Petrology of the Ordovician I- and S-type granitoids of the NE sector of the North Patagonian Massif

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

Córdoba

Congreso; XIX Congreso Geologico Argentino; 2014

Ordovician granitoids of the NE sector of the North Patagonian Massif are known as the Valcheta Pluton which intrudes the Nahuel Niyeu Formation and as the Punta Sierra Plutonic Complex which intrude the late Cambrian El Jagüelito Formation nearby Sierra Grande town and Hiparsa Mine. Moreover, around Gonzalito Mine, Ordovician granites and granodiorite orthogneisses are encompassed in the Mina Gonzalito Metamorphic Complex (Varela et al. 2011, among others). The aim of this contribution is to present an advance of an in progress major and trace elements, and Sr-Nd data study of the Ordovician magmatism. A total of 36 chemical analyses were considered. Sampling strategy involved well recorded- Ordovician granite localities in Valcheta, Arroyo Salado and Punta Sierra. These results were combined with geochemical data published by Busteros et al. (1998), Pankhurst et al. (2006), and Gozálvez (2009). The Punta Sierra Plutonic Complex (Busteros et al.1998) is built up by a complex magmatic association of orthoclase bearing granodiorites, and a minor diorite/tonalite facies with different degrees of magmatic hybridization as evidenced by abundant mafic microgranular enclaves (Arroyo Salado, Playas Doradas and Hiparsa Mine granites s.l.), and biotite granites (Punta Sierra granites). U-Pb SHRIMP zircon ages (Pankhurst et al. 2006) on a hybrid tonalite yielded 475±6Ma for the Arroyo Salado, whereas 474±6 Ma and 476±6 were calculated for Playas Doradas Granite and Sierra Grande Granodiorites respectively. Monzogranites of Punta Sierra yielded a WR Rb-Sr age of 488±22 Ma (Varela et al. 1998). The Valcheta Pluton  crops out west of Valcheta town as a more than 40 km long NE-SW trending belt composed of small and isolated outcrops of pink to white medium grained muscovite bearing leucogranites in the central part that grade to finer grained facies towards the east of the belt. In the westernmost part of the belt leucogranites are mostly grey, and may exhibit biotite traces. There, scarce garnet bearing tonalites were found and are under study. Cooling ages for the leucogranites were bracketed between Middle Ordovician and Silurian (430-470 Ma, K-Ar and Ar-Ar on Ms, López de Luchi et al. 2008, Rapalini et al. 2013 and references). Granitoids of the Punta Sierra Plutonic Complex are separated based on TAS diagram in (1) Qtz-diorites, (2) Tonalites-Granodiorites and (3) Granites, that define a calc alkaline mostly magnesian metaluminous  to  peraluminous  I  ?type  series.  The  Qtz-diorites  group  is  restricted  to  mafic  microgranular enclaves in the Arroyo Salado group with SiO2  ~55%, FeOt  and CaO >6% and Mg#~50. Punta Sierra Granite and minor granites in Arroyo Salado and Sierra Grande units are peraluminous to highly peraluminous (1.10<ASI<1.36) whereas the Mina Gonzalito granites are slightly metaluminous (ASI<1.1). Granites from Punta Sierra and Arroyo Salado are characterized by relatively high Sr (150ppm), Y (20-30ppm) and low Nb (14ppm). Granodiorites show REEt  (70-120ppm), slightly higher (La/Yb)N  (6-9), (Tb/Yb)N  (1-1.8) and lower Rb/Sr (<1) than the granites (La/Yb)N  (3.5-5) (Tb/Yb)N  (1-1.1) and Rb/Sr (1-1.5). All the units of PSPC show86Sr/87Sr (470) from 0.70596-0.70833, Epsilon Nd from-2.63- to -3 and TDM (2 stages) from 1.42 to 1.45 Ga. Ms-leucogranites of Valcheta Pluton are peraluminous (ASI 1.14-1.24) ferroan calc-alkaline S-Type with 72-76 %SiO2. Trace element absolute values and ratios allow a distinction between two groups, the leucogranites of the central part from those of the borders of the belt. The former shares the chemical features of the granites of the Punta  Sierra  Plutonic  Complex  whereas  the  latter  are  characterized  by  higher  P2O5   (015-0.2),  lower  Ba (100ppm), Sr (70ppm), Nb(<10ppm), Y(<10ppm), La/Yb)N (2) and (Tb/Yb)N  (0.8), higher Rb (140-160 ppm) and Rb/Sr (2-3). The low Sr Valcheta Granite shows 86Sr/87Sr (470) of 0.7149, Epsilon Nd is-5 and TDM (2 stages)1.58 Ga whereas the high Sr granites are comparable to the granites Punta Sierra Plutonic Complex in terms of Nd isotopes but the 86Sr/87Sr (470) is very low, 0.7047. Multi-element patterns for the Ordovician granites normalized to bulk continental crust vary from flat crustal-like compositions (granites from Punta Sierra PlutonicComplex and high Sr granites of Valcheta Granite) to more spiky and depleted for the low Sr leucogranites of Valcheta Granite. The