Many evolved peraluminous granite plutons contain either andalusite (Al2SiO5) or topaz (Al2SiO4(OH,F)2), but some plutons contain both of these A/NK [mol. Al2O3/(Na2O+K2O)] = ∞ phases. We present the results of experiments conducted to locate the andalusite-topaz boundary in water-saturated peraluminous haplogranite melts in T-P-Xspace, where T = 700—650°C, P = 200 MPa. andX(A/NK = 1.1, 1.2, 1.3, 1.4, and F = 0, 1, 2, 4 wt.%).The starting materials are synthetic K-Na-Al-Si gels, with added H2O and AgF, with seeds of natural andalusite and topaz, and with run times of 5—7 days. Phase identification is by electron microprobe. All experimental run products contained quartz. For all values of bulk A/NK, F concentrations ⩽ 1 wt.% favour andalusite stability, whereas F concentrations ⩾ 1 wt.% favour topaz stability. For bulk A/NK ⩽ 1.2 and intermediate F concentrations, no A/NK = phase is present; for bulk A/NK ⩾ 1.3, quench or metastable mullite formed. Topaz reaction rims on naturally occurring andalusite, frequency of naturally occurring assemblages, general chemical considerations, and experimental evidence all suggest that some topaz is the product of a peritectic reaction between an early primary magmatic andalusite and a late F-enriched melt. The appearance of such topaz is an important mineralogical expression of F activity in the melt, and may be an indicator for high field-strength Sn-W-U-Mo polymetallic mineralization.

Most granites result from partial melting within the crust. Granite melts produced at the lowest temperatures of partial melting mainly comprise close to equal amounts of the haplogranite components Qz, Ab and Or, with H2O. Many felsic granites were formed by partial melting under such conditions and are low-temperature types, with crystals of zircon and other restite minerals present in the initial magma. Such magmas evolve in composition, at least initially, through fractionation of that restite. If one of the four haplogranite components either becomes depleted or too low in amount to contribute further to the melt, then melting may proceed to higher temperatures without a contribution from that component. Melting will advance to significantly higher temperatures if there is a critical deficiency in one or more components and a high-temperature granite magma forms, in which zircon is completely soluble. Such magmas are extracted from the source in a completely molten state and may evolve by fractional crystallisation. They are monzonitic, tonalitic or A-type, depending on whether the critical deficiency occurred in the Qz, Or or H2O component. If the Ab component is critically deficient, as in pelitic rocks, the rocks may be infertile for granite production. The control that source rock compositions exert on both the physical and chemical properties of granite magmas provides a unifying element in granite gen