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sobreimpuesto en sedimentitas del Complejo Quebradagrande
Table of Contents
Only the abstracts and keywords of the manuscripts accepted up to now have been included.
The Permian – Triassic history of magmatic rocks of the northern Andes (Colombia and Ecuador): Supercontinent assembly and disassembly
By Richard SPIKINGS and Andre PAUL
Manuscript accepted October 29, 2018
Northwestern South America and its conjugate margins record the Permian assembly of Pangaea, its Triassic fragmentation and opening of the proto–Caribbean ocean, and the onset of the Andean cycle at ~209 Ma. We review Permian and Triassic magmatic rocks exposed in the cordilleras and dispersed inliers in Colombia and Ecuador, and present a large geochronological, geochemical, isotopic and thermochronological database. These data are used to develop a model for the evolution of rocks within Colombia and Ecuador during the formation and destruction of Pangaea. Similar data has been assembled from studies of the southern North American and western Caribbean Plates, as well as Venezuela and further south within South America, and a large–scale reconstruction for western Pangaea is provided. Permian magmatic rocks in Colombia and Ecuador (288–253 Ma) formed within a continental arc system which extended from at least southern North America to southern Perú. The Permian arc within northwestern South America was dismembered during Cenozoic interactions with the Caribbean Plate, causing some blocks to be transferred eastwards. Compression and regional metamorphism at ~250 Ma is best recorded in the Sierra Nevada de Santa Marta, and represents the final stages of amalgamation and thickening of western Pangaea. Continental rifting prevailed within southern North America and the entire western margin of South America during 245–216 Ma. Significant back–arc extension in northwestern South America lead to a rift–to–drift transition in Colombia and Ecuador, forming oceanic lithosphere of the proto–Caribbean. Rifting failed south of the Huancabamba Deflection, and is preserved as Triassic basins in Perú, western Argentina and Chile. Triassic rifting represents the early fragmentation of western Pangaea, and the attenuation of its margin may be a prelude to complete separation by enhancing mantle upwelling, inducing a large igneous province and weakening the crust within a tensile regime.
The petrologic nature of the "Medellín Dunite" revisited: An algebraic approach and proposal of a new definition of the geological body
By Antonio GARCIA–CASCO, Jorge Julián RESTREPO, Ana María CORREA–MARTÍNEZ, Idael Francisco BLANCO–QUINTERO, Joaquín Antonio PROENZA, Marion WEBER and Lidia BUTJOSA
The “Medellín Dunite”, the main ultramafic body of the Central Cordillera of Colombia, constitutes a fragment of oceanic lithospheric mantle formed at a back–arc basin/incipient arc scenario emplaced onto the western continental margin of Pangea during Triassic time. This body has been classically, and is still considered, mainly of dunite composition. However, in spite of two subsequent metamorphic imprints that obscure the primary mantle mineralogical composition, there is petrographic and geochemical evidence that points to a harzburgitic nature of the unit. In order to overcome the petrographic effects of medium–T metamorphism, metasomatism and serpentinization, we analyzed published and new major–element geochemical data by means of algebraic methods to approximate the mantle mineralogical composition of ultramafic rocks. The restored mantle mineralogy clearly indicates that the body is mainly of harzburgitic composition, and therefore we propose that the term “Medellín Dunite” should no longer be applied to avoid terminological confusion. Furthermore, a phase–relation approach in simple systems for the metamorphic evolution allows us to identify the main reason for the contradictory terminology used so far: olivine is paragenetic (stable) with tremolite and talc during medium–T (ca. 600 °C) metamorphic imprint undergone by the body. During this initial metamorphic event, characterized by full hydration (as opposed to the late–stage serpentinization), mantle pyroxenes reacted out and medium–T olivine formed while high–T olivine persisted metastably as a likely consequence of moderate temperature and sluggish diffusion kinetics. On the other hand, we analyze two likely geodynamic scenarios to provide a common context of metamorphism for the ultramafic body and associated metabasites (Aburrá Ophiolite): (i) ocean–floor metamorphism and (ii) intra–backarc subduction–initiation metamorphism. The latter allows a new tectonic view of the Aburrá Ophiolite, formed by tectonic units from the upper and downgoing plates on a nascent active plate margin. For all these reasons, we propose the new term “Medellín Metaharzburgitic Unit” in order to combine in a single term the original high–T mantle composition, its subsequent metamorphic transformation and the independent tectonic character of the ultramafic body.
