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Petrology and geochemistry of meta-ultramafic rocks in the Paleozoic Granjeno Schist, northeastern Mexico: Remnants of Pangaea ocean floor

SONIA ALEJANDRA TORRES SANCHEZ Uwe Jenchen JOSE RAFAEL BARBOZA GUDIÑO JUAN ALONSO RAMIREZ FERNANDEZ DARIO TORRES SANCHEZ (2017)

"The Granjeno Schist is a meta-volcanosedimentary upper Paleozoic complex in northeastern Mexico. We suggest different tectonic settings for metamorphism of its serpentinite and talc-bearing rocks based on petrographic and geochemical compositions. According to the REE ratios (LaN/YbN = 0.51 –20.0 and LaN/SmN = 0.72–9.1) and the enrichment in the highly incompatible elements Cs (0.1 ppm), U (2.8 ppm), and Zr (60 ppm) as well as depletion in Ba (1 – 15 ppm), Sr (1 –184 ppm), Pb (0.1 –14 ppm), and Ce (0.1 –1.9 ppm) the rocks have mid-ocean ridge and subduction zones characteristics. The serpentinite contains Al-chromite, ferrian chromite and magnetite. The Al-chromite is characterized by Cr# of 0.48 to 0.55 suggesting a MORB origin, and Cr# of 0.93 to 1.00 for the ferrian chromite indicates a prograde metamorphism. We propose at least two serpentinization stages of lithospheric mantle for the ultramafic rock of the Granjeno Schist, (1) a first in an ocean-floor environment at sub-greenschist to greenschist facies conditions and (2) later a serpentinization phase related to the progressive replacement of spinel by ferrian chromite and magnetite at greenschist to low amphibolite facies conditions during regional metamorphism. The second serpentinization phase took place in an active continental margin during the Pennsylvanian. We propose that the origin of the ultramafic rocks is related to an obduction and accretional event at the western margin of Pangea."

Article

Ultramafic rocks Serpentinite Granjeno Schist Northeastern Mexico Gondwana Pangea CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOLOGÍA

Origen y evolución del basamento cristalino del Complejo Iglesias en la región central de Los Andes de Mérida, Venezuela

Origin and evolution of the crystalline basement of the Iglesias Complex in the central Mérida Andes, Venezuela

MARIA DANIELA TAZZO RANGEL (2019)

El basamento metamórfico de Los Andes de Mérida en el oeste de Venezuela está constituido por paragneises, esquistos, ortogneises, anfibolitas y metagranitoides del Paleozoico Temprano del Complejo Iglesias. En este trabajo, se seleccionaron dos áreas en la región central de Los Andes de Mérida para una revisión y actualización del mapeo geológico del Complejo Iglesias, integrando también análisis petrográficos y una sólida base de datos de geocronología, química e isotopía, para así dilucidar la historia magmática y metamórfica de estas rocas. En este sentido, en esta tesis se presenta geocronología U-Pb en zircón, geocronología Rb-Sr en micas, geoquímica en roca total, isotopía Lu-Hf en zircón, isotopía Sm-Nd y Lu-Hf en roca total e isótopos de O en zircón para rocas seleccionadas de dicho basamento metamórfico. Los resultados obtenidos se enmarcan en los modelos tectónicos propuestos para la esquina noroeste del supercontinente Gondwana y se correlacionan con rocas paleozoicas equivalentes en otros fragmentos corticales peri-Gondwánicos. Los paragneises y esquistos en el Complejo Iglesias se componen de protolitos pelíticos-psamíticos y volcaniclásticos, localmente alternados con lentes de anfibolita que tienen un origen volcánico o volcaniclástico. Las edades de zircones detríticos sugieren que la sedimentación de los protolitos de los paragneises abarca de ~520 Ma a ~490 Ma, mientras que las firmas químicas e isotópicas sugieren una cuenca trasarco como ambiente de depositación. Los protolitos de los ortogneises de biotita-granate intrusionaron a los paragneises a ~487 Ma, durante el metamorfismo Barroviano (M1) que alcanzó la anatexis a ~467 Ma. Posteriormente, orto- y paragneis fueron intrusionados por magmas máficos a intermedios derivados del manto y de la corteza inferior (ahora ortogneises de hornblenda y diques de anfibolita en los mismos) en un ambiente tectónico de trasarco a ~460-455 Ma. Un nuevo régimen compresivo condujo a un metamorfismo de alto grado (M2) a ~450 Ma, seguido de fusión por descompresión a ~430-420 Ma (M3). Finalmente, se produjo un metamorfismo en la facies de esquistos verdes superior a ~250 Ma (M4). Los metagranitoides ordovícicos y triásicos son probablemente sintectónicos al metamorfismo M2 y M4, respectivamente. Los eventos tectono-termales del Paleozoico Temprano están relacionados con ciclos alternos de avance y retroceso de la losa de subducción en un arco continental activo, ampliamente reconocido como la ...

