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Two main magmatic events occurred during the Mesozoic evolution of the Atlas System, the first one dated to Late Triassic-Early Liassic (200-195 Ma), tied to the opening of the Central Atlantic Ocean (CAMP - Central Atlantic Magmatic Province - Event), the second dated from Middle-Late Jurassic to Early Cretaceous (~ 165-110Ma), within which K-Ar ages seem to cluster into two distinct groups, 175-155 Ma and 135-110 Ma. The post-Liassic magmatic rocks, possibly representing the last magmatic phases related to the CAMP event, are specific for the High Atlas domain, with the emplacement of (1) subvolcanic intrusive complexes, often cropping out in the cores of anticlinal ridges, and (2) basaltic lava flows, interbedded in the Middle-Late Jurassic to Early Cretaceous red beds. Their magmatic signatures, ranging from transitional basalt series to weakly/moderately alkaline series, are typical of continental intraplate magmatism and are usually considered as rift-related. In our work, we studied magmatic bodies (1) cropping at the core of anticlines and/or (2) interbedded in the red beds of the High Atlas, in the chain and at its southern front. The relationships observed between the magmatic products (2) and both the stratigraphic succession and structures, faults and folds, point to their possible emplacement as sills and to a development of the magmatism within an active, Jurassic-Cretaceous, compressive stress field. In particular, we present the first results obtained on two mafic magmatic bodies outcropping in correspondence of the South Atlas front. They are intruded at the core of anticlines folding Late Jurassic-Cretaceous rocks, or are interbedded within the vertical Late Jurassic-Cretaceous succession as a sub-effusive body. We present an important first radiometric 40Ar-39Ar dating obtained on a lava flow rock sample; the obtained age of 119 Ma, allows us to refer it to the second magmatic event of the High Atlas. We also present preliminary petrographic and geochemical compositional data of collected rock samples that display porphyritic and fluidal textures in the lavas and porphyroid textures with poikilitic crystals set in an ophitic matrix in dykes and sills. The studied rocks display a restricted compositional range, falling across the fields of basalts, trachybasalts and basaltic trachyandesites with alkaline to subalkaline affinity. We report here major and trace elements and isotopic compositions (87Sr/86Sr and 143Nd/144Nd) obtained on representative lava flow, dyke and sill rock samples.

Monte Amiata is a small volcano composed by trachytic to olivine latitic lava flows and domes emplaced in a very short time between 305 and 231 ka. The main petrographic features are represented by the occurrence of i) abundant rounded magmatic enclaves increasing in dimension and quantity passing from early to late erupted Monte Amiata volcanic rocks, ii) large sanidine megacrysts, mainly confined in the second stage of activity characterised by the emplacement of exogenous domes and massive lava flows, and iii) mafic olivine latitic lava flows, with intermediate compositions between the early silica-rich volcanic rocks and the most mafic rounded magmatic enclaves hosted by the Monte Amiata volcanic rocks. The occurrence of rounded magmatic enclaves testifies fresh magma injection and stirring within a differentiated magma reservoir. This triggered the pouring out of the viscous trachydacitic resident magma. A reverse differentiation pathway is observed with time of magma emplacement, which is accompanied by the decrease of silica contents and increase of MgO and compatible elements passing from early trachydacites to final olivine-latites. The same timely reverse differentiation pathway is observed among magmatic enclaves, with the most mafic terms hosted by final olivine-latitic lava flows. Fine-grained rounded magmatic enclaves, indeed, range in composition from potassic trachybasalt (absarokite) to olivine-latite. The overall geochemical and isotopic features agree with a mixing process between a highly differentiated (i.e., high silica), and partially crystallised, high-K calc-alkaline end- member and a mafic ultrapotassic magma possibly leucite-bearing. Absence of leucite in the Amiata rocks and enclaves is due to high-silica activity of derived magmas caused by the high-silica end-member of the mixing process.

The duration of the Mt. Amiata volcanic activity is still a matter of debate, in spite of the presence of several geochronological data in the literature. We performed new 40Ar-39Ar dating on the sanidinegroundmass pairs of the upper stratigraphic units: Dome and massive Lava flows Complex (DLC) and Olivine Latitic final Lavas (OLF). The aim was twofold: to check the reliability of sanidine ages as geochronometer in these products, questioned in the literature, and to better define the chronology of the late activity of this volcano. Ages obtained on coexisting sanidine and groundmass of the Dome and massive Lava flows Complex (DLC) samples overlap within errors, demonstrating that sanidine crystals recorded reliable emplacement ages in these rocks. The Olivine Latite final lavas (OLF) display a different scenario, where the groundmass has an age younger than that of the sanidine, which is xenocrystic and, evidently, retains inherited Ar. Preferred ages for analysed DLC samples are comprised between 301 and 294 ka, an interval of time too short to resolve the ages of the four dome samples taken into account. The Ermeta lava is about 60 ka younger. We propose that the majority of Mt. Amiata volcanic rocks were emplaced in a narrow interval of time, whilst a temporal gap, which needs more detailed constraints, exists with at least one of the Olivine Latite final lavas (OLF).

This paper and the associated 1:50,000 geological map are devoted to describe the geological features of the Monte Amiata region. The tectono-stratigraphic setting of Monte Amiata region includes, from bottom to top, 1) the pre-Neogene stack of tectonic units, made up of Tuscan, Sub-Ligurian and Ligurian Tectonic Units, 2) the Neogene sedimentary deposits and 3) the Plei -stocene Radicofani and Monte Amiata volcanoes. The pre-Neogene stack of tectonic units includes, from bottom to top, the Tuscan Nappe, belonging to the Tuscan Domain, and Canetolo Tectonic Unit, belonging to the Sub-Ligurian Domain. These tectonic units, regarded as representative of the thinned continental margin of the Adria plate, are topped by the Santa Fiora and Ophiolitic Tectonic Units, interpreted as remnants of the Ligure-Piemontese oceanic basin and its transition to the Adria continental margin. All the tectonic units of the pre-Neogene stack have been affected by folds and thrusts originated during the convergence related to the Europe-Africa motion during the Middle Eocene-Early Miocene. Subsequently, these tectonic units were affected by a widespread reduction of thickness of their successions due to low-angle normal faulting related to the Middle Miocene extensional tectonics. The Neogene sedimentary deposits unconformably overlie the pre-Neogene stack of tectonic units. They consist of Upper Miocene to Pliocene continental and marine sediments, filling the Cinigiano-Baccinello, Velona, and Siena-Radicofani basins, adopting an informal hierarchy of different stratigraphic units where the first order units are synthems. The Pleistocene Radicofani and Monte Amiata volcanoes are made up by high-K basaltic andesitic to shoshonitic volcanic rocks and by trachydacitic to trachytic and olivine-latitic volcanic rocks, respectively. The geological mapping has provided evidences of a complex tectonic setting resulting from a long-lived history shifting from Cretaceous to Early Miocene compressive events to Middle Miocene extensional tectonics and Late Miocene-Pleistocene contractional and extensional events during which the Pleistocene magmatic activity occurred. In this regard, the Monte Amiata region can be regarded as a key area where the final result of a 200 Ma long geological history of the Northern Apennines is exposed.