Petrology of Fe‐Mg‐carpholite‐bearing metasediments from NE Oman

Abstract

Abstract Fe‐Mg carpholite occurs in metasediments of tectonically disrupted basement, shelf and foreland basin units that structurally underlie the Semail ophiolite in NE Oman. In the lower grade, structurally higher units, Fe‐rich carpholite coexists with paragonite, quartz, illite, kaolinite and chlorite, whereas in deeper units, Fe‐Mg carpholite occurs with pyrophyllite, sudoite, phengite and/or chloritoid. Mineral compositions in these units indicate that chlorite is more magnesian than coexisting Fe‐Mg carpholite at low temperatures and pressures but, at higher metamorphic grades, X Mg decreases in the order sudoite > carpholite > chlorite > chloritoid. This suggests a reversal in Fe‐Mg partitioning between Fe‐Mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite + quartz to pyrophyllite and at X Mg = 0.35. Phase relations and mineral equilibria indicate that the P‐T conditions of formation of the Fe‐Mg‐carpholite‐bearing rocks of NE Oman range from 280–315° C, 3–6 kbar for the structurally highest units to 325–440° C, 6–9.5 kbar for the deepest units, indicating a systematic down‐section increase in metamorphic grade. Textural relations in these rocks, interpreted in the context of pertinent equilibria, are consistent with the clockwise P‐T paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks.