Relationships between intermediate and acidic rocks in orogenic granitoid suites: petrological, geochemical and isotopic (Sr, Nd, Pb) data from Capo Vaticano (southern Calabria, Italy)

Abstract

Major, trace element and isotopic data are reported for Hercynian granodiorites and tonalites from Capo Vaticano, Calabria, with the aim of clarifying the relationships between acidic and intermediate lithologies in orogenic intrusive suites. These granodiorites (SiO 2 = 70.5%–73.5%) are peraluminous, relatively depleted in Rb, U, Th, and Pb, and display a large variation in many geochemical and isotopic parameters. REE show fractionated, cross-cutting chondrite-normalized patterns with small but significant negative Eu anomalies. Initial (290 Ma) Sr and Nd, and present-day Pb isotope ratios are: ( 87 Sr 86 Sr ) 290 = 0.7105–0.7110 ; ( 143 Nd 144 Nd ) 290 = 0.51175–0.51221 ; 206 Pb 204 Pb = 18.35–18.50 ; 207 Pb 204 Pb = 15.64–15.76 ; 208 Pb 204 Pb = 38.51–39.03 . Pb isotope ratios determined for separated feldspars show similar 207 Pb 204 Pb and 208 Pb 204 Pb and higher 207 Pb 204 Pb ratios compared to the present-day values for the whole rock. The tonalites (SiO 2 = 57.6%–67.3%) are typically calc-alkaline in composition and most of them are weakly peraluminous. They display large heterogeneities in trace-element abundances, REE patterns and Nd and Pb isotope ratios, both between and within individual intrusive units. Sr, Nd and Pb isotopic compositions display a range of values close to that of the associated granodiorites ( 87 Sr 86 Sr ) 290 = 0.7099–0.7111 ; ( 143 Nd 144 Nd ) 290 = 0.51185–0.51224 ; 206 Pb 204 Pb = 18.31–18.56 ; 207 Pb 204 Pb = 15.65–15.72 ; 208 Pb 204 Pb = 38.41–39.59 . The obtained data indicate that the tonalites and granodiorites share many compositional features which point to a genetic linkage. Likewise, many petrological, geochemical and field data exclude the possibility that all the granodiorites derive from intermediate magmas by any common evolutionary process, such as crystal-liquid fractionation or assimilation-fractional crystallization (AFC), and point to a different genesis for the two rock types. An origin of the granodiorites by the mixing of two separate acidic melts, one of crustal anatectic origin and one probably derived from an intermediate magma by crystal/liquid fractionation or AFC, is suggested by the data presented. The tonalites appear to have been generated by interaction of a mafic magma with crustal end-member(s) with isotopic and geochemical signatures similar to those of the granodiorites. The large involvement of crustal material in the genesis of the Capo Vaticano granitoids is responsible for the many common compositional characteristics observed in the intermediate and acidic rocks. However, a contribution from mantle components is also indicated by petrological and geochemical data, as well as by the occurrence of mafic enclaves commonly present in the tonalites. Nevertheless, the geochemical and isotopic features of such a mantle component appear to be greatly diluted by a large quantity of crustal material involved in the genesis of the studied granitoids.