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Low-temperature recrystallization of Franciscan greywackes from Pacheco Pass, California

Published online by Cambridge University Press:  05 July 2018

S. Maruyama ,
J. G. Liou  and
Y. Sasakura
S. Maruyama
Affiliation:
Department of Geology, Stanford University, Stanford, California 94305, USA
J. G. Liou
Affiliation:
Department of Geology, Stanford University, Stanford, California 94305, USA
Y. Sasakura
Affiliation:
Department of Earth Science, Faculty of Science, Toyama University, Toyama 930, Japan
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Abstract

Low-temperature metamorphism of the Franciscan complex at Pacheco Pass occurs at P-T conditions near the so-called jadeite isograd. Along Highway 152 in the Diablo Range, from west to east, four distinct prograde metamorphic zones are defined: (I) albite-quartz zone, (II) albite-rich, albite-quartz-clinopyroxene assemblage, (III) clinopyroxene-rich, albite-quartz-clinopyroxene assemblage, and (IV) clinopyroxene zone. Metamorphic pyroxenes are restricted to zones II, III, and IV in metagreywackes, but metabasites contain augite-rich pyroxenes even in zone I. With increasing grade, the compositions of pyroxene in metagreywackes change in XJd from nearly 100 in zone II to about 60 in zone III. The diopside component has little effect on the XJd of pyroxene. The apparent change of XJd reflects differences in pressure and temperature rather than in bulk rock composition. In metagreywackes, at the onset of zone II, albite breaks down to form Jd100 according to the reaction Ab = Jd+Qz. With increasing grade, this reaction leads to less jadeitic pyroxene. From zones II to III, a continuous reaction is delineated: Qz+2 pyroxenes (Jd80Ac15Aug5)=pyroxene (Jd60Ac30Aug10) + Ab.

The metamorphic temperatures are estimated to be about 170°C in zones I and II, and about 230°C in zones II and III. The temperature variation recorded in a single rock is probably less than 30°C The pressure estimate depends on the choice of experimental data for the jadeite-albite-quartz curve; it ranges from 4.5 to 8 kbar. A gently folded thermal structure of the Cretaceous subduction zone metamorphism is postulated for this area.

Keywords

metamorphismgreywackesFranciscan complexPacheco PassCalifornia
Type
Research Article
Information
Mineralogical Magazine , Volume 49 , Issue 352 , June 1985 , pp. 345 - 355
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1985

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Footnotes

*

Present address: Department of Earth Science, Toyama University, Toyama 930, Japan.

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