古亚洲洋俯冲板片断离与后造山伸展:贺根山缝合带火山岩年代学和地球化学证据

Paleo-Asian Ocean subducted slab breakoff and post orogenic extension: evidence from geochronology and geochemistry of volcanic rocks in the Hegenshan suture zone

本文以出露于贺根山缝合带迪彦庙蛇绿混杂岩北侧的乌兰拜其白音高老组流纹岩-粗面岩为研究对象,通过野外地质调查和岩石学、地球化学、锆石U-Pb年代学研究,探讨岩石成因类型、构造环境和古亚洲洋俯冲板片断离与后造山伸展过程。岩石地球化学研究表明,乌兰拜其地区白音高老组流纹岩具有较高的SiO2、Al2O3、K2O和Na2O+K2O含量,以及较高的Ga/Al比值,相对贫CaO、MgO、Sr、Ba、Eu、Ti和P,稀土元素配分曲线为右倾海鸥式分布,负铕异常显著。该区粗面岩属于钾玄岩系列,具有高碱(Na2O+K2O)、高K2O、高Al2O3、低TiO2,富集Rb、Ba、U和轻稀土元素,亏损Nb、Ta和Ti,稀土元素配分曲线为右倾式分布。岩石学和岩石地球化学特征表明,乌兰拜其地区白音高老组流纹岩为迪彦庙-白音布拉格蛇绿混杂岩带早白垩世后造山A型花岗岩浆的喷出相-A型流纹岩,其形成于后造山伸展构造环境;粗面岩为后造山型钾玄质火山岩,与洋壳俯冲作用有关,形成于俯冲板片断离-后造山伸展构造背景。流纹岩和粗面岩锆石LA-ICP-MS U-Pb同位素测年,获得^(206)Pb/^(238)U加权平均年龄分别为140.4±1.2Ma和134.9±1.1Ma,表明白音高老组流纹岩-粗面岩喷发于早白垩世,反映了贺根山缝合带早白垩世后造山A型花岗岩浆-钾玄质岩浆作用事件。结合贺根山缝合带的壳幔电性结构特征和晚古生代蛇绿岩-岛弧岩浆岩、中生代后造山A型岩浆岩的时空分布与演化关系,认为根山缝合带三叠纪-早白垩世后造山伸展岩浆作用与古亚洲洋俯冲板片断离作用有关,提出了该区后造山A型花岗岩浆-钾玄质岩浆作用的俯冲板片断离-后造山伸展地球动力学模式。

Based on field geological surveying, petrology, geochemistry and LA-ICP-MS zircon U-Pb geochronology of the Wulanbaiqi rhyolite and trachyte of the Baiyingaolao Formation exposed in the northern part of the Diyanmiao ophiolitic melange zone of the Hegenshan suture, this paper discusses the petrogenesis and tectonic setting of the volcanic rocks, and the slab breakoff-post orogenic extension of the Paleo-Asian Ocean. Petrogeochemical studies show that the rhyolite of the Baiyingaolao Formation in Wulanbaiqi area has high SiO2, Al2O3, K2O and Na2O+K2O contents, higher Ga/Al ratios but poor in CaO, MgO, Sr, Ba, Eu, Ti and P contents. The chondrite normalized REE distribution patterns in rhyolites are of right-inclined Gull-wing shape with remarkably negative Eu anomalies. The trachyte belong to shoshonite series, which is high in Na2O+K2O, K2O and Al2O3 and low in TiO2, obviously enriched in Rb, Ba, U and LREEs, and depleted in Nb, Ta and Ti. The chondrite normalized REE distribution patterns in the trachytes are of right-inclined shape. Petrological and geochemical characteristics show that the rhyolites were the extrusive igneous rocks(A-type rhyolites) of the Early Cretaceous post orogenic A-type granite magma which occur in the Diyanmiao-Baiyinbulage ophiolitic melange zone of the Hegenshan suture, formed in post-orogenic extensional tectonic setting. The trachytes are the post orogenic shoshonitic rocks, were related to the previous oceanic subduction, and formed in the subsequent subducted slab breakoff-post orogenic extension tectonic setting. The zircon LA-ICP-MS U-Pb dating for the rhyolite and trachyte yielded a weighted mean 206Pb/238U age of 140.4±1.2 Ma and 134.9±1.1 Ma respectively, indicating that the rhyolite and trachyte of the Baiyingaolao Formation erupted in the Early Cretaceous, reflecting the Early Cretaceous post orogenic A-type and shoshonitic magmatism event in the Hegenshan suture zone. Combined with the crust-upper mantle electrical structure in the Hegenshan suture and the time-space evloution of the Carboniferous ophiolites, Carboniferous-Permian island arc rocks and Triassic-Early Cretaceous post orogenic A-type magmatic rocks in the Erenhot-Hegenshan suture, it is considered that post orogenic extensional tectonic-magmatism of the Erenhot-Hegenshan suture zone was related to the subducted slab break off of the Paleo-Asian Ocean during the Triassic-Early Cretaceous. The geodynamic model of the subducted slab break off-post orogenic extension of the post orogenic A-type granitic and shoshonitic magmatism is proposed.