胶东地区玲珑双向变质核杂岩的均衡隆升:运动学涡度的指示OA北大核心CSTPCD

Early Cretaceous extensional structures around the Linglong batholith in the northwestern Jiaodong Peninsula belongs to a bivergent metamorphic core complex(MCC).The MCC is bounded by the Zhaoping and Jiaojia detachment shear zones in the east and west respectively.It is not clear whether or not isostatic rising also contributes to exhumation of the bivergent MCC.In this study,kinematic vorticity measurements were conducted to mylonite samples from different localities of the two boundary shear zones by means of the rigid porphyroclast aspect ratio(RPAR)methods,(including therigid grain net and wallis plot methods)and the quartz c-axis fabrics-finite strain ratio(RXZ/β)method.The Zhaoping shear zone gives vorticity values of 0.57～0.73 from the RPAR measurements,and 0.80～0.93 from the RXZ/βmeasurements.The Jiaojia shear zone yields vorticity values of 0.58～0.74 from the RPAR measurements,and 0.79～0.93 from the RXZ/β measurements.For the same mylonite samples,the two methods give different vorticity values.The two shear zones give higher thinning rates from the RPAR measurements than those from the RXZ/β measurements.Theoretical analysis and other examples suggest that vorticity values obtained from the RPAR measurements represent earlier deformation whereas those obtained from the RXZ/β measurements indicate later deformation.It is suggested therefore that the boundary detachment shear zones in the Linglong bivergent MCC evolved with reduction in pure shear component due to isostatic rising and resultant steepening of the shear zones caused.The isostatic rising took place at a later stage of the Linglong MCC evolution,and resulted from emplacement of the syn-tectonic,Guojialing-stage granodiorites of the Early Cretaceous age.Our results show that both bivergent and asymmetrical MCCs share similar evolution with isostatic rising at their later stages that has important contribution to exhumation of middle-lower crust.