Late Triassic to Jurassic magmatism in Colombia: Implications for the evolution of northern margin of South America
By Julián Andrés LÓPEZ–ISAZA and Carlos A. ZULUAGA
Diverse Jurassic magmatic arcs of the Colombian Andes: Constraints from petrography, geochronology and geochemistry
By Gabriel RODRÍGUEZ–GARCÍA, Ana María CORREA–MARTÍNEZ, Gilberto ZAPATA–GARCÍA, María Isabel ARANGO–MEJÍA, Gloria OBANDO–ERAZO, Juan Pablo ZAPATA–VILLADA and José Gilberto BERMÚDEZ–CORDERO
Jurassic evolution of the northwestern corner of Gondwana: Present knowledge and future challenges in studying Colombian Jurassic rocks
By Germán Alonso BAYONA–CHAPARRO, Camilo BUSTAMANTE, Giovanny NOVA and Ana Milena SALAZAR–FRANCO
This paper summarizes actual knowledge of metamorphic, plutonic, volcanic, carbonate and clastic sedimentary Jurassic rocks that are exposed from northern Perú to Venezuela. This compilation allows an evaluation of three tectonic models that have been proposed for the evolution of the northwestern corner of Gondwana: An extensional model, a subduction–dominated model, and the along–marginal migration of blocks model, that last of which considers the interaction of western subduction and the north–south separation of continental blocks. We conclude that (i) the Jurassic evolution of this orthogonal margin cannot be represented in a single paleogeographic map that represents a dominant geodynamic process; (ii) future analyses must consider the superposition of both Pacific subduction and proto–Caribbean extensional processes; (iii) extensional basins in Guajira, the Perijá range and the Mérida Andes include the sedimentary record of predominantly proto–Caribbean extension, whereas western–subduction processes are recorded by a batholith chain that extends from southern Ecuador to the Santa Marta Massif; and (iv) a Middle Jurassic unconformity separates Lower to Middle Jurassic sedimentary and volcanic successions, which are related to subduction magmatism and the separation of the North and South American Plates, from Upper Jurassic continental and marine deposits in extensional basins along the northern margin, which record the opening of the proto–Caribbean sea. Future geochemical studies in Jurassic intrusive bodies should be able to evaluate the contamination of Triassic versus Grenvillian and older continental crust. Metamorphic studies should concentrate on the petrology and the pressure–temperature–time (P–T–t) paths. The chronostratigraphic framework of sedimentary basins should be improved by resuming paleontological investigations and geochronological analysis at the base and top of volcanoclastic rocks. Sedimentological analysis should focus on establishing the geometry of sedimentary basins, the relationship of basin generation with magmatic centers, and documenting the record of paleo–climate indicators in order to establish possible paleo–latitudinal variations of tectonic blocks. Paleomagnetic studies should be conducted at different localities in Lower – Middle Jurassic rocks to test whether tectonic blocks are static or record northward translations. The strong decrease in magmatic activity during the Late Jurassic time should be explained within a regional tectono–magmatic framework.
140 million years of tropical biome evolution
By Carlos JARAMILLO
October 29, 2018
The origin and development of Neotropical biomes are central to our understanding of extant ecosystems and our ability to predict their future. During the Cretaceous, biomass of tropical rainforests was mostly dominated by gymnosperms and ferns, forest structure was poorly stratified and the canopy was open and dominated by gymnosperms. Extant tropical rainforests first developed at the onset of the Cenozoic, as a result of the massive extinction of the Cretaceous – Paleocene boundary. Paleocene rainforests were multistratified, with an angiosperm–dominated canopy that had high photosynthetic potential. Tropical climate has followed global patterns of warmings and coolings during the last 60 my. Rainforest diversity has increased during the warmings while it has decreased during coolings. Several extant biomes, including paramos, cloud forest, savannas, and dry/xerophytic forest, have increase significantly during the late Neogene at the expense of the reduction of the rainforest. Timing and drivers of these changes are still unknown but seem to be related to the onset of our modern, cool–state climate since the onset of the Pleistocene, 2.6 Ma.
