High-Mg extrusive rocks from the Romeral Zone ophiolites in the southewestern Colombian Andes

Abstract

In the southwestern Colombian Andes, high-Mg extrusive rocks occur within the Mesozoic (?Late Cretaceous) ophiolitic fragments from the Romeral fault zone, a major suture between the continental domain of the Central Cordillera, and the oceanic domain of the Western Cordillera. The high-Mg rocks include picrites and picritic basalts, and are associated in places with peridotites showing transitional to extrusive textures. Plagioclase-free picrites characterized by quench textures have been found in two localities, and occur as pillow-lava (Rio Boloblanco) or as clasts of monogenic and polygenic volcanic breccias (El Encenillo). They show similar olivine-phyric micro-spinifex textures with groundmass consisting of hopper and plate olivine (or Ca-poor pyroxene at El Encenillo), skeletal clinopyroxene and glass. Hyaloporphyritic texture occurs in the Rio Boloblanco picrites at pillow rims. Chemically, these picrites are characterized by MgO contents of >20%, and are distinguished by the content of TiO 2 and incompatible elements in a high-Ti type (Rio Boloblanco) and a very high-Ti type (El Encenillo), the latter particularly remarkable for the high contents of Ti, Zr and Nb which give evidence of an alkaline tendency. Phase chemistry is consistently different between the two picrite types, but comparably high Al contents (up to 14%) of skeletal pyroxenes suggest that the magmas were erupted and cooled in similar conditions. Olivine-phyric picritic rocks characterized by microlitic non-quenched groundmasses occur as clasts of volcanic breccias at El Encenillo (picritic basalts), and are present with comparatively high frequency within major (Los Azules) and minor (Upper Chapungo Valley) ophiolitic fragments from the Romeral Zone, either as lavas and volcanigenic breccias. These rocks are mostly richer in olivine than the quench-textured picrites, contain modal plagioclase, and relatively low-Al pyroxenes. As regards chemistry, a high-Ti and very low-Al group (Los Azules, Upper Chapungo Valley) and a very high-Ti group (El Encenillo) can also be distinguished within these picrites. A comparison with the ultramafic volcanic rocks from Gorgona Island, in the nearby Pacific domain off Colombia, among which the unique Phanerozoic komatiites were discovered, indicates close similarities between the Rio Boloblanco picrites and some Gorgona Island komatiites (olivine-phyric komatiites with micro-spinifex groundmass) regarding their petrochemical features. It is suggested that they record a unique event related to the same tectonic environment, and consequently that they belong to the same structural unit, developed in the Late Cretaceous and extending from continental Colombia to the Pacific domain. This is in contrast with previous subdivisions of the ophiolitic complexes from southwestern Colombia into distinct age groups progressively younger from the Central Cordillera toward the Pacific Ocean.