Physicochemical study of skarn formation in pelitic rock, Costabonne peak area, eastern Pyrenees, France

Abstract

The Costabonne skarn complex was emplaced in the lower part of the Cambrian Canaveilles Formation (eastern Pyrenees) in contact with a Hercynian granitic stock. Skarns have developed from calcium-poor (<1% CaO) pelitic rocks. A zonation can be clearly seen: schists (Z0), biotite zone (Z1), amphibole zone (Z2), pyroxene zone (Z3), and garnet zone (Z4). Two superposed successions of transformations are observed: (1) muscovite→feldspar→ garnet and (2) biotite→amphibole→pyroxene. Calculation of mass transfer indicates an important exchange of CaO and K₂O and, in lesser amounts, Fe₂O₃, MgO and MnO. SiO₂, Al₂O₃, FeO, TiO₂, the heavy rare earths (HREE) and most of the trace elements remain constant; whereas the contents of Na₂O, Rb, Sr, Ba, and the light rare earths elements (LREE) are reduced. According to fluid inclusion data and mineral compositions, the metasomatic fluid was mainly$H_2 O(X_{H_2 O} > 0.99)$ with minor CO₂, CH₄, N₂, H₂, O₂, H₂S, HCl and HF. Ca, Na, and K were the most abundant cations. Temperature and pressure conditions are estimated to be at least 550° C and 1.7 to 2.0 kb, respectively. The totality of the observed transformations may be described in the system CaO-K₂O-SiO₂-Al₂O₃-MgO-H₂O with the minerals quartz, muscovite, phlogopite, tremolite, diopside, grossular, K-feldspar, and anorthite. With CaO and K₂O taken as perfectly mobile components, SiO₂, Al₂O₃, and MgO as determining inert components and H₂O as excess component, the reactions leading to the skarn formation can be represented in a$\mu _{CaO} - \mu _{{\rm K}_2 O}$ diagram. The succession of zones is shown to take place with increasing μ_CaO from the schist (Z0) up to the garnet zone (Z4). The nature of the feldspar (plagioclase or K-feldspar) depends on the value of$\mu _{{\rm K}_2 O}$ relative to μ CaO.