Characteristics of granitic pegmatite magmas

Following Tuttle and Bowen's (1958) reference point of an igneous origin for granites near the thermal minimum in the system Ab-An-Or-Qtz, low inferred primary crystallization temperatures coupled with coarse crystals and mineralogical zonation fostered concepts of granitic pegmatite magmas as special entities with viscosities lowered by extreme enrichments in fluxing components including B, P, and F. The vast majority of granitic pegmatites, however, possess simple haplogranite compositions with accessory quantities of flux-bearing minerals (tourmaline, apatite, fluorite, etc). A paucity of alteration within or outside of individual bodies implies that little material is gained or lost during consolidation and, hence, that whole-rock compositions closely approach the bulk compositions of the primary pegmatite magmas. This is true even for more evolved pegmatites that contain abundant B-, P-, or F-rich minerals. Granitic pegmatites, therefore, crystallize from highly viscous liquids, for which graphic feldspar-quartz intergrowths constitute key evidence.

The concept of flux-rich magmas has been rejuvenated recently by reports of exotic melt inclusions (MI) within quartz from pegmatites. The high flux contents of these MI, however, are 1-2 orders of magnitude greater than both their abundance in the bulk rock and the concentrations indicated by relevant buffering mineral equilibria. One possibility is that the flux-rich MI represent boundary layer liquids trapped during rapid crystal growth from viscous granitic compositions.

If formed in the presence of flux-rich liquids, crystalline phases might be anomalously enriched in ordinarily incompatible B, P, and/or F. Experiments entailing rapid growth of alkali feldspars (Afs) from flux-rich liquids yield commensurately enriched crystals with B and P contents much higher than those found in the natural Afs. If a flux-rich melt is present during the consolidation of granitic pegmatites, even as narrow boundary layers along crystal surfaces, then an important main phase of the pegmatites, the Afs, do not record it.