Contact Metamorphism and Decarbonation in the Gangdese Arc: A Petrological, Geochemical, and Phase Equilibria Study

The potential correlation between long-term climate changes on Earth and the intensity of continental arcs, particularly their considerable CO2 emissions, implies considerable climatic impacts from these petro-tectonic processes. Contact metamorphism of sedimentary carbonates converts the country rocks into calcsilicate and releases substantial CO2. The widespread carbonate sequences deposited on major continental margins where arc activities culminated in the Cretaceous to Paleogene give rise to a hypothetical causal relationship between the magma-carbonate interaction and the Cretaceous Thermal Maximum. While isotopic and modeling evidence supports this hypothesis, direct field-based studies constraining the extents and rates of decarbonation would help assess aureole-scale CO2 fluxes. A wide range of lithology within contact aureoles, from calcsilicates to marble, in the Cretaceous Gangdese Arc in southern Tibet reveals distinctive behaviors of contact metamorphism. The first step to approach the question is the understanding of phase relations. Current phase diagrams detailing metacarbonate systems can be further refined to include additional components and minerals at lesser pressures. Metamorphic conditions such as pressure, temperature and the fluid composition can be estimated from the phase diagrams provided in this thesis. This thesis then presents comprehensive data obtained from the study of two exemplary endmember cases from the Lhasa terrane in southern Tibet. In the case of calcsilicate aureole, diffusion modeling across compositional zones within garnet and diopside suggests brief metamorphic alteration. As these calcsilicate samples demonstrate a large fraction of carbon loss, the corresponding carbon fluxes could be enormous, comparable to fluxes from other contemporary tectonic units. In contrast, studies on marble aureoles suggest fluid behaviors can vary according to rock rheology, thereby affecting the extent of carbon release. Thermobarometric and isotopic measurements from veins surrounded by recrystallized marble suggest that brittle carbonate country rock adjacent to short-lived intrusions facilitates rapid fluid migration and minimal decarbonation reactions. We speculate that, in comparison, ductile country rocks in the vicinity of long-lived intrusions seem to promote extensive fluid-rock interactions, leading to the creation of calcsilicate aureoles. Collectively, this thesis adds to our understanding of decarbonation processes through contact metamorphism from various perspectives, such as the duration and degree of decarbonation, fluid-rock interactions, and phase relations in metacarbonate rocks.

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.