A geochronological and petrological study of anatectic paragneiss and associated granite dykes from the Day Nui Con Voi metamorphic core complex, North Vietnam: constraints on the timing of metamorphism within the Red River shear zone

The Red River shear zone (RRSZ) is a major left-lateral strike-slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike-slip and normal faults, which stretches for >1000km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high-grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial IndiaAsia collision. Analysis of a weakly deformed anatectic paragneiss using PT pseudosections constructed in the MnONa2OCaOK2OFeOMgOAl2O3SiO2H2OTiO2O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ UThPb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of similar to 805 degrees C and similar to 8.5kbar between c. 38 and 34Ma. Exhumation occurred along a steep retrograde PT path with final melt crystallizing at the solidus at similar to 5.5kbar at similar to 790 degrees C. Further exhumation at similar to 640700 degrees C and similar to 45kbar at c. 31Ma occurred at subsolidus conditions. UPb geochronological analysis of monazite from a strongly deformed pre-kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 +/- 1.0Ma prior to the IndiaAsia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 +/- 0.4Ma. Monazite grains from a cross-cutting post-kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 +/- 1.4Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 8369Ma is suggested to be the result of fluid-assisted recrystallization following the emplacement of magmatic units.