Abstract

Oxygen isotope analyses have been obtained for coexisting minerals in several blue-schist-facies metamorphic rocks from California, Oregon, and New Caledonia. Detailed isotopic studies were made on a continuous exposure of schist in Ward Creek, California, previously described by Coleman and Lee (1962). The oxygen isotope fractionations among coexisting minerals in a variety of rock types, including metasediments and metabasalts, are systematic and larger than those measured in pelitic schists metamorphosed at the grade of biotite zone or higher. Therefore, these Ward Creek rocks (termed Type III) must have formed at lower temperatures than have such pelitic schists.

Evidence for significant isotopic equilibration and homogenization is observed in the Ward Creek sequence. Six different metasediments and metavolcanics collected within 25 m of one another show almost identical mineral δ;-values: quartz (15.8 to 16.3), aragonite (13.1 to 13.3), glaucophane (9.8 to 10.0), muscovite (10.9 to 11.3), lawsonite (9.3 to 9.5), and garnet (8.0 to 8.4), given as per mil enrichment in O18 relative to mean ocean water. These rocks seem to have reached equilibrium at about the same temperature in contact with abundant metamorphic pore fluids.

Not all the Ward Creek rocks have completely equilibrated with the postulated metamorphic pore fluids. In particular, the metacherts seem to have been relatively impermeable to the aqueous fluids during metamorphism, as indicated by the large δ-values of quartz in such rocks (17 to 19.2) and by their appreciably higher Fe+3/Fe+2 ratios. Gradients in O18/O16 and Fe+3/Fe+2 have been generally “smoothed out” in the rocks during metamorphism, but the process has gone to completion only locally. Cherts and limestones have apparently been lowered in O18 content by 10 to 15 per mil, and the metabasalts are enriched by 3 to 4 per mil over their unmetamorphosed parent rocks.

Using the calibrated quartz-muscovite and quartz—CaCO3 geothermometers, the measured quartz-muscovite and quartz-aragonite fractionations indicate essentially “concordant” temperatures of formation for the Type III rocks of 270° to 315° C. Inasmuch as aragonite is part of the equilibrium assemblage, these rocks must therefore have been metamorphosed at pressures of at least 6.4 to 7.0 kb. Oxygen isotope fractionations for the mineral pairs aragonite-lawsonite, quartz-muscovite, and quartz-glaucophane progressively decrease from Type II (low-grade) through Type III to Type IV (high-grade tectonic blocks) metabasalts.

The higher-grade blueschists from New Caledonia exhibit quartz-muscovite and quartz-glaucophane fractionations similar to the Type IV metabasalts at Ward Creek, indicating temperatures of formation of 400° to 550° C. Thus, glaucophane-bearing metamorphic rocks apparently form over a temperature range of 200° C to 550° C, encompassing the probable temperature range of the entire greenschist and epidote-amphibolite facies. This suggests that glaucophane schists should be separated into at least two metamorphic facies, a lower-grade, lawsonite-aragonite blueschist facies and a higher-grade, epidote-rutile blueschist facies, both representing higher pressures than are attained during ordinary low-rank and middle-rank regional metamorphism.

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