The nature of fluids associated with granulite fades metamorphism in the Okiep Copper District, Namaqualand, South Africa

摘要

The Namaqualand Metamorphic Complex (NMC) is a well-exposed mid-Proterozoic low-P amphibolite-granulite facies terrane flanking the Archaean Kaapvaal Craton of southern Africa. In the Okiep Copper District (OCD) an early volcano-sedimentary supracrustal sequence (1600 - 1300 Ma) records peak metamorphic conditions of 800 - 850℃ and 5kbar in cordierite - garnet - K-feldspar - quartz, hercynitic spinel - quartz, osumilite (metapelite), and orthopyroxene - clino-pyroxene - plagioclase ± hornblende (metabasite) assemblages. A well-defined isobaric cooling path is constrained by retrograde coronas of garnet, sillimanite, and biotite around pre-existing prograde minerals. The supracrustals are intruded by at least 3 distinct magmatic suites, of which the first two are granitic and the last comprises crustally-contaminated mafic intrusions. Of these, the Little Namaqualand Suite (1220 -1170 Ma) is deformed by the regional D_2 deformation, while the later Spektakel and Koperberg Suites (1060 - 1030 Ma) are late-D_2 and D_3. Late-Kibaran crustal thickening and magma-tism at 1060 - 1030 Ma was responsible for, and coeval with, the development of peak metamorphic conditions in the region. Palaeofluids have been studied in healed micro-fractures within a variety of rock types in the region, with an emphasis placed on samples derived from the D_3-related steep structures. Most palaeofluids are concentrated in fluid inclusion planes which have orientations in two major directions, N15 -35°E and N160°E. The first orientation corresponds broadly with the principal compressive stress direction prevailing during the D_2 and D_3 stages of deformation. Fluid inclusions have been characterized by microthermometry and Raman microspectroscopy in order to reconstruct the P-V-T-X conditions of fluid migration during brittle deformation associated with the principal metamorphic stage. Results show that the region is characterized by diverse fluid populations dominated by H_2O-CO_2-N_2 mixtures. Fluids appear to have been trapped both during prograde metamorphism as well as along the isobaric cooling path that characterizes the NMC. Fluid characteristics and isochoral intersections for the NMC are similar to the pattern of fluid evolution observed in other low-P high-T granulite facies terranes.