. Geochemistry of the Yaminué metaigneous complex

LÓPEZ DE LUCCHI, M.G.; MARTÍNEZ DOPICO, C.; RAPALINI, A.; RAPELA, C.W.; PANKHURST, R.J

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Geológica Argentina

The Yaminué MetaIgneous Complex (YIC) (Rapalini et al. 2013 and references therein which extends south ofRamos Mexía station (40º30?05??S-67º15?42??W) down to the Yaminué village (40º57?03?S?-67º10?27??W), is a780 km2  sheeted orthogneissic complex. Foliated equigranular or porphyritic biotite- and scarce biotite- amphibole granitoids are interlayered with locally coarse grained foliated equigranular or porphyric biotite-muscovite leucogranite. Undeformed N to NNE trending leucogranitic dykes are present in the south. The host of the Yaminué MetaIgneous Complex is represented in the south by marble plus scarce amphibolites. A steeplydipping NNW mylonite belt controls this contact. In the north the host rock is a metaclastic biotite-plagioclase-muscovite gneiss with detrital? ages of ca 380 Ma (Tohver et al. in prep). Based on ages and lithology we propose to separate the Yaminué MetaIgneous Complex in three sectors: northern, central and southern. The Yaminué MetaIgneous Complex in the northern sector forms WNW-ENE shallowly NE  (in few cases SW) dipping sheet like bodies of a coarse to medium grained porphyritic biotite (±amphibole) granodiorite and biotite monzogranite (López de Luchi et al. 2010) that are separated by sub-concordant sheets of a fine-grained equigranular to porphyritic leucogranite and some pegmatite dykes. In the central sector coarse to medium grained biotite (±amphibole) tonalite to granodiorite are interlayered with biotite-muscovite leucogranite. In both sectors titanite is common even in samples lacking amphibole. In the southern sector sheeted units which exhibit a dominant N to NNE S2 penetrative planar fabric and shallowly dip to the east or west, are made up by coarse to medium grained porphyritic biotite granodiorite-monzogranite interlayered with a highly deformed medium- grained to pegmatoid equigranular to porphyritic muscovite-biotite leucogranite which in some layers contains garnet (López de Luchi et al. 2010). Sub-concordant sheets of mostly finer grained biotite tonalite orthogneiss are observed. For many years this unit was considered of Precambrian age based on low quality radimetric dating for rocks located in the central part (see López de Luchi et al. 2010 for details). Basei et al. (2002) calculated U-Pb conventional zircon ages of 305±31, 281± 29, 276±11 Ma and 244±14 Ma, for rocks located in the central and southern parts. New accurate SHRIMP dating allowed to recognize Ordovician ages for the southern sector (Rapalini et al. 2013), Permian-Late Permian for the granodiorite and tonalite of the central sector (Chernicoff et al. 2013, Pankhurst et al. 2014) and Early Triassic ages for the granodiorite of the northern sector (Tohver et al. in prep).8 new samples of the Yaminué MetaIgneous Complex were analyzed for whole-rock major- and trace elements at Activation Laboratories (Canada). This new data are combined with published data from Varela et al. (2005 and Pankhurst et al. (2014)All the studied samples are I-type granitoids that can be separated in each sector in two groups based on SiO2 content of 70.5%. All the studied granitoids are enriched in LILE (large ion lithophile elements) compared to HFSE (high field strength elements), which is a general characteristic of calc-alkaline granitoids.Rocks with SiO2 lower than 70.5 % are I-type metaluminous to slightly peraluminous with A/CNK <1.1, calc- alkaline and most of them plot within the magnesian series field in the FeOtot/ (FeOtot+ MgO) versus SiO2 diagram reflecting hydrous, oxidizing magmas. Ordovician granodiorites are low to moderately peraluminous on the basis of the A-B diagram (Villaseca et al. 1998) and exhibit a restricted range of SiO2 (66-69%), higher CaO, MgO and TiO2 and lower K2O than in the central sector where granodiorites are metaluminous and have higher total alkalies which make them plot in the limit between granites and quartz monzonites in the TAS diagram whereas  the  only  available  data  for  the  northern  sector  (SiO2   ca.  70%)  show  major  elements  contents intermediate between those of  the other sectors with similar SiO2 content. On average Ordovician granodiorites are lower in total REE, Rb and Nb than those of the central sector. (La/Yb)N varies from 6-24 and (Tb/Yb)N from1.6-2.for the southern granodiorites whereas these   ratios   range from   4-40 and 1.4-1.6 respectively for thecentral sector. In N-MORB normalized plots all the granodiorites show Nb, Sr and Ti troughs. REE chondrite normalized plots are very similar for the granodiorites of the central and southern sector whereas the only sample of the northern sector show a smooth Eu/Eu* (0.8) and a concave upward pattern for the HREE. Initial 87Sr/86Srranges between 0.7046 and 0.7063 and TDM (two stages) is 1.5 Ga and epsilon Nd is -4 for the Ordovician granodiorites. Permian granodiorites exhibit initial 87Sr/86Sr of 0.7074 and TDM  (two stages) of 1.4 Ga and epsilon Nd -5.Tonalites (SiO2 around 62%, two data) of the central sector are metaluminous to moderately peraluminous. Although both are amphibole bearing and have titanite major and trace elements differ between them especially in the Sr, La, Ce, Sm, Eu and HREE contents. Initial 87Sr/86Sr is 0.7084 whereas TDM (two stages) of 1.7 Ga is the older for the studied rocks and epsilon Nd of -9, the less radiogenic. Barometric constraints obtained from Al-in hornblende barometers for amphibole of one of the tonalities yielded 6-7 kbars for the core of amphibole and a mean 5.5 Kbar for the borders.Monzogranites cover a range of SiO2 (72.5-76) and are located in the field of the fractionated peraluminous granites in the A-B plot of Villaseca et al. (1998). In the southern sector granites are magnesian but are located in the alkali-calcic trend (Frost et al. 2001) due to their high K2O (6.4-7.7). They also show low CaO (0.7-1.3), MgO (0.01-0.25), Fe2O3t (0.4-0.9) and TiO2  (< 0.14): Granites from the central and northern sector which are distinctly higher in Na2O (3.5-4.5) and lower in P2O5 are calc-alkaline and straddle the field from magnesian to ferroan (Frost et al. 2001). Zr and Nb contents are higher for the granites of the northern and central sector whereas Rb, Ba and Sr exhibit variable contents. (La/Yb)N  is 5 and (Tb/Yb)N  1.2 for the southern granites whereas these ratios range from 2-15 and 0.6-1.4 for the central and northern sectors.  In N-MORB normalized patterns are similar but Permian granites show a peak in Th and a more pronounced trough at P. In REE plots Ordovician granites show a smooth positive peak at Eu whereas Permian granites mostly differ in REEt which are higher for the central sector and Eu/Eu*varies from 0.4-0.7. Permian Granites show TDM  of 1.6 Ga andepsilon Nd of -7 and initial 87Sr/86Sr of 0.7081.In trace elements discriminant diagrams (Pearce et al. 1984, Pearce 1996) the Ordovician granitoids plot within the volcanic arc and syncollisional granitoid fields. Although Permian granitoids of the central and northern sector plot in the same fields some rocks of the central sector are located in the within-plate field. The area inwhich the studied samples plot corresponds also to the post-collision granitoids field (Pearce 1996). The post-collision granites are the most difficult to classify, since some show subduction-like mantle sources with many characteristics of volcanic arc granites, and others show within plate granite character (Pearce 1996). In the Rb/30-Hf-Ta plot (Harris et al. 1986) samples are located in the volcanic arc and the late collisional fields with only one leucogranite from the northern sector located in the syn-collisional field.