Structural and metamorphic evolution of the Day Nui Con Voi massif, Northeast Vietnam

Abstract

The Day Nui Con Voi massif in North Vietnam is the southernmost part of the Red River Shear Zone in Southeast Asia. It is a narrow NW-SE trending high-grade metamorphic zone composed of amphibolite, migmatite, mylonite and gneiss. Structural analyses suggest that Day Nui Con Voi experienced at least 3 generations of ductile deformation. The oldest Dn-2 event formed NNW-SSE striking upright folds. The Sn-2 foliations parallel sillimanite and mymerkite indicated the Dn-2 event should occurred around amphibolite facies (>600 °C) metamorphic condition. The Dn-1 event is associated with subhorizontal folds with subhorizontal fold axial plane Sn-1 within the central region of the massif. The mineral assemblages of sillimanite + biotite + quartz + plagioclase + K-feldpar + muscovite suggest that the temperature of Dn-1 deformation is around 600 to 550°C. The Dn-1 structures are refolded by Dn, which formed a dome like structure with NW-SE striking Sn fold axial plane. Sillimanite boudinage necks filled by muscovite indicates that the Dn happened at lower temperature (<550oC). The youngest ductile deformation formed mylonite zone with left-lateral kinematic indicators on both limbs of the dome. Structural and petrological analyses suggest that left-lateral shearing happen synchronously with doming event (Dn deformation). Our new 40Ar/39Ar age data shows the cooling age of hornblende about 26 Ma, phlogopite from 23 to 24 Ma and K-feldspar from 21 to 23 Ma. Muscovite represents the age of both cooling and crystallization ages from 22 to 24 Ma. Basing on different age of different minerals relating to different closure temperatures, cooling history of the Day Nui Con Voi massif is reconstructed. Combining results of petrological analyses and cooling history, we constrains that left-lateral shearing event likely occurred during 26 ~ 22 Ma causing rapid cooling of the massif. From these conclusions, we propose that strike slip movement along Red River Shear Zone may not relate to opening of the South China Sea.The Day Nui Con Voi massif in North Vietnam is the southernmost part of the Red River Shear Zone in Southeast Asia. It is a narrow NW-SE trending high-grade metamorphic zone composed of amphibolite, migmatite, mylonite and gneiss. Structural analyses suggest that Day Nui Con Voi experienced at least 3 generations of ductile deformation. The oldest Dn-2 event formed NNW-SSE striking upright folds. The Sn-2 foliations parallel sillimanite and mymerkite indicated the Dn-2 event should occurred around amphibolite facies (>600 °C) metamorphic condition. The Dn-1 event is associated with subhorizontal folds with subhorizontal fold axial plane Sn-1 within the central region of the massif. The mineral assemblages of sillimanite + biotite + quartz + plagioclase + K-feldpar + muscovite suggest that the temperature of Dn-1 deformation is around 600 to 550°C. The Dn-1 structures are refolded by Dn, which formed a dome like structure with NW-SE striking Sn fold axial plane. Sillimanite boudinage necks filled by muscovite indicates that the Dn happened at lower temperature (<550oC). The youngest ductile deformation formed mylonite zone with left-lateral kinematic indicators on both limbs of the dome. Structural and petrological analyses suggest that left-lateral shearing happen synchronously with doming event (Dn deformation). Our new 40Ar/39Ar age data shows the cooling age of hornblende about 26 Ma, phlogopite from 23 to 24 Ma and K-feldspar from 21 to 23 Ma. Muscovite represents the age of both cooling and crystallization ages from 22 to 24 Ma. Basing on different age of different minerals relating to different closure temperatures, cooling history of the Day Nui Con Voi massif is reconstructed. Combining results of petrological analyses and cooling history, we constrains that left-lateral shearing event likely occurred during 26 ~ 22 Ma causing rapid cooling of the massif. From these conclusions, we propose that strike slip movement along Red River Shear Zone may not relate to opening of the South China Sea.