THERMAL PULSE INDUCED BY EMPLACEMENT OF RAMBA LEUCOGRANITES IN SOUTHERN TIBET

The thermal histories of Himalayan leucogranites provide critical information to unravel post-collisional geodynamic processes. The Ramba Dome is located at the intersection of the Tethyan Himalaya leucogranite belt and Yadong-Gulu Rift, which hosts several generations of granitic intrusions. Of these, the 8 Ma two-mica granites and garnet leucogranite dykes are the youngest endmember of Himalayan leucogranites. A contact aureole develops in the metasedimentary country rocks that yield identical 8 Ma metamorphic ages and show the same deformation as the granites (Liu et al., 2014 Lithos). In this study, we focus on the carbonaceous staurolite schist at ~1.7 km from the intrusion-country rock contact, to constrain the thermal history of the magmatic intrusion that marked the cessation of leucogranite magmatism. The schist contains euhedral garnet and staurolite porphyroblasts, in a foliated matrix of muscovite + biotite + chlorite + plagioclase + quartz + graphite. The garnet features a distinctive bell-shaped Mn profile; Mg#[=Mg/(Mg+Fe)] increases toward the rims where a minor downturn is seen. The compositional maps of garnet display patchy dendritic high-Ca domains, and inherent high-Na zones aligned parallel to the foliation. In contrast, the staurolite shows weak compositional variations from the inclusion-free core to inclusion-rich rim. In particular its, ZnO contents scatter between 0.18 and 0.25 wt.% with no well-defined trends. In a graphite-bearing equilibrium phase diagram, the isopleths of garnet rim intersect at 542 °C, 2.9 kbar, consistent with an independent thermometer based on the Raman spectra of carbonaceous materials (RSCM; 543.5 ± 5.4 °C). The P-T condition lies within the narrow low- variance zone of g + st + bi + mu + chl + q, bracketed by the staurolite-in and chlorite-out boundaries, indicating minimal overstepping of staurolite nucleation and growth. On the other hand, the garnet high-Ca domains near the core indicate 530 °C at 2.8 kbar, about 45 °C higher than the garnet-in boundary (~485 °C). The overstepping corresponds to a chemical affinity 500 J/mol O for garnet nucleation, comparable to previous estimates. The sharp boundaries of the high-Ca domains suggest limited diffusion modification at the peak temperature. Given a step-function initial condition, diffusion simulation of the profiles yields a time scale ~1.4 Myr. The short thermal pulse involves advective heat transfer by leucogranites emplacement, followed by rapid cooling and exhumation toward the end of Himalayan magmatism.