The nature(i.e., sub-oceanic, sub-arc or subcontinental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the ...The nature(i.e., sub-oceanic, sub-arc or subcontinental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re-Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr~# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr~#(~0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites(Day et al., 2017;Parkinson and Pearce, 1998);two samples have spinel Cr~# higher than 0.6, which is probably ascribed to intense melt-rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnetfacies melting followed by 10% –18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites(Hellebrand et al., 2002) and implies deep initial melting(> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites(commonly <80 km in origin). The Dazhuqu peridotites have unradiogenic 187 Os/188 Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle(PUM)(Meisel et al., 2001). All but one samples yield relatively younger Re depletion ages(TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re-Os isotopes and highly siderophile element(HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites(Day et al., 2017) and the Oman southern massifs(Hangh?j et al., 2010) but are distinct from noncratonic sub-continental lithospheric mantle(SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision.展开更多
基金financially supported by the National Natural Science Foundation of China(grants 41673038,41521062)the Key Research Program of Frontier Sciences from CAS(QYZDB-SSW-DQC032)the Open Fund Project of State Key Laboratory of Lithospheric Evolution(201707)
文摘The nature(i.e., sub-oceanic, sub-arc or subcontinental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re-Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr~# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr~#(~0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites(Day et al., 2017;Parkinson and Pearce, 1998);two samples have spinel Cr~# higher than 0.6, which is probably ascribed to intense melt-rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnetfacies melting followed by 10% –18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites(Hellebrand et al., 2002) and implies deep initial melting(> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites(commonly <80 km in origin). The Dazhuqu peridotites have unradiogenic 187 Os/188 Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle(PUM)(Meisel et al., 2001). All but one samples yield relatively younger Re depletion ages(TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re-Os isotopes and highly siderophile element(HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites(Day et al., 2017) and the Oman southern massifs(Hangh?j et al., 2010) but are distinct from noncratonic sub-continental lithospheric mantle(SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision.
