496 ± 3 Ma zircon ion microprobe age for pre-Hercynian granite, Central Iberian Zone, NE Portugal (earlier claimed 618 ± 9 Ma)

Abstract

Ion microprobe (SIMS) dating of zircon from the Miranda do Douro orthogneiss, Central Iberian Zone, Hercynian Iberian belt, defines an Early Ordovician U–Pb age of 496.0 ± 2.6 Ma (95 % conf., MSWD = 1.14) for magmatic zircon crystallization in its granitic protolith. The age contrasts with an earlier, conventional (ID-TIMS) U–Pb zircon age of 618 ± 9(95 % conf.) Ma, now thought to be an artefact of the complex zircon population. Individual SIMS ages for zircon from the rock range from 2700 to 180 Ma and comprise inherited and magmatic zircon, both concordant and common Pb-enriched, and younger, reset ages. The ID-TIMS study seems to have misinterpreted this heterogeneous population as a cogenetic suite consisting of magmatic zircon and its age-reset equivalents produced by recent Pb loss. The 496 ± 3 Ma SIMS age represents the weighted average for 26 magmatic zircon domains located by careful inspection of cathodoluminescence, secondary electron and optical microscopy images of ∼ 700 individual zircon crystals. Inherited zircon is widespread, ranging in age from 2700 to 550 Ma, with age clusters, which are statistically indistinguishable from those known from pre-Hercynian granitic basement material elsewhere in the Central Iberian Zone. Including the present age information, 582 ± 4 Ma (95 % conf., MSWD = 1.02, n = 13) and 619 ± 9 Ma (95 % conf., MSWD = 0.93, n = 7) appear as prevailing inherited zircon age components in basement intrusions in the Central Iberian Zone.