The evolution of fluorine-rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite

Taylor, R.P. and Fallick, A.E. (1997) The evolution of fluorine-rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite. Terra Nova, 9(3), pp. 105-108.

Abstract

Topaz granite is alkali-feldspar granite that contains essential albite, quartz, K-feldspar, lithium-mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a 'low-P₂O₅ subtype' (P₂O₅ < 0.1 wt%, Al₂O₃ < 14.5 wt%, SiO₂ > 73 wt%) and a 'high-P₂O₅ subtype' (P₂O₅ > 0.4 wt%, Al₂O₃ > 14.5 wt%, SiO₂ < 73 wt%), the δ18O values of which indicate a dichotomy of source rock: the low-P₂O₅ subtype (δ¹⁸O < 10‰) having a meta-igneous protolith and the high-P₂O₅ subtype (δ¹⁸O > 10‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal-melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near-minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fallick, Professor Anthony
• Authors: Taylor, R.P., and Fallick, A.E.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Terra Nova
• Publisher: Wiley-Blackwell Publishing Ltd.
• ISSN: 0954-4879
• ISSN (Online): 1365-3121
• Published Online: 25 November 2003