T201 From the Micro to Macro in Metamorphic Geology: Constraining Tectono-Metamorphic Processes with High-Resolution Approaches

Thursday, 29 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

THERMAL PULSE INDUCED BY EMPLACEMENT OF RAMBA LEUCOGRANITES IN SOUTHERN TIBET

Paper number: T201-14
Program: Technical Sessions
Time: 4:30 PM – 4:40 PM

Authors

  • Ozan Akca Presenting Author
    University of Toronto, Toronto, ON
  • Xu Chu
    University of Toronto Department of Earth Science, Toronto, ON
  • Ivano Gennaro
    University of Toronto, Toronto, ON

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-356906

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