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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Contrasting melt percolation and melt-rock reactions in the Parece Vela back-arc oceanic lithosphere, Philippine Sea: A mineralogical perspective

    Yanhui DongXiaohan GongLiyan Tian
    14页
    查看更多>>摘要：The accretion and evolution of oceanic lithosphere beneath back-arc spreading ridges remain less understood compared with mid-oceanic ridges (MOR). Here, we investigate mantle peridotites and gabbros recovered from the southernmost side of the Parece Vela back-arc basin, Philippine Sea. The peridotites contain multiple types of clinopyroxenes (Cpx) and amphiboles (Amp) which record contrasting melt percolation and melt-rock reaction processes. Type-1 Cpx shows primary texture and light rare-earth elements (LREE)-depleted pattern, which can be reproduced by —8-10% fractional melting of depleted MORB mantle. Type-2 Cpx occurs in the deformed peridotites (SY224-1280-2a &-2b) and shows less depleted LREE at similar middle to heavy REE (M-HREE)-contents compared with type-1 Cpx. Such features reflect re-equilibration with MORB-type percolating melts, which is further evidenced by elevated TiC>2 and Cr# in coexisting spinels and crystallization of associated MORB-type gabbros. Type-3 Cpx occurs in the undeformed peridotites (SY225-1255-1 &-2) and is texturally replaced by type-1 Amp. They show variably enriched large ion lithophile elements (LILE) and LREE at similar M-HREE contents compared with type-1 Cpx, which reflect an open-system (flux) melting and reaction process with a melt flux enriched in LILE and LREE. Type-1 Amp is almost magnesio-hornblende and shows trace-element pattern characteristic of supra-subduction amphiboles, which supports the hydrous melt that formed type-3 Cpx probably originated from subducted slab. In contrast, type-2 Amp occurs in a leucocratic vein in sample SY225-1255-1 and is characterized by nearly flat M-HREE pattern at 600-200 x CI, LREE depletion and marked negative Eu anomaly. These features are consistent with crystallization from an amphibole-saturated melt formed by extreme differentiation of MORB-type melts. Our data, together with previous observations, suggest co-occurrence of hydrous MORB-type and slab-derived melt percolations beneath the Parece Vela spreading ridge, which may affect the chemical evolution of back-arc oceanic lithosphere different from MOR.
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    Petrogenesis delineation of the felsic intrusive rocks in the eastern North China Craton: Implications for crustal evolution and geodynamic regimes

    Houxiang ShanMingguo ZhaiXiaoping Lu
    14页
    查看更多>>摘要：As die dominant component of the continental crust, Archean intrusive granitoids can provide significant insights into the growth and evolution of the early crust. In this study, we report a systematic geochronological and geochemical study on a tonalite-trondhjemite-granodiorite (TTGi)-granites (G2)quartz monzonite (QM) assemblage in the northeastern North China Craton (NCC). Zircon U-Pb dating shows that they were emplaced during 2506-2549 Ma. Their geochemical features, such as REE concentrations, patterns and ratios, super-chondritic Nb/Ta ratios, and similarity to the HP-MP TTGs, suggest residues of garnet-amphibolite-to eclogite-facies. TTGs in Group I show high Sr and low Y concentrations and negligible/pronounced negative Eu anomalies, reflecting rare or a small quantity of plagioclase residue. Positive Eu anomalies of Group II possibly indicate involvement of varying amount of accumulative plagioclase, and that garnet fractionation also plays a role. High MgO, Mg#, Cr and Ni, and positive enKO varying from juvenile to evolved signatures, suggest that the TTGs were generated under a continental arc setting. The granitic rocks (G2-QM) were generated by melting of predominant TTGs and subordinate metasedimentary rocks, with late-early Neoarchean TTGs, Mesoarchean TTGs, and Eoarchean crustal material in the NCC involved. Isotopic evidence shows that 2.7-2.9 Ga crustal reworking predominates the formation process of the granitoids. The study area witnessed a long period of crust extraction from Eoarchean to the end of Neoarchean. Subduction-related magmatisms at late Neoarchean are widespread in the world.
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    Secular evolution of the subcontinental lithospheric mantle beneath Indian cratons: Insights from geochemistry and geochronology of the Precambrian mafic dykes

