A Sequence of Pan-African and Hercynian Events Recorded in Zircons from an Orthogneiss from the Hercynian Belt of Western Central Iberia—an Ion Microprobe U–Pb Study

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(2s) Ma and 582 5(2s) 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(2s) 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(2s) 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(2s) Ma and 582 5(2s) 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.