The P–Fe diagram for K-feldspars: A preliminary approach in the discrimination of pegmatites

Abstract

Pegmatites are extremely coarse-grained and heterogeneous rocks in which quantitative measurements of mineral proportions and chemical compositions of the whole rock are virtually impossible to acquire. Thus, conventional criteria such as bulk compositions and modal mineralogy used for the classifications of igneous rocks simply cannot be applied for pegmatites. An alternative is to use the mineralogical and chemical attributes of K-rich feldspars, the only mineral that is omnipresent in pegmatites. We have used this approach to test a possible discriminant among four groups of pegmatites on the basis of major petrological features, such as the abundance of quartz, feldspars, micas and phosphates. Group I is represented by relatively flux-poor, and silica-poor pegmatites, in most cases with hypersolvus feldspars, devoid of quartz and with minor biotite, which are common in rift settings as in the Coldwell Alkaline Complex in northwestern Ontario, Canada. Group II comprises relatively flux-poor, silica-rich pegmatites with quartz, subsolvus feldspars and biotite as major primary minerals, typically occurring in the asymmetric collisional Grenville Orogeny. Group III comprises relatively flux-rich, silica-rich P-poor pegmatites with quartz, subsolvus feldspars, and muscovite as the major primary minerals. Finally, group IV consists of relatively flux-rich, silica-rich, P-rich pegmatites with the same previous major minerals as in group III but with abundant phosphates. Group III and IV are found in most symmetric collisional orogens, such as in the Eastern Brazilian Pegmatite Province as the result of the collision of cratons mainly formed by igneous and metamorphic rock of Archean and Early Proterozoic age. We have selected specimens of blocky perthitic K-rich feldspar from the inner part of thirty-one pegmatites belonging to these four categories occurring worldwide to cover a wide range of mineralogy, geological age, geotectonic setting and geographical positions. Concentrations of major elements (Si, Al, K, Na, Ca, Fe, Mg, Mn, Ti and P) were obtained by X-ray fluorescence (XRF), and those of minor and trace elements (P, Fe, Li, Ge, Ga, Rb, Sr, Ba, Tl, Pb, Y, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were established by laser-ablation inductively coupled plasma - mass spectrometry (LA-ICP-MS), in areas free of coarse Na-feldspar veins or patches. We show that the four groups have very different average values of the minor and trace elements. However, only the cations occupying tetrahedral sites, particularly the Fe and P, are sufficiently immobile to show distinct differences among pegmatites. Hence, we propose a P–Fe diagram to discriminate among the four groups of pegmatites, as a possible criterion with which to classify pegmatites.