A sequence of Pan-African and Hercynian events recorded in zircons from an orthogeneiss from the Hercynian belt of western central Iberia - An ion microprobe U-Pb study

Abstract

Sensitive high-resolution ion microprobe U-Pb dating shows that a biotite orthogneiss from the Hercynian belt of western central Iberia contains 1000-300 Ma zircon. Older, 1000-570 Ma ages within this range represent inherited, detrital material among which four age components may be recognized: ∼980 Ma, ∼830 Ma, 616 ± 10(2σ) Ma and 582 ± 5(2σ) Ma. This inherited zircon commonly forms cores that are surrounded by rims yielding Late Pan-African ages, identical to those found in slender, prismatic, and some stubbier, bi-pyramidal, euhedral crystals. This is the predominant type of zircon with an average age of 546 ± 3(2σ) Ma, thought to have been formed during the main magmatic crystallization stage of the granitic protolith of the gneiss. Local deuteric replacements of magmatic zircon yield a virtually identical average age of 547 ± 5(2σ) Ma, suggesting rapid magmatic cooling, typical of shallow intrusive settings. Many zircon crystals have very thin, low-Th/U rims with an age of ∼315 Ma, suggested to represent the gneissification of the granitic rock during the Hercynian orogeny. The abundance in the gneiss body of Al-rich restitic material and inherited, detrital zircon suggests that the granitic magma was formed by anatectic melting of a metasedimentary source rock complex. The age of the youngest inherited, detrital zircon constrains the sedimentation age of the (youngest parts of the) anatectic source rock complex to the Late Neoproterozoic (<582 ± 5 Ma) and leaves a maximum period of ∼40 Myr for metamorphism and anatexis of the source rock complex, and migration and intrusion of the granitic magma. Among the inherited, detrital zircon the 616 ± 10(2σ) Ma and 582 ± 5(2σ) Ma ages are by far the most abundant, and might be derived from the West African craton, where such ages are common for main-stage Pan-African complexes. The older, ∼980 Ma and ∼830 Ma inherited zircon ages are absent or rare in the West African craton and a derivation from the Amazon craton (Grenvillian, Sunsas orogens), where such ages are common, is an alternative. © Oxford University Press 2004; all rights reserved.