Unravelling the complex interaction between mantle and crustal magmas encoded in the lavas of San Vincenzo (Tuscany, Italy). Part II: Geochemical overview and modelling

This work reports a geochemical overview and modelling of the lavas erupted similar to 4.4 Ma ago at San Vincenzo (Tuscan Magmatic Province, TMP). Although these lavas cover a relatively small area (similar to 10 km(2)), they show very large geochemical variations caused by the interaction of mantle-derived and crustal-anatectic magmas. The lavas consist of peraluminous rhyolites (Sr-82/Sr-86((i)) up to 0.726) hosting primarily variably sized magmatic enclaves with shoshonite/latite compositions (Sr-82/Sr-86((i)) down to 0.708). New whole-rock data for a large shoshonite enclave show high concentrations of LREE, LILE, and tetravalent HFSE, coupled with pentavalent HFSE depletions and enrichments in compatible elements such as Cr and Co. The chondrite-normalised REE pattern is strongly fractionated and characterised by a negative Eu anomaly (Eu/Eu* = 0.79). Hybridisation and AFC models suggest that the shoshonite enclave is the result of 12% rhyolite contamination of a mantle-derived magma akin to the potassic trachybasalt/shoshonite lavas of Capraia Island (similar to 4.6 Ma; TMP), following an 18.5% assimilation of Late Triassic metasediments (13% evaporite and 5.5% carbonate) and 56% fractionation of clinopyroxene (39%), plagioclase (10%), and biotite (7%). Each rhyolite sample is characterised by mineral-scale isotopic disequilibria (e.g., Sr-82/Sr-86((i)) = 0.711-0.726), glass inclusions with large K2O/Na2O variations (1.1-3.4) and a poli-thermobarometric history of crustal melt production at eutectic conditions. A multi-parametric approach accounting for K2O/Na2O (1.3-2.2), Sr-82/Sr-86((i)) (0.713-0.725), Sr (104-311 ppm) and Rb (294-403 ppm) whole-rock variations, allowed us to divide the anatectic (A) rhyolites into five groups (A1, A2.1, A2.2, A2.3, A3). Group Al shows the highest Sr-82/Sr-86((i)) ratios and the lowest values of Sr, K2O/Na2O and Rb. It is related to A2.1 and A3 rhyolites by positive K2O/Na2O-Rb and K2O/Na2O-FeO correlations. These three rhyolite groups crop out in the south of San Vincenzo and are interpreted to derive from mixing and ascent of peraluminous magma batches locally extracted at progressively higher temperatures from the heterogeneous Paleozoic metamorphic basement. The A2.3 rhyolites show the highest Rb concentrations (up to 403 ppm) and inverse Sr-Rb and K2O-FeO correlations, suggesting reactions involving the incongruent melting of a biotite-bearing crustal source, which was previously depleted in low temperature melting components such as muscovite and albite. Most of the A2.3 rhyolites crop out in the northern SVVC area and appear spatially and geochemically related to the A2.2 rhyolites by similarly low Sr-82/Sr-86((i)) ratios. However, A2.2 rhyolites show lower Rb contents (296-316 ppm) and geochemical correlations (e.g., Sr-82/Sr-86((i))-Rb, Sr-Rb, MgO-FeO) clearly indicating hybridisation (mixing) with the shoshonite mantle derived magma (Rb = 109-122 ppm). Finally, we propose a four-stage model for the magma plumbing system of San Vincenzo, produced by the uprising of a mantle-derived magma in a normally faulted (and doubled) crustal basement characterised by two sections of Paleozoic siliciclastic rocks separated by Late Triassic evaporite-carbonate sequences. (C) 2015 Elsevier B.V. All rights reserved.