Origin of alkali-feldspar granites: An example from the Poimena Granite, northeastern Tasmania, Australia

Abstract

The Lottah Granite is a composite pluton of tin-mineralised, strongly peraluminous alkali-feldspar granite which intrudes the Poimena Granite, a major component of the mid-Devonian Blue Tier Batholith of northeastern Tasmania. Earlier workers interpreted the Lottah Granite as a metasomatised differentiate of the Poimena Granite. The Poimena Granite is a slightly peraluminous, felsic, I-type biotite granite which contains restite minerals and shows linear trends on Marker plots, both consistent with restite separation. The mineralogy, chemical variation, and isotopic characteristics of the Lottah Granite are consistent with origin as a magma genetically unrelated to the host granite. The Lottah Granite contains sanidine, albite, topaz, zinnwaldite and other minerals consistent with crystallisation from a melt. It is richer than the Poimena Granite in Li, Rb, Sn, Ga, F, and P, and trends on variation diagrams are distinctly different ( e.g., SiO 2 decreases as Na 2O, Rb, Li and F increase) from those of the Poimena Granite. Furthermore, Rb-Sr isotopic dating indicates that the Lottah Granite was emplaced about 10 Ma after the Poimena Granite, and initial Sr and Nd isotope ratios indicate that the Lottah Granite was derived from a higher- 87Sr/ 86Sr (0.715 ±.002, cƒ. 0.70933 ±.0008), higher-ϵNd (−2.15 to –2.75, cƒ. about -6) source composition. Chemical and mineralogical evolution of the Lottah Granite conform to the experimental behaviour of Li-F-rich melts, and indicate a possible crystallisation temperature range as extreme as 750-430°C. Vertical chemical zonation of the Lottah Granite is consistent with fractional crystallisation and accumulation of residual liquids at the top of the magma chamber. Many other examples of alkali-feldspar granite, and much of the associated mineralisation, are probably also of essentially primary magmatic origin rather than of metasomatic or hydrothermal origin as commonly interpreted. They may also be genetically unrelated to granites with which they are associated.