拉萨地块北部去申拉组火山岩:班公湖-怒江特提斯洋南向俯冲的产物?

Qushenla Formation volcanic rocks in north Lhasa block:Products of Bangong Co-Nujiang Tethy's southward subduction

拉萨地块广泛分布有中生代的岩浆活动,研究它们对于认识特提斯洋的演化和理解整个青藏高原的形成过程有着重要的启示。本文首次对出露于拉萨地块北缘的去申拉组火山岩进行了详细的主、微量及同位素地球化学研究,结果显示去申拉组火山岩主要包括亚碱性的玄武岩、玄武安山岩、安山岩,主体为中钾钙碱性系列;其明显亏损高场强元素(Nb、Ta、Ti等),相对富集大离子亲石元素(Rb、Ba、Sr等),具有岛弧火山岩的特征;稀土元素配分模式主体表现为轻稀土元素富集右倾型,少数具有较平坦的配分模式,无Eu异常;具有低Sr、高Nd(εNd:0.32～5.29)同位素特征,表明其源于亏损的地幔源区,暗示其为俯冲带地幔楔部分熔融的产物。结合区域地质资料及前人的研究成果,认为去申拉组火山岩代表了特提斯班公湖-怒江洋盆沿拉萨地块北缘南向俯冲消减的产物,从而为中生代班公湖-怒江洋盆的南向俯冲消减提供了直接的岩石学证据。

Studying the Mesozoic volcanic rocks widely exposed in Lhasa block can better understand the geological evolution of the Tethys and construction of the whole Tibet Plateau. In present study,major,trace element compositions and isotopic results of Qushenla Formation in north Lhasa block have been reported firstly. The volcanic rocks from Qushenla Formation mainly consist of medium-K calc-alkaline series,including basalt,basalt-andesite and andesite. In the spider diagram,they show the character of enrichment of LILE (Rb,Ba,Th,U) and depletion of HFSE (Nb,Ta,Ti),similar to those of an island arc volcanic rock. The REE patterns show enrichment of LREE,including some flat ones,no Eu anomaly. Low Sr isotopic compositions and relatively high Nd isotopic compositions (especially positive εNd:0.32~5.29) provide the clearly evidences that these volcanic rocks derived from the depleted mantle wedge. Considering field survey data and previous research results,we suggest that the Qushenla Formation volcanic rocks were resulted from the southward subduction of Bangong Co-Nujiang ocean,which provide direct petrology evidences of Bangong Co-Nujiang ocean's southward subduction in Mesozoic.