Provenance of Middle Ordovician detritus in the Argentine Precordillera

ASTINI, R.A.; THOMAS, W.A.; MCCLELLAND, W.C.

Mendoza

Congreso; Gondwana 12; 2005

Academia Nac. de Ciencias

Indications of provenance for the syn-orogenic clastic wedge in the Precordillera are consistent with deformed Grenville-age rocks along the present eastern margin of the Precordillera. The age range, although not exclusive, is consistent with Laurentian basement (Grenville), older Laurentian provinces represented in syn-rift detritus, and Iapetan syn-rift igneous rocks. Detrital ages from the post-rift and pre-Ocloyic passive-margin succession, including La Laja Formation (Early to Middle Cambrian) and the Early Cambrian Cerro Totora Formation, are consistent with a sedimentary cover atop a Laurentian basement in the Precordillera. Neither Ordovician volcanic/plutonic rocks of Famatina nor metamorphic rocks of the Pampean Ranges are represented in the detritus, as clasts or as detrital grains. Neither is there detritus from the abundant Ordovician bentonites (volcanic ash beds) in the Ordovician stratigraphy of the Precordillera. In contrast, Ordovician igneous detrital ages dominate the sedimentary deposits in the back-arc (east of Famatina). Detrital ages and proportions in the Precordillera Ocloyic clastic wedge differ from those in the closest peri-Gondwana regions at Famatina. Dominant composition and provenance of the clastic wedge suggests a barrier possibly related to back-thrusting and unroofing of the leading edge of the Precordillera terrane, during accretion to Gondwana, preventing input of Ordovician detritus into the lower-plate peripheral foreland. Recent discoveries of Ordovician I- and S-type granites within the Grenville-age metamorphic basement imply a higher degree of paleogeographic complexity than previously realized, possibly including multiple relatively parallel subduction zones between a constellation of smaller Laurentia-derived terranes that are presently telescoped within the Central Andes.