    Om Prakash PandeyDebajyoti Paul
    20页
    查看更多>>摘要：The Indian Shield is comprised of the Dharwar, Bastar, Singhbhum, Bundelkhand, and Aravalli Cratons, intruded by distinct mafic dyke swarms of different generations (ca. 2.8-0.8 Ga). Most of these dykes are tholeiitic basalt to basaltic-andesite, including boninite. Some other subordinate dyke rocks are of komatiitic, picritic, and andesitic compositions. The vast areal extent of these dykes indicates that they are remnants of Precambrian Large Igneous Provinces (LIPs). The present study reviews the existing geochronological and geochemical data of various Precambrian mafic dyke swarms intruding in all Indian cratons, to track temporal changes in composition of the underlying subcontinental lithospheric mantle (SCLM). Most of these dykes have crust-like abundances of incompatible trace-element. Even the primitive dykes (Mg# = 82-64) exhibit crust-like incompatible element patterns. However, some also have depleted mantle-like abundances of incompatible elements. Most of the dykes also have high concentrations of compatible trace elements (e.g., Cr and Ni). The age-corrected radiogenic Nd isotope compositions (eNdffi) of these dykes vary between the upper continental crust and depleted mantle ε_(Nd) growth arrays. The elemental composition and Swao) of these dykes suggest their derivation from a heterogeneous SCLM comprising enriched (metasomatized) and depleted mantle components. The negative as well as positive ε_(Nd) values exhibited by the ca. 2.8 Ga (oldest) dykes suggest the presence of enriched mantle materials distributed within a depleted-SCLM already by ca. 2.8 Ga. The enrichment must have occurred before ca. 2.8 Ga due to subduction-released fluids. Later, the SCLM beneath Indian cratons evolved with a composition consisting of an enriched/metasomatized mantle component and a depleted mantie component.
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    Fluid-rock interactions at shallow depths in subduction zone: Insights from trace elements and B isotopic composition of metabasites from the Mariana forearc

    Haiyang LiuYing-Yu XueTinggen Yang
    21页
    查看更多>>摘要：To reveal the spatial variations of slab-derived fluids and to trace the in-situ dehydration in the shallow subduction zone, we investigated the petrography, mineral chemistry, and whole-rock B isotopes of metabasites that were recovered from the Fantangisha and Asut Tesoru serpentinite mud volcanoes and originated from the shallow subduction channel at the forearc of the Mariana subduction zone. The alteration mineral assemblages in the investigated metabasites suggest zeolite-to prehnite-pumpellyite-facies metamorphism and lawsonite-blueschist fades metamorphism beneath Fantangisha and Asut Tesoru seamounts, respectively. The fluid mobile elements (e.g., B, As, Sb, Pb) are preserved in the low grade metamorphic phyllosilicate minerals (e.g., glauconite, pumpellyite, celadonite), thus fixing the B concentrations of the subducted oceanic crust during shallow subduction (<18 km). Both B concentrations (16.7 to 43.9 ng/g) and δ~(11)B values (-5.0 to +3.2%o) of the investigated metabasites are significantly higher than fresh normal mid-ocean ridge basalts (N-MORB) and ocean island basalts (OIB), and are generally comparable to the uppermost altered oceanic slab. Notably, the recovered metabasites from the Mariana forearc exhibit a decreasing trend in δ~(11)B values with increasing distance to the trench from Fantangisha (62 km) through Asut Tesoru (72 km) to South Chamorro (78 km) Seamounts. This trend together with Rayleigh dehydration modeling indicate that the ~(11)B-enriched aqueous fluids were released from the subducting slab during prograde metamorphic dehydration. The estimated B isotopic compositions of the slab-derived fluids released at arc magma genesis depths are generally comparable to that of the Mariana arc lavas. However, the fluids released by dehydration of subducted sediments at shallow depths should be characterized by lower δ~(11)B values than fluids released from the slab at depths of magma genesis beneath the island arc. Then, the recycling of the hydrated forearc mantle is necessary to explain the high δ~(11)B values of the Mariana arc lavas. While, the variable B isotope compositions of Mariana arc lavas should be controlled by the different ratios of sediment/AOC ratios.
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    Cretaceous thermal evolution of the closing Neo-Tethyan realm revealed by multi-method petrochronology

