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Sabatini volcanic complex, Italy

volcano number: none (not Holocene)

summit elevation: 612 m (Monte Rocca Romana)

location: XXXXX°N, XXXXX°E

The following is summarized from De Rita (1993).

View of the northern part of Lago di Bracciano, the largest structure of the Sabatini complex. The lake fills a volcanotectonic depression formed about3.7 Ma ago. Monte Rocca Romana, a postcaldera stratocone is visible on the right (northern) side of the depression. Taken from the rim of the Sacrofano basin, in late April 1996.

The Sabatini volcanic complex, with the Bracciano volcanotectonic depression in its central part, is probably the most complex of the areas featured here. Its activity began more than 0.6 Ma ago, contemporaneoulsy with the other alkaline-potassic volcanic centers of Lazio. The various explosive centers of the complex developed on a vast plain constituted by Plio-Pleistocene clayey and sandy sediments, bordered in the W by the Monti della Tolfa and by acidic lava domes of the Tolfa-Ceriti-Manziana volcanic district whose activity had ceased shortly before. The E border is constituted by the meso-cenozoic sedimentary formations of M. Soratte, and more southwards, by the Monti Cornicolani. A former sedimentary high in the central portion of the Sabatini complex lies now buried under about 200 m of volcanics.

The first eruptive activity was mainly explosive and occurred in the E part of the complex, building a volcanic edifice named "Morlupo-Castelnuovo di Porto". The majority of volcanic deposits in that zone actually stem from that center which itself lies buried beneath more recent volcanics and edifices. The explosivity of its activity was due to interaction of the trachytic to phonolitic magma with deep groundwater.
While activity was ongoing at the Morlupo-Castelnuovo di Porto center, eruptive activity began also more towards the W, from what is probably one of the most important eruptive centers of the Sabatini complex, the Sacrofano edifice. Its activity spanned quite a long period, from 0.6 to 0.37 Ma and produced large volumes of ash flow tuffs.
The pyroclastic flows from the Sacrofano center reach distances of 30-40 km from their source, covering a large portion of what is now the northern part of Roma. These ash flow tuffs yielded (and still yield) the primary material for building construction in that city, thanks to its consistency (cementation) by zeolitization. The occurrence of these vast ignimbrites have left a fairly smooth surface in the area they covered.

A notable paroxysmal eruptive phase of Sacrofano occurred about 0.4 Ma ago, when large volumes of air fall pyroclastics were erupted from the central eruptive vent and smaller ones; this activity furthermore produced lava flows. All products of that phase were undersaturated, potassium (K)-rich magmas.
During this time interval, many other eruptive centers were active in the area occupied by the Sabatini complex. Major volumes of tephritic phonolite and phonolitic tephrite lavas were emitted from regional fissures and a few major scoria cones N and S of the present depression of Lago di Bracciano. During the relatively brief period from 0.4 to 0.25 Ma ago, about 15 per cent of the total volume of Sabatini volcanics were erupted.

Volcanic landscape on the northern side of Lago di Bracciano, seen from the castle ruins of Trevignano. A nice outcrop is visible in the lower left corner of the photo. Deposits are mostly from a small explosion crater lying out of the photo to the right whose eruption broke through a lava flow or dome ejecting meter-sized lithic blocks of that lava.

It is presently believed that this culminating phase of Sabatini volcanism was related to high stress connected with extensional tectonics that controlled the evolution of the Tyrrhenian margin of the Appennine peninsula during that period. Due to tectonic activity and simultaneous emptying of magma reservoirs, there ensued the collapse of the volcano-tectonic depression now occupied by Lago di Bracciano and the subsidence of the structural high of Baccano-Cesano. During the paroxysmal phase of activity, during and/or after the collapse of the Bracciano basin, ash flows were erupted from circumferential fissures before very violent hydromagmatic explosions occurred from vents around the collapse basin.

After the culminating phase, about 0.37 Ma ago, the Sacrofano eruptive center entered its final eruptive stage. After violent hydromagmatic explosions and the emplacement of the Sacrofano pyroclastic flow unit, the upper part of the central edifice collapsed, forming a vast calderic depression with low rims.
Following the end of Sacrofano activity, minor eruptive activity occurred from vents in the E part of the complex, being mainly hydromagmatic. The tuff rings of Monte Razzano and Monte Sant'Angelo as well as the complex center of Baccano were formed during this period. The last Baccano eruptions occurred 40 ka ago, but still more recently, small-scale eruptive episodes occurred from the Martignano, Stracciacappa and Cese eruptive centers.

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