Zr- AND Ba-RICH MINERALS FROM THE PONTE NOVA ALKALINE MAFIC-ULTRAMAFIC MASSIF, SOUTHEASTERN BRAZIL: INDICATION OF AN ENRICHED MANTLE SOURCE

Abstract

Zirconium-and Ba-rich minerals are found in gabbroic rocks from the Ponte Nova alkaline mafic-ultramafic massif in southeastern Brazil. The unusual mineralogical assemblage includes zirconolite, baddeleyite, Ba-rich alkali feldspar, and Baand Ti-rich biotite. Zirconolite of the Ponte Nova massif has higher levels of Zr (up to 1.172 apfu) than those registered in other terrestrial rocks and a prominent enrichment in the light rare-earth elements. Baddeleyite contains small quantities of Hf, Ti, and Fe. The Ba-rich alkali feldspar and Ba-and Ti-rich biotite contain up to 9.25 and 7.35 wt% BaO, respectively, and the biotite contains up to 12.01 wt% TiO 2 . In the different intrusions of the Ponte Nova massif, such an unusual assemblage typifies the residual magma after the crystallization of clinopyroxene and olivine from previously enriched basanitic parental magma. The different trends of enrichments in REE and Th + U found for zirconolite of the intrusions of the Ponte Nova massif provide a better understanding of the variable degrees of enrichment of incompatible elements of the distinct gabbroic bodies. A lithospheric mantle source enriched in incompatible elements by the metasomatic action of volatile-rich fluids and with the presence of phlogopite or amphibole (or both) and other minor accessory phases could explain the presence of the Zr-and Ba-rich minerals in this gabbroic massif.

Alkaline magmatism in the Amambay area, NE Paraguay: The Cerro Sarambí complex

“…For the Cerro Sarambí phlogopite-biotites, both Ti-Tschermak-and Ti-vacancy coupled substitutions are the key operative mechanisms (Fig. 4B and C), as also concluded by Azzone et al (2009) for the Ti-rich biotites from the Ponte Nova massif. The Ti-oxy substitution cannot be tested because of the unknown H 2 O content of the micas, although the low totals suggest that this mechanism may not be relevant.…”

COEXISTING Ba-FELDSPAR AND MELILITE IN A MELAFOIDITE LAVA AT MT. VULTURE, ITALY: ROLE OF VOLATILES AND ALKALINE EARTHS IN BRIDGING A PETROLOGICAL INCOMPATIBILITY

“…Vulture (together with the Prete della Scimmia melilitite neck). Nkoumbou et al (1995), Conticelli et al (1997), Gaeta (1998) and Azzone et al (2009) analyzed hyalophane at Mt. Etinde (Cameroon), Sabatini and Alban Hills (Italy) and Ponte Nova (Brazil) alkaline rocks, but no coexisting melilite has been reported.…”

Preliminary investigation of Ba-rich sanidine in phonolites of Badmer, Rajasthan

“…Accordingly they have been reported from alkaline rocks such as leucitites, analcitites, basanites, phonolites and lamprophyres and ultrabasic rocks (Cundari, 1979, Zhang et al, 1993a, Deer et al, 2001, Chakhmouradian and Mitchell, 2002, Azzone et al, 2009, Fergusson and Cundari, 1982, Flohr and Ross 1990, Nephelinite from Cameroon, (Nkoumbou et al, 1975), from granitic suites (Nemec, 1975, Yanev et al, 1998, Essene et al, 2005. Essene et al (2005) also described in greater detail celsian and hyalophane from metamorphic rocks and from some igneous rocks.…”

“…On triangular diagram (Fig. 8) Na-K-Ba (Azzone et al, 2009) the compositions of Ba-rich sanidines from Badmer phonolites plot in igneous field. feldspars from melaphonolites of Badmer complex, no definite conclusions are drawn here as a detailed investigation of melanephelinite-melaphonolite and felsicphonolite association is contemplated.…”