Fluid Inclusions Evidence for Fluids Associated with Recrystallization of Quartzites In the EJB Aureole, California

The Eureka Valley-Joshua Flat-Beer Creek pluton (EJB) in the White Mountains of California is a composite pluton of monzodiorite-monzonite-quartz monzonite composition. The EJB intruded a Neoproterozic and early Cambrian sedimentary shelf sequence, already metamorphosed to greenschist facies. Wall rocks of the contact aureole have a subvertical dip and consist primarily of quartzites, marbles, and psamites, and lesser amounts of schists.

The aureole shows varying degrees of recrystallization and deformation. For example, in the eastern aureole, quartzites closest to the contact show the least amount of recrystallization with preserved rounded quartz grains that lack crystallographic preferred orientations (CPO's). A marble unit that lies between the intrusion and the quartzites may have prevented fluid flow from the pluton or kept the activity of H₂O low with the release of CO₂. Our hypothesis is that low H₂O activity prevented recrystallization of the quartzite. In contrast, farther from the pluton contact, quartzites are more completely recrystallized with few remnant grain shapes and show well-defined CPO's, suggesting a higher activity of H₂O.

Both primary and secondary fluid inclusions are analyzed to constrain the compositions of fluids that were responsible for the contrasting styles of deformation and recrystallization. Preliminary results from microthermometry of secondary and pseudo-secondary inclusions in highly recrystallized quartzite samples show H₂O-dominated inclusions with about 25% CO₂. The amount of CH₄ is minor as the lowest first-melting temperature of the carbonic phase is only -57.5º C. On the other hand, the first-melting of the aqueous phase is as low as -41.5º C, indicating the presence of non-sodium salts, including salts of Ca and Mg that may have been derived from dolomitic marbles.