4 research outputs found
Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley, Sierra de Valle Fértil, Argentina: Implications for the evolution of magmas in the lower crust of the Famatinian arc
This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ∼ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO2 ∼ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO2) of the parental magma estimated between +0.8 and −0.6 log fO2 units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.Fil: Otamendi, Juan Enrique. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba; ArgentinaFil: Cristofolini, Eber Ariel. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba; ArgentinaFil: Tibaldi, Alina MarÃa. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba; ArgentinaFil: Quevedo, F. I.. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; ArgentinaFil: Baliani Cocitto, Ignacio. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentin
Geology of the sierra de Chávez in central-western section of the Sierra de Valle Fértil, San Juan: Metamorphism and tectonic evolution in the lower crust of the Famatinian arc
La geologÃa de la sierra de Chávez está constituida por rocas Ãgneas y metamórficas intercaladas complejamente a todas las escalas, y corresponderÃan a la unidad máfica definida en la sierra de Valle Fértil. Cinco asociaciones litológicas han sido identificadas en éste complejo: diorÃtica, gábrica, metasedimentaria, milonÃtica y granÃtica-pegmatÃtica. Las estimaciones termobarométricas realizadas en las migmatitas indican condiciones de equilibrio para el pico metamórfico (M1) de 876 °C y 8,3 Kbar y reflejarÃan el efecto del magmatismo durante el OrdovÃcico Temprano (489 - 470 Ma). Las presiones aquà estimadas corresponden a las mayores paleo-profundidades reconocidas en todo el arco magmático Famatiniano. Las relaciones de campo y el análisis estructural permitieron establecer la presencia de varias fábricas magmáticas y tectónicas, las cuales son: i) estructuras magmáticas (S0) interpretadas como una foliación magmática primaria, ii) foliación submagmática a subsólida de alta temperatura y foliación metamórfica (S1), iii) foliación milonÃtica estabilizada en facies de anfibolita (S2a) y esquistos verdes (S2b) y iv) foliación frágil (S3) definida como un clivaje de fractura centimétrico. (S0) y (S1) están asociadas al primer estadÃo de deformación (D1) reconocido en la zona, desarrollado en condiciones de alta temperatura (facies de granulita) y coetáneo con el magmatismo y el pico metamórfico (M1). (S2a,b) y (S3) transponen a las estructuras Ãgneas y metamórficas generadas durante el evento D1, y se asocian a un segundo y tercer estadÃo de deformación (D2-D3), representando una evolución progresiva de la deformación desde dominios dúctiles a frágiles, vinculadas a las etapas de cierre y exhumación del arco Famatiniano.The geology of the Sierra the Chavéz is constituted by igneous and metamorphic rocks that appear complexly inter-layered at all scales, and belong to the mafic unit from defined in the Sierra de Valle Fértil. Five lithological associations have been identified in this complex, which are: dioritic, gabbroic, metasedimentary, mylonitic and granitic-pegmatitic. Thermobarometric estimates performed in metasedimentary migmatites yield peak (M1) metamorphic conditions of 876 °C and 8.3 Kbar, and are interpreted to reflect Early Ordovician magmatism (~ 489-470 Ma). Maximum estimated metamorphic pressures correspond to one of the deepest crustal paleo-depths found all over the Famatinian arc. Field observations and structural analyses allow recognizing the presence of several magmatic and tectonic fabrics, which are: i) magmatic layering (S0) interpreted as primary magmatic foliation, ii) sub-magmatic to sub-solid high temperature foliation and metamorphic foliation (S1), iii) mylonitic foliations first formed in amphibolite (S2a) and then re-worked under green schist facies (S2b), and iv) brittle foliation (S3) occuring as centimeter-spaced cleavage fracture. Both (S0) and (S1) foliations are associated to a first stage of deformation (D1) that developed at high temperature conditions (granulite facies) and was broadly coeval with the magmatism and the metamorphic peak (M1). (S2a,b) and (S3) structures transposes the igneous and metamorphic fabrics developed during the D1 event, and are related to a second and third stage of deformation (D2-D3) which represent a progressive evolution of the deformation from ductile to fragile regimens related to the stage of closure and exhumation of the Famatinian arc.Fil: Cristofolini, Eber Ariel. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Tibaldi, Alina MarÃa. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Otamendi, Juan Enrique. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Martino, Roberto Donato. Universidad Nacional de Córdoba; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Baliani, Ignacio. Universidad Nacional de RÃo Cuarto; Argentin
Infernal structure of the migmatites of the northern portion of the Sierra de Valle Fértil, San Juan
