PETROGRAFÍA Y GEOQUIMICA DEL NEIS DE NECHÍ
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Universidad Industrial de Santander
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El Neis de Nechí aflora dentro del terreno Tahami, al occidente de la Falla Otú-Pericos en la Serranía de San Lucas; se extiende como una franja de 35 km de largo por 10 km de ancho, desde El Porvenir (al sur) hasta el corregimiento Méjico (al norte); comprende neises y granofels cuarzo feldespáticos con anfíbol y biotita; presenta sectores donde la roca conserva el aspecto ígneo del protolito y sectores con estructura néisica con bandas granulares intercaladas con bandas néisicas. El metamorfismo regional sobre impuesto alcanzó la facies anfibolita baja, representada por la asociación cuarzo-plagioclasa-biotita-epidota-esfena y los minerales heredados del protolito ígneo son: hornblenda-cuarzo-ortoclasa-plagioclasa. Dentro de la unidad se observan vetas de cuarzo lechoso con mineralización de oro y polisulfuros y diques de pegmatitas de cuarzo y plagioclasa. La geoquímica de óxidos mayores y elementos trazas ubican las rocas dentro de la serie calcoalcalina alta en K, formadas a partir de un protolito ígneo intermedio, localizándolas en el diagrama TAS en los campos de las tonalitas y dioritas. El comportamiento de los elementos traza y las tierras raras indican que el protolito se desarrolló en un ambiente de arco, con enriquecimiento en elementos traza ligeros y empobrecimiento de los trazas pesados, con afinidad hacia los magmas magnesianos. La edad U/Pb obtenida en circones de 277,3 ± 3,0 Ma (Pérmico), fue interpretada como edad de intrusión del granitoide; si se asocia con los resultados geoquímicos, permite sugerir la existencia de un arco plutónico de margen continental relacionado a subducción, que intruyó un basamento más viejo, el cual fue sometido a metamorfismo regional durante el Triásico, afectando todo el conjunto y arrojando una edad de metamorfismo de 236,4 ± 6,6 Ma similar al del Complejo Cajamarca.
The Nechi’s Gneiss outcrops within the Tahami terrain to the west of the Otu-Pericos fault, and It’s located in the San Lucas range. It extends from El Porvenir, in the southern part, as a band of 35 Km length by 10 Km wide, to the Mejico’s town in the northern part. It is composed by quartzfeldespathic gneisses and granofels with amphibole and biotite, in some areas, these rocks present igneous aspect from their protolith and sectors with gneissic structure with granular bands interspersed with gneissic bands. The subsequent regional metamorphic reached low amphibole facies, is represented by quartz-plagioclase-biotite- epidote- titanite association, and the inherited igneous minerals from protolith are: hornblende, quartz, orthoclase, plagioclase. Inside the unit, milky quartz veins with gold and polysulphures mineralizations, and pegmatites dikes of quartz and plagioclase are observed. The majors oxides chemistry and traces elements, place these rocks among of the high in K calc-alkaline series, which are formed from an intermediate igneous protolith, located in the TAS diagram in the tonalities and diorites fields. The traces elements and rare earth elements behaviour suggests that the protolith was developed in an arc environment with enrichment in light traces elements and impoverishment of heavy traces, with affinity to magnesian magmas. The U/Pb zircons age is 277.3± 3.0 Ma (Permian), it was interpreted as the granitoide’s intrusion age. This age and the geochemistry analysis allow to suggest the existence of a plutonic arc of continental margin, related to subduction, which intruded the oldest basement that was submitted to regional metamorphism during the Triassic; affecting the whole unit, besides of giving a metamorphism age of 236.4 ± 6.6 Ma, similar to Cajamarca Complex.
The Nechi’s Gneiss outcrops within the Tahami terrain to the west of the Otu-Pericos fault, and It’s located in the San Lucas range. It extends from El Porvenir, in the southern part, as a band of 35 Km length by 10 Km wide, to the Mejico’s town in the northern part. It is composed by quartzfeldespathic gneisses and granofels with amphibole and biotite, in some areas, these rocks present igneous aspect from their protolith and sectors with gneissic structure with granular bands interspersed with gneissic bands. The subsequent regional metamorphic reached low amphibole facies, is represented by quartz-plagioclase-biotite- epidote- titanite association, and the inherited igneous minerals from protolith are: hornblende, quartz, orthoclase, plagioclase. Inside the unit, milky quartz veins with gold and polysulphures mineralizations, and pegmatites dikes of quartz and plagioclase are observed. The majors oxides chemistry and traces elements, place these rocks among of the high in K calc-alkaline series, which are formed from an intermediate igneous protolith, located in the TAS diagram in the tonalities and diorites fields. The traces elements and rare earth elements behaviour suggests that the protolith was developed in an arc environment with enrichment in light traces elements and impoverishment of heavy traces, with affinity to magnesian magmas. The U/Pb zircons age is 277.3± 3.0 Ma (Permian), it was interpreted as the granitoide’s intrusion age. This age and the geochemistry analysis allow to suggest the existence of a plutonic arc of continental margin, related to subduction, which intruded the oldest basement that was submitted to regional metamorphism during the Triassic; affecting the whole unit, besides of giving a metamorphism age of 236.4 ± 6.6 Ma, similar to Cajamarca Complex.
Keywords
Petrography, Geochemistry, metamorphism, Nechi Gneiss, Petrografía, geoquímica, metamorfismo, Neis de Nechí