Melanite-bearing nepheline syenite fragments and Ar-40/Ar-39 age of phlogopite megacrysts in conduit breccia from the Pocos de Caldas Alkaline Massif (MG/SP), and implications

Autor(es)

Farias Vlach, Silvio Roberto ^[1] ; Gustavo Jose Ulbrich, Horstpeter Herberto ^[1] ; Costas Ulbrich, Mabel Norma ^[1] ; Vasconcelos, Paulo Marcos ^[2]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Dept Mineral & Geotecton, Inst Geosci, Sao Paulo, SP - Brazil

^[2] Univ Queensland, Sch Earth Sci, Brisbane, Qld - Australia

• Tipo de documento: Artigo Científico
• Fonte: BRAZILIAN JOURNAL OF GEOLOGY; v. 48, n. 2, p. 391-402, APR-JUN 2018.
• Citações Web of Science: 2

Resumo

The Pocos de Caldas Alkaline Massif, located in southeast Brazil, is composed of tinguaites, as well as phonolites and nepheline syenites (NeS), together with assorted pyroclastic rocks. The latter constitute deposits within the Vale do Quartel, and show fragments of alkaline rocks (NeS, tinguaites), basaltic and ultramafic volcanics/subvolvanics, sandstones, and quartz-feldspathic rocks from the crystalline basement. NeS fragments with melanite, previously unknown, and cm-sized fragments of phlogopite megacrysts from related conduit breccia were chosen for detailed studies. The NeS fragments are medium-to coarse grained and contain K-feldspar (Or 95-83), secondary albite (Ab 97), altered nepheline, clinopyroxene (diopside, minor hedenbergite) and partially altered, zoned, idiomorphic melanite. These NeS fragments are typically metaluminous, and point to the existence of similar intrusions hidden at depth, an important input to the massif's petrology. The phlogopite megacrysts are compositionally homogeneous, with high Ti, Mg and Al contents; Ba and F are minor components. Two phlogopite aliquots were dated and yielded identical Ar-40/Ar-39 plateau ages of 87.1(+/- 0.5) Ma and 86.6(+/- 0.5) Ma, which represent minimum and maximum values for phlogopite crystallization and breccia emplacement, respectively. Previously obtained ages for the Pocos de Caldas rocks are concentrated in the 75-86 Ma range interval. (AU)

• Processo FAPESP: 12/06082-6 - A província magmática Paraná-Etendeka no Brasil: relações temporais e petrológicas entre o magmatismo toleítico e alcalino e suas implicações geodinâmicas
• Beneficiário: Excelso Ruberti
• Modalidade de apoio: Auxílio à Pesquisa - Temático