, and this tectonic evolution may have contributed to the reconstruction of the Columbia supercontinent.

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 ​wt.%), P2O5 (<0.05 ​wt.%), K2O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu∗ ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N ​= ​0.10–0.27 and (La/Yb)N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.

Eclogites in peridotite massifs in the Western Gneiss Region, Scandinavian Caledonides: Petrogenesis and comparison with those in the Variscan Moldanubian Zone

Eclogite lenses and boudins are volumetrically minor, but petrologically important, features of peridotite massifs worldwide. In the Western Gneiss Region of the Scandinavian Caledonides, eclogites in the Almklovdalen and Raubergvik peridotites originated as basaltic to picrobasaltic dikes, comprising both olivine–normative and nepheline–normative types, with a wide variation in Mg–number from 34 to 65. Positive anomalies for Pb and Sr and negative anomalies for Zr and Hf reflect a subduction signature in the basic melts, and rare–earth element modelling requires 20% to 70% fractional crystallization, combined with 20% to 70% assimilation of peridotite. Clinopyroxenes in eclogites have a wide variation in εNd(0) from +68 to −26, which is comparable to that for associated garnet peridotites and pyroxenites, +55 to −38, and a range in 87Sr/86Sr from 0.7021 to 0.7099, which is much larger than that in peridotites and pyroxenites, 0.7014 to 0.7033.Plagioclase and amphibole inclusions in eclogite garnet provide evidence for prograde metamorphism, which attained a maximum temperature of ~775 °C and pressure of ~25 kb. Such conditions are allofacial with those of associated garnet peridotites and pyroxenites, which equilibrated at ~825 °C and ~37 kb. Eclogites yield mixed Sm-Nd isochron ages, as do the peridotites and pyroxenites, but ages in eclogites are <1000 Ma, and those in peridotites and pyroxenites are >1000 Ma. Three eclogites yield Ordovician U-Pb ages for rutile at 440 ± 12, 445 ± 51, and 480 ± 29 Ma, which are coeval with the Taconic Orogeny and are consistent with a Laurentian provenance for the host peridotites.Eclogites in both Norwegian and Czech peridotites originated from melts passing through a mantle wedge above a subduction zone, and both suites exhibit subduction geochemical signatures, although they differ dramatically in petrogenesis. Eclogites in Norwegian peridotites initially crystallized as relatively low–pressure, plagioclase–bearing basaltic or gabbroic dikes and subsequently recrystallized to high–pressure eclogite, whereas most eclogites in Variscan Moldanubian peridotites crystallized directly from magmas at high pressure to produce eclogite facies assemblages.

Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India: Implications for petrogenesis

The Naga Hills Ophiolite (NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite, diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx. Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features. The basalts are characterized by 44.1–45.6 wt.% of SiO2 with 28–38 of Mg#, and the gabbros by 38.7–43.7 wt.% of SiO2, and 26–79 of Mg#. The ultramafic rocks are characterized by 37.4–52.2 wt.% of SiO2, and 80–88 of Mg#. In multi-element diagrams (spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr, Nb, Sr and a gentle negative anomaly of P. However, the rare earth element (REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE (LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite (IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340–4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97–1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources (fertile and refractory).

U–Pb zircon geochronology and geochemistry of Neoproterozoic granitoids of the Maevatanana area, Madagascar: implications for Neoproterozoic crustal extension of the Imorona–Itsindro Suite and subsequent lithospheric subduction

Voluminous Neoproterozoic granitoid sheets of the Imorona–Itsindro Suite are important components of exposed basement in west-central Madagascar. Here, we report precise new zircon U–Pb ages and whole-rock geochemistry for granitoids within the Maevatanana area of Madagascar. The new laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating undertaken during this study indicates that Antanimbary granitoid and Antasakoamamy granitoid were emplaced at 747 ± 9 Ma and 729 ± 9–727 ± 8 Ma, respectively. Geochemically, the Antanimbary granitoids show poor Nb, Ta anomalies, pronounced positive Zr anomalies, and are K-rich (K2O/Na2O > 1), but the Antasakoamamy granitoids are relatively depleted in Nb, Ta, show slightly negative Zr anomalies, and are Na-rich (Na2O/K2O > 1). Both suites contain zircons with strongly negative εHf(t), indicating participation of much older (Palaeoproterozoic and Archaean) crust. Their geochemical characteristics, along with the use of various discrimination diagrams, reveals that crustal delamination and asthenospheric upwelling resulted in crustal extension of the region before ~747 Ma, with subsequent lithospheric subduction and arc magmatism after 729–727 Ma.

