Abstract
In different areas of the Hercynian in the Iberian Peninsula some reactions are repeatedly observed in pelites, these reactions are: Staurolite + muscovite + quartz = Biotite+ Al 2 SiO 5 + H 2 O Garnet + muscovite = Al 2 SiO 5 + biotite+ quartz Biotite + Al 2 SiO 5 + quartz = Cordierite + K feldspar+Hp In order to examinate the P-T stability fields of these, and other similar reactions, aH the univariant equilibria in multisystems with Gt-Cd-St-Bi-Mu-Als-Qz-HP, Gt-St-Bi-Mu-Fk-AIs-QzHp and Cd-St-Bi-Mu-Fk-AIs-Qz-H 2 O in Kp-FeO-AI 2 O r SiO 2 -H 2 O (KFASH) system have been calculated, and their corresponding P-T grids have been constructed. The expansion of these reactions into divariant surfaces through the P-T-X (Fe-Mg) space was made by studying the assemblage Gt-Cd-St-Bi-Mu-Fk-AIs-Qz-HP in K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (KFMASH) with a Fe/Fe+Mg relationship Gt > St > Bi > Cd such as observed in most of natural pelites. A resultant grid was obtained by combining those obtained in the aboye systerns. This grid has been P-T located for P H 2 0 = P 19 near QFM buffer, and excess muscovite and quartz conditions Reaction slopes in this grid were calculated within different P-T surroundings from thermodynamic data as weH as by considering the existing experimental data. In addition to the stability fields of reactions the P-T-X Fe-Mg theoric relations for three univariant and thirteen divariant reactions have been obtained. The grid confirms the imposibility of staurolite-K feldspar and Garnet-Cordierite-Muscovite coexistence, as weH as the extension of the stability fields for Garnet-Staurolite, Cordierite-Staurolite and Garnet-Cordierite assemblages in muscovite-poor metapelites.
Highlights
Studies on mineral equilibria in pelitic rocks and their stability fields location in P-T or P-T-X diagrarns for both low-medium grade and high-grade conditions were abundant in the last two decades
+sillimanite +quartz-Cordierite +K feldspar +H20 have been observed in metapelites in a number of locations across the Hercynian belt in the Iberian peninsula like in thermal domes and their peripheries (Durán, 1985; Martínez & Rólet, 1988; Martínez el al., 1988)
Sorne reactions between the phases considered in this paper have been theoretically or experimentally studied by different authors, the following paragraphs are mainly addressed to the study of their P-T stability fields
Summary
Studies on mineral equilibria in pelitic rocks and their stability fields location in P-T or P-T-X diagrarns for both low-medium grade (fj. Brown, 1975; Day, 1976; Thompson, 1976 a,b) and high-grade conditions were abundant in the last two decades. Sorne reactions between the phases considered in this paper have been theoretically or experimentally studied by different authors, the following paragraphs are mainly addressed to the study of their P-T stability fields. These fields have been constructed in KFASH, KMASH and KFMASH systerns witb PH20 = P, and excess quartz. From data of table 2, the different invariant points for the multisystem have been constructed by the method of Schreinemaker (1915-1925), taking into account the thermodynamic restrictions due to slopes. The invariant point [St] of fig. 2B is lacated at higher
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