藏南林周盆地设兴组砂岩及其中玄武岩夹层的地球化学与成因

印度与亚洲大陆的碰撞是青藏高原演化的重要构造事件,碰撞过程被记录在拉萨地块南部的晚白垩世到古新世的沉积-岩浆作用中。林周盆地的晚白垩世设兴组及其之后不整合覆盖的林子宗火山岩,是解析碰撞过程的重要记录。本文对设兴组最高层位的砂岩和玄武岩夹层进行了岩石学、地球化学和年代学研究,探讨了岩石成因和构造意义。设兴组砂岩属于杂砂岩,碎屑物质主要来自中酸性岩浆岩源区;锆石Hf同位素指示设兴组大部分碎屑物质来源于盆地北面的中部拉萨地块,少部分来自盆地南部的冈底斯岩基;砂岩中最年轻的碎屑锆石年龄指示林周盆地设兴组是在98Ma之后接受沉积的。以夹层产出在设兴组顶部的玄武岩和玄武安山岩,富集轻稀土元素、亏损重稀土元素、弱负Eu异常,强烈富集Ba、Th、U、Pb等大离子亲石元素,显著亏损Nb、Ta等高场强元素,属于高钾钙碱性玄武岩系列,与典型安第斯型玄武岩特征吻合。玄武岩和玄武安山岩的锆石均为捕获锆石,其最年轻碎屑锆石年龄限定了设兴组玄武岩的喷发晚于110Ma。综合分析表明,林周盆地晚白垩世时期为夹持在冈底斯岩浆弧与中部拉萨地块之间的弧后盆地,新特提斯洋壳晚白垩世俯冲到冈底斯弧和弧后盆地之下,大约在98～110Ma之后喷发到林周盆地的很少量中基性岩浆构成了设兴组顶部的玄武岩和玄武安山岩夹层,是新特提斯洋俯冲相关的幔源岩浆作用。林周盆地设兴组(晚于98Ma)与上覆的林子宗火山岩(底部约为65Ma)之间呈大约33Myr的构造间断,可能代表了冈底斯弧的碰撞之前的隆升剥蚀过程。

Geochemistry and petrogenesis of sandstones and their basaltic interlayers of Shexing Formation from Linzhou basin, South Tibet

The collision between India and Asia was the most important tectonic event in the evolution of the Tibetan Plateau, which was recorded in the Late Cretaceous to Paleogene sedimentary and magmatic rocks in the southern Lhasa terrane. The Late Cretaceous Shexing Formation strata and later eruption of the Linzizong volcanic succession (ca. 65~45Ma) is the best candidate to show the collisional processes. This study provides new petrological, geochemical and zircon U-Pb geochronological data on the sandstones and basaltic interlayers collected from the highest level of Shexing Formation in the Linzhou basin in southern Tibet. The materials of the Shexing greywacke were from the felsic magmatic source region. Hf isotope implies that the majority of the sediments came from the central Lhasa terrane to the north of the basin, and others from the Gangdese arc batholiths to the south of the basin. The youngest age of the detrital zircons in Shexing sandstone constrains the deposition were later than 98Ma in Linzhou basin. The interlayers close to the top of the Shexing Formation are basalts and basaltic andesites, with enriched light rare earth elements (REE), depleted heavy REE, weak negative Eu anomaly, strongly enriched large ionic lithophile elements (LILEs, such as Ba, Th, U, Pb), and depleted high-field strength elements (HFSEs, such as Nb, Ta). These basalts and basaltic andesites are high potassium calc-alkaline basalt series, with the affinity of the typical Andes-type basalts from continental arc. The detrital zircons from these interlayers define their eruption to be later than 110Ma. Our results suggest that the Linzhou basin is the back-arc tectonic setting, which located between the Gangdese arc and the central Lhasa terrane during the Late Cretaceous. The Neo-Tethys Ocean subducted beneath the Gangdese arc and the back-arc system and triggered the mantle partial melting to produce the eruption of the basalts and basaltic andesites after 98~110Ma. The interval of 33Myr between the upper most Shexing Formation (98Ma) and the Linzizong volcanics (65Ma), may represent the uplift and erosion processes of the Gangdese arc before the collision of India and Asia.