Sr-Nd-Pb isotopic and major and trace element compositions of the Yufu-Tsurumi volcanic rocks: implications for the magma genesis of the Yufu-Tsurumi volcanoes, northeast Kyushu, Japan

Abstract

We have characterized the Sr Nd Pb isotopes and major and trace element compositions of 10 Quaternary volcanic rocks from the Yufu and Tsurumi volcanoes in northeast Kyushu, Japan. The enriched incompatible elements, negative Nb, and positive Pb and Sr anomalies are generally interpreted to be from island arc affinities of the lavas. However, the LREE/HREE ratio of the lavas (La/Yb = 8.5 ± 1.3) is greater than that from the island arc intermediate volcanics from northeast Japan (La/Yb = 3.8 ± 0.6), suggesting a different origin. Sr compositions fell within the range of typical adakites. Other lavas were classified as normal island arc type magmas. These results suggest that a partial melting of the subducting Philippine Sea Plate played a role in the genesis of the Yufu Tsurumi volcanic rocks. The Sr, Nd, and Pb isotopic compositions show evidence of mixing between melts, derived from oceanic basaltic crust and subducted terrigenous sediments for the adakite magma, whereas another source is required to explain the enrichment in Sr Nd isotopes and depletion in Pb isotopes of the island arc type magma. Although the island arc type magma appears to have a different source, a mixing of the adakite magma with this magma is apparent from our observations of the trace elements and isotopes.

Some first-order observations on magma transfer from mantle wedge to upper crust at volcanic arcs

“…through upwelling of hotter asthenospheric mantle, frequently associated with partial melting of the slab edges and resulting adakite-type volcanism at the surface (Yogodzinski et al 2001). Thus, the following arcs are classified as irregular on basis of slab discontinuities: CAS, where adakite-type volcanism and high surface heat flux are observed in the south at Lassen and Mount Shasta, east of the Mendocino triple junction and above the southern edge of the subducting Juan de Fuca plate (Baker et al 1994;Borg et al 1997); MEX, where slab detachment is evidenced by a lack of seismicity directly beneath the arc, and where eastward propagating late Miocene OIB-type and adakitic volcanism occurred due to asthenospheric mantle upwelling (Ferrari 2004), consistent with the present-day surface heat flux increase towards the west; NCH, where ongoing work indicates slab tearing as far south at 218S (Rietbrock et al 2006); NEJ, where a slab window in the Philippine Sea plate widens toward the backarc north of Mount Fuji (Ishida 1992;Mazzotti et al 1999), and where an extensive slab crack beneath the Hokkaido corner (Katsumata et al 2003) widens towards the backarc (Lundgren & Giardini 1990), consistent with elevated surface heat flux in both the central Honshu backarc and northern Honshu; and SWJ, where the slab ruptures between Honshu and Kyoshu (Zhao et al 2002) with slab melting proposed to result in adakitetype volcanism in SW Honshu (Morris 1995;Kimura et al 2005) and in NE Kyushu (Sugimoto et al 2006). Slab windows and tears have also been identified in the westernmost Aleutians (Yogodzinski et al 2001), south of the Central American volcanic front in southeastern Costa Rica and Panama (Johnston & Thorkelson 1997), beneath Tierra del Fuego north and east of the Austral Volcanic Zone (Gorring & Kay 2001), and in the southern Ryukyu arc (Lin et al 2004).…”

Across‐Arc Diversity in Rhyolites From an Intra‐oceanic Arc: Evidence From IODP Site U1437, Izu‐Bonin Rear Arc, and Surrounding Area

“…The range of isotopic values found in Eocene‐Oligocene crust is denoted by the gray fields, representing modern‐day isotopes measured on Eocene‐Oligocene tephras collected in the Izu‐Bonin fore‐arc (Straub et al, ) and Eocene basement rocks from IODP Site U1438 (Yogodzinski et al, ); relevance of Eocene‐Oligocene crust is discussed later in the text. The range of trace element and isotopic values found in SW and NE Japan Arcs is denoted by the yellow field, representing modern‐day analyses (Hanyu et al, ; Hanyu et al, ; Kimura et al, ; Sugimoto et al, ). Estimated isotopic ratios for average subducted sediment and average altered oceanic crust are plotted as maroon stars, with data from Tollstrup et al ().…”

“…However, Japanese tephras have higher Th contents and/or Ba/La ratios than all Site U1437 tephras included in this study (Figure ). In addition, Unit II rhyolites have Nd and Sr isotope ratios similar to those from the Izu‐Bonin rear arc but dissimilar to Japanese lavas (Figure ) (Hanyu et al, , ; Sugimoto et al, ).…”

Lower crustal H2O controls on the formation of adakitic melts

“…Based on the occurrence of deep long-period tremors, the young slab is thought to dehydrate beneath Shikoku in the forearc (Obara, 2002). Chemically, the arc changes northeastward from the typical calcalkaline Aso volcano in central Kyushu to eruptives yielding increasingly high Sr/Y ratios at Kuju and Yufu volcanoes in northern Kyushu (Kita et al, 2001;Sugimoto et al, 2006), and Aonoyama, Sanbe, and Daisen volcanoes in western Honshu (Morris, 1995). Residual garnet signatures have previously been ascribed to partial melting at the leading edge of the young subducting slab, bolstered by hot mantle upwelling from depth (Morris, 1995;Sugimoto et al, 2006).…”