HREE Contents Research Articles

The generation and evolution of Archean continental crust in the Dunhuang block, northeastern Tarim craton, northwestern China

Tonalite–trondhjemite–granodiorite (TTG) preserved in Archean cratons can provide insights into the generation and evolution of the early continental crust. In this study, typical TTG gneisses from the Dunhuang block in the northeastern Tarim craton were studied in detail regarding their geochemistry and geochronology to constrain the generation and evolution of the Archean continental crust in this region. These TTG gneisses are characterized by high contents of SiO2 (68.3–71.6%), Al2O3 (15.3–16.9%), Na2O (4.43–4.85%), low K2O/Na2O ratios (0.20–0.37) and a very low HREE content (Yb<1ppm) and show two-stage Nd isotope model ages of ∼3.06–2.84Ga. Zircon U–Pb analyses reveal that these TTG gneisses were formed ∼2.7–2.6Ga ago, as shown by inherited magmatic zircon cores, and were later altered by Paleoproterozoic (∼2.0–1.9Ga) and early Paleozoic (∼430Ma) high-grade metamorphic events. Two samples show positive ɛHf (t) values of 1.5–5.4 for magmatic zircons with ages of ∼2.7–2.6Ga and give a two-stage Hf isotope model age of ∼2.95Ga, while one sample exhibits negative ɛHf (t) values of −3.4 to −7.2 for magmatic zircons with ages of ∼2.7–2.6Ga and gives a two-stage Hf isotope model age of ∼3.4Ga, suggesting that the Paleoarchean and Mesoarchean Eras were important periods for the generation of juvenile continental crust in the Dunhuang block. Lastly, based on analyses of previous studies, we speculate that the Tarim craton has been subjected to episodic crustal growths at ∼3.4Ga, ∼3.2Ga, ∼2.95Ga, ∼2.8Ga and ∼2.6Ga and reworking events at ∼2.7–2.6Ga and ∼2.5Ga

Mineral Chemistry of Clays Associated with the Jhilmili Intertrappean Bed in the Eastern Deccan Volcanic Province: Palaeoenvironmental Inferences and KTB Transition

Abstract Jhilmili intertrappean bed (~13 m thick) attains its significance with the recent discovery of brackish water ostrocod and planktonic foraminifera fossils (Keller et al. 2009; Khosala et al. 2009). Present XRD data revealed abundance of montmorillonite &amp;gt; montmorillonite/chlorite mixed layer &amp;gt; palygorskite in five physically distinct litho-units namely: (a) lower chocolate brown siltstone with green patches, followed by (b) brick red clayey siltstone, (c) greenish grey clay, (d) yellowish brown clay and (e) uppermost olive grey to dark brown silt layers in the succession-represent higher weathering indices and annual precipitation, reflecting cyclic, but longer spells of weathering. Occurrence of M/C mixed layer with the smectite in Jhilmili area is suggestive of their derivation from the later, whilst montmorillonite to palygorskite transformation is ascribed to the drastic changes in the humid to arid climate, where former served as a source of Al and some of the Si and Mg ions for the later. Jhilmili and Anjar clays represent similar charge occupancies at different sites, but later contains relatively higher amount of palygorskite, formed in the arid environment. Majority of the trace elemental data plots for Jhilmili clays lie within the upper and lower limits of infra (Lametas)-/inter-trappeans clays. The continuous release of Cu throughout the succession (mainly in the palygorskite dominated clays) indicates oxidizing conditions. PAAS normalized REE data plots for these clays show progressive enrichment in the HREE contents in the lower part, but, upper part of the succession is marked by positive cerium anomaly, reflecting oxidizing conditions prevailed at the later stage of the succession. These conditions continued, but, were not conducive to HREE enrichment as evidenced by their depletion in the upper part of the succession. The Ce anomaly observed in the middle part of the succession is similar to that form by continental weathering of the basalt, ascribed to Ce precipitation in the oxic environment, thus suggestive of drastic changes in the oxidizing conditions. Thermodynamic data-sets for Jhilmili clays show I/S mixed layer and celadonite compositions, whereas, Jabalpur infra-/inter-trappean clays correspond to Mg smectites and celadonite end members, thus, representing compositional commonality with those of the other clays derived from the continental weathering of basalt protolith. Jhilmili smectites and smectite-chlorite mixtures show compositional similarity with the dioctahedral and trioctahedral smectites and the smectites formed at 250ºC, having compositions between trioctahedral smectite and chlorite, thus, assigning high temperatures for their formation, where the heat required for the formation of these clays was possibly derived from the hydrothermal fluids, associated with the Deccan volcanism.

