古岛弧地体的俯冲是南羌塘增生杂岩形成的重要机制:来自日湾茶卡洋岛的证据

Subduction of the Paleozoic intra-oceanic arc terrane as an important mechanism for the formation of the South Qiangtang accretionary complex: evidence from Riwanchaka intra-oceanic arc

关键词:  日湾茶卡组  洋内岛弧地体  南羌塘增生杂岩  碎屑锆石

Key words:  Riwanchaka Formation  Intra-oceanic arc terrane  south Qiangtang accretionary complex  detrital zircon

摘要:

洋内岛弧及微陆块的俯冲增生是形成增生杂岩的重要机制。本文通过对南羌塘地区日湾茶卡组进行野外实测地质剖面，开展沉积特征、古生物化石、碎屑组分模式、碎屑锆石测年等研究，发现：（1）日湾茶卡为近源沉积，最年轻碎屑锆石年龄峰值为325~375 Ma，但在龙木错-双湖古特提斯大洋周边陆块均未发现源区，其真正物源应为其下伏的望果山组火山岩；（2）日湾茶卡组内珊瑚化石丰度虽然高，但分异度非常低，其沉积位置应是一个相对突出的孤立位置。根据日湾茶卡组下伏望果山组火山岩所具有的洋内岛弧地球化学特征，并与同期SSZ型蛇绿岩组成的类似洋内俯冲的大地构造体系对比，本文认为日湾茶卡组与其下伏的望果山组火山岩共同组成了泥盆纪—石炭纪由洋内俯冲形成的古岛弧地体。根据碎屑锆石分布型式的相似性，本文进一步认为猫儿山地区部分南羌塘增生杂岩的源岩为日湾茶卡组。因此，日湾茶卡洋岛应曾经历过俯冲增生作用：浅部发生前端“刮削作用”形成冈玛错地区有变形但无变质的日湾茶卡组及望果山组，俯冲到深部的日湾茶卡组则发生高压变质作用并在后期折返至增生杂岩的浅部层次。因此，本文认为在南羌塘增生杂岩的形成过程中，日湾茶卡古岛弧地体的俯冲与增生也起到了重要的作用。

Abstract:

Subduction and accretion of intra-oceanic island arc and microcontinent are important mechanisms for the formation of accretionary complex.The present paper deals, on the basis of field measurements of the Riwanchaka Formation in south Qiangtang, with sedimentary facies, fossils, detrital modal analysis and detrital zircon dating. According to regional comparison, we conclude that: (1) the Riwanchaka Formation is not derived from adjacent terranes surrounding Longmu Co-Shuanghu Paleo-Tethys, but from the underlying volcanic rocks of Wangguoshan Formation. (2) Although the coral fossils in the Riwanchaka Formation are abundant, the differentiation is extremely low, suggesting that its depositional position should be a relatively isolated location. In the light of the geochemical signatures of intra-oceanic arc of the Wangguoshan Formation, which should belong to an intra-oceanic system with the SSZ ophiolites with similar age, we suggest that they are originally a Devonian-Carboniferous ancient arc terrane that developed by intra-oceanic subduction.In terms of the similarity of the detrital zircondistribution patterns, we suggest that the part of the South Qiangtang accretionary complex in Maoershan region should be derived from the Riwanchaka Formation.Therefore, the Riwanchaka intra-oceanic arc terrane should have experienced subduction and accretion during the formation of south Qiangtang accretionary complex. The frontal offscrapping resulted in the deformation of the Riwanchaka Formation and Wangguoshan Formationinin the shallow part of the accretionary wedge. While in the deeper part of it, the Riwanchaka Formation was underthrustedand experienced high-pressure metamorphism, and exhumed by later tectonic process.Therefore, the subduction and accretion of the ancient intra-oceanic arc terrane played an important role in the formation of the accretionary complex.