The metamorphic basement of the Mérida Andes in western Venezuela is constituted of Early Paleozoic paragneiss, schist, orthogneiss, amphibolite, and metagranitoid of the Iglesias Complex. In this work, two areas of the Central Mérida Andes were selected for a revision and update of geologic mapping of the Iglesias Complex, integrating petrographic analyses and a robust database of geochronology, chemistry, and isotopic data to constrain the magmatic and metamorphic history of these rocks. Therefore, U–Pb (zircon) and Rb-Sr (mica) geochronology, whole-rock geochemistry, Hf-Nd isotope systematics, and O isotopes of these rocks are presented in this thesis. The results are framed in the tectonic settings proposed for Northwestern Gondwana and correlated with equivalent Paleozoic rocks in other peri-Gondwanan crustal fragments. Paragneiss and schist in the Iglesias Complex are derived from pelitic-psammitic and volcaniclastic protoliths, locally alternating with amphibolite lenses which may be related to a volcanic or volcaniclastic origin. Detrital zircon ages suggest that sedimentation of paragneiss protoliths spans ~520 Ma to ~490 Ma, whereas chemical and isotopic signatures are suggestive of a back-arc setting sedimentation. Biotite-garnet orthogneiss protoliths intruded paragneiss at ~487 Ma, during Barrovian metamorphism (M1) that reached anatexis at ~467 Ma. Paragneiss and orthogneiss were later intruded by mafic to intermediate lower-crust and mantle-derived magmas (now hornblende orthogneiss and amphibolite) in a back-arc setting at ~460-455 Ma. A new compressional setting led to high-grade metamorphism (M2) at ~450 Ma, followed by decompression melting at ~430-420 Ma (M3). Finally, an upper greenschist-facies overprint in the metamorphic complex occurred at ~250 Ma (M4). Ordovician and Triassic metagranitoids are probably syn-tectonic to metamorphism M2 and M4, respectively. The Early Paleozoic tectonothermal events are related to alternating cycles of advance and retreat of the subduction slab in an active continental arc, broadly recognized as the Famatinian orogeny, which formed along the proto-Andean margin of Western Gondwana. The Permo-Triassic event is possibly related to the final amalgamation of crustal blocks to form the Pangea supercontinent. The Iglesias Complex has several similarities to rocks of the same age in other crustal fragments adjacent to Northwestern Gondwana, such as the Santander Massif in Colombia, the Acatlán Complex ...

Doctoral thesis

Andes de Mérida, Complejo Iglesias, geocronología, geología isotópica, metamorfismo, anatexis, Orogenia Famatiniana, aglutinamiento de Pangea Mérida Andes, Iglesias Complex, geochronology, isotope geology, metamorphism, anatexis, Famatinian Orogeny, Pangea agglutination CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA CIENCIAS DE LA TIERRA Y DEL ESPACIO GEOLOGÍA PETROLOGÍA ÍGNEA Y METAMÓRFICA PETROLOGÍA ÍGNEA Y METAMÓRFICA