Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Abstract

The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip-dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocenelate Pleistocene-age volcanic rocks in the western Aleutians. These are termed the ''Adak-type'' and ''Piip-type'' magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (ϳ59% SiO 2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks. Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age.

Late Triassic high-Mg andesite/dacite suites from northern Hohxil, North Tibet: Geochronology, geochemical characteristics, petrogenetic processes and tectonic implications

“…5c). The andesite samples exhibit high wt.%), Mg # (64-72), and Cr (272-531 ppm) and Ni (66.7-636 ppm) contents, which are similar to typical Cenozoic high-Mg andesites in arc settings, e.g., "adakitic" and "Piip" high-Mg andesites in the Aleutian islands (Alaska) and the Kamchatka Strait to the west of the Aleutian islands (e.g., Yogodzinski et al, 1994Yogodzinski et al, , 1995Yogodzinski et al, , 2001Yogodzinski and Kelemen, 1998), bajaites in Baja California (Mexico) (Calmus et al, 2003;Saunders et al, 1987), sanukitoids in the Setouchi Volcanic Belt (SW Japan) (Shimoda et al, 1998;Tatsumi, 2001;Tatsumi and Ishizaka, 1981) and boninites of the Bonin Islands (Japan) (e.g., Taylor et al, 1994;Umino, 1986) (Fig. 6a-e).…”

“…5 for other data sources. Xu et al, 2008;Yogodzinski et al, 1994Yogodzinski et al, , 1995Yogodzinski and Kelemen, 1998).…”

“…The compositional disequilibrium characteristics of the augites suggest that the Hohxil high-Mg andesites were likely derived by interaction between melts and mantle (e.g., Rapp et al, 1999;Yogodzinski and Kelemen, 1998). However, the origin of the melts requires further clarification, given that similar melts have been attributed to partial melting of delaminated or subducted continental crust (e.g., Gao et al, 2004;Wang et al, 2006bWang et al, , 2008cXu et al, 2008), subducted basaltic oceanic crust, or sediments (e.g., Kay et al, 1993;Kelemen et al, 2003;Shimoda et al, 1998;Tatsumi, 2001;Tatsumi and Hanyu, 2003;Yogodzinski and Kelemen, 1998;Yogodzinski et al, 1994Yogodzinski et al, , 1995.…”

“…The data for bajaites in the Baja area, California, Mexico are from Calmus et al (2003) and Saunders et al (1987). The data for Adakitic high-Mg andesites of Adak Island (Aleutian islands) and the Kamchatka Strait to the west of Aleutian islands are from Yogodzinski et al (1995Yogodzinski et al ( , 1998Yogodzinski et al ( , 2001, and references therein; the data for Piip-type high-Mg andesites of the Piip volcano (Aleutian islands) are from Yogodzinski et al (1994). HMA-high Mg andesites.…”

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

“…Garnet enriched in Y and HREE is, therefore, a necessary residual phase during melting of basalt, while plagioclase enriched in Sr is minor or absent in the residue (Rapp and Watson, 1995). Partial melting of hydrated basaltic rocks at low pressures (≤8 kbar; Beard and Lofgren, 1991;Rushmer, 1991) do not produce liquids in equilibrium with garnet, whereas garnet + clinopyroxene ± (Kay,1978) Cook adakite (Stern and Killian, 1996) Cerro Pampa adakite (Kayetal., 1993) Precambrian basement of SE China [data from Yuan and Zhang (1992) (Kay, 1978;Yogodzinski et al, 1995); Cook Island, Chile (Puig et al, 1984); Baja of California, Mexico (Rogers et al, 1985;Aguillón-Robles et al, 2001); Sierra Madre, Mexico (Cameron and Cameron, 1985); Skagway, Alaska (Barker et al, 1986); El Valle and La Yeguada Volcanoes, Panama (Defant et al, 1991); Cerro Pampa, Argentina (Kay et al, 1993); Mindanao, Philippines (Sajona et al, 1993); Southwest Japan (Morris, 1995); Austral Andes (Stern and Killian, 1996); Quimsacocha, Ecuador (Beate et al, 2001). Partial melts ( ) of alkali basalt at 12-38 kbar, from Rapp et al, 1991;Rapp, 1995;Rapp and Watson, 1995.…”