POST-LATE MESOZOIC TECTONIC EVOLUTION OF THE NORTHERN MOGOK METAMORPHIC BELT (MMB) AND NEO-TETHYS: INSIGHTS FROM MIGMATITE COMPLEX STAGNATION

Migmatites are extensively found throughout the northern Mogok metamorphic belt (MMB) in the southern Tengchong block of western Yunnan, China. These migmatites illustrate crustal reworking and petrological diversity in the middle crust. High-grade metamorphism and inversion tectonics document melt and residue separation during various tectonic stages of the MMB. Petrological and geochemical analyses were conducted on amphibolite, granodiorite, gneiss, and leucogranite near the Donghe pluton. U-Pb dating of leucogranitic gneiss (06CG20) indicates a sedimentary origin from Cretaceous granites (124 Ma and 73 Ma), transforming into S-type granite around 65 Ma, followed by sillimanite-grade migmatization between 45 and 40 Ma.

Geochemical characteristics, such as high SiO₂ content (65-88%), low melting temperatures (666-810 °C) estimated from Al₂O₃/TiO₂ thermometry, and CaO/Na₂O ratios of diatexite (0.56-1.17) and leucogranites (0.16-0.77), indicate these rocks represent the neosome of a regional migmatite complex, originating from the partial melting of 65 Ma S-type granite within the Meso-Tethys accretionary wedge. Low SiO₂ content (50-60%), high melting temperatures (920-1100 °C), and CaO/Na₂O ratios of metadiorite (4.35) and amphibolite (1.40-2.03) suggest these rocks are the concentrated restite with hornblende dehydration breakdown reactions within the melanosome. Diatexites in the neosome and mesosome generally have high Na₂O+CaO but low K₂O compared to anatectic melts and melanosomes.

Thermal chronological history shows a cooling path from approximately 900 °C at 65 Ma to 750 °C at 53 Ma, then stabilizing between 750-700 °C from 53 to 35 Ma. The 65 Ma magmatism is linked to the Neo-Tethys slab subduction. Migmatization occurred from 65 Ma to 40 Ma, concurrent with the Donghe detachment (D₂) due to crustal thinning and mantle upwelling as the Neo-Tethys slab rolled back and broke off. The Donghe detachment reactivated as a thrust system (D₃) between 35 and 25 Ma due to the collision of Greater India with the Tengchong block.

The key to crustal reworking is the tectonic inversion from extensional detachment to thrusting during the Eocene. This inversion led to the remelting and differentiation of sediments weathered from subduction-related magmatic rocks, transforming the Tengchong block from an island arc into a metamorphic rock basement (MMB).