The Lower Tertiary volcanic belts of Northwestern Patagonia

RAPELA, C.W.; SPALLETTI, L.A.; MERODIO J.C.; ARAGÓN, E.

Catania

Conferencia; IAVCEI. Scientific Assembly, Symposium, on "Potassic Volcanism",; 1985

IAVCEI

The Patagonian Volcanic Province (PVP) includes two main volcanic series , a Cordilleran series (CS) to the west and a Plateau Basaltic Series (PBS) to the east. The protacted volcanism of the CS (Paleocene –Present) is well documented along the Patagonian Andes and neighboring areas between 40º and 43º SL. Two main periods of volcanic activity have been detected in the Cs. The oldest one occurs east of the Cretaceous Andean Batholith and extended from the Paleocene up to the Lower Oligocene. The second episode began after the intrusion of the Miocene Andean Granite and occurred simultaneously with a Mid – Miocene to Pliocene shift of the volcanic belt towards the Pacific. The Lower Tertiary CS appears as two ensialic belts striking NNE-SSW. In the southernmost sector the belts are clearly separated. Towards the north, they convergence due to a strong deflection in the strike of the Eastern Belt. A widespread andesitic volcanism characterizes the Western (Andean) Belt. Exposures of these volcanic are remmanents of eroded composite stratovolcanoes and associated fissural eruptions. Lavic-pyroclastic and lava flow facies are typically defined in this belt. They are arranged n stratigraphic sequence with the lava flow facies at the top. The lavic-pyroclastic facies vary from basalt to dacite. Andesite is the main lithological type in both facies. The Easten Belt has a bimodal composition, but is dominated by a silicic volcanism. These volcanics ocurr as ignimbrites, plinian, obsidian and ubvolcanic facies. Their field arrangement suggest that they ar probably related to calderas. The composition of the silicic volcanism is mainly rhyolitic. The silicic eruptions of the Eastern Belt were followed by a more restricted basic volcanism. It ranges in composition from basalt to basaltic andesite. Significant variations in rock associations, defined by average silica content as well as variations in the alkali and iron have been detected along and across the Lower Tertiary CS Belts. Along the western and Eastern Belts K2O decreases as FeOt increase southward for a given silica content. There are gentle variations from high-K calcalkaline (transitional to alkaline) series in the north (~40º SL) towards normal-K calcalkaline (41º and 42º SL) and tholeiitic normal-K series (42º30’ SL) in the Western Belt (figure 1). The transition from calcalkaline for the Pliocene-Recent volcanism at the same latitude by López-Escobar (1984). The silicic volcanism of the eastern Belt is characterized by a transition from high-K in the north to normal-K calcalkaline series in the south. The generation of iron enriched calcalkaline magmas along the whole Cenozoic is a significant characteristic of the Cordilleran Series of the PVP. The comparison of potassium content across the belts at given latitude shows that K2O tends to increase towards the continent. Differences in K2O content between the belts tend to narrow at the southern end of the Lower Tertiary CS. Temporal variations were determined by a superposition in selected cross sections within the convergence zone of both belts. The volume ratio of the basic and intermediate to silicic rocks increases with decreasing relatives age, while the sequences show progressive iron enrichment. Geological and geophysical evidence indicate a link between the nature and thickness of the underlying sialic crust and the spatial distribution of rock association. Ignimbrite suites appear only where the continental crust was thick enough to allow complex magmatic differentiation. The tholeiitic andesites and dacites of the southernmost part of the western belt evolved within a relative thin crust, meanwhile andesitic magmas became more potassic northward as the crust thickened. The temporal evolution of the CS seems to be related to recurrent variations of the tectonic regime during the Cenozoic. The subduction  geometry nd the convergence rate are the main factors controlling the tectonic regime (papela el al., 1983, 1984)