Clinopyroxene–a mineral telescoped through the processes of blueschist facies metamorphism |
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| Authors: | S MARUYAMA JG LIOU |
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| Institution: | Department of Geology, Stanford University, Stanford, CA 94305, U.S.A. |
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| Abstract: | Abstract Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan-shaped medium-grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P and/or T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature. The first formed clinopyroxene (Jd20Aug65Ac15) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low XJd content towards acmite. At a composition around Jd20Aug30Ac50, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite-omphacite join and clinopyroxene increases in XJd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite- and pumpellyite-zones, ranging from XJd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor & Coleman (1968) for these zones does not support the low-temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980). The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure-increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone. |
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| Keywords: | Key-words: blueschist clinopyroxene meta-basite Ward Creek Cazadero California |
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