小秦岭金矿田中生代构造演化与矿床形成

作为中国金矿主产地之一,小秦岭变质核杂岩经历两期不同性质的伸展。第一期为沿周缘拆离断层发育、方向与造山带平行的同造山伸展,上盘向WNW运动,活动时代为距今135～123Ma,属燕山期陆内造山形成的地壳增厚和岩浆活动共同作用的结果。第二期为退化变质糜棱岩带和正断层组成的变质核杂岩内部伸展构造,代表造山后进一步隆升导致的垮塌,时代为距今120～106Ma。小秦岭变质核杂岩内部发育与垮塌伸展同期的挤压性逆冲断层,由造山后残余挤压作用和构造剥蚀导致的伸展驱动力降低所致。小秦岭中蚀变千糜岩型金矿受退化变质糜棱岩带控制,成因为典型的伸展控矿机制,石英脉型金矿产于内部逆冲断层,成矿机制与小秦岭变质核杂岩垮塌伸展过程中的构造反转相关。

MESOZOIC TECTONIC EVOLUTION AND ORE-DEPOSITS FORMATION IN THE GOLD MINE FIELD OF XIAOQINLING

The Xiaoqinling metamorphic core complex experienced two stages of extension with different mechanism. The first stage took place along the detachment fault developing along the margin of the complex. This stage of extension occurred simultaneously with the orogeny and its direction was parallel with the orogenic belt, i.e., the upper plate moved toward WNW. Ages for this stage were 135 ～ 123 Ma ago, which was resulted from the crustal thickening and magmatism caused by the Yanshanian intraplate orogenic movement. The second stage extension formed retrograded mylonitic zones and normal faults within the complex, which represented the collapse of the complex caused by the further uplift in an age ranging from 120 to 106 Ma ago. The contractional thrusting occurred in Xiaoqinling at the same time as the collapse extension within the complex. It was caused by the residual orogenic compression and the reduction in extensional driving force due to tectonic denudation. In Xiaoqinling, the altered phyllonitic gold deposits were controlled by the retrograded mylonitic zones and their mineralization was dominated by the extensional tectonics. The quartz-vein gold deposits were formed in the thrusts within the Xiaoqinling metamorphic core complex. Their formation might have a direct relation to the tectonic inversion during the collapse of the Xiaoqinling metamorphic core complex.