INGEOSUR   20376
INSTITUTO GEOLOGICO DEL SUR
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Evolution of a high-grade ductile shear zone through the analysis of textural and compositional changes. An example from the Famatinian Orogen, Sierra de la Huerta, San Juan, Argentina.
Autor/es:
BRIGIDA CASTRO DE MACHUCA; SERGIO DELPINO; ABERRA MOGESSIE; SANDRA PONTORIERO; LORENA PREVILEY; ERNESTO BJERG
Lugar:
Río Cuarto, Córdoba, Argentina.
Reunión:
Congreso; 14º Reunión de Tectónica; 2009
Institución organizadora:
Universidad Nacional de Río Cuarto
Resumen:
A lower granulite to upper amphibolite facies ductile shear zone with NNE trending and E dippinglower granulite to upper amphibolite facies ductile shear zone with NNE trending and E dipping mylonitic foliation has been recognized in the Arenosa Creek, Sierra de La Huerta, province of San Juan,foliation has been recognized in the Arenosa Creek, Sierra de La Huerta, province of San Juan, affecting an outcrop of quartzose metagabbro-metadiorite (PI-Hbl-Bt-Qtz±Cpx±Opx-opaques) which is part of the Famatinian magmatic arc of the Western Sierras Pampeanas. Kinematic analysis of the Arenosa Creek Shear Zone (ACSZ) indicates reverse-sense of shear with a minor component of dextral displacement. Thisan outcrop of quartzose metagabbro-metadiorite (PI-Hbl-Bt-Qtz±Cpx±Opx-opaques) which is part of the Famatinian magmatic arc of the Western Sierras Pampeanas. Kinematic analysis of the Arenosa Creek Shear Zone (ACSZ) indicates reverse-sense of shear with a minor component of dextral displacement. Thismagmatic arc of the Western Sierras Pampeanas. Kinematic analysis of the Arenosa Creek Shear Zone (ACSZ) indicates reverse-sense of shear with a minor component of dextral displacement. Thisreverse-sense of shear with a minor component of dextral displacement. This is compatible with the kinematics of other ductile deformation zones in the area, coliecUvely forming a network.compatible with the kinematics of other ductile deformation zones in the area, coliecUvely forming a network.. On the basis of paragenetic associations and deformation textures, at least three overprintedbasis of paragenetic associations and deformation textures, at least three overprinted deformation events can be differentiated in the ACSZ: a) a relict ductile event developed entirely within theevents can be differentiated in the ACSZ: a) a relict ductile event developed entirely within the granulite facies field , at temperatures exceeding 700 ·C and in the probable range of pressures between 5 tofacies field , at temperatures exceeding 700 ·C and in the probable range of pressures between 5 to 6 kbar; b) a two-stage mylonitic deformation event occurring under condiUons of high to medium-grade metamorphism, at temperatures that ranges between >550-600 ·C and pressures in the interval 6-7 kbar; c); b) a two-stage mylonitic deformation event occurring under condiUons of high to medium-grade metamorphism, at temperatures that ranges between >550-600 ·C and pressures in the interval 6-7 kbar; c)at temperatures that ranges between >550-600 ·C and pressures in the interval 6-7 kbar; c) a brittle deformaUon event developed at low greenschist facies, probably at temperatures below 400 ·C.deformaUon event developed at low greenschist facies, probably at temperatures below 400 ·C. Evidences of relict high-grade ducUle event are well preserved in the quartzose metagabbm-metadiorite (protolith) notwithstanding it is affected by a stronger mylonitic high- to medium-grade ductile deformation thatrelict high-grade ducUle event are well preserved in the quartzose metagabbm-metadiorite (protolith) notwithstanding it is affected by a stronger mylonitic high- to medium-grade ductile deformation thatnotwithstanding it is affected by a stronger mylonitic high- to medium-grade ductile deformation that sensibly obliterate them, generating typical mylonitic fabrics. Significant modal and chemical changes are appreciated between the proto lith and its myloniticmylonitic fabrics. Significant modal and chemical changes are appreciated between the proto lith and its myloniticmodal and chemical changes are appreciated between the proto lith and its mylonitic products. Nevertheless, mineral compositions of main components such as plagioclase remain essentially constant. Amphibole composition shows a slight decreasing in the Mg/(Mg+Fe2+) ratio as deformation. Nevertheless, mineral compositions of main components such as plagioclase remain essentially constant. Amphibole composition shows a slight decreasing in the Mg/(Mg+Fe2+) ratio as deformationdecreasing in the Mg/(Mg+Fe2+) ratio as deformation increases. The textural and compositional changes experienced by the quartzose metagabbro-meta diorite during mylonitization allow reconstructing its tectono-metamorphic evolution as well as the metamorphicncreases. The textural and compositional changes experienced by the quartzose metagabbro-meta diorite during mylonitization allow reconstructing its tectono-metamorphic evolution as well as the metamorphicallow reconstructing its tectono-metamorphic evolution as well as the metamorphic conditions achieved. Age dating ('°Ari"'Ar hornblende) of the mylonite formed at upper amphibolite-facies conditions gives deformation ages that vary from 441.9±1.9 Ma to 438.7±1.9 Ma. These ages are internally consistent and compatible broadly with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.dating ('°Ari"'Ar hornblende) of the mylonite formed at upper amphibolite-facies conditions gives deformation ages that vary from 441.9±1.9 Ma to 438.7±1.9 Ma. These ages are internally consistent and compatible broadly with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.that vary from 441.9±1.9 Ma to 438.7±1.9 Ma. These ages are internally consistent and compatible broadly with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.in time and space.