Petrogenesis and geochronology of Cretaceous adakitic, I- and A-type granitoids in the NE Yangtze block: Constraints on the eastern subsurface boundary between the North and South China blocks

The position of the subsurface boundary between the North China block (NCB) and South China block (SCB) has been debated, mainly using evidence from surface geology and geophysical observations. Here, petrochemical and geochronological data on four early Cretaceous granitic plutons from a focused area east of the Tan–Lu fault, NE Yangtze block, are reported to provide constraints on their petrogenesis and tectonic affinity. The Chuzhou intermediate intrusions consist mainly of quartz monzonite and granodiorite, and formed at ca. 125–127Ma. They have high MgO, Al2O3, Sr, and low Rb, Y and Yb contents, together with high Sr/Y and La/Yb ratios, indicating an adakitic affinity. They also show strongly negative whole-rock εNd(t) and zircon εHf(t) (−26 to −16) values, and old Hf crustal model ages (2.2–2.8Ga). Coupled with their negative Nb–Ta anomalies, high K2O/Na2O (0.91–1.18) and low Ce/Pb (1.08–5.40) ratios, these data suggest that the Chuzhou adakites were derived from thickened Archean to Paleoproterozoic lower crust. In contrast, the Fanchang, Qingyang and Huangshan intermediate–silicic complexes are dominantly composed of I-type granodiorite and quartz monzonite and A-type monzogranite and alkali feldspar granite. The I-type intrusions (126–138Ma) were emplaced slightly earlier than the A-type intrusions (121–129Ma), and distributed around A-type intrusions with clear intrusive boundaries. The I- and A-type rocks have lower MgO, CaO, Co, Sr, and higher Rb, Nb, Th and HREE contents than the adakitic rocks, with strong depletion in Ba, Sr, P, Eu and Ti for the A-type rocks. They have higher whole-rock εNd(t) (−9 to −5) and zircon εHf(t) (−13–0) values, and younger Nd model ages (TDM2=1.1–1.4Ga) and zircon Hf model ages (Tcrust=1.2–2.0Ga) than the adakitic rocks. These features indicate that the I- and A-type granites originated by partial melting of Mesoprotorozoic–Neoproterozoic lower crust, followed by fractional crystallization of plagioclase, K-feldspar and ferromagnesian minerals. The completely different isotopic compositions suggest that the Chuzhou adakites and three I- and A-type complexes were derived from different lower crustal domains, the former with NCB affinity and the latter with SCB affinity. Thus, a subsurface boundary between the NCB and SCB in the area east of the Tan–Lu fault can be constrained to a position between the Chuzhou and Ningwu–Fanchang area, possibly near Nanjing city. The association of Cretaceous bimodal volcanic rocks, within-plate mafic rocks and rift basins, suggests that the early Cretaceous rocks in the NE Yangtze block were produced in a lithospheric extensional setting, probably related to mantle-derived magma underplating and lithospheric thinning.

