The final assembly of Gondwana: the evidence from the Sierras Pampeanas

RAPELA, C.W.; PANKHURST, R.J; CASQUET, C.; FANNING, C.M; BALDO, E.G; GONZÁLEZ-CASADO, J.M.; GALINDO, C.; DAHLQUIST, J.A.

Oslo, Noruega

Congreso; 33th International Geological Congress; 2008

International Union of Geological Sciences

Key evidence for the Neoproterozoic-Early Palaeozoic assembly of Gondwana can be obtained from the Sierras Pampeanas, along a modern flat-slab segment of the Andes. The main episode of terrane accretion took place during the Early Cambrian (540-515 Ma), where the thick Ediacaran siliciclatic sequence of the Pampean belt, dominated by ¡­600 Ma ("Brazilean") and ¡­1100 Ma ("Namaqua-Natal") zircon provenance components, underwent an oblique dextral collision with the Western Sierras Pampeanas (WSP) block. The Pampean magmatic rocks include 540-530 Ma calcalkaline granites (¥åNdt= -3.8 to -6.6; TDm = 1740-1540 Ma), 520-515 migmatites and S-type granites (¥åNdt = -5.1 to -6.1; TDm = 1704-1630 Ma) and oceanic crust basic and ultrabasic remnants. Previously envisaged as a proto-Pacific accreted terrane, the colliding WSP was in fact the southern block (present coordinates) of the Amazonian group of continents, that included the Arequipa block, and Rio Apa, that were amalgamated with Congo-San Francisco and Rio de la Plata cratons along the Transbrasiliano mobile belt in Early Cambrian times. After a brief quiescence period, an ensialic basin developed to the east of the WSP, filled with Latest Cambrian to Llanvirn epiclastic deposits, interbedded with volcaniclastic and shallow marine siltstones. The coeval widespread 500-463 Ma magmatism ¡§Cthe Famatinian episode- is composed of I-type suites (most granites, granodiorites and gabbros show ¥åNdt = -5 to -6, rare gabbros reached ¥åNdt = -2.4), minor S-type granites (¥åNdt = -6.1 to -6.9) and subordinate isolated plutons of Na-rich granites located in the Pampean belt foreland (TTG suites, ¥åNdt = +1.6 to -2). Most of the Famatinian magmatic rocks show TDm ages between 1700 and 1500 Ma and Nd isotopic signatures indicating derivation from a whole Palaeoproterozoic lithospheric section, that included lower and upper crust sources as well as the sublithospheric mantle. Both the Famatinian (with the exception of the minor TTG suites) and the Pampean episodes reworked old lithospheric sources, with very little addition of juvenile material. U-Pb SHRIMP zircon detrital age patterns show however, that while in the Ediacaran metasediments of the Pampean the typical 2200 Ma component of the now adjacent R¡§¨£o de la Plata Craton is absent, detritus with this component appear in the Ordovician sediments of the Famatinian basins. This suggests protracted relative displacement of the colliding blocks during and probably after the Pampean collision. The Grenville age blocks of the WSP and mid-crustal sections of the Famatina basins show an overprinted ¡­ 465-430 Ma metamorphic event, that probably indicate the docking/displacement of the Precordillera terrane, a later arrived component of the Amazonian craton group. The evolution culminate with the intrusion of conspicuous 380-340 Ma postorogenic granites (¥åNdt = -1.1 to -4.0; TDm = 1229-1443 Ma) along the Early Palaeozoic suture.