Genesis of Roccastrada Volcanic Rocks (Central Italy): Inferences from Melt Inclusions Analyses

Per. Mineral. (1999), 68, 1, 69-80 An InternationalJournal of PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Genesis of Roccastrada volcanic rocks (central Italy): inferences from melt inclusions analyses pAOLA MARIANELLI* and P AOLO CARLETTI Dipartimento di Scienze della Terra, via Santa Maria 53, I-56126 Pisa, Italy Submitted Janu(//y I999- Accepted March I999 ABSTRACT. - Melt inclusions analyses were di Roccastrada, suggeriscono che questi fusi non carried out on quaternary rhyolitic rocks from hanno subito processi di differenziazione Roccastrada volcano (Tuscany, central Italy). The significativi nei momenti successivi alia loro data confirm the anatectic origin for the magma that generazione. Quarzo, sanidino, plagioclasio e gave rise to the Roccastrada volcanism and cordierite sono cristallizzati da fusi peralluminosi, a evidenced no involvement of basic component in the temperature nell' intervallo 755-820°C, e a pressioni petrogenesis of these melts. Similar compositions of di circa 150-180 MPa. melt inclusions trapped in phenocrysts and of residual glass matrix of Roccastrada rhyolites, suggest that these melts did not undergo to important KEY WORDS: Roccastrada, melt inclusions, differentiation processes after their generation. microthermometric measurements, crusta! Quartz, sanidine, plagioclase and cordierite anatexis. crystallized from peralominous melts, at temperatures in the range of of 755-820°C, and at pressure of about 150-180 MPa. INTRODUCTION RIASSUNTO. - Sono state studiate le inclusioni Since Middle Miocene, the Northern sector silicatiche nei cristalli nelle rocce dell' apparato of the A pennine belt was affected by the vulcanico di Roccastrada (Toscana, Italia centrale). emplacement of silicic plutonic rocks at I dati raccolti confermano che il magma che ha dato relatively shallow depth in the continental crust origine al vulcanismo di Roccastrada si e originato and by volcanic activity that produced mainly per anatessi. Inoltre l'esame di questi dati permette di escludere il coinvolgimento di un termine basico silicic and crusta} derived potassic rocks. These nella petrogenesi di questi fusi. Le composizioni intrusive rocks and volcanic products are delle inclusioni silicatiche intrappolate nei cristalli e commonly known in the literature as the dei vetri residuali della massa di fondo delle rioliti Tuscan Magmatic Province (PMT, Innocenti et al. , 1992 and references therein). The silicic magmas of the PMT are Corresponding author, e-mail: [email protected]. considered to have been generated by anatectic 70 P. MARIANELLI and P. CARLETTI processes occurring in the adriatic continental crust and affected by widespread mixing with mantle-derived liquids (Serri et al., 1991). An alternative two-stage petrogenetic model is invoked by Poli (1992) to explain geological, petrographical and geochemical characteristic of some granitoids of the PMT. During the first stage, a mixing plus crystal fractionation process involves a crusta! peraluminous magma and a basic magma of probable subcrustal origin. During the second stage, a simple mixing process between the same crusta! peraluminous magma and different evolved magmas, derived from the basic magma during the first stage, occurs. In this framework the Roccastrada products could represent the acid end-member (Poli, 1992), originated by pure crusta! anatexis (Giraud et al., 1986). Peculiar geochemical features of the products induce Fig. 1. - Geological sketch of the studied area Giraud et al. ( 1986) to interprete the formation of these rhyolites not simply by partial melting Stephens et al., 1983; Clocchiatti and Massare, of a Paleozoic garnet-bearing micashist source, 1985; Anderson et al., 1989; Hervig and but with a selective contamination of pure Dunbar, 1989). We have analyzed MI in antectic melts by fluids derived from the deeper quartz, cordierite and feldspar phenocrysts, crust. with the aim to collect information on the The Roccastrada volcanic complex consists temperature and pressure of crystallization of exclusively of cordierite-bearing rhyolitic Roccastrada magma and possibly on the lavas, outpoured along a regional fault system genesis and the evolution of these magmas. (Mazzuoli, 1967) about 2. 5 Ma ago (Innocenti et al., 1992). Pinarelli et al. (1989), on the basis of geochemical data, distinguished these GEOLOGICAL AND VOLCANOLOGICAL OUTLINES products into two groups, the High-Rubidium rhyolites (HRb) and the Low-Rubidium The Roccastrada volcanic complex is located rhyolites (LRb), cropping out at East and at in South Tuscany at about 40 km NW of the West of the Mt. Alto ridge, respectively (fig. Mount Amiata Quaternary volcano, and it is 1). These geochemical characteristics distributed over an area of about 100 km2. The 7 6 associated to 8 Srf8 Sr value variations are volcanites mainly rest on Triassic-Paleozoic probably related to a different degree of partial formations, belonging to the Monticiano melting of the crusta! source (Pinarelli et al., Roccastrada tectonic units (Costantini et al., 1989; Innocenti et al., 1992). 