HIGH-MG ANDESITES IN THE SETOUCHI VOLCANIC BELT, SOUTHWESTERN JAPAN: Analogy to Archean Magmatism and Continental Crust Formation?

Abstract

The occurrence of unusual high-Mg andesite (HMA) characterizes the Setouchi volcanic belt in SW Japan, which was activated at 13.7±1.0 Ma by subduction of the young and hot Shikoku Basin lithosphere into the high-temperature upper mantle. This tectonic setting may be analogous to the thermal regime during Archean times, which suggests more ubiquitous production of HMA. A plausible process that can comprehensively account for the petrological and geochemical characteristics of Setouchi HMAs involves partial melting of subducting lithosphere, subsequent melt-mantle interactions, and final equilibration with the upper-most mantle. HMAs and more differentiated andesites, which are coined sanukitoids, are distinct in that they are phenocryst-poor (<10%), compact, and nearly anhydrous, despite HMA magmas originally containing ∼7 wt% H2O, and commonly form composite lava flows. One mechanism for explaining these features is formation of a mostly solidified HMA pluton, remelting of the HMA pluton by intrusion of a high-temperature basaltic magma, consequent production of a nearly dry HMA magma, and mixing of this HMA magma with overlying residual felsic melts during ascent to form a zoned magma reservoir.

Geochronology and geochemistry of the high Mg dioritic dikes in Eastern Tianshan, NW China: Geochemical features, petrogenesis and tectonic implications

“…The geochemistry of the diorite dikes is consistent with them being derived from a depleted mantle source, similar to that proposed for other sanukitoids (Beyth et al, 1994;Heilimo et al, 2010;Martin, 1999;Martin et al, 2009Martin et al, , 2005. The sanukitoid HMA suite is widely considered to be generated through interaction of a mantle peridotite with a silicic melt derived from partial melting of a subducting oceanic slab and/or sediment (e.g., Yogodzinski et al, 1994;Tatsumi, 2001Tatsumi, , 2006Tatsumi and Hanyu, 2003;, consistent with experimental results that show that sanukitoid can be produced by partial melting of harzburgitic or lherzolitic mantle (Tatsumi, 2001;Tatsumi and Ishizaka, 1982).…”

“…The dikes are characterized by intermediate SiO 2 , low TiO 2 , relatively high Al 2 O 3 and MgO contents, with Mg# generally higher than 56 (56-59) similar to sanukitoids. Field observations, mineralogical, geochemical, whole rock Sr-Nd and zircon Hf isotopic data suggest that the dioritic dikes were generated by (Shimoda et al, 1998;Tatsumi, 2006;Tatsumi and Ishizaka, 1982) and references therein.…”

“…The inhomogeneous eHf(t) and eNd(t) values suggest that The Rb, Sr, Sm, Nd, U, Th, and Pb concentrations used in Table 4 are from Table 1. the sediment was added during ascent rather than in the source area which would tend to produce more uniform values. In summary, the geological setting, geochemistry and isotopic characteristics of the dioritic dikes suggest that they were generated by partial melting of mantle wedge that assimilated minor sediment ($3%) during the upwelling process analogous to sanukitoids in Japan (Shimoda et al, 1998;Tatsumi, 2006;Tatsumi and Ishizaka, 1982).…”

Middle Jurassic MORB-type gabbro, high-Mg diorite, calc-alkaline diorite and granodiorite in the Ando area, central Tibet: Evidence for a slab roll-back of the Bangong-Nujiang Ocean

“…In addition, sediment-derived melts could notably enhance La/Sm ratios of the magma (Fig. 9c), but it could not change Ba/Th ratios (Tatsumi, 2006). The high-Mg diorites have relatively high La/Sm, low Ba/Th.…”

K-Ar ages of high-magnesian andesite lavas from northern Kyushu, Japan

“…The origin of HMA magma is closely associated with the anomalous tectonic settings such as the subduction of a young and hot oceanic plate and backarc opening (Tatsumi, 2003(Tatsumi, , 2006.…”