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., DÉCIMA, F.

S. S.de Jujuy

Congreso; 17 Congreso Geológico Argentino; 2008

Asociación Geológica Argentina

This study focuses on the analyses of the Lower Ordovician sequences cropping out 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 lithotype associations recognized uncover valuable clues to understand their genesis and to reconstruct the evolution of the NW Argentina Ordovician arc. The Arenigian volcanic-sedimentary deposits analyzed crops out along a 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 thick) build up an anticlinal structure, with a subhorizontal axis trending southeast- northwest. In the sequence, psammitic and pelitic levels host an Arenigian brachiopod?s fauna (Décima 2006). Among the lithotypes recognized in the profile there are lavas with autoclastic facies (autobreccias, hyaloclastite breccias and pillow breccias), volcanic breccias, lapilli-tuffs, volcanic sandstones and siltstones, and minor resedimented tuffs (Figs. 2 and 3). Lavas and autoclastic members represent the more abundant lithotypes and are mainly composed by basalts and andesites. 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 fluidal lava matrix (hypocrystalline or glassy) or are surrounded by calcite. The dacites ? rhyodacites and autoclastic related facies are less extensive, and correspond to gray ? greenish blocky lava (18 m thick). Pillow fragment breccias are monomictic; basaltic units containing pillows and pillow fragments dispersed in a hyaloclastite matrix crop out 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, with generally finer-grained margins represented by altered glass. Volcanic breccias are fine grained to massive and the laminated levels are pelitic to psammitic, green to grayish-green, varying in thickness from 1 to 20 cm, and forming sequences up to 20 m thick. 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 show partial alteration to chlorite. Lapilli-tuffs show accretionary and armoured lapilli varying from millimeters to 3 cm. Volcaniclastic breccias (Fig. 4) can be up to 20 m thick and are represented by blocky mudstones ? sandstones; they contain 10 - 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 frequently pass into volcaniclastic sandstones and ash turbidites at the upper levels and show erosion structures and load casts at the base. Volcanic mudstones, siltstones and sandstones are ash and volcanic clast 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. Main deposits represent 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 fossil content reflects a shallow water depositional environment. The sequence, including water-settled fall and gravity-current levels, especially at the upper levels of the column, may have developed 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 proportion of volcaniclastic deposits in the sequence, especially at the upper levels, indicates the involvement of an efficient fragmentation processes as well as repeated erosion 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.