GEOCHEMISTRY AND ARGON THERMOCHRONOLOGY OF THE VARISCAN SILA BATHOLITH, SOUTHERN ITALY - SOURCE ROCKS AND MAGMA EVOLUTION

The Sila batholith is the largest granitic massif in the Calabrian-Peloritan Arc of southern Italy, consisting of syn to post-tectonic, calc-alkaline and metaluminous tonalite to granodiorite, and post-tectonic, peraluminous and strongly peraluminous, two-mica +/-cordierite +/- Al silicate granodiorite to leucomonzo-granite. Mineral 40Ar/39Ar thermochronologic analyses document Variscan emplacement and cooling of the intrusives (293-289 Ma). SiO2 content in the granitic rocks ranges from -57 to 77 wt%; cumulate gabbro enclaves have SiO2 as low as 42%. Variations in absolute abundances and ratios involving Hf, Ta, Th, Rb, and the REE, among others, identify genetically linked groups of granitic rocks in the batholith: (1) syn-tectonic biotite +/- amphibole-bearing tonalites to granodiorites, (2) post-tectonic two-mica +/- Al-silicate-bearing granodiorites to leucomonzogranites, and (3) post-tectonic biotite +/- hornblende tonalites to granodiorites. Chondrite-normalized REE patterns display variable values of Ce/Yb (up to approximately 300) and generally small negative Eu anomalies. Degree of REE fractionation depends on whether the intrusives are syn- or post-tectonic, and on their mineralogy. High and variable values of Rb/Y (O.40-4.5), Th/Sm (0.1-3.6), Th/Ta (0-70), Ba/Nb (1-150), and Ba/Ta (approximately 50-2100), as well as low values of Nb/U (approximately 2-28) and La/Th (approximately 1-10) are consistent with a predominant and heterogeneous crustal contribution to the batholith. Whole rock deltaO-18 ranges from approximately +8.2 to +11.7 parts per thousand; the mafic cumulate enclaves have the lowest deltaO-18 values and the two-mica granites have the highest values. DeltaO-18 values for biotite +/- hornblende tonalitic and granodioritic rocks (9.1 to 10.8 parts per thousand) overlap the values of the mafic enclaves and two-mica granodiorites and leucogranites arc (10.7 to 11.7 parts per thousand). The initial Pb isotopic range of the granitic rocks (Pb-206/Pb-204 approximately 18.17-18.45, Pb-207/Pb-204 15.58-15.77, Pb-208/Pb-204 approximately 38.20-38.76) also indicates the predominance of a crustal source. Although the granitic groups cannot be uniquely distinguished on the basis of their Pb isotope compositions most of the post-tectonic tonalites to granodiorites as well as two-mica granites are somewhat less radiogenic than the syn-tectonic tonalites and granodiorites. Only a few of the mafic enclaves overlap the Pb isotope field of the granitic rocks and are consistent with a cogenetic origin. The Sila batholith was generated by mixing of material derived from at least two sources, mantle-derived and crustal, during the closing stages of plate collision and post-collision. The batholith ultimately owes its origin to the evolution of earlier, more mafic parental magmas, and to complex interactions of the fractionating mafic magmas with the crust. Hybrid rocks produced by mixing evolved primarily by crystal fractionation although a simple fractionation model cannot link all the granitic rocks, or explain the entire spectrum of compositions within each group of granites. Petrographic and geochemical features characterizing the Sila batholith have direct counterparts in all other granitic massifs in the Calabrian-Peloritan Arc. This implies that magmatic events in the Calabrian-Peloritan Arc produced a similar spectrum of granitic compositions and resulted in a distinctive type of granite magmatism consisting of coeval, mixed, strongly peraluminous and metaluminous granitic magmas.