Distinguishing crustal segments in the North Patagonian Massif, Patagonia

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

Göttingen, Alemania

Congreso; Latein Amerikan Kolloquium of Earth Sciences 2009; 2009

DFG; Universidad de Göttingen

Throughout the Paleozoic the North Patagonian Massif (NPM), northern Patagonia has been proven to be a place where granitic magmatism and mixing of crustal melts have been a repetitive geological event. In order to evaluate their sources we integrated new Nd data with all the reliable previous data (Pankhurst et al., 2006; Varela et al., 2005; Varela et al., 2000, Rapela et al., 1992, Dalla Salda et al., 1991) for the main Paleozoic igneous, sedimentary and metamorphic events in the NPM and recalculated them under the assumptions made by Wu et al. (2003) for the model ages calculations. The oldest crustal segment is identified in the early Cambrian fine to medium-grained metasediments of the Nahuel Niyeu, El Jagüelito and Mina Gonzalito formations in the northeastern border of the NPM and in the Lower Carboniferous schists in its western border (Cushamen Formation) as formed in Upper Paleoproterozoic between 1.6 to 2.1 Ga with a repetitive 1.6-1.8 Ga interval within. In northeastern NPM, Valcheta and Sierra Grande areas, Ordovician granitoids intrude the low grade metaclastic units. These granites show TDM model ages between 1.45-1.57 Ga and εNd(t) ~ -2.7 suggesting a subordinate juvenile supply to the bulk crustal segment. This fact agrees with both microgranular enclaves and field and textural evidence of magma hybridization (Rapalini et al., 2009). Isolated Ordovician stocks north Rio Colorado river (Pankhurst et al., 2006), have bulk TDM model ages (~1.6 Ga) and crust-derived εNd(t) ca. -5, suggesting crustal continuity between the NPM and Pampia terrane. Upper Paleozoic magmatism is represented in Valcheta by the deformed granitoids of the Yaminué Complex (YC) and the Navarrete Plutonic Complex (NPC). The YC exhibits TDM values between 1.5 to 1.6 Ga, mean fSm-Nd values -0.34 to -0.53 and spread of εNd(t) between -3.6 to -6.1. These values typify different degrees of recycling in the bulk crustal segment as well as being a reflection of the different protoliths involved. However, all cases show a main continental crustal recycling as demonstrated by the crustal εNd(t). Regarding the Permian magmatism, NPC shows distinct episodes of mixing of unequal parts of an old recycled crust with new inputs of a slightly depleted mantle-derived magmas as demonstrated by the wide range in εNd(t) and TDM ages 1.3- 1.6 Ga. TDM model ages for the calc-alkaline granodiorite of the Aranda facies (recently defined by López de Luchi et al., 2009) at Pto. Navarrete show values of 1.3 Ga and relatively radiogenic εNd(t) values (-2.08) suggesting a rejuvenation yielded by a new magma input to the already recycled bulk crustal segment. The age of this bulk crustal segment could be considered as younger than 1.6 Ga as a result of the processes of magma addition during Ordovician times and the melting homogenization evidenced by the Yaminué Complex. In contrast, the high K-calc-alkaline series of the Upper Permian San Martin Granodiorite shows 1.5Ga TDM ages and εNd(t) -5.3, which could be explained as pure  recycling of the bulk crust. The Colohuincul Complex (CC) has been always recognized as the oldest rocks on the western NPM, even after the recent revealing menu of Upper Silurian-Devonian Rb-Sr and U-Pb ages obtained by Varela et al. (2005) and Pankhurst et al. (2006), among others. Collected data for the CC involves the well-established Devonian orthogneisses, migmatites and granodiorites that crop out in San Martín de los Andes, Sañicó, Laguna del Toro and Sierra del Medio. These rocks yielded TDM model ages between 1.6 to 1.75 Ga, besides an isolated value of 2.1 Ga in a metadioritic enclave in Sañicó area. The εNd(t) indicate a consistent mean -5 and isolated much more negative values in the Sañicó-Comallo area. Widespread devonian tonalithic magmatism show slightly different Sm-Nd isotopic parameters: model