西藏桑日县马门晚白垩世O型埃达克岩年代学、地球化学及构造意义

Chronology,geochemistry and tectonic Significance of Late Cretaceous O-Type Adakite in Mamen,Sangri County,Tibet

通过对出露于桑日县马门一带的火山岩开展全岩地球化学、锆石U-Pb年代学、Sr-Nd同位素研究,结果表明马门火山岩形成于90.24±0.41Ma,高Si O2(56.11%71.36%)、Al2O3(14.27%16.06%)、Na2O(2.804.93)、Sr(327.24969.15),低Y(8.1614.54)、Yb(0.881.27<1.9)及高Sr/Y比值(27.8772.98)为特征,显示O型埃达克岩地球化学特征。岩石属钙碱性系列,富集LREE、亏损HREE,富集大离子亲石元素Rb、K、Ba,亏损高场强元素Nb、Ti、P,无明显负Eu异常,Sr、Nd同位素较一致,εNd>0,表明源区以俯冲洋壳为主,并可能卷入了少量俯冲富硅沉积物。通过与南冈底斯埃达克岩对比,认为马门火山岩可能是新特提斯洋脊俯冲背景下,由俯冲消减的新特提斯洋壳上部部分熔融形成,源区可能还卷入了少量俯冲富硅沉积物。

Petrogeochemical、zircon U-Pb chronology and Sr-Nd isotope studies of volcanic rocks in Mamen area,the results is the volcanic rocks formed in 90.24±0.41 Ma,the characteristic is high Si O2(56.11%71.36%)、Al2 O3(14.27%16.06%)、Na2 O(2.804.93)、Sr(327.24969.15)、Sr/Y(27.8772.98),low Y(8.1614.54)、Yb(0.881.27<1.9),the O-type adakite geochemical characteristics.The rock belongs to calc-alkaline series,enrich in LILE and depleted in HFSE,no significant negative Eu anomaly,and Sr-Nd isotopes are more consistent,εNd>0,which indicates that the sources of these rocks were derived from subducted oceanic crust probably with a minor proportion of subducted rich silicon sediments.By comparing with adkitic rocks from South Gangdese,we hold that the volcanic rocks resulted from partial melting of the upper part of oceanic crust during Neo-tethys northward subduction,and the source area may be involved a minor proportion of subducted rich silicon sediments,in Mamen area,Sangri country.