Geochemical cycling during subduction initiation:Evidence from serpentinized mantle wedge peridotite in the south Andaman ophiolite suite  

作　　者：Abhishek Saha M.Santosh Sohini Ganguly C.Manikyamba Jyotisankar Ray Jagadish Dutta 

机构地区：[1]CSIR-National Institute of Oceanography,Dona Paula,Goa 403 004,India [2]School of Earth Science and Resources,China University of Geosciences,Beijing,100083,China [3]Department of Earth Sciences,University of Adelaide,Adelaide,SA 5005,Australia [4]Department of Earth Science,Goa University,Taleigao Plateau,Goa 403206,India [5]CSIR-National Geophysical Research Institute,Uppal Road,Hyderabad,500 007,India [6]Department of Geology,University of Calcutta,35,Ballygunge Circular Road,Kolkata,700 019,India

出　　处：《Geoscience Frontiers》2018年第6期1755-1775,共21页地学前缘（英文版）

基　　金：the funds from Council of Scientific and Industrial Research(CSIR)to CSIR-National Institute of Oceanography through the MLP-1703 and GAP 2175 project;supported by Foreign Expert funding from CUGB and Professorial position at the Adelaide University

摘　　要：The ophiolite suite from south Andaman Islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here, we present new major, trace, rare earth(REE) and platinum group(PGE) element data for serpentinized and metasomatized peridotites(dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al_2 O_3/TiO_2 > 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with(La/Sm)N > 1 and(Gd/Yb)N <1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of(i) hydration, metasomatism and serpentinization of depleted, MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and(ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signatures collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd(4.4-12.2 ppb) with highΣPPGE/∑IPGE(2-3) and Pd/Ir(2-5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic meltThe ophiolite suite from south Andaman Islands forms part of the Tethyan Ophiolite Belt and preserves the remnants of an ideal ophiolite sequence comprising a basal serpentinized and tectonised mantle peridotite followed by ultramafic and mafic cumulate units, basaltic dykes and spilitic pillow basalts interlayered with arkosic wacke. Here, we present new major, trace, rare earth(REE) and platinum group(PGE) element data for serpentinized and metasomatized peridotites(dunites) exposed in south Andaman representing the tectonized mantle section of the ophiolite suite. Geochemical features of the studied rocks, marked by Al_2 O_3/TiO_2 > 23, LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns with(La/Sm)N > 1 and(Gd/Yb)N <1, suggest contributions from boninitic mantle melts. These observations substantiate a subduction initiation process ensued by rapid slab roll-back with extension and seafloor spreading in an intraoceanic fore-arc regime. The boninitic composition of the serpentinized peridotites corroborate fluid and melt interaction with mantle manifested in terms of(i) hydration, metasomatism and serpentinization of depleted, MORB-type, sub-arc wedge mantle residual after repeated melt extraction; and(ii) refertilization of refractory mantle peridotite by boninitic melts derived at the initial stage of intraoceanic subduction. Serpentinized and metasomatized mantle dunites in this study record both MOR and intraoceanic arc signatures collectively suggesting suprasubduction zone affinity. The elevated abundances of Pd(4.4-12.2 ppb) with highΣPPGE/∑IPGE(2-3) and Pd/Ir(2-5.5) ratios are in accordance with extensive melt-rock interaction through percolation of boninitic melts enriched in fluid-fluxed LILE-LREE into the depleted mantle after multiple episodes of melt extraction. The high Pd contents with relatively lower Ir concentrations of the samples are analogous to characteristic PGE signatures of boninitic magmas and might have resulted by the infiltration of boninitic melt

关 键 词：South Andaman OPHIOLITE MANTLE PERIDOTITE METASOMATISM GEOCHEMICAL cycling SUBDUCTION initiation 

分 类 号：P[天文地球]