Two phases of Paleoproterozoic metamorphism in the Zhujiafang ductile shear zone of the Hengshan Complex: Insights into the tectonic evolution of the North China Craton
Two phases of Paleoproterozoic metamorphism in the Zhujiafang ductile shear zone of the Hengshan Complex: Insights into the tectonic evolution of the North China Craton
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横山杂岩朱家坊韧性剪切带古元古代两期变质作用：洞察华北克拉通构造演化
DOI：
10.1016/j.lithos.2019.02.001
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发表时间：
2019
期刊：
影响因子：
3.5
通讯作者：
Zhang Jian
中科院分区：
文献类型：
--
作者：
Qian Jiahui;Yin Changqing;Wei Chunjing;Zhang Jian
朱家坊韧性剪切带位于横山杂岩中部，是研究华北造山带古元古代构造演化的关键地区。对朱家坊韧性剪切带中的变长角闪岩和石榴角闪岩进行了详细的变质作用和年代学分析。变质岩中保存有两期矿物组合：石榴子石云母片岩中早期蓝晶石-金红石组合和晚期绿泥石-十字石组合;硅线石云母片麻岩中包裹体型白云母（高硅）+蓝晶石组合和晚期硅线石组合。石榴子石在变岩偶尔表现出明显的两阶段分带特征的扩散核与不规则的镁铝榴石（Xpy）和钙铝榴石（Xgr）的内容和生长边缘与Xpy和Xgr向外增加。P-T假剖面中最大Xgrin石榴石核和包裹体型白云母中Si含量的等值线表明，早期矿物组合经历了中高压型变质作用，压力高达12-14 kbar，温度为700-750 °C。晚期组合和石榴石环的生长分带预示了晚期分离的顺时针P-T轨迹，峰值条件为6.5 ± 0.2 kbar/620 ± 10 °C（中低压型）。石榴角闪岩主要由石榴子石、角闪岩、斜长石、钛铁矿和石英组成，除局部的冕状结构外，无晚期矿物组合的叠加。相模拟表明，岩石经历了高角闪岩相变质作用，峰值条件为10.5 ± 0.8 kbar/770 ± 50 °C，这与变岩的早期变质作用大致一致。变岩锆石U-Pb定年得到1.96- 1.92Ga和1.87- 1.86Ga两个变质年龄组，分别代表两个变质阶段的时间。衡山-五台地区可能分别在1.95Ga和1.85Ga发生了两次独立的造山事件。造山带的老阶段可能是大陆碰撞的结果，而新阶段可能是板内变形的结果。形成于较老造山带（1.95Ga）的高级岩石的最终折返应与较年轻的变形/变质事件有关。
The Zhujiafang ductile shear zone is located in the central Hengshan Complex, a key area for understanding the Paleoproterozoic tectonic evolution of the Trans-North China Orogen, North China Craton. Detailed metamorphic and geochronological analyses were carried out on metapelite and garnet amphibolite from the Zhujiafang ductile shear zone. The metapelite preserves two phases of mineral assemblages: early kyanite-rutile-bearing assemblage and late chlorite-staurolite-bearing assemblage in garnet–mica schist, and inclusion-type muscovite (high-Si) + kyanite assemblage and late sillimanite-bearing assemblage in sillimanite–mica gneiss. Garnet in the metapelite occasionally exhibits pronounced two-stage zoning characteristic of a diffusion core with irregular pyrope (Xpy) and grossular (Xgr) contents and a growth rim with Xpyand Xgrincreasing outwards. The isopleths of the maximum Xgrin garnet core and Si content in inclusion-type muscovite in the P–T pseudosections suggest that the early mineral assemblages underwent medium-high-pressure type metamorphism with pressures up to 12–14 kbar at 700–750 °C. The late assemblages and the growth zoning of garnet rim predict a late separated clockwise P–T path with peak conditions of 6.5 ± 0.2 kbar/620 ± 10 °C (medium-low-pressure type). The garnet amphibolite is mainly composed of garnet, hornblende, plagioclase, ilmenite and quartz, without overprinting of late mineral assemblages except for localized corona textures. Phase modeling suggests that the rock has experienced high-amphibolite facies metamorphism with peak conditions of 10.5 ± 0.8 kbar/770 ± 50 °C, which is broadly consistent with the early-phase metamorphism of metapelite. Zircon Usingle bondPb dating on metapelite yields two metamorphic age groups of 1.96–1.92 Ga and 1.87–1.86 Ga which are interpreted to represent the timing of the two separated phases of metamorphism. Two separated orogenic events may have occurred respectively at ~1.95 Ga and ~1.85 Ga in the Hengshan–Wutai area. The older phase of orogeny was resulted from continental collision and the younger phase of orogeny may be caused by within-plate deformation. The final exhumation of the high-grade rocks formed in the older (i.e. 1.95 Ga) orogeny should be related with the younger deformation/metamorphic event.