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Epidote Group Minerals in Low–Medium Pressure Metamorphic Terranes

Institut für Mineralogie, Petrologie und Geochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 23 b, D-79104 Freiburg, Germany, rodney.grapes@minpet.uni-freiburg.de, paul.hoskin@minpet.uni-freiburg.de

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	INTRODUCTION

 
Epidote group minerals are common in metamorphosed mafic to intermediate igneous rocks, quartzofeldspathic sediments and calc-alumina silicate (marl) rocks of higher grade zeolite to medium grade amphibolite facies of low–medium pressure contact and regional metamorphic terranes (i.e., pressure and temperature conditions below the calcite-to-aragonite transition). Within any one rock, epidote composition in terms of Fe³⁺/(Fe³⁺ + Al) can be variable, but in general, is limited by whole-rock composition, such that epidote group minerals in metabasite lithologies are more Fe-rich than those in marls that tend to be more Al-rich and typically include zoisite. Because of their wide range of P-T stability, epidote group minerals of variable composition may form in a single rock during several stages of metamorphic re-equilibration. Slow rates of intra-crystalline Fe³⁺-Al exchange, especially at low temperatures, preserve complex zonation patterns in individual grains that can serve as a "tape-recorder" providing evidence for continuous or discontinuous prograde and retrograde reactions and the P-T-fluid-redox conditions of metamorphism. Thus, relic lower grade epidote (typically Fe-rich) often form cores over which new (typically less Fe-rich) higher grade epidote rims form. In such a case, the boundary between the two generations of epidote may be sharp or gradational depending on the temperature history of the rock and Fe³⁺-Al diffusivity. Often compositional differences are blurred across the boundary. In addition to zoning is the spread of individual epidote grain compositions within a rock (even on a thin-section scale). This is related to variation in the composition of coexisting phases or reactants (e.g., quartz; Ca-Al silicates such as plagioclase, margarite, lawsonite; mafic silicates such as chlorite, pumpellyite, amphiboles; other Ca-Fe³⁺ silicates such as prehnite, andraditic garnet; carbonates; Fe-oxides; relict volcanic glass), which serve as compositional micro-domains in which epidote may form. Thus, where epidote is formed . . . [Full Text of this Article]

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