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岩石学报 2010
Zircon SHRIMP U-Pb dating and its geological significance of Lujiacun quartz-monzonite porphyry in Shangri-la County, northwestern Yunnan Province, China
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Abstract:
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.
