The complexity of volcanism in central Italy animated the scientific debate during the last decades. The peculiar potassium-rich magmatism of the Roman Province (Peccerillo, 2005) aimed numerous scientific contributions focused on the petrology of the various volcanic districts. Among these, the Sabatini Volcanic District (hereafter SVD) is one of the largest, being characterized by a number of explosive eruptions emplaced during the last 800 kyr that produced pyroclastic deposits cropping out in a widespread area at north of Rome (~1800 km2, Sottili et al., 2010). During these explosive eruptions (mostly known as yellow tuffs and red tuffs), large volumes of phonolitic magmas were emplaced (an average of 10 km3 dense rock equivalent of magma). The interest for these pyroclastic deposits (quarried since the ancient Roman age) promoted detailed studies, mainly on stratigraphy and geochronology (Karner et al., 2001; Sottili et al., 2010), whereas petrological studies are scarce. The petrological studies are, indeed, limited to lava flows and scoria cones with primitive chemical composition (i.e., Cundari, 1979; Conticelli & Peccerillo, 1992; Conticelli et al., 1997) that represent only a small fraction (less than 10%) of the total volume of the emplaced products. One of the major features of SVD pyroclastic deposits is the textural variability of juvenile clasts. These deposits are commonly characterized by a transition from crystal-poor juvenile clasts at their bottom, toward crystal-rich ones at the top. In general, phonolitic volcanism offers numerous examples of pyroclastic successions showing analogue textural variations of the juvenile component, often accompanied by chemical variation of the juvenile clasts. These variations are commonly interpreted in the light of compositional variation of the erupted magma, resulting from the compositional and/or thermal layering of shallow magma chambers. However, zoning models based on compositional variations invoked for these magmatic systems, do not strictly apply in the case of SVD eruptions, given that no significant chemical variation is observed between the crystal-poor and the crystal-rich juvenile fraction. In addition to the problem of textural variations of the juvenile component, it comes up the paradox on the genesis of crystal-poor, differentiated magmas. Crystallization is intrinsic in the differentiation, but crystal fractionation may be not so obvious in a differentiated magma (i.e., low contrast of densities between melt and crystal, high viscosity of the melt). Hence, mechanisms alternative to crystal settling need to be found to explain the formation of crystal-poor, differentiated magmas. In this study, the products from large explosive eruptions of SVD were collected and investigated in detail. Phase equilibria experiments, coupled with MELTS simulations (Ghiorso & Sack, 1995), were used to constrain both differentiation and pre-eruptive conditions of the phonolitic system. Moreover, temperature gradient experiments were performed with the aim to mimic conditions occurring in a thermally zoned magma chamber and explain the observed textural variation in the deposits. Through the coupling of natural and experimental data, magmatic processes occurring in the shallow, thermally zoned magma system of SVD were modelled, addressing the problems of melt differentiation, crystal-melt separation and achievement of pre-eruptive conditions of phonolitic magmas feeding large explosive eruptions.
Magma differentiation in shallow, thermally zoned magma chambers: the example of Sabatini Volcanic District (central Italy)
MASOTTA, MATTEO
2012-01-01
Abstract
The complexity of volcanism in central Italy animated the scientific debate during the last decades. The peculiar potassium-rich magmatism of the Roman Province (Peccerillo, 2005) aimed numerous scientific contributions focused on the petrology of the various volcanic districts. Among these, the Sabatini Volcanic District (hereafter SVD) is one of the largest, being characterized by a number of explosive eruptions emplaced during the last 800 kyr that produced pyroclastic deposits cropping out in a widespread area at north of Rome (~1800 km2, Sottili et al., 2010). During these explosive eruptions (mostly known as yellow tuffs and red tuffs), large volumes of phonolitic magmas were emplaced (an average of 10 km3 dense rock equivalent of magma). The interest for these pyroclastic deposits (quarried since the ancient Roman age) promoted detailed studies, mainly on stratigraphy and geochronology (Karner et al., 2001; Sottili et al., 2010), whereas petrological studies are scarce. The petrological studies are, indeed, limited to lava flows and scoria cones with primitive chemical composition (i.e., Cundari, 1979; Conticelli & Peccerillo, 1992; Conticelli et al., 1997) that represent only a small fraction (less than 10%) of the total volume of the emplaced products. One of the major features of SVD pyroclastic deposits is the textural variability of juvenile clasts. These deposits are commonly characterized by a transition from crystal-poor juvenile clasts at their bottom, toward crystal-rich ones at the top. In general, phonolitic volcanism offers numerous examples of pyroclastic successions showing analogue textural variations of the juvenile component, often accompanied by chemical variation of the juvenile clasts. These variations are commonly interpreted in the light of compositional variation of the erupted magma, resulting from the compositional and/or thermal layering of shallow magma chambers. However, zoning models based on compositional variations invoked for these magmatic systems, do not strictly apply in the case of SVD eruptions, given that no significant chemical variation is observed between the crystal-poor and the crystal-rich juvenile fraction. In addition to the problem of textural variations of the juvenile component, it comes up the paradox on the genesis of crystal-poor, differentiated magmas. Crystallization is intrinsic in the differentiation, but crystal fractionation may be not so obvious in a differentiated magma (i.e., low contrast of densities between melt and crystal, high viscosity of the melt). Hence, mechanisms alternative to crystal settling need to be found to explain the formation of crystal-poor, differentiated magmas. In this study, the products from large explosive eruptions of SVD were collected and investigated in detail. Phase equilibria experiments, coupled with MELTS simulations (Ghiorso & Sack, 1995), were used to constrain both differentiation and pre-eruptive conditions of the phonolitic system. Moreover, temperature gradient experiments were performed with the aim to mimic conditions occurring in a thermally zoned magma chamber and explain the observed textural variation in the deposits. Through the coupling of natural and experimental data, magmatic processes occurring in the shallow, thermally zoned magma system of SVD were modelled, addressing the problems of melt differentiation, crystal-melt separation and achievement of pre-eruptive conditions of phonolitic magmas feeding large explosive eruptions.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
