Abstract

This study presents the first geochronological results, apart from previously obtained mica ages, from metamorphic basement complex occurring within the internal zone of the Alpine chain of Algeria. The Rb–Sr whole-rock and U–Pb zircon data reveal the existence of an episode of anatectic magmatism around 510 Ma ago that corresponds to late Pan-African orogenic events.A deep crustal fault is thought to represent a major phase of shearing associated with Hercynian blastomylonitic metamorphism and the production of granitic remelts at 271 ± 12 Ma. Reworking of an ancient basement is demonstrated by the presence of inherited radiogenic lead in zircons from lower Palaeozoic orthogneisses and metarhyolites: upper intercepts on concordia plots yield ages of 1950 ± 100 and 1321 ± 30 Ma, respectively. Apart from the dating results, which may provide new insight into the geological history of the region, this study also provides general implications for the interpretation of geochronological data in metamorphic and igneous formations.Firstly, the effect of the transformation of granite to orthogneiss appears to produce a dispersion of Rb–Sr analytical points between two apparent isochrons of 565 ± 50 and 465 ± 52 Ma. Such ages may also correspond to two separate groups of granitic protoliths that are difficult to distinguish in the field because of the superimposition of a gneissose fabric. In fact, the true emplacement age of these metagranites is given by the U–Pb zircon ages around 510 Ma. The Rb–Sr whole-rock systems of these orthogneisses have been opened during subsequent metamorphism, since model initial 87Sr/86Sr ratios calculated at 510 Ma are sometimes too low (< 0.700) to be explained by heterogeneity of the source materials.Secondly, the influence of fractional crystallization of a granitic melt on the behaviour of inherited zircon xenocrysts has been investigated. This involves the U–Pb study of further zircon separates from rocks having the same emplacement age but differing degrees of differentiation. An orthogneiss showing relative little magmatic differentiation (SiO2 = 65%) contains zircons that have a component of inherited radiogenic lead and that yield a lower intercept age of 506 ± 3 Ma. Two other orthogneisses showing far greater differentiation (SiO2 = 70 and 78%) contain zircons that lack any inherited radiogenic lead and that yield an upper intercept on concordia plots of 514 ± 18 Ma. This behaviour can be explained in the light of published experimental data on the dissolution of zircons in granitic liquids.

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