Search

Field relationships and structural studies combined with in situ U-Th-Pb dating of monazite from Las Chacritas pluton (LCP), Sierra de Humaya, provide insight into the emplacement of peraluminous magmas triggered by lateral shortening of the host rock and magmatic stoping at a mid-crustal level of a retro-arc zone in a convergent orogen. Modal and chemical compositions indicate that the LCP is composed of two main igneous units of peraluminous granitoids. The predominant two-mica granitoids were generated by interaction of crustal rocks with mafic or mafic-derived magmas and/or crystal-rich magmas that entrained residual phases, whereas less abundant leucocratic granitoids may have been originated by partial melting of metasedimentary rocks. The calculated crystallization age of 474 ± 4 Ma is consistent with Ordovician ages (477-470 Ma) of the high grade metamorphic rocks, indicating concomitant magmatism and metamorphism during the Famatinian orogeny. The LCP was emplaced in the middle crust at a maximum depth of ~ 14.5 km, where attendant fracturing and ductile deformation were active. Field evidence shows strong temporal and spatial relationships between host rock ductile deformation and the emplacement of the pluton such as folding and strike deflection of the host rock layering and folded concordant leucocratic sheets with magmatic fabrics. This suggests that material transfer processes like lateral wall rock displacement (lateral shortening) was a viable mechanisms for the emplacement of the LCP. However, cross section restoration and field evidence such as wall-rock xenoliths and intrusive truncations of the host rock foliation and fold traces suggest that magmatic stoping was a complementary mechanism to create the necessary space for the emplacement of the LCP. This work supports previous studies showing that participation of multiple material transfer processes are the rule rather than the exception in the emplacement of plutons. Fil: Larrovere, Mariano Alexis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientificas y Transferencia Tecnológica de Anillaco; Argentina. Universidad Nacional de la Rioja; Argentina Fil: Alasino, Pablo Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Cientificas y Transferencia Tecnológica de Anillaco; Argentina. Universidad Nacional de la Rioja; Argentina Fil: de Los Hoyos, Camilo Raul. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Willner, Arne P.. Ruhr-universität Bochum; Alemania

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Geochronological and Sm–Nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm–Nd results for the metamorphic basement suggest that the TDM age interval of 1.8–1.7 Ga, which is associated with the less radiogenic values of εNd(540) (−6 to −8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger TDM age and a more positive εNd(540) value. Pampean pre-540 Ma granitoids form two clusters, one with TDM ages between 2.0 and 1.75 Ga and another between 1.6 and 1.5 Ga. Pampean post-540 Ma granitoids exhibit more homogenous TDM ages ranging from 2.0 to 1.75 Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. TDM values for the Ordovician Famatinian granitoids define a main interval of 1.8–1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger TDM ages ranging from 1.3 to 1.0 Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic εNd(540) values that range between 0.5 and −4 and TDM ages younger than 1.3 Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism. Fil: Steenken, André. Universität Greifswald; Alemania Fil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina Fil: Drobe, Malte. Universität Göttingen; Alemania Fil: Wemmer, Klaus. Universität Göttingen; Alemania Fil: Siegesmund, Siegfried. Universität Göttingen; Alemania

The central-eastern part of the Sierra de Velasco (Sierras Pampeanas, NW Argentina) is formed by the large Huaco (40 x 30 km) and Sanagasta (25 x 15 km) granite massifs and the small La Chinchilla stock (2 x 2 km). The larger granites intrude into Ordovician metagranitoids and crosscut Devonian (?) mylonitic shear zones, whereas the small stock sharply intrudes into the Huaco granite. The two voluminous granites are biotitic-muscovitic and biotitic porphyritic syeno- to monzogranites. They contain small and rounded tonalitic and quartz-dioritic mafic microgranular enclaves. The small stock is an equigranular, zinnwaldite- and fluorite-bearing monzogranite. The studied granites are silica-rich (SiO2 > 70%), potassium-rich (K2O > 4%), ferroan, alkali-calcic to slightly calk-alkalic, and moderately to weakly peraluminous (A/CNK = 1.06-1.18 Huaco granite; 1.01-1.09 Sanagasta granite; 1.05-1.06 La Chinchilla stock). They have moderate to strong enrichments in several LIL (Li, Rb, Cs) and HFS (Nb, Ta, Y, Th, U) elements, and low Sr, Ba and Eu contents. U-Pb monazite age determinations indicate Lower Carboniferous crystallization ages: 350-358 Ma for the Huaco granite, 352.7 ± 1.4 Ma for the Sanagasta granite and 344.5 ± 1.4 Ma for the La Chinchilla stock. The larger granites have similar eNd values between ?2.1 and ?4.3, whereas the younger stock has higher eNd of ?0.6 to ?1.4, roughly comparable to the values obtained for the Carboniferous San Blas granite (?1.4 to ?1.7), located in the north of the sierra. The Huaco and Sanagasta granites have a mainly crustal source, but with some participation of a more primitive, possibly mantle-derived, component. The main crustal component can be attributed to Ordovician peraluminous metagranitoids. The La Chinchilla stock derives from a more primitive source, suggesting an increase with time in the participation of the primitive component during magma genesis. The studied granites were generated during a post-orogenic period in a within-plate setting, possibly as a response to the collapse of the previous Famatinian orogen, extension of the crust and mantle upwelling. They are part of the group of Middle Devonian-Lower Carboniferous granites of the Sierras Pampeanas. The distribution and U-Pb ages of these granites suggests a northward arc-parallel migration of this mainly post-orogenic magmatism with time. Fil: Grosse, Pablo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; Argentina Fil: Söllner, Frank. Ludwig-Maximilians-Universität; Alemania Fil: Báez, Miguel Adolfo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Toselli, Alejandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Rossi, Juana Norma. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: de la Rosa, J. D.. Universidad de Huelva; España

The evolution of the provenance areas for Late Neoproterozoic,Cambrian and Early Ordovician sedimentary and meta-sedimentaryrocks of north central and northwest Argentina is discussed using 123maximum ages of detrital zircons from 42 samples from this andpreviously published studies. Most detrital zircon ages fall into twogroups: 1,200–900 Ma and 670–545 Ma. These ages are essentiallyidentical for the non- to very low grade metamorphic lateNeoproterozoic to Early Cambrian Puncoviscana Formation and thelow to high grade metamorphic rocks of Eastern Sierras Pampeanas.Hence, both units are related to similar provenance areas at thesame time of sedimentation. The time span from zirconcrystallization in the Earth’s crust to exhumation and erosion may bevery long. This is important when determining maximum ages ofsedimentary rocks. Variation of zircon maxima may also beinfluenced by concurrent sedimentary cover of proposed provenanceareas. For the late Mesoproterozoic to early Neoproterozoic zirconage group, an active mountain range of the southwest BrazilianSunsás orogen is the most probable provenance area. The younger,late Neoproterozoic zircons are related to the continuouslydeveloping mountains of the Brasiliano orogen of southwest and southcentral Brazil. Young zircons, up to 514 Ma, from fossil-bearingPuncoviscana and Suncho Formation outcrops are related to late EarlyCambrian volcanism contemporaneous with sedimentation. Thissituation continues through the Late Cambrian to the EarlyOrdovician, but the Sunsás orogen provenance diminishes as possibleRío de la Plata craton origins become important.Keywords West Gondwanaland – Neoproterozoic – Cambrian – Detrital Fil: Miller, Hubert. Ludwig-Maximilians-Universität; Alemania Fil: Adams, Christopher. GNS Science; Nueva Zelanda Fil: Aceñolaza, Florencio Gilberto. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo; Argentina Fil: Toselli, Alejandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina