19 Mineralogy and Petrology

Tuesday, 24 April 2012
8:40 AM - 11:40 AM
Room 204

TOWARDS UNDERSTANDING EARLY VARISCAN METAMORPHISM AND GARNET SPECIES IN THE GUILLERIES MASSIF, CATALAN COSTAL RANGES, NORTHEASTERN SPAIN

Paper number: 19-2
Program: Technical Sessions
Time: 9:00 AM – 9:20 AM

Authors

Abstract

The Guilleries massif of northeastern Spain rises above its Tertiary cover along Neogene normal faults. The lack of an alpine overprint, given its location south of the Alpine Pyrenean axial zone, qualifies it as an ideal location to study the early stages of the Variscan orogeny. Pelitic schist and gneiss of the Osor Formation are characterized by AND+CRD and BT+GRT+SIL at medium- and high-grades, respectively. The massif records a low-pressure/high-temperature metamorphic field gradient. In garnet porphyroblasts, folded S1 inclusion trails with monazite are truncated by the regionally dominant S2 cleavage. AND+CRD grade rocks and S2 are truncated by the 323 Ma Susqueda Diorite where contact metamorphism reached OPX+GRT+CRD grade. BT- and two-mica granites, ranging from meter-sized dikes and sills to mm-sized veins, trending northeast with crystallization ages of ca. 300 Ma, are the youngest intrusives. All large structures and metamorphic zones are cut by Neogene normal faults. This study addresses two questions: Is the metamorphic gradient generated by one regional metamorphic event related to the intrusion of Carboniferous granites or does the Guillerieries massif preserve a series of older smaller events of early-Variscan metamorphism unrelated to Carboniferous magmas? Is it possible to date the evidence of early metamorphism preserved within S1 inclusions?

To resolve these questions, five unique types of garnets from the Osor Formation were analyzed. A: small, idiomorphic, and occurring along the boundaries of quartz veins in S2. Monazite inclusions are present. B: smaller, elongated, and retrograded. These garnets and inclusions are parallel to S2. C: large, sub-idiomorphic, and occurring in the pelitic matrix. These are retrograded with plagioclase coronas. Inclusion trails define S1 suggesting that nucleation of C-garnets occurred during early stages of deformation and metamorphism and growth halted before S2 development. D: large, nodular, and found along quartz vein boundaries in massive quartz-rich psammite. E: large, euhedral idiomorphic grains in the contact aureole of the Susqueda diorite. In this study, through U-Pb geochronology and documenting relic mineral assemblages, fabrics, and P-T conditions, the early stages of the Variscan orogeny are illuminated.