Reactions and physical conditions during metamorphism of Liassic aluminous black shale and marls in central Switzerland

摘要

The aluminous Liassic (lower Jurassic) shales, slates, and marls of the Central Swiss Alps were investigated by electron microprobe (EMP) and scanning electron microscopy (SEM) to determine the physical conditions of low grade metamorphism. The samples investigated experienced metamorphism from the diagenetic (100 ℃) to the epizonal grade (45 ℃). Temperatures estimated from Fe-Mg exchange equilibria between chlorite and chloritoid agree with calcite-dolomite geothermometry and previous workers' estimates. In the upper anchizone, calcite-dolomite temperatures record peak temperatures ca. 330 ℃. Epizone rocks in the Urseren Zone record ca. 400-450 ℃. Calculations of T-X phase equilibria in the CaO-MgO-FeO-Al_2O_3-SiO_2-H_2O-CO_2-CH_4 system with excess quartz and graphite were used to predict the stability fields of aluminous minerals and coexisting fluid compositions. The assemblage Sud + Dol + Qtz + Gr is stable over a temperature interval of 300-360 ℃. Calculations predict that, in CO_2-bearing fluids, reactions that form the index minerals margarite, sudoite, and chloritoid predominantly involve carbonate minerals. Margarite is not predicted to form in fluids dominated by CH_4. Temperatures recorded by T-X phase equilibria in the anchizone (330 ℃) agree well with calcite-dolomite thermometry. Temperatures recorded by T-X phase equilibria in the epizone (330-360 ℃) are significantly lower than calcite-dolomite temperatures (400-480 ℃). The calculated fluid composition in the anchizone is CO_2-rich (y_(CO_2) = CO_2/(CO_2 + H_2O) = 0.2), whereas the epizone fluids are richer in H_2O (y_(CO_2) = 0.02-0.06). Fluid-flux calculations based on phase equilibria and reaction progress indicate that there was significant fluid infiltration in both the anchizone and epizone. The anchizone samples recorded a flux of ca. 6 * 10~3 cm~3 fluid/cm~2 rock with a flux increase to at least 2 * 10~5 cm~3/cm~2 in the epizone. These values are comparable to calculated fluxes during Devonian regional metamorphism in New England. Results show that careful mineral analysis can yield data appropriate for the estimation of sub-greenschist facies metamorphic conditions in aluminous low-grade rocks.