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

AbstractComposite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.

Post-orogenic extension in the eastern part of the Jiangnan orogen: Evidence from ca 800–760 Ma volcanic rocks

We present a systematic geochronological and geochemical study on ca 800–760Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at ca 805Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The ca 800–790Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na2O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes (ɛNd(t) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO2 contents, negative anomalies in Zr and Hf, and high values of Mg# and ɛNd(t) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO2 contents and low positive ɛNd(t) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The ca 760Ma mafic rocks from the Puling bimodal volcanics generally have low TiO2 contents (<0.9wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO2 content (2.41wt%) and high ɛNd(t) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period ca 800–760Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.

Trace element chemistry of mineral inclusions in eclogitic diamonds from the Premier (Cullinan) and Finsch kimberlites, South Africa: Implications for the evolution of their mantle source

Although diamonds of eclogitic paragenesis are commonly encountered in the productions of many Southern Africa kimberlites, the nature and evolution of the protolith to eclogitic diamonds are still poorly understood. There is some evidence that these protoliths (and possibly also the diamonds) may be related to subduction of oceanic crust, although this is not a universally accepted view. In order to further investigate the protolith/diamond relationship, garnets and (in some cases) clinopyroxene inclusions in 23 diamonds from Premier mine and 16 diamonds from Finsch were analysed for their trace element composition. From both mines a strong correlation between the garnet Ca content and the chondrite-normalised rare earth element (REE) pattern is evident. Garnets with comparatively low Ca content are characterised by REE patterns which show a steady increase in abundance from light rare earths (LREE) to heavy rare earths (HREE). With increasing Ca content in garnet, the abundance of LREE (La, Ce, Pr, and Nd) as well as the middle rare earths (MREE; Sm, Eu, Gd, and Tb) progressively increases, ultimately giving the trace element pattern a distinct ‘humped’ appearance. Bulk-rock trace element abundance patterns have been reconstructed from measured trace element contents in garnet as well as calculated trace element concentrations in clinopyroxene, based on known clinopyroxene–garnet partition coefficients ( Harte and Kirkley, 1997). At both Premier and Finsch, the low-Ca group samples (2.6 to 5.0 wt.% CaO in garnet) are LREE depleted, and have relatively flat calculated bulk-rock trace element abundance patterns at approximately 10 times chondrite concentrations, but with marked positive Sr and negative Zr anomalies. The intermediate-Ca group samples (5.2 to ∼9 wt.% CaO in garnet) are LREE depleted, show Sr and Zr anomalies, have somewhat higher concentrations of Zr and MREE, and have HREE contents that overlap with the low-Ca group ( Fig. 6). High-Ca group samples (∼ 9 to 14.8 wt.% CaO in garnet) are LREE depleted, show Sr and Zr anomalies, are MREE-enriched, and have HREE contents that are slightly less than the low- and intermediate-Ca group samples. Based on both the calculated bulk eclogite trace element abundances and their patterns, as well as previously published radiogenic isotope data, our preferred model of protolith evolution for the eclogitic diamonds from Premier and Finsch is one in which both the major and trace element chemistry of the inclusions are ultimately inherited from low-pressure oceanic protoliths, consisting of varying mixtures of oceanic basalt + cumulate gabbro for diamonds from both Premier and Finsch. Of particular importance in the current data are the presence of marked negative Zr anomalies, marked positive Sr anomalies, and a general absence of Eu anomalies in all compositional groupings. The Zr anomaly can arise in reconstructed bulk eclogite trace element abundance patterns if rutile is not included in the calculations, but the Sr anomalies (coupled with an absence of Eu anomalies) can only be explained through the mixing of oceanic gabbro and mid-ocean ridge basalt. The averaged eclogite bulk trace element compositions for Premier and Finsch are also markedly similar to that of clinopyroxene in a typical cumulate gabbro, and a role for cumulate clinopyroxene in protolith evolution may therefore also be inferred. It is likely that prior to and during diamond crystallisation, the major and particularly the trace element compositions of the high-pressure eclogite source rock to these diamonds may have been slightly modified by metasomatic fluids and melts. However large-scale fluid- or melt-related metasomatic processes are not indicated.

