Melting Of Mantle Wedge Research Articles

Paleoproterozoic arc basalt-boninite-high magnesian andesite-Nb enriched basalt association from the Malangtoli volcanic suite, Singhbhum Craton, eastern India: Geochemical record for subduction initiation to arc maturation continuum

The Singhbhum Craton of eastern India preserves distinct signatures of ultramafic-mafic-intermediate-felsic magmatism of diverse geodynamic affiliations spanning from Paleo-Mesoarchean to Proterozoic. Here we investigate the 2.25Ga Malangtoli volcanic rocks that are predominantly clinopyroxene- and plagioclase-phyric, calc-alkaline in nature, display basalt-basaltic andesite compositions, and preserve geochemical signatures of subduction zone magmatism. Major, trace and rare earth element characteristics classify the Malangtoli volcanic rocks as arc basalts, boninites, high magnesian andesites (HMA) and Nb enriched basalts (NEB). The typical LILE enriched-HFSE depleted geochemical attributes of the arc basalts corroborate a subduction-related origin. The boninitic rocks have high Mg# (0.8), MgO (>25wt.%), Ni and Cr contents, high Al2O3/TiO2 (>20), Zr/Hf and (La/Sm)N (>1) ratios with low (Gd/Yb)N (<1) ratio, TiO2, and Zr concentrations. The HMA samples are marked by moderate SiO2 (>54wt.%), MgO (>6wt.%), Mg# (0.47) with elevated Cr, Co, Ni and Th contents, depleted (Nb/Th)N, (Nb/La)N, high (Th/La)N and La/Yb (<9) ratio, moderate depletion in HREE and Y with low Sr/Y. The NEBs have higher Nb contents (6.3–24ppm), lower magnitude of negative Nb anomalies with high (Nb/Th)pm=0.28–0.59 and (Nb/La)pm=0.40–0.69 and Nb/U=2.8–34.4 compared to normal arc basalts [Nb=<2ppm; (Nb/Th)pm=0.10–1.19; (Nb/La)pm 0.17–0.99 and Nb/U=2.2–44 respectively] and HMA. Arc basalts and boninites are interpreted to be the products of juvenile subduction processes involving shallow level partial melting of mantle wedge under hydrous conditions triggered by slab-dehydrated fluid flux. The HMA resulted through partial melting of mantle wedge metasomatized by slab-dehydrated fluids and sediments during the intermediate stage of subduction. Slab-melting and mantle wedge hybridization processes at matured stages of subduction account for the generation of NEB. Thus, the arc basalt-boninite-HMA-NEB association from Malangtoli volcanic suite in Singhbhum Craton preserves the signature of a complete spectrum of Paleoproterozoic active convergent margin processes spanning from subduction initiation to arc maturation.

The distribution, geochronology and geochemistry of early Paleozoic granitoid plutons in the North Altun orogenic belt, NW China: Implications for the petrogenesis and tectonic evolution

Abundant early Paleozoic granitoid plutons are widely distributed in the North Altun orogenic belt. These rocks provide clues to the tectonic evolution of the North Altun orogenic belt and adjacent areas. In this paper, we report an integrated study of petrological features, U–Pb zircon dating, in situ zircon Hf isotope and whole-rock geochemical compositions for the Abei, 4337 Highland and Kaladawan Plutons from north to south in the North Altun orogenic belt. The dating yielded magma crystallization ages of 514Ma for the Abei Pluton, 494Ma for the 4337 Highland Pluton and 480–460Ma for the Kaladawan Pluton, suggesting that they are all products of oceanic slab subduction because of the age constraint. The Abei monzogranites derived from the recycle of Paleoproterozoic continental crust under low-pressure and high-temperature conditions are products of subduction initiation. The 4337 Highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust. The Kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive εHf(t) values, and the Kaladawan monzogranite-syenogranite are derived from partial melting of Neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle. These results, together with existing data, provide significant information about the evolution history of oceanic crust subduction during the 520–460Ma. The initiation of subduction occurred during 520–500Ma with formation of Abei Pluton; subsequent transition from steep-angle to flat-slab subduction at ca.500Ma due to the arrival of buoyant oceanic plateaus, which induces the formation of 4337 Highland Pluton. With ongoing subduction, the steep-angle subduction system is reestablished to cause the formation of 480–460Ma Kaladawan Pluton. Meanwhile, it is this model that account for the temporal–spatial distribution of these early Paleozoic magmatic rocks in the North Altun orogenic belt.

A late Neoarchaean–early Palaeoproterozoic crustal thickening event in the eastern North China Craton: petrological and geochronological evidence from Eastern Hebei terrane

