Epidote Group Minerals in Low-Medium Pressure Metamorphic Terranes

Abstract

Grapes & Hoskin 302 state, as epidote Fe 3+ -content is strongly dependent on ƒ O 2 (Holdaway 1972;Liou 1973). Again, consideration of these variables is relevant both to whole-rock compositions and sub-domains in rocks. For example, in sub-greenschist facies metabasites, epidote with different compositions may occur in amygdules with or without calcite, and in the rock matrix where it may replace pumpellyite or has replaced volcanic glass and grown together with pumpellyite, chlorite, etc. In the two sub-domains (amygdules and matrix), X CO 2 , X H 2 O and ƒ O 2 can signifi cantly differ.These interrelated controls, as well as other kinetic factors, determine the composition of single growth-generation epidote at any particular pressure and temperature. Nevertheless, general trends of epidote compositional changes in a number of metamorphic terranes have established that in metabasaltic and quartzofeldspathic rocks epidote tends to become more aluminous with increasing grade (Miyashiro and Seki 1958;Apted and Liou 1983). On the other hand, zoisite and clinozoisite, for example in meta-marl and Ca-rich metapelite, become more Fe-rich with increasing metamorphic grade.This chapter reviews the changes in composition, the textures, epidote-producing and consuming reactions, conditions of oxidation, fl uid composition, and P-T conditions during zoisite and clinozoisite-epidote formation in various rock types from selected low to medium pressure regional and contact metamorphic terranes. The review is intended to be broad but not comprehensive; it is focused on occurrences where epidote composition and textual data for a range of related rocks within a single terrane are available. Additional data for a range of epidote group mineral occurrences can be found in the summary of Deer et al. (1997). Data on compositional varieties such as Mn-(excluding piemontite; see Bonazzi and Menchetti 2004), Sr-and Cr-bearing members of the epidote group are included. Evidence for composition gaps in the zoisite-clinozoisite and clinozoisite-epidote series for natural occurrences is discussed in relation to experimental verifi cation of miscibility gaps in the epidote group series. Finally, critical mineral associations and reactions relevant to epidote group mineral paragenesis in metabasaltic and Al-marl rocks are summarized. Nomenclature, mineral abbreviationsIn this chapter, epidote group mineral compositions are expressed as 100(Fe 3+ /(Fe 3+ + Al)) and denoted as X Fe throughout the text. Mineral abbreviations used in the text and fi gures are listed in Table 1. EPIDOTE GROUP MINERALS IN DIFFERENT LITHOLOGIES MetabasiteKarmutsen Metabasite, Vancouver Island, Canada. In the low grade Karmutsen metabasite, Vancouver Island, epidote is one of the most common Ca-Al silicates and occurs in amygdules, veins and as fi ne-grained aggregates in the rock matrix. Cho et al. (1986) show that in low-variance (calcite-free, 3-phase assemblages with excess chlorite + quartz) amygdule assemblages of epidote, pumpellyite, chlorite, quartz laumo...

Chromian epidote in omphacite rocks from the Sambagawa metamorphic belt, central Shikoku, Japan

“…Our data for Ca − , Sr − , and REE − rich epidote types fill the composition gap field in the Al − Cr − Fe series noted by Grapes and Hoskin (2004).…”

Experimental Subsolidus Studies on Epidote Minerals

“…In quartz + aluminosilicate saturated metapelite, a variety of model reactions as previously discussed will defi ne the reactions between anorthite plagioclase , grossular garnet , and (clino)zoisite epidote . In fact, an extended fi eld of epidote ss in metapelite and metagranitoid is confi rmed by natural occurrences (see Grapes and Hoskin 2004), however, due to relatively low CaO-contents, epidote in metapelite is only a minor constituent. Johannes (1980) investigated melting and subsolidus equilibria involving zoisite in the system K 2 O-CaO-Al 2 O 3 -SiO 2 -H 2 O (KCASH).…”

“…The extent of one and two phase fi elds along the zoisite-epidote pseudo-binary join have been mainly proposed on the basis of fi eld studies (e.g., Raith 1976;Enami and Banno 1980;Franz and Selverstone 1992; see also Grapes and Hoskin 2004) as experiments specifi cally devoted to the solution of this problem are scarce (Heuss-Aßbichler and Fehr 1997;Brunsmann et al 2002). Brunsmann et al (2002), using equilibrium overgrowths on zoisite and clinozoisite seeds, attempted to reconcile experimental data and natural evidences in the schematic T-X diagram (Fig.…”

Metamorphosed cumulate gabbros from the Støren Group of the Upper Allochthon, northern Western Gneiss Region, Norway: petrology and metamorphic record

“…Green hornblende mantles and rims having >1 Al per formula unit (n = 48) have an average of 0.26 Na + K (91% in A-sites, 8% in M4) and 1.35 Al (27% octahedral, 73% tetrahedral). Leake et al (1997) with increasing grade, commonly recorded by core to rim zoning in epidote (Grapes & Hoskin, 2004). This is partly caused by prograde pumpellyite breakdown on entering the greenschist facies.…”