Occurrence and chemical composition of amphiboles and related minerals in corundum-bearing mafic rock from the Horoman Peridotite Complex, Japan

A corundum-bearing mafic rock in the Horoman Peridotite Complex, Japan, was derived from upper mantle conditions to lower crustal conditions with surrounding peridotites. The amphiboles found in the rock are classified into 3 types: (1) as interstitial and/or poikilitic grains (Green amphibole), (2) as a constituent mineral of symplectitic mineral aggregates with aluminous spinel at grain boundary between olivine and plagioclase (Symplectite amphibole) and (3) as film-shaped thin grains, usually less than 10 μm in width, at grain boundary between olivine and clinopyroxene (Film-shaped amphibole). The Film-shaped amphibole is rarely associated with orthopyroxene extremely low in Al 2O 3, Cr 2O 3 and CaO (Low-Al OPX). These minerals were formed by infiltration of SiO 2- and volatile-rich fluids along grain boundaries after the rock was recrystallized at olivine-plagioclase stability conditions, i.e. the late stage of the exhumation of the Horoman Complex. Chondrite-normalized rare earth element patterns and primitive mantle-normalized trace-element patterns of the Green amphibole and clinopyroxene are characterized by LREE-depleted patterns with Eu positive and negative anomalies of Zr and Hf. These geochemical characteristics of the constituent minerals were inherited from original whole-rock compositions through a reaction involving both pre-existing clinopyroxene and plagioclase. We propose that the fluids were originally rich in a SiO 2 component but depleted in trace-elements. Dehydration of the surrounding metamorphic rocks in the Hidaka metamorphic belt, probably related to intrusion of hot peridotite body into the Hidaka crust, is a plausible origin for the fluids.

Rb–Sr and Sm–Nd isotopic systematics of the Hayachine–Miyamori ophiolitic complex: Melt generation process in the mantle wedge beneath an Ordovician island arc

The Hayachine–Miyamori (HM) ophiolitic complex in the Kitakami Mountains, northeastern Japan consists of ultramafic tectonite and cumulate members. The most fertile lherzolites have mineral and trace element compositions similar to those of abyssal peridotites. They show 350–430 Ma Nd depleted mantle model ages, which are within the range of the K–Ar emplacement ages obtained from intrusive gabbroic rocks, suggesting a partial melting event just before the emplacement. The measured 143Nd/ 144Nd ratio of clinopyroxene in the tectonite peridotites shows positive correlation with 147Sm/ 14 4Nd and decreases with increasing refractoriness, which cannot be explained by a simple melting and melt extraction to a various extent followed by radiogenic ingrowth. It clearly suggests influx of a melt/fluid enriched in highly incompatible trace elements during melting. Time corrected isotopic compositions of the HM complex exhibit a clear island arc signature with uniform initial isotopic ratio ( 87Sr/ 86Sr = 0.7035–0.7041, ε Nd = + 7.8–+ 5.0). Application of an open-system melting model to the observed trace element abundances in clinopyroxene suggests influx of three distinct agents to the HM mantle with the following characteristics: (1) moderate enrichment in highly incompatible elements with negative anomalies of Sr and Zr; (2) extensive enrichment of highly incompatible elements with positive Sr and negative Zr anomalies; and (3) extensive enrichment of highly incompatible elements with positive anomalies of Sr and Zr. These characteristics cover a variety of slab-derived components proposed in the literatures, suggesting the agents responsible for the open-system melting in the HM ophiolite might represent full spectrum of slab-derived components from back-arc to fore-arc regions of the Ordovician island arc system.

