北澜沧江结合带晚白垩世花岗岩成因及其地球动力学意义

Genesis and Its Geodynamic Significance of Late Cretaceous Granites in North Lancang River Suture

摘要

北澜沧江结合带花岗岩的研究主要集中于印支期，对白垩纪花岗岩却鲜有报道.在野外地质调查基础上，对藏东类乌齐地区新发现的花岗岩进行了岩石地球化学、锆石U-Pb定年和Hf同位素研究.结果显示，2件花岗岩样品成岩年龄分别为75.06±0.82 Ma（MSWD=1.90）、74.89±0.65 Ma（MSWD=1.05），为晚白垩世岩浆活动的产物；花岗岩含白云母（~5%），具高SiO₂（69.07%~69.39%）、富K₂O（5.31%~5.77%）、低Mg^#（0.30~0.33）的特点，A/CNK比值为1.11~1.15，富集大离子亲石元素和LREE，亏损高场强元素和HREE，轻、重稀土元素强烈分馏，负Eu异常显著，属过铝质S型花岗岩，其源岩为变泥质岩和变质杂砂岩.岩石具分布较为分散的锆石ε _Hf（t）值（-4.6~1.1）和古老的Hf同位素二阶段模式年龄（T _DM2，1.07~1.43 Ga）.综合分析认为，北澜沧江结合带晚白垩世花岗岩是拉萨-南羌塘地体后碰撞伸展构造背景下，由加厚岩石圈拆沉引发软流圈物质上涌和减压熔融形成的幔源镁铁质岩浆底侵于古老地壳，诱发其部分熔融，并与之混合形成的母岩浆，再经历一定程度分离结晶作用而形成，指示晚白垩世藏东类乌齐地区造山演化处于后期造山带破坏阶段.

Abstract

Studies on granites in the North Lancang River suture were mainly focused on the Indosinian period, but little on the Cretaceous. Based on the field geological survey, the petrogeochemistry, zircon U-Pb dating and Hf isotope of the newly discovered granites in Leiwuqi area, East Tibet are carried out. The results show that the diagenetic ages of the two granites are 75.06±0.82 Ma (MSWD=1.90) and 74.89±0.65 Ma (MSWD=1.05), respectively, which are the products of Late Cretaceous magmatic activity. The granites contain muscovite (~5%) and are characterized by high SiO₂ (69.07%-69.39%), rich K₂O (5.31%-5.77%) and low Mg^# (0.30-0.33), with A/CNK ratio of 1.11-1.15, and are enriched in large ion lithophile elements (LILE, e.g., Rb, Th, U and Pb) and LREE, depleted in high field strength elements (HFSE, e.g., Ba, Nb, Sr, P and Ti) and HREE, with strong fractionation of light and heavy rare earth elements and significant negative Eu anomaly. These features suggest that the granites belong to the peraluminaceous S-type granite, and the source rocks are metamorphic argillaceous rocks and greywacke. The zircon ε _Hf(t) value of the granite (-4.6 to 1.1) varies widely and are of ancient two-stage Hf isotopic model ages (T _DM2, 1.07-1.43 Ga). Comprehensive analysis shows that the Late Cretaceous granites in the North Lancang River suture were generated by a degree of separation crystallization of parent magma, derived from mantle derived mafic magma formed by the upwelling and decompression melting of asthenosphere materials caused by the delamination of the thickened lithosphere, underplating into the ancient crust, inducing the melting of the ancient crust and mixing with it, which indicates that the orogenic evolution of the Leiwuqi area in East Tibet was in the late orogenic destruction stage during Late Cretaceous.

关键词

S型花岗岩 / 晚白垩世 / 后碰撞 / 地球化学 / 锆石U-Pb定年与Hf同位素 / 北澜沧江结合带 / 岩石学

Key words

S-type granite / Late Cretaceous / post-collision / geochemistry / zircon U-Pb dating and Hf isotope / North Lancang River suture / petrology

基金资助

中国地质调查局项目(DD20230449;DD20221741)

国家自然科学基金项目(91955208)

第二次青藏高原综合科学考察研究(2019QZKK0902)

中国地质科学院探矿工艺研究所科研结余资金项目(2022004)