扬子板块西北缘碧口地块变形变质作用序列——松潘—甘孜造山带伸展垮塌事件的意义

Deformation and Metamorphism Sequence of Bikou Terrane in the Northwest Margin of Yangtze Plate:Implications for Extension Collapse and Transition of Songpan-Garze Orogenic Belt

碧口地块位于秦岭-大别、松潘-甘孜造山带与扬子板块的交汇部位。根据碧口地区各套地层构造变形样式的不同，将地层自下而上划分为三个构造地层单元：①新元古界基底杂岩，主要由新元古界碧口群火山岩系和横丹群浊积岩系组成，杂岩体核部变形较弱，边部发育韧性剪切变形，并以一条由超糜棱岩带组成的基底拆离断层为顶界，与上覆地层分隔。基底杂岩经历了蓝片岩相到硬柱石-钠长石-绿泥石相的变质作用。②古生界中间韧性流变层，由志留系-泥盆系的糜棱岩或千糜岩组成，岩层中发育多期构造面理（S1、S2、S3）和典型的固态流变构造，变质程度普遍达中绿片岩相。③中生界沉积盖层，包括碧口地块以北的三叠系关家沟组和小面积出露的侏罗系-白垩系，以脆性破碎带和小型脆性断层构造为主。拆离断层与各套构造地层中的韧性剪切和流变构造-同反映了区域性上盘向SSE剪切的几何学和运动学特征，证实了碧口地区中生代206～154Ma伸展垮塌事件的存在，从而建立了碧口地块的隆升模式。研究结果表明，松潘-甘孜在中生代造山作用后的区域伸展作用下形成了拆离断层和中地壳韧性流变层，造成了地层缺失和地壳减薄，基底杂岩在造山带的腹陆隆升并露出地表。

The Bikou terrane lies at the junction of Qinling orogenic belt, Songpan-Garze orogenic belt and Yangtze Plate. Three tecton-stratigraphic units have been identified within this terrane. （1） the Neoproterozoic basement complex, composed mainly of volcanic rocks of Bikou Group and turbidite of Hengdan Group, has weak deformation in the core but strong ductile shear deformation along the margin. The basement is separated from its overlying strata by a detachment fault composed of ultramylonite. Metamorphic grade ranges from blueschist facies to lawsonite-albite -chlorite facies within the basement complex. （2） the Paleozoic middle ductile slab is composed of Silurian-Devonian mylonite and phyllonite, in which develops three stages of foliations （S1, S2, S3）. The middle ductile slab underwent metamorphism of mid-greenschist facies. （3） the Mesozoic sedimentary cover, including Triassic Guanjiagou Formation to the north of Bikou terrane and Jurassic-Cretaceous in some small areas, is characterized by brittle fault and fracture belt without metamorphism. The geometry and kinematics features of up to-the-SSE shear within the detachment fault and strata prove that regional extension collapse developed in Bikou area in Mesozoic, 206-154Ma. In conclusion, detachment fault and middle ductile slab resulted from the regional extension during the Mesozoic orogenesis in Songpan-Garze, which caused thinning and selective removal of the strata, as well as the uplift and exhumation of the basement complex in the hinterland of orogenic belt.