Unraveling an antique subduction process from metamorphic basement around Medellín city, Central Cordillera of Colombian Andes

Abstract

In the surroundings of Caldas and El Retiro cities (Colombia) metamorphic rocks derived from basic and pelitic protoliths comprise the Caldas amphibole schist and the Ancón schist respectively. Subordinated metamorphosed granite bodies (La Miel gneiss) are associated to these units, and The El Retiro amphibolites , migmatites and granulites crops out eastwards of these units, separated by shear zones. The Caldas amphibole schist and the Ancón schist protoliths could have been formed in a distal marine reduced environment and amalgamated to the South American continent in an apparent Triassic subduction event. The El Retiro rocks are akin to a continental basement and possible include impure metasediments of continental margin , whose metamorphism originated granulite facies rocks and migmatites as a result of the anatexis of quartz–feldspathic rocks. The metamorphism was accompanied by intense deformation, which has juxtaposed both migmatites and granulite blocks. Afterward, heat and fluid circulation associated with the emplacement of minor igneous intrusions resulted in intense fluid–rock interaction, variations in the grain size of the minerals and, especially, intense retrograde metamorphic re-equilibrium. Thermobarometric estimations for the Caldas amphibole schist indicate metamorphism in the Barrovian amphibolite facies . The metamorphic path is counter-clockwise, but retrograde evolution could not be precisely defined. The pressures of the metamorphism in these rocks range from 6.3 to 13.5 kbar, with narrow temperature ranging from 550 to 630 °C. For the Ancón schist metapelites the P – T path is also counter-clockwise, with a temperature increase evidenced by the occurrence of sillimanite and the cooling by later kyanite . The progressive metamorphism event occurred at pressures of 7.6–7.2 kbar and temperatures of 645–635 °C for one sample and temperature between 500 and 600 °C under constant pressure of 6 kbar. The temperature estimated for these rocks varies between 400 and 555 °C at pressures of 5–6 kbar in the retrograde metamorphic path. The El Retiro rocks evidence strong decompression with narrow variation in temperature, showing pressure values between 8.7 and 2.7 kbar at temperatures of 740–633 °C. These metamorphic fragments of the basement in the Central Cordillera of the Colombian Andes could represent a close relationship with an antique subduction zone . ► The protoliths of the studied rocks correspond to a volcano-sedimentary sequences intercalated with pelitic and C–Al-rich layers. ► The high-grade event exhibits similar protoliths as described for the amphibolite facies units, indicating a more proximal environment. ► Caldas and Ancón rocks suggest an evolution in a subduction environment accompanied by the development of thrust shear zones. ► The El Retiro high-grade rocks suggest that an initial rifting event possibly occurs in a transpressional–transtensional regime.