Intracrustal Derivation of Na-Rich Andesitic and Dacitic Magmas: An Example from Volcán Ollagüe, Andean Central Volcanic Zone

Abstract

Andesitic and dacitic lavas erupted from Quaternary stratovolcanoes in the Central Volcanic Zone (CVZ) of the Andes have major and trace element compositions similar to proposed melts of subducted mid-ocean ridge basalt and Na-rich trondjhemites, tonalites, and granodiorites (TTG) in Early Archean high-grade gneiss terrains. Suites of lavas from individual volcanoes have average La/Yb of 25-45, Sr/Y of 30-65, Yb contents of 1.5-1.1 ppm, and Na O contents > 3.8 wt %, with lavas erupted on the volcanic front displaying the most affinity to the Early Archean suites. Because slab-melting is not involved in the derivation of the CVZ magmas, these data indicate that rocks with Early Archean TTG compositions can also be produced through intracrustal differentiation processes. Isotopic, major element, and REE data from Volcan Ollagiie are consistent with a model incorporating contamination of mantle-derived basaltic magmas in garnet-bearing deep continental crust to produce basaltic andesite magma, followed by fractionation of a plagioclase-dominated mineral assemblage in shallow crustal magma chambers to produce andesitic and dacitic magma with compositional features similar to Na-rich TTG. Slight offset between parental basaltic andesite and andesite samples on diagrams illustrating variation of REE suggest that the shallow crustal magma chambers are density stratified with basaltic andesite at the base overlain by zones of andesitic and dacitic magma. Furthermore, strongly increasing La/Yb and decreasing '43Nd/'44Nd ratios with decreasing Yb contents for basaltic andesite samples indicate that the deep continental crust beneath Ollagiie is probably more silicic and enriched in incompatible trace elements relative to crust beneath volcanoes located farther west along the present arc front of the central Andes. Based on these results, it is suggested that contamination of mantle-derived basaltic magmas in garnet-bearing mafic continental crust is an alternative mechanism to slab-melting to produce the Early Archean TTG suites and younger rocks with similar compositions. This model and the data summarized here may provide support for models advocating relatively early growth of the earth's continental crust.

The petrogenesis of sodic island arc magmas at Savo volcano, Solomon Islands

“…Savo (approximately 14 km; Petterson et al 2003) is much thinner than in those models that involve extensive crustal interaction (Atherton and Petford 1993;Feeley and Hacker 1995). DeLong et al (1975) compiled data for sodic magmas in intra-oceanic arcs and observed that these rock types occur in a number of specific tectonic settings: 1) near lateral edges of subduction zones where hinge faulting is occurring (Bering, Fiji, Grenada) and 2) where fracture zones and ridges are being subducted at a high angle (Kanaga, Aoba and Ambrym, New Georgia, Iwo-jima).…”

“…Sodic magmas (where Na 2 O−2 > K 2 O) are unusual occurrences in arcs, although they may be generated by a number of mechanisms, including: partial melting of subducted slabs; partial melting of underplated basaltic crust (Atherton and Petford 1993); assimilation of basaltic lower crustal material (Feeley and Hacker 1995); partial melting of mantle metasomatised by aqueous fluids (Kamenov et al 2008;McInnes and Cameron 1994) and/or melts from subducted slabs (Kepezhinskas et al 1995); and small degrees of partial melting of the mantle in truncated melt columns (Hole and Saunders 1996).…”

Mesozoic adakites in the Lingqiu Basin of the central North China Craton: Partial melting of underplated basaltic lower crust

“…Chondrite and PM values are from Sun and McDonough (1989). Defant and Drummond, 1990) cling lower continental crust in the mantle (e.g., Xu et al, 2002), and/or assimilationfractionation-crystallization (AFC) process (e.g., Feeley and Hacker, 1995). Either partial melting of subducted oceanic slab or AFC of a basaltic magma is unlikely for the Baiqi adakitic rocks based on the following observations.…”

Extremely high-Na adakite-like magmas derived from alkali-rich basaltic underplate: The Late Cretaceous Zhantang andesites in the Huichang Basin, SE China.

“…Three petrogenetic models have been proposed for the generation of adakitic magmas: (1) partial melting of subducting slab (e.g., Defant and Drummond, 1990), (2) partial melting of basaltic lower crust (e.g., Atherton and Petford 1993), and (3) fractionation and contamination of mantle-derived magmas with garnet-bearing lower continental crust (e.g., Feeley and Hacker, 1995).…”

“…Fractionation and contamination of mantlederived magmas with garnet-bearing lower crust was proposed to interpret petrogenesis of the adakite-like rocks in Volcan Ollague, Andean Central Volcanic Zone (Feeley and Hacker, 1995). Garnet as a separated phase rapidly depletes a magma in Y and HREE with little effect on the Sr and LREE contents, and results in high Sr/Y and La/Yb ratios in the residual magma.…”