The Matsue Formation: Evidence for gross mantle heterogeneity beneath Southwest Japan at 11 Ma

Abstract

Middle Miocene basalts and basaltic andesites of the Matsue Formation outcrop within a 5 km radius of Matsue city in eastern Shimane Prefecture. Despite their limited outcrop and age (11.0 2 1.5 Ma), they show a wide range in s7Sr-s6Sr(0.70370-0.70593), 143Nd-144Nd(0.512904-0.512471) and large ion lithophile element (LILE) contents, but a relatively narrow range for some high field strength elements (HFSE) such as Nb and Ti. These basalts and andesites can be divided into three groups based on petrography, major element, trace element and isotope chemistry. Although onF group has uncfergone some fractional crystallization, isotope chemistry precludes linkage of the groups by a closed-system process. Crustal contamination can explain isotope chemistry, but is not consistent with trace element variations. The most satisfactory model is eruption of two compositionally distinct magmas, with limited magma mixing and fractional crystallization. Published experimental work shows that one end-member resulted from shallow melting of upwelling mantle at -25 km. The simultaneous eruption of the other end member magma in the same area points towards a heterogeneous mantle. The isotopic composition of Matsue Formation basalts and andesites covers the entire range of