A PROPOSAL TO SPLIT-OFF THE SO-CALLED ‘SILGARÁ FORMATION’ (SANTANDER MASSIF, COLOMBIA) SUPPORTED ON DETRITAL U-Pb ZIRCON AGES
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Universidad Industrial de Santander
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La Formación Silgará (s.l.) aforante en el Macizo de Santander (Cordillera Oriental de Colombia) en la franja Matanza-Cachirí, presenta litologías metamórfcas fundamentalmente metapelíticas y metasemipelitas, las cuales alcanzaron el máximo pico de metamorfsmo en la facies Esquistos Verdes. Circones detríticos con edades U-Pb entre 906,5±10,5 Ma y 1.610,3±9,8 Ma, permiten determinar que la máxima edad de depositación del protolito de ésta unidad es Neo-proterozóico (Toniano). Contrariamente, la franja de rocas metamórfcas Piedecuesta-Aratoca (relacionada en trabajos previos con la denominada Formación Silgará s.l., propuesta desde la década de los 70s), presentan dos grupos de litologías diferentes: una compuesta fundamentalmente de metapsamitas, semipelitas y metabasitas (hacia la base), las cuales alcanzaron el máximo pico de metamorfsmo en la facies anfbolita; y otra de metapsamitas, semipelitas hacia la parte superior (en facies sub-esquistos verdes). Circones detríticos con edades U-Pb entre 506,7±9,3 Ma y 2.586,9±10,2 Ma, en cuarcitas de la facies anfbolita, apuntan a que la máxima edad de depositación del protolito de estas litologías metamórfcas estaría entre el Cámbrico temprano (Terreneuviano) a Cámbrico medio; mientras que en las cuarcitas de la facies sub-esquistos verdes, las edades entre 451,6±7,7 Ma y 1.611,5±13,6 Ma, sugieren que su máxima edad de depositación del protolito es Ordovícico tardío (Katian) y una historia paleogeográfca y sedimentológica muy diferente, en comparación con las litologías previamente referidas.
Apoyados en las diferencias antes referidas, se propone escindir la Formación Silgará (s.l.) en tres unidades diferentes: Esquistos del Silgará s.s. (unidad fundamentalmente pelítica y semipelita), Esquistos del Chicamocha (unidad compuesta fundamentalmente de metapsamitas, metapelitas y metabasitas; y posiblemente litologías calco-silicatadas?, presentes en otros franjas metamórfcas del MS) y Filitas de San Pedro (unidad compuesta fundamentalmente de metapsamitas, metapelitas; y localmente muy posiblemente litologías meta-volcanoclásticas?). El pico de metamorfsmo de las unidades Esquistos del Silgará (s.s.) y del Chicamocha, se relacionan con el evento orogénico principal Fammatiniano (localmente referido como Quetame-Caparonensis, de edad Ordovícico temprano); mientras que el pico de metamorfsmo de la unidad Filitas de San Pedro, se relacionaría con el evento orogénico menor Fammatiniano (de edad Silúrico). Aunque los unidades Esquistos del Silgará (s.s.) y del Chicamocha, al parecer alcanzaron el máximo pico de metamorfsmo durante el evento orogénico principal Fammatiniano, aquí se propone su escisión, considerando sus diferencias litológicas (además de las diferencias ya referidas). No obstante, se recomienda un análisis comparativo más detallado entre las diferentes franjas metamórfcas esquistosas presentes en el Macizo de Santander (en términos de sus litologías predominantes y sus máximas edades estratigráfcas, entre otros aspectos), para fnes de soportar o descartar ésta división. En lo referente a la unidad Filitas de San Pedro (aquí propuesta), su escisión (individualización) es muy evidente y necesaria, debido a las diferencias litológicas y edad de metamorfsmo, respecto a las unidades previamente referidas. No se descarta la existencia de eventos tectono-termales más jóvenes en el Macizo de Santander (especialmente de tipo dinamo-térmico), considerando la presencia localizada de rocas sedimentarias, de edad Paleozóico tardío, afectadas por pizarrosidad.
