U–Th–Pb SHRIMP ages and oxygen isotope composition of zircon from two contrasting late Variscan granitoids, Nisa-Albuquerque batholith, SW Iberian Massif: Petrologic and regional implications

Abstract

The late Variscan Nisa-Albuquerque batholith in the SW Iberian Massif, consists of a dominant very coarse-grained porphyritic S-type monzogranite to syenogranite (Nisa monzogranite) surrounding a discontinuous central core that includes contrasting very fine-grained I-type tonalite–granodiorite (Aldeia da Mata tonalite). The batholith is located at the boundary between the Central Iberian and Ossa-Morena Zones, a complex segment of crust that was subject to both Cadomian and Variscan tectonism. Variscan zircons in the Nisa monzogranite can be broadly classified into three texturally and chemically distinct types with mutually indistinguishable SHRIMP 206Pb/ 238U ages: 1) high-U, low-Th/U (< 0.1) outermost overgrowths (307.4 ± 4.0 Ma); 2) moderate U and Th/U zircon with concentric zoning occurring both as inner overgrowths and whole grains (305.4 ± 6.2 Ma)";; and 3) texturally discordant cores (309.0 ± 4.6 Ma). Many other cores have ages in the ranges 2.56–1.85 and 0.66–0.51 Ga. The overgrowths and Variscan cores with low Th/U have uniformly high δ 18O (9.5 ± 0.2‰). Variscan cores with moderate Th/U have a wide range of δ 18O (6.7–10.9‰). Cores older than 500 Ma have an even wider range of composition (4.4–10.0‰). Zircon from the central Aldeia da Mata tonalite, in contrast, contains no inherited cores, has moderate to high Th/U (0.5–1.8), and is uniform in 206Pb/ 238U age (306.2 ± 3.0 Ma) and δ 18O (7.4 ± 0.3‰). The zircon in the Nisa monzogranite records a history of magma genesis involving mixing between 1) a metaluminous magma progressively contaminated by a small sedimentary component, and 2) a more voluminous peraluminous magma originating from a largely metasedimentary source. The inherited zircon age pattern closely matches the age pattern of detrital zircon in early Paleozoic sediments from North Africa. The zircon in the Aldeia da Mata tonalite records nothing of the age of the magma's source rocks, but the moderately high δ 18O does preclude derivation of the magma directly from the mantle. Both the chemical and isotopic compositions of the tonalite zircon make it highly unlikely that the tonalite magma was a component in the monzogranite magma mixture.