The Pleistocene Monte Amiata volcano is part of the Radicofani-Monte Amiata volcanic system, an igneous complex responsible of a wide domed area in which the two volcanic centres cluster the central portions of its main axis. The Monte Amiata and Radicofani volcanoes are characterised by different eruption styles and rocks. The Radicofani centre outpoured few mantle-derived shoshonitic lava flows at about 1 Ma, whilst the Monte Amiata outpoured differentiated trachy-dacitic to trachytic lavas and domes at about 300 ka. The differentiated magmas of Monte Amiata derived by the fractional crystallisation and crustal assimilation from a Tuscanytype mantle-derived silica-saturated magma, similar in composition to the Radicofani shoshonite. Subsequent complex interaction between silica-saturated melts and newly arrived leucite-bearing magmas provided a further element of complexity. The occurrence in the Monte Amiata rocks of rounded magmatic enclaves testifies this complexity. The arrival of Roman-type mafic magma within the differentiated magmatic reservoir lead to mingling and mixing with the viscous and extremely differentiated trachytic resident magma to form magma batches with intermediate compositions and give the thermal-energy to induce convective motions within the magma chamber. Large K-feldspar megacrysts do occur in both host rocks and rounded enclaves of Monte Amiata volcano, suggesting that they are solid witnesses of this interaction processes. Alternatively, they can are also hypothesised as xenocrysts derived by ancient crustal derived granitoids, introducing a fourth element of igneous complexity. To tackle this issue we report a detailed mineralogical, petrographic, microthermometric, chemical and isotopic study on K-feldspar megacrysts and their melt inclusion with the aim to investigate their nature. Major and trace element compositions of K-feldspar megacrysts show evidence for interaction between magmas found in correspondence with the oscillatory zoning and resorbed bands of the crystal. Isotope chemistry investigation shows the presence of a higher Sr-radiogenic megacryst core (0.71338) compared to the most-differentiated rock outcropped (0.71300). Microthermometric approach show a glass entrapment temperature of about 1050°C and a CO2-rich vapour phase entrapped at ≈ 1-2 kbars. Our results support the presence of a high Sr-radiogenic magma not erupted to the surface, located at the top of the magmatic chamber and in a melt-impregnated regime condition (< 20% melt), which simulates the presence of a plutonic partially-molten intrusive body. The extent of the mixing process to the outcropped trachytic differentiated body is justified by the presence of melt inclusions with CO2-rich vapour phase in sanidine pheno- and mega-crysts.
Sanidine megacrysts from Monte Amiata: elemental, Sr-isotope and melt inclusions studies
Valeriani L.;Conticelli S.
2022-01-01
Abstract
The Pleistocene Monte Amiata volcano is part of the Radicofani-Monte Amiata volcanic system, an igneous complex responsible of a wide domed area in which the two volcanic centres cluster the central portions of its main axis. The Monte Amiata and Radicofani volcanoes are characterised by different eruption styles and rocks. The Radicofani centre outpoured few mantle-derived shoshonitic lava flows at about 1 Ma, whilst the Monte Amiata outpoured differentiated trachy-dacitic to trachytic lavas and domes at about 300 ka. The differentiated magmas of Monte Amiata derived by the fractional crystallisation and crustal assimilation from a Tuscanytype mantle-derived silica-saturated magma, similar in composition to the Radicofani shoshonite. Subsequent complex interaction between silica-saturated melts and newly arrived leucite-bearing magmas provided a further element of complexity. The occurrence in the Monte Amiata rocks of rounded magmatic enclaves testifies this complexity. The arrival of Roman-type mafic magma within the differentiated magmatic reservoir lead to mingling and mixing with the viscous and extremely differentiated trachytic resident magma to form magma batches with intermediate compositions and give the thermal-energy to induce convective motions within the magma chamber. Large K-feldspar megacrysts do occur in both host rocks and rounded enclaves of Monte Amiata volcano, suggesting that they are solid witnesses of this interaction processes. Alternatively, they can are also hypothesised as xenocrysts derived by ancient crustal derived granitoids, introducing a fourth element of igneous complexity. To tackle this issue we report a detailed mineralogical, petrographic, microthermometric, chemical and isotopic study on K-feldspar megacrysts and their melt inclusion with the aim to investigate their nature. Major and trace element compositions of K-feldspar megacrysts show evidence for interaction between magmas found in correspondence with the oscillatory zoning and resorbed bands of the crystal. Isotope chemistry investigation shows the presence of a higher Sr-radiogenic megacryst core (0.71338) compared to the most-differentiated rock outcropped (0.71300). Microthermometric approach show a glass entrapment temperature of about 1050°C and a CO2-rich vapour phase entrapped at ≈ 1-2 kbars. Our results support the presence of a high Sr-radiogenic magma not erupted to the surface, located at the top of the magmatic chamber and in a melt-impregnated regime condition (< 20% melt), which simulates the presence of a plutonic partially-molten intrusive body. The extent of the mixing process to the outcropped trachytic differentiated body is justified by the presence of melt inclusions with CO2-rich vapour phase in sanidine pheno- and mega-crysts.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
