Regional O-, Sr-, and Pb-isotope relationships in late Cenozoic calc-alkaline lavas of the Andean Cordillera

Abstract

Quaternary-Recent volcanism in the Andes has occurred in three regions: 45–33°S, 28–16°S and 2°S–5°N, each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics. Isotope ratios of O and Sr are lowest and those of Pb least variable in the southern volcanic zone (SVZ) where medium-K lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent. The SVZ lavas were probably derived from an asthenospheric mantle source above a shallow Benioff zone. Parental basaltic magmas rose largely unmodified through thin continental crust, where differentiation occurred by low-pressure crystal fractionation without concurrent modification of isotopic composition. The slight enrichment of 206 Pb in the northern volcanic zone (NVZ), where the crust is thin and the Benioff zone deep, suggests a greater subduction-zone component for parental medium-K magmas. The slight 18 O enrichment suggests a small amount of lower crustal interaction. Isotope ratios of O and Sr are highest and those of Pb most variable in the central volcanic zone (CVZ). Parental magmas in the CVZ were probably generated within the same mantle source region as those in the SVZ and NVZ. Subsequently, during transit through the exceptionally thick continental crust of the central Andes, high-K magmas were produced by a combination of bulk contamination in the lower crust and later low-pressure fractional crystallization-assimilation processes in the upper crust which altered both chemical and isotopic compositions.

Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

“…A calculation shows that, in order to obtain the high LILE concentrations of Pichincha lavas, considering their global partition coefficients close to zero (= 0.01) and a mean degree of partial melting of 10 %, concentrations of Rb, Ba and K in the oceanic crust should be as high as 6 ppm, 80 ppm and 0.2 %, respectively. These are typical values for altered MORB [Tatsumi and Kogiso, 1997] Stern and Killian, 1996] and the Central Volcanic Zone [James, 1982;Hawkesworth et al, 1982;Harmon et al, 1984;Davidson et al, 1990] hand, the hypothesis of derivation of Pichincha adakites from the subducted altered oceanic crust is reinforced by their Sr isotopic signature close to 0.7040 and also typical of altered MORB [Spooner, 1976;Tatsumi and Kogiso, 1997]. A contribution of the subducted sediments to the source of Pichincha lavas could alternatively explain the enrichments in the more incompatible elements and particularly in Ba.…”

Geochemistry and petrology of lavas from San Jose volcano, Southern Andes (33.DEG.45'S).

“…Compared with the andesites, the basaltic an desites (SJ 4-1) is depleted in LREE (La equals to 60 X chondrites) but similar to the first four andesites in HREE. All the samples have similar (Kuno 1968) and solid line is the boundary between the tholeiitic and calc-alkaline fields proposed by Irvine and Baragar (1971 Isotopic studies carried out during the past ten years reveal that while the Central Andes Pliocene-Quaternary calc-alkaline volcanic rocks are characterized by high 87Sr/86Sr ratios (0.7049-0.7100; James, 1981;Deruelle et al, 1983;Notsu and Lajo, 1984;Harmon et al, 1984), the Southern Andes volcanics, south of 360S, have relatively low 87Sr/86Sr ratios (0.7035-0.7045; Munizaga and Mantovani, 1976; The San Jose samples fall on the crystal fractionation line, suggesting that their parental magmas were subjected to a crystal fractionation process involving, at least, plagioclase, clinopyroxene and hornblende. The intersection of both lines suggests that the primary magmas were generated by a low degree of melting (2%) of mantle peridotite.…”

“…Table 4. Rb-Sr isotopic compositions of San Jose Klerkx et al, 1977;Drake, 1981;Stern, 1982;volcanic rocks Hickey et al, 1982;Deruelle et al, 1983;Harmon et al 1984; According to present knowledge, the 87Sr/ 86Sr ratios of the Southern Andes basaltic and andesitic rocks are relatively high (0.7049 0.70551), between 33'S and 34'S (this paper; Stern et al, 1984a;Fig. 6 L .…”

“…6 L . Lopez-Escobar et al Deruelle et al, 1983;Harmon et al ., 1984;Notsu and Lopez, in prep. (Stern et al, 1984b). Thus one of the lowest 87Sr/86Sr ratios of the whole Southern Andes are found in volcanic rocks located around latitude 37'S.…”

ELA-ICP-MS U?Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu?Au deposit: implications for porphyry-related mineralization

“…Inherited Paleozoic zircon populations are common in mineralized porphyry Cu intrusions of South America (e.g., Zentilli et al 1994;Cornejo et al 1997;Richards et al 1999). In spite of other radiogenic isotopic evidence that suggests minimal contamination by old crustal rocks (e.g., Tilton et al 1981;Harmon et al 1984;Pankhurst et al 1988;Walker et al 1991;Kay et al 1999), the presence of these old zircons provides clear evidence of at least some crustal interaction in the generation of porphyry Cu magmas (Richards et al 1999).…”