Coeval S- and I-type granites in the Sierra de Ancasti, Eastern Sierras Pampeanas, Argentina

RAPELA, C.W.; FANNING, C.M.; BALDO, E.G; DAHLQUIST, J.A.; PANKHURST, R.J.; MURRA, J.A.

Mendoza, Argentina

Congreso; GONDWANA 12; 2005

From the chemical evolution of the granitic rocks in the Sierra de Ancasti (Reissinger, 1983), and for the Eastern Sierras Pampeanas in general (Rapela et al., 1990), it has usually been assumed that metaluminous granites, gabbros and mafic rocks were intruded earlier than peraluminous suites, commonly represented by two-mica granites. To test this assumption we have studied a restricted area in south-eastern Sierra de Ancasti, using SHRIMP for U-Pb zircon geochronology as well as determining the provenance of the metasediments into which the granites suites are intruded. The metamorphic rocks are dominated by medium to high-grade metasediments of the Ancasti and Portezuelo formations. Banded schists with a N–S foliation exhibit 0.5 cm biotite-rich layers alternating with 1–2 cm light coloured granoblastic layers of Qtz + Pl + Bt. We have analysed a typical schist sample of the Ancasti Formation, with the following mineral assemblage: Bt(XMg0.52, TiO2 1,6%) - Qtz – Pl(An20) – Ms – Grt(Alm65, Sp19) – Ilm/Mag - Tur – Ap – Zrn – (Chl) P–T estimates of the thermal peak (M2) are T= 535 ± 20 ºC and P= 2.5-3.5 kb. SHRIMP U–Pb analyses of 76 detrital zircons yield dates that are mostly dispersed within two ranges: Late Neoproterozoic (545–685 Ma, 38% of the grains, with the most prominent peaks at ca. 570 Ma and 600 Ma) and Mesoproterozoic (900–1100 Ma,  42% of the grains, prominent peaks at ca. 1020 Ma and 960 Ma). There is also less conspicuous noise in the Early Neoproterozoic (700–830 Ma, 9%) and Palaeoproterozoic (1850–2050 Ma, 8%), with a single Archean grain at 2520 Ma. The banded schist is intruded by the S-type Las Cañadas two-mica felsic granodiorite. The zircons from this granodiorite are complex elongate grains with sub-euhedral to euhedral terminations. CL images reveal dark, high–U rims/overgrowths to zoned low–U cores. Some grains are dominated by zoned components, interpreted as totally of older inherited component(s). These inherited components yield ages of ca. 510 Ma and ca. 560Ma, whilst a single zoned core gave ca. 1140 Ma. The dark high-U rims are considered to constrain the time of granite formation: selected rims give a crystallization age of 471 ± 5 Ma (Concordia age). In contrast, the Ramblones tonalite is a foliated I-type granite, with abundant mafic enclaves and a metaluminous assemblage: Pl – Qtz – Bt ± Hb, with Aln – Ep – Zrn - Ap and Op as accessory minerals. Zircon grains from this tonalite are mostly clear, euhedral elongate crystals with bipyramidal terminations. The CL images reveal a mostly simple zoned igneous internal structure. However, in some grains there are discordances within the oscillatory zoning and there is the possibility of more than one magmatic crystallisation event: e.g., the zoned centre of one grain gave an age of ~955 Ma. The remaining analyses of zoned igneous zircon form a slightly skewed distribution with the dominant age at 466 ± 5 Ma. The crystallization ages of these two contrasting granites overlap within analytical uncertainty, indicating that both were intruded during the Mid Ordovician. These ages are at the younger end of the interval determined for the Famatinian granites by U–Pb (495–460 Ma, Pankhurst et al., 2000). Coeval I- and S-type magmatism is now known across the entire E–W extent of the Famatinian belt from the Sierra de Ancasti to Villa Castelli, west of the Sierra de Famatina. Regardless of their composition and emplacement at medium to low pressures, these granites are derived from sedimentary and igneous sources and record the presence of Mesoproterozoic and Neoproterozoic inherited components that have the same age ranges as detrital zircons in the host metasedimentary sequences.   Pankhurst. R.J., Rapela, C.W. & Fanning, C.M., 2000. Age and origin of coeval TTG, I- and S-type granites in the Famatinian belt of NW Argentina. Transactions of the Royal Society of Edinburgh: Earth Sciences, 91, 151-168. Rapela, C.W., Toselli, A., Heaman, L.& Saavedra, J., 1990. Granite plutonism of the Sierras Pampeanas: An inner cordilleran Paleozoic arc in the Southern Andes. In Kay, S.M & Rapela, C.W. (Eds.): "Plutonism from Antarctica to Alaska", Geological Society of America, Special Paper 241: 77-90. Ressinger, M., 1983. Evolución geoquímica de las rocas plutónicas. In Aceñolaza, F.G., Millar, H. & Toselli, A. (Eds), Geología de la Sierra de Ancasti. Münstereche Forschungen zur Geologie und Paläontologie, 59: 101-112.