Two Tertiary metamorphic events recognized in high‐pressure metapelites of the Nevado‐Filábride Complex (Betic Cordillera, S Spain)

Abstract

AbstractTwo phengite–garnet‐bearing metapelites from the Nevado‐Filábride Complex, Betic Cordillera, one from the Ragua unit (sample 23085) and the other (sample 23098) from the Calar Alto unit, were studied in detail to elucidate their metamorphic evolution. We calculated various P–T pseudosections for different O2 and H2O‐CO2 contents and Fe3+/Fe2+ ratios with PERPLE_X . On the basis of the compositions of the garnet core and the highest Si content in potassic white mica, different peak pressures of 12.7 ± 0.7 kbar at 525 ± 10°C (23085) and 17.4 ± 1.2 kbar at 526 ± 17°C (23098) resulted (errors refer to an estimated 1σ range). The suggested clockwise P–T loop was followed in both cases by another loop caused by a reheating process. On the basis of garnet rim compositions, peak temperatures of 596 ± 10°C at 10.7 ± 1.4 kbar (23085) and 629 ± 11°C at 8.5 ± 0.7 kbar (23098) resulted, supported by Zr‐in‐rutile thermometry (23085: 591 ± 11°C; 23098: 607 ± 10°C). In situ dating of monazite (23085) with the electron microprobe yielded ages between c. 48 and 14 Ma. On the basis of the monazite composition and histogram analysis, two age populations with mean ages of 40.2 ± 1.7 (1σ) and 24.1 ± 0.8 Ma could be defined. Low Y2O3 contents (<0.6 wt%) in monazite point to its formation after initial growth of garnet. The older generation was therefore assigned to the formation of the garnet core and the early P–T loop. The younger monazite population was related to the late P–T loop. A geodynamic scenario is hypothesized that assigns the Eocene metamorphism to subduction of a continental margin to depths of 50–65 km, followed by exhumation in an exhumation channel accompanied by mixing of slices of different rock types including mantle rocks. This process ended with stacking of such slices causing reheating of the studied rocks in the Early Miocene. The noted time gap of c. 15 Ma between subduction and nappe stacking is similar to the continent–continent collisional orogeny of the Himalaya.