科研之友
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摘要
Tibetan Cenozoic syn-collisional potassic-ultrapotassic igneous rocks provide unique insights into the processes and origins of metasomatism in the upper mantle, as well as continental subduction and plateau uplift. Crustal recycling in the magma source of the Tibetan potassic-ultrapotassic rocks has been well-documented. However, the nature of the metasomatic agents and the timing of mantle metasomatism are still disputed. Oxygen isotopes are a powerful tracer for identifying any recycled crustal material in the mantle due to the significant fractionation caused by surface water-rock interaction. Here we present an integrated in-situ study of oxygen isotopes and the major/trace elements of olivine in the Cenozoic potassic-ultrapotassic rocks and mantle xenoliths from the western Kunlun area of the northwestern Tibet. Olivines from mantle xenoliths have oxygen isotope compositions that range from elevated delta O-18 values (5.83 +/- 0.78-5.97 +/- 0.40 parts per thousand) to values (5.09 +/- 0.56-5.10 +/- 0.46 parts per thousand) that are indistinguishable from typical mantle olivine values of 5.18 +/- 0.28 parts per thousand (Mattey et al., Earth Planet Sci Lett 128:231-241, 1994). Elevated olivine delta O-18 values reflect mantle metasomatic processes by an O-18-rich agent, although a few olivine rims show higher delta O-18 values caused by reactions with host lavas. In contrast, olivine oxygen isotope compositions of potassic-ultrapotassic rocks are higher than typical "mantle" values and those of the mantle xenoliths (6.51 +/- 0.74-7.52 +/- 0.24 parts per thousand). There is no sign of crustal contamination, and fractional crystallization did not change the isotopic compositions of the studied potassic-ultrapotassic igneous rocks, thus their high-delta O-18 signature reflects the nature of the primary melts and their source region. Enrichments in olivine O-18 and clinopyroxene Sr isotopic compositions, as well as the enriched trace element compositions of clinopyroxene and orthopyroxene in the mantle xenoliths, indicate that they have been highly metasomatized by silicate materials with a minor carbonate component from recycled Indian continental crustal component, and the enriched lithospheric mantle is the magma source of potassic-ultrapotassic rocks. High olivine delta O-18 values are a common feature of Tibetan Cenozoic potassic-ultrapotassic rocks, consistent with the mantle metasomatic agents that were derived directly from recycled continental crust material via Indian continental subduction. Our study reinforces the profound link between continental subduction, mantle processes and generation of Tibetan Cenozoic potassic-ultrapotassic rocks.
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单位中国科学院; 桂林理工大学
