Formation and preservation of fresh lawsonite: Geothermobarometry of the North Makran Blueschists, southeast Iran

A low-grade metamorphic Coloured Melange in North Makran (SE Iran) contains lenses and a large klippe of low temperature, lawsonite-bearing blueschists formed during the Cretaceous closure of the Tethys Ocean. The largest blueschist outcrop is a >1,000m thick coherent unit with metagabbros overlain by interlayered metabasalts and metavolcanoclastic rocks. Blueschist metamorphism is only incipient in coarse-grained rocks, whereas finer grained, foliated samples show thorough metamorphic recrystallization. The low-variance blueschist peak assemblage is glaucophane, lawsonite, titanite, jadeite +/- phengitic mica. Investigated phase diagram sections of three blueschists with different protoliths yield peak conditions of similar to 300-380 degrees C at 9-14kbar. Magnesio-hornblende and rutile cores indicate early amphibolite facies metamorphism at >460 degrees C and 2-4kbar. Later conditions at slightly higher pressures of 6-9kbar at 350-450 degrees C are recorded by barroisite, omphacite and rutile assemblages before entering into the blueschist facies and finally following a retrograde path through the pumpellyite-actinolite facies across the lawsonite stability field. Assuming that metamorphic pressure is lithostatic pressure, the corresponding counterclockwise P-T path is explained by burial along a warm geothermal gradient (similar to 15 degrees C/km) in a young subduction system, followed by exhumation along a cold gradient (similar to 8 degrees C/km); a specific setting that allows preservation of fresh undecomposed lawsonite in glaucophane-bearing rocks.