First evidence of lamprophyric magmatism within the Subbetic Zone (Southern Spain)

摘要

Two drillings carried out at Cerro Prieto (Province of Málaga), together with additional geophysical data, revealed the existence of an igneous body formed of rock-types previously unknown in the Subbetic zone. The recovered rocks, emplaced under hypoabyssal conditions, are predominantly porphyric, containing olivine, diopside and TiO2-rich phlogopite phenocrysts (up to 1-2 mm in size) within a micro-to-hypocrystalline groundmass componed of alkali-feldspar, diopside, phlogopite and abundant magnetite, and could be classified as “alkali minettes” lamprophyres. They contain numerous xenocrysts corroded by the magma and centimetric ultrafemic xenoliths deriving from the mantle. Clinopyroxenes yield crystallisation temperatures from about 1150 to 1320º C and pressures ranging from about 4 to 17 kbar, suggesting 50 km as the minimum depth of the magma sources. The chemical compositions of these lamprophyres are similar to intra plate alkali-basalts, derived from oceanic-island-basaltic-type highly metasomatized mantle sources. 40Ar/39Ar dating of a phlogopite mineral separate gave an age of 217±2.5 Ma. However, these rocks are more similar to the Permian alkaline lamprophyres in the Spanish Central System than to the Mesozoic dolerites and basalts widespread throughout the Subbetic Zone. We propose that the Cerro Prieto subvolcanic event represents the onset of a widespread magmatic phase induced by the post-Hercynian extensional tectonic activity that also affected the whole South-Iberian Paleomargin, within a geodynamic context that ultimately led to the opening of the Atlantic and the Neotethys oceans, accompanied by intrusion of basic magmas along their continental margins.