New evidence on the nature of the Frontal Cordillera ophiolitic belt — Argentina

New petrographic and chemical data of volcanic rocks from the Metales Belt, Rio de las Tunas area, in the Frontal Cordillera of Argentina are presented. The rocks display a broadly basaltic composition. Two varieties of basaltic dikes can be differentiated. One group, whose mineralogy comprise essentially tremolite-actinolite, plagioclase, pyroxene and limited amounts of olivine as the main phases, display metamorphic textures. The other group show porphyritic textures consisting of zoned plagioclase and K-feldspar. Zoned orthopyroxenes are scarce. Chemically, the rocks are similar to the Caradocian basaltic rocks (the ofiolitas famatinianas) from the Precordillera, San Juan Province. REE chondrite-normalised patterns and overall abundances, along with other petrologic and geochemical similarities, suggest a common mode of origin for both groups of basaltic rocks. Their geochemical and petrographical characateristics are consistent with a continental-scale extension environment during Eopalaeozoic times that could include an oceanic rift in an early stage of development, a transitional ridge segment or oceanic intraplate magmas, similar to those erupted in oceanic islands.