Temporal geochemical evolution of Neogene volcanism in northern Baja California (27°–30° N): Insights on the origin of post-subduction magnesian andesites

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“…10 and 14C), which has a much lower Nb-Ta abundance, positive Ba and Sr anomalies, no Eu anomaly, and stronger depletion toward the heavy REEs (La/Yb n = 13.3). The composition of this sample is similar to those of samples from Sierra Libre, in the central Sonora coast (Till et al, 2009), and also coincides relatively well with that of clinopyroxene-bearing andesite from Santa Rosalía (Conly et al, 2005), and Mg-andesite reported for the Borja and Jaraguay volcanic fi elds (Pallares et al, 2008) Miocene silicic rocks points to variable magma generation processes acting during this event.…”

“…Oligocene and Early Miocene silicic to bimodal volcanism of the SMO silicic large igneous province in mainland Mexico, as well as the Middle Miocene intermediate Comondú Group in Baja California, has for many years simply been interpreted as the expression of suprasubduction arc magmatism (e.g., Sawlan and Smith, 1984;Sawlan, 1991;Hausback, 1984;Umhoefer et al, 2001), whereas the appearance of more heterogeneous magma types (e.g., adakites, Nb-enriched basalts, magnesian andesites) after 12 Ma has been associated with the development of the oblique-divergent plate boundary (see reviews in Pallares et al, 2008;Calmus et al, 2010). Implicit in this view is the idea that a given tectonic setting should be promptly and clearly refl ected in magma composition.…”

Fossil slabs attached to unsubducted fragments of the Farallon plate

“…2). Although there is debate about their origin (24,25), the bajaites are produced where the top edge of the slab with the basaltic crust lies directly below at a depth typical for the formation of arc volcanism. Most of the high-Mg andesites were produced millions of years after subduction ceased.…”

High-Mg adakitic rocks and their complementary cumulates formed by crystal fractionation of hydrous mafic magmas in a continental crustal magma chamber

“…They are enriched in LREEs, LILEs and Pb, and are depleted in HFSEs. Such island arc-type trace element patterns in basalts and basaltic andesites are attributable to the involvement of recycled crustal components in the mantle source (Grove et al, 2002) or partial melting of enriched lithospheric mantle (Pallares et al, 2008;Xu et al, 2004). Geological observations suggest that the northern part of the NCC was tectonically in a post-collisional extensional regimes in the early Mesozoic (Ma et al, 2012) after the final closure of the Palaeo-Asian Ocean at 300-250 Ma (Xiao et al, 2003).…”