PETROGRAPHY AND MINERAL CHEMISTRY OF THE EARLY PALEOZOIC METAMORPHIC BASEMENT NORTH OF VALCHETA TOWN, RÍO NEGRO

MARTÍNEZ DOPICO, C.I.; LÓPEZ DE LUCHI, M.G.; RAPALINI, A.E.

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Geológica Argentina- UNC- UNRC

Metaclastic and metaigneous rocks west (up to Musters Station) and north of Valcheta town, Río Negro (40°41´S 66° 10´W) were originally encompassed in the Nahuel Niyeu Formation (NNF, Caminos 1983; Greco et al.2013). In this area metamorphic rocks include schists, motted schists, phyllonites, fine grained metasandstones,orthogneisses and scarce orto and para-amphibolites. According to Caminos (2001), the metamorphic gradeincreases up to greenschist facies towards the east. The age of the metamorphic rocks is only constrained in thearea by the intrusion of the undeformed Valcheta Pluton. Ordovician high-T cooling ages were widely obtainedfor pink and grey Ms- leucogranites of the Valcheta Pluton (470±2 Ma K-Ar/ Ms, Gozálvez 2009; 449±7 Ma KAr/Ms, López de Luchi et al. 2008). No detrital or metamorphic age is available for the metamorphic rocks.Moreover, they exhibit a deformational and metamorphic history more complex than the units located to the westalong the course of the Arroyo Salado, where the type locality for the NNF is. The youngest detrital zircons ofthe NNF, very low grade fine grained metasandstone in Puesto Navarrete (40º35?S, 68º34?W), south of NahuelNiyeu town, yielded 515 Ma (Pankhurst et al. 2006). This youngest detrital age was further confirmed aroundPuesto Aranda with detrital ages between 507 and 515 Ma (Rapalini et al. 2013). This short contribution is anattempt to report two different metamorphic assemblages observed in the metaclastic rock assemblage west andnorth of Valcheta and to present mineral chemistry data of part of this unit.25 km west of Valcheta town and across the national route 23, Ordovician Ms- leucogranites (from localityVAL74 40°36´S, 66° 22´W to locality VAL311 40°37´ S, 66° 11´W), intrude metaclastic -metaigneous rocks.The metaclastic rocks are green to dark green spotted schist and phyllonites that consists of biotite, muscovite,chlorite, quartz, plagioclase, tourmaline, opaque minerals and zircon. In certain areas, 1-2 cm oversized crystals(andalusite/ cordierite?) widely replaced by sericite and white micas occur. Very tight anastomosed foldedfoliation is defined by the alternation of leucocratic layers of mostly polygonal quartz and darker layers made upby biotite, chlorite, muscovite, minor tourmaline and sericite/white micas aggregates together with very smallsubhedral grains of quartz and minor subhedral multiply twinned plagioclase. Equilibrium mineral association isBt+Ms+Qtz+Alb+Op (biotite zone; folded Sx). The biotite is subhedral and partially replaced by muscovite andchlorite. Several samples exhibit poikiloblastic (quartz-feldspar) ghost fish-like crystals totally replaced bysericite and white micas (andalusite/ cordierite?). Inclusions are often rotated to the biotite- muscovite layeringwhich mimic the limits of the ghost crystal. A very well defined internal layering of quartz-feldspar and sericite(Sx-2) rotated respect to the main foliation (Sx) argues for a pre or intertectonic growth of these replaced(andalusite/ cordierite?) blasts (Sx-1). The mineralogy associated to Sx-2 seems to be(muscovite?)