HP and UHT metamorphic associations in the Day Nui Con Voi，northwestern Vietnam: The consequence of convergence between the Indian and the Eurasian plates

Abstract:

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 Cenozoic RRSZ activity. The DNCV in northwestern Vietnam predominantly consists of HP granulite facies metapelites, and lenticular enclaves of HP mafic granulites, 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. The peak assemblage of the HP metapelites includes garnet + kyanite + ternary feldspar + quartz + rutile. The peak assemblage of the HP mafic granulites is garnet + clinopyroxene + plagioclase + quartz. Meanwhile, the peak assemblage of the calc-silicate granulites is high-Ca garnet (Grs>50) + diopside + scapolite + quartz. On the other hand, characteristic UHT metamorphic assemblage occur as the coexistence of spinel and quartz which indicates the metamorphic temperature is higher than 900℃. The peak assemblage of the UHT rocks in DNCV is corundum + spinel + garnet + sillimanite + quartz.

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 30Ma. 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.