The Colli Albani mafic caldera (Roma, Italy): Stratigraphy, structure and petrology

摘要

New results for the Colli Albani volcano (Roma, Italy) surveyed for the Geological Map of Italy at 1:50,000 scale (CARG Project), integrated with previous data, provide insights on caldera evolution. The Colli Albani, a quiescent volcano, became active at ～600 ka. Eruptive compositions are consistently mafic ( < 50% SiO_2); nevertheless, morphology and the dominantly explosive eruptive style match those of felsic calderas. The volcano is composite, containing multiple superposed edifices or lithosomes. The oldest edifice (Vulcano Laziale (VL), ca. 600-350 ka) is a 1600 km~2 plateau of low aspect ignimbrites (VEI 5-7) with a central caldera. After the last large eruption ( > 50 km~3 deposits), forming the Villa Senni Eruption Unit ignimbrites at ca. 355 ka, two edifices were built within the caldera: (1) The horseshoe-shaped Tuscolano-Artemisio (TA) composite edifice (or lithosome) consists of coalescing, peri-caldera, fissure-related scoriae cones interbedded with lava flows; the fissure system forms two segments controlled by regional fractures; (2) The steep-sided Faete stratovolcano (949 m a.s.l.) filled the caldera. The TA and Faete lithosomes partly interfinger and were emplaced at ～350-260 ka. Their products indicate reduced eruption rates relative to the VL period and a change to effusive and mildly explosive eruptions. The most recent and still active phase of phreatomagmatic activity formed overlapping maars and tuff cones along the western and northern slopes of the volcano, collectively named Via dei Laghi composite lithosome. The Colli Albani caldera is poly-phase: (1) a piecemeal caldera is associated with large volume ignimbrites of the VL edifice; the present shape of the caldera is related to the Villa Senni eruption; 2) the TA composite edifice, erupted from peripheral-caldera fissures, is unrelated to explosive phases of caldera collapse: the TA final products cover a morphologically stable caldera wall. The peripheral fractures feeding the TA composite edifice are interpreted as volcano-tectonic structures activated during the late stage downsag of the caldera. Reduced eruption rates during the TA and the Faete stages (10~(-1) km~3/1 ka respect to > 10~0 km~3/1 ka for the VL edifice) suggest a reduced recharge of the magma chamber that may have induced prolonged deflation and downsagging of the caldera floor and the opening of outward dipping peripheral fractures. By this interpretation, the TA edifice represents the surface expression of ring dykes at depth. The absence of similar fissure-structures along the western caldera rim may relate to the deep geometry of the ring-faults dipping inward in those areas and therefore not favourably oriented for magma intrusion during a period of general subsidence. By contrast, the following and still active phreatomagmatic phase, that has emplaced the Via dei Laghi composite edifice, is located right on the western side of the caldera, and may therefore relate to resurgent conditions. Classical petrological and PERs (Pearce Elements Ratios) analyses indicate that lavas are co-genetic and show a differentiation trend up through stratigraphy driven by crystal fractionation of the lc-cpx paragenesis, and by assimilation of upper crust, consituted by up to 6000 m thick up-thrusted Mesozoic-Cenozoic carbonatic successions.