PERIODICO�di�MINERALOGIA�-��Miocene�to�Recent�Plutonism�and�Volcanism�in�the�Tuscan�Magmatic�Province�(Central�Italy)

Monte Amiata “Castel del Piano”Basal Lavas (1° Period of Activity)

Roberto Mazzuoli*, Diego Perugini** and Giampiero Poli***Department of Earth Sciences, University of Pisa, Via S. Maria 53, 56126, Pisa, Italy

**Department of Earth Sciences, University of Perugia, Piazza Università, 06100, Perugia, Italy

This stop is focused of the first phase of volcanic activity of Monte Amiata. In this phase,basal trachydacite lavas were emitted in a relatively narrow time span from the ENE-WSWtrending fault system. The main structural elements recognizable on the field arerepresented by massive lava flows separated by an auto-brecciated zone representing thebase of the flow. Figure 1 and 2 show transversal and longitudinal sections, respectively,of multiple overlapping lava flows.

Figure 1. Transversal section of multiple overlapping lava flows. To better appreciate the structure of theoutcrop, the auto-brecciated zone has been separated from the upper and lower massive lava flows withwhite dashed lines.

In particular, in figure 1 are recognizable two superimposed massive units separated bya brecciated zone whereas figure 2 reports a ramping lava flow on the right side and abrecciated zone on the left side. Within the brecciated zone large blocks of the massive lavaflow can be observed. In both the massive and brecciated units blocks of sedimentary rockshaving different dimensions, probably derived from the sedimentary substratum over which

Auto-brecciated zone

Massive lava flow

Massive lava flow

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PERIODICO�di�MINERALOGIA�-��Miocene�to�Recent�Plutonism�and�Volcanism�in�the�Tuscan�Magmatic�Province�(Central�Italy)

the lava flows emplaced, are commonly observed. An example of such blocks can beobserved in figure 2 in correspondence of the dark zone on the left side of the figure.

Figure 2. Transversal section of a massive (right side) and auto-brecciated (left side) lava flow. Note thedark coloured area on the left side of the picture that represents a block of sedimentary rock deriving fromthe substratum.

The massive unit has a medium grained porphyritic texture with phenocrysts ofplagioclase, sanidine, biotite, orthopyroxene and scarce clinopyroxene set in a glassy and/or perlitic groundmass (fig. 3).

Figure 3. Microphotograph showing the typical glassy perlitic aspect of the massive lava flow.

Lava tongues

200 m�µ

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PERIODICO�di�MINERALOGIA�-��Miocene�to�Recent�Plutonism�and�Volcanism�in�the�Tuscan�Magmatic�Province�(Central�Italy)

The brecciated unit contains the same phenocrysts constituting the massive unit butdiffers from the latter because crystals are always fractured, rounded and alined along wellvisible flow lines constituted by alternate dark and light coloured bandings (fig. 4).

Figure 4. Fractured and rounded phenocrysts alined along flow lines constituting the auto-brecciated unit.

In samples from both the massive and brecciated units are commonly observedmetasedimentary xenoliths containing abundant green spinel (fig. 5).

Figure 5. Metasedimentary xenolith containing abundant green spinel in a sample from the massive unit.

200 m�µ

200 m�µ

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