华北中部造山带五台-恒山地区古元古代变质作用与构造演化
Paleoproterozoic Metamorphism and Tectonic Evolution in Wutai-Hengshan Region, Trans-North China Orogen

华北克拉通古元古代造山带的时空分布与构造属性尚有很大争论，一种观点认为华北克拉通从新太古代至古元古代受大洋俯冲闭合控制，在1.85～1.95Ga之间先后发生3次陆-陆碰撞形成3条造山带，即孔兹岩带、胶-辽-吉带和中部造山带；另一种观点强调华北克拉通这3条元古代造山带在1.80～1.98Ga之间经历了相同的漫长演化过程，指示当时特有的热俯冲和碰撞环境.通过总结中部造山带中的五台-恒山地区变质作用研究进展，阐述该区古元古代造山时代与构造属性.五台-恒山地区的主要变质岩石-构造单元包括恒山杂岩、五台杂岩和滹沱群.恒山杂岩和五台杂岩主体为新太古代TTG片麻岩和表壳岩，它们在古元古代晚期经历了两期变质作用改造.第一期变质作用为中压型，是由于陆-陆碰撞导致弧后伸展盆地闭合、地壳加厚造山所致，从南向北形成一个递增序列：包括五台群下部和南恒山杂岩南部的低角闪岩相、南恒山杂岩北部高角闪岩相和北恒山杂岩的高压麻粒岩相，其压力峰期所对应的地热梯度为～15℃/km.变质锆石所记录的年龄峰值随着变质程度增高而降低，依次为～1.95Ga、～1.92Ga和～1.85Ga，这是因为在变质过程中锆石生长受流体和熔体行为控制：在亚固相线下，变质锆石可记录峰期变质年龄，而在超固相线条件下记录伴随熔体结晶的退变质年龄.由此确定该区中压相系变质作用压力峰期时间为～1.95Ga，对应地壳加厚造山的峰期.加厚地壳由于重力均衡导致变质岩从深部地壳折返至中部地壳，在P-T轨迹上表现为压力峰期之后发生等温减压(ITD)至0.5～0.7GPa，岩相学上表现为峰期石榴石分解形成斜长石“白眼圈”等，指示缺流体条件.南恒山北部高角闪岩相岩石中的变质锆石记录的折返时间为～1.92Ga，指示第一次造山结束.第二期变质作用为中-低压型，系为板内变形所致，表现为折返至中地壳的岩石伴随挤压型剪切变形和流体注入形成平衡矿物组合.朱家坊韧性剪切带就是这次板内变形的强构造域，其中也记录了顺时针型P-T轨迹，但所反映的地壳加厚程度有限，第二期变质-变形峰期时间为～1.85Ga.由于朱家坊韧性剪切带左行走滑，导致北恒山麻粒岩地体抬升.五台-恒山地区在1.80～1.96Ga之间经历两期变质-变形事件，这一认识或对讨论华北克拉通其他地区的古元古代造山带演化有一定启示意义.
The tempospatial distribution and tectonic attribute of the Paleoproterozoic orogens of the North China Craton (NCC) are strongly controversial. One model argues that three times of continental collision occurred sequentially during the period of 1.85-1.95Ga in the NCC, resulted in the formation of three Paleoproterozoic orogens, i.e. the Khondalite belt, Jiao-Liao-Ji belt and the Trans North China orogen (TNCO). The other model contests that the above three orogens may have experienced the same long lived evolutional history within the period of 1.80-1.98Ga, suggesting particular hot subduction and collision in Paleoproterozoic. Summarizing the recent advances of the study on metamorphism in the Wutai-Hengshan region, this paper presents the time and tectonic attribute of the Paleoproterozoic orogeny. The main litho-structural units in the Wutai-Hengshan region include the Hengshan complex, Wutai complex and Hutuo Group. The Hengshan and Wutai complexes are recognized to have undergone two phases of metamorphism. The first phase was identified to be a medium-pressure type, which was attributed to resulting from a crust thickening orogeny related to the closure of back arc basins due to continental collision. A progressive metamorphic series can be revealed, including, from south to north, the low amphibolite facies of the lower Wut