Cordillera Of Colombian Andes Research Articles

El Escondido, a newly identified silicic Quaternary volcano in the NE region of the northern volcanic segment (Central Cordillera of Colombia)

El Escondido (1650m.a.s.l.) is a silicic Quaternary volcano recently identified on the Central Cordillera of Colombian Andes. It is located about 50km NE from the northern segment of active volcanoes. It has been recognized as a polygenetic pyroclastic-dome ring complex structure, whose eruptive products derived from a crater-like semi-circular depression of ~1.7km in diameter. This crater formed at the contact of the Granitic Florencia Stock (54.9±1.8my) with the Paleozoic metamorphic basement, and at the intersection of the NNESSW trending Palestina fault system with an EW trending fault. The preserved volcano morphology and the volcanic succession associated with it were defined as the El Escondido volcanic center. The eruptive products are alternating silicic (mostly dacitic) pyroclastic density current deposits, pumice fall deposits and ballistic projectiles, which suggest high explosiveness. Two 14C radiogenic dates obtained on carbonized wood fragments of the middle part of the eruptive sequence displayed ages of 36,030±380 BP and 33,550±280 BP suggesting two major eruptive episodes of the volcano about 2500years apart. The latest eruptive activity is inferred to be the emplacement of lava domes to the south of the crater-like structure, which are responsible for the partial destruction of the pyroclastic ring in that sector. The products emitted by El Escondido correspond to medium K andesites to dacites of the calc-alkaline series and petrographically to amphibolic andesites. Towards the western and eastern sector of the volcano, there are chlorinated thermal waters with temperatures up to 45°C. The first monitoring of the volcanic activity, carried out by the Colombian Geological Survey (CGS) through the Volcanological and Seismological Observatory of Manizales (VSOM), has not detected so far unusual volcano-seismic activity. The previously unreported El Escondido volcano represents one of the northernmost occurrence of Quaternary volcanism in the whole Andean cordillera, thus, its study may have wide implications regarding the geodynamics of the region and subduction zone, the correct assessment of geologic hazards and the identification of natural resources, like geothermal reservoirs and mineral deposits of economic interest.

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

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.