北祁连南缘右行韧性走滑剪切带的同位素年代学及其地质意义

Geochronology of the dextral strike ductile shear zone in south margin of the Northern Qilian Mountains and its geological significance

北祁连南缘右行韧性走滑剪切带位于祁连地块与北祁连俯冲碰撞杂岩带边界 ,长约 80 0km ,走向NWW SEE ,面理向北陡倾 ,中西部宽 5～ 6km ,东部由四条呈帚状撒开的强应变带组成。构造指向及向南东低角度倾伏的拉伸线理揭示出韧性剪切带的右行走滑和转换挤压性质。TIMS法测定的单颗粒锆石U Pb上交点年龄为 96 5～ 95 6Ma ,代表韧性剪切带原岩———基底变质岩的变质时代。糜棱岩中钾长石、黑云母单矿物40 Ar/ 3 9Ar同位素测年结果及与地层和岩浆活动的关系表明韧性剪切带形成于 4 4 0～ 380Ma。北祁连南缘右行韧性走滑剪切带是在祁连加里东造山带形成过程中 ,祁连地块与阿拉善地块间斜向碰撞诱发大规模转换挤压作用的产物。

The dextral strike-slip ductile shear zone in the northern Qilian Mountains is located at the boundary between Qilian terrane and north Qilian Caledonian subduction-collision complex belt. It is about 800 km long, 5 to 6 km wide, striking NWW-SEE and dipping north. The dextral strike slipping and transpressive deformation are revealed by the senses of shear and the stretching lineations plunging low-angled to SE. The upper intersection age for single zircon U-Pb isotopic isochrone with TIMS method is 965～956 Ma, which represents the metamorphic age of the metamorphic rock of basement in Qilian block. The results of^(40)Ar/^(39)Ar age dating to the K-feldspar and biotite from mylonites, and the relationships among ductile shear zone, strata and intrusions indicate that the ductile shear zone formed between 440 and 380 Ma. The dextral strike-slip ductile shear zone in south margin of north Qilian Mountains was formed by a large-scale transpression resulted from the continent-continent oblique collision between Qilian block and Alxa block in Qilian Caledonian orogeny.