西藏南木林盆地林子宗群火山岩年代学和地球化学特征
摘要
In order to better constrain the age framework, petrogenesis and evolution history of the Linzizong volcanic succession (LVS), this study focuses on the petrography, zircon U-Pb geochronology and whole-rock geochemical characteristics analyses of LVS in Namling basin in the central-southern Lhasa Terrane.Results show that the periods of eruptive cycles of LVS in Namling basin are 62.8 Ma to 57.0 Ma, 52.0 Ma to 50.2 Ma and 49.7 Ma, respectively.With the evolution of time, the lithofacies have changed.Dianzhong Formation is dominated by the eruption facies, Nianbo Formation is dominated by the eruption-sedimentary facies, while Pana Formation is dominated by the eruption-overflow facies.Meanwhile, it shows the trend of evolution from andesitic to acidic, from calc-alkaline to high K calc-alkaline to shoshonite series.The dacite rocks of Dianzhong Formation are calc-alkaline series, which are enriched in LILE, depleted of HFSE, with obvious negative anomalies of Nb, Ta and Ti, showing typical arc volcanic characteristics.The rhyolite rocks of Nianbo Formation and Pana Formation are calc-alkaline to high K calcalkaline series with occurrence of shoshonite series, which are enriched in LILE, and depleted in HFSE.In addition to the obvious negative anomalies of Nb, Ta and Ti, there are also significant negative anomalies of Ba, Sr and P, showing the characteristics of collision to post-collision volcanic rocks.The above features suggest that the dacite rocks of Dianzhong Formation may be the product of partial melting of the overlying crust induced by the undercutting of mantle-derived magma in the arc region during the northward subduction of the Neo-Tethys Ocean.While, the rhyolite rocks of Nianbo Formation and Pana Formation are the products of further separation and crystallization of dacite rocks in the early stage of the Indian-Asian continental collision.In summary, LVS is an important archive for recording this system transformation from subduction to collision.