| 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 |
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| 引用本文： | 魏春景.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[J].地球科学,2018,43(1):24-43. |
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| 作者姓名： | 魏春景 |
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| 作者单位： | 造山带与地壳演化教育部重点实验室, 北京大学地球与空间科学学院, 北京 100871 |
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| 基金项目： | 国家自然科学基金41172055国家自然科学基金41430207 |
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| 摘 要： | 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1.85~1.95 Ga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1.80~1.98 Ga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~1.95 Ga、~1.92 Ga和~1.85 Ga，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~1.95 Ga，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0.5~0.7 GPa，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~1.92 Ga，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~1.85 Ga.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1.80~1.96 Ga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.
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| 关 键 词： | 变质作用 P-T轨迹 恒山杂岩 五台杂岩 构造演化 华北中部造山带 岩石学 |
| 收稿时间： | 2017-08-22 |
Paleoproterozoic Metamorphism and Tectonic Evolution in Wutai-Hengshan Region,Trans-North China Orogen |
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| Abstract: | The tempo-spatial 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 1.85-1.95 Ga 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 1.80-1.98 Ga, 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 Wutai Group and the southern part of the south Hengshan complex, the high-amphibolite facies of the northern part of the south Hengshan complex and the high-pressure granulite facies of the north Hengshan complex, with their pressure peak conditions corresponding to a geothermal gradient of~15℃/km. With increasing in metamorphic grade, the peak ages from metamorphic zircons show a decreasing trend, which are~1.95 Ga, ~1.92 Ga and~1.85 Ga for the low-amphibolite, high-amphibolite and granulite facies rocks respectively. This is because zircon growth during metamorphism depends on the behaviors of fluids and melts, which results in that metamorphic zircons tend to record the time of the peak stages for metamorphism under subsolidus conditions, but to date the retrograde stage with melt crystallization during metamorphism under suprasolidus conditions. As a result, the pressure peak stage of the medium-pressure type metamorphism is determined to be~1.95 Ga, also equivalent to the peak stage of the crust-thickening orogeny. Due to gravity isostasy in the thickening crust region, metamorphic rocks may exhume from deep crust to middle crust, displaying as the post-peak isothermal decompression (ITD) to 0.5-0.7 GPa in P-T paths, and suggested from the 'white eye socket' textures formed by the decomposition of peak garnet under fluid-absent conditions. The metamorphic zircon ages of 1.92 Ga from the high-amphibolite facies rocks of the northern part of the south Hengshan complex were interpreted to represent the decompression of medium-pressure rocks, also constraining the termination of the first-phase orogeny. The second phase of metamorphism is inferred to be a low-pressure type, resulted from deformation within-plate, and displaying as the formation of equilibrium assemblages that overprinted the exhumed medium-pressure rocks triggered by compressional shearing and fluid infiltration. The Zhujiafang shearing belt could be a high-strain zone produced in this within-plate deformation, which also records a clockwise P-T path, but indicating limited crustal thickening. The second phase of metamorphism and deformation was constrained to occur mainly at~1.85 Ga. The sinistral strike-slip of the Zhujiafang shearing belt may have resulted in the uplifting of the north Hengshan granulite terranes. Two phases of metamorphism and deformation were proposed to have occurred within the period of 1.80-1.96 Ga in the Wutai-Hengshan region, which could be significant for discussing the Paleoproterozoic orogenic evolution from the other domains in the NCC. |
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