Two-stage partial melting and contrasting cooling history within the Higher Himalayan Crystalline Sequence in the far-eastern Nepal Himalaya

Abstract

The timing of partial melting and the pressure–temperature (P–T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P–T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil+Grt+Bt+Kfs+Pl+Qtz and Ky+Sil+Grt+Bt+Ms+Pl+Qtz migmatites at different structural level of the HHCS display ages of c. 33–28Ma (Early Oligocene) and c. 21–18Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P=c. 7–10kbar and T=c. 730–780°C, and at P=c. 8–14kbar and T=c. 720–770°C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27–23Ma (Late Oligocene) and at c. 18–16Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P=c. 4–7kbar and T=c. 650–725°C. Early Miocene Ms–Bt leucogranites with two successively overgrown zircon rims at c. 18.3±0.3Ma and c. 16.3±0.2Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P–T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P–T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30–40°C/My) than the Early Oligocene (c. 15–25°C/My) migmatites, inferred from peak-T conditions and FT (c. 6Ma for both migmatites) and U–Pb zircon ages. The P–T–t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.