Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications
ABSTRACT: The Poços 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, t...
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doaj-402ad9033a0540b2bfe5bdf3f9ca3d282020-11-24T21:40:25ZengSociedade Brasileira de GeologiaBrazilian Journal of Geology2317-469248239140210.1590/2317-4889201820170095S2317-48892018000200391Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implicationsSilvio Roberto Farias VlachHorstpeter Herberto Gustavo Jose UlbrichMabel Norma Costas UlbrichPaulo Marcos VasconcelosABSTRACT: The Poços 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 40 Ar/ 39 Ar 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 Poços de Caldas rocks are concentrated in the 75-86 Ma range interval.http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2317-48892018000200391&lng=en&tlng=enMelanite-bearing nepheline syenite fragmentsphlogopite megacrystsconduit breccia40 Ar/ 39 Ar agePoços de Caldas Alkaline Massif |
collection |
DOAJ |
language |
English |
format |
Article |
sources |
DOAJ |
author |
Silvio Roberto Farias Vlach Horstpeter Herberto Gustavo Jose Ulbrich Mabel Norma Costas Ulbrich Paulo Marcos Vasconcelos |
spellingShingle |
Silvio Roberto Farias Vlach Horstpeter Herberto Gustavo Jose Ulbrich Mabel Norma Costas Ulbrich Paulo Marcos Vasconcelos Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications Brazilian Journal of Geology Melanite-bearing nepheline syenite fragments phlogopite megacrysts conduit breccia 40 Ar/ 39 Ar age Poços de Caldas Alkaline Massif |
author_facet |
Silvio Roberto Farias Vlach Horstpeter Herberto Gustavo Jose Ulbrich Mabel Norma Costas Ulbrich Paulo Marcos Vasconcelos |
author_sort |
Silvio Roberto Farias Vlach |
title |
Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications |
title_short |
Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications |
title_full |
Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications |
title_fullStr |
Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications |
title_full_unstemmed |
Melanite-bearing nepheline syenite fragments and 40Ar/39Ar age of phlogopite megacrysts in conduit breccia from the Poços de Caldas Alkaline Massif (MG/SP), and implications |
title_sort |
melanite-bearing nepheline syenite fragments and 40ar/39ar age of phlogopite megacrysts in conduit breccia from the poços de caldas alkaline massif (mg/sp), and implications |
publisher |
Sociedade Brasileira de Geologia |
series |
Brazilian Journal of Geology |
issn |
2317-4692 |
description |
ABSTRACT: The Poços 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 40 Ar/ 39 Ar 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 Poços de Caldas rocks are concentrated in the 75-86 Ma range interval. |
topic |
Melanite-bearing nepheline syenite fragments phlogopite megacrysts conduit breccia 40 Ar/ 39 Ar age Poços de Caldas Alkaline Massif |
url |
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2317-48892018000200391&lng=en&tlng=en |
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