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Journal of Petrology Advance Access published online on July 1, 2004
Journal of Petrology, doi:10.1093/petrology/egh026
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Received November 8, 2002
Accepted February 23, 2004

Article

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

H. P. ZECK 1*, M. T. D. WINGATE 1, G. D. POOLEY 2, J. M. UGIDOS 3

1 TECTONICS SPECIAL RESEARCH CENTRE, SCHOOL OF EARTH AND GEOGRAPHICAL SCIENCES M004, THE UNIVERSITY OF WESTERN AUSTRALIA, CRAWLEY, WA 6009, AUSTRALIA
2 CENTRE FOR MICROSCOPY AND MICROANALYSIS M010, THE UNIVERSITY OF WESTERN AUSTRALIA, CRAWLEY, WA 6009, AUSTRALIA
3 DEPARTAMENTO DE GEOLOGÍA, UNIVERSIDAD DE SALAMANCA, FACULTAD DE CIENCIAS, PLAZA DE LA MERCED, E 37008 SALAMANCA, SPAIN

* To whom correspondence should be addressed. E-mail: zeck.frappier{at}wanadoo.fr.


  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: ~980Ma, ~830 Ma, 616 ± 10(2{sigma}) Ma and 582 ± 5(2{sigma}) 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(2{sigma}) 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(2{sigma}) 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(2{sigma}) Ma and 582 ± 5(2{sigma}) 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.

Keywords: SIMS U-Pb Zircon dating; inherited, magmatic, deuteric and metamorphic granitic zircon; anatectic granite; Iberian Hercynian belt.
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