ages indicates 1.4 Ga and gNd(t) calculations yielded a -4 mean value in spread range. The Sm-Nd trace of paleozoic basement of the Northern Patagonian Andes in Chile and Argentina was studied by Lucassen et al. (2004), who found single stage-TDM model ages between 1.6 to 2.1 Ga. This data as well as ours confirm that the bulk crustal age at Early Paleozoic times would be 1,6 to 2,1 Ga partially  in agreement with the bulk crustal background age observed at the same moment in the northeastern area. This may allow us to propose a mechanism of main crustal recycling with a little new juvenile addition to the bulk crustal background as a responsable of the Devonian tonalithic invasion and concomitant migmatization event. Lucassen et al. (2004) also provides a Rb-Sr isochron for a migmatite near San Martín de los Andes that points towards a 368 Ma age. We cannot discard some kind of only-recycling processes in local areas as reflected by the upper crustal ISr signature and 1.5 Ga model age of the gneisses in Lago Curruhué (Pankhurts et al., 2006). The scenario for this juvenile addition, homogenization and recycling would be a thermal event congruent with an evolved arc and evidenced by the widespread devonian tonalites, gneisses and tonalithic migmatites that crop out in the area as previously mentioned by Godoy et al. (2008). Also based on the Sm-Nd signature, this Devonian magmatic event could be extended further south to Gastre, where Pankhurts et al. (2006) obtained a reliable U-Pb 371± 3 Ma age in granites. South Bariloche, near 41°30´S crop out the Early Carboniferous (~320Ma) amphibolites and granodiorites of the Cañadón de la Mosca arc- related rocks (Pankhurst et al., 2006), which may typify a subduction-processed subcontinental lithosphere in the region where the amphibolites are believed to be derived from a mantle wedge slightly contaminated with subducted sediments. This is evidenced by their positive to slightly negative gNd(t) values (-0.6 to + 2.8), closer to the depleted mantle source, and lower ISr (0.703-0.704). All the samples show Neoproterozoic TDM model ages (0.8-1.1 Ga). This evidence states that new inputs of a juvenile magma to the crustal segment were added without significative contamination. The Early Permian igneous suites of the Mamil Choique Sierra and Rio Chico area exhibit typical medium K- calcalkaline tendences as well as mean 1.25- 1.34 Ga TDM model ages. The gNd(t) values for these rocks rank between -3.2 to -5.2 with almost the same fSm-Nd (-0.45). Considering the succeeding events of juvenile additions plus the all the events of recycling that have been produced in the continental crust, the bulk crustal composition during Permian times would have change from the one in early paleozoic times.  U/Pb ages of the oldest rocks exposed in the MNP hold considerably different values in the northeastern corner, where metasedimentary units have proven to have a minimum Cambro-Ordovician inheritance and Famatinian metamorphism, respect to the western, where minimum reliable ages of metamorphism indicate Devonian times. However, Pb and Sm/Nd whole rock isotopic compositions delineate a whole one basament domain. The crust in both areas´ were apparently formed during the Paleoproterozoic between the 1.7 to 2.1 Ga, and then subsequently recycled with minor additions of juvenile material during the Neoproterozoic. The first stage of crustal growth is evidenced from the Ordovician granites whose material were extracted from a fairly Mesoproterozoic depleted-mantle. Meanwhile in the western belt little Mesoproterozoic juvenile addition can be relied on due to certain εNd(t) values and field evidence. Hence, for these times the main crustal-forming mechanism would be the recycling of the bulk crust. On the other hand, the coeval Carboniferous to Permian intrusions in northeastern and western of the NPM were formed as an old Mesoproterozoic already recycled crust partially contaminated with a fairly low proportion of juvenile material, probably proportioned by the granites and amphibolites of the Cañadón de la Mosca.