Origin of cordierite-bearing monzogranites from the southern Central Iberian Zone – Inferences from the zoned Sierra Bermeja Pluton (Extremadura, Spain)

Abstract

Voluminous peraluminous (biotite, biotite–cordierite and cordierite–muscovite-bearing) granitic plutons were emplaced in the southern part of the Central Iberian Zone (Iberian Massif) during the waning stages of the Variscan Orogeny. The role of the mantle and the nature of the crustal sources involved in the generation of these remarkable granites are still a matter of discussion. To address these issues, a detailed study of the Sierra Bermeja Pluton, a ~60 km2 cordierite-bearing monzogranite intrusion representative of the regional Nisa–Alburquerque–Los Pedroches Magmatic Alignment, has been carried out. This pluton was formed by an inward sequence of three distinct but near contemporaneous magma pulses: Outer Unit (OU), Middle Unit (MU) and Inner Unit (IU). The nested pluton is cut by a number of syn-plutonic vaugneritic dykes, while lamprophyres were distinctly later.The age of the Pluton has been established at ca. 306 Ma (309 ± 3 to 304 ± 3 Ma) by LA-ICP-MS U–Pb dating of zircon and monazite. The peraluminosity (A/CNK = 1.01–1.33) and P2O5 contents (0.19–0.35 wt%) of the monzogranitic units increase inwards (OU → MU → IU). This is accompanied by an overall decrease in TiO2, MgO, CaO, and FeOt, and most trace-elements, including REE. The three units display similarly low 87Sr/86Sr306Ma ratios (0.7033–0.7046) but decreasing εNd306Ma (OU: ~ −2.7, MU: ~ −3.8, IU: −7.5 to −4.7). Vaugneritic rocks have homogeneous composition with similar 87Sr/86Sr306Ma ratios (~0.7043) to those of the monzogranites but higher εNd306Ma (~ +1.1) values. Whole-rock geochemical data imply a partial melting of mainly Cambro–Ordovician felsic metaigneous rocks (OU) with increasing proportion of Neoproterozoic–Lower Cambrian metasediments of the Schist–Greywacke Complex (MU) that formed, almost exclusively, the source of the IU. Thermobarometric data suggest that melting processes progressed upward through the geological record and magma emplacement took place at <3 kbar, with crystallization temperatures of 710–810 °C. Geochemical modelling implies that each monzogranitic unit could have evolved independently at the emplacement level by a 20–26% fractional crystallization of a mineral assemblage constituted by Kfs + Pl > Bt >> Ap + Mnz + Xtm. The present results could be extensible to other cordierite-bearing granites of the southern Central Iberian Zone although forthcoming experimental work and detailed comparisons are required to confirm such extrapolation.