苏鲁高压—超高压变质带的折返构造及折返机制

Exhumation Structure and Mechanism of the Sulu Ultrahigh-pressure Metamorphic Belt, Central China

苏鲁高压—超高压变质带的折返构造是由韧性剪切叠覆构造岩片组成,具NWW—SEE向剪切矢量及SEE向NWW的剪切指向,与折返构造伴随的高压和超高压退变质反应过程与石英从高温—中温—低温的组构模式吻合。150～100 Ma期间的伸展事件包含了北界韧性伸展转换性剪切带及莱阳盆地的形成、苏鲁高压—超高压变质带北部花岗岩侵位、折返面理弯曲形成背形构造及伴随的韧-脆性正滑构造。多学科的综合研究表明,240～220 Ma扬子板块巨量物质往北深俯冲于北中国板块之下,220～200 Ma高压—超高压变质岩石整体快速折返,折返板片中保存的自上而下变质岩石单元序列与剪切叠覆构造岩片的物质组成序列基本一致。提出苏鲁高压—超高压变质折返板片呈上拱的舌形体,变形分解表明苏鲁高压—超高压变质板片是在“挤出”机制下折返及受后期伸展事件的改造。

The exhumation structure of the Sulu high-ultrahigh-pressure (HP-UHP) metamorphic terrain consists of superimposed ductile shear slices. The stretching lineations are WNW-ESE and the shearing direction is from ESE to WNW. The UHP retrograde metamorphic reaction accompanying with the exhumation structure is coincident with the quartz LPO models from high temperatures (>650℃) through middle temperatures (400 ~650℃) to lower temperature (<400℃). During the period of 150~100 Ma, the extension events involved the formation of both the transtension shear zone at the northern boundary and the Laiyang Cretaceous basin, the emplacement of the granites, and the formation of an anticline form with ductile-brittle normal slip structures due to curved exhumed foliations. Integrated multidisciplinary investigations indicate that a huge amount of material from the Yangtze plate was subducted northwards beneath the North China plate in 220~ 240 Ma, and then the whole HP-UHP slab was exhumed rapidly in 220 ~ 200 Ma. That is why the metamorphic rock sequence from top to bottom preserved in the exhumed slab is coincident with the rock sequence in the superimposed shear slices. Such seismic data reveal that the appearance of the exhumed Sulu HP-UHP metamorphic slab is characterized by a doming tongue structure. Deformation decomposition shows that the Sulu HP-UHP slab was exhumed by extrusion mechanism and was modified by the post-exhumation extension event.