En la porción norte de la Sierra de Valle Fértil se observa la presencia de rocas migmáticas, las cuales han sido clasificadas de acuerdo a su fábrica en metatexitas y diatexitas, y de acuerdo a su protolito en pelÃtica y grauváquicas - semipelÃticas. Las relaciones de campo y el análisis estructural detallado de las rocas migmáticas y los resisters asociados a las mismas, permiten establecer la presencia de estructuras pre-migmáticas (S0-SR) interpretadas como estratificación sedimentaria y foliación metamórfica relÃctica con el desarrollo de recristalización estática. Asimismo, el desarrollo de fábricas migmáticas (S1a,b,c) y (P1a,b), asà como también la generación de granitos antécticos, se asocian a una etapa de deformación (D1) desarrollada en condiciones de alta temperatura (facies de granulita) y coetánea con el magmatismo. Por último, se reconoce la presencia de fábricas post-migmáticas (S2a y S2b) asociadas a un evento deformacional (D2) que registran una evolución bajo condiciones dúctiles-frágiles, las asociadas a las etapas de cierre y exhumación del arco Famatiniano a la que se le sobreimpone una estructura planar de carácter frágil. De esta manera, se establece que el arco magmático Famatiniano se desarrolló en una secuencia sedimentaria espesa, depositada y consolidada desde el Cámbrico medio, la cual no registra una historia deformacional previa al ingreso del plutonismo OrdovÃcico, sino que por el contrario todas las estructuras definidas se asocian a los periodos de migmatización, cierre y exhumación final del mencionado arco.In the northern portion of the sierra de Valle Fértil, there are migmatitic rocks which have been classifed as metatexites and diatexites according to their fabric, and as metapelitic and metasemipelitic-metagreywacke in function of their protoliths. Field relationships and a detailed structural analysis of the migmatites and the resisters associated to them, allow recognizing the presence of pre-migmatitic structures (S-SR) interpreted as sedimentary bedding and relictic metamorphic foliation, this last one related to the buried processes. The generation of anatectic granites related to migmatites with foliations (S1abc) and (P ,b), are associated to a deformational phase (D ) developed at high temperature conditions (granulite facies) and contemporary with the magmatism. Lastly, postmigmatic fabrics (S and S 2b) related to a deformational event (D ), show an evolution under ductile-brittle conditions related to the cióse and exhumation stage of the Famatinian are. This fabric is superimposed by a planar structure of brittle character (S ). Thus, we establish that the Famatinian magmatic are is developed in a thick sedimentary sequence, deposited and consolidated since the mid-Cambrian. This sequence does not record a previous deformational history to the emplacement of Ordovician magmatism. In contrast, all the preserved structural features are related to metamorphism (migmatization) and exhumation during the are development.Fil: Tibaldi, Alina MarÃa. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas FisicoquÃmicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Cristofolini, Eber Ariel. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas FisicoquÃmicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Otamendi, Juan Enrique. Consejo Nacional de Investigaciones CientÃficas y Técnicas. Centro CientÃfico Tecnológico Conicet - Córdoba. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas FisicoquÃmicas y Naturales. Instituto de Ciencias de la Tierra, Biodiversidad y Ambiente; ArgentinaFil: Baliani Cocitto, Ignacio. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentina. Ministerio de Ciencia, TecnologÃa e Innovación Productiva. Agencia Nacional de Promoción CientÃfica y Tecnológica. Fondo para la Investigación CientÃfica y Tecnológica; ArgentinaFil: Gaido, Gisela Andrea. Universidad Nacional de RÃo Cuarto. Facultad de Ciencias Exactas, FisicoquÃmicas y Naturales. Departamento de GeologÃa; Argentin
Reconstruction of the Early Ordovician Famatinian arc through thermobarometry in lower and middle crustal exposures, Sierra de Valle Fértil, Argentina
The crustal structure of the Famatinian paleoarc is reconstructed by determining the metamorphic crystallization P–T conditions from metasedimentary rocks at various structural levels in the Valle Fértil section. The bulk section exhibits a 15-km-thick arc crustal section. Thermobarometry shows that nested tonalitic and granodioritic plutons constructed the arc crust at depths 6.6 km/s is located at deeper depths in the Famatinian arc than in Talkeetna arc. The thickness of a crustal layer dominated by plutonic rocks with low seismic wave velocities (< 6.2 km/s) is 10 km thinner than the crustal layer with similar physical properties in the Sierra Nevada batholith. A putative model for the whole Famatinian arc suggests a total crustal thickness between 30 and 35 km with three distinct layers.Fil: Tibaldi, Alina MarÃa. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Otamendi, Juan Enrique. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Cristofolini, Eber Ariel. Universidad Nacional de RÃo Cuarto; Argentina. Consejo Nacional de Investigaciones CientÃficas y Técnicas; ArgentinaFil: Baliani, Ignacio. Universidad Nacional de RÃo Cuarto; ArgentinaFil: Walker, Barry A.. University of Washington; Estados UnidosFil: Bergantz, George W.. University of Washington; Estados Unido