ABSTRACTDetermining an age framework for Precambrian crystalline rocks and associated granulite-facies metamorphism of the inner blocks in the North China Craton (NCC) is important for determining the tectonic setting and evolution of the craton during the Neoarchaean–Palaeoproterozoic. The Eastern Hebei terrane (EHT), located in the Eastern Block of the NCC, is composed of tonalitic-trondhjemitic-granodioritic (TTG) gneisses and potassium-rich granitoids, along with rafts of supracrustal rocks that are intruded by basic dikes. TTG gneisses in the EHT yield crystallization ages of 2516–2527 Ma. The oldest age of inherited zircons from a mylonitic TTG gneiss is ~2918 Ma. Granulite-facies supracrustal metamorphic rocks in the Zunhua high-grade meta-greenstone belt indicate an andesitic/basaltic protolith that was formed at ~2498 Ma. A syn-deformational granite in the Jinchangyu greenschist-facies shear zone yields a crystallization age of ~2474 Ma. Metamorphism of the supracrustal rocks and mylonitic greenschist took place at ~2461 and ~2475 Ma, respectively. Rare earth elements (REE) patterns and slightly negative Nb and Ta anomalies indicate that the magmatic precursors of the supracrustal rocks might be derived from partial melting of a sub-arc mantle wedge and metasomatized by fluids derived from a subducting slab. These rocks plot in the island arc basalts (IAB) field on a La/Nb vs. La diagram, further supporting this interpretation. The microstructures of a garnet–two-pyroxene granulite indicate an approximately clockwise P-T path. The crystallization ages of the TTG gneisses represent periods of the major crustal growth in the NCC, and the granulite- and greenschist-facies metamorphism indicates an orogenic event that involved crustal thickening at ~2.47 Ga.

Neoarchean Andean-type active continental margin in the northeastern North China Craton: Geochemical and geochronological evidence from metavolcanic rocks in the Jiapigou granite-greenstone belt, Southern Jilin Province

The Jiapigou granite-greenstone belt of Southern Jilin Province is located at the northeastern margin of the North China Craton (NCC). The Neoarchean metavolcanic rocks of this region provide strong lines of evidence in petrology and geochemistry for Neoarchean crust–mantle geodynamic evolution and significant crustal growth events. The metavolcanic rock assemblage is dominated by amphibolite, hornblende/garnet biotite plagioclase gneiss, and minor felsic gneiss. Zircon LA–ICP–MS U–Pb isotopic analyses reveal that these supracrustal rocks may be formed by two episodes of magmatism. The first magmatic episode occurred at ∼2688Ma and was recorded by minor felsic gneiss. The second episode occurred at ∼2588–2536Ma and was marked by a major lithological assemblage of metavolcanic rocks in this area. All the supracrustal rocks experienced amphibolite facies metamorphism at ∼2510–2480Ma.Except for the felsic gneiss, seventeen metavolcanic rock samples can be geochemically classified into three groups. Group #1 has tholeiitic basaltic chemical composition, and is characterized by slightly LREE-enriched chondrite-normalized REE pattern with low (La/Yb)N ratio and weakly negative Nb anomalies, similar to those of primitive arc tholeiite. Their magmatic precursor was most likely generated by partial melting of upper sub-arc mantle wedge slightly metasomatized by slab-derived fluids or melts in a subduction initiation environment. Group #2 with basaltic composition is akin to typical calc-alkaline basalt (CAB), showing fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. The magmatic precursor of the Group #2 was derived from partial melting of a sub-arc mantle wedge at relatively deeper levels, significantly metasomatized by slab-derived fluids or melts. Group #3 show chemical composition of andesite to dacite, and is distinguished by wide variations in SiO2, MgO and Mg# values, strongly fractionated chondrite-normalized REE patterns with higher (La/Yb)N ratios, low Yb and Y contents, and negative Nb, Ta, Ti and P anomalies, analogous to the Phanerozoic adakites. They might be derived from magma mixing between the mantle wedge melts metasomatized by slab-derived melts with perhaps an ancient crustal contribution.The felsic gneiss shows positive zircon εHf(t2) values (+2.2 to +4.5), together with ∼2.7Ga metamorphic volcanic rocks in the neighboring area, indicating a ∼2.7Ga crustal growth episode. Most of the metavolcanic rocks display positive εHf(t2) values of +0.4 to +7.4, the highest of which is close to the value of contemporaneous depleted mantle, suggesting a significant crustal growth event at ∼2.59–2.54Ga. Combined with numerous ∼2.83–2.65Ga inherited zircons and ∼2688Ma felsic gneiss, the Jiapigou granite-greenstone belt evolved at an Andean-type active continental margin, recording the transition from primitive arc to mature arc.

Petrogenesis of Middle–Late Triassic volcanic rocks from the Gangdese belt, southern Lhasa terrane: Implications for early subduction of Neo-Tethyan oceanic lithosphere

The Gangdese belt is dominantly composed of igneous rocks that formed during the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane and has played a crucial role in understanding the pre-collisional evolution of southern Tibet. This paper presents new geochronological and geochemical (whole-rock major and trace element and Sr–Nd and zircon Hf isotope) data for recently identified volcanic rocks exposed in Changguo area, southernmost part of the Lhasa terrane. Zircon U–Pb dating from six samples yields consistent ages of 237.1±1.1Ma to 211.7±1.5Ma for magma emplacement through volcanic eruption, showing the Middle–Late Triassic magmatic activity in the southernmost Gangdese Belt. The Changguo volcanic rocks are mainly composed of basaltic and andesitic rocks and exhibit LILE enrichment and HFSE depletion. They also exhibit relatively uniform Nd–Hf isotopic compositions (εNd(t)=+5.20 to +7.74 and εHf(t)zircon=+10.2 to +15.9). The basaltic magmas were likely sourced from partial melting of sub-arc mantle wedge that was metasomatized by not only the aqueous fluid derived from subducting altered oceanic crust but also hydrous melt derived from subducting seafloor sediments, and subsequently experienced fractional crystallization and juvenile crustal contamination during ascent. The andesitic magmas were generated by partial melting of mafic–ultramafic metasomes through melt/fluid–peridotite reaction at slab–mantle interface. Taking into account the temporal and spatial distribution of the Early Mesozoic magmatic rocks and regional detrital zircon data, we further propose that the northward subduction of Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane commenced by Middle Triassic.

Late Neoproterozoic layered mafic intrusion of arc-affinity in the Arabian-Nubian Shield: A case study from the Shahira layered mafic intrusion, southern Sinai, Egypt

The Shahira Layered Mafic Intrusion (SLMI), which belongs to the late Neoproterozoic plutonic rocks of the Arabian-Nubian Shield, is the largest layered mafic intrusion in southern Sinai. Field relations indicate that it is younger than the surrounding metamorphic rocks and older han the post-orogenic granites. Based on variation in mineral paragenesis and chemical composition, the SLMI is distinguished into pyroxene-hornblende gabbro, hornblende gabbro and diorite lithologies. The outer zone of the mafic intrusion is characterized by fine-grained rocks (chilled margin gabbroic facies), with typical subophitic and/or microgranular textures. ifferent rock units from the mafic intrusion show gradational boundaries in between. They show some indications of low grade metamorphism, where primary minerals are transformed into secondary ones. Geochemically, the Shahira layered mafic intrusion is characterized by enrichment in LILE relative to HFSE ( e.g. Nb, P, Zr, Ti, Y), and LREE relative to HREE [(La/Lu)n= 4.75–8.58], with subalkaline characters. It has geochemical characteristics of pre-collisional arc-type environment. The geochemical signature of the investigated gabbros indicates partial melting of mantle wedge in a volcanic-arc setting, being followed by fractional crystallization and crustal contamination. Fractional crystallization processes played a vital role during emplacement of the Shahira intrusion and evolution of its mafic and intermediate rock units. The initial magma was evolved through crystallization of ornblende which was caused by slight increasing of H2O in the magma after crystallization of iquidus olivine, pyroxene and Ca-rich plagioclase. The gabbroic rocks crystallized at pressures between 4.5 and 6.9kbar (~15–20km depth). Whereas, the diorites yielded the lowest crystallization pressure between 1.0 to 4.4Kbar (<10km depth). Temperature was estimated by several eothermometers, which yielded crystallization temperatures ranging from 835o to 958oC for the gabbros, and from 665o to 862oC for the diorites. Field, petrological, geochemical and mineralogical characteristics of the SLMI are akin to the Egyptian layered mafic-ultramafic ntrusions of volcanic-arc setting, not ophiolitic rocks.

Origin of the late Early Cretaceous granodiorite and associated dioritic dikes in the Hongqilafu pluton, northwestern Tibetan Plateau: A case for crust–mantle interaction

We present a detailed study of geochronology, mineral chemistries, bulk-rock major and trace element abundances, and Sr–Nd–Hf isotope compositions of the granodiorite and associated dioritic dikes in the Hongqilafu pluton at the northwestern margin of the Tibetan Plateau. The granodiorite and dioritic dikes yielded zircon U–Pb ages of ~104Ma and ~100Ma, respectively. The dioritic dikes comprise varying lithologies of gabbroic diorite, diorite porphyry and granodiorite porphyry, exhibiting a compositional spectrum from intermediate to felsic rocks. Their mineral compositions display disequilibrium features such as large major element compositional variations of plagioclase, clinopyroxene and amphibole crystals. These dioritic dikes are enriched in incompatible elements (Ba, Rb, Th, U, K) and Sr–Nd–Hf isotopes (87Sr/86Sri: 0.7066 to 0.7071, εNd(t): −5.3 to −7.4, εHf(t): −3.6 to −6.2). We suggest that the dioritic dikes were most likely derived from partial melting of mantle wedge metasomatized by the subducted/subducting seafloor with a sediment component, followed by AFC processes with fractional crystallization of clinopyroxene, amphibole and plagioclase and assimilation of lower continental crust. The mantle-wedge derived magma parental to the dioritic dikes underplated and induced the lower continental crust to melt, forming the felsic crustal magma parental to the granodiorite with mantle melt signatures and having more enriched isotope compositions (87Sr/86Sri: 0.7087 to 0.7125, εNd(t): −9.5 to −11.6, εHf(t): −10.3 to −14.1) than those of the dioritic dikes. The Hongqilafu pluton is thus the product of mantle–crust interaction at an active continental margin subduction setting over the period of several million years. This understanding further indicates that the closure timing of the Shyok back-arc basin and the collision between the Kohistan–Ladakh Arc and the Karakoram Terrane may have taken place later than ~100Ma.

Effect of hydrothermal circulation on slab dehydration for the subduction zone of Costa Rica and Nicaragua

Dehydration of subducting oceanic plates is associated with mantle wedge melting, arc volcanism, intraslab earthquakes through dehydration embrittlement, and the flux of water into the mantle. In this study, we present two-dimensional thermal models of the Costa Rica–Nicaragua subduction zone to investigate dehydration reactions within the subducting Cocos plate. Seismic and geochemical observations indicate that the mantle wedge below Nicaragua is more hydrated than that below Costa Rica. These trends have been hypothesized to be due to a variation in either the thermal state or the hydration state of the subducting slab. Despite only small variations in plate age along strike, heat flow measurements near the deformation front reveal significantly lower heat flow offshore Nicaragua than offshore Costa Rica. These measurements are interpreted to reflect an along-strike change in the efficiency of hydrothermal circulation in the oceanic crust. We parameterize thermal models in terms of efficient and inefficient hydrothermal circulation and explore their impact on slab temperature in the context of dehydration models. Relative to models without fluid flow, efficient hydrothermal circulation reduces slab temperature by as much at 60°C to depths of ∼75km and increases the predicted depth of eclogitization by ∼15km. Inefficient hydrothermal circulation has a commensurately smaller influence on slab temperatures and the depth of eclogitization. For both regions, the change in eclogitization depth better fits the observed intraslab crustal seismicity, but there is not a strong contrast in the slab thermal structure or location of the main dehydration reactions. Consistent with other studies, these results suggest that observed along-strike differences in mantle wedge hydration may be better explained by a northwestward increase in the hydration state of the Cocos plate before it is subducted.

Different sources involved in generation of continental arc volcanism: The Carboniferous–Permian volcanic rocks in the northern margin of the North China block

New zircon U–Pb dating results on the Carboniferous–Permian volcanic rocks in the northern margin of the North China block (NCB) indicate their eruption during the Early Carboniferous to Late Permian from 347±3Ma to 258±1Ma and a slight decrease of the upper limits of the volcanic sequences from west to east. They have a main rock association of basalt, basaltic andesite, andesite, dacite, rhyolite, tuff, and tufaceous sandstone. Most of them have calc-alkaline compositions and exhibit variable SiO2 contents from 48.2wt.% to 77.1wt.%. There is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams, indicating that they are not bimodal in composition. The Carboniferous–Permian volcanic rocks exhibit subduction-related geochemical features such as negative Nb and Ta anomalies of mafic to intermediate rocks on primitive mantle-normalized diagrams, indicating they were formed in an Andean-type continental arc during southward subduction of the Paleo-Asian oceanic plate beneath the northern NCB. However, their wide range of whole-rock Sr–Nd and zircon Hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks. Geochemical and Sr–Nd–Hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust; however, most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust. There is an increasing input of crustal materials from the Carboniferous to Permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences. The results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments, mantle wedge and arc continental crust could be involved in their generation. For the Permian andesite, dacite and rhyolitic volcanic rocks, there is a striking correlation between their Sr–Nd–Hf isotopic compositions and those of the underlying basement. Sr–Nd–Hf isotopic differences between the Permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the North China craton suggest that mixture and homogenization of the magmas in the melting–assimilation–storage–hybridization (MASH) zone within this continental arc was incomplete, and incomplete homogenization of magmas is likely a common feature of many MASH zones within continental arcs. Our results on the volcanic rocks also show that the final closure of the Paleo-Asian Ocean along the Solonker Suture Zone has not been finished prior to the Middle Permian and the decease of the Paleo-Asian oceanic subduction beneath the northern NCB in its middle-western parts is a little earlier than that in its eastern part.

Geochemical characteristics of gold bearing boninites and banded iron formations from Shimoga greenstone belt, India: Implications for gold genesis and hydrothermal processes in diverse tectonic settings

This paper is the first documentation of boninites from the Kudrekonda Formation of Shimoga greenstone belt (SGB) in western Dharwar Craton (WDC), India and the occurrence of native gold in the associated quartz veins. We also investigate the gold mineralization hosted in the banded iron formations (BIFs) of Ganajur, Karajgi and Palavanahalli areas of the SBG. The Kudrekonda metavolcanic rocks are composed of clinopyroxene, orthopyroxene and plagioclase with secondary amphibole. The geochemical attributes of these rocks, based on MgO, TiO2 Ni, Cr and Co contents, Al2O3/TiO2, (Gd/Yb)N and (La/Sm)N ratios, classify them as boninites and their relative LILE-HFSE abundance suggests magma generation in an intraoceanic arc setting. The Gd/Yb and Dy/Yb ratios suggest parent magma origin through partial melting of a spinel lherzolite mantle source. Geochemical signatures of the boninites suggest high temperature, shallow level melting of refractory mantle wedge metasomatized by fluids derived through dehydration of oceanic slab during subduction initiation. Quartz veins present in the Kudrekonda boninites contain gold grains showing affinity towards quartz-carbonate vein type greenstone hosted occurrences. EPMA data record 88.5–90.3wt.% Au content with 7.7–8.4wt.% Fe, 0.05–2.1wt.% S and negligible Ag. The Au is hosted in Fe-oxides. Bulk chemical analyses show 4.1–684.4ppb Au and 139.4–2186.4ppb Ag in the studied samples. The gold mineralization of Kudrekonda boninites is syngenetic, mesothermal, convergent margin orogenic type. Decarbonation, desulphidization and metamorphism of subducted oceanic lithosphere and devolatilization of metasomatized enriched mantle wedge in intraoceanic arc setting are the viable processes that contributed to gold genesis in the Kudrekonda boninites. Devolatilization during prograde metamorphism, hydrothermal fluid generation, syntectonic gold remobilization and migration of gold enriched fluids through shear zones account for Au transportation and precipitation in quartz veins emplaced within the boninites. The BIFs of Ganajur, Karajgi and Palavanahalli areas of Shimoga greenstone belt dominantly comprise alternating Si-rich layers of quartz and Fe-rich hematite and magnetite layers. Geochemical characteristics of the BIF samples including their depleted ƩREE, positive Eu anomalies, negative to negligible Ce anomalies, and superchondritic Y/Ho ratios suggest that the BIFs were (i) derived by chemical sedimentation from paleo-ocean enriched in dissolved iron and silica with marked contributions from hydrothermal fluids of volcanic origin, and (ii) deposited in an off-shelf marine environment proximal to a mid-oceanic spreading centre where hydrothermal activity induced release of Eu and Y enriched hydrothermal solutions under low oxygen fugacity conditions. Native gold occurs in banded strata-bound disseminated sulphide lenses compositionally characterized as pyrite, arsenopyrite, chalcopyrite and galena. Mineral chemical analyses reveal 73–100% Au content with variable concentrations of Ag (0.02–15.2wt.%). Geochemical compositions of BIFs are characterized by pronounced enrichment in Au (706–15,831ppb) and Ag (630–2435ppb). Gold mineralization in Shimoga BIFs is epigenetic, epithermal type akin to Homestake type iron-formation hosted vein and disseminated lode gold deposit. Gold genesis in these BIFs is attributed to migration of oxygen depleted, reducing, sulphidic–auriferous fluids of hydrothermal origin to the site of BIF deposition that served as chemical traps for the auriferous fluids. The Kudrekonda boninites represent accretion and preservation of a slice of older oceanic crust onto a relatively younger, active continental margin through arc–continent collision during Neoarchean time.

Boninitic metavolcanic rocks and island arc tholeiites from the Older Metamorphic Group (OMG) of Singhbhum Craton, eastern India: Geochemical evidence for Archean subduction processes

The metavolcanic rocks of the Paleo-Mesoarchean Older Metamorphic Group (OMG) from Jagannathpur area of the Singhbhum Craton occur as enclaves within the Singhbhum granites (SBG). The primary mineralogical composition of these rocks has been overprinted by plagioclase–chlorite–tremolite–actinolite assemblage reflecting lower to medium greenschist facies metamorphism. Disseminated sulphides and minor amount of chromite are present as opaque phases. Clinopyroxene compositions correspond to augite. Major, trace and rare earth element compositions of these rocks are characterized by high Mg# (80–82), MgO (30.6–32.9wt.%), Ni (490–1262ppm) and Cr (2404–4385ppm) contents, high Al2O3/TiO2 (49–67), Zr/Hf (10–100) ratios with low TiO2 (0.1–0.2wt.%), Zr (1.0–3.1ppm) contents, (La/Sm)N >1 and (Gd/Yb)N <1. These geochemical features in conjunction with distinct negative Nb, Ta, Zr, Hf and Ti anomalies on N-MORB normalized multi-element diagram, relative enrichment in LILE and La collectively attest to boninitic signatures and magma generation in an intraoceanic subduction zone setting. These boninites are associated with low Ti island arc tholeiites (IAT) that have geochemical characteristics inherited from subduction zone processs. Relative MREE depletion in the OMG boninites compared to LREE and HREE is reflected by their high ((La/Sm)N=0.7–6.7) and low (Gd/Yb)N=0.4–0.6) ratios. These REE signatures corroborate a multi-stage petrogenetic process involving a refractory mantle source that was previously depleted by melt extraction episodes and subsequently enriched by subduction-related slab-dehydration process. The LILE and LREE replenishment of the refractory mantle wedge is attributed to influx of slab-derived hydrous fluids that lowered the solidus temperature and triggered partial melting of mantle wedge at shallow level under hydrous conditions. A mantle domain of spinel peridotite composition has been inferred for partial melting and generation of magmas parental to OMG boninites and IAT. The PGE chemistry of the studied boninites and IAT, marked by higher Pd and Pd/Ir over lower Pt and Pt/Pd and S-undersaturated nature attest to fluid-fluxed metasomatism of mantle wedge and sulphide retention in the residual mantle due to high fluid pressure and high degree of mantle melting at shallow depth.

Experimentally determined distribution of fluorine and chlorine upon hydrous slab melting, and implications for F–Cl cycling through subduction zones

Fluorine and chlorine are volatile elements known to be enriched in primitive arc magmas, and variations of F/Cl ratios can carry information about slab devolatilization processes. Recent experiments on the fractionations of these elements suggest that aqueous fluid has limited capacity to enrich the magma source region in F. Hence, it is difficult to explain observations of primitive arc magmas particularly rich in F. To complement previous experimental studies, we examined the fractionation of fluorine and chlorine during hydrous partial melting of subducting slab. Element-doped phase equilibria experiments were carried out in a complex chemical system at conditions equivalent to potential slab melting temperatures (750–1000°C) across the amphibolite to eclogite facies transition (1.3–3GPa). Partition coefficients of F and Cl between hydrous silicic melts and minerals were determined by electron microprobe and/or ion probe. Fluorine is compatible in amphibole (DFamp/glass=1.18–1.85), and incompatible in garnet (0.034–0.140), clinopyroxene (0.059–0.505), and allanite (0.205–0.504). Hence, amphibole is an important F host, and can retain significant quantities of F in the solid residue of partial melting. On the contrary, Cl is incompatible, with DClmineral/glass generally decreasing from amphibole (0.079–0.625; one outlier at 1.87) to allanite (0.163), clinopyroxene (0.066–0.158), and garnet (0.031–0.153; outlier at 0.492). As a result, Cl is easily mobilized during partial melting. Fluorine and chlorine release during slab melting have been quantified by applying our partition coefficients to a non-modal batch melting model. The model shows that amphibole plays a key role in F/Cl fractionation during partial melting, while F/Cl is close to that of source for the melting of amphibole free eclogite. Moreover, the results from a flux-melting model employing several source compositions are compared to F and Cl abundances in primitive arc magmas. The observed variations are best described by variable additions of slab-derived aqueous fluids and hydrous melts to the mantle wedge melting region.

Middle Neoproterozoic (∼845 Ma) continental arc magmatism along the northwest side of the Jiangshan–Shaoxing suture, South China: Geochronology, geochemistry, petrogenesis and tectonic implications

This paper presents the first detailed zircon U–Pb chronology, major and trace element, and Nd–Hf isotope geochemistry of two Neoproterozoic plutons (Shanhou and Jiangshan) along the northwest side of the Jiangshan–Shaoxing suture, Zhejiang province, South China. The Shanhou pluton consists of gabbros, dioritic rocks and monzogranites. SHRIMP and LA-ICP-MS zircon U–Pb dating indicates that the gabbros (∼846Ma), dioritic rocks (∼845Ma) and monzogranites (∼847Ma) were emplaced simultaneously. The Jiangshan pluton consists mainly of dioritic rocks that were also emplaced in the middle Neoproterozoic with a SHRIMP zircon U–Pb age of ca. 842Ma. The Shanhou gabbros are mainly high-K calc-alkaline, and are enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They have positive ɛNd (T) (2.0–3.5) and ɛHf (T) (in-situ zircon) (3.3). The Shanhou and Jiangshan dioritic rocks as well as the Shanhou granites are also high-K calc-alkaline, enriched in LREE and LILE and depleted in HFSE with marked negative Ta–Nb anomalies. They also show positive ɛNd (T) (0.1–4.7) and ɛHf (T) (in-situ zircon) (0.8–5.0). Detailed elemental and isotopic data suggest that both plutons were formed in a continental arc setting with gabbros derived from partial melting of subduction-modified mantle wedge. Such gabbroic magmas underwent fractionation crystallization of clinopyroxene+amphibole, forming the less felsic dioritic rocks. Progressive fractionation crystallization of amphibole+plagioclase from the evolved melts with a little crustal assimilation produced the more felsic dioritic rocks and granites. Our new data suggest that final amalgamation between the Yangtze and Cathaysia blocks postdated 842Ma.

Early Cretaceous arc volcanic suite in Cebu Island, Central Philippines and its implications on paleo-Pacific plate subduction: Constraints from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

The Philippine island arc system is a collage of amalgamated terranes of oceanic, continental and island arc affinities. Here we investigate a volcanic suite in Cebu Island of central Philippines, including basalt, diabase dike, basaltic pyroclastic rock and porphyritic andesite. LA-ICP-MS U–Pb geochronology of zircon grains from the porphyritic andesite and pyroclastic rock yielded ages of 126±3Ma and 119±2Ma, respectively, indicating an Early Cretaceous age. The age distribution of the detrital zircons from river sand in the area displays a peak at ca. 118Ma, close to the age of the pyroclastic rock. The early Cretaceous volcanic rocks in the central Philippines were previously regarded as parts of ophiolite complexes by most investigators, whereas the Cebu volcanics are distinct from these, and display calc-alkaline affinity and island arc setting, characterized by high LREE/HREE ratios and low HFSE contents. These features are similar to the Early Cretaceous arc basalts in the Amami Plateau and east Halmahera in the northernmost and southernmost West Philippine Basin respectively. Zircon Hf isotopes of the pyroclastic rocks show depleted nature similar to those of the Amami Plateau basalts, implying the subducted Pacific-type MORB as probable source. Zircon Hf isotopes of the porphyritic andesite show slight enrichment relative to that of the pyroclastic rocks and MORB, indicating subducted sediments as a minor end-member in the source. The Hf isotopic compositions of the volcanic rocks are also reflected in the detrital zircons from the river sands. We propose that the volcanic rocks of Cebu Island were derived from partial melting of sub-arc mantle wedge which was metasomatized by dehydration of subducted oceanic crust together with minor pelagic sediments. Within the tectonic environment of Southeast Asia during Early Cretaceous, the volcanic rocks in Cebu Island can be correlated to the subduction of paleo-Pacific plate. The Early Cretaceous volcanic suites in Cebu Island along with the Early Cretaceous arc volcanic rocks, ophiolites and boninites in the southern–western–northern margin of West Philippine Basin are inferred to have formed within single subduction zone before the opening of the Philippine Sea Plate.

Tectonic Evolution of the Meso‐Tethys in the Western Segment of Bangonghu‐Nujiang Suture Zone: Insights from Geochemistry and Geochronology of the Lagkor Tso Ophiolite

AbstractThe subduction of the Bangonghu‐Nujiang Meso‐Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu‐Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu‐Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA‐ICP‐MS zircon U‐Pb and 39Ar/40Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum‐group element (PGE) data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic (161.2 ± 2.7 Ma – 165.4 ± 3.5 Ma), and experienced a second tectonic emplacement during the Early Cretaceous (137.90 ± 6.39 Ma). The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu‐Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second‐order expanding center of the obduction dish. This led to inter‐arc expansion, followed by the formation of inter‐arc and back‐arc basins with island arc features, which are represented by ophiolites around the Shiquanhe‐Lagkor Tso‐Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc‐ocean basin systems are developed.

Platinum Group Elements (PGE) geochemistry of komatiites and boninites from Dharwar Craton, India: Implications for mantle melting processes

High MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for platinum group elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. Though their higher PGE abundance is economically significant in mineral exploration studies, their lower concentrations are also valuable geochemical tools to evaluate petrogenetic processes. In this paper an attempt has been made to evaluate the PGE geochemistry of high MgO volcanic rocks from two greenstone belts of western and eastern Dharwar Craton and to discuss different mantle processes operative at diverse geodynamic settings during the Neoarchean time. The Bababudan greenstone belt of western and Gadwal greenstone belt of eastern Dharwar Cratons are dominantly composed of high MgO volcanic rocks which, based on distinct geochemical characteristics, have been identified as komatiites and boninites respectively. The Bababudan komatiites are essentially composed of olivine and clinopyroxene with rare plagioclase tending towards komatiitic basalts. The Gadwal boninites contain clinopyroxene, recrystallized hornblende with minor orthopyroxene, plagioclase and sulphide minerals. The Bababudan komatiites are Al-undepleted type (Al2O3/TiO2=23–59) with distinctly high MgO (27.4–35.8wt.%), Ni (509–1066ppm) and Cr (136–3036ppm) contents. These rocks have low ΣPGE (9–42ppb) contents with 0.2–2.4ppb Iridium (Ir), 0.2–1.4ppb Osmium (Os) and 0.4–4.4ppb Ruthenium (Ru) among Iridium group PGE (IPGE); and 1.4–16.2ppb Platinum (Pt), 2.8–19ppb Palladium (Pd) and 0.2–9.8ppb Rhodium (Rh) among Platinum group PGE (PPGE). The Gadwal boninites are high-Ca boninites with CaO/Al2O3 ratios varying between 0.8 and 1.0, with 12–24wt.% MgO, 821–1168ppm Ni and 2307–2765ppm Cr. They show higher concentration of total PGE (82–207ppb) with Pt concentration ranging from 13 to 19ppb, Pd between 65 and 180ppb and Rh in the range of 1.4–3ppb compared to the Bababudan komatiites. Ir, Os and Ru concentrations range from 0.6 to 2.2ppb, 0.2 to 0.6ppb and 1.4 to 2.6ppb respectively in IPGE. The PGE abundances in Bababudan komatiites were controlled by olivine fractionation whereas that in Gadwal boninites were influenced by fractionation of chromite and sulphides. The Al-undepleted Bababudan komatiites are characterized by low CaO/Al2O3, (Gd/Yb)N, (La/Yb)N, with positive Zr, Hf, Ti anomalies and high Cu/Pd, Pd/Ir ratios at low Pd concentrations suggesting the derivation of parent magma by high degrees (>30%) partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. The komatiites are geochemically analogous to Al-undepleted Munro type komatiites and their PGE compositions are consistent with Alexo and Gorgona komatiites. The S-undersaturated character of Bababudan komatiites is attributed to decompression and assimilation of lower crustal materials during magma ascent and emplacement. In contrast, the higher Al2O3/TiO2, lower (Gd/Yb)N, for Gadwal boninites in combination with negative Nb, Zr, Hf, Ti anomalies and lower Cu/Pd at relatively higher Pd/Ir and Pd concentrations reflect high degree melting of refractory mantle wedge under hydrous conditions in an intraoceanic subduction zone setting. Higher Pd/Ir ratios and S-undersaturation of these boninites conform to influx of fluids derived by dehydration of subducted slab resulting into high fluid pressure and metasomatism of mantle wedge.

Petrology and geochemistry of late Carboniferous hornblende gabbro from the Awulale Mountains, western Tianshan (NW China): Implication for an arc–nascent back-arc environment

The Awulale Mountains, the conjunction point of the Tarim Plate, Junggar Plate and Yili-Central Tianshan Block, are the crucial place to decipher the tectonic evolution of the western Tianshan. We report results of petrologic, whole-rock geochemical, Sr–Nd isotopic data and in-situ zircon U–Pb and Hf isotopes for the Wuling (poikilitic) hornblende gabbro from the Awulale Mountains, western Tianshan. Two stage crystallization processes are applied to interpret its mineral composition. Zircon U–Pb ages indicate that the Wuling hornblende gabbro was emplaced during the late Carboniferous (313Ma±3Ma). The Wuling hornblende gabbro is characterized by tholeiitic affinity with low 87Sr/86Sr ratios, positive εNd(t) and zircon εHf(t) values, resembling its coeval mafic rocks in the Awulale Mountains. They are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), indicating a subduction-related setting for their generation and their mantle source were mainly fluxed by fluids released by the down-going slab. The flat REE and depleted Sr–Nd–Hf isotopic signatures imply a shallow and heterogeneous mantle source with the involvement of a deep MORB-like component. We propose that an arc–nascent back-arc system had been developed between the Nalati Mountains and the Awulale Mountains during the late Carboniferous, where the magma derived from decompression melting of MORB-like mantle caused by the slab roll-back and flux-induced melting of mantle wedge were both contributed to their generation. The arc-type features of them were inherited from the strong subduction input at the initiation of back-arc rifting. However, the back-arc rifting ceased at the end of late Carboniferous because of the compressional stress from the collision between the Tarim Plate and the Yili-Central Tianshan Block.

Geochronology and geochemistry of Early Jurassic volcanic rocks in the Erguna Massif, northeast China: Petrogenesis and implications for the tectonic evolution of the Mongol–Okhotsk suture belt

The Mongol–Okhotsk suture belt played an important role in the tectonic evolution of northeast Asia during the Mesozoic. However, few studies have examined the influence of this tectonic belt on the geological evolution of northeast China. In this paper, we present zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf–O isotopic data for Early Jurassic volcanic rocks in the Erguna Massif of northeast China, with the aim of constraining the evolution of the Mongol–Okhotsk suture belt and its influence on the tectonic history of China during the Early Jurassic. Zircon U–Pb dating indicates that the trachybasalt and basaltic andesite in the study area were erupted between 193±5Ma and 181±9Ma (i.e., in the Early Jurassic). These Early Jurassic volcanic rocks belong to the high-K calc-alkaline series and are enriched in large ion lithophile elements and light rare earth elements, as well as being depleted in heavy rare earth elements and high field strength elements such as Nb and Ta. The rocks show a small negative Eu anomaly. The zircon εHf (182Ma) values of the volcanic rocks range from −1.9 to +5.1, corresponding to TDM1 values of 640–901Ma and TDM2 values of 901–1345Ma. Zircons from two volcanic rocks yield δ18O values of 7.2‰±1.5‰ (n=19) and 6.6‰±0.7‰ (n=35). Geochemically, these Early Jurassic volcanic rocks are similar to those from active continental margin settings, and their primary magmas could have been derived from the partial melting of a lithospheric mantle wedge modified by fluid from a subducted slab. The discovery of Early Jurassic calc-alkaline volcanic rocks in the Erguna Massif, together with the coeval porphyry Cu–Mo deposits, indicates that an active continental margin existed in the Erguna area during the Early Jurassic. Taken together, we conclude that southward subduction of the Mongol–Okhotsk oceanic plate took place beneath the Erguna Massif during the Early Jurassic.

Post-collisional high-K calc-alkaline volcanism in Tengchong volcanic field, SE Tibet: constraints on Indian eastward subduction and slab detachment

Post-collisional high-K calc-alkaline volcanism occurred in the Tengchong volcanic field in the southeastern Tibetan Plateau from 8 Ma to AD 1609. The volcanic rocks have chondrite-normalized rare earth element (REE) patterns characterized by enrichment in light REE (LREE), a flat heavy REE (HREE) pattern and weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element diagrams indicate enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and negative Ta–Nb–Ti anomalies. Incompatible trace element and Sr–Nd–Pb isotope compositions reflect the suprasubduction-zone fluid enrichment of the asthenospheric mantle wedge beneath the Burma–Tengchong terrane as a consequence of eastward underthrusting of the Indian continental lithosphere following the India–Asia collision at 55 Ma. Sr–Nd isotope mixing modelling indicates that amounts of the Indian slab-derived fluid range from 1.0 to 4.6% in the asthenospheric mantle source. Incompatible trace element modelling calculation results show 16% melting of the enriched asthenospheric mantle wedge beneath the Burma–Tengchong terrane. Three parameters (i.e. India-derived fluid, melting degree and residual minerals in the mantle source) constrain the composition of the Tengchong volcanic field mafic magmas. Trace element and Sr–Nd isotopic mixing modelling indicate that the post-collisional mafic magmas in this volcanic field are the product of slab detachment of the eastward subducted Indian continental lithosphere, which may be linked to a significant change in the convergence angle between India and Asia since 8 Ma.

Consideraciones geoquímicas y petrogenéticas para establecer la evolución magmática del Complejo Volcánico Nevado del Huila

The Nevado del Huila Volcano Complex is made of rocks that have intermediate to acid composition, calc-alkaline affinity and middle content in K2O. According to SiO2 content, there are three geochemical types: andesites ss, dacitic andesites and dacites. Adakitic tendence in more recent lavas and more dacitic composition. Behaviors of mayor elements, REE, LFSE and HFSE are typical of orogenic andesites of active continental margin. Isotopic composition Sr-Nd resemble to volcanoes of ZVN Andes (Ecuador’s volcanoes, Galeras and Nevado del Ruiz volcanoes from Colombia). Homogeneous isotopic rate Sr-Nd (87Sr/86Sr = 0,7041-0,7042 and 143Nd/144Nd = 0,51279-0,51283) are typical of mantle melts, with limited taking part of derived components of subduced slab or continental crust. Compatible and incompatible trace elements indicate that fractional crystallization is petrogenetic process that has controlled magmatic evolution. Besides to reject others mechanisms: crustal contamination and magmas mixing. These rocks have probably been generated from partial melting of enriched mantle wedge by components of subduced slab. Later fractional crystallization drove magmatic differentiation of this volcano complex.