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

The Dengfeng and Taihua Complexes are well-exposed Neoarchean to Paleoproterozoic units in the southern segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating shows that the Dengfeng Complex records two episodes (2568±11Ma and 2306±6Ma) of tonalite–trondhjemite–granodiorite (TTG) magmatism. All of the TTG rocks are characterized by high SiO2 (66.7–75.4wt%), Na2O (3.20–5.06wt%) and relatively low MgO (0.40–1.78wt%). The Late Neoarchean TTG gneisses have very low contents of HREE (YbN=0.69–2.75) and Y (1.73–7.07ppm), with moderate [La/Yb]N (24.1–53.8) and high Sr/Y (65.1–291.3) ratios. The Early Paleoproterozoic TTG gneisses have low contents of HREE (YbN=2.93–6.37) and Y (6.7–11.0ppm), with moderate [La/Yb]N (10.1–27.0) and Sr/Y (10.6–52.1) ratios. Both suites show pronounced negative Nb–Ta, P and Ti anomalies but positive Sr and Pb anomalies. The Late Neoarchean TTG gneisses all have similar bulk-rock Nd and zircon Hf model ages with mainly positive ɛNd(t), and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole. The Early Paleoproterozoic TTG gneisses have extremely variable ɛNd(t) (−6.23 to +4.23) and heterogeneous zircon ɛHf(t) (−3.3 to +3.1), which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet. The Taihua Complex in the Xiaoqinling area records three episodes of Early Paleoproterozoic TTG magmatism (2.48Ga at Caotan, 2.31Ga at Houjiacun and 2.16Ga at Bayuan), younger than the Taihua Complex in the Lushan area (2.85–2.72Ga). All rocks have relatively low contents of HREE (YbN=1.03–8.32) and Y (2.84–24ppm), with moderate [La/Yb]N (8.7–88.4) and Sr/Y (19.8–125.8) ratios, and show negative Ta–Nb and Ti anomalies and positive Sr and Pb anomalies. The Caotan gneisses at 2.48Ga and the Houjiacun TTG gneisses at 2.31Ga have low Mg# (0.14–0.45), low Cr (<42ppm) and Ni contents (1–21ppm), with variable but overall positive ɛNd(t) and ɛHf(t) values, and were derived from the partial melting of thickened lower crust with residual garnet and amphibole. The younger Bayuan TTG gneisses at 2.16Ga have low SiO2 (57.11–64.89wt%), high MgO (2.64–4.62wt%), Cr (100–247ppm) and Ni (32–80ppm), with negative whole rock ɛNd(t) and zircon ɛHf(t) values, resulted from the partial melting of delaminated lower crust that interacted with peridotitic mantle.The geochronology of the Dengfeng Complex (in the Dengfeng area) and the Taihua Complex (in the Lushan, Xiong’er and Xiaoqinling areas) reveals at least four magmatic episodes in the southern segment of the TNCO from the Late Mesoarchean to Early Paleoproterozoic (2.85–2.72Ga, 2.57–2.48Ga, 2.34–2.30Ga and 2.20–2.07Ga). The rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock ɛNd(t) and zircon ɛHf(t) values, suggesting two episodes of crustal growth formed in a subduction tectonic setting. The magmatic rocks of the third episode consist of both the juvenile and pre-existing crustal materials with variable whole rock ɛNd(t) and zircon ɛHf(t) values, which were generated in a subduction zone during the initial assembly of the NCC within the Columbia supercontinent cycle. The final episode of magmatism lacks juvenile materials with whole rock ɛNd(t) and zircon ɛHf(t) values being consistently negative. These may have resulted from the orogenic collapse. The episodic continental growth recorded in the southern segment of the TNCO was caused by subduction and consequent orogeny, consistent with global supercontinent cycles within the Late Archean and Early Paleoproterozoic.

Petrological and geochemical characteristics of Paleoproterozoic ultramafic lamprophyres and carbonatites from the Chitrangi region, Mahakoshal supracrustal belt, central India

A number of ENE–WSW trending Paleoproterozoic dykes and plugs of mafic, ultramafic, alkaline and carbonatite rocks intrude Mahakoshal supracrustal belt (MSB), which is a part of the Central Indian Tectonic Zone (CITZ). Best exposures of these intrusions are found in the eastern parts of the MSB, particularly in and around Chitrangi area. Many of these intrusions have greenschist facies mineral composition and show sharp contact with supracrustal rocks. However, igneous textures, such as porphyritic/glomeroporphyritic, are still preserved in the form of partly pseudomorphed olivines, phlogopites and pyroxenes. Striking feature observed in some ultramafic samples is the presence of melanite garnet and rounded or elliptical carbonate ocelli. The petrographic characteristics suggest occurrence of carbonate-rich ultramafic lamprophyres; close to aillikite composition. Coarse-grained carbonatites show hypidiomorphic texture and mostly composed of calcite with appreciable amount of silicate minerals like clinopyroxene, phlogopite and olivine (often pseudomorphed by calcite, amphibole and chlorite). It is difficult to establish any direct genetic relationship between carbonatite and ultramafic lamprophyre samples on the basis of their chemistry; they were likely derived from distinct parental melts. High Mg# (up to ~78), and high Ni and Cr contents (up to ~1700 and ~1100, respectively) and low HREE concentration in few ultramafic lamprophyre samples apparently suggest their derivation from a near-primary mantle-derived melts originated at great depths. Geochemistry and presence of carbonate ocellae in ultramafic lamprophyre samples suggest genesis of these silicate rocks and associated carbonatites through liquid immiscibility, however possibility of their derivation through vein-plus-wall-rock melting model cannot be ignored. A multi-stage veined mantle melting model is suitable in the latter case. It is suggested that early stages of rifting in the Mahakoshal region due to lithospheric thinning caused by possible plume activity provided suitable conditions for the genesis of ultramafic lamprophyre (possibly aillikitic) and carbonatitic melts which ultimately crystallized as dykes and plugs.

