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

作者：Vilalva Frederico C J^*; Vlach Silvio R F; Simonetti Antonio

来源：The Canadian Mineralogist, 2013, 51(2): 313-332.

DOI：10.3749/canmin.51.2.313

• nacareniobsite-(Ce)
• britholite-(Ce)
• EPMA and LA-ICP-MS analysis
• peralkaline granites
• Morro Redondo Complex
• Graciosa Province
• Brazil

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.

• 出版日期2013-4