Ediacaran - Earliest Cambrian arc-tholeiite and adakite associations of the Malcocinado Formation (Ossa-Morena Zone, SW Spain): Juvenile continental crust and deep crustal reworking in northern Gondwana

Abstract

The Ossa-Morena Zone of the SW Iberian Massif encloses an arc-related assemblage ascribable to subduction processes that encompass the latest Cryogenian-early Cambrian. It started ca. 645 Ma ago and is recorded by coeval N-MORB and E-MORB type magmatic rocks. Mature stages of the arc generated the Malcocinado Formation volcano-sedimentary complex. We study 15 coherent andesite units (usually lava flows) from this unit with a focus on their whole-rock geochemical and isotopic signatures and their geochronology. Lavas consist of sub-alkali andesites and basaltic andesites enriched in Th and LREE, depleted in P, Nb and, to a lesser extent, in Ti, Zr and Yb. Their εNd(t) – Mg# relations disclose three igneous associations: 1) arc-tholeiites derived from fluid-modified garnet lherzolites; 2) silica-poor adakites generated by partial melting of basic/ultrabasic sills; and 3) hybrid rocks generated by mixing of evolved melts from the previous associations plus variable assimilation of volcano-sedimentary country rocks. Formation of youngest lavas is dated ca. 534 Ma. It involved partial melting of enriched garnet lherzolite accompanied by deep crustal reworking of previous, subduction-related igneous rocks. Subduction was either still active at ca. 534 Ma or, likely, stopped some time before. The relationships with regional low-pressure metamorphism and ophiolite emplacement point to interaction between oceanic ridge/transform lithosphere and the trench at a steep subduction scenario. We envisage an active margin involving Gondwana hyper-extended crust North of the West African Craton and the Tuareg Shield.