    Regina HoltmannJesus Munoz-MontecinosSamuel Angiboust
    19页
    查看更多>>摘要：A Cretaceous paleo-accretionary wedge, the Ashin Complex, now exposed along the Zagros suture zone in southern Iran, exhibits mafic, metasedimentary and ultramafic lithologies. Field, geochemical and petrological observations point to an anomalous high-temperature event that gave rise to the formation of peritectic (trondhjemitic) melts associated with restitic garnet-bearing amphibolites. Lu-Hf isotopic dating of centimetre-sized garnet in amphibolite-facies metasediments yielded a crystallization age of 113.10 ± 0.36 Ma, possibly representing the age of prograde to near-peak metamorphic conditions. SHRIMP U-Th-Pb zircon dating from trondhjemitic leucosomes yields crystallization ages of 104 ± 1 Ma, interpreted as the age of the temperature peak, which occurred in the upper amphibolite-facies (c. 650-680 °C at 1.1-1.3 GPa), according to thermodynamic modelling and Ti-in-zircon thermometry. Rutile crystals from two leucosomes yield Zr-in-rutile temperatures in the range of 580-640 °C and a LA-ICP-MS U-Pb age range from 85 to 112 Ma, interpreted as a consequence of partial re-equilibration during incipient cooling. A late static recrystallization event is indicated by the presence of sodic-calcic clinopyroxene, sodic amphibole, Si-rich phengite, titanite overgrowths after rutile and lawsonite within former leucosomes and late fractures. This mineral assemblage is a typical blueschist-facies (high pressure-low temperature) paragenesis and is interpreted as reflecting long-term isobaric cooling that occurred until the end of the Cretaceous as a consequence of increasing slab thermal age. This first report of a melting event in the Zagros paleo-accretionary wedge reveals the presence of a transient, abnormally high thermal gradient of c. 18 °C/km that occurred at c. 105-113 Ma. We speculate that this could be explained by the subduction of a thermal anomaly such as a seamount chain, a transform fault system or, more likely, a spreading ridge under the southern Iranian margin. Indeed, paleogeographic reconstructions of the Tethyan realm suggest the entrance of the Northern Tethyan basin ridge into the subduction zone shortly after 120 Ma.
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    Weakening the lower crust: conditions, reactions and deformation

    Tommaso TacchettoChris ClarkTimmons Erickson
    18页
    查看更多>>摘要：The impact of fluid infiltration on the deformation mechanisms that facilitate the development of lower-crustal ductile shear zones is evaluated through a multiscale structural, geochemical, and thermobaric analysis undertaken across a shear zone/wall-rock interface exposed on the island of Rad0y in the Bergen Arcs (western Norway). At the outcrop scale, the shear zone is characterized by a strain gradient reflected in the progressive evolution from weakly-deformed coronitic gabbroic anordiosite to finer-grained foliated amphibolite characterized by a distinct mineral lineation, shear bands, and a-type porphyroblasts. Electron backscattered diffraction (EBSD) crystallographic orientation data from the coronitic gabbroic anorthosite define an initial stage of shear localization whereby most of the deformation is accommodated by crystal plasticity within plagioclase accompanied by grain size reduction through subgrain rotation recrystallization. As deformation proceeds, complementary to increasing fluid-rock interaction, the replacement of the anhydrous mineral assemblage results in strain partitioning and the development of a heterogeneous ductile shear zone. At the grain scale, the distinct CPO of amphibole, epidote and kyanite suggests deformation being dominated by crystal plastic mechanisms. U-Pb age data obtained from zircon grains within the Caledonian shear zone cluster at 883 ± 3 Ma consistent with ages derived from the granulite facies assemblage. Phase equilibria modelling indicates conditions of deformation within the shear zone at —600 °C and —11 kbar, consistent with mid-crustal levels at amphibolite facies conditions. Conversely, geochemical data from garnet of the shear zone characterized by the absence of Eu anomaly, point to mineralogical reactions having initially occurred at higher pressure conditions. This study highlights the key role of fluid infiltration and metamorphic reactions on strain localization processes which can facilitate the ductile deformation of the original assemblage and ultimately contribute to the rheological weakening of an anhydrous and refractory lower crust.
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    Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif—Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula

