Zircon U-Pb geochonology and Hf isotopes of the Early Cretaceous Rena-Co rhyolites from southern margin of Qiangtang, Tibet, and their implications
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摘要: 在青藏高原的演化历史中,班公湖-怒江洋的俯冲方向一直存在争议。现有的岩浆作用时空展布表明,大量早白垩的岩浆作用分布在班公湖-怒江缝合带(以下简称为班怒带)以南,但是近年来在该缝合带以北也发现了少量同时代的岩浆作用。本文研究了靠近班公湖-怒江蛇绿岩带以北、改则县北部热那错地区的流纹岩,获得了岩石的锆石U-Pb年龄和Hf同位素成分。热那错流纹岩年龄为~110Ma,与相邻地区报道的岩浆岩活动和缝合带以南的北拉萨地体地区大范围出露的早白垩世岩浆岩同期产出。岩石具有不均一且偏正的εHf(t)特征,与北拉萨地体同期岩浆岩Hf同位素成分相似。本文综合考虑了班怒带两侧发育同期岩浆活动、且南侧极大量而北侧很少量发育的特征,认为热那错流纹岩的成因可以置于班公湖-怒江洋向南俯冲的总体模式中,南向俯冲的班公湖怒江岩石圈在~110Ma发生板片断离,可以同时解释分布于缝合带两侧的早白垩世岩浆活动。Abstract: The direction of subduction of Bangong Lake-Nujiang oceanic crust has long been a debate in the topic of geological evolution history of Tibetan Plateau. Informed research shows that large amount of Cretaceous igneous activity spreads widely on northern Lhasa terrane, south of Bangong Lake-Nujiang suture zone. However, small magmatic activity of the same era was discovered on the north side of the suture zone. An isotopic and geochronologic study of this paper presents zircon LA-ICP-MS U-Pb data and Hf isotopic composition of Rena-Co rhyolites from the north side of Bangong Lake-Nujiang suture zone, north of Gerze. It shows that Rena-Co rhyolites erupted and crystallized at the age of ~110Ma, which is the similar era to the reported igneous rocks nearby and large Cretaceous magmatic activity of northern Lhasa subterrane. εHf(t) values of Rena-Co rhyolites vary within a wide range and most of them are positive, which are comparable with those igneous rocks of northern Lhasa subterrane in the same era. The magmatic activity in the same era is discovered on both sides of the suture zone, and the scale in the south is much larger than that in the north, indicating that the petrogenesis of Rena-Co rhyolites is related to the south direction subduction of Bangong Lake-Nujiang ocean floor and the break-off of the subducted slab at ~110Ma. This model explains Cretaceous magmatic activity on both sides of Bangong Lake-Nujiang suture.
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