Petrogenesis of the Santa Eulália Plutonic Complex (Alentejo, Portugal): contribution from recent elemental and isotopic (Sr-Nd) geochemical data

Abstract

The Santa Eulalia Plutonic Complex (SEPC) is a late-Variscan granitic massif with elliptical contours intersecting the NW-SE Variscan dominant regional structures in the most northerly sectors of the Ossa-Morena Zone (Iberian massif). The host rocks of this plutonic complex are meta-sedimentary formations generated between Proterozoic and Lower Paleozoic, some of them strongly affected by contact metamorphic processes related to the SEPC intrusion, and now recognized as a set of hornfels with a wide compositional range. SEPC has two main granitic facies which present different compositions and textures. It can be classified as a ring igneous complex composed, from the rim to the core, by a medium- to coarse-grained pink granite (G0-group), with biotite ( horneblende) as the dominant mafic mineral, involving large (kilometric) elongated masses of mafic (gabbroic) to intermediate (granodioritic) rocks, here called M-group, and a central gray medium-grained granite (G1-group), typically monzonitic, with biotite as the main mafic mineral, sometimes showing a slight porphyritic tendency, namely in a few outcrops located in transitional zones, near the contact with the peripheral G0-type ring. Magnetic Susceptibility (MS) analysis sustain major genetic differences between pink and gray SEPC granites. G0 facies present relatively high values of magnetic susceptibility (Km>10-3 SI) compatible with the presence of magnetite, while the low values registered in G1-group (Km<10-4 SI) are consistent with the presence of ilmenite. Distinct magnetic behaviour of these two granitic facies also reveal contrasting redox conditions during the magma genesis and cooling process (Sant’Ovaia et al., 2011). Elemental geochemistry confirms significant differences between G0 and G1 facies. G0 granites represent more evolved liquids (>SiO2 wt.% and <MgO wt.%) with dominant metaluminous character, plot close to A-type fields (Whalen et al., 1987), and some samples show negative Eu anomalies. Instead, G1 facies are typically monzonitic granites with a slight peraluminous character and no significant Eu anomalies (Lopes et al., 1998). Relationships between SiO2 vs. (Na2O+K2O-CaO) indicate that G0-group and G1-group have alkali-calcic and calc-alkaline character respectively; both granitic groups, should be classified as magnesian according to SiO2 vs. FeOt/(FeOt+MgO) correlation (Frost et al., 2001). Recent Sr and Nd isotopic data obtained for 15 samples (5 per group of facies) were used considering an age around 290 My (Pinto, 1984). The least evolved gabbro of the M-group has a mantle Nd isotopic signature (ɛNd290=+1.7) representing the highest value of the wide ɛNdi range revealed by this group of facies (-3.0<ɛNd290<+1.7). Representative samples of G0 granite, belonging to the external ring, show a narrower range of ɛNdi values (-2.3<ɛNd290<-0.9) which is included in the previous M-group ɛNdi range. G0-group also plots in the mantle-crust Sr-Nd isotopic alignment apparently defined by mafic-intermediate rocks of M-group. In contrast, the grey granites placed in the central part of the SEPC (G1-group), diverge from this trend, showing clearly lower ɛNd290 values (-5.9<ɛNd290<-4.7) which suggest petrogenetic processes involving dominant crustal melts and/or primary magmas strongly contaminated by crustal fractions. Also (87Sr/86Sr)290 vs. (1/Sr) projection points to a likely ACF-type differentiation process showing that crustal assimilation were probably more important during the petrogenesis of the G1-facies than in the magmatic event which provided the association between M and G0 groups, probably dominated by fractional crystallization mechanisms with relatively lower contribution of crustal assimilation.