The Nature, Origin, and Thermal Influence of the Granite Source Layer of Central Iberia

Abstract

The Central Iberian Zone comprises low‐grade metamorphic zones alternating with Variscan thermal domes apparently equilibrated at the same pressure. The Variscan thermal domes are composed of authochtonous to allochtonous granites and anatectic complexes that represent remnants of the granite source layer. The anatectic complexes consist of ∼90% high‐grade metasediments and ∼10% Cadomian ($$543\pm 6$$ Ma) felsic metaigneous rocks, the latter being extremely rare in low‐grade metamorphic areas. The high‐grade metasediments form part of the Schist‐Greywacke Complex, the dominant formation of central Iberia, but compared with equivalent rocks from low‐grade metamorphic areas, they are more fertile (67% vs. 31% of haplogranitic component), have a higher heat production (2.7 vs. 1.5 μW m−3), are isotopically more juvenile (0.7108 vs. 0.7179 87Sr/86Sr350 Ma; −3.3 vs. −3.7 ε540 Ma[Nd]), and contain younger pre‐Variscan zircons ($$544\pm 14$$ m.yr. old). These differences reveal that the high‐grade metasediments were derived from ∼540‐m.yr.‐old felsic igneous rocks, whereas the low‐grade metasediments were derived from recycling older sedimentary rocks. A model of 1D transient geotherms for the postcollisional crust of central Iberia reveals that, after 25–30 m.yr. of thermal maturation, the high‐grade metasediments of the source layer can melt if they were buried in the middle crust. The model provides a sensible explanation for the most relevant features of Variscan magmatism in this area, in particular, (1) the ∼50‐m.yr. time span recorded by the source layer zircons; (2) the lack of subduction‐related magmas and the scarcity of mantle magmas; (3) the lack of almost any intrusive activity during the first ∼30 m.yr. after crustal thickening, followed by (4) the sudden formation of huge volumes of crustally derived granite magmas, which (5) happened simultaneously all over central Iberia and (6) marked the beginning of the gravitational collapse of the orogen. Therefore, we propose that the Variscan thermal domes of central Iberia occurred where the Paleozoic sequence of the Schist‐Greywacke Complex derived from a Cadomian felsic igneous protolith because of the elevated heat production and fertility of the metasediments.