    Jihyuk KimJung-Woo ParkMi Jung Lee
    21页
    查看更多>>摘要：The Triassic period is when abundant late-orogenic magmatism occurred in the Korean peninsula, but the characteristics of the lithospheric mantle at that time beneath the Yeongnam Massif have not been well-defined because of the scarcity of mafic intrusions. We report zircon U—Pb and phlogopite 40Ar/39Ar ages, petrography, and geochemistry of the Triassic (227.6 Ma) ultrapotassic igneous rocks in the Yeonhwa I Mine (Yeonhwa Ultrapotassic Intrusions, YUI) emplaced into the Taebaeksan Basin in the Yeongnam Massif. The YUI occur as two stocks with different lithologies: phlogopite-rich and amphibole-rich, where the former underwent carbonate-melt interaction along the margin in contact with the carbonate formadon. The two lithologies share similar compositional and isotopic characteristics; however, the phlogopite-rich YUI better retains its mantle-derived features, having idiomorphic high-Mg# (up to 91.9) phlogopite and diopside phenocrysts and bulk concentrations of high MgO (Mg# = molar 100 Mg/[Mg + Fe] >70), Cr (208-1150 ppm), and Ni (48-219 ppm). The enrichment of the YUI in H20 (phlogopite-rich), silica contents (50.2-56.9 wt%), and large ion lithophile elements (LILE; K, Rb, Sr, Ba, Pb), together with depletion of high fields strength elements (HFSE; Ti, Zr, Nb, Hf, Ta), are typical of the ultrapotassic rocks in orogenic settings. The elemental and isotopic data imply that the YUI originated from pardal melts of die phlogopite-bearing lithospheric mantle near the spinel-garnet transition zone. The extremely enriched isotopic ratios (87Sr/86Sr(t) = 0.70756 to 0.71227, eNd(t) =-8.82 to-11.0) suggest a highly enriched mantle source, possibly more enriched than the contemporary pelagic sediments. Considering that the Yeongnam Massif was in a subduction system during Permian-Triassic, not a continental collisional belt, the enrichment was inherited from a combination of both young, subduction-related and ancient, pre-existing metasomes in the continental lithospheric mantle. Our data confirm the presence of the enriched, volatile-rich lithospheric mantle beneath the Triassic Yeongnam Massif. This enriched origin contrasts with earlier (c. 240-280 Ma) Yeongnam arc plutons that originated from the depleted asthenospheric mande (εNd(t) >+2). Since the enrichment is also characteristic of most of the c. 220-240 Ma igneous rocks around the Korean peninsula, such lithospheric mantle lithology was widespread during die Late Triassic.
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    Dynamics of Early Neoproterozoic accretion, west-central India: II —1.65 Ga HT-LP and-0.95 Ga LT-HP metamorphism in Godhra-Chhota Udepur, and a tectonic model for Early Neoproterozoic accretion

    Anwesa BanerjeeN. PrabhakarNicole Sequeira
    21页
    查看更多>>摘要：The N/NNE-striking Aravalli Delhi Fold Belt (ADFB) and the E-striking Central Indian Tectonic Zone (CITZ) converge at the Godhra-Chhota Udepur (GC) sector, west-central India. Analyses of mesoscale deformation structures and metamorphic phase equilibria in the basement and supracrustal rocks are integrated with geochronological-geochemical data (accompanying article) to address the dynamics of the Early Neoproterozoic CITZ-ADFB accretion. In the GC sector, —1.65 Ga granulite fades anatectic gneisses, ~2.5 Ga and 1.03-1.02 Ga granitoids, and greenschist to amphibolite fades allochthonous supracrustal rocks constitute a tectonic melange (D2 deformation). The lithodemic units are traversed by networks of W/WNW-striking steep-dipping trans-pressional shear zones with sinistral kinematics (D3). The shallow-dipping melange with top-to-the south kinematics is intruded by post-D2 to syn-D3 0.95-0.93 Ga granitoids. Mn-NCKFMASH P-T pseudosection analyses of the anatectic gneisses with pre-D2 garnet + cordierite-bearing leucosomes suggest the rocks evolved along a clockwise P-T patii in the range of 5-6 kbar and 680-720 °C. By contrast, the Early Neoproterozoic (0.95-0.93 Ga) chlorite + phengite (Si up to 3.32 apfu) + clinozoisite + quartz ± biotite ± garnet schists in the melange attained pre/syn-D2 peak metamorphic conditions (10-12 kbar, 450-500 °C). NCKFMASH pseudosection analyses of the —0.95 Ga phengite-bearing schists indicate the supracrustal rocks evolved along a high-P, low-T clockwise path; phengite-poor micas defining the D3 fabric (Si up to 3.04 apfu) attest to post-peak decompression in the schists. The —2.5 Ga ADFB granites accreted with the —1.65 Ga HT-LP anatectic gneisses, —1.03 Ga granites, and the —0.95 Ga LT-HP supracrustal rocks of the CITZ during D2 thrusting. The crustal convergence continued with the emplacement of post-D2 0.95-0.93 Ga granitoids that culminated with the nucleation of W/ WNW-striking D3 transpressional shear zones. This broad contemporaneity among felsic plutonism, LT-HP metamorphism in the supracrustal rocks, and the D2-D3 shortening are interpreted to be the result of a switch in subduction polarity between 1.03 and 0.93 Ga during oblique ADFB-CITZ convergence.
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    Building of arc crust during Silurian subduction in the southern Paleo Asian Ocean: Constraint from mineralogy of the pyroxenitic-gabbroic suite in the Chinese East Tianshan

