Abstract
Abundant Li-Cs-Ta aplite-pegmatite dykes were emplaced in the western Central Iberian Zone of the Iberian Massif during the Variscan Orogeny. Their origin and petrogenetic relationships with the widespread granitoids have led to a currently rekindled discussion about anatectic vs. granitic origin for the pegmatitic melts. To deal with these issues, the aplite-pegmatite dykes from the Tres Arroyos area, which constitute a zoned pegmatitic field related to the Nisa-Alburquerque granitic batholith, have been studied. This work comprises a complete study of Nb-Ta-Sn oxides’ mineralogy, whole-rock geochemistry, and U-Pb geochronology of the aplite-pegmatites that have been grouped as barren, intermediate, and Li-rich. The most abundant Nb-Ta-Sn oxides from Tres Arroyos correspond to columbite-(Fe), columbite-(Mn) and cassiterite. Niobium-Ta oxides show a marked increase in the Mn/(Mn+Fe) ratio from the barren aplite-pegmatites up to the Li-rich bodies, whereas variations in the Ta/(Ta+Nb) ratio are not continuous. The probable factors controlling fractionation of Mn/Fe and Ta/Nb reflected in Nb-Ta oxides may be attributed to the crystallization of tourmaline, phosphates and micas. The lack of a progressive Ta/Nb increase with the fractionation may be also influenced by the high F and P availability in the parental pegmatitic melts. Most of the primary Nb-Ta oxides would have crystallized by punctual chemical variations in the boundary layer, whereas cassiterite formation would be related to an undercooling of the system. Whole-rock composition of the distinguished lithotypes reflects similar tendencies to those observed in mineral chemistry, supporting a single path of fractional crystallization from the parental Nisa-Alburquerque monzogranite up to the most evolved Li-rich aplite-pegmatites. The age of 305 ± 9 Ma, determined by LA-ICP-MS U-Pb dating of columbite-tantalite oxides, reinforces the linkage of the studied aplite-pegmatites and the cited parental monzogranite.
Highlights
Niobium-tantalum-tin (Nb-Ta-Sn) oxides are characteristic phases in evolved granitic systems, since Nb, Ta and Sn behave as incompatible elements during magmatic fractionation (e.g., [1,2,3].In this sense, pegmatites, as one type of these granitic systems, are of great interest and represent an important target for the mining of columbite-tantalite (CT) group minerals and other strategic elements (e.g., [1,4,5,6])
In the Tres Arroyos field, cassiterite occurs as an accessory phase and shows different textural features depending on the aplite-pegmatite type (Table 1)
In the Tres Arroyos field, cassiterite occurs as an accessory phase and shows different textural features depending on the aplite-pegmatite type(a)(Table
Summary
Niobium-tantalum-tin (Nb-Ta-Sn) oxides are characteristic phases in evolved granitic systems, since Nb, Ta and Sn behave as incompatible elements during magmatic fractionation (e.g., [1,2,3].In this sense, pegmatites, as one type of these granitic systems, are of great interest and represent an important target for the mining of columbite-tantalite (CT) group minerals and other strategic elements (e.g., [1,4,5,6]). The presence of cassiterite seems to be a marker of the degree of magmatic differentiation, since it usually appears to be restricted to the most evolved pegmatites according to the classification of Černý and Ercit [18] This model of the genetic linkage between granite plutons and pegmatites, as residual melts derived from fractional crystallization in large-scale granitic plutons (e.g., [19]), is supported by field relationships and petrographic observations, combined with mineralogical, geochemical, and geochronological data in some pegmatite provinces (e.g., [20,21,22]). The pegmatites are interpreted to be of anatectic origin, meaning that they crystallized directly from melts formed by a low-degree of partial melting of high-grade metamorphic rocks (e.g., [23,24,25])
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