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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Terminal stage of divergent double subduction: Insights from Early Cretaceous magmatic rocks in the Gerze area, central Tibet

    An-Bo LuoJian-Jun FanDe-You Sun
    19页
    查看更多>>摘要：Divergent double subduction (DDS) plays an important role in facilitating tectonic processes such as ocean closure, accretion and amalgamation of magmatic arcs, and continental growth. However, DDS geodynamics remain poorly constrained, especially its evolution and the accompanying magmatism at the terminal stage. According to geological and geophysical data, the Meso-Tethys Ocean (also known as the Bangong-Nujiang Tethyan Ocean) in central Tibet has been considered a Mesozoic example of DDS, which provides a rare opportunity to evaluate the geodynamic processes of the terminal stage of a DDS system. Here, we present new geochronological, geochemical, and isotopic data of volcanic rocks from the Gerze area in the western segment of the Bangong-Nujiang suture zone (BNSZ). Combined with previous data, we found that the ca. 105 Ma magmatic rocks in the Gerze area are distributed not only along the two sides of the BNSZ but also as manifold types. On the northern side, the magmatism ca. 105 Ma varied, including A-type rhyolites, bajaitic latites, high-Nb basalts, N-MORB-type basalts, adakites and bimodal volcanic rocks, which originated from sources including a sinking oceanic slab, asthenospheric mantle, metasomatized mantle and overlying continental crust. The lithology on the southern side only includes A-type rhyolite and magnesia-rich andesite and adakitic intrusions. Spatial and compositional variations in ca. 105 Ma magmatism should have been the response to slab breakoff, which may have occurred only in the northern branch of the Meso-Tethys oceanic slab after ocean closure. Our results not only verify the existence of slab breakoff in the classical DDS model but also emphasize that breakoff can cause upwelling of the subslab asthenosphere, which leads to a complex magmatic response.
      原文链接:
    • NSTL

    Co-Ni decoupling indicates fluid exsolution during the formation of podiform chromitites: Insights from the Luobusa ophiolite, southern Tibet

    Yuan HeXiangkun ZhuJianxiong Ma
    13页
    查看更多>>摘要：Podiform chromitites typically occur in harzburgites near the petrological Moho in ophiolite sections, and are petrologically and economically significant. Hydrous fluids may have an important role in the formation of podiform chromitites, although unambiguous evidence for the involvement of such fluids is rare. To examine whether hydrous fluids are involved in the formation of podiform chromitites, we measured the Ni, Co, Mn, and Zn contents of olivines and chromites in representative samples from the Luobusa ophiolite, southern Tibet, taking account of the different geochemical behaviors of these elements in melts and hydrous fluids. In the harzburgites, the olivine (01) has a limited compositional range (2605-2919 ppm Ni, 117-135 ppm Co, 736-1005 ppm Mn, and 28.9-35.7 ppm Zn), whereas the chromite (Chr) has a relatively variable composition (718-1239 ppm Ni, 364-493 ppm Co, 1314-2733 ppm Mn, and 1411-1900 ppm Zn). From the dunite lenses to the dunite envelopes, and to the chromitites, both olivine and chromite exhibit significant decreases in Co (Ol = 137 to 45 ppm; Chr = 542 to 168 ppm), Mn (Ol = 889 to 273 ppm; Chr = 2426 to 862 ppm), and Zn (Ol = 37.7 to 4.0 ppm; Chr = 1746 to 206 ppm) contents, and an increase in Ni contents (Ol = 3009 to 5946 ppm; Chr = 413 to 1462 ppm). These features cannot be fully explained by subsolidus re-equilibration, partial melting, fractional crystallization, and melt-rock reactions. Olivine relicts in chromite indicate the dissolution of pre-existing olivine in dunitic channels. The olivine dissolution may have been due to the water-rich nature of the parental melts of the chromitites. These observations and mass-balance calculations suggest that, during the formation of podiform chromitites, chromite crystallization was accompanied by olivine dissolution and exsolution of a hydrous fluid phase. This resulted in the preferential transfer of Ni into the melt and olivine relicts, and Co, Mn, and Zn into the fluid phase. As such, the chromite and olivine in the chromitites became Ni-rich and Co-, Mn-, and Zn-poor, which led to Ni-Co decoupling.
      原文链接:
    • NSTL

    Generation of continental crust by remelting of enriched oceanic crust in accretionary orogen: Geochemical evidence of granitoids in the Tongbai Orogen, Central China

