EARLY ORDOVICIAN PERALUMINOUS MAGMATISM IN THE SIERRA DE PIE DE PALO, (WESTERN SIERRAS PAMPEANAS): GEOTECTONIC IMPLICATIONS.

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

Mendoza

Congreso; GONDWANA 12 CONFERENCE: GEOLOGICAL AND BIOLOGICAL HERITAGE OF GONDWANA; 2005

C.W. RAPELA (CONICET-UNLP) Y L. A. SPALLETTI (CONICET-UNLP), COORDINADORES-EDITORES.

The Sierra de Pie de Palo (SPP) is considered to represent the Proterozoic basement of the Precordillera terrane (PT) of alleged Laurentian derivation. The PT was accreted to the proto-Andean margin of Gondwana in the Mid Ordovician during the Famatinian orogeny, through eastward subduction under an ensialic magmatic arc of Early Ordovician age (e.g., Thomas & Astini, 1996). At this time, two enigmatic garnet-bearing two-mica granite bodies, the El Indio and the Difunta Correa plutons, were emplaced in the eastern part of the SPP; their host rocks belong to the Difunta Correa metasedimentary sequence, a Neoproterozoic cover to a reactivated Grenvillian (1.0-1.2 Ga) basement (Galindo et al., 2004). The bodies are roughly concordant with the external foliation and are variably mylonitized. The granites show a high SiO2 content, between 65.45 and 76.41% and plot in the granite field of the Ab-An-Or diagram. Their mean ASI is 1.10, i.e., they conform to a slightly peraluminous suite. Analysed garnets from these granites largely belong to the almandine¨Cspessartine series, which constitutes 90-92.5 mole% of the total, and are compositionally comparable to garnets from other peraluminous plutons. AlIV content in biotite is remarkably high (mean = 2.76 for the Difunta Correa pluton and 2.84 for the Indio pluton), suggesting that biotite equilibrated with an Al-rich phase such as garnet. Textural and chemical evidence point to a primary origin for the white micas analysed. U¨CPb SHRIMP zircon crystallization ages of the two plutons are 481 ± 6 Ma (El Indio pluton, Pankhurst and Rapela, 1998) and 470 ± 10 Ma (Difunta Correa pluton, new age). e(Nd)t values calculated at these ages are -3.6 and -2.6 respectively. TDM model ages are 1.48 Ga for the El Indio pluton and 1.41 Ga for the Difunta Correa pluton. These data suggest that the parental magma was probably derived by the partial melting of older lithosphere, with little or no asthenospheric contribution. The Indio and the Difunta Correa plutons were emplaced early during the Famatinian tectono-thermal event (peak of metamorphism at ca. 465 Ma, Casquet et al., 2001). The fact that granitic magmatism of peraluminous type has not been reported so far from lower plate tectonic settings suggests that the SPP was more probably part of the upper plate, as already argued by Galindo et al. (2004). This interpretation is contrary to present views that the SPP was part of the basement of the Precordillera terrane (Ramos, 2004 and references therein). Moreover, the high P/T conditions inferred for the Famatinian metamorphism in SPP (Casquet et al., 2001) were not favourable for crustal melting because the metamorphic geotherm does not intersect the granite solidus at any point. In consequence magmatism was necessarily generated in the upper plate and emanated from nearby areas with lower P/T metamorphic conditions, probably near the magmatic arc where middle crust melting was conspicuous (e.g., Sierra de las Chacras and Sierra de Valle Fértil, Baldo et al., 2001).