In situ laser ablation ICP‐MS analyses of dimict diogenites: Further evidence for harzburgitic and orthopyroxenitic lithologies

AbstractTrace element concentrations in pyroxene, plagioclase, and olivine were measured in five diogenite breccias previously identified as containing distinct harzburgitic (ol+opx) and orthopyroxenitic (opx) lithologies (dimict). Three samples show two distinct populations of pyroxene trace element abundances, supporting their classification as dimict. These three meteorites show increases in Y, Yb, and HREE concentrations from harzburgitic to orthopyroxenitic pyroxenes, supporting the hypothesis that the lithologies are related through fractional crystallization whereby harzburgite olivine and pyroxene crystallized from the magma first followed by orthopyroxenite pyroxene. Depletions in LREE and Eu concentrations in the orthopyroxenitic lithology are most likely due to equilibration with LREE and Eu‐rich phases, likely plagioclase, which is found primarily in that lithology.Two samples do not show evidence supporting a dimict classification. Large pyroxene trace element variation in one sample indicates that it is polymict, while uniform trace element distribution in the other suggests that it may be a monomict breccia.

Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over <0.5km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges of Al2O3, Na2O, Cr2O3 and TiO2 contents. Primary spinels show a large variation of Cr# [=Cr/(Cr+Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr)+LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3– <10% melting for type I lherzolites and ~15% for harzburgites. Hornblendes are enriched in fluid-mobile elements relative to HFSE+U inherited from their precursor Cpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (<5cm) mineralogical and chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incremental melt that formed during decompression melting of a normal MORB mantle source. The studied peridotites possibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.

Petrogenesis of serpentinites from the Franciscan Complex, western California, USA

Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n=12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values >0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (<0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ18O values of +5.4 to +7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (−107 to −90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ18O values (+7.2 to +9.5‰) likely due to low-T interaction with meteoric water at high fluid–rock ratios. Most of the serpentinites have δ37Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ37Cl value (+1.7‰) possibly from interaction with a slab-derived fluid.

Element mobility during continental collision: insights from polymineralic metamorphic vein within UHP eclogite in the Dabie orogen

AbstractMetamorphic dehydration and partial melting are two important processes during continental collision. They have significant bearing on element transport at the slab interface under subduction‐zone P–T conditions. Petrological and geochemical insights into the two processes are provided by a comprehensive study of leucocratic veins in ultrahigh‐pressure (UHP) metamorphic rocks. This is exemplified by this study of a polymineralic vein within phengite‐bearing UHP eclogite in the Dabie orogen. The vein is primarily composed of quartz, kyanite, epidote and phengite, with minor accessory minerals such as garnet, rutile and zircon. Primary multiphase solid inclusions occur in garnet and epidote from the both vein and host eclogite. They are composed of quartz ± K‐feldspar ± plagioclase ± K‐bearing glass and exhibit irregular to negative crystal shapes that are surrounded by weak radial cracks. This suggests their precipitation from solute‐rich metamorphic fluid/melt that involved the reaction of phengite breakdown. Zircon U–Pb dating for the vein gave two groups of concordant ages at 217 ± 2 and 210 ± 2 Ma, indicating two episodes of zircon growth in the Late Triassic. The same minerals from the two rocks give consistent δ18O and δD values, suggesting that the vein‐forming fluid was directly derived from the host UHP eclogite. The vein is much richer in phengite and epidote than the host eclogite, suggesting that the fluid is associated with remarkable concentration of such water‐soluble elements as LILE and LREE migration. Garnet and rutile in the vein exhibit much higher contents of HREE (2.2–5.7 times) and Nb–Ta (1.8–2.0 times) than those in the eclogite, indicating that these normally water‐insoluble elements became mobile and then were sunken in the vein minerals. Thus, the vein‐forming agent would be primarily composed of the UHP aqueous fluid with minor amounts of the hydrous melt, which may even become a supercritical fluid to have a capacity to transport not only LILE and LREE but also HREE and HFSE at subduction‐zone metamorphic conditions. Taken together, significant amounts of trace elements were transported by the vein‐forming fluid due to the phengite breakdown inside the UHP eclogite during exhumation of the deeply subducted continental crust.

FLUORINE-RICH XENOTIME FROM THE WORLD-CLASS MADEIRA Nb-Ta-Sn DEPOSIT ASSOCIATED WITH THE ALBITE-ENRICHED GRANITE AT PITINGA, AMAZONIA, BRAZIL

The Madeira deposit (Sn, Nb, Ta) at Pitinga, Amazonia, in Brazil, is associated with the albite-enriched facies of the A-type Madeira granite (~1,820 Ma). Fluorine (cryolite), Y, REE, Li, Zr, and Th are potential by-products of the disseminated ore. We studied the xenotime from the core albite-enriched (CAG), transitional albite-enriched granite (TAG) and pegmatitic albite-enriched granite (PAG). In all these rocks, xenotime is magmatic, has high HREE contents (0.37 to 0.54 atoms per formula unit; LREE are not detected), and low U, Th and Ca content. The CAG xenotime is the richest in REE, followed by that in the PAG and TAG units. Fluorine was detected in all xenotime crystals from CAG (up to 5.10%) and from PAG (up to 1.40%) and in most crystals from TAG (up to 0.68%). Where the xenotime is richer in F, the Na and Si increase, the P, Ca, and the effectiveness of a thorite-type substitution decrease. Fluorine controls the ratio REE/Y; the richer the xenotime in F, the richer it is in REE, in particular Er and Yb. The cell parameters a and c are significantly shortened in F-rich xenotime. There is no evidence of OH in the structure. We contend that fluorine substitutes for O to form PO3F tetrahedra, as in bobdownsite. The formation of the PO3F tetrahedra in the xenotime accounts well for the compatibility between the crystallographic characteristics and the chemical composition.

Anatomy of garnets in a Jurassic granite from the south-eastern margin of the North China Craton: Magma sources and tectonic implications

The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (i) euhedral garnet in mafic biotite and garnet rich enclave (Grt I), (ii) coarse-grained garnet (Grt II) in the host granite, and (iii) small euhedral garnet in aplite (Grt III). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt II. Grt III has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt II. Grt III has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ18O values of <3.8‰ in most of Grt I, 3.8–4.7‰ in most Grt II (for inclusion-free garnets), and typically >4.7‰ in Grt III. Some of the Grt II and Grt III display two distinct zonings with cores having similar major and trace element compositions to Grt I.Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U–Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (∼160Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210–236Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie–Sulu orogen (230Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events.Based on the zircon U–Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peritectic garnet, whereas Grt II and III are probably the results of magmatic dissolution–precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie–Sulu orogen representing the South China Craton.

Late Cretaceous subduction initiation and Palaeocene–Eocene slab breakoff magmatism in South-Central Anatolia, Turkey

The Late Cretaceous Alihoca ophiolite in the Inner Tauride suture zone (ITSZ) of South-Central Turkey represents part of a single ophiolitic thrust sheet that originated from the Inner Tauride ocean. The ophiolite contains upper mantle peridotites, cumulate wehrlites, layered-to-isotropic gabbros, and microgabbroic-to-doleritic dikes. An ophiolitic mélange beneath the Alihoca ophiolite includes blocks of limestone, peridotite, dolerite, basalt, and deep-sea sedimentary rocks (radiolarite, chert) in a matrix comprising sheared serpentinite and mudstone. Isotropic gabbro and dolerite dike rocks show enrichment in Sr, K, Rb, Ba, and Th (LILE) and depletion of Ta, Nb, Zr, Ti, and Y (HFSE), indicating an island arc tholeiite (IAT) affinity. Relatively younger dolerite rocks display low TiO2 (<0.5 wt.%) contents, concave REE profiles with low HREE concentrations, and high LREE values, typical of boninitic affinities. The Alihoca ophiolite, hence, displays an IAT to boninitic geochemical progression in its magmatic evolution, reminiscent of many other Tethyan ophiolites in the region. It represents the remnant of a forearc oceanic crust, which developed during the early stages of subduction within the Inner Tauride ocean. Volcanic, volcano-sedimentary, and sedimentary rocks of the Ulukışla–Çamardı basin north of the ITSZ disconformably overlie the mafic-ultramafic rocks of the Alihoca ophiolite. Pillowed and massive lavas of the latest Cretaceous–Palaeocene Ulukışla Formation have alkaline basalt-to-basaltic andesite compositions, displaying relatively enriched LILE and LREE patterns with negative Nb and Ta anomalies. These geochemical features suggest that magmas of the Ulukışla–Çamardı volcanic rocks formed from partial melting of a metasomatized lithospheric mantle. This melting event was triggered by the influx of asthenospheric heat through a slab breakoff-induced window in the downgoing Tethyan oceanic lithosphere.

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015±4.4Ma, 966.5±3.5 and 808±3.1 respectively. The plagiogranite shows high SiO2, Na2O and extremely low K2O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO2 and moderate Na2O, together with high K2O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO2 moderately higher Na2O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.

Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Hima- layan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillima- nite. Peak metamorphic conditions were 8-9 kbar and *800 C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by micro- beam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of meta- morphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to meta- morphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales charac- terizes melting which occurred over a period of *15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (*0.6 Ma). An older *36 Ma high-grade event is recorded in an al- locthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic dis- continuity. The higher structural levels reached melting and peak conditions later (*26-23 Ma) than the lower structural levels (*31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.

Late-Neoarchean magmatism and metamorphism at the southeastern margin of the North China Craton and their tectonic implications

In the present study, two gneiss xenoliths from the Mesozoic Jiagou intrusion at the southeastern margin of the North China Craton (NCC) are examined for in situ zircon SHRIMP U–Pb dating in combination with zircon trace element and Hf isotope analyses. Cathodoluminescence (CL) images and trace element data reveal that most zircons from the dated samples display distinct core–rim structures, i.e., magmatic cores and metamorphic rims. The cores show typical igneous characteristics with oscillatory growth zoning and high rare earth element (REE) contents, whereas the metamorphic rims are characterized by spherical to oval shape, and high and homogeneous luminescent intensity. Zircon SHRIMP U–Pb dating results suggest that the xenoliths formed at 2.55–2.64Ga and experienced high-grade metamorphism at 2.48–2.49Ga. The lower HREE contents, negative Eu anomalies, high Th/U ratios (generally >0.5) and high Ti-in-zircon temperatures (>800°C) of metamorphic zircon rims suggest that the 2.48–2.49Ga event represents an episode of granulite facies metamorphism. These results demonstrate that the lower-crustal rocks in the studied area experienced late-Neoarchean magmatism and subsequent metamorphism. Positive ɛHf(t) values of +2 to +5, with model ages younger than 3.0Ga and formation ages of 2.55–2.64Ga for igneous zircons from the xenoliths indicate that they were derived from juvenile crustal sources and strongly suggest the existence of significant late-Neoarchean crustal growth in the region.

Petrogenetic implications of magmatic garnet in granitic pegmatites from Southern Norway

The magmatic garnet of seven granitic NYF (niobium–yttrium–fluorine-enriched) granitic pegmatites from the Froland and Evje–Iveland areas in southern Norway were studied with respect to their major- and trace-element composition and intracrystalline distribution of major and minor elements. The increasing average MnO/(FeO + MnO) values of the garnet grains investigated reflect the increasing fractionation of pegmatites from abyssal heavy REE to muscovite rare-element REE pegmatites. At a crystal scale, the MnO/(MnO + FeO) values show various trends controlled by coexisting Mn–Fe-consuming minerals. Back-scattered electron imaging revealed a great variety of structures, including large-scale (>100 μm) concentric growth-induced zoning, fine-scale oscillatory growth and replacement overgrowths. The structures predominantly correlate with the distribution of Y and HREE in the crystals. It is presumably the first time that such diversity has been reported from magmatic garnet originating from one area. The average Y and HREE concentrations are related to the bulk composition of the pegmatite-forming melt, whereas the intracrystalline zoning reflects the absence or presence of Y-bearing minerals or, in the case of the Slobrekka pegmatite, diffusion-controlled crystal growth. Sharp drops of the Y and HREE content record the abrupt change of “normal” peraluminous melt composition to a Na-rich aqueous silica-bearing fluid enriched in F, Rb, Cs, Ta, Mn in the case of the Solas and Hovasen pegmatites. These Na-rich aqueous silica-bearing fluids are responsible for the formation of “amazonite”–“cleavelandite” replacement units. The regional implications are, first, that the Froland pegmatites are characterized by a shorter range of pegmatite fractionation. The parent melts seem to be more primitive with respect to the differentiation of a granitic magma compared to the Evje–Iveland pegmatites, as reflected by the elevated Ca content and the smaller negative Eu anomaly of garnets at Froland. Rough estimates of the bulk composition of the pegmatites could not reveal such a difference. Second, garnet of the Evje–Iveland pegmatites shows a wider range of chemical variability and patterns of Y–HREE zoning compared to the Froland samples. Thus, the gradients of pegmatite fractionation are much stronger within the Evje–Iveland field. The Evje–Iveland melts were probably more enriched in volatiles, being partially responsible for the crystallization of common rare-metal and REE minerals.

Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone

Enrichment of the mantle wedge above subduction zones with fluid mobile elements is thought to represent a fundamental process in the origin of arc magmas. This “subduction factory” is typically modeled as a mass balance of inputs (from the subducted slab) and outputs (arc volcanics). We present here a new method to model fluid mobile elements, based on the composition of peridotites associated with supra-subduction ophiolites, which form by melt extraction and fluid enrichment in the mantle wedge above nascent subduction zones.The Coast Range ophiolite (CRO), California, is a Jurassic supra-subduction zone ophiolite that preserves mantle lithologies formed in response to hydrous melting. We use high-precision laser ablation ICP-MS analyses of relic pyroxenes from these peridotites to document fluid-mobile element (FME) concentrations, along with a suite of non-fluid mobile elements that includes rare earth and high-field strength elements. In the CRO, fluid-mobile elements are enriched by factors of up to 100× DMM, whereas fluid immobile elements are progressively depleted by melt extraction. The high concentrations of fluid mobile elements in supra-subduction peridotite pyroxene can be attributed to a flux of aqueous fluid or fluid-rich melt phase derived from the subducting slab. To model this enrichment, we derive a new algorithm that calculates the concentration of fluid mobile elements added to the source:Cwr,add=[Ccpx-obs/[[Dcpx/(Dbulk-PF)]∗[1-(PF/Dbulk)](1/P)]]-[C0,wr]where Cwr,add=concentration of FME added to mantle wedge during a given melt increment, Ccpx-obs=concentration of observed pyroxene, Dcpx and Dbulk=mineral and bulk partition coefficients, P=melt proportion, and F=melt fraction required to model the observed MREE–HREE concentrations. Application of this algorithm to CRO peridotites shows that fluid influx must be continuous with open system melting, which allows us to calculate FME concentrations for small melt increments. Addition of the calculated FME concentrations to depleted MORB mantle (DMM) asthenosphere or refractory arc mantle (RAM) results in pooled magmas that match primitive arc tholeiites and boninites.

Comparison of fluorite geochemistry from REE deposits in the Panxi region and Bayan Obo, China

Panxi region in west Sichuan province is one of the most economically significant REE mineralization belts in China, and includes the large Maoniuping and Daluxiang deposits and the minor Lizhuang deposit. The REE mineralization in these deposits is spatially and temporally associated with carbonatite–syenite complexes. Large proportional fluorites and REE minerals occurring as veins intrude Cretaceous granite and Oligocene syenite in Maoniuping, and Oligocene syenite and carbonatite in Lizhuang, and Miocene syenite in Daluxiang. Fluorite is also one of main gangue minerals in the world-class Bayan Obo REE deposit. We present a comparison of the trace element and isotopic compositions of fluorites from four REE deposits in the Panxi region and Bayan Obo. The fluorites from Maoniuping and Daluxiang are characterized by variable REE patterns, with either LREE enrichment or LREE depletion relative to MREE. Typically they have a larger range in La/Ho compared to Y/Ho ratios, and pronounced positive Y anomaly relative to chondrite-normalized REE patterns. Their REE distribution patterns are controlled by fluoride-complexes and the loss of separate LREE-rich minerals. Different Y/Ho (ca. 73 vs. 108) and initial Sr isotopic (ca. 0.7061 vs. 0.7077) ratios are observed between the fluorites from Maoniuping and Daluxiang, reflecting their different source compositions. This contrasts with the fluorites from Maoniuping and Lizhuang, which have similar initial Sr isotopes, and appear to be cogenetic. However, the Lizhuang fluorite shows a consistent depletion of LREE relative to MREE, as well as lower Y/Ho ratios and higher HREE content than that in Maoniuping. In this respect the Lizhuang fluorite may have precipitated from a late-stage fluid following abundant fluorite and REE mineral deposition in Maoniuping. Carbonate, more than fluoride complexing, appears to have a stronger control on REE fractionation in the Lizhuang fluorites.The fluorites from three deposits in Panxi region show uniform initial Sr and Nd isotopic compositions similar to their associated carbonatites, but differ from ore-veins found intruding wall rocks, e.g. granite in Maoniuping and syenite in Daluxiang. This is not consistent with a model for fluorite formation involving interaction of F-rich, carbonatite-exsoloved fluid with wall rocks. Instead, the fluorite in Panxi region may precipitate from a residual carbonthermal fluid, which was dominated by Ca, CO2 but also contained F, H2O and REE, and derived from the fractioned carbonatitic magma. Fluorite deposition produced a sharp drop in the activity of F−, which destabilized the REE fluoride complexes and caused deposition of REE minerals. In Bayan Obo, the fluorite typically has higher La/Ho than that in Panxi region and is characterized by a consistent LREE enrichment relative to MREE and negligible to positive Y anomalies. This is consistent with the compositional change of the hydrothermal fluids, which were infiltrated by external F-, LREE-rich fluids. The 87Sr/86Sr of Bayan Obo fluorite is relatively low radiogenic, and has a large range (0.7038–0.7065): similar characteristics to the carbonatite dykes found near the ore bodies. This supports a model for fluorite and REE mineral genesis involving the interaction of a carbonatite-derived fluid and the ore-hosted dolomitic marble.

