藏南仲巴地区早白垩世日朗组物源分析及其构造意义

藏南仲巴地区早白垩世日朗组出露于特提斯喜马拉雅北亚带,整体为黄绿色火山岩屑砂岩,局部层位可见页岩与泥岩,分析为一套深海海底扇沉积组合。本文仔细分析了日朗组砂岩岩石学特征及鲍马序列和槽模沉积构造等沉积学特征,结果表明:日朗组砂岩成分成熟度和结构成熟度均不高,具有近源物源的特点;槽模构造古水流数据统计表明古流向由南向北,指示物质组分来源于南侧特提斯喜马拉雅和/或印度克拉通。砂岩碎屑组分统计结果表明日朗组的物源区构造背景属于克拉通内部及石英再旋回区。碎屑锆石U-Pb年龄频谱图对比进一步表明其物源区为印度稳定大陆边缘,外加一套早白垩世火山碎屑物质的输入。仲巴地区日朗组物源特征反映了印度大陆北缘早白垩世由深部断裂引起的一次强烈的火山事件,可能与印度大陆从澳大利亚-南极大陆裂解有关。

Provenance analysis and its tectonic significance of Early Cretaceous Rilang Formation in Zhongba area, southern Tibet

The Early Cretaceous strata, Rilang Formation, exposes in the northern zone of Tethyan Himalaya in Zhongba area, southern Tibet. Rilang Formation is dominated by yellow-green volcaniclastics, but shale and mudstone are also visible in some layer. A detailed study on the sandstone petrologic characteristics and sedimentary characteristics (such as Bouma sequence and flute cast) was carried on. Rilang Formation is a set of deep-sea submarine fan sedimentary assemblage. Both compositional maturity and textural maturity of the sandstone are not high, indicating that Rilang Formation has a proximal provenance. The calculation of paleocurrent data from flute cast shows that the paleocurrent direction of Rilang Formation is from south to north, indicating that the material components came from the south, Tethyan Himalaya and/or Indian craton. The statistical results of sandstone fragment components show that the tectonic setting of provenance may belong to croton interior and quartzose recycled area. Further evidence of detrital zircon U-Pb dating of the sandstone from Rilang Formation proves that Rilang Formation has a mixed sediment source, with material derived both from stable continental margin of Indian Plate and a new source of Lower Cretaceous volcanic rocks. The provenance characteristics of Rilang Formation in Zhongba area is interpreted to be the result of a volcanic event in the northern margin of the Indian Continent during the Early Cretaceous, which was brought about by a deep-seated fracture. This volcanic event was most probably related to the break-up of Indian Continent from the Australia-Antarctic supercontinent.