Geochemical features of two rock types in the lower basic crust of the Serre (Calabria): a preliminary study

In the Serre massif (southern Calabria, Italy) a Variscan crust section crops out consisting of: i) a middleto low-grade metamorphic rocks in the upper segment, ii) an about 13 km thick “layer” of granitoids and iii) 7-8 km thick lower crust. The deep crust forms the lower part of the section and includes from the bottom to the top: a) layered metagabbros including meta-peridotites; b) felsic and mafic granulites with interleaved metapelites; c) migmatitic metapelites with interleaved metabasites, rare marbles and felsic orthogneisses. The aim of this study is to characterize the geochemistry and the evolution of the Neoproterozoic to Early Cambrian basic magmatism represented by metagabbros and metabasites. The reconstruction of the geodynamic setting in which the basic magmas were emplacement could allow a better understanding of the tectonic evolution of the peri-Gondwana terraines having memory of the West African craton. The effects of the Hercynian metamorphism have modified the mineralogical assemblage and, in some case, have induced partial melting modifying the original compositions. Petrographic and geochemical (major and trace elements) analyses have been performed on 20 samples collected at the base of the deep crust in the gabbroic portion. Two groups of basic rocks have been defined on the basis of the petrographic features. The first group characterized by coarse grained and isotropic texture, consists of gabbros and Qtz-gabbro containing Pl+Opx+Cpx+Amph±Qtz±Grt, rare crystals of biotite can be also present. Frequently, amphibolites with green or brown hornblende form thick layers. On the other hand leucocratic portions showing trondhjemitic composition are interspersed within the main gabbroic body. The second group is characterized by a medium grained and anisotropic texture formed by Pl+Opx+Bt+ k-feld±Cpx±Qtz±Grt. Peculiar characteristics of this rock-type are the abundance of biotite and the presence of pockets of granitic melts having eutectic composition. In both groups, few porphyroblastic garnet crystals occur including amphibole, pyroxene, plagioclase, ± biotite and ± quartz. Frequently, garnet crystals are rimmed by Opx+Pl+Amph or Amph+Pl symplectitic corona in biotite free-rock types or by Bt+Pl±Qtz symplectitic corona in biotite bearing-rock types. The two groups of rocks have variable chemical compositions owing to the variability of their components. The former are subalkaline rocks (Na2O + K2O = 1.17-4.84%) with a content of K2O around 0.47%, whereas the latter are alkaline rocks (Na2O + K2O = 4.88-6.77%) with a content of K2O around 3.9%. The metagabbros show lower Rb, Sr and Nb contents (Rb = 0-12 ppm; Sr = 68-592 ppm; Nb = 0.5-13 ppm) than the Bt-bearing metabasites (Rb = 10-210 ppm; Sr = 389-1044 ppm; Nb = 8-25 ppm). The contents of the other major elements are quite similar despite varying in a large range (SiO2 = 41.53- 60.38% vs. 45.75-51.16%; Al2O3 = 14.71-20.75% vs. 14.72-19.48%; FeO = 6.29-13.99% vs. 8.13-12.54%; MgO = 2.59-9.84% vs. 4.72-7.99%; CaO = 6.82-15.49% vs. 5.73-8.96%) for the presence of cumulitic portions and of differentiated rock type, in addition the Variscan partial melting events complicate the chemical variability. The first data (ACF and A’KF diagrams) indicate a common origin from basic magmas for the two rock types so the alkali enrichment for biotite bearing-rocks can be connected with host rock interaction or due to permeation of melts derived from wall rocks in Variscan times. Different hypotheses about the origin and evolution of metabasic rocks of the Serre, can be proposed.