The transition from magmatic to hydrothermal crystallization conditions of a shallow-level granitic pluton

Abstract Many thermobarometric methods applied to granitic composition rocks that crystallized at < 5 kbar yield temperature estimates ~ 50 to 100 °C lower than the widely used haplogranite water-saturated solidus. To address thermobarometric discrepancies, we investigated a shallow-level granitic pluton that is not enriched in fluxing elements such as Li, B, P, or F. The Rito del Medio pluton (RDMP) near Questa, New Mexico contains numerous minerals and inclusions suitable for thermobarometric estimates, and it has abundant miarolitic cavities that represent the final crystallization stages. Occurrences of coexisting melt and fluid inclusions show that groundmass minerals crystallized from a water-saturated magma. After groundmass crystallization, the pluton transitioned to a fluid-dominated system manifested by the crystallization of freestanding minerals contained in the miarolitic cavities. The granite contains mica, feldspar, quartz, primary fluid inclusions in quartz, and accessory minerals including garnet with quartz inclusions. We used minerals and their inclusions in both paragenetic contexts to track changes in P, T, and mineral and fluid compositions that accompanied the magmatic-to-hydrothermal transition. Resultant univariant curves from thermobarometry for the groundmass minerals converge at ~ 1.9 to 2.1 kbar and ~ 590 to 625 °C indicating final magmatic crystallization. To address discrepancies between thermobarometric results and the haplogranite solidus, we performed crystallization experiments at 2 kbar, which show that RDMP compositions magmas complete crystallization at temperatures ~ 620–625 °C. Univariant curves for thermobarometric approaches applied to the RDMP miarolitic cavity minerals converge at ~ 1.5 to 2.1 kbar and ~ 500 to 525 °C defining the transition to hydrothermal crystallization conditions.