青藏高原羌塘中部蜈蚣山印支期花岗岩的地球化学特征和黑云母40Ar-39Ar年龄

本松错岩基是羌塘中部规模最大的花岗岩复合岩基,面积超过1800km2,由石炭纪、三叠纪和侏罗纪3个不同时代的花岗岩岩体组成,记录了羌塘中部不同时期的岩浆活动,是研究羌塘盆地构造演化的重要窗口。蜈蚣山花岗岩位于本松错复合岩基北部,前人认为其时代为侏罗纪,但是近期在蜈蚣山地区侏罗纪花岗岩中发现有少量印支期花岗岩出露,岩性主要为花岗片麻岩和二长花岗岩,可能为侏罗纪花岗岩的捕虏体。地球化学研究表明,二长花岗岩属高钾钙碱性过铝质花岗岩,形成于同碰撞环境,与区域内其它印支期中酸性岩浆岩类似,共同构成龙木错-双湖-澜沧江板块缝合带同碰撞—后碰撞岩浆弧。此外还对花岗片麻岩片麻理中的黑云母做了40Ar-39Ar测年,获得了175.8Ma±1.1Ma的定年结果,与其围岩侏罗纪花岗岩年龄相近,推测花岗片麻岩是印支期花岗岩受后期侵入的侏罗纪岩浆改造后的产物,本松错复合岩基应当是中酸性岩浆岩多期侵入的产物。

Geochemical and 40Ar-39Ar isotope chronological characteristics of Wugongshan Indo-Chinese granites in central Qiangtang, Tibetan Plateau

Bensongcuo batholith occupies an area of over 1800km2 and is the greatest granite multiple batholith in central Qiangtang. It consists of granites of three different ages (Carboniferous, Triassic and Jurassic) and possesses material records of magmatism of different epochs, so it can serve as a key showcase for studying the tectonic setting of Qiangtang basin. Wugongshan granites, located in the north of Bensongcuo batholith, were previously thought to have been formed in Jurassic; nevertheless, a few Indo-Chinese granites composed mainly of monzogranite and granitic gneiss were recently found in Wugongshan area, which may be inclusions of Jurassic granites. Geochemical studies indicate that monzogranites are high-potassium peraluminous calc-alkaline granites formed in collisional tectonic setting, similar to other Indosinian intermediated-acid rocks in this area. These intermediated-acid rocks compose the collisional or post-collisional magmatic arc of the Lungmu Co-Shuanghu-Lancang River suture zone. Moreover, 40Ar-39Ar dating was carried out for biotites from the granite gneiss and the dating results show that the granitic gneiss resulted from the reformation of Indo-Chinese granites by Jurassic magmatism at 175.8±1.1Ma, and hence the Bensongcuo batholith should serve as the material record of multi-stage intrusion of intermediate-acid magmatic rocks.