Are Si-Rich Phengite Inclusions In Eclogitic Garnet Evidence of Ultrahigh Pressure Conditions?

Mineral inclusions in garnet can contribute to elucidate the early metamorphic history of rocks. For instance, Si-rich phengite was reported to be enclosed in eclogitic garnet of the Franciscan Formation, California, and the Dabie Shan, eastern China. Geothermometry with this inclusion mineral and garnet and omphacite core compositions yielded pressures as high as 45 kbar. This was taken as evidence that dense crustal material can be exhumed even after very deep subduction.

Eclogites from the Franciscan Formation, for which early P-T conditions of about 30 kbar and 500°C were reported on the basis of phengite (3.6 Si p.f.u.) enclosed in garnet core, were re-investigated applying the computer program PERPLE_X to thermodynamically calculate P-T pseudosections. The bulk rock compositions of these eclogites are relatively poor in Al (< 14 wt.% Al₂O₃) and Si (< 50% SiO₂). It turned out that biotite is a stable potassium phase at temperatures of 400 and 500°C up to water pressures of about 12 and 17 kbar, respectively, in such basic rock compositions. The breakdown of this phase with rising pressures leads to phengite and garnet and/or chlorite. This phengite shows Si contents around 3.6 p.f.u., for instance, at temperatures of 450°C, when Mn-rich garnet cores were formed, and pressures around 18 kbar. Isobaric heating to 600-650°C resulted in an assemblage with phengite with the lowest Si content observed (~3.4 p.f.u.). Subsequent slight cooling at nearly the same pressure caused formation of the matrix assemblage in the studied eclogites from the Franciscan Formation accompanied by corrosion of garnet.

The deduced P-T evolution is suggested to be the result of mass transport within a subduction channel system. Furthermore, the process of exhumation was related to oceanic crustal material subducted not deeper than 70 km in contrast to previous assumptions.