藏北尼玛地区白垩纪岩浆岩对班公湖-怒江缝合带演化的制约

班公湖-怒江缝合带及其两侧广泛分布白垩纪岩浆岩,这些岩浆活动记录了班公湖-怒江特提斯洋俯冲至闭合以及拉萨-羌塘板块碰撞过程。为了约束该缝合带在早-晚白垩世的演化过程,本文对缝合带中段尼玛地区花岗岩进行岩相学、地球化学、锆石年代学和Hf同位素研究。尼玛北部虾别错花岗岩侵入到中生代地层中,发育石英闪长质包体。锆石U-Pb定年结果表明寄主花岗岩和包体形成于早白垩世(122Ma和121Ma)。这些锆石均具有正的ε_Hf(t)值,分别为+2.4~+7.0和+3.0~+5.1。寄主花岗岩具有高硅和高钾钙碱性特征,属于准铝质-弱过铝质系列。包体相对低硅,属于中钾钙碱性准铝质系列。寄主花岗岩和包体具有相似的微量元素分布,如均亏损Nb、Ta和Ti,富集Th、U和Pb。综合分析,虾别错寄主花岗岩和包体是壳幔熔体混合作用的产物。尼玛南部张乃错花岗岩侵入到古生代地层里。锆石U-Pb年龄为97Ma,形成于晚白垩世。锆石ε_Hf(t)值在+2.2~+6.0之间。张乃错花岗岩具有高硅特征,属于高钾钙碱性弱过铝质系列。岩体显著亏损Ba、Sr、Ti和Eu,富集Rb、Th、U和Pb等元素。该花岗岩来源于新生地壳部分熔融,并在后期经历结晶分异。结合区域地质概况,虾别错早白垩世花岗岩(和包体)形成于班公湖-怒江特提斯洋闭合过程,而张乃错晚白垩世花岗岩形成于洋盆闭合之后拉萨-羌塘板块碰撞背景。尼玛地区早-晚白垩世岩浆活动记录了班公湖-怒江缝合带从洋盆闭合到拉萨-羌塘板块挤压碰撞的演变过程。

Cretaceous magmatic rocks in the Nyima area, North Tibet: Constraints for the tectonic evolution of the Bangong-Nujiang suture zone

Cretaceous magmatism is widely distributed within and on both sides of the Bangong-Nujiang suture zone.These rocks record the subduction to closure history of the Bangong-Nujiang Tethys Ocean and the collisional history between Lhasa and Qiangtang terranes.In this study,petrological,geochemical,and zircon U-Pb and Hf isotopic analysis of granites outcropped in the Nyima area are performed to constrain the evolutionary process of the suture zone during the Early to Late Cretaceous.The Xiabie Co granites,located in the north of Nyima,have quartz dioritic enclaves and intrude into the Mesozoic strata.The host granites and enclaves yielded zircon U-Pb ages of 122 Ma and 121 Ma,respectively,with corresponding positiveε_Hf(t)values ranging from+2.4 to+7.0 and from+3.0 to+5.1.The host granites have high silica,and they are high K calc-alkalineand metaluminous to weakly peraluminous.In contrast,the enclaves have low silica,and are medium K calc-alkalineand metaluminous.Similar trace element distribution is shown in the host granites and enclaves,such as depletion of Nb,Ta and Ti,and enrichment of Th,U and Pb.A comprehensive analysis suggested that the host granites and enclaves were mixing products of crust and mantle-derived magmas.The Zhangnai Co granites,exposed in the south of Nyima,intrude into the Palaeozoic strata.These rocks have zircon U-Pb ages of 97 Ma and positiveεHf(t)values of+2.2~+6.0,indicating that they were formed in the Late Cretaceous.These granites are enriched in silica and belong to high K calc-alkaline and weakly peraluminous series.They are significantly depleted in Ba,Sr,Ti and Eu,and enriched in Rb,Th,U and Pb.The Zhangnai Co granites were derived from partial melting of a juvenile crust,and experienced fractional crystallization.In combination with regional geology,the Xiabie Co granites and enclaves were formed in the closing period of the Bangong-Nujiang Tethys Ocean in the Early Cretaceous,while the Zhangnai Co granites were melting products during the Lhasa-Qiangtang collision in the Late Cretaceous.The Early to Late Cretaceous magmatism thus records the evolutionary history from ocean closure to continental collision between Lhasa and Qiangtang in the Bangong-Nujiang suture zone.