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Istituto di Geoscienze e Georisorse     
Rottura A., Caggianelli A., Campana R., Del Moro A. Petrogenesis of Hercynian peraluminous granites from the Calabrian Arc, Italy. In: European Journal of Mineralogy, vol. 5 (4) pp. 737 - 754. Schweizerbart, 1993.
 
 
Abstract
(English)
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%) aud high Al₂O ₃ (A/CNK = 1.20-1.40) 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 e1ement 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; ∊_(Ndˌ), 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 c1assical S-type granites ofthe 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 geoehemieal trends and REE patterns. An ori gin from a distinct calc-alkaline magma batch, involving both crustal and subcrustal components, is favoured as the most realistic petrogenetic model.
URL: http://www.schweizerbart.de/j/ejm/
Subject peraluminous granotoids
petrogenesis
crystal-liquid unmixing
mixed origin
Hercynian magmatism
Calabrian Arc
Italy


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