    Yu JinjieMao JingwenChen Baoyun
    14页
    查看更多>>摘要：Arc lower crustal cumulates are significant in investigation of arc differentiation in the lower level of an island arc. The Silurian pyroxenitic-gabbroic suite in the Chinese East Tianshan represents a lithological assemblage of the basal to lower arc crust. This suite comprises, in the order of evolution, olivine websterite (ol-websterite), olivine and plagioclase clinopyroxenite (ol-and pl-clinopyroxenite), and amphibole gabbro (amph-gabbro). Pyroxenite are dominated by cumulus clinopyroxene with interstitial olivine. Amphibole is present in all these rocks, occurring either as coronas of pyroxene and olivine or as oikocrysts, implying a H20-rich parental melt. Plagioclase occurs as veinlets cutting cumulus clinopyroxene in the pl-clinopyroxenite, revealing that it crystallizes totally later than clinopyroxene. Spinel ranges from chromite in the ol-websterite and ol-clinopyroxenite to pleonaste in other evolved rocks, revealing an aluminous differentiation trend. Olivine in the pyroxenite ranges in Fo from 64 to 86 mol%, averagely 7-15 Mg# units lower than coexisting clinopyroxene (79-93). Clinopyroxene shows a chemical trend from low AI2O3 (<2 wt%) in the ol-websterite to high Al2O3 (up to 6.5 wt %) in the pl-clinopyroxenite. All clinopyroxene has incompatible trace elements similar to those of clinopyroxene from typical arc lower crustal cumulates. All plagioclase is extremely calcic (An >83 mol%, up to 99 mol%), generally coupled with iron-rich olivine (Fo <76 mol%) in the studied rocks. Such distinct mineral compositions are typical of arc lower crustal cumulates and ascribed to the crystallization sequence of hydrous arc magmas at high pressure. The pressure for the formation of the pyroxenitic rocks is estimated to be 6-8 kbar using the two-pyroxene geobarometer, a preferred pressure for clinopyroxene crystallization coupled with high H2O content. In the stability field of clinopyroxene crystallization, olivine crystallizes from the interstitial melt but later than clinopyroxene, having unexpectedly lower Fo than Mg# of clinopyroxene. Meanwhile, plagioclase crystallization is totally suppressed with the residual magmas evolving towards highly aluminous, crystallizing spinel with a pure pleonaste composition. Upon saturation in plagioclase with decreasing pressures, plagioclase will acquire extremely calcic compositions due to the aluminous and hydrous nature of the residual arc magmas. The incompatible trace element compositions of the Silurian primitive magmas, estimated from the most primitive clinopyroxene (Mg# = 92-93), show selective enrichment in large ion lithophile elements relative to high field strength elements with an exception of depletion in Ba and low Nb/Ta ratios, implying a magmatic deviation from metasomatized mantle modified by aqueous fluids dehydrated from rutile-bearing slab. The Silurian arc lower crust was built by crystal fractionation and accumulation from hydrous arc basaltic magmas at 6-8 kbar of the lower crustal pressure.
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    Late Eocene subduction initiation of the Indian Ocean in the North Sulawesi Arc, Indonesia, induced by abrupt Australian plate acceleration

    Xiaoran ZhangTse-Ning HuangSun-Lin Chung
    14页
    查看更多>>摘要：Subduction of oceanic lithosphere is the main driving force of plate tectonics on Earth, yet, how subduction began (i.e. subduction initiation; SI) remains enigmatic, due to limited SI examples. Here, we report new whole-rock geochemical and zircon U-Pb-Hf isotopic data of arc magmatism from the North Sulawesi Arc, Indonesia, to constrain the petrogenesis, magmatic tempo, and tectonic setting, as well as the timing and trigger of SI. Zircon U-Pb dating yielded crystallization ages of —29.7-16.7 Ma for parental magmas, which are calc-alkaline I-type in nature and show typical subduction-related signatures. Geochemical data and high positive zircon emit) values indicate magma generation by partial melting of preexisting mafic basement rocks. Our new results, together with literature data, allow the identification of an Indian SI in the Late Eocene, as evidenced by the last vanishing of BAB-type mafic basement rocks at ~37 Ma and the first appearance of calc-alkaline felsic arc magmatism at —30 Ma in the North Sulawesi Arc, as well as —36-32 Ma ophiolite and-34-29 Ma metamorphic sole in the nearby East Sulawesi Ophiolite. Such a temporal duration (maximum of —7 Ma) from SI to typical arc magmatism is identical to that (—8-7 Ma) of the Izu-Bonin-Mariana system. Given the contemporaneousness of a sudden increase (from —4-5 to —8-9 cm/y) in northward drifting rate of the Australian plate, we propose that Late Eocene Indian SI was probably linked to the sharp Australian plate acceleration. Such plate reorganization may have played a significant role in driving SI in Earth history.
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