Petrogenesis of Miocene ultrapotassic rocks in the western Lhasa terrane, Tibetan Plateau: evidence

来源 :2014年中国地球科学联合学术年会 | 被引量 : 0次 | 上传用户:lz3163
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  Major and trace elements,whole rock Sr–Nd–Pb isotopic data,zircon U–Pb geochronology,and in situ zircon Hf–O isotopic compositions have been determined for the ultrapotassic lavas from Chajiasi,Dangreyong lake,Xuru lake,and Chazi in the western Lhasa terrane of the Tibetan Plateau.The results provide new insights into the nature of the source region of the lavas and their petrogenesis,and also into the mechanism of crust–mantle interaction related to the India–Eurasia continental collision.The lavas are mainly trachyte and trachyandesite.Secondary ion mass spectrometer (SIMS) U–Pb analyses of zircons from the lavas yield Miocene ages of 24–11 Ma,consistent with a range of previously published ages from other parts of the Tibetan Plateau.The lavas are characterized by high SiO2,K2O,and MgO contents,and they are enriched in light rare earth elements and large ion lithophile elements,and depleted in high field strength elements.They have highly radiogenic Sr (87Sr/86Sr(i)=0.715922–0.732492) and Pb (206Pb/204Pb=18.4547–18.8885,207Pb/204Pb=15.7394–15.7848,208Pb/204Pb=39.3997–39.6433),together with negative values of εNd(t) (–12.8 to –11.3).Altogether,these data indicate the lavas were derived from an enriched mantle with some crustal contamination.Magmatic zircons from the Chajiasi and Dangreyong lake areas display δ18O values of 6.7‰ to 11.3‰ and εHf(t) values of –17.0 to –7.9 with relatively old Hf crustal model ages of 1.61 to 2.19 Ga,indicating little or no involvement of juvenile crust and/or new mantle-derived materials.However,magmatic zircons from the Xuru lake and Chazi areas exhibit δ18O values in the range 6.8‰ to 8.5‰ and εHf(t) values from –13.1 to –6.3,with relatively old Hf crustal model ages varying from 1.50 to 1.93 Ga.The negative correlation between δ18O and εHf(t) values indicates a two-component mixing of mantle and crust-derived melts.All the εHf(t) and 176Hf/177Hf values fall around the line of the lower crust,indicating a lower crustal source.Taking into account various other geochemical features,we conclude that the parental magmas of the lavas were derived from the partial melting of sub-continental lithospheric mantle (SCLM) previously modified by subducted Indian mafic lower crust.We propose a new model for the generation of ultrapotassic magmas in the Lhasa terrane,and suggest that the Indian continental crust was subducted beneath the Lhasa terrane,as far north as the Bangong–Nujiang suture,in the early–middle Miocene.
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