کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
12121895 | 1353702 | 2019 | 102 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Evolution of nascent mantle wedges during subduction initiation: Li-O isotopic evidence from the Luobusa ophiolite, Tibet
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
علوم زمین و سیارات
ژئوشیمی و پترولوژی
پیش نمایش صفحه اول مقاله
چکیده انگلیسی
The Li-O isotopes of olivine and pyroxene from the mantle sequence of the Luobusa ophiolite, a fragment of Neo-Tethyan forearc lithosphere in Tibet, reveal a series of magmatic processes and geochemical evolution in a nascent mantle wedge during subduction initiation. Olivine grains from the ophiolitic mantle sequence display δ18O values similar to those of normal mantle (+5.18â¯Â±â¯0.2â°), but have large δ7Li variations, â¼+4â° to +13â° in harzburgites and â¼+1â° to +11â° in high-Cr chromitites. Clinopyroxene grains in cpx-bearing harzburgites have finger-like protrusions and LREE-depleted patterns comparable to those of clinopyroxene in abyssal peridotites, and also show high Li concentrations (â¼2-8â¯Î¼g/g) and negative δ7Li values (â¼â15â° to â8â°), which are beyond the ranges of normal upper mantle (<1.3â¯Î¼g/g and +4â¯Â±â¯2â°, respectively). Our dataset suggests that the nascent Luobusa harzburgitic mantle wedge was penetrated by limited amounts of slab-derived fluids at an early stage, leading to decoupling of high δ7Li but normal mantle-like δ18O values of the mantle rocks. The hydrated harzburgites were later refertilized by depleted mantle-derived melts. This process generated the cpx-bearing harzburgites with light-δ7Li clinopyroxene, and also imposed normal mantle-like Li isotopic signatures on some previously hydrated high-δ7Li mantle areas at deeper levels. Subsequent melting in both the high-δ7Li and normal mantle-like Li isotopic mantle regimes generated the parental magmas of the high-Cr chromitites with high-Ca boninitic affinities, accounting for the large δ7Li variation of olivine in the chromitites. Slab rollback below the nascent mantle wedge induced asthenospheric upwelling and a high geothermal gradient, which played a more direct role than slab dehydration in triggering the magmatic events of refertilization and formation of the high-Cr chromitites. Numerical modeling of inter-mineral Li diffusion in cpx-bearing harzburgites suggests an interval of <1000â¯years between the two magmatic events and rapid slab rollback before the generation of the high-Cr chromitites. Intra-oceanic emplacement of the Luobusa proto-forearc lithosphere was possibly finished within 106 years after the chromitite formation.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Geochimica et Cosmochimica Acta - Volume 245, 15 January 2019, Pages 35-58
Journal: Geochimica et Cosmochimica Acta - Volume 245, 15 January 2019, Pages 35-58
نویسندگان
Peng-Fei Zhang, Mei-Fu Zhou, Paul T. Robinson, Julian A. Pearce, John Malpas, Qiong-Ying Liu, Xiao-Ping Xia,