Late Precambrian and Late Paleozoic granites from the amphibolite facies basement of the Peloritani Mountains (NE Sicily) and Aspromonte Massif (SW Calabria) have been studied, both by geochemical investigations and SHRIMP zircon U-Pb dating, to shed light on the processes and sources involved in granitoid magma production during the poly-orogenic evolution of that segment of the crust. Studies of zircon from an augen gneiss, the granite protolith of which was emplaced at 545 ± 5 Ma, and a 300 ± 4 Ma leucogranodiorite, indicate that both granitic rocks originated by polyorogenic partial melting of metasedimentary rocks derived from the same sources. First, both rocks contain a large amount of inherited zircon, commonly as cores rimmed by a thick igneous zircon overgrowth. The inheritance ranges in age from Early Paleoproterozoic to latest Neoproterozoic, with main clusters at c. 0.55 and c. 0.63 Ga, and minor ones at c. 0.95 and c. 2.5 Ga. The broad range of inheritance ages indicates a source containing a diverse zircon population, as is typical of sedimentary rocks. Secondly, the detrital zircon cores in a biotite paragneiss, host rock of the late Precambrian granite, have the same age pattern as that of the inherited cores in the granites, providing clear evidence that both granites originated by partial melting of that, or a closely related, paragneiss. The information from zircon has been integrated with new and pre-existing geochemical data on the two granite suites. Both are dominantly strongly peraluminous and show similar compositions and trends in chemical variation diagrams. Both have a post-collisional signature in multi-element and tectonomagmatic discrimination diagrams. REE patterns for the Paleozoic granites are on the whole more fractionated, suggesting equilibration with a source containing residual garnet. Restitic garnet-rich metasedimentary rocks are abundant in the lower crust of southern Calabria, possibly representing the granulite counterpart of the amphibolite facies paragneisses from the shallower crustal portions exposed in the studied area. On the other hand, Sr and Nd isotopic data suggest a possible involvement of a mantle-related component in the genesis of the granitoids. Investigation of the possible sources and processes involved in the generation of the entire compositional spectrum of the studied Precambrian and Paleozoic granitoids included a comparison with the compositions of different experimental melts, taking into account the possible effects of interactions between metasedimentary-derived melts and mantle-derived melts or restitic components. The Paleozoic granite compositions mostly fall in the fields of melts produced from greywacke-pelite sources, with a weak trend along the reaction curves representing hybridization of metagreywacke with basaltic melt at high pressure. The augen gneisses appear to share the same metasedimentary source, but to have had more interaction with a basaltic component at low pressure. In contrast, despite the high abundance of inherited zircon in both granites, attesting to the presence of restitic components in the granitic magmas, no clear evidence for mixing processes involving individual rock forming minerals emerges from the geochemical data. This study reveals that poly-orogenic partial melting of the same crustal sequence, in the same tectonic context, has given rise to similar granitoid associations with distinct chemical compositions, and that zircon probably provides the most straightforward and reliable evidence of the source-granite link.
|Titolo:||Poly-orogenic melting of metasedimentary crust in post-collisional setting: the zircon inheritance-rich late Cadomian and late Hercynian granites of the southern Calabria-Peloritani Orogen (southern Italy)|
|Data di pubblicazione:||2011|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|