Contact metamorphism in the Los Santos W skarn (NW Spain)

• Published in: Mineralogy and petrology Vol. 90; no. 1-2; pp. 109 - 140
• Main Authors: Timón, S. M., Moro, M. C., Cembranos, M. L., Fernández, A., Crespo, J. L.
• Format: Journal Article
• Language: English
• Published: Wien Springer Nature B.V 01-05-2007
• Subjects: Calcite; Cambrian; Carbon dioxide; Carbonates; Distillation; Dolomite; Fluorine; Geology; Metamorphic rocks; Metamorphism; Mineralogy; Permian; Petrology; Rocks; Sediments; Spain
• ISSN: 0930-0708; 1438-1168
• DOI: 10.1007/s00710-006-0166-0

The intrusion of the Lower Permian Los Santos-Valdelacasa granitoids in the Los Santos area caused contact metamorphism of Later Vendian-Lower Cambrian metasediments. High grade mineral assemblages are confined to a 7 km wide contact aureole. Contact metamorphism was accompanied by intense metasomatism and development of skarns, and it generated the following mineral assemblages: diopside, forsterite, phlogopite (+/- clintonite) and humites and spinel-bearing assemblages or diopside, grossular, vesuvianite +/- wollastonite in the marbles, depending on the bulk rock composition. Cordierite, K-feldspar, andalusite and, locally, sillimanite appear in the metapelitic rocks. Mineral assemblages of marbles and hornfelses indicate pressure conditions ranging from 0.2 to 0.25 GPa and maximum temperatures between 630 and 640 degrees C. 13C and 18O depletions in calcite marbles are consistent with hydrothermal fluid-rock interaction during metamorphism. Calcites are depleted in both 18O ([delta]18O = 12.74[per thousand]) and 13C ([delta]13C = -5.47[per thousand]) relative to dolomite of unmetamorphosed dolostone ([delta]18O = 20.79[per thousand] and [delta]13C = -1.52[per thousand]). The [delta]13C variation can be interpreted in terms of Rayleigh distillation during continuous CO2 fluid removal from the carbonates. The [delta]18O values reflect hydrothermal exchange with an externally derived fluid. Microthermometric analyses of fluid inclusions from vesuvianite indicate that the fluid was water dominated with minor contents of CO2 (+/-CH4 +/- N2) suggesting a metamorphic origin. Fluorine-bearing minerals such as chondrodite, norbergite and F-rich phlogopite indicate that contact metamorphism was accompanied by fluorine metasomatism. Metasomatism was more intense in the inner-central portion of the contact aureole, where access to fluids was extensive. The irregular geometry of the contact with small aplitic intrusives between the metasediments and the Variscan granitoids probably served as pathways for fluid circulation. [PUBLICATION ABSTRACT]