Cenozoic Thermal Evolution And Exhumation Of The Day Nui Con Voi Massif In Northwestern Vietnam

The Red River shear zone(RRSZ)is the most significant geologic discontinuity in Southeast Asia.The Day Nui Con Voi(DNCV)massif at the southernmost part of the shear zone bears records of the Cenozoic RRSZ activity.The whole DNCV massif went though strongly shear deformation and high-grade metamorphism.It developed strongly mylonitizion in its early stage of progressive shear deformation,and different scales of A-type folds and regional dome structure were formed by the bending of mylonite foliation in its late stage of progressive shear deformation.The development of a large number of different scales of A-type folds throughout the shear deformation indicated that the shear deformation of DNCV was accompanied by compression simultaneously.The massif in northwestern Vietnam predominantly consists of HP granulite facies metapelites,and lenticular enclaves of HP mafic granulites and calc-silicate granulites,and UHT aluminum-rich rocks in the metapelites.HP granulites are characterized by the key associations of garnet-kyanite-ternary feldspar(in metapelites)and garnet-clinopyroxene-plagioclase-quartz(in mafic granulites).In the latter case,rocks are typically orthopyroxene-free.On the other hand,characteristic UHT metamorphic assemblages occur as the coexistence of spinel and quartz which indicates the metamorphic temperature is higher than 900?.P-T conditions of peak metamorphism within HP granulite facies are: T = 750~850?,P = 12.5~14kbar.The temperature of UHT metamorphism is higher than 900?,the pressure is higher than 9~9.4 kbar.SHRIMP U-Pb dating of zircons from the HP mafic granulites and UHT rocks gave minimum ages of ca.59 Ma and 40 Ma,respectively.HP metapelites gave ca.60~50Ma and 30 Ma.The above facts suggest the DNCV massif experienced HP granulite facies at its peak metamorphism at ?59 Ma.Thermal disturbance on the local scale to UHT metamorphism occurred at ?40 Ma.Then isothermal decompression to medium-pressure granulite facies occurred at ?30Ma,which may reflect a rapid exhumation.The HP rocks are interpreted to be resulted from crustal thickening due to far-field compression during Indian-Eurasian convergence.However the high heat flow for UHT metamorphism was possibly attributed to upwelling of asthenosphere triggered by delamination of the thickened crust.The delamination may also be responsible for subsequent generation of high potassic magmas along the RRSZ belt.The low-temperature exhumation of DNCV included two stages,namely rapid exhumation in the early stage(24~28Ma,cooling rate is 37.5?/My)and slow exhumation in the late stage(24Ma till now,cooling rate is 3.13?/My),which is suggested by the Fission Track dating of Apatite and Zircon.During the rapid exhumation stage,transverse and longitudinal uneven cooling rates may be related to the non-uniformity of dome structure formed by the partial material flow.Early stage of DNCV rapid exhumation was directly related to the tectonic uplift-cooling of induced by RRSZ shear deformation,and late stage of DNCV slow exhumation due to of uncovering and cooling during deep crustal rapid uplift.The time node of DNCV massif transition from rapid exhumation to slow exhumation,ca.24 Ma,marked the end of RRSZ shear deformation in DNCV region.This is in accord with diachroneity of the end of Ailaoshan and Diancangshan regional shear deformation.