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Título

MINERAL CHEMISTRY AND AMPHIBOLE-PLAGIOCLASE GEOTHERMOBAROMETRY OF GRANITOIDS FROM THE LAGOA DA MEIA LUA SUITE, SÃO GABRIEL BLOCK, WESTERN DOM FELICIANO BELT, RS

Texto do resumo

An integrated textural and chemical approach on plagioclase, K-feldspar, amphibole, biotite, epidote, titanite, and Fe-Ti oxides was conducted to investigate chemical contrasts, as well as to estimate crystallisation conditions and geodynamic implications for two granitoids (Cerca de Pedra and Buriti) of the post-collisional Lagoa da Meia Lua Suite (LMLS) (700-680 Ma). The LMLS rocks are intrusive mainly into calc-alkaline, arc-related, Tonian dioritic, tonalitic, and granodioritic orthogneisses of the Cambaí Complex (750-720 Ma), São Gabriel Block, western Dom Feliciano Belt. The Cerca de Pedra Granitoids (CPG) are fine- to medium-grained heterogranular biotite tonalites and biotite granodiorites with anhedral feldspars and irregular aggregates of mafic minerals (M’ 8-15). The Buriti Granitoids (BG) are medium- to coarse-grained, heterogranular to locally porphyritic, biotite quartz monzodiories, biotite tonalites and biotite granodiorites (M’ 10-18) with subhedral to euhedral feldspars. The CPG shows the primary assemblage plagioclase + quartz + K-feldspar + biotite ± amphibole ± titanite ± epidote ± magnetite. The BG also shows this primary assemblage except for magnetite. The Fe-Ti oxide in the BG is ilmenite. Biotite is the main mafic phase and amphibole is an acessory phase in both granitoids. Other acessories are allanite, apatite and zircon. Epidote and titanite are also found as secondary phases, with sericite, chlorite and carbonate. CPG plagioclase crystals are oligoclase, with cores (An21-27) slightly more calcic than rims (An18-27). K-feldspar composition is Ab7-8Or92-93. The BG plagioclase crystals are mainly andesine, with weak normal zoning (core An30-34; rim An30-32) and some reversely zoned crystals (core An30-31; rim An31-34). K-feldspar composition is Ab7-9Or91-93. CPG and BG amphiboles are calcic and classify mainly as magnesio-hornblende, tschermakite and magnesiohastingsite in the former, and ferrohornblende, ferrotschermakite and ferropargasite in the latter. The CPG amphibole has a relatively higher Mg/(Mg + Fe2+) ratio (0.50-0.59) than that of the BG (0.32-0.43). CPG biotite shows Fe2+/(Fe2+ + Mg) ratio (0.47-0.52), and its composition transitions between phlogopite and annite. The BG biotite has Fe2+/(Fe2+ + Mg) ratio (0.60-0.69) and plots towards a siderophyllite composition. Magmatic epidote forms subhedral to euhedral inclusions in the biotite of both granitoids. CPG and BG epidotes have Ps28-30 and Ps18-20 [Ps = molar Fe3+/(Fe3+ + Al) x 100] and low TiO2 (0.01-0.16 and 0.15-0.26 wt. %, respectively). CPG titanites have Al + Fe3+ (0.06-0.10 apfu) and Ti (0.92-0.95 apfu). CPG magnetite has a nearly end-member composition with very low TiO2 contents (0.04 – 0.08 wt. %; Mol% Usp. 0.00-0.27). It forms subhedral to euhedral crystals with straight grain boundaries against biotite, plagioclase, quartz, and titanite. The Fe-Ti oxide in the BG is ilmenite found as inclusions in plagioclase. The composition is TiO2 52.5 wt. %, FeOt 44.5 wt. % and MnO = 3.0 wt. %. Mineral assemblages, textural relationships, mineral chemistry, and amphibole-plagioclase geothermobarometry indicate near-solidus crystallisation conditions (P = 5.0-5.8 kbar; T = 692-716 ºC), and high oxygen fugacity (fO2) for the CPG. On the other hand, the BG has crystallised under higher pressures (6.4-6.9 kbar), lower temperatures (666-706 ºC), and intermediate fO2.

Palavras Chave

Post-collisional; São Gabriel Block; Lagoa da Meia Lua Suite; mineral chemistry; Geothermobarometry