San Venanzo intrappenninic volcanic complex, Part I: textural
constraints and preliminary definition of volatiles in
S. Campagnola1, F. Lucci1, C. Romano1 & J. White2
1 Geological Science Department, University of Roma Tre, Rome, Italy
2Department of Geography and Geology, Eastern Kentucky University, U.S.A
The San Venanzo volcanic complex is located 30 km NNE of the Vulsini Complex on the western edge of
Tiber Valley Graben, where the tectonic settings is dominated by active extensional faults (NNW-SSE
direction) and trastensional systems (with NE-SW main direction). This complex represents one of the most
important and well preserved intrappenninic kamafugitic center and can be subdivided into three major
vents: San Venanzo, Pian di Celle and Celli.
Pian di Celle represents the major vent (< 2 Kmq) of the San Venanzo complex; it has been classified as an
explosive-effusive center since it is possible to recognize a first phase with production of lapilli-ash tuff
(surge like stratification), channelized unwelded pyroclastics with bread-crust bombs and abundance of
sedimentary lithics (carbonates of Umbria-Marche Succession) and stratified tephra beds. The pyroclastic-
ring is overlain, in continuity, by the final production of a kamafugitic/venanzitic
(Lct+Ol+Mel+Kls+Phl+Cpx) lava flow.
The field analysis has highlighted three major features within this venanzitic lava-flow:
• One first episode, recognized only at the vent (SV3), characterized by a strongly vesiculated
• A well developed macrocrystalline facies (PEG) with venanzitic mineral association enriched in
Ap+Bt+Kls+Cc, visible at the head of the venanzitic flow.
• Microamygdules (< 3 cm in diameter) with mineralogy similar to the macrocrystalline facies,
observed in the whole lava flow (SV4, SVX).
Given the widespread occurrence of volatiles in these eruptions (first phase characterized by explosive event,
second phase represented by emission of lavas enriched in vesicles and amygdules), we decided to
investigate in greater details volatile distribution and evolution during the eruptive phases via textural
analyses. For this purpose, we selected samples from different phases recognized in the lava flow: the
scoriaceous bed (SV3), the macrocrystalline facies (PEG), a microcrystalline lava with presence of
micrometric amygdules (SVX), and the last emitted portion of venanzite lava at the vent (SV4).
Textural analyses have been performed following the method proposed by Shea et al. (2010).
In order to fully investigate all vesicle sizes, three different magnifications were chosen (20x, 40x, 100x),
increasing statistically the number of images for every magnification step. Different percentages of each
phase (vesicles and amygdules-minerals) were calculated to obtain preliminary constraints on the
relationship between vesicle and crystal distribution.
Preliminary results indicate that:
• the total percentage of vesicles plus amygdules is constant throughout the flow, (16-19% with a
mean value of 17.2%);
• the highest vesicles contents (17%) are obtained for scoriaceous venanzitic bed (in which amygdules
• the maximum value of amygdule-like assemblage (15.3%) is related to the macrocrystalline facies
• SVX and SV4 samples, present a vesicle content of approximately 4% and an average of 12% of
amygdules, corresponding to the microcrystalline sample and lava at vent that represents the last
The continuous evolution of the textural features investigated, the similar total percentage of vesicles plus
amygdules throughout the flow, and the absence of structural controls or correlation between fragile
elements and macrocrystalline facies (Lucci et al., 2011), suggest that the macrocrystalline facies cannot be
interpreted as a dykelet intrusion of lava (Stoppa et al., 1997). Alternatively, on the basis of our preliminary
reports on vesicle distribution in the lava body, and in agreement with the models by Melnik (2000), Spieler
et al. (2003) and Mueller et al. (2004), we suggest that the effusive phase of the San Venanzo
kamafugitic/venanzitic lava is originated from a continuous discharge of a fluid-enriched magma. The
activity initiated by the discharge of a bubbly flow in which volatiles were completely exsolved from the
magma, generating the scoriaceous lava present in proximal location. Following this initial event,
macrocrystalline (PEG) and microcrystalline (SVX and SV4) lavas, with the low vesicles percentage (4%),
were erupted, representing the magma, enriched in dissolved volatiles, approaching to the volatile-exsolution
level. Macrocrystalline facies, (PEG), could be interpreted as a macroamygdula itself and represents the first
emission of lava after the scoriaceous event.
Chemical and petrographical analyses of the macrocrystalline amygdules also support this interpretation and
are presented in a second related abstract (Lucci et al, 2013, this volume).
Textural analyses of the crystal distribution along the flow are underway to further investigate the dynamic
and evolution of San Venanzo activity.
Distribution of intrappenninic kamafugitic centers. Geological map of Pian di Celle ring (San Venanzo
Volcanic Complex) and schematic sketch of Venanzite lava flow (Modified from Lucci et al. 2011).
Table showing vesicles and amygdules content in venanzite selected samples: for every sample is presented
i) position (from the top to the bottom, in order of emission from the vent) along venanzitic flow (modified
from Lucci et al. 2011), ii) representative binarized image used in texture analysis (black as vesicles, dark
gray as amygdules), iii) schematic plot for cumulative vesicle and amygdule distributions in analyzed
Lucci F., Cozzupoli D. & Brizzi S. (2011): The “Uncompahgritic Pegmatite” at San Venanzo volcano
(central Italy): an example of late crystallization controlled by fluids carried in the venanzite lava flow. –
Geoitalia 2011, Poster.
Melnik O. (2000): Dynamics of two-phase conduit flow of high viscosity gas-saturated magma: large
varations sustained explosive eruption intensity. – Bulletin of Volcanology, 62: 153-170.
Mueller S., Melnik O., Spieler O., Scheu B. & Dingwell D.B. (2004): Permeability and degassing of dome
lavas undergoing rapid decompression: An experimental determination. – Bulletin of Volcanology, 67(2):
Shea T., Houghton B.F., Gurioli L., Cashman K.V., Hammer J.E. & Hobden B.J. (2010): Textural studies of
vesicles in volcanic rocks: An integrated methodology – Journal of Volcanology and Geothermal Research,
Spieler O., Dingwell D.B. & Aldibirov M. (2003): Magma fragmentation speed: An experimental
determination – Journal of Volcanology and Geothermal Research, 129:109-123.
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association: the melilitolite from Pian di Celle, Italy – Mineralogy and petrology, 61: 27-45.