Arc to rift transitional volcanism in the Santa Rosalía Region, Baja California Sur, Mexico

Middle to late Miocene volcanic rocks in the Santa Rosalía basin region, Baja California Sur, record the transition from arc- to rift-related volcanism associated with the initial opening of the Gulf of California. New K–Ar age dates have led to the identification of three distinct sequences: (1) 24 to 13 Ma arc lavas of the Andesite of Sierra Santa Lucia (ASL) suite; (2) 13 to 10 Ma Santa Rosalía dacite (SRD), representing the final products of subduction-related volcanism; and (3) a diverse rift-related volcanic assemblage that erupted between 11 and 7.7 Ma, which consists of: (1) 11 to 9 Ma Boléo basalts (BTB) to basalt andesites (BBA); (2) the El Morro tuff (EMT), a 10.5- to 7.9-Ma lapilli tuff to ignimbritic unit; and (3) 11 to 10 Ma clinopyroxene-dominant and 9 to 7.7 Ma hornblende-dominant high-K andesites of the Cerro San Lucas (CSL) suite. The BTB-BBA suite is equivalent to regional early rift tholeiitic basalts, while the CSL suite is representative of the regional high-Mg andesite (bajaite) lavas. K–Ar geochronology of EMT tuff has constrained the timing of initial basin sedimentation to after 8 Ma. All lava suites are calc–alkaline with arc-like signatures and are characterized by LILE and LREE enrichment and N-MORB-like patterns of HFSE and HREE. Such signatures are consistent with melts being derived from a mantle that was metasomatized by slab-derived aqueous fluids and silicic melts. Nb–Ta–Ti depletions that characterize the ASL and CSL suites are evidence of aqueous fluid metasomatism, since fluids were depleted in these elements due to residual rutile in the dehydrating slab. Evidence supporting the partial melting of an adakite-altered mantle comes from the relative depletion in HREE of CSL lavas and adakite-like characteristics of both SRD and CSL lavas. ASL lavas are generated from the partial melting of the sub-arc wedge after extensive slab-derived aqueous fluid metasomatism, followed by extensive calc–alkaline crystal fractionation (±amphibole accumulation) and variable degrees of crustal assimilation. SRD are interpreted to be slab melts that hybridized a significant mantle component. BTB, BBA and CSL rift lavas are the products of moderate degrees of partial melting of an adakite and aqueous fluid metasomatized mantle, with the principal control on melt chemistry being the extent of adakite contamination and degree of partial melting. Clinopyroxene-bearing CSL lavas appear to have undergone varying degrees of mixing with co-eruptive SRD melts