1988); only in the western and southern sector In this paper we present chemical data on of the area, the lavas overlie the Ligurian units melt inclusions (MI) trapped in phenocrysts of and Neogenic deposits. These products form these rhyolites. During magmatic numerous outcrops, situated west and east of crystallization droplets of melt may be trapped the Monte Alto ridge, which is made up of as a primary inclusions in phenocrysts of quarzites of Trias (fig. 1). The volcanic volcanic rocks and therefore they may be complex is characterized by numerous lava representative of the melt from which the domes associated with a few lava flows. There phenocrysts grew (Clocchiatti, 1971; Beddoe- is no evidence of exp1osive activity in the Genesis of Roccastrada volcanic rocks (central Italy): inferences from melt inclusions analyses 71 TABLE 1 thickness of about 100 meters. In this area the lavas, generally glassy, are intensely Whole rock analyses and CIPW norms of autobrecciated and affected by oxidation selected samples processes. From Roccastrada village to Rocca Sam2le R25 R14 R110 R30 di Torri, small lava domes are roughly aligned Si02 74.19 74.11 73.75 75.17 along a NW -SE direction. This trend Ti02 0.27 0.23 0.29 0.23 corrisponds to the main Apenninic fault Al203 13.82 14.31 13.99 13.85 Fe203 2.24 2.01 2.23 2.00 system, not easily recognizable in the field in MnO 0.04 0.04 0.04 0.04 this area due to the presence of a clay-rich M gO 0.40 0.25 0.52 0.24 Neogenic deposit. CaO 1.03 0.89 1.00 0.82 Na20 2.89 2.95 2.74 2.81 Analyses of aerial photos and field data K20 4.98 5.04 5.30 4.69 revealed that the majority of the Roccastrada P20s 0.14 0.18 0.14 0.13 volcanites were erupted along fractures with NW-SE Apenninic trend. The largest volumes CIPW norm Q 34.66 34.59 34.19 37.81 of lavas, however, were emitted in c 2.12 2.78 2.26 2.91 correspondence of Mt. Sassoforte and the or 28.95 29.19 31.32 27.3 Seguentina Valley, where the Apenninic and ab 24.11 24.45 23.19 23.44 an 4.1 3.14 4.05 3.17 Antiapenninic regional trends (NW-SE and hy 2.55 2.05 3.08 2.03 NE-SW) match. Furthermore, the field mt 0.99 0.88 1.12 0.89 observations suggest that in late Pliocene time, il 0.51 0.44 0.55 0.44 during the volcanic activity which gave rise to ap 0.33 0.43 0.33 0.31 ag 1.68 2.03 2.28 the Roccastrada volcanites, the morphology was very similar to the present one. Probably Mt. Alto ridge already was representing at that Roccastrada area, except for local explosions time a morphological high that conditioned the occurring during dome emplacement, which distribution of the outpoured lava. gave rise to very limited pyroclastic deposits Relationships between the tectonics and (Rocca di Torri and Torniella units). volcanism indicate that the tectonic activity The main volcanic structures recognizable along these fault systems continued after the west of M. Alto are (fig. 1): 1) the large lava lava emplacement. 2 dome of Mt. Sassoforte (about 2.7 km ) dislocated by two fault systems; 2) the large 2 lava flow extending for about 11 krn south of ANALYTICAL METHODS Mt. Sassoforte with a thickness ranging between 2 and 10 meters, rarely exceeding 10 Fragments of samples were pulverized and meters; 3) the small dome of Roccatederighi analyzed by X-ray fluorescence for major 2 (0. 2 km ) with subvertical flow-bands in the elements with the method described by outer portions and a massive lava core Franzini et al. (1975). characterized by large sanidine phenocrysts and Crystals selected for microthermometric and altered cordierite crystals of 0. 3/0.5 cm. On the compositional studies were prepared in double Eastern flank of M. Alto the volcanites are polished wafers. Several heating experiments characterized by a series of lava domes and on quartz phenocrysts were carried out with an lava flows outpoured from fractures with a optical heating stage designed in the Vernadsky NW-SE direction (fig. 1). In the northern sector Insitute of Geochemistry of Moscow (described (Torniella Area, fig. 1) small, occasionally by Sobolev et al., 1980). The temperature was isolated lava domes with scarce and limited, measured with a Pt-Pt90Rh10 thermocouple and usually autobrecciated, lava flows are found. In controlled by the melting point of pure gold the Seguentina Valley lavas reach a maximum and silver. The rate of heating was varied as a 72 P. MARIANELLI and P. CARLETTI function of the rate of transformation in perlitic-fluidal, or microgranophyric textures. inclusions. Due to the high viscosity of the The Roccatederighi lava dome is generally melt, a very low rate of heating and long made up of microcrystalline lava with a well plateau were used: temperatures of 800°C were developed granophyric texture, especially in reached only after 6 to 10 hours.

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