Pressure–temperature evolution of lawsonite‐bearing eclogites, Pinchi Lake, British Columbia

Blueschists and retrogressed eclogites are located along the Pinchi Fault Zone (near 54°30′N and 124°W) in central British Columbia. The Pinchi Fault separates rocks of contrasting geological histories, and the blueschists and eclogites occur with ultramafic rocks as fault‐bounded blocks. The retrogressed eclogites occur as tectonic blocks, now in glacial debris, a few metres across; blueschists occur in coherent kilometre‐sized tracts. Eclogites contain garnet–omphacite–rutile–quartz, with glaucophane, lawsonite and titanite. Some of the lawsonite appears to be stable with omphacite and garnet. Tectonic blocks of eclogite from two different localities have recorded different P–T histories. In one tectonic block, P–T estimates for garnet inclusions in clinopyroxene and lawsonite ( c . 565° C, >13.1 kbar) suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite. A decrease in temperature, as recorded in garnet rims and matrix clinopyroxene, resulted in crystallization of lawsonite and other retrogressive minerals. Later crystallization of stilpnomelane (locally pseudomorphing garnet), howieite, winchite and actinolite was triggered by an influx of fluid under P–T conditions outside the stability field of garnet. Lawsonite appears to have been stable, suggesting a minimum pressure of about 3 kbar. In the second tectonic block, clinopyroxene inclusions in garnet suggest temperatures near 350° C ( P > 10 kbar) and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450° C at pressures above c . 12 kbar.