relatively flat HREE pattern of all the Ordovician granitoids with (Tb/Yb)N <2 suggest an absence of garnet in the source and therefore middle crustal levels of partial melting. Tectonic setting as inferred from trace elements indicates a transition from arc to syncollisional magmatism for the Valcheta Granite and two granites of Punta Sierra Plutonic Complex.Martínez Dopico et al. (2010) considered that the sources for the tonalite and granodiorite of Punta SierraPlutonic Complex underwent hybridization during a low pressure melting process involving the mixing of mafic and crustal /felsic precursors. Granites of Punta Sierra Plutonic Complex would derive from a greywacke or igneous source that could represent the crustal/felsic component of the tonalite-granodiorite. In any case Punta Sierra Plutonic Complex would involve sources affected by recycling processes as indicated by the relatively high (-3) epsilon Nd. Conversely, sources for the low Sr granites of Valcheta Pluton are pelitic, specifically Ms- rich pelites as indicated by the high Rb/Sr and low Al2O3/TiO2 (Martínez Dopico et al. 2010). The higher phosphorous content of the low Sr peraluminous granites of the Valcheta Pluton could result either from their high peraluminosity or from the inheritance from a P-rich source. Pelitic shales, the inferred source for the low Sr Valcheta Pluton exhibit high P content which would be reflected in the derived granitic partial melts since P is more soluble in peraluminous compositions. Calculated cooling ages for Valcheta granites are older in the central part that corresponds to the rocks that are comparable with the granites of the Punta Sierra Plutonic Complex whereas they are younger for the low Sr granites. Taking together the inferred pelitic source, the inferred syn-collisional setting and the younger cooling age we could suggest that the low Sr granites result from the stacking of different sources with pelitic melting at middle pressure assisted by hydrous fluids.Punta  Sierra  Plutonic  Complex  emplacement  would  postdate  a  515-476  Ma  M1  metamorphic.  At  Mina Gonzalito amphibolite-grade metamorphism is dated at 472±5Ma, U-Pb SHRIMP (Pankhurst et al. 2006). These quartz-feldspathic gneisses were considered as deeper crustal equivalents of both the El Jaguelito and Nahuel Niyeu formations based on zircon provenance data (Pankhurst et al. 2006) which indicate that sedimentation is younger ca 520Ma (Rapalini et al. 2013). Punta Sierra Plutonic Complex granitoids, intruding the very low grade metamorphic rocks of the El Jaguelito Fm. are bracketed between 475-483 Ma (Pankhurst et al 2006, Varela et al. 2009 and references). The older Rb-Sr age corresponds to the Punta Sierra biotite granite whereas the  younger  ages  were  calculated  for  granodiorites.  Although  the  closer  constraint  for  the  age  of  the metamorphic peak of the low grade Nahuel Niyeu Fm. in the Valcheta area is provided by the younger detrital zircon age of 515Ma (Pankhurst et al. 2006, Rapalini et al. 2013) no data are available for the host of Valcheta Granite which underwent a different metamorphic evolution (see Martínez Dopico et al, this Congress). Cooling ages of the Valcheta pluton are broadly consistent with the U-Pb and Rb-Sr of Punta Sierra Plutonic Complex. Whereas the older Ar-Ar ages of the Valcheta pluton almost overlap with the crystallization ages of the granodioritic plutons of Punta Sierra Plutonic Complex the low Sr granites yielded the youngest cooling ages. Considering the youngest available biotite ages of ca 435 Ma in the Sierra Grande area (Weber 1983) and the ca430 Ma muscovite age for the east of Valcheta Pluton (Rapalini et al. 2013), a long-lasting Ordovician thermal event associated with shallow emplacement could be inferred. The inferred age of the metamorphic peak for the Gonzalito gneiss suggests some time overlap which contradicts the field evidence of metamorphic xenoliths of the host, development of thermal aureoles and clear?cut contacts with both the low grade El Jaguelito Formation and the metamorphic units of the eastern sector of the Nahuel Niyeu Formation. In summary Ordovician magmatism of the NE sector of the North Patagonian Massif developed after a metamorphic event and would represent either an active margin related magmatism which grades in time to syn-collisional or it could be related with post-collisional magmatism in which different sources are involved during a general decompressive path. The lack of penetrative deformation that is characteristic of these granitoids could result from the high levels of the crust in which they are emplaced.