滇西北中甸陆家村石英二长斑岩的锆石SHRIMP定年及其意义

滇西北中甸陆家村石英二长斑岩位于义敦-中甸岛弧带最南端、金沙江断裂带与扬子地台交汇部位,侵入于中三叠统浅变质的砂泥质板岩、泥质粉砂岩及蚀变玄武岩中。锆石SHRIMPU-Pb定年结果表明,两件锆石增生边的年龄分别为35.6±1.4Ma(MSWD=0.36,N=4)、36.7±0.8Ma(MSWD=1.80,N=8),平均为36.2±1.3Ma(MSWD=0.35,N=12),代表了岩浆侵位结晶时代;继承性锆石年龄在413～61.2Ma之间变化,分别代表了早期岩浆侵位和捕获基底围岩碎屑锆石的年龄,反映其多种来源并经历了复杂的地质演化。结合岩体锆石SHRIMP定年结果、产出地质背景及区域对比,认为陆家村石英二长斑岩浆侵位于印-亚大陆晚碰撞阶段,是藏东-金沙江-红河新生代富碱斑岩带的组成部分,其形成与喜马拉雅期陆陆碰撞导致壳幔过渡层大规模侧向迁移引起的地幔上涌和中下地壳部分熔融有关,是青藏高原东缘构造转换带对主碰撞带造山作用过程响应的记录。该认识为深入了解中甸-丽江构造-岩浆-成矿作用提供了重要约束,也确认本区具有斑岩型矿床的找矿潜力。

Zircon SHRIMP U-Pb dating and its geological significance of Lujiacun quartz-monzonite porphyry in Shangri-la County, northwestern Yunnan Province, China

Lujiacun quartz-monzonite porphyry in Shangri-la County of northwestern Yunnan Province, intruded into the Middle-Triassic low-grade metamorphic rocks including slate, argillaceous siltstone and meta-basalt, bounded on the north, southwest and east by the Yidun-Zhongdian arc belt, the Jinsha River tectonic belt, and the Yangzi block respectively. It is obvious that the granitic magma is very important for studying the basement and tectonic evolution of this area. Zircon SHRIMP U-Pb dating on two samples of the quartz-monzonite porphyry show that, the latest growth rim of zircon grains has the concordant ages of 35.6±1.4Ma (MSWD=0.36, N=4) and 36.7±0.8Ma (MSWD=1.80, N=8) respectively, and with the weighted mean age of 36.2±1.3Ma (MSWD=0.35, N=12), representing the magma emplacement and/or crystallization ages for the Lujiacun pluton. However, some of the crystal zircons contain rounded or unregular cores in shape, and it is explained that the rounded zircons were captured from the wall rocks during the upwelling of the granitic magma or detrial zircons from partial melting of the lower crust. Based on the SHRIMP dating, the inherited core zircons yield ages from 413Ma to 61.2Ma, indicates that the Lujiacun quartz-monzonite porphyry has multiple sources which was closely related with the ancient basement rocks and/or metasediments under complicated geodynamic setting. Combinating with its zircon dating results and geological occurrence characteristics, and comparing to the regional evolution sequences, it is strongly reflected that the Lujiacun quartz-monzonite porphyry and associated gold-polymetallic deposits can be the prominent represent of the eastern Xizang-Jinsha River-Red River porphyry belt, formed and emplaced during the later stage of collision between Indian and Eurasian plate, and be controlled by the evolution of Indo-Asian collisional orogen and tectonomagmatism, displayed the long-distance effects in the structural transform zone of the main collisional orogenic setting since the Palaeocene. Episodically stress relaxation during tectonically transforming from transpressional ( 65~40Ma) to transtensional ( 24~17Ma) regimes probably caused multiple magmatic intrusions, which most eventually result in the mantle upwelling caused by the large-scale lateral migration of crust-mantle transitional layer and partial melting of mid- to lower-crustal rocks within the eastern Indo-Asian continental collision zone, and the time about 36Ma is the main reheating event period of the porphyry-hydrothermal metallogenic system. These conclusions provided key constraints for understanding deeply the tectonic-magmatic-metallogenic processes of the Zhongdian-Lijiang porphyry metallogenetic belt. Consequently, there is great gold-polymetallic prospective reserves of porphyry-type deposits in this ore district and surrounding areas.