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

Abstract

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-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 AE 1.0 Ma prior to the India-Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 AE 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 AE 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. GeochronologyInsight into the tectonothermal evolution of the DNCV region has been obtained through multisystem geochronology performed on various lithological

Did Oligocene crustal thickening precede basin development in northern Thailand? A geochronological reassessment of Doi Inthanon and Doi Suthep

“…Ratios of Pb/Pb, Pb/U and Pb/Th were normalized to monazite reference materials Manangotry monazite (559 ± 1 Ma ID-TIMS age; Paquette et al, 1994); Stern (512.1 ± 1.9 Ma, ID-TIMS age; Palin et al, 2013b); and Moacyr (~ 515.6 ± 1.4 Ma, ID-TIMS age, Palin et al, 2013b).…”

“…Candidate monazite grains were then analyzed at NIGL following the methodology described in Palin et al (2013b).…”

Low–temperature eclogite–facies bauxite from the Red River shear zone in Vietnam: Its evolution and significance

“…These reports imply that younger monazite grains in the analyzed samples were most probably formed by fluid infiltration at the Cretaceous period. Although such ages were rarely reported from the Red River shear zone, a recent investigation reported~83-69 Ma as monazite U-Pb lower intercept ages from high-grade metamorphic rocks in the eastern belt (Palin et al, 2013). Based on the internal structures of monazite grains and the age of granitoids in northern Vietnam (87.3 ± 1.2 Ma; Roger et al, 2012), the age was speculated as the timing of fluid-assisted monazite recrystallization due to the emplacement of granitoids (Palin et al, 2013), which is consistent with our interpretation.…”

“…Pelitic gneisses are widespread and garnet + sillimanite is one of the most common assemblages. Peak metamorphic conditions for the pelitic gneiss have previously been estimated to be 0.5-0.7 GPa at 630-720°C (Nam et al, 1998), 0.7-0.9 GPa at 780-830°C (Gilley et al, 2003), and 0.85 GPa at 800°C (Palin et al, 2013); all corresponding to amphibolite-to granulite-facies conditions. These P-T estimates were obtained from the eastern side of the Red River ( Fig.…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…It is one of the few methods that allows investigation of the petrological and chemical evolution of deep-crustal rocks, as it provides an opportunity to define equilibrium relationships between melt and solid mineral species for particular bulk compositions at a given set of P-T conditions e.g., [13,14]. Several previous studies have utilised this technique to explore the effects of reintegrating melt back into lower-crustal rocks [16,18,19,41,78,79]. However, rarely do studies attempt to assess the veracity of the melt reintegration process to create valid subsolidus rock compositions.…”