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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 >1000 km 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 India- Asia collision. Analysis of a weakly deformed anatectic paragneiss using P- T pseudosections constructed in the MnO- Na2O- CaO- K2O- FeO- MgO- Al2O3- SiO2- H2O- TiO2- O ( MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U-Th-Pb 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 ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P-T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640-700 °C and ~4-5 kbar at c. 31 Ma occurred at subsolidus conditions. U-Pb 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 ± 1.0 Ma prior to the India- Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross-cutting post-kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83-69 Ma is suggested to be the result of fluid-assisted recrystallization following the emplacement of magmatic units. [ABSTRACT FROM AUTHOR]

Zircons gneisses and migmatites collected from the Antarctic Peninsula have different core–rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent. Two samples contain inherited zircon that is partially dissolved. In these samples, the 176Hf/177Hf rations of the inherited zircon and new magmatic zircon rims are, on average, indistinguishable and consistent with in situ melting. In such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source. Variation in 176Hf/177Hf within the magmatic zircon rims from grain to grain suggests that Hf isotopes were only partially homogenized during melt migration; alternatively, zircon growth may have taken place within small volumes of partial melt. Other samples do not preserve textural evidence for zircon dissolution during melt generation; in these samples the 176Hf/177Hf values of the inherited zircon and new magmatic zircon rims are different. The zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon. Whether this relates to a separate juvenile source or, alternatively, is derived from minerals other than zircon at the source, cannot be resolved. Inherited zircon, irrespective of age, has been strongly influenced by the reworking of a juvenile Late Mesoproterozoic source, suggesting that such crust underlies the Antarctic Peninsula. Our results therefore suggest that Hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts. [ABSTRACT FROM AUTHOR]