Metamorphic evolution and exhumation of high-pressure assemblages of the Cordillera Real (Ecuador)
Abstract
Detailed petrological investigation of garnet-bearing assemblages from three areas along the strike of the Cordillera Real, Ecuador, reveal both similarities and differences in a generally complex metamorphic evolution. Metasedimentary packages in Monte Olivo, Papallacta, and Baños areas each display texturally-early low T/high P assemblages characterized by Ph (XCel≤0.5)+Qz±Cld±Pg±Ky±Rt assemblages included in garnet interiors. Phase equilibria models typically converge around T=550±50 °C and P=1.7±0.2 GPa, placing the early metamorphism within the eclogite facies. Packages from the Agoyan unit commonly have thin rims of compositionally and texturally-distinct garnet, with inclusions of Ms-Ep-Chl-Ttn, surrounding early HP garnet cores. Albite is not observed as inclusions in garnet, but commonly forms large porphyroblasts that include the matrix foliation. These assemblages yield similar temperatures (~500 °C), but distinctly lower pressures (0.9-1.0 GPa), compared with that preserved in the garnet cores. The Cuyuja unit has only a single garnet type, commonly with spiral inclusion trails, and yields P-T conditions of 500±40 °C and P=1.2±0.1 GPa.
Preliminary LA-ICP-MS U-Pb geochronology on titanite replacing rutile or within the matrix foliation by, indicates a distinct difference in the timing of exhumation across the strike of the Cordillera. The pooled lower intercept age from Agoyan unit samples is 234±7 Ma, whereas Cuyuja unit samples yield 136±10 Ma. The former ~234 Ma age is coeval with or slightly predates the timing of high T/low P metamorphism in other parts of the cordillera, potentially associated with post-thickening extension. The ~136 Ma age is coeval with eclogite-facies metamorphism in the Raspas complex of southwest Ecuador, emplacement of the Peltetec ophiolite along the western edge of the Ecuadorian Cordillera, and the presumed collision of the Alao volcanic arc. Thus, there appears to be two distinct phases of low T/high P metamorphism (collision/thickening) recorded within the Cordillera Real, each of which correspond with established tectono/metamorphic events elsewhere in the norther Andes. Additional petro-chronological work is in progress, and should enable the complex metamorphic evolution within the Cordillera Real to be more clearly defined.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0126F
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY