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 (two-mica 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 SiO2 contents varying from 67 to 75%, low mafics (TiO2 + FeO(t) + MgO < 3.9%) and high Al2O3 (A/CNK = 1.201. 40) with low Rb and high Ba contents and strongly fractionated REE patterns (La(n)/Yb(n) = 34 and Yb(n) = 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; is-an-element-of NdI 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.
PETROGENESIS OF HERCYNIAN PERALUMINOUS GRANITES FROM THE CALABRIAN ARC, ITALY RID A-1229-2011
CAGGIANELLI, Alfredo;
1993-01-01
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 (two-mica 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 SiO2 contents varying from 67 to 75%, low mafics (TiO2 + FeO(t) + MgO < 3.9%) and high Al2O3 (A/CNK = 1.201. 40) with low Rb and high Ba contents and strongly fractionated REE patterns (La(n)/Yb(n) = 34 and Yb(n) = 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; is-an-element-of NdI 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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
