Petrogenesis of the Monte Croce granitoids in the context of Permian magmatism in the Southern Alps, Italy RID A-1229-2011

The Monte Croce pluton (MCP), in the central-eastern Southern Alps, belongs to a large calc-alkaline magmatic association of Permian age also including the Bressanone, Ivigna, Cima d'Asta and Monte Sabion granitoid intrusions, as well as the Atesina volcanics. The MCP was intruded into the low-medium grade basement of the Southern Alps, at depths corresponding to a crystallization pressure of about 3.1 +/- 0.6 kbar. Rb/Sr biotite dating yields a value of 273 +/- 3 Ma for the intrusion age. The MCP consists of medium-grained to porphyritic biotite +/- hornblende granodiorite/monzogranite (GD) containing mafic microgranular enclaves (MME) of tonalitic to granodioritic composition. Field, petrographic and mineralogical evidence strongly supports a model of extensive interaction between partially crystallized mafic and felsic magmas, with incorporation, fragmentation and dispersal (i.e. mingling +/- mixing) of the mafic component into the granitic host magma. The occurrence of mafic enclaves having distinct textures and particular compositions for the Fe-rich biotite and amphibole, suggests that at least two pulses of hybridized intermediate magmas were involved in the petrogenesis of the Monte Croce granitoids. The GD and the hosted MME are metaluminous to weakly peraluminous granitoids with high-K calc-alkaline affinity, mostly following linear trends on Harker diagrams. On chondrite-normalized spider diagrams, both GD and MME show patterns with low HFSE/(LILE, LREE) ratios and distinct Ba, Nb, Sr, P, and Ti negative spikes, similar to those of orogenic suites. Initial Sr-87/Sr-86 ratios range from 0.7086 to 0.7114 in the GD, and from 0.7080 to 0.7099 in the MME. epsilon(Ndi) values range from -6.0 to -7.6 in the GD, and from -4.9 to -7.1 in the MME. Field and petrographic evidence, as well as geochemical trends and isotopic data, are all consistent with a complex petrogenetic model involving magma mingling/mixing coupled with chemical exchange, plus local crystal fractionation. The tectonic significance of the MCP granitoids is interpreted in the context of the Permian magmatism of the Southern Alps. Geochemical features, similar to those of are-related suites, point to an orogenic signature. Palaeogeographic restorations, however, do not show evidence of active subduction processes. Furthermore, geological and tectonic evidence, as well as the timing of intrusion, indicate that the magmatism is related to late/post-orogenic lithospheric extension. Therefore, the calc-alkaline affinity and the orogenic signature of these Permian granitoids might reflect the involvement of mantle source(s) modified by previous subduction processes.