ISSN:
1365-3121
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1365-3121.1997.00124.x
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