Contrasting high-Mg, high-K rocks in Central Iberia: the appinite—vaugnerite conundrum and their (non-existent) relation with arc magmatism

Abstract

The central zone of the Iberian Massif contains abundant granitic batholiths, mostly consisting of peraluminous granodiorites to granites, with minor volumes of high-Mg and high-K ultramafic, mafic, and intermediate rocks (UMI rocks), belonging to two associations: vaugnerites and appinites. The UMI rocks crop out as small stocks within anatectic domains or granite bodies, as enclaves (scattered, clusters or swarms), even as syn-plutonic dikes. SHRIMP zircon U–Pb ages range from ~ 291 Ma to ~ 328 Ma, being broadly coeval with the granites and slightly younger than the anatexites that host them. The vaugnerites are alkaline, with Na2O + K2O concentrations similar to shoshonites, whereas the appinites are comparable to high-K calc-alkaline series, but in no way can they be considered arc-rocks indicative of subduction. Compared with arc-related shoshonites and high-K calc-alkaline rocks, the Iberian UMI rocks, especially the vaugnerites, are much more enriched in mica-compatible elements such as Li, Rb, Cs, Be, Sn, Nb, etc. as well as in Th and U. They also have extraordinarily elevated production of radiogenic heat, the appinites about 1.5 µWm−3, and the vaugnerites about 3.9 µWm−3. These values contrast with the average heat production of the upper mantle (0.02 µWm−3) and the lower continental crust (0.6 µWm−3) and are even higher than the average continental crust (1.4 µWm−3) indicating extensive crustal contamination. The negative whole-rock eNdt, usually between − 2 and – 6, the elevated initial 87Sr/86Sr, usually > 0.706, and the old Nd model ages (1 Ga to 1.5 Ga), i.e., much higher than the zircon crystallization ages, reinforces this idea. Despite both rock associations being affected by crustal contamination, the processes that operated were different. Whereas appinites may have resulted from bulk contamination with anatexites plus crystal fractionation, the vaugnerites require contamination with supercritical fluids resulting from biotite breakdown. Accordingly, we propose that the production of appinites or vaugnerites depends on the fertility of the metamorphic rocks they traversed en route from the mantle to the middle crust: if the host rock was fertile and partially molten, the intrusion of mafic magma increased the melt fraction so permitting bulk contamination to produce hybrid appinite magmas; if the rock was infertile, the fluids released from micas breakdown during the metamorphism could migrate as such to the mafic magmas, contaminating them to yield vaugnerite magmas.