Lawsonite eclogites from the Pinchi Lake area, British Columbia—new P– T estimates and interpretation

Abstract

Lawsonite eclogites from Pinchi Lake, British Columbia, occur as tectonic blocks associated with lawsonite, aragonite, and jadeite-bearing blueschists. One tectonic block from Tezzeron Lake shows extensive lower-temperature retrogression. The block from Beaver Lake Road (BLR) shows less retrogression and the garnet crystals contain abundant inclusions of lawsonite and omphacite suggesting these minerals were stable throughout most of the crystallization history. Compositional profiles and element maps of garnets show: (1) Mn enrichment in cores with decrease toward rims; (2) little change in Ca from core to rim; (3) general decrease in Fe/Mg from core to rim. Phase diagram sections for sample BLR-3 and garnet-core compositions suggest garnet-core growth at 505 °C and 24.3 kbar (1 kbar = 0.1 GPa). Temperature estimates based upon garnet–clinopyroxene Fe–Mg exchange geothermometry are, for inclusions in garnet at 25 kbar, 450 ± 30 °C (4 spots); 435 ± 15 (10); and 445 ± 25 (6). For garnet rims and matrix clinopyroxene the results are: 515 ± 50 °C (2); 440 ± 25 (5); 475 ± 20 (2). These results suggest very little if any statistically significant changes in temperature during crystal growth. Estimates of pressure are based upon several different calibrations of the garnet–clinopyroxene–phengite equilibria. Using the calibration of Krogh-Ravna we obtain pressures of: 24.2–26.8 kbar, compatible with that estimated for garnet-core growth in BLR-3. For the glaucophane = talc + jadeite geobarometer, the activity relations for glaucophane are not well constrained. At 450 °C a pressure of 26 kbar is calculated (one sample). A pressure of 25 kbar at 450 °C suggests depths of ~ 90 km, leading to a P– T gradient of ~ 5 °C km − 1 . This gradient passes through the inferred stability field of lawsonite eclogites and the calculated stability fields of lawsonite for Pinchi Lake samples. This is very near the P– T stability field of coesite, which is characteristic of Ultra-High-Pressure metamorphism.