Origin and petrogenetic implications of tourmaline-rich rocks in the Sierra Nevada (Betic Cordillera, southeastern Spain)

摘要

Tourmaline-rich rocks (up to 60% tourmaline) associated with low-medium grade metamorphic assemblages occur in the Sierra Nevada area (Betic Cordillera, southeastern Spain). Tourmaline appears in a variety of forms: (1) stratiform tourmalinites; (2) quartz-tourmaline nodules; (3) porphyroclasts in felsic orthogneisses; and (4) disseminations in psammopelitic meta-sediments and gneisses. Tourmaline within these lithologic groups exhibits textural and chemical variations that reflect complex premetamorphic growth under open-system conditions, and subsequent changes due to Alpine regional metamorphism. Microprobe analyses of the tourmalines reveal a wide compositional variation between schorl and dravite end members with variable contents of X-site vacancies (av. 0.084-0.225 apfu), Ca (av. 0.095-0.269 apfu), and excess of Al (up to 6.588 apfu) compared with the theoretical value of 6 in ideal schorl and dravite. The amount of Ca may be significant in porphyroclasts from the gneisses. Fe/(Fe + Mg) ratios for tourmalines in tourmalinites, metasediments, and gneisses range from 0.34 to 0.95, 0.16 to 0.92, and 0.28 to 0.97, respectively. Na/(Na + Ca) ratios are also variable, mostly ranging from 0.5 to 0.9. Many of the tourmalines have complex chemical and colour zoning patterns, including significant fluctuations in Al, Fe, Mg, Na, Ca, Ti, and F. Based on petrographic and chemical data, three generations of tourmaline have been established. The first generation corresponds to magmatic-postmagmatic tourmaline that is represented by tourmaline porphyroclasts within the orthogneisses. The second generation of tourmaline formed during tourmalinization of psammopelitic rocks giving rise to tourmalinites. The third generation of tourmaline is represented by cellular textures, pale reaction rims and overgrowths developed during the Alpine regional metamorphic overprint. (C) 2002 Elsevier Science B.V. All rights reserved. [References: 76]