PARENTAL MELTS IMPRINTED IN PLUTONIC ROCKS OF THE VOYKAR OPHIOLITE, POLAR URALS: EVIDENCES FROM CLINOPYROXENE GEOCHEMISTRY

The plutonic rocks (dunites, wehrlites, clinopyroxenites and gabbronorites) of the Voykar ophiolite are associated with mantle tectonites and a diabase dike complex. Clinopyroxenes of the plutonic rock units were analyzed for trace elements. The abundances of moderately incompatible elements (heavy rare-earth elements and Y) in these clinopyroxenes display two distinct levels (relatively enriched and depleted) with no crystal fractionation trends.enriched clinopyroxenes occur both in ultramafics and in gabbronorite. In the ultramafic plutonic rocks they have Mg# = 89 and high Cr content (>3800 ppm), while in the gabbronorites they have Mg# <77 and low Cr abundance (<1000 ppm). The presumable silicate liquid ( melt-1 ) equilibrated with the enriched clinopyroxenes should be only slightly depleted in incompatible elements relatively to MORB. The melt-1 migration through the hot mantle restite resulted in metasomatic wehrlite and diopside-bearing dunite with enriched clinopyroxenes. In upper horizons of the plutonic section the melt-1 formed gabbronorite bodies.depleted clinopyroxenes occur in wehrlite and clinopyroxenite and have Mg# = 89-85 with intermediate Cr abundances (1300-3600 ppm). They were equilibrated with a melt-2 significantly depleted in incompatible elements relatively to MORB, with marked positive Sr and negative Zr anomalies. The olivine+chromite and olivine+clinopyroxene cumulates are related to this melt. The melt-2 incompatible-element pattern closely matches with that of least evolved Voykar dikes and suggests a supra-subduction type magma.clinopyroxene geochemistry reinforces a supra-subduction nature of the Voykar magmatic complexes. The parental melts imprinted in the plutonic rocks show a geochemical evolution of mantle source from slightly depleted relatively to those of MORB through strongly depleted with high Sr/Zr, i.e. large-ion lithophile / high-field strength elements ratios.specific features of the Voykar magmatism are: (1) discrete and compositionally contrast magma influx into the lower crust of a supra-subduction spreading zone; (2) continuous melt percolation through aggregates of accumulating crystals resulted in limited variations within distinct depleted and enriched HREE abundances in clinopyroxene.

Petrogenesis of Silali volcano, Gregory Rift, Kenya

Basalts of the Silali volcano are of high-Fe transitional affinity and range from plagioclase-phyric varieties, probably formed by selective accumulation, to aphyric. They have compositional similarity to oceanic island basalts, but are isotopically variable and have strong, negative Zr anomalies on chondrite-normalized plots. Major and trace element and isotopic evidence indicates that the basalts represent several liquid lines of descent, each equilibrated at crustal pressures. All Silali basalts are relatively evolved; primitive basalts were probably held close to the Moho, where they evolved by gabbro fractionation to produce basalts with less than 8% MgO. These magmas were subsequently held at several levels in the crust, including the rift-axial lava-sediment sequence. Fractionation was along a high-Fe trend through mugearite to metaluminous, two-feldspar trachyte and thence to peralkaline, one-feldspar, silica-oversaturated and -undersaturated trachytes. Magmas intermediate in composition between basalt and trachyte were very rarely erupted at Silali, but their evolution can be deduced from an extensive suite of dolerite blocks, which contain residual glasses varying from mugearitic to peralkaline phonolitic compositions. Magma mixing between basalt and trachyte has been common at Silali, implying complexity of the plumbing system. Evolution of the trachytes was by fractional crystallization combined with assimilation of crust during or after fractionation.