Metamorphic evolution of Triassic rocks from the transition zone between the Malaguide and Alpujarride complexes (Betic Cordilleras, Spain)

A complete sequence of mineral assemblages ranging from late diagenesis to the epizone has been identified in Triassic rocks from "intermediate units" between the Alpujarride and the Malaguide complexes (Betic Cordillera, Spain). These units appear as a set of tectonic slices, the uppermost showing lithological characteristics similar to the Malaguide complex, which change as depth increases, towards lithologies typical of the Alpujarride complex. The study was carried out by X-ray diffraction, optical microscopy, electron microprobe and X-ray fluorescence. The mineralogical composition shows a clear vertical evolution: A dickite-bearing assemblage, characteristic of the upper slices is replaced by sudoite +/- pyrophyllite-bearing assemblages, these by trioctahedral chlorite-rich assemblages, and finally by paragonite-bearing assemblages. Examination of bulk-rock chemistries indicates that most of the mineralogical changes are the result of the increasing metamorphic grade, with the exception of chloritoid; its growth does appear to be partly controlled by the rocks chemistry (it is Al2O3 dependent). The evolution of the metamorphic assemblages occurred prior to emplacement of the slices by thrusting and reveals a progressive transition in metamorphic grade between both complexes, although some discontinuities in the KI values between consecutive slices are observed.