Zircon ages of the metavolcanic rocks and metagranites of the Ollo de Sapo Domain in central Spain: implications for the neoproterozoic to early palaeozoic evolution of Iberia

摘要

Dating the pre-Middle Ordovician metavolcanic rocks and metagranites of the Ollo de Sapo Domain has, historically, been difficult because of the small compositional variation, the effects of the Variscan orogeny and, as revealed in this paper, the unusually high fraction of inherited zircon components. The first reliable zircon data (U-Pb ion microprobe and Pb-Pb stepwise evaporation) indicate that the Ollo de Sapo volcanism spanned 495 +/- 5 Ma to 483 +/- 3 Ma, and was followed by the intrusion of high-level granites from 483 3 Ma to 474 4 Ma. In both metavolcanic rocks and metagranites, no less than 70-80% of zircon grains are either totally Precambrian or contain a Precambrian core overgrown by a Cambro-Ordovician rim. About 80-90% of inherited zircons are Early Ediacaran (602-614 Ma) and derived from calc-alkaline intermediate to felsic igneous rocks generated at the end of the Pan-African arc-continent collision. In the Villadepera region, located to the west, both the metagranites and metavolcanic rocks also contain Meso-Archaean zircons (3.0-3.2 Ga) which ultimately originated from the West African Craton. In the Hiendelaencina, region, located to the east, both the metagranites and metavolcanic rocks lack Meso-Archaean zircons, but they have two different inherited zircon populations, one Cryogenian (650-700 Ma) and the other Tonian (850-900 Ma), which suggest older-than-Ediacaran additional island-arc components. The different proportion of source components and the marked variation of the Sr-87/Sr-86(init) suggest, at least tentatively, that the across-arc polarity of the remnants of the Pan-African arc of Iberia trended east-west (with respect to the current coordinates) during Cambro-Ordovician times, and that the passive margin was situated to the west.