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

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This chapter focuses mainly on modes of occurrences of zoisite, clinozoisite-epidote and piemontite in various rock types from selected subduction zones and HP to UHP metamorphic terranes. The amount of information on these rocks has increased dramatically during the last 20 years and it is virtually impossible to completely review all. We therefore restrict this review mainly to those localities where we have our own experience, and add some information from other localities. Epidote minerals in metamorphic rocks that followed classical barrovian- or Buchan-type metamorphism are reviewed by Grapes and Hoskin (2004). Nevertheless, as the P-T conditions encountered during subduction zone and Barrovian-type metamorphism are not thus distinct in the P-T range of the greenschist, blueschist, to epidote-amphibolite facies transition, some overlaps between Grapes and Hoskin (2004) and this chapter were found to be unavoidable. As subduction zone metamorphism represents also a potential prograde path of many HP and UHP rocks we will first review some selected and well studied examples of subduction zone metamorphism and then prograde into the field of the peculiar HP and UHP metamorphic rocks.

Specifically, this chapter deals with the following major subjects: (1) Prograde sequences from lawsonite to epidote zone blueschists and the corresponding epidote-forming reactions are described from the Franciscan, New Caledonian and Sanbagawa metamorphic rocks; these examples are used to illustrate the role of epidote minerals in typical subduction zone metamorphic successions. (2) Phase relations of zoisite-bearing assemblages in calcareous metasediments are briefly reviewed for eclogite facies rocks in the European Alps. (3) The control of bulk rock composition on paragenesis and compositions of UHP phases is exemplified by various zoisite-bearing UHP eclogites from Dabie (central China) with an AFM diagram. (4) The common break down of plagioclase in HP and UHP rocks in the presence of H2O to form the assemblage zoisite + kyanite + sodic clinopyroxene + quartz is illustrated in some HP to UHP coronitic metagranites and metagabbros. (5) The common but selective occurrence of epidote minerals in Sulu-Dabie eclogite, quartzite and gneiss may be related to the oxidation of supracrustal rocks by a fossil hydrothermal system. (6) The pseudomorphic replacements of epidote/zoisite + amphibole + biotite + sodic plagioclase after garnet and of zoisite + albite after kyanite as indicators for a metasomatic and retrograde overprint during exhumation are described using kyanite-phengite-coesite eclogites from Dabie. (7) The coexistence of lawsonite and epidote in some HP metabasites is related to bulk composition, Æ’O2 and metastable persistence. (8) Composition of epidote minerals from buffered assemblages is sensitive to P, T and Æ’O2, and its use as a geobarometer or geothermometer is discussed. (9) Epidote minerals are a key to discuss fluid-rock interaction and partial melting of metamorphic rocks at high-P conditions. (10) Unusually high Sr and REE contents in epidote minerals including allanite suggest that they are the main reservoirs of these elements in subduction zones and HP to UHP environments. (11) Sector-zoning of zoisite and epidote and their petrological and mineralogical significance is also briefly discussed.


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