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Jiankang Li, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Zhenghui Chen, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Denghong Wang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
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Pegmatite deposits with clear mineral zoning, big crystal and enrichment of rare metal strongly illustrate that they usually formed in relatively closed system.
Stable structure environment is necessary for the closed system. Therefore, pegmatite deposits usually form in the stable phase of orogenic process, in reverse they can trace the stable stage of orogenic process. Jiajika, Xuebaoding, Ke¡¯eryin and Danba pegmatite type rare metal or muscovite deposits, locating in western Sichuan province of China, provide a new means to trace the evolvement of Songpan-Ganzi orogenic zone that puzzles geologists until now.
The four deposits underwent bi-directional constringency and folding orogeny raise by collision between blocks in Mesozoic (Xu et al., 1992). Xuebaoding locating in the north of the orogenic zone and Ke¡¯eryin deposit locating in the center-north of the orogenic mainly underwent the contracting stress from north to south produced by the collision between North China Block and Yangtse Block. Jiajika deposit locating in the west of orogenic zone mainly underwent the contracting stress from west to east produced by the collision between Qiangtang-Changdu Block and Yangtse Block. Danba deposit locating in the center-east of the orogenic zone underwent the tectonic stress from north to south and from west to east (Hou et al., 2002). In this paper, the muscovite 40Ar/39Ar dating showed that Jiajika deposit formed in about 198∼195Ma, Xuebaoding deposit formed in about 191∼189Ma, Ke¡¯eryin deposit formed in about 152Ma, and Danba deposit formed in about 125∼114Ma.
By the tracing of pegmatite, the evolvement of Songpan-Ganzi orogenic zone in Mesozoic can be depicted as following. In Indo-Chinese epoch, North China Block and Qiangtang-Changdu Block collided with Yangtse Block respectively. Then, tectonic stress transfered from the outer to inner of the orogenic zone. In this process, Jiajika, Xuebaoding, and Ke¡¯eryin region underwent tectonic and magmatic movement in turn, and came into stable stage successively. In the end, bi-directional tectonic stress converged in Danba region. After tectonic doming and metamorphism, Danba region came to stable stage in the middle-late of Yanshan epoch, too.
The tracing result illustrates that stable state exists during the strong tectogenetic movement and magmatic movement in Songpan-Ganzi orogenic in Mesozoic. These stable periods provide favorable condition for some mineralization. For example, Carlin type gold deposit in the western-north Sichuan, Jinmuda gold deposit in Markan, and Yangliuping Cu-Ni-PGE deposit have similar spatio-temporal condition to Xuebaoding, Ke¡¯eryin, and Danba deposit respectively. Thus, they all formed in stable phase of orogenic process.
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