RETROGRADE EVOLUTION OF QUARTZ SEGREGATIONS FROM THE DOS-PICOS SHEAR ZONE IN THE NEVADO-FILABRIDE COMPLEX (BETIC CHAINS, SPAIN) - EVIDENCE FROM FLUID INCLUSIONS AND QUARTZ C-AXIS FABRICS

Synkinematic quartz veins are ubiquitous in the shear zone separating the Veleta unit from the Calar Alto unit in the internal part of the B etic Cordilleras. They have been studied with respect to quartz c-axis fabrics, microstructures and fluid inclusions. Veins were probably ge nerated during syn-metamorphic stacking of the units at P = 500-600 MP a and T = 400-500 degrees C. Quartz displays two groups of microstruct ures in the shear zone: (1) older coarse-grained mosaics (CGM) resulti ng from exaggerated grain growth; and (2) younger fine-grained mosaics (FGM) developed at the expense of the former. The fine-grained mosaic s show polygonal granoblastic and elongate mosaic microstructures in g eneral, with ribbon microstructures often found near the boundary of t he units. Fluids contained in secondary inclusions vary from high sali nity brines to different types of CO2-brine mixtures and low density C O2 fluids. Differences in composition and P-T trapping conditions are indicated for the different types of inclusions. Some fluid inclusions are older than the FGM, whereas others are younger, thus constraining the P-T conditions at which the two microstructural events took place . Fluid inclusion evidence suggests conditions of P-fluid > 170 MPa an d T greater than or equal to 370-430 degrees C for the CGM and P-fluid greater than or equal to 20-80 MPa and T > 340 degrees C for the FGM. The quartz c-axis fabrics dealt with here correspond to the second re crystallization event, as little evidence of older fabrics is preserve d in the shear zone. C-axis patterns vary across the shear zone from s lightly asymmetrical type I crossed girdles in the hanging wall and fo otwall to more asymmetrical crossed girdles at the boundary of the uni ts. This indicates a correlative increase in the magnitude of the hete rogeneous shear strain in the same direction. Most of the deformation is concentrated at the top of the Veleta unit. The sense of movement i s top to the west, in agreement with other kinematic markers. The quar tz c-axis fabrics resulted from dynamic recrystallization during simpl e shear. The retrograde P-T path inferred from fluid inclusion analysi s, along with other geological and geochronological evidence, indicate s that this deformation is coeval with a reduction in the crustal over burden. Geochronological and stratigraphic data show that the proposed Dos Picos extensional detachment, separating the Calar Alto and Velet a units, took place during the early Miocene, synchronous with the int ense thinning of the Nevado-Filabride Complex and of the whole contine ntal crust underlying the Alboran Basin.