Original paper
Petrogenesis of Hercynian peraluminous granites from the Calabrian Arc, Italy
Rottura, Alessandro; Caggianelli, Alfredo; Campana, Raffaellamaria; Moro, Aldo Del
European Journal of Mineralogy Volume 5 Number 4 (1993), p. 737 - 754
71 references
published: Jul 22, 1993
manuscript accepted: Feb 26, 1993
manuscript received: Mar 25, 1992
DOI: 10.1127/ejm/5/4/0737
Abstract
Abstract The Capo Rasocolmo (CR) and Villa San Giovanni (VSG) granitoids, in the Southern Calabrian Arc (CLA), form post-tectonic unzoned intrusions in migmatitic paragneisses. They are strongly peraluminous (twomica aluminosilicate-bearing), showing significant small-scale mineralogical and chemical heterogeneities with compositions ranging from leucotonalite to monzogranite. The CR and VSG granitoids are characterized by SiÜ2 contents varying from 67 to 75%, low mafics (TiO2 + FeOt + MgO < 3.9%) and high AI2O3 (A/CNK = 1.20 - 1.40) with low Rb and high Ba contents and strongly fractionated REE patterns (Lan/Ybn = 34 and Ybn - 3,7 on average). They show broad trends in major and trace element compositions which define linear regressions on variation diagrams. The granitoids have an "S"-type signature in terms of their enclaves, mineralogy and zircon typology, whereas their geochemical characteristics are similar to those of late- to post-collision granites. Initial Sr isotopic ratios (0.7094 - 0.7107) are lower than that of the regionally exposed basement rocks and similar to that of the coeval hybrid calc-alkaline granitoids; GNdi values range from -6.7 to -9.1. Petrographic and geochemical evidence indicate a crystal-liquid unmixing process as the dominant mechanism involved in granitoid evolution. A two-stage model was tested on the CR granitoids, involving fractional crystallization plus assimilation followed by partial solid-liquid segregation during the emplacement of the granite magma; the model accounts very well for the observed geochemical and isotopic variations. The ultimate origin of the CR and VSG granitoids investigated in the present study. A direct derivation from shale-like protoliths, suggested by field data, mineralogy, is consistent with REE patterns and experimental melting data, but is not supported by Ba-Rb decoupling and the Sr isotopic data. A comparison with the classical S-type granites of the Lachlan Fold Belt also excludes an S-type source for the CR and VSG granitoids. A direct derivation of the CR and VSG granitoids from a coeval calc-alkaline suite, through low pressure fractionation processes, is not consistent with the geochemical trends and REE patterns. An origin from a distinct calc-alkaline magma batch, involving both crustal and subcrustal components, is favoured as the most realistic petrogenetic model.
Keywords
peraluminous granitoids • petrogenesis • crystal-liquid unmixing • mixed origin • Hercynian magmatism • Calabrian Arc • Italy.