The Sierras Pampeanas of NW Argentina- Growth Stages of the Pre-Andean Margin of Gondwana

PANKHURST, R.J; RAPELA, C.W.; SAAVEDRA, J.

Estrasburgo

Simposio; EUG 9; 1997

European Union of Geosciences

The Precambrian and Palaeozoic rocks of the Sierras Pampeanas of NW Argentina record the growth of the pre-Andean margin of Gondwana. We present the results of a multy-disciplinary transect across the southern part if this zone and integrate the conclusions with recent work in the surrounding areas. Three roughly N-S zones are recognized: The western zone consists a carbonate platform, exposed in the Precordillera, with a basement of high – grade gneisses and migmatites that have yielded Grenvilian ages, together representing a terrane that rifted from Laurentia and was accreted to the Gondwana margin during mid-Ordovician times. The eastern zone (Sierras de Córdoba) has a complex polyphase tectono-magmatic history, in which the earliest episodes, including mafic magmatism, have been overprinted bye Cambrian (530 Ma) granulite-grade metamorphism (ca. 6 Kb and 800ºC). Subsequent widespread anatexis resulted in the formation of extremely cordierite-rich restitic rocks, garnet-absent migmatites and S-type granites at 522± 2 Ma. These events may correspond to the collision and accretion of an earlier “Pampea” terrane to the Proterozoic craton. They were followed by a peraluminous trondhjemite suite and Devonian-Early Carboniferous post-orogenic magmatism. The central zone (the sierras of southern La Rioja province) consists of voluminous granitoid complexes, intruded into distal clastic low-grade metasediments related to the Cambrian event (the latter are also present in the extreme west of  the eastern zone). The granitoids range from metaluminous to strongly peraluminous, and constitute a volcanic arc calc-alkline series, emplaced through early Ordovician times up to 460 ± 5 Ma. They are an extension of the magmatic arc system previously recognized in the Sierra de Famatinina and the Puna to the north (in both of which Lower Ordovician volcanic rocks are interbedded with shallow marine sediments). Isotopic compositions in the granitoids, including initial 87Sr/86Sr ratios mostly in the range 0.709- 0.711 and εNdt, values of -5 to -7, indicate development of this arc on the western margin of the Cambrian continent, which has a c. 1600 Ma lower crust according to Nd model ages. Magmatism in the extended Famatinian arc has essentially the same time span as the Precordillera and records subduction during the approach of the Laurentian terrane. The granitoids of the central zone abut against Laurentian gneisses to the west along a belt of magic igneous rocks and amphibolites that may represent the Ordovician suture.