+biotite+opaque+quartz. Probably close to M1 peak a thermal overprint related to the Ordovician(?) or older intrusions (e.g. Tardugno Granodiorite; Rapalini et al. 2013) might have controlled the growth ofandalusite/ cordierite(?) blasts. Deformation that might have generated S2 locally led to a phyllonite and to theretrogression to chlorite and sericite (muscovite) from the original Bt-Ms ±And assemblage. Most of the sampleslack the replaced ghost like crystal (andalusite?). In those samples S1 is parallel to S2. Similar texture andstructure in NNF were tracked up to east of the Nahuel Niyeu town.24 km north Valcheta town, metaclastic rocks (sample V46) are Ms-schists intruded by the Permian Mina SanMartin pluton (U-Pb SHRIMP ~260 Ma, Pankhurst et al. 2006 among others). Rocks are fine grained mediumgrade Ms- schist (locality V46) that consists of an interlaying of mostly muscovite flakes, minor biotite andgarnet (both partially replaced by chlorite), tourmaline, magnetite, opaque minerals and very fine grained sericiteaggregates (which replace a poikiloblastic relictic mineral/ andalusite?) and layers mostly composed of quartzand lesser amounts of plagioclase, apatite, zircon and sericite (garnet zone). The foliation (115º to 150º/ >75º)isdepicted by the preferred orientation of elongated ameboidal individuals as well as boudins of quartz andmuscovite and biotite flakes and sericite replaced-ghost minerals. Garnet is euhedral as well as biotite. Themineralogy associated to S1 is biotite+opaque+quartz+garnet. Deformation that might have generated S2 leadsto a phyllonite and to the retrogression to chlorite, opaque minerals and sericite.Biotite and garnet mineral chemistry at two localities VAL74 and V46 was determined using a JEOL SuperprobeJXA-8900-M at the Luis Brú Electron Microscopy Center, Complutense University, Madrid, Spain. Microprobeanalyses of biotite indicate amount of Ti is 0.17-0.22 (a.p.f.u.) in the westernmost sample (VAL74) and 0.15-0.18 (a.p.f.u.) in the easternmost sample (V46). Phlogopite-annite type is the dominant constituent in biotite in both localities. Garnet internal composition, do not significantly change from core (Alm56-Sps22-Prp16-Grs6)-torim(Alm57-Sps23-Prp17-Grs3). Garnet is Fe-rich with an almandine component between 55-58%. Thetemperatures of metamorphic peak were estimated based on the Ti-in-biotite geothermometer (Henry et al.,2005). This thermometer is used for peraluminous metapelites and is based on the saturation surface of Ti innatural biotites when equilibrated at 4-6 Kbar. These calculations are valid for biotite compositions in the rangeXMg=0.275-1.0, Ti=0.04-0.6 apfu and temperatures between 480-800ºC. In both samples biotite is in equilibriathe S1- paragenesis with quartz, plagioclase, opaque minerals and, probably, muscovite (garnet). S1 paragenesisis retrograded to the low-temperature S2 paragenesis. Under these guidelines the mean of maximumtemperatures for the dark-green phyllites and phyllonites (VAL74) locality is 605±24ºC, almost identical to thevalue obtained for sample V46 where the mean of the temperature is 595±24ºC.In sample Ms- schist (locality V46), garnet is a common mineral in equilibria with biotite (S1). The Fe-Mgexchange in the garnet-biotite pair has been widely applied to parameterize temperatures in several metamorphicgrades and rocks according to the equation proposed by Ferry and Spear (1978). The metamorphic reaction inwhich Grt-Bt thermometer is based is Prp + Ann = Alm + Phl. Two cases of garnet in contact with biotite wereconsidered for the application of three different calibrations for this thermometer. Both pairs show similar valuesfrom core to rim between 570 and 600ºC. Non-zoned garnets are not uncommon when homogenization begins attemperatures above 600ºC considering grain size and the duration of the metamorphic conditions (Woodsworth1977). Thermometry indicates V46 garnets were grown at conditions around 600ºC, which means the growthzoning could have been homogenized by the metamorphic episode. On the other hand, if the replaced mineralwere andalusite that would set a pressure of 4 Kbar as maximum for the metamorphic episode. Therefore, theseinferred metamorphic conditions (T≈600ºC; P