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Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-Pressure Metamorphism

Department of Earth and Planetary Sciences, Nagoya University, Nagoya 464-8602, Japan

J. G. Liou and C. G. Mattinson

Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Epidote minerals—the monoclinic epidote group minerals together with the orthorhombic polymorph zoisite—are important Ca-Al-silicates in many metabasites, metapelites and metacherts that are characterized by high P/T ratios. Such high P/T ratios are typical for subduction zones and the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism during continent-continent collisions (e.g., Liou 1973, 1993). All of these P-T conditions can be described by geothermal gradients between 5 and 20°C/km, that therefore provide a rough framework for the P-T conditions covered by this review (Fig. 1). Depending on the actual thermal structure of a subduction zone, the subducting plate will encounter subgreenschist, greenschist, blueschist, epidote-amphibolite, amphibolite, HP granulite, and/or eclogite facies conditions during its travel down into the mantle (Fig. 1). The P-T regime of the eclogite facies can further be subdivided into amphibole eclogite, epidote eclogite, lawsonite eclogite, and dry eclogite facies (Fig. 1). HP metamorphism refers to metamorphic pressure in excess of ~1.0 GPa and includes parts of the blueschist, epidote-amphibolite, and HP granulite facies as well as the eclogite facies (Fig. 1). UHP refers to the metamorphism of crustal rocks (both continental and oceanic) at P high enough to crystallize the index minerals coesite and/or diamond. HP and UHP metamorphism are separated conveniently by the quartz-coesite equilibrium which implies a minimum P > 2.7 GPa at T > 600°C for UHP metamorphism (Fig. 1). The equilibrium boundary for the graphite-diamond transition can be used to further subdivide the UHP region into diamond-grade and coesite-grade. The stability of coesite and other UHP minerals in a metamorphic regime requires abnormally low temperatures at depths greater than 100 km. Such environments can be attained only by the subduction of cold oceanic crust-capped lithosphere ± pelagic sediments or of continental crust.

View larger version (44K): [in this window] [in a new window]   Figure 1. P-T regimes of . . . [Full Text of this Article]

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