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中國東部大別山造山帶熱定年學研究及其在構造演化之意義

Thermochronological Study of the Dabie Orogenic Belt in East China and Its Tectonic Implictions

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摘要


本文針對大別山造山帶內出露的片麻岩、火成岩脈及火山岩,進行氬-氬定年及地球化學分析工髼。研物定年結果得到~241-91 Ma的年代,代表岩體接受峰值變質作用及抬升冷卻的年代。岩脈及火山岩記錄了~148-100 Ma的冷卻年代,此結果與造山帶內廣布的駐鎂-超鐵鎂質及花崗岩質深成岩具相同的年代,隱示晚侏羅紀至早白堊紀(~148-100 Ma),造山帶內發生大規模的岩漿活動。由年代與封存溫度得到的冷卻曲線,顯示造山炎各岩石構造單位具有幾乎一致的冷卻曲線及三階段的冷卻歷史。冷卻曲線的一致性,示造山帶在三疊紀至白堊紀以整體的型式自地殼深度剝蝕隆露至地表,並具有5.6-17.7、1.3-2.7及5.0-9.8˚C/My等快、慢、快三階段的冷卻歷史。火山岩及深成岩體的地球化學特徵顯示為一高鉀質鉀玄岩系及高鉀鈣鹼岩系的岩石組合,同時微量元素具大離子半徑元素富集、高場力元素貧乏及鉛正異常現象,並有地殼物質混染的情形,類似大陸地區岩石圈張裂環境下,引發岩石圈地函部份熔融的產物,為張裂型的地質環境。 綜而言之,大別山造山帶岩層的隆露由二個不同的地質環境所完成的。早期為壓縮作用,可能源於三疊紀至早侏羅紀時的板塊隱沒作用,同時產生超高壓變質作用,並使超高壓岩層隆露至中、下地殼;早侏羅紀至早白堊紀期間,超高壓岩層大致是停滯在中、下地殼;而晚侏羅紀或早白堊紀時,由於岩石圈地函的去層作用,或因隱沒板塊之斷脫作用,導致岩石圈地函上湧,引發大規模的岩漿活動,使得岩石圈處於伸張環境。由於高壓及超高壓岩層晚期冷卻作用的發生時間,與岩石圈伸張作用的時間相符,因此,推論岩石圈伸張作用,是使得大別山造山帶超高壓/高壓變質岩從中、下地殼快速隆露至地表的主要機制。

並列摘要


This study has performed systematic Ar-Ar dating and geochemical analyses on minerals separates from gneisses, dyke rocks and volcanics across the Dabie ultrahigh pressure (UHP) metamorphic terrane. The measured ages of minerals separated from gneisses have the ages of 241-91 Ma that May represent cooling ages of the metamorphic rocks after peak metamorphism. The ages of the dyke rocks at~150-100 Ma are consisitent with those of granitic and gabbroic intrusives in the same terrane dated by others. Thus, a regional-scale, Jurassic-Cretaceous, magmatic activity can therefore be well defined. The cooling paths evidently imply that the Dabie Shan terrane was exhumed as a “single” tectonic block. The shoshonitic and bi-modal volcanics, with characteristic features of LREE enrichment and negative Nb, Ta, and Ti anomalies in the spidergrams, are quite similar to those of “post-orogenic” magmatic rocks in an extensional tectonic regime. In summary, durig Triassic to early Jurassic, continental crust was subducted to mantle depth resulting in the ultrahigh pressure metamorphism and afterward been exhumed to the middle or lower crust under the compressional tectonic regime. From early Jurassic to early Cretaceous, the UHPM terrane staed in the middle or lower crustal depth. On late Jurassic or early Cretaceous, there was a large-scale magmatic activities in the orogen that resulted from partial melting of the lithosphere due to “lithospheric delamination”, “removal of thickened lithospheric mantle” or “slab breakoff” during mountain collapse. This has caused the exhumation of the ultrahigh-pressure metamorphic rocks from the middle-lower crustal depths to the surface.

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