Metamorphic evolution of Raspas complex (Ecuador) and its relation with a J-K belt of melanges in NW of the South American plate.

Abstract

<p>High-pressure complexes along the Earth's surface provide evidence of the processes involved in both the crystallization of rocks in the subduction channel and its exhumation. Such processes are key to understand the dynamics and evolution of subduction zones and to try to reconstruct P-T trajectories for these complexes.</p><p>Previous studies on the Raspas complex (southern Ecuador) agree to state that it is composed of metamorphic rocks, mainly blueschists and eclogites, containing the mineral assemblage: glaucophane + garnet + epidote + omphacite + white mica + rutile ± quartz ± apatite ± pyrite ± calcite; which stabilized in metamorphic conditions of high pressure and low temperature. Additionally, the Raspas Complex has been genetically related to accretion and subduction processes of seamounts, which occurred in South America during the Late Jurassic - Early Cretaceous interval; and the exhumation of the complex was related to subduction channels. However, the evidence presented in the existing literature makes little emphasis on the reconstruction of thermobarometric models for the rocks of this complex.</p><p>By combining petrographic observations, whole-rock chemistry, and mineral chemistry in this work; it was possible to determine that pressure values of 10 ± 3 Kbar and temperature values of 630 ± 30 ° C, (obtained by simulations with THERMOCALC®) correspond to an event of retrograde metamorphism, suffered by the complex during its exhumation. This theory is complemented by the specific textures (that suggest this retrograde process) observed during petrographic analysis, such as amphibole replacing pyroxene, garnet chloritization, plagioclase crystallization and rutile replacement by titanite.</p><p>The results obtained, together with the thermobarometry data published for the Arquía complex in Colombia, allow us to establish a P-T trajectory, that may suggest a genetic relationship between these two complexes as a result of the tectonic processes associated with an active subduction margin that affected the NW margin of the South American plate at the end of the Jurassic.</p><p> </p>

Crystal-Chemical and Structural Characterization of Omphacite in High-Pressure Eclogites From the Arquía Complex on Southwestern Pijao, Central Cordillera (Colombian Andes)

“…Na-pyroxene omphacite is a high-pressure mineral index of the eclogite metamorphic facies. According to previous studies (e.g., Maresch and Abraham, 1981;Bakirov et al, 1998;Miyagi and Takasu, 2005;Kabir and Takasu, 2010;Arrieta-Prieto et al, 2020), the average Na content in "fresh" eclogites worldwide is about 8.0 wt%, whereas in the present study we report an average Na content of 5.0 wt%. This fact could be interpreted as a product of the intense retrogradation and chemical imbalance exhibited by the Arquía Complex, which in light of these results becomes a highly limiting factor in geochemical, thermobarometric and geochronological models.…”

“…Eclogites are widely spread in metamorphic belts of high-pressure and/or ultrahigh-pressure worldwide (e.g., Miyagi and Takasu, 2005;Kabir and Takasu, 2010;Nakano et al, 2010;Li et al, 2016;Arrieta-Prieto et al, 2020;Xie et al, 2020;Feng et al, 2021;Schorn et al, 2021;Wang et al, 2021). They are important witnesses of subduction-collision processes (e.g., Sizova et al, 2010;van Hunen and Moyen, 2012).…”