大陆板片多重性俯冲与折返的动力学模式——苏鲁高压超高压变质地体的折返年龄限定

A dynamic model for sequential subduction and exhumation of a continental slab: Age Constraints on the timing of exhumation of the Sulu HP-UHP metamorphic terrane

苏鲁高压-超高压变质地体自南而北由高压(HP)、很高压(VHP)和超高压(UHP)变质叠覆岩片组成,前者依次叠覆在后者之上,岩片之间的界限为韧性剪切带.根据超高压变质岩片中角闪岩相岩石与高压变质岩片中绿片岩相岩石的黑云母和白云母Ar-Ar和Rb-Sr测年新结果,结合前人在该区所做的锆石SHRIMP U-Pb、全岩Sm-Nd、Rb-Sr等测年数据综合分析表明,超高压变质岩石的峰期变质年龄为240～220 Ma,折返年龄为220～200Ma;而高压变质岩石的峰期变质年龄大于258 Ma,起始折返年龄为258 240 Ma,折返年龄比超高压变质岩石早30～40Ma.这说明扬子板片并不是整体俯冲和折返的.由于具组分和密度差异,俯冲板块的不同部位沿岩性或构造界面先后分片俯冲和折返,在北苏鲁超高压变质板片开始俯冲时,南苏鲁高压变质板片已开始折返.

From south to north, three parallel metamorphic imbricated slices compose the Sulu high to ultrahigh pressure metamorphic terrane. They are high pressure （HP） slice, very high pressure （VHP） slice, and ultrahigh pressure （UHP） slice, scparated by ductile shear zones. Using ^40Ar/^39Ar and whole rock Rb-Sr techniques, the authors analyzed micas from amphibolite facies rocks in the UHP slice and from greenschist facies rocks in the HP slice to constrain the timing of exhumation of the Sulu HP-UHP metamorphic belt. Together with previous results from zircon U/Pb, whole rock Sm-Nd and Rb-Sr measurements, the data obtained show that peak metamorphism and subsequent exhumation in the UHP zone occurred in 240--220 Ma and 220-200 Ma, respectively. In the liP zone, however, peak metamorphism and the onset of exhumation occurred earlier than 258 Ma and in 258--240 Ma, respectively. These data have led us to propose a new model for the formation and exhumation of the Sulu HP-UHP rocks. In this model, .sequential subduction and exhumation of different parts of the Yangtze slab occurred in different periods, depending upon vertical variations in the physical properties of the subducted continental crust.