The Predazzo Intrusive Complex (PIC), a Ladinian plutonic body located in the Southern 31 Alps (NE Italy), is made up of a 4.5 km 3 gabbroic to syenitic and syenogranitic intrusion, 32 basaltic to latitic volcanic products (about 6 km 3 in volume) and by an extended dike swarm 33 intruding both intrusive and volcanic rocks. An extensive field survey of the complex, 34 followed by detailed petrographic and geochemical analyses, allowed the identification of 35 three different magmatic units: a Shoshonitic Silica Saturated Unit (SS), 3.1 km 3 in volume, a 36 Shoshonitic Silica Undersaturated Unit (SU), 0.3 km 3 in volume, and a Granitic Unit (GU), 37 1.1 km 3 in volume. K-affinity, marked Nb and Ti negative anomalies and a strong Pb 38 enrichment are distinctive markers for all PIC lithotypes. A general HFSE (Th, U, Pb), LREE 39 (La, Ce, Pr, Nd) and Na enrichment characterises the SU suite with respect to the SS series. 40 Mass balance calculations, based on major and trace element whole rock and mineral 41 compositions, have been used to simulate the fractionation process of SS and SU suites, 42showing (i) the complexity of the evolutionary stages of the PIC and (ii) the analogy between 43 the calculated subtracted solid assemblages and the natural cumulitic lithotypes outcropping 44 in the area. The field relationships between the various portions of the intrusive complex, the 45 volcanic products and the dike swarm define the temporal evolution of the PIC, in which the 46 SS magma batch was followed by the GU and later on by the SU intrusion. The presence, in 47 both eastern and western portions of the complex, of a transitional magmatic contact between 48 the intrusive rocks of the SS suite and the volcanics is not in favour of the hypothesis of a 49 caldera collapse to explain the ring-like shape of the PIC.
Carbon in the upper mantle controls incipient melting of carbonated peridotite and so acts as a critical driver of plate tectonics. The carbon-rich melts that form control the rate of volatile outflux from the Earth's interior, contributing to climate evolution over geological times. However, attempts to constrain the carbon concentrations of the mantle source beneath Oceanic Islands and Continental Rifts is complicated by pre-eruptive volatile loss from magmas. Here, we compile literature data on magmatic gases, as surface expression of the pre-eruptive volatile loss, from 12 Oceanic Island and continental Rift volcanoes. We find the levels of carbon enrichment in magmatic gases correlate with the trace element signatures of the corresponding volcanic rocks, implying a mantle source control. We use this global association to estimate that the mean carbon concentration in the upper mantle, down to 200 km depth, is approximately 350 ppm (range, 117-669 ppm). We interpret carbon mantle heterogeneities to reflect variable extents of mantle metasomatism from carbonated silicate melts. Finally, we find that the extent of carbon enrichment in the upper mantle positively correlates with the depth at which melting starts. Our results imply a major role of C in driving melt formation in the upper mantle.
We present the first complete petrological, geochemical and geochronological characterization of the oldest lamprophyric rocks in Italy, which crop out around Predazzo (Dolomitic Area), with the aim of deciphering their relationship with Triassic magmatic events across the whole of the Southern Alps. Their Mg# of between 37 and 70, together with their trace element contents, suggests that fractional crystallization was the main process responsible for their differentiation, together with small-scale mixing, as evidenced by some complex amphibole textures. Moreover, the occurrence of primary carbonate ocelli suggests an intimate association between the alkaline lamprophyric magmas and a carbonatitic melt. 40Ar/39Ar data show that the lamprophyres were emplaced at 219·22 ± 0·73 Ma (2σ; full systematic uncertainties), around 20 Myr after the high-K calc-alkaline to shoshonitic, short-lived, Ladinian (237–238 Ma) magmatic event of the Dolomitic Area. Their trace element and Sr–Nd isotopic signatures (87Sr/86Sri = 0·7033–0·7040; 143Nd/144Ndi = 0·51260–0·51265) are probably related to a garnet–amphibole-bearing lithosphere interacting with an asthenospheric component, significantly more depleted than the mantle source of the high-K calc-alkaline to shoshonitic magmas. These features suggest that the Predazzo lamprophyres belong to the same alkaline–carbonatitic magmatic event that intruded the mantle beneath the Southern Alps (e.g. Finero peridotite) between 190 and 225 Ma. In this scenario, the Predazzo lamprophyres cannot be considered as a late-stage pulse of the orogenic-like Ladinian magmatism of the Dolomitic Area, but most probably represent a petrological bridge to the opening of the Alpine Tethys.
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