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Petrology of an arc-Oceanic Crust Contact Zone in the Laohushan Back-arc Basin, the Eastern Section of the North Qilian Mountains, NW China

引用

Acta Geologica Sinica(English Edition) 2002年第1期76卷 1-14页

作者：杨宏仪吴燕民吴才来 Department of Earth Sciences Cheng Kung University Tainan 70101 China Institute of Geology Chinese Academy of Sciences Beijing 100037 China

A contact zone sandwiched between an arc and an oceanic crust was discoveredin the Laohushan area in the present study. It consists of a series of north-dipping imbricatedthrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 kmin an E-W direction and measures about 1-3 km wide. Lithologically, it can be divided into foursubzones. Subzone 1 consists of meta-andesite and metasandstone; subzone 2, psammitic schists;subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro,epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: lowgreenschist facies in subzone 1 - > high greenschist facies in subzone 2 - > low amphibolite fadesin subzone 3 - > epidote amphibolite facies in subzone 4. Petrographic and geochemical evidenceshows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceaniccrustal rocks. The metamorphic mineral assemblages and especially mineral chemistry of the bluishgreen amphibole from the pelitic schists and epidote amphibolite of subzone 4 suggest that the rocksof the contact zone were metamorphosed at a pressure of up to 0.69 GPa. It is thought that theLate-Mid Ordovician oceanic lithosphere of a back-arc basin was underthrust northerly beneath an arcto a depth of 20-23 km, where the basaltic rocks and gabbro were converted to epidote amphiboliteand metagabbro respectively. Then, the root rocks of the arc and these metamorphosed oceanic rockswere brought up to shallower depths by thrust faults to form a contact zone between the arc and theoceanic crust in the Laohushan area.

关键词： contact zone thrust sheets metamorphism arc rocks ocean crust mini-suture

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