NACARENIOBSITE-(Ce) AND BRITHOLITE-(Ce) IN PERALKALINE GRANITES FROM THE MORRO REDONDO COMPLEX, GRACIOSA PROVINCE, SOUTHERN BRAZIL: OCCURRENCE AND COMPOSITIONAL DATA

Author(s)

Vilalva, Frederico C. J. ^[1] ; Vlach, Silvio R. F. ^[1] ; Simonetti, Antonio ^[2]

Affiliation

^[1] Univ Sao Paulo, Inst Geosci, BR-05508080 Sao Paulo - Brazil

^[2] Univ Notre Dame, Dept Civil Engn & Geol Sci, Notre Dame, IN 46556 - USA

• Document type: Journal article
• Source: CANADIAN MINERALOGIST; v. 51, n. 2, p. 313-332, APR 2013.
• Web of Science Citations: 11

Abstract

Nacareniobsite-(Ce), Na3Ca3(REE)Nb(Si2O7)(2)OF3, and britholite-(Ce), (Na, Ca, REE)(10)(Si, P)(6)O-24(OH, F)(2), formed during the late stages of crystallization of the most evolved peralkaline granites from the Morro Redondo Complex, Graciosa Province (Southern Brazil). Nacareniobsite-(Ce) compositions are very close to ideal, with Na2O and Nb2O5 contents up to 11.1 and 15.5 wt.%, respectively, corresponding to an average formula of Na-3.0(Ca2.75Sr0.01)(Sigma 2.76) (REE, Y)(1.16) (Nb0.95Ta0.02Ti0.04)(Sigma 1.01) Si3.99O14(O1.11F2.89)(Sigma 4). Compared to the host rock composition, trace-element patterns for nacareniobsite-(Ce) indicate enrichment in REEs and Sr, and depletion in alkali metals and Zr. The available chemical data are in excellent agreement with a complete solid solution series between rinkite and nacareniobsite-(Ce), controlled by {[}M+4M2+](-1){[}M5+Na+] and, in part, {[}M2+](-2){[}REE3+Na+] and {[}M+4M2+](-1){[}REE3+](2) exchange vectors. Britholite-(Ce) compositions are amongst the most (REE, Y)-enriched reported to date, with REE + Y contents up to 73.6 wt.% oxide. REE and trace element distributions relative to the host rock exhibit some similarities with those observed for nacareniobsite-(Ce); however, Ba, Th, U, and Pb contents are relatively higher. The observed compositions agree very well with {[}PM2+F](-1){[}Si(REE, Y)(OH)], {[}M2+](-2){[}Na(REE, Y)], and {[}M2+(Th, U)](-1){[}(REE, Y)](2) exchange vectors. Crystallization of both minerals was favored by the F-, Na-, and HFSE-enriched nature and moderate-to-strong peralkaline signature of the melt, at temperatures below 600 degrees C in a relatively reduced environment. (AU)

• FAPESP's process: 08/00562-0 - Mantle-crust interaction and the genesis and evolution of the A-type: post-collisional magmatism: geochemical and isotopic records from the Graciosa Province, S-SE Brazil
• Grantee: Silvio Roberto Farias Vlach
• Support Opportunities: Regular Research Grants