VOLCANO STRATIGRAPHY AND 40AR/39AR GEOCHRONOLOGY OF THE MACUSANI IGNIMBRITE FIELD - MONITOR OF THE MIOCENE GEODYNAMIC EVOLUTION OF THE ANDES OF SOUTHEAST PERU

A total of 1400 m of ignimbritic ash-flow tuffs distributed in five cross-sections and extending from the pre-Neogene base to the uppermost flows were studied in the Miocene Macusani ignimbrite basin (southeast Peru). Volcano-stratigraphic interpolation and precise 40Ar/39Ar dating of sanidine, biotite, and rhyolitic glass from the ash-flow tuffs give rise to a southwest-northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units, i.e., (1) 10.0 +/- 0.5; (2) 7.8-8.0 +/- 0.1; (3) 7.5 +/- 0.1; (4) 7.3 +/- 0.1; (5) 6.8-7.0 +/- 0.1; and (6) 6.7 +/- 0.1 Ma. These delimit two brief eruption episodes at 10 +/- 1 and 7 +/- 1 Ma, which are sensibly synchronous with the Quechua 2 and Quechua 3 compressional events characterizing the tectonic regime in the Central Andes. This close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the Miocene tectono-magmatic evolution of the Cordillera Oriental. The stratigraphic relationships of the uranium mineralization of the Macusani field are defined for the first time: the stratiform-stratabound occurrences are restricted to three main cooling units dated at 7.8, 7.5 and 6.9-6.8 Ma which constitute a maximum age for uranium deposition. A total of 1400 m of ignimbritic ash-flow tuffs distributed in five cross-sections and extending from the pre-Neogene base to the uppermost flows were studied in the Miocene Macusani ignimbrite basin (southeast Peru). Volcano-stratigraphic interpolation and precise 40Ar/39Ar dating of sanidine, biotite, and rhyolitic glass from the ash-flow tuffs give rise to a southwest-northeast correlation chart subdivided into six volcanic cycles or fundamental cooling units, i.e., (1) 10.0 +/- 0.5; (2) 7.8-8.0 +/- 0.1; (3) 7.5 +/- 0.1; (4) 7.3 +/- 0.1; (5) 6.8-7.0 +/- 0.1; and (6) 6.7 +/- 0.1 Ma. These delimit two brief eruption episodes at 10 +/- 1 and 7 +/- 1 Ma, which are sensibly synchronous with the Quechua 2 and Quechua 3 compressional events characterizing the tectonic regime in the Central Andes. This close relationship between tectonic pulses and felsic magmatic activity adds a supplementary constraint to models of the Miocene tectono-magmatic evolution of the Cordillera Oriental. The stratigraphic relationships of the uranium mineralization of the Macusani field are defined for the first time: the stratiform-stratabound occurrences are restricted to three main cooling units dated at 7.8, 7.5 and 6.9-6.8 Ma which constitute a maximum age for uranium deposition.