Circa 546 Ma plume-related dykes in the ∼1 Ga Novillo Gneiss (east-central Mexico): Evidence for the initial separation of Avalonia

The ∼1 Ga Novillo Gneiss exposed in an inlier in the front ranges of the Laramide belt in east-central Mexico is cut by an unmetamorphosed NE-trending mafic dyke swarm. The gneiss represents the northernmost inlier of the Oaxaquia terrane that underlies the backbone of Mexico. Titanite and biotite from metasedimentary rocks yielded ages of 928 ± 2 Ma (concordant U-Pb age), and duplicate ⁴⁰Ar/³⁹Ar plateau ages of 697 ± 7 and 697 ± 10 Ma, which are inferred to represent cooling through ∼660 and ∼300-350 °C, respectively. On the other hand, hornblende from cross-cutting, NE-trending mafic dykes yielded a ⁴⁰Ar/³⁹Ar plateau age of 546 ± 5 Ma, which is inferred to closely post-date the time of intrusion. Combined with published ages (U-Pb zircon, Sm-Nd garnet, K-Ar hornblende) the data suggest that, following granulite facies metamorphism at 982 ± 6 Ma (8.9-9.7 kbar and 730-775 °C), the Novillo Gneiss cooled at a rate of ∼1.45 °C/my. If this latter rate is extrapolated it would place the rocks at the surface (30 °C) by ∼497 Ma, about 50 my after dyke emplacement. Geochemistry of the dykes indicates that they are Fe-rich tholeiites with: (i) low MgO, but high TiO₂ and other HFSE, (ii) smooth mantle-normalized incompatible trace element patterns peaking at Nb and La, and (iii) intraplate tectonic affinities. These features are typical of enriched tholeiites associated with plumes. Current Precambrian paleogeographic reconstructions place Avalonia against Oaxaquia and subsequently separate Avalonia by way of ridge-trench collision at ∼550 Ma giving rise to the unique Avalonian Cambrian fauna and a rapid phase of early Cambrian subsidence. These events are contemporaneous with emplacement of the Novillo mafic dykes, suggesting that the plume-related magmatism and separation of Avalonia from Oaxaquia are linked.