Petrology,geochemistry and thermobarometry of the northern area of the Flamenco pluton,Coastal Range batholith,northern Chile. A thermal approach to the emplacement processes in the Jurassic andean batholiths |
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| Institution: | 1. Departamento de Geología, Universidad de Atacama, Copayapu 485, Copiapó, Chile;2. Unidad Asociada de Petrología Experimental, CSIC-Universidad de Huelva, Campus El Carmen, 21071 Huelva, Spain;1. CONICET-INGEOSUR, San Juan 670, UNS, Bahía Blanca, 8000, Argentina;2. Universidad Nacional del Sur, Departamento de Geología, San Juan 670, UNS, Bahía Blanca, 8000, Argentina;3. Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstrasse 18, D-70174, Germany;1. Departamento de Geografia, Universidade Federal de Minas Gerais, Antônio Carlos, 6627, Pampulha, Belo Horizonte, MG, 31270-901, Brazil;2. Departamento de Geografia, Universidade do Estado do Rio de Janeiro, Dr. Francisco Portela, 1470, Patronato, São Gonçalo, RJ, 24435-005, Brazil;3. Departamento de Geologia, Universidade Federal Fluminense, Passos da Pátria, 156, São Domingos, Niterói, RJ, 24210-240, Brazil;4. Instituto de Física, Universidade Federal Fluminense, General Milton Tavares de Souza, sala A1-01, Campus da Praia Vermelha, Gragoatá, Niterói, RJ, 24210-346, Brazil;1. Earth Sciences Dpt., Venezuelan Foundation for Seismological Research, FUNVISIS, Quinta Funvisis, El Llanito, Caracas, Venezuela;2. Dpt. of Geology, School of Geology, Mines & Geophysics, Universidad Central de Venezuela –UCV, Ciudad Universitaria, Los Chaguaramos, Caracas, Venezuela;3. Gabinete de Neotectónica y Geomorfología, INGEO-FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, San Juan Province, Argentina;4. CONICET, Argentina;5. INGEMA-FCN e IML-Universidad Nacional de Tucumán, Argentina;6. Chisñanco SRL, Argentina;7. Departamento de Geofísica y Astronomía, FCEFyN-UNSJ, Av. Ignacio de La Roza y Meglioli, 5400 San Juan, San Juan Province, Argentina;8. Formerly at FUNVISIS, Independent Geological Engineer, Venezuela;1. Museu Nacional/UFRJ, Quinta da boa Vista S/N – São Cristovão, 20940-040 Rio de Janeiro, RJ, Brazil;2. TEKTOS – Geotectonics Research Group, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A, Maracanã, 20559-900 Rio de Janeiro, RJ, Brazil;3. LAGIR – Isotope Geology and Geochronology Laboratory, Faculdade de Geologia, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524/4006-A, Maracanã, 20559-900 Rio de Janeiro, RJ, Brazil;4. Petrobras S.A. – Ilha do Fundão, Quadra 7, 20179-900, Rio de Janeiro, Brazil |
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| Abstract: | The Flamenco pluton is part of a N–S alignment of Late Triassic to Early Jurassic intrusive belt comprising the westernmost part of the Coastal Range batholith in northern Chile. The Jurassic-Cretaceous voluminous magmatism related to subduction in the western active continental margin of Gondwana is emplaced in the predominantly metasedimentary Paleozoic host-rocks of the Las Tórtolas formation, which in the northern area of the Flamenco pluton present an intense deformation, including the Chañaral mélange.Geochemically, the Flamenco pluton shows a wide compositional variability (SiO2 between 48wt % and 67wt %). Gabbros, Qtz-diorites and tonalites, mesocratic and leucocratic granodiorites are classified as calc-alkaline, calcic, magnesian and metaluminous magmatism. Flamenco granitoids define cotectic linear evolution trends, typical of magmatic fractionation processes. Geochemical trends are consistent with magmas evolved from undersaturated and low-pressure melts, even though the absence of transitional contacts between intrusive units precludes in-situ fractionation. Although some granodioritic samples show crossed geochemical trends that point to the compositional field of metasediments, and large euhedral prismatic pinnite-biotite crystals, typical Crd pseudomorph, are observed in contact magmatic facies, geochemical assimilation processes are short range, and the occurrence of host-rocks xenoliths is limited to a few meters from the pluton contact.A thermal approach to the emplacement process has been constrained through the thermobarometric results and a 2D thermo-numerical model of the contact aureole. Some Qtz-diorites and granodiorites located in the north area of the pluton exhibit granulitic textures as Hbl-Pl-Qtz triple junctions, poikiloblastic Kfs and Qtz recrystallization. The Hbl–Pl pairs have been used for the thermobarometric study of this metamorphic process, resulting granoblastic equilibrium temperatures between 770 and 790 °C, whereas Hbl–Pl pairs in domains that preserve the original igneous textures yield temperatures above 820 °C. This is characteristic of self-granulitization processes during the sequential emplacement of composite batholiths.In addition, the thermal modeling was used in order to compare the expected and observed thermal contact aureole of the intrusive body. Model P-T conditions have been established between 3 and 4 kbars (extracted from the thermobarometric results), and temperatures between 1159 °C (liquidus temperature for a tonalitic composition) and 992 °C (fixed at the rheological threshold of a 50% crystal fraction). The thermal modeling estimates a homogeneous contact aureole, where the established temperatures for the melting reactions in the host-rocks are located at distances between 200 and 650 m from the magma chamber boundary, whereas the temperatures for Crd stabilization extend 1500 m far from the contact in the case of the emplacement at liquidus temperatures and 4 kbars. According to field observations, the contact aureole presents a scarce development in the northern area of the Flamenco pluton, with few migmatite outcrops and less than 1 Km in thickness for Crd-schists. However, in the southern contact, partially melted rocks are described at distances up to 2 km from the Flamenco pluton boundary.The processes of self-granulitization and the differences between the observed and calculated (by the thermal modeling of one single pulse) contact aureole suggests a process of incremental emplacement for the Flamenco pluton, by accretion of magmatic pulses from north to south (in its current position), where the thermal maturity reached through the repeated magmatic intrusion generates a more extensive area of high-grade metamorphism. |
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| Keywords: | Thermal modeling Contact aureole Self-granulitization Coast Range batholith Northern Chile |
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