Tectonostratigraphic terranes in Colombia: An update. Second part: Oceanic terranes
By Jean–François TOUSSAINT and Jorge Julián RESTREPO
Detrital U–Pb provenance, mineralogy, and geochemistry of the Cretaceous Colombian back–arc basin
By Javier GUERRERO, Alejandra MEJÍA–MOLINA and Jose OSORNO
Biomicrite, marlstone, and shale properties. Exploration of non–conventional hydrocarbons from the Cretaceous Colombian back–arc basin
Cretaceous record from a Mariana to an Andean–type margin in the Central Cordillera of the Colombian Andes
By Agustín CARDONA, Santiago LEÓN, Juan S. JARAMILLO, Victor VALENCIA, Sebastián ZAPATA, Andrés PARDO–TRUJILLO, Axel SCHMITT, Dany MEJÍA and J.C ARENAS
Barremian deposits of Colombia: A special emphasis on marine successions
By Pedro PATARROYO
Plesiosaurs, palaeoenvironments and the Paja Formation Lagerstätte of central Colombia: An overview
By Leslie Francis NOÈ and Marcela GÓMEZ–PÉREZ
October 29, 2018
The Cretaceous Paja Formation of the Alto Ricaurte of the Eastern Cordillera of central Colombia was laid down under an epicontinental sea during Hauterivian – Aptian times. The Paja Formation epicontinental sea was home to a diverse, and now well–preserved, pelagic marine fauna that includes members of Plesiosauria, other marine reptiles, fish and ammonites. However, the benthic fauna is depauperate, preserving just a few thin–shelled bivalves and evidence of microbial mats. This suggests dysoxic–anoxic bottom waters, separated from oxic surface waters by a chemocline–pycnocline. The exceptional preservation of the Paja Formation fauna makes the Alto Ricaurte a unique Lower Cretaceous marine vertebrae Lagerstätte. Previous palaeoenvironmental interpretations of the Paja Formation, based on observations of the gypsiferous, dark mudrock sequence, suggested an intertidal evaporitic (sabkha) environment. However, integration of sedimentological, palaeobiological, taphonomic and diagenetic data provides evidence for deeper water conditions. The exquisite preservation and articulation of the skeletons of large marine reptiles, three–dimensionally preserved fish, beautifully ornamented ammonites, and delicate plants, do not accord with a sabkha environment. Sabkha is typical of mid–latitude, dryer climates under the descended limb of the Hadley atmospheric cell, rather than a wet tropical equatorial location of the Paja Formation. Mineralogical arguments used to infer the presence of sabkha are not primary depositional features, but due to secondary migration of mineral–rich fluids. These fluids probably had their source in the earliest Cretaceous topographic high now beneath the Sabana of Bogotá, and were driven by hydraulic pressure generated by volumetric changes due to hydration of anhydrite into gypsum due to the post–Cretaceous rise of the northern Andes mountain chain. The separation of primary and secondary diagenetic features is thereby critical for understanding the evolution of the Paja Formation sedimentary basin in the Alto Ricaurte.
Two Cretaceous subduction events in the Central Cordillera: Insights from the high P–low T metamorphism
By Camilo BUSTAMANTE and Andrés BUSTAMANTE
Manuscript accepted October 29, 2018
The scarcity of high–pressure metamorphic rocks at the Earth’s surface due to the specific conditions required for their formation and preservation makes it difficult to access the information about subduction zones that they can provide. The northern Andes are characterized by several occurrences of blueschists and, in more minor proportions, eclogites, whose origins are yet to be unraveled. The metamorphic rocks found herein include the Pijao amphibolitized eclogites, Barragán blueschists and associated garnet–amphibolites, and Jambaló blueschists found in Colombia as well as the Raspas Metamorphic Complex in Ecuador. All these rocks have been correlated into a single Late Cretaceous high–pressure metamorphic belt based on regional geochemistry and geochronological data. A compilation of the most recent whole–rock geochemistry and Ar–Ar and Lu–Hf ages from the three high–pressure sequences in Colombia indicates that at least two different subduction events have been recorded in the Central Cordillera of Colombia. The first event, involving subduction and collision, occurred at ca. 130–120 Ma and is represented by the Pijao, Barragán, and Raspas high–pressure rocks, which have N–MORB–like protoliths and are contemporaneous with the end of the arc–related magmatism of the northern Andes, related to an oblique convergence between the Farallón Plate and the continental margin of South America. The second event of subduction is represented only by the Jambaló blueschists at ca. 70–60 Ma, whose protolith is akin to basalt formed in a plume–influenced intra–oceanic arc that was accreted to the continental margin. No reliable correlation is possible for these rocks as yet.
Keywords: blueschist, eclogite, northern Andes, high–pressure metamorphism.