Ordovician volcanic - arc deposits in the northern Famatina System, Argentina: basaltic-andesitic and dacitic hydrovolcanism in a shallow submarine basin.

CISTERNA, C.E., COIRA, B. Y DÉCIMA, F.

Jujuy, Argentina

Congreso; XVII Congreso Geológico Argentino.; 2008

AGA

This study focuses on the analyses of the Lower Ordovician sequences outcropping along the sierra de Las Planchadas, as part of the Famatina Belt (Fig. 1a). Deciphering their textural and compositional characteristics deserves special attention, because the different lithotypes associations recognized uncover valuable clues to understand their genesis and to reconstruct the evolution of the NW Argentina Ordovician arc. The present study was focused on the Arenigian volcanic-sedimentary deposits crops out in the Sierra de Las Planchadas (northern Famatina System),  along the  profile in the quebrada Larga ? Gallina Muerta area (27° 47` 00" - 27° 49` 18" S and 68° 04`52" - 68° 02`27" W) (Fig. 1b).  These sections (approximately 1300 m in thickness) build up an anticlinal structure, with subhorizontal south-eastern - north-western trend axis. At the psamitic and pelitic levels in the sequence was recognized an Arenigian brachiopod?s fauna (Décima, 2006). Among the lithotypes recognized in the profile there are: lavas (mainly basalts and andesites) with their autoclastic facies (autobreccias, hyaloclastite breccias and pillow breccias), volcanic breccias, lapilli-tuffs, volcanic sandstones and siltstones, and minor resediment tuffs (Fig. 2, 3). The lavic and autoclastic members represent the more abundant lithotypes and are mainly composed by basalts and andesites and their autoclastic facies. Coherent lavas crop out as subconcordant tabular to irregular bodies (1.5-10 m thick) and show porphyritic to aphanitic textures and primary fluidal structures. The autoclastic facies are composed by clasts with different morphology, varying from 30-40 cm to a few millimeters. Autobreccias are slabby, massive or flow foliated with jagged ends or curviplanar margins, developing jigsaw-fit textures. In many cases, clasts are enclosed in a lavic flow matrix (hypocrystalline or glassy) or are surrounded by calcite. The dacites ? riodacites and autoclastic related facies outcrop in minor, and correspond to gray ? greenish blocky lava (18m thick). Pillow fragment breccias are monomictic, basaltic units containing pillows and pillow fragments dispersed in a hyaloclastite matrix are outcropping at the upper sections of the analized sequence, forming massive levels of nearly 15 metres.  Pillows are black to dark gray in colour, range nearly 50 cm in length, generally finer-grained margins represents by altered glass. Volcanic breccias are fine grain to massive in laminated levels, pelitic to psamitic, green to grayish-green, varying in thickness from 1 to 20 cm and can sum to nearly 20 meters. Plagioclase crystalloclasts and angular to subangular volcanic clasts (basalts and andesites) vary in size from 3 mm to 1.5 cm. In the fine mesostasis, glass shards partially altered to chlorite. Lapilli-tuffs shows accretionary and armoured lapilli varying from millimeters to 3 cm. Volcaniclastic breccias (Fig. 4) build up deposits can be up to 20 m thick and are represented by blocky mudstones ? sandstones; they contain between 10 and 40% of angular to subangular clasts, in a range that varies from 2 to 30 cm in diameter of dacitic, andesitic and basaltic rocks. They used to pass into volcaniclastic sandstones and ash turbidites at the upper levels and show erosion structures and load marks at the base. Volcanic mudstones, siltstones and sandtones are ash and volcanic clasts rich, massive or normally graded, with a greenish-gray color, and parallel-laminated levels. Fine tuffs show frequently accretionary lapilli. The Ordovician sequence studied records the evolution of a volcanic arc in a submarine environment. These deposits show the records of a mainly effusive volcanism under subaqueous conditions, with significant volumes of fragmented lavas. The hyaloclastites are valuable indicators of the emplacement of lava into subaqueous setting and the fossils content reflects a shallow water depositional environment. The sequence, including water-settled fall and gravity-current levels especially at the upper levels of the profile may be developing as a result of repeated syn-volcanic sedimentation events. So on, an exclusive volcanic provenance was recognized even in the epiclastic deposits pointing to a continuous magmatic activity of the arc and an intermittent intrabasinal contribution related to the instability events. The very high percentage of volcaniclastic deposits in the sequence, especially at the upper levels, indicates the involvement of an efficient fragmentation processes as well as repeated erosional events during the evolution of the Ordovician basin. Moreover, the great variability in texture, dimension and shape of the clasts supports the occurrence of autoclastic fragmentation implying different degrees of lava ? seawater interaction.