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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Main Authors: Silvio Roberto Farias Vlach, Horstpeter Herberto Gustavo Jose Ulbrich, Mabel Norma Costas Ulbrich, Paulo Marcos Vasconcelos
Format: Article
Language:English
Published: Sociedade Brasileira de Geologia
Series:Brazilian Journal of Geology
Subjects:
Online Access:http://www.scielo.br/scielo.php?script=sci_arttext&pid=S2317-48892018000200391&lng=en&tlng=en
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spelling 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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