An International Journal of CHAPTER 3 Monte Amiata Volcanic Products
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Per. Mineral. (2003), 72, SPECIAL ISSUE: Miocene to Recent... , 169-181 http://go.to/permin An InternationalJournal of : _. PERIOD! CO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, ArchaeometJ)' and Cultural Heritage CHAPTER 3 Monte Amiata volcanic products and their inclusions CH!ARA CR!STIANI1 and ROBERTO MAZZUOLI1* 1 Dipartimento di Scienze della Terra, Universita di Pisa, ViaS. Maria, 53, Pisa, 56 126, Italy 3.1 HISTORICAL PERSPECTIVES Visconti, then the Aldobrandeschi family, and finally, in 1416, it was taken over by Siena. Monte Amiata is the highest mountain of Entrance to the town is through a battlemented Tuscany (1750 m.a.s.l.). This solitary gateway; inside are the remains of a castle mountain lies in the very heart of the region, erected by the Aldobrandeschi, and in the and rises between the valleys of Orcia, Fiora, central Piazza Matteotti there is the ancient and Paglia rivers. It is well known for its Palazzo Pretorio. mercury sulphide (cinnabar) mines; some of the deposits were discovered by the Etruscans and, until recently, they formed a substantial 3.2 GENERAL REMARKS component of the local economy. There was human settlement on the mountain in pre The Mount Amiata is the youngest and the historic times, as testified by the Pittura southernmost volcanic complex belonging to dell' Arciere on the top of the mountain. This the Tuscan magmatic province, excluding the was also due to the abundance of water springs Tolfa complex. It is located about 30 Km North at the contact between the volcanic products of Vulsini volcanoes, the northernmost Roman and Oligocene flysh. The Etruscans dominated volcanic complex. The M. Amiata volcanic 900 300 from BC up to BC. In the river Fiora massif, consists essentially of felsic effusive 2 valley there are lots of signs of prehistoric and products covering an area of about 85 km and Etruscan settlement. resting on rocks of different age: Tuscan series Then Romans conquered this area and at the (Palaeozoic basement, Triassic limestones, falling of the Roman empire the area was Cretaceous-Oligocene terrigenous formations), godforsaken for long time until the Low the Ligurid sequence (mainly ophiolites) and Middle Age. At Abbadia San Salvatore there is Neogene sediments (Fig. 1). The eruptive 7 43 an abbey founded in by the Longobard activity of M. Amiata volcano occurred in an King Rachis and rebuilt in a Romanesque style area (comprising also the Radicofani volcano) 1036. in Piancastagnaio was originally the uplifted during Quaternary time, most probably property of the abbey, but then it changed due to the emplacement of a magma body at 5- hands and came under the control of the 6 km of depth (Barberi et al., 1994). The volcanic activity developed in four phases * Corresponding author, E-mail: [email protected] mainly along a fault system with ENE-WSW 170 C. CRISTIANI and R. MAZZUOLI direction (normal to the Apennine trend; Fig. 1; deep morphology and assumed the typical Mazzuoli and Pratesi, 1963; Barberi et al., structures of viscous lavas. These features are 1971; Van Bergen, 1985; Ferrari et al., 1996). well visible in all marginal sectors of the basal The emplacement of these products occurred in volcanic complex. The structural elements of a relatively narrow span of time ranging from these lavas, recognizable on the field (i.e in 303 to 209 Ka (Ferrari et al., 1996). southern sector at Marroneto and in the North Eastern sector at Castel Del Piano cimitery) 3.3 FIELD CHARACTERISTICS are: a) massive lava usually laminated for the AND VOLCANOLOGY flowing b) tongues usually laminated (ramps) uprising among c) brecciated lava very The first phase of volcanic activity gave rise common at the base of the flow and among the to trachydacite lavas (BT: basal tracydacites) ramps. The complex is still object of different outpoured from the ENE-WSW trending fault interpretations. Old studies (Rittmann, 1958; system (Fig. 1). These lavas, although viscous, Marinelli, 1961) referred to B T as flowed in narrow valleys far from the vents for rheoignimbrite, which is an ignimbrite that DillBasal trachydacites (BT) Trachydacitic Domes (TO) Upper Trachydacites (UT) • Final Latites (FL) /Major faults / / / ·Eruptive faults • STOP ....... roads Leccio Castel del Piano ·· - :·' · ' . ······:: ... ;,..: S.Fiora Fig. 1 Geological sketch map of Mt. Amiata (modified from Mazzuoli and Pratesi, 1963 and Ferrari et al., 1996). Monte Amiata volcanic products and their inclusions 171 developed secondary flowage after the In figure 2 published ages with relative depostion, due to high temperature. More errors are reported for the four Amiata recent studies (Ferrari et al., 1996) propose that complexes (Ferrara and Tonarini, 1985; BT was initially characterized by an explosive Laurenzi and Villa, 1991; Barberi et al., 1994). activity that progressively decreased its energy, It is shown that Amiata volcano has been with final emission of blocky lava flows. The emplaced into two main phases: 290 ± 18 Ka described field characters, as well as the and 202 ± 8 Ka. In the first period were relevant thickness in respect to the extension of emplaced the basal Trachydacites and the BT volcanics, however, led to exclude that domes and lavas, whereas in the second period these products can be considered ignimbrites. were emplaced the Upper Trachydacites and Furthermore in the M. Amiata area no the final Latites. pyroclastic products associated with BT lavas The Mt. Amiata volcanites contain has been recognised in the field. metamorphic xenolith and/or magmatic The second period of activity is inclusions. The latter are very scanty or lacking characterised by the emplacement of in the basal trachydacites and abundant in the trachydacite lava domes (TD: Trachydacitic other samples rock types, whereas the Domes) and minor lava flows usually metamorphic xenoliths are particularly outpoured by the margins of the domes (Fig. abundant in the basal products. 1 ). These products were mainly erupted along the same ENE-WSW fault and along a fault with conjugate direction. The tree cover on the 3.4 PETROGRAPHY AND GEOCHEMISTRY majority of the volcanites, makes difficult to OF LAVAS recognise domes and lava flows structures. An attempt to distinguish them on the basis of In Table 1 modal analyses of Amiata rocks photogeologic studies can be found in Fen·ari are reported. The basal trachydacites (BT) have et al. (1996). However the few outcrops on the medium grained porphyritic texture with border of the volcanic complex show phenocrysts of plagioclase, sanidine, biotite, structures characteristic of exogenous domes. orthopyroxene and scarce clinopyroxene set in The most common superficial aspect of these a generally glassy and perlitic groundmass. volcanic units is constituted by blocks Apatite, zircon and ilmenite are the common somewhere sheeted, which represent the accessory phases. Resorbed xenocrysts of autobrecciated margins of the domes. quartz may somewhere occur. The The third period of activity produced trachydacitic domes (TD) contain large and trachydacitic lava flows (UT: Upper abundant phenocrysts of sanidine reaching up 5 Trachydacites) outpoured from the southern cm in length, plagioclase, orthopyroxene flank of two lava domes: Poggio Biello and somewhere rimmed by clinopyroxene, Corno di Bellaria. The easternmost of these clinopyroxene, biotite and sporadic olivine. flows, even if trachydacitic in composition and Microcrystals of clinopyroxene, biotite and similar to the previous products, extends quite rarely of orthopyroxene are found in a far from the vent southward (Fig. 1 ). generally glassy groundmass. The accessory The M. Amiata volcanism ends with the minerals are apatite, zircon and ilmenite. Under eruption of two small latitic lava flows (FL: microscope the upper trachydacite lava flows Final Latites) from the same eruptive fracture (UT) are similar to the basal trachyidacites (Fig. 1 ). It is noteworthy that although the Mt. whereas the final latites (FL) contain Amiata magmas are felsic they did not give phenocrysts of plagioclase, sanidine, ortho- and explosive products as, on the other hand, all the clinopyroxene, biotite, olivine and sporadic others Tuscan anatectic magmas (e.g. S. xenocrysts of quartz. In the glassy groundmass Vincenzo and Roccastrada). microcrystals of plagioclase, sanidine, 172 C. CRISTIANI and R. MAZZUOLI 340 320 2 300 � 2 � � 280 1 �� �& 1 � 260 m � T � 240 6 _L 220 6 BT D TO 9 200 0 UT � 180 0 FL ? 160 0 2 4 6 8 10 12 14 16 Samples Fig. 2 Published age data of M. Amiata volcanics. Triangles: Basal trachydacite (BT); squares: Trachy-dacitic domes (TD); rhombs: Upper trachydacites (UT); circles: Final latites (FL). Data sources: Ferrara and Tonarini, 1985; Laurenzi and Villa, 1991; Barberi et al., 1994. TABLE 1 clinopyroxene and minor magnetite are usually found. Modal analyses of selected samples ji·om Mt. In Table 2a are reported selected chemical Amiata volcanic units. BT: Basal analyses of M. Amiata volcanites. On TAS Trachydacites, TD:trachydacitic D01nes, diagram (Fig. 3) the products belonging to the UT: Upper Trachydacites, FL: Final Latites first three periods of activity (BT, TD and UT, BT TD UT FL respectively) fall in calc-alkaline field of Irvine and Bm·agar (1971) and show a relatively N° samples 20 21 12 12 narrow cluster in the trachydacitic field, Pl g 17.4 14.5 18.2 10.1 whereas the final lava flows have a latitic San 15.1 10.9 15.7 4.2 composition and more alkaline character. On Opx+Cpx 3.3 3.9 4.3 8.65 the K20-Si02 diagram (Fig. 4A; Peccerillo and Bi o 2.3 3.3 3.1 2.4 Taylor, 1976) all the rocks belong to the Ace* 0.5 1 0.7 0.95 Gnd 61.4 66.4 58 73.7 shoshonitic series. Tot al 100 100 100 100 In figure 4 B-D Harker' s diagrams of selected elements are reported.