Unravelling a permian volcanic succession: paleoenvironments and tectonic in the Valganna area (Southern Alps)

In the Southern Alps, at Permian times, post-Variscan extension is commonly associated with widespread plutonic and volcanic activities [1, 2]. We addressed a poorly investigated succession outcropping in the western Southern Alps between Lugano and Maggiore lakes (Valganna area), The Permian volcanic to subvolcanic (Valganna granophyre) sequence is almost tectonically undisturbed. In the area 3 volcanic series [1, 3, 7] were investigated and our field and petrographic observations suggested different environments of subaqueous lacustrine and subaerial emplacement. Series I (basal series): this extrusive series, directly deposited upon a non-conformity surface over the Scisti dei Laghi basement [4], consists of a few metres-thick volcaniclastic mass-flow followed by lithic-crystal and crystal tuffs interbedded with thin cinerite levels. The M. Piambello rhyolite lava flow ends this series. Series II: it starts with andesite to dacites agglomerates in turn covered by tuff and cinerite levels, the Alpe Tedesco rhyolite flow and a final ignimbrite (P. Ganna ignimbrite), whose geometrical relationships with the youngest Valganna granophyre are unclear. Series III: it starts with the M. Piambello dacite lava flow followed by lithic-crystal and crystal tuffs and a rhyolitic ignimbrite. The whole of volcanic products from the three series show high-K calc-alkaline affinity with similar PM- and REE-normalized patterns, which are characterized by LILE and LREE enrichment relative to HFSE and HREE, respectively. Whereas the andesite to dacite extrusive products show very similar geochemical behaviour, the most outstanding differences are among the acidic volcanic products. The rhyolite tuffs and ignimbrites, as well as the very homogeneous rhyolite lava flow of the Series II, mainly differ from those of the Series I and III in: 1) higher K2O, Rb, Nb and Y and lower Na2O, MgO, CaO, TiO2, Sr and Zr contents; 2) more pronounced negative Ba, Nb, Sr, P, Zr and Ti and positive Th, K, Pb spikes in PM- normalized multielement diagram; 3) higher HREE, but similar LREE, contents that lead to a flatter REE pattern with a more pronounced Eu anomaly. In Valganna, the petrography of the volcanic complex reflects different environments of emplacement. The basal extrusive series (1800-2000 m-thick) was probably emplaced in a shallow lacustrine environment, whereas the Series II could reflect a subaerial apron. The Series III suggest a syntectonic emplacement along an active lineament at the boundary of the paleo-lake (Series I) and its shoulders (Series II). Petrographic and geochemical data reveal an igneous evolution by fractional crystallization within each series together with density selection during the emplacement. The enrichment in LILE and LREE observed from Series I to III could origin in the different abundance of the crustal component or in different source levels in magma genesis. Moreover, the association of high-K-calc-alkaline (Series I and III) with K-rich transitional Series II is consistent with a transtensional setting, probably driven by Permian strike-slip faults at regional scale, as already reported for the Collio and Orobic basins [2, 5]. In progress SHRIMP U-Pb geochronological dating of the key-units will allow to make regional scale correlations with volcanic rocks of similar age and evolution [1, 2, 6]. References. [1] Schaltegger U. & Brack P. (2007): Int. J. Earth Sci., 96, 1131-1151; [2] Cassinis G., Cortesogno L., Gaggero L., Perotti C.R., Ronchi A. (2007): Per. Mineral, 76, 25-47; [3] Bakos F., Del Moro A., Visonà D. (1990): Eur. J. Mineral, 2, 373-383; [4] Boriani A. Giobbi Origoni E., Borghi A., Caironi V. (1990): Tectonophysics, 182, 103-118; [5] Cassinis G. & Perotti C.R. (2007): Palaeoworld, 16, 140-172; [6] Cortesogno L., Cassinis G., Dallagiovanna G., Gaggero L., Oggiano G., Ronchi A., Seno S., Vanossi M. (1998): Lithos, 45, 305-328; [7] Riccardi, M.P. (1988) unpublished Diploma Thesis.