    Yadi ZhuangYuanbao WuWenxiang Zhang
    13页
    查看更多>>摘要：Oceanic arcs are known as the critical sites to generate new continental crust. However, additional processes are needed to transform mafic oceanic crust to mature continental crust. Ascertaining the petrogenesis of granitoids therein is vital to understand such a maturation process. In this study, we carried out a combined study of in-situ zircon U-Pb dating and Hf-0 isotopic analyses, as well as whole-rock geochemical and Sr-Nd isotopic compositions for the Changwan granitoids in the Erlangping oceanic arc unit of the Tongbai orogen. This intrusion mainly consists of biotite granite with subordinate trondhjemite. LA-ICPMS and SIMS zircon U-Pb dating yielded identical U-Pb ages of ca. 460 Ma, representing the intrusion ages. Whole-rock analyses demonstrated that the Changwan granitoids have relatively high SiO2 (72.71-77.14 wt%) and Al2O3 (12.61-14.24 wt%), but low MgO (0.31-0.74 wt%) contents and Mg# (32.0-40.2). The relatively high K2O (2.81-3.63 wt%) contents and K2O/Na2O values (0.76-1.03) of the granitic samples suggest they belong to middle-K weakly peraluminous I-type granite, while the trondhjemitic samples with low K2O (1.06-1.96 wt%) and K2O/Na2O values (0.23-0.42) are tholeiitic to middle-K series. In both cases, the samples show arc-type trace element distribution patterns. They are characterized by depleted Sr-Nd-Hf isotopic compositions with (~(87)Sr/~(86)Sr); of 0.7038-0.7050, ε_(Nd)(t) of +0.52 ～ +2.85, zircon ε_(Hf)(t) of +3.4 ～ +9.6, and relatively low zircon δ~(18)O values of 4.71 ± 0.17-4.97 ± 0.18‰. These isotopic fingerprints suggest the Changwan granitoids were derived from a relatively depleted source that had been hydrothermally altered under high temperature conditions. The most likely candidate is the accreted Erlangping lower oceanic crust which might have been heterogeneously modified in a supra-subduction zone. This is further verified by the trace element modeling on potential mafic source rocks enriched in incompatible elements in the Erlangping unit. We suggest that the high K2O/Na2O granitic samples were derived from K-rich segment of the Erlangping lower oceanic crust whilst the low K2O/Na2O trondhjemites from K-poor part. Therefore, our results show that partial melting of enriched oceanic crust in supra-subduction zone plays an important role for crust maturation in accretionary orogens.
      原文链接:
    • NSTL

    Microtextural evolution of chrome spinels in dunites from Mayodia ophiolite complex, Arunachal Pradesh, India: Implications for a missing link in the 'two-stage' alteration mechanism

    Sankhadeep RoyDebaditya BandyopadhyayTomoaki Morishita
    25页
    查看更多>>摘要：Although occurring as an accessory phase in mafic-ultramafic rocks, the universal presence of chrome spinel in ophiolitic settings bears importance in deciphering the tectonic setting of the ophiolite. Compositional and textural modification of chrome spinel via post-magmatic processes (metamorphism and fluid-induced alteration) is widely reported from host chromitites and peridotites, however, well-documented studies from dunite in ophiolitic settings are sparse. Here we present a detailed petrographic and geochemical account of accessory chrome spinels from previously unreported massive dunite bodies of Mayodia ophiolitic complex, Arunachal Pradesh, India, that portrays a microtextural evolution during alteration/metamorphism. Although having massive appearance, the bodies are crudely foliated and deformed. Severe alteration obliterated primary spinel chemistry as well as pre-alteration mineralogical assemblages. From textural and chemical point of view, the disseminated accessory chrome spinel grains are classified into four broad groups: (1) less chloritized spinel, (2) heavily chloritized spinel, (3) patchy-zoned spinel, and (4) homogeneous spinel. The chloritized spinels are relatively coarse grained, compositionally enriched in Cr and Fe~(2+) (ferrous chromite). They contain Cr-rich chlorite inclusions and are surrounded by a mixed matrix of Cr-rich chlorite and Cr-rich serpentine. Unzoned, homogeneous spinels appear as aggregates and are devoid of chlorite inclusions. They are compositionally different from the chloritized spinels, being enriched in Fe~(3+) and depleted in Cr (ferrian chromite). The patchy-zoned spinels have a non-uniform zoning with patches of ferrous chromite floating within ferrian chromite, separated from each other by micro fracture networks. Adapting a novel technique, we reconstruct bulk rock composition based on global data compilation, which has been applied to thermodynamic modelling. Comparing our integrated results with global examples we infer that fluid-induced alteration (chloritization) caused the formation of chloritized spinels from their primary counterpart. Later oxidizing fluid interacted with the chloritized spinels and converted them to homogeneous spinel; supporting the two-stage alteration model. We interpret the patchy zoning as an arrested intermediate stage between chloritized spinel and homogeneous spinel, preserving the characters of both stages. From a regional perspective, we postulate that the chrome spinel microtextures evolved between ～780 °C and ～550 °C during amphibolite fades retrogression of the dunite bodies, possibly related to exhumation of Mayodia ophiolitic complex.
      原文链接:
    • NSTL