Petrological and mineralogical analysis of the metamorphic zones in Cap de Creus, NE Spain

Abstract

The Cap de Creus massif in the northeastern Pyrenees consist of LP–HT greenschist to upper amphibolite facies metasediments and granitoid bodies. The region is subdivided into five metamorphic zones. These zones have been studied extensively but P–T estimates are limited. Earlier studies used general P–T relations to determine pressure and temperature conditions of the metamorphic zones. In this study, P–T conditions were determined based on specific bulk rock compositions from each metamorphic zone in order to give new insights on the relations between the different zones. Optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and major and trace element analyses were performed to determine mineral assemblages and bulk rock compositions for samples from each of the metamorphic zones. The whole-rock chemistry of the samples was determined by X-ray fluorescence (XRF) and Laser Ablation Inducted Coupled Plasma Mass Spectrometry (LA-ICP-MS). Thermodynamic Perple_X modelling was applied to estimate metamorphic conditions from each zone. This resulted in P–T conditions of 1.0–2.5 kbar and 400–500°C in the lowest grade (chlorite) zone and up to 3.8–4.3 kbar and 650–680°C in the highest grade (sillimanite–K-felspar) zone. In the pelitic rocks of this zone, unstable muscovite reacted with cordierite to form new biotite and prismatic sillimanite, possibly at temperatures close to the solidus. Trace element data indicate that no partial melting took place in the highest-grade samples. Actual contact metamorphism did not take place in the samples, but temperature could have increased locally thanks to the heat of intruding granitoid bodies. Retrograde metamorphism created a new muscovite phase when temperatures dropped below at least 655°C again. As retrograde conditions reached greenschist facies conditions, temperature probably dropped below at least 500°C. Over a distance of 2.75 km, a metamorphic field gradient was determined at 62°C/km.