THE PETROGENESIS OF THE GARNET MENZERITE-(Y) IN GRANULITE FACIES ROCKS OF THE PARRY SOUND DOMAIN, GRENVILLE PROVINCE, ONTARIO

The recent discovery of menzerite-(Y), a garnet containing essential Y, in Mesoproterozoic supracrustal granulites from the southwestern Grenville province has prompted an investigation into the petrogenesis of these rocks, and their Y + HREE-bearing phases, through a high-grade metamorphic cycle. A majority of samples from the supracrustal sequence have bulk compositions that are similar to that of average Proterozoic andesitic and basaltic volcanic rocks. Light REE concentrations in samples from the menzerite-(Y)-bearing locality are similar to that of average upper continental crust, whereas HREE concentrations are considerably higher. Extrapolating from the REE concentrations in detrital zircon cores and bulk Zr content, primary zircon could have contributed only ~6% Y and ~14% Yb to the whole-rock budget. Thus another phase, most likely xenotime, was the main carrier of Y + HREE in the precursor and the source of these constituents for menzerite-(Y). Microstructural relations indicate that menzerite-(Y) crystallized early, in equilibrium with oligoclase, ferrosilite, quartz, clinopyroxene, and iron oxides, then broke down during partial melting, and finally was overgrown by Y + HREE-enriched euhedral almandine as the rocks underwent further heating and burial. Phase-equilibrium modeling constrains conditions for menzerite-(Y) stability to pressures less than 8.5 kbar and temperatures between 550 and 780 °C (at pressures above 5 kbar) for the bulk composition of the host rock. The new constraints on P-T conditions of prograde and peak metamorphism, when combined with other recently acquired data, define a counterclockwise P-T-t path for the PSD that records post-depositional burial of magmatic arc rocks to lower crustal depths between ca. 1250 and 1160 Ma. Peak temperature conditions were followed by a period of isobaric cooling to subsolidus conditions (by ca. 1145 Ma) and subsequent nappe-style transport (at ca. 1100 Ma) onto the Laurentian margin during the Grenville orogeny.

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province, South China: Constraints from Zircon U‐Pb Geochronology, Geochemistry and Nd Isotopes

Abstract:Mesozoic granitic intrusions are widely distributed in the Nanling region, South China, Yanshanian granites are closely connected with the formation of tungsten deposits. The Xihuashan granite is a typical representative of tungsten‐bearing granite. The Xihuashan granite consists mainly of medium‐grained porphyritic biotite granite, medium‐grained biotite granite and fine‐grained two‐mica granite, which correspond to LA‐ICP‐MS zircon U‐Pb ages of 155.5±0.4 Ma, 153.0±0.6 Ma and 152.8±0.9 Ma, respectively. Rocks from the Xihuashan mining area displays high SiO2 (73.85% to 76.49%) and Na2O+K2O contents (8.09% to 9.43%), belonging to high‐K calc‐alkaline series. They are metaluminous to weakly peraluminous with A/CNK values ranging from 0.96 to 1.06. All granites in this study area are rich in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti, Nb and Eu, especially depleted in medium‐grained biotite granite and fine‐grained two‐mica granite. The medium‐grained porphyritic biotite granites usually have high LREE concentrations, whereas medium‐grained biotite granite and fine‐grained two‐mica granite displays high HREE contents. Our geochemical data reveal that the studied rocks are highly fractionated I‐type granite. The magma underwent strong magma differentiation with decreasing temperature and increasing oxygen fugacity, which may explain the formation of three types of distinct granites. Variations of Rb, Sr and Ba concentrations in different type granites were controlled by fractional crystallization of biotite and feldspar. Fractional crystallization of monazite, allanite and apatite resulted in LREE changes in granite, and formation of garnet mainly caused HREE changes. Granites from the Xihuashan mining area have relatively high εNd(t) values (–9.77 to –11.46), indicating that they were probably generated by partial melting of underlying Proterozoic metasedimentary rocks with minor addition of juvenile crust or mantle‐derived magmas.