WHAT WE CAN INFER FROM ANCIENT VOLCANIC AND VOLCANICLASTIC ROCKS: THE ~ 2.00-1.85 GA AMAZONIAN VOLCANISM

Paleoproterozoic Amazonian rocks record one of the best-preserved ancient complex of magmatic episodes on Earth. All these volcanic/plutonic rocks are attributed to the Uatumã Supergroup that covers an area of more than 1,200,000 km2. Massive intermediate and bedded rhyolitic lava flows, porphyritic felsic rocks, low- to high-grade welded ignimbrites and rheo-ignimbrites and different kind of volcaniclastic deposits characterise this magmatism. Despite we focused our studies mainly on pyroclastic deposits and magmatic breccias, we here report also deposits that are directly related to volcanic environments. We divide our “clastic” deposits in primary and secondary volcaniclastic rocks. The first group include all those rocks related to a primary volcanic activity such as lavic autobbreccias, rheoignimbrite autobreccias, pyroclastic flow deposits, conduit breccias, welded ignimbrites etc.. The secondary volcaniclastic rocks consist of all those deposits that are driven by non-volcanic activity such as polimict or monomictic conglomerates, giant massive debris flows, fluvial braidplain deposits, rock avalanches, etc.. The lack of continuum outcrops and the dense forest cover strongly difficult the recognition of the rocks and consequently the origin of these deposits, although some informations can be provided. The present contribution documents the extremely well preserved architecture of these basaltic-intermediate to felsic volcanic and volcaniclastic rocks and detailes and classifies their textural features. Here we also try to distinguish between the different transport and emplacement mechanisms associated to different effusive and explosive eruptive styles and sedimentary environment related to the Paleoproterozoic volcanism widely distributed in this region. Acknowledgments Thanks to CAPES/CNPq proj. 402564/2012-0; CNPq/CT-Mineral (Proc.550.342/2011-7) and INCT-Geociam (573733/2008-2)-CNPq/MCT/FAPESPA/PETROBRAS.