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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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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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    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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    Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

    Qiang-Qiang ZhangXiao-Ying GaoYong-Fei Zheng
    22页
    查看更多>>摘要：It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected die composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify tbis issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with D_(Ba)~(K-feldspar/biotite) of 2.5-3.0 and D_(Sr)~(K-feidspar/piagiociase) values of 1.4-1.9 and ilmenite can preserve more Nb and Ta than biotite with D_(Nb)~(biotite/ilmenite) values of 0.07-0.1 and D_(Ta)~(biotite/ilmenite) values of 0.04-0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and ortbopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.
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    Subduction initiation in the Neo-Tethys and formation of the Bursa ophiolite in NW Turkey

    Guang-Yao XinYang ChuBen-Xun Su
    17页
    查看更多>>摘要：Subduction of oceanic lithosphere is the primary driver for plate tectonics on earth, but a paucity of geological records hinders our understanding of how subduction is initiated. Two general mechanisms have been proposed-induced and spontaneous nucleation of subduction zones. For both mechanisms, subduction inception begins with initial lower plate burial, but whether upper plate extension and magmatic activity coincide with, or are later than, lower plate underthrusting is the key to identifying spontaneous or induced subduction initiation. Here we present geological, geochemical and geochronological characteristics of mafic dikes and amphibolites from the Bursa ophiolite in NW Turkey, to identify the mechanism of the subduction initiation of the Neo-Tethys Ocean. The mafic dikes are depleted in light rare earth elements (LREE) and slightly enriched in large-ion lithophile elements (LILE), which together with high ε_(Nd)(t) values and low ~(207)Pb/~(204)Pb ratios, indicate a MORB-like mantle source similar to the earliest forearc basalts (FAB) of the Izu-Bonin-Mariana subduction initiation system. Zircon U-Pb geochronology shows a crystallization age of 89.8 ± 0.9 Ma. In contrast, the amphibolites are enriched in LREE and LILE, and have low ε_(Nd)(t) values and high ~(207)Pb/~(204)Pb ratios, suggesting an E-MORB-like geochemical affinity. Zircon U-Pb ages of the amphibolites reveal that the metamorphism occurred at 96.4 ±1.9 Ma, representing the initial burial of the lower plate. Thus, subduction predated upper plate extension and formation of the Bursa oceanic crust by 9.4-3.8 Myr, arguing for induced subduction of Neo-Tethyan lithosphere that formed the Bursa ophiolite.
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    Cretaceous basalt-andesite sequence in the Southern Pamir: arc—back-arc architecture at the Pamir Plateau genetically related to the northward flat subductions of the Neo-Tethys Ocean

    Chuan-Lin ZhangHaibo ZouXiao-Qiang Liu
    13页
    查看更多>>摘要：The Pamir-Karakorum is one of the most important regions for our understanding the Tethys evolution. Although it is commonly known that the northward subduction of the Neo-Tethys Ocean induced the formation of the magmatic arc in Southern Pamir, the temporal-spatial configuration of the arc—back-arc during the Neo-Tethys evolution remains unclear. In this study, we report sedimentary feature, petrography, zircon U—Pb ages and Hf—O isotope compositions as well as bulk-rock and mineral geochemistry of the well-preserved Cretaceous volcanic sequence in Chinese Wahan Corridor at South Pamir. The volcanic sequence is mainly composed of phenocrysts-bearing basalt-andesites with minor rhyolite and volcaniclastic rock. Zircon U—Pb ages reveal the basalt-andesite sequence erupted at 100-98 Ma. Bulk-rock geochemistry defines their potassic calc-alkaline signature and both their rock association and elemental geochemistry share most features of the continental arc volcanic sequences. On the other hand, they have variably enriched Nd isotope compositions with ewd(t) varying from-5.9 to-9.6, coupled with their enriched zircon Hf isotope signature with ε_(Hf)(t) between-13 to-8. Whole-rock geochemistry and zircon oxygen isotope compositions (average δ~(18)O = 7.74 ± 0.14) demonstrate that the primitive magma of the basalt-andesites were derived from partial melting of a metasomatized sub-continental lithospheric mantle and underwent variable assimilation fractional crystallization. Late Jurassic to Cretaceous volcanic-sedimentary sequences and coeval voluminous arc-like granodiorite and granites are widely distributed along the Southern Pamir. In the Central Pamir and further north, late Jurassic to Cretaceous sequences resemble features of back-arc basins. Taken together, we suggest that a northward low-angle or flat subduction of the Neo-Tethys Ocean along the Shyok zone was geodynamically related to the formation of the broad magmatic arc in South Pamir-Karakorum and the back-arc basins north of the South Pamir.
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    High-and low-Mg adakitic rocks in southern Tibet: Implication for the crustal thickening and geodynamic process in the late Cretaceous

    Zhenzhen WangMingchun DongZhidan Zhao
    14页
    查看更多>>摘要：The history of crustal thickening is critical for comprehending the evolution process of the Tibetan plateau. The genetic relationship and spatiotemporal distribution of adakitic rocks with high Sr/Y, (La/Yb)N ratios may impose fundamental constraints on specific geodynamic and crustal thickening processes. The Songka intrusive suite consists of adakitic rocks with high-Mg adakite (monzonites), low-Mg adakitic rocks (quartz monzonites), and gabbroic-diorites. Their geochemical characteristics, pedogenesis, and geodynamic implication is discussed in this paper. Songka high-Mg adakites formed when subducting Neo-Tethyan oceanic slab melts were assimilated by mantle peridotite at elevated temperatures (average Ti-in-zircon temperatures of sample SK1319 are 830 °C). Low-Mg adakitic rocks exhibit markedly different geochemical characteristics than those formed in the juvenile thickened lower crust. The least-squares mass balance and trace element simulations indicate that Songka low-Mg adakitic samples were formed at a degree of 60-65% fractional crystallization of melts resembling Songka high-Mg adakites. Songka gabbroic diorites are formed in the metasomatic mande wedge. The genetic relationship between these rocks indicates that the crust of die southern Tibet did not appear to thicken obviously during the early stage of late Cretaceous (100-90 Ma). The intensive emplacement of late Cretaceous high-Mg adakites (Mg# >50) and mafic rocks (SiO2 < 52%) in the southern Lhasa terrane around 95-90 Ma, together with the slow convergence velocity of the Indian-Asian continent, can be explained by die response of rapid crustal thickening and Neo-Tethyan slab rollback at 95-90 Ma.
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