Osmium and lead isotope investigation of magmas within the long-lived Pichincha Volcanic complex from the Northern Andean Volcanic zone (Ecuador)

Abstract:

The study of arc magmas most often stumbles on deciphering both the nature and the composition of the subduction components involved in magma genesis. In Ecuador, the subduction of the Carnegie ridge, appears as a key parameter accounting for the temporal chemical changes highlighted in the quaternary lavas, whose composition shifted from calc-alkaline to adakitic [1-3]. Moreover, the adakitic signature is only observed in an area located above the Carnegie ridge subduction [4, 5]. Re-Os and Pb isotopic compositions of lavas from the Pichincha volcano were investigated, in order to document the nature and origin of this transition. The Pichincha Volcanic Complex consists of (1) an ancient, eroded edifice (the Rucu, 1.1-0.15 Ma), and (2) a younger edifice (Guagua, 60-11 ka). This structural evolution is correlated with significant variations of trace element abundances, mainly reflecting major processes of magmatic mixtures [1, 2].

In addition to Pichincha lavas, we also analysed the subducted oceanic basalts and sediments (Amadeus campaign), as well as samples of the Pichincha basement. In a ¹⁸⁷Os/¹⁸⁸Os vs. 1/Os diagram, Pichincha basalts define a positive trend ranging between an unradiogenic Os component (i.e., peridotitic mantle) and a radiogenic basaltic Os component with low Os content, which is consistent with Carnegie ridge basalt composition. On another hand, the sediments and basement samples plot away from this trend, indicating that crustal contamination contribution remained insignificant. This conclusion is also supported by the low ²⁰⁷Pb/²⁰⁴Pb ratios in the Guagua compared to Rucu lavas. Finally, in the Guagua lavas, the high ¹⁸⁷Os/¹⁸⁸Os ratios positively correlate with the adakitic character (high Sr/Y and La/Yb).

[1] Samaniego S. et al. CMP 160 (2010) 239-260

[2] Schiano P. et al. CMP 160 (2010) 297–312

[3] Hidalgo S. et al. Lithos 132-133 (2012) 180–192

[4] Bourdon E. et al. J. Petrol. 43 (2002) 199–217

[5] Martin H. et al. Lithos 198-199 (2014) 1-13