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

Abstract

Nacareniobsite-(Ce), Na 3 Ca 3 (REE)Nb(Si 2 O 7 ) 2 OF 3 , 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 Na 2 O and Nb 2 O 5 contents up to 11.1 and 15.5 wt.%, respectively, corresponding to an average formula of Na 3.0 (Ca 2.75 Sr 0.01 ) ∑2.76 (REE,Y) 1.16 (Nb 0.95 Ta 0.02 Ti 0.04 ) ∑1.01 Si 3.99 O 14 (O 1.11 F 2.89 ) ∑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 +4 M 2+ ] −1 [M 5+ Na + ] and, in part, [M 2+ ] −2 [REE 3+ Na + ] and [M +4 M 2+ ] −1 [REE 3+ ] 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 [PM 2+ F] −1 [Si(REE,Y)(OH)], [M 2+ ] −2 [Na(REE,Y)], and [M 2+ (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 °C in a relatively reduced environment.