NEOPROTEROZOIC A-TYPE MAGMATISM IN THE BASEMENT OF THE PRECORDILLERA TERRANE, SIERRA DE PIE DE PALO, ARGENTINA: EVIDENCE OF RODINIA RIFTING?

BALDO, E.; CASQUET , C.; PANKHURST, R.J.; RAPELA, C.W.; GALINDO, C.; DAHLQUIST, J.; MURRA, J.; FANNING, C.M.

Mendoza

Conferencia; GONDWANA 12 CONFERENCE: GEOLOGICAL AND BIOLOGICAL HERITAGE OF GONDWANA.; 2005

CONICET, Academia Nacional de Ciencias, ANPCyT

The Sierra de Pie de Palo (SPP) in the Western Sierras Pampeanas of Argentina is taken by many as the main outcrop representing the Proterozoic basement of the Precordillera Terrane, an alleged exotic block of Laurentian derivation that was accreted to the southwestern margin of Gondwana (present coordinates) in the Mid Ordovician (Astini et al., 1995; Casquet et al., 2001). However, the provenance of the Western Sierras Pampeanas basement still remains controversial. In south-western SPP, mylonitic orthogneisses are found intermingled with strongly folded and sheared rocks of the Neoproterozoic Difunta Correa metasedimentary sequence (Galindo et al., 2004, Rapela et al., 2005). Deformation and metamorphism under amphibolite?epidote facies conditions took place in the Mid Ordovician during the Famatinian orogeny (Casquet et al., 2001). In Quebrada Derecha (31º 35´ 52´´S; 68º 13´ 57´´W) orthogneiss contains 1?3 cm sized σ-type porphyroclasts of K-feldspar set in a fine-grained dynamically recrystallized matrix. Kinematic markers suggest a reverse, top-to-the-SW, sense of movement along one of several Famatinian thrusts that can be recognized in the SPP. The mineral assemblage is Kfs (Or 95, Ab4-5 ) ? Pl (An3) ? Qtz ? Bt (XFe= 0.83) ? Grt (Alm40, Grs44, Sp24, Py0.7) ? Am(Fe-pargasite, K2O + Na2O = 4.8 % ) ? Ep (Ps42) - Ttn ? Aln ? Zrn ? Mnz ? Ap ? Py - Mag. Textural evidence suggests that after igneous crystallization the rocks underwent metamorphism (Grt + Bt + Ep), followed by shearing and dynamic recrystallisation under lower-T conditions. The igneous protoliths of the Quebrada Derecha orthogneiss were metaluminous monzogranites (SiO2 = 72? 74%, ASI = 0.90?0.97). They fall in the field of A-type granites with the following geochemical characteristics: 104*Ga/Al = 3.5?3.9, agpaitic index (AI) = 0.85?0.88, total alkalis = 7.6?8.2%, Fe2O3total/MgO =11.2?15.5% and CaO = 0.98-1.61%. Trace element contents are also distinctive of A-type granites, particularly the relatively high content of HFS elements such as Y (52?64 ppm), Nb (22?30 ppm), Ta (1.62?2.34 ppm), Ga (23?25 ppm), Zr (338?516 ppm), and LREEtotal (256?269 ppm). REE patterns show moderate slopes with [La/Yb]N = 4.9?6.1 and a remarkable negative Eu anomaly (Eu/Eu* = 0.36?0.54). The monzogranitic protoliths of the orthogneiss can be classified as within-plate granites on a Y vs. Nb diagram. Moreover, the high 143Nd/144Nd value at the time of crystallization (εNd774 = + 4.2) suggests that parental magma had an important asthenospheric component. Zircons from a sample of this orthogneiss are mostly subhedral, as complete crystals and fragments. CL imaging shows simple zoned cores, often with oscillatory-zoned overgrowths suggesting discontinuous igneous growth: embayments indicate high-temperature metamorphic erosion. Measurements in both cores and overgrowths yield a consistent mean U?Pb SHRIMP zircon age of 774 ± 6 Ma for crystallization of the igneous protolith. This is the first evidence for a magmatic activity in the Neoproterozoic in the alleged basement of the Precordillera Terrane, but A-type granites of similar Neoproterozoic age (765 to 680 Ma) are also found in the central and southern Appalachians (Tollo et al., 2004). This A-type magmatism probably represents one of several episodes of crustal rifting that eventually led to the break-up of Rodinia.