Three-Phase Perthite and the Progressive Predominance of Albite in the Cap de Creus LCT Pegmatite Swarm, Catalunya, Spain

The Cap de Creus swarm of 400 or so dikes is a well-known and well-exposed LCT granitic pegmatite in Catalunya, northeastern Spain. Four categories of dikes are recognized across the zoned metamorphic belt, from Type-I pegmatites in the sillimanite-K-feldspar zone to Type-IV in the chlorite-muscovite zone. The dikes were emplaced close to 299 million years ago, and a fluid-circulation event dated at 292 m.y. accompanied the intrusion of the nearby Roses granite. We chose 20 representative samples (six from Type I, five from Type II, five from Type III, and four from Type IV) to determine how closely the feldspar assemblages had achieved equilibrium compositionally and structurally. We acquired step-scanned X-ray diffraction scans of the 20 samples and refined the spectra using the Rietveld full-pattern approach. In the time available, the magmatic K-feldspar equilibrated fully compositionally with albite down to a low temperature. Structurally, the pseudomonoclinic K-feldspar (now orthoclase) did invert to well-ordered microcline in the time available only in rare cases. Also present are imperfectly ordered microcline and relict orthoclase. The progression of Al–Si order in K-feldspar requires repeated solution-and-redeposition steps. Dissolution in the ambient aqueous fluid is very sluggish in peraluminous systems like this one. As a result, eleven of the 20 samples contain three-phase perthite (Mcc + Or + Ab), which is clearly a metastable assemblage. The presence of CO2 in the fluid phase hindered progress in the ordering reaction. In the transition from Type-I to Type-IV dikes, the proportion of K-feldspar progressively decreases, such that albite eventually predominates or becomes the only feldspar to coexist with muscovite. The steady buildup of albite at the expense of K-feldspar is a predictable result of ionic equilibria in the system K2O–Na2O–Al2O3–SiO2–H2O. In most cases, the assemblage crossed the K-feldspar-out curve before the ordering reaction was complete.