LA-ICP-MS U–Pb columbite ages and trace-element signature from rare-element granitic pegmatites of the Pampean Pegmatite Province, Argentina

Abstract

Crystals of the columbite-group minerals (CGM) from 23 rare-element granitic pegmatites of the Pampean Pegmatite Province (PPP) were analyzed by LA-ICP-MS technique. The resulting database shows weighted mean 206Pb/238U ages between 530 and 335 Ma defining two time windows, T1 and T2. T1, from 490 to 440 Ma, spans 75% of the ages of LCT rare-element pegmatites formed during the Famatinian collisional orogeny of peri-Gondwana location. T2, from 370 to 340 Ma, comprises postorogenic pegmatites with lithium-caesium‑tantalum (LCT) and niobium‑yttrium‑fluorine (NYF) or mixed NYF-LCT signatures. The CGM ages of the PPP orogenic pegmatites show a shifting from older in the north to younger in the south. The major elements composition of the CGM shows preference for high #Mn at variable but usually moderate to high #Ta. The normalized, with the Global CGM median, trace elements composition of the CGM of most orogenic LCT (Li-Cs-Ta) pegmatites shows an enrichment in Bi, Pb and U, a remarkable depletion in Sn and sometimes a slight decrease in W (Y ± Yb). The postorogenic pegmatites show CGM with a general enrichment in Y (±Yb), W, Pb, U, and occasionally in Sn. The REEN diagrams of CGM from the northern districts of the PPP show patterns with LREEN < MREEN > HREEN and strong negative Eu anomalies. In the southern districts, there is a bimodal behavior in the total content and distribution of the normalized patterns of REE. The CGM from some pegmatites of the Conlara Pegmatite district have similar features to the ones from the northern districts. It seems that the kind of pegmatite has no definitive control in the type of pattern. The other group shows low contents of REEN mostly comprised between 0.01 and 10 times the chondrite values, flattened patterns, weak to moderate negative anomalies of Eu, and occasionally a weak Ce anomaly. In the southern Totoral district, the CGM of the different kinds of pegmatites also have approximately similar, though relatively lower, total REEN contents and patterns with LRREN < MREEN < HREEN. The petrogenesis of the orogenic LCT pegmatites is linked to the differentiation of S-type leucogranites, likely of low temperature, generated by a low degree of partial melting of the upper crustal sequences of micaceous protoliths. The postorogenic pegmatites are of NYF (Nb-Y-F) or mixed signatures, linked likely to the A-type monzogranitic batholiths. The combined data of ages, trace element and REE of the CGM from the PPP give a typical signature to its Ta ore minerals.