The metamorphic lithologies from the Silgará Formation (s.l.) outcropping along the Matanza-Cachirí strip in the Santander Massif (Colombian Eastern Cordillera) are mainly constituted by metapelites and metasemipelites that reached the thermal metamorphic peak in the range of green schist facies. Detrital zircons from these metamorphic lithologies yielded U-Pb ages from 906.5 ± 10.5 to 1610.3 ± 9.8 Ma, suggesting a maximum age of deposition at Neoproterozoic time. On the other hand, the Piedecuesta-Aratoca metamorphic rocks strip (also linked previously to the so-called Silgará Formation) is composed by two groups of lithologies: The lower sequence which is constituted mainly by meta-psamites, semipelites and metabasites with the thermal metamorphic peak at amphibolite facies and; the upper sequence which are basically meta-psamites and semipelites that reached the thermal metamorphic peak at sub-green schist facies. Detrital zircons from the quartzites in amphibolite facies rocks yielded U-Pb ages from 506.7± 9.3 to 2586.9±10.2 Ma, suggesting a maximum age of deposition at Early to Middle Cambrian time; whereas U-Pb zircon ages determinated from the meta-sandstones from the upper sequence yielded ages from 451.6±7.7 to 1611.5±13.6 Ma, suggesting a maximum age of deposition at Late Ordovician (Katina) time. These geochronology data point out that these both sequences have different paleogeographic and sedimentological history. Supported by the aforementioned differences, it is proposed here to split-off the older Silgará Formation (s.l.) in the following three different units: the Silgará schists s.s.(constituted by metapelitic and semipelitic rocks), the Chicamocha schists (mainly constituted by metapsammites, metapelites and metabasites, and possibly metacalcsilicates? rocks outcropping in some others metamorphic strips from the Santander Massif) and the San Pedro phyllites (constituted by metapsamites and metapelites and possibly metavolcanoclastic lithologies?). The thermal metamorphic peak of the Silgara (s.s.) and Chicamocha Schists units must be related to the main Fammatinian orogenic event (locally known as Quetame-Caparonensis, early Ordovician in age), whereas the metamorphic peak for the San Pedro Fillites unit should be related to the minor Fammatinian orogenic event (Silurian in age). Despite both The Silgará Schists (s.s.) and Chicamocha Schists units reached probably the thermal metamorphic peak during the Fammatinian Orogeny, here is proposed to split-off these units, because of these lithological differences (among some others aforementioned characteristics). Anyway, in order to avoid or to support this idea, there is a need to develop new studies, involving more metamorphic strips in order to identify better their lithologies and their maximum deposition ages. The split-off of the San Pedro Phyllites Unit from the Silgara Formation (s.l.) is evident and necessary task, considering their lithologies, metamorphic ages, among others features, compared with the Silgara Schists (s.s.) and Chicamocha Schists Units.Younger tectono-thermal events (mainly dynamo-thermal) are not ruled out, taking into account the local presence of late Paleozoic sedimentary rocks in the Santander Massif, affected by a slaty cleavage
The metamorphic lithologies from the Silgará Formation (s.l.) outcropping along the Matanza-Cachirí strip in the Santander Massif (Colombian Eastern Cordillera) are mainly constituted by metapelites and metasemipelites that reached the thermal metamorphic peak in the range of green schist facies. Detrital zircons from these metamorphic lithologies yielded U-Pb ages from 906.5 ± 10.5 to 1610.3 ± 9.8 Ma, suggesting a maximum age of deposition at Neoproterozoic time. On the other hand, the Piedecuesta-Aratoca metamorphic rocks strip (also linked previously to the so-called Silgará Formation) is composed by two groups of lithologies: The lower sequence which is constituted mainly by meta-psamites, semipelites and metabasites with the thermal metamorphic peak at amphibolite facies and; the upper sequence which are basically meta-psamites and semipelites that reached the thermal metamorphic peak at sub-green schist facies. Detrital zircons from the quartzites in amphibolite facies rocks yielded U-Pb ages from 506.7± 9.3 to 2586.9±10.2 Ma, suggesting a maximum age of deposition at Early to Middle Cambrian time; whereas U-Pb zircon ages determinated from the meta-sandstones from the upper sequence yielded ages from 451.6±7.7 to 1611.5±13.6 Ma, suggesting a maximum age of deposition at Late Ordovician (Katina) time. These geochronology data point out that these both sequences have different paleogeographic and sedimentological history. Supported by the aforementioned differences, it is proposed here to split-off the older Silgará Formation (s.l.) in the following three different units: the Silgará schists s.s.(constituted by metapelitic and semipelitic rocks), the Chicamocha schists (mainly constituted by metapsammites, metapelites and metabasites, and possibly metacalcsilicates? rocks outcropping in some others metamorphic strips from the Santander Massif) and the San Pedro phyllites (constituted by metapsamites and metapelites and possibly metavolcanoclastic lithologies?). The thermal metamorphic peak of the Silgara (s.s.) and Chicamocha Schists units must be related to the main Fammatinian orogenic event (locally known as Quetame-Caparonensis, early Ordovician in age), whereas the metamorphic peak for the San Pedro Fillites unit should be related to the minor Fammatinian orogenic event (Silurian in age). Despite both The Silgará Schists (s.s.) and Chicamocha Schists units reached probably the thermal metamorphic peak during the Fammatinian Orogeny, here is proposed to split-off these units, because of these lithological differences (among some others aforementioned characteristics). Anyway, in order to avoid or to support this idea, there is a need to develop new studies, involving more metamorphic strips in order to identify better their lithologies and their maximum deposition ages. The split-off of the San Pedro Phyllites Unit from the Silgara Formation (s.l.) is evident and necessary task, considering their lithologies, metamorphic ages, among others features, compared with the Silgara Schists (s.s.) and Chicamocha Schists Units.Younger tectono-thermal events (mainly dynamo-thermal) are not ruled out, taking into account the local presence of late Paleozoic sedimentary rocks in the Santander Massif, affected by a slaty cleavage
Keywords
Colombia, Santander Massif, metamorphism, detrital zircon U-Pb geochronology, Colombia, Macizo de Santander, metamorfismo, geocronología U-Pb en circones detríticos