The Timescale and Carbon Flux Recorded by Skarn Garnet From Gangdese Arc, Southern Tibet

Abstract

Contact metamorphism of carbonate rocks in response to fluid infiltration releases carbon dioxide (CO2), potentially affecting Earth's carbon budget over geological timescales. This process was notably active in the Cretaceous when the closure of the Neo-Tethys Ocean led to extensive arc magmatism in southern Tibet, coincident with a greenhouse climate interval. These arc plutons intersected carbonate sequences and formed widespread calcsilicate rocks. The mineral assemblages of these rocks record both the progression and duration of the CO2 release. In this study, we focus on a representative aureole in southern Tibet that was metamorphosed by a ∼64 Ma granitoid pluton of the Gangdese Batholith. The aureole presents lithologic zonation, with the inner garnet-wollastonite zone indicating equilibrium with a water-rich fluid at ∼525°C. Large garnet porphyroblasts feature cross-cutting veins of secondary garnets that illustrate several stages of interface-coupled dissolution-reprecipitation processes facilitated by fluid infiltration. Diffusion modeling for Mg concentration profiles across vein-host garnet yields a brief high-temperature stage (25–150 kyr)—a duration still significantly overestimated considering factors that may accelerate diffusion in garnet, such as lattice parameters and hydration. Such short timescale aligns with conduction modeling of a cooling pluton, which, combined with the bulk-rock mass balance analysis, indicates a local carbon flux comparable to fluxes at other modern tectonic settings. The metamorphic decarbonation, potentially episodic, in the continental arc may have been an important endogenic carbon source in the late Cretaceous.