Iberian late-Variscan granitoids: Some considerations on crustal sources and the significance of “mantle extraction ages”

A suite of post-tectonic granitoids (mostly peraluminous, broadly I-type granodiorites and monzogranites) and mafic rocks from NW Iberia with crystallization ages between ca. 309 and 290 Ma has been investigated for Sm–Nd isotopes and inherited zircon content in order to constrain the nature of their source rocks. εNd values (at 300 Ma) vary from −0.2 to −5.9 and TDM values range from 1.01 to 1.58 Ga. Inherited (xenocrystic) zircons yielded ages ranging from 458 to 676 Ma, with 90% of data between 490 and 646 Ma, corresponding to Neoproterozoic (mostly Ediacaran), Cambrian and Ordovician ages. Only three highly discordant analyses yielded ages older than 650 Ma. Based on the data reported herein and relevant data from the literature we contend that post-tectonic granitoids of the Iberian Variscan Belt (with exception of the scarce anatectic S-type granitoids) were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca. 0.9 and 1.1 Ga. I-type granitoids and mantle-derived mafic rocks both underwent varying degrees of contamination by a metasedimentary lower crust depleted in pre-650 Ma zircon (through previous melting episodes) with a time-integrated Sm–Nd evolution different to that of the metaigneous lower crust. Participation of this metasedimentary crust in the genesis of these granitoids may account for Nd isotopic variability and Nd model ages well in excess of 1.2 Ga.

Research highlights▶ Post-tectonic I-type granitoids in Iberia were generated mainly by melting of lower crustal metaigneous protoliths. ▶ Zircon inheritance indicates that these protoliths were mostly formed during magmatic events related to the Ediacaran-Cambrian Cadomian orogeny in northern Gondwana. ▶ The Sm-Nd isotopic composition of these granitoids suggests that the ultimate source for their protoliths was a sub-continental lithospheric mantle enriched between 0.9 and 1.1 Ga. ▶ Contamination with lower crustal metasedimentary protoliths explains TDM values in excess of ca. 1.1 Ga.