Plutonismo e vulcanismo sul versante italiano delle Alpi centrali ed orientali

Schiavinato, Giuseppe

Atti della Accademia nazionale dei Lincei. Rendiconti della Classe di scienze fisiche, matematiche e naturali, Série 8, Tome 52 (1972) no. 4, pp. 523-538

Résumé

During the last ten years a new survey based on uniform geological assumptions has been made of a large area in the Central Alps, resulting in completing the Geological Map of Italy 1 : 100,000 as well as in careful, new investigations of some smaller areas of outstanding interest. With the support of these new data it is now possible to point out some peculiar features of the plutonic and volcanic activities which took place both during the Hercynian and the Alpine orogenesis on the Italian side of the Central and Eastern Alps. The overall characteristics of the "Hercynotype" volcano-plutonic associations may be summarized as follows: (1) significant breadth of the occurrences, all dating back to late Hercynian times (270-300 my); to the South of the Insubric line l.s. they appear to be in their normal geological setting within the South-alpine schists, whilst to the North they form part of basament nappes; (2) great abundance of granites/rhyolites; (3) on the one hand migmatites within or around the plutonic rocks, and on the other hand plenty of anatexis phenomena with transitional boundaries developed in the granitoid complexes; (4) contact metamorphic aureoles with andalusite and cordierite often widespread round the plutons; (5) scarcity of basic or ultrabasic rocks related to the initial magmatism; (6) a rather intimate association between volcanic and plutonic rocks; (7) abundance of ignimbrites among the vulcanites: they are related to an explosive volcanic activity in a subaerial environment. With the exception of the Adamello massif, located near the meeting point of the Tonale line with the Giudicarie line within the south-alpine scists and the Paleozoic and Mesozoic formations, the plutonic rocks related to the alpine orogenesis all lie to the North of the great tectonic line which divides the Pennides and Austrides from the southern Alps. In the same region many important ophiolitic masses outcrop also, bearing evidence of a significant initial (Mesozoic) basic magmatic activity. The main characteristics of the "Alpinotype" plutonic rocks are: (1) they stretch along a narrow belt fringing the Insubric line l.s.; they are aged synorogenic to late-orogenic; (2) the dominant rocks are granodiorites, quartz-diorites and tonalites, then granites and, sometimes, mafic rocks such as gabbrodiorites and gabbros; (3) migmatic and anatectic phenomena are rare, except in the Val Masino-Bregaglia massif where they are widespread and impressive; (4) significant differentiation processes acted within some plutons; moreover the contact metamorphic aureols appear to be relatively wide when considering the size of the plutons and the type of the country rocks; (5) basic rocks which can be referred to the initial ophiolitic magmatism (100-200 my) are present; (6) genetical associations between volcanic and plutonic rocks are completely lacking. The SW-Veneto volcanic province show distinctive, widespread occurrences of an activity of the late-orogenic type that began during the Austrian phase of the Alpine folding (Upper Cretaceous) and lasted until the Oligocene. These activities are connected with a large net of deep faults having a NW-SE trend. Radiometric dates on the extremely differentiated lavas of the Euganean district confirm the succession in time of magmas from basaltic to rhyolitic compositions, whilst the available petrological and petrochimical data testify in favour of a simatic origin of the parental magma, which evolved to the other magmas by a process of contact anatexis.