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

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

CONICET, Academia Nacional de Ciencias, ANPCyT

From the chemical evolution of the granitic rocks in the Sierra de Ancasti (Reissinger, 1983) and in general for the Eastern Sierras Pampeanas (Rapela et al., 1990), it has been usually assumed that metaluminous granites, gabbros and mafic rocks are intruded earlier than peraluminous suites, commonly represented by two-mica granites. In order to test this assumption we have studied a restricted area in the south-eastern sector of the Sierra de Ancasti, using SHRIMP 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. The N?S foliation of banded schists show alternation of 0.5 cm biotite-rich layers and 1?2 cm light coloured granoblastic layers of Qtz + Pl + Bt. A typical schist sample of the Ancasti Formation that we have examined has 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 estimations at the thermal peak (M2) indicate T= 535 ± 20 ºC and P= 2.5?3.5 kb. SHRIMP U?Pb analyses of 76 detrital zircons from this sample yield dates that are mostly dispersed within two main age ranges: in the Late Neoproterozoic (545?685 Ma, 38% of the grains, most prominent peaks at ca. 570 Ma and 600 Ma) and the 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 to euhedral terminations; CL images reveal dark, high U rims/overgrowths to zoned low U cores. Some grains are dominated by zoned components that are interpreted to be totally of older inherited component(s). These inherited components yield ages of about 510 Ma and 560Ma, whilst the analysis of a single zoned core gave ~1140 Ma. The dark high U rims component are considered to constrain the time of granite formation and a concordia age for selected rims gives a crystallisation age of 471 ± 5 Ma. A foliated I-type granite, the Ramblones tonalite, has abundant mafic enclaves and in contrast a metaluminous assemblage defined by 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. There are however discordances within the oscillatory zoning in a number of grains and there is the possibility of more than one single magmatic crystallisation event as evidenced by the zoned centre to one grain with an age of ~955 Ma. The remaining analyses of zoned igneous zircon form a slightly skewed distribution with the dominant age at 466 ± 5.0 Ma. Crystallization ages of these contrasting granites overlap to within analytical uncertainty, indicating that both were intruded during the Middle Ordovician. These ages are at the younger limit of the interval determined for the Famatinian granites (495?465 Ma, Pankhurst et al., 2000), and constrain the maximum E?W extension of the Famatinian belt, with coeval I- and S-type magmatism 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 were derived from sedimentary and igneous sources and record the presence of Mesoproterozoic and Neoproterozoic inherited components. The ages of these inherited components are in turn the dominant age intervals for detrital zircons in the host metasedimentary sequences.