Feldspar thermometry in pegmatites: truth and consequences

Abstract

Temperatures of crystallization for all or portions of three thin granitic pegmatite dikes in southern California are derived from feldspar solvus thermometry, with supporting data from the K/Cs ratio of K-feldspar, the extent of Al/Si order in K-feldspar, and the texture of granophyre found along the margins of dikes. Although K-feldspars become perthitic and increasingly ordered toward the centers of dikes, their ratio of K/Cs falls from margin to core along trajectories that reflect fractional crystallization from silicate melt without subsequent interaction with an aqueous solution in an open system. A few sporadic samples that record loss of Cs, and consequent rise in K/Cs, validate the test of fidelity that the perthites generally retain their igneous compositions. Feldspar solvus thermometry from these three dikes indicates that their pegmatite-forming melts crystallized at ~\u2009375–475 °C. Those low temperatures are consistent with the occurrence of granophyric plagioclase–quartz intergrowths along the borders of pegmatites, thick and thin, that arise from thermal quenching of their melts against much cooler host rocks, and hence at much shallower depths than the igneous sources of the pegmatite-forming melts. The temperature profiles are nearly isothermal across the pegmatites, but where variation exists, apparent temperatures are highest along their margins or in their central domains (cores). Plagioclase shows normal fractionation of decreasing An content from margins to center, which mimics the line of descent with cooling down the solidus and solvus surfaces. However, the fractionation trends in the feldspars are attributable to their isothermal crystallization far from the equilibrium of the liquidus at a highly undercooled state, not to crystallization upon cooling on the solidus surface.