Intra-crustal recycling and crustal-mantle interactions in

Abstract

In situ O-isotope compositions of detrital, inherited and melt-precipitated zircons with Neoproterozoic to Ordovician ages are presented to assess the crustal evolution of the North Gondwana margin. Different groups of pre-Mesozoic rocks from SW Iberia were targeted: i) Ediacaran paragneisses and meta-greywakes of the Ossa-Morena Zone – the Serie Negra Group deposited at ~ 560 Ma in a Cadomian magmatic arc setting (Pereira et al., 2008); ii) Early to Middle Cambrian orthogneisses and volcaniclastic rocks of the Ossa-Morena Zone – Evora Massif igneous complexes related to ensialic rifting at ~ 530–500 Ma (Pereira et al., 2008, Chichorro et al., 2008); iii) Late Cambrian to Early Ordovician volcaniclastic rocks and granites of the Ossa-Morena–Central Iberian transition zone – the Urra Formation and Portalegre granite formed at ~ 495– 488 Ma in a extensional setting (Solá et al., 2008); iv) Carboniferous granitoids (Nisa and Arraiolos granites) containing inherited zircons with Cambrian to Ordovician ages (Solá, this volume). A compilation the results for the period ~ 3.4 Ga to ~ 450 Ma reveals that: a) Archean zircons show little variation in d18O, with most values lying between 4.7 and 7.5‰, (average 6.2‰) comparable with usual d18O of zircons from Archean elsewhere (e.g., Valley et al., 2005); b) the range of d18O in Paleoproterozoic grains increases between 2.1 and 1.8 Ga with d18O >7.5‰, indicating increasing supracrustal recycling, but at ~ 1.8 Ga the d18O has mantle-like values (<5.1‰), documenting a crustal growth episode at this time; c) rare Mesoproterozoic grains have mildly evolved d18O values in the range 5.6–7.1‰); d) Tonian grains have low d18O values (4.2–5.6‰) typical of mantle-derived juvenile magmas but also higher values of 9.9‰ suggesting intra-crustal recycling; e) Cryogenian–Ordovician zircons show more variable and higher d18O values (~4 to >10‰), indicating great diversity and mixing of sources through intra-crustal recycling and crust–mantle interactions; f) some d18O values near to or below mantle composition (5.3 ± 0.3‰) were recorded at ~ 590 Ma (Ediacaran) suggesting input of mantle material into the crust; g) a decrease in variance of d18O occurs from 575 Ma to the Ediacaran/Cambrian boundary, suggesting a relative decrease in the magmatic contribution of surface-derived material; h) in Cambrian times, the average d18O is higher in the 536–520Ma interval (7.0‰) than in the 520–488 interval (6.2‰), which can be taken as a signal of gradual opening of the system to mantle-derived, mafic, rift-related igneous complexes; i) higher values of d18O (>7.5 ‰) recorded at ~ 623–574 Ma and 490–470Ma mark periods of pronounced increase in crustal recycling.