Exploring hindered decarbonation in contact metamorphism: A glimpse into marble aureoles in Southern Tibet

Abstract

Fluid-driven contact metamorphism in the Gangdese Arc, resulting from Late Cretaceous intrusions, led to a diverse range of aureoles in carbonate sequences in southern Tibet. These include extensive carbon loss in calc-silicate rocks with minor calcite remnants and minimal carbon loss in predominantly pure marble formations. Many studies connecting contact metamorphism and decarbonation flux focus on metacarbonate aureoles comprising calc-silicate assemblages, while marble formations represent a less prominent endmember. To investigate the factors determining decarbonation extent, we analyzed the thermobarometric, geochemical, and isotopic signatures of two representative marble aureoles in southern Tibet. Our findings suggest that isobarically exsolved magmatic fluid from a neighboring ∼64 Ma granodiorite pluton led to limited fluid-rock interaction in the marble aureole. Based on the petrologic observations, we present a conceptual model to elucidate this process. The heating and fluid infiltration were brief, likely due to short-lived intrusion events. Consequently, a thin ductile layer was formed adjacent to the pluton, while the majority of the country rock remained brittle. The lithostatic fluid in the ductile layer likely escaped through structurally weak zones, creating veins along the boundary between the pluton and marble. The fluids traversed the brittle-ductile transition experienced a sudden pressure drop from lithostatic in the ductile rock to hydrostatic in the brittle regime. The boiling fluid escaped by brecciating the brittle country rock and resulted in brecciation and stockwork of veins. In both cases, most fluid escaped through pathways, preventing infiltration into the country rock and decarbonation reactions. In contrast, a longer-lived heat source would have produced more extensive ductile country rocks, allowing for widespread fluid infiltration and calc-silicate formation. Thus, the thermal history of the intrusion and the rheology of the country rock significantly influence the extent of decarbonation in contact aureoles.