Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

Abstract

Fluid inclusions in quartz veins within Proterozoic metamorphic rocks in the Black Hills, South Dakota, were examined by microthermometry and Raman spectroscopy to assess the evolution of fluid compositions during regional metamorphism of organic-rich shales and late-orogenic magmatism, both of which were related to the collision of the Wyoming and Superior crustal blocks. Fluid inclusions occur in veins that began to be generated before or during regional compression and metamorphism that reached at least garnet-grade conditions, and in veins within the aureole of the Harney Peak Granite (HPG), where temperatures reached second-sillimanite grade conditions. Early veins in the schists have undergone recrystallization during heating and deformation that modified the composition of early CH 4 or CO 2 and N 2-dominated inclusions. These fluids were apparently trapped under conditions of immiscibility with a saline aqueous fluid phase. They are interpreted to represent components generated during maturation of organic matter and dehydration of phyllosilicates during incipient metamorphism at reducing fO 2 conditions. Most inclusions in the quartz veins are, however, secondary CO 2-bearing. They imply a transition to higher fO 2 conditions with increasing temperature of regional metamorphism. The fO 2 conditions may have been controlled by the mineral assemblage in the host metapelites. The prevalence of bimodal distributions of trapped CO 2–N 2 and aqueous endmembers in the biotite and garnet zones also suggests that two immiscible fluid phases existed during the regional metamorphism. In the aureole of the HPG, graphite was evidently consumed by influx of magmatic fluids. CO 2–H 2O fluid inclusions dominate, but they have significantly less N 2 than inclusions at lower metamorphic grades. All inclusions define secondary trails in mostly unstrained quartz. The bimodality of inclusion compositions is not as well defined as at lower grades, with many inclusions containing intermediate CO 2–H 2O compositions. This suggests that a single fluid phase existed at the high temperatures in the granite aureole, but then unmixed during cooling. A set of late quartz veins with graphitized and tourmalinized selvages in the granite aureole contains CH 4-bearing inclusions with little N 2. The existence of CH 4 in these inclusions is attributed to complexing of magmatic B with hydroxyl anions taken from the CO 2–H 2O fluid phase, effectively causing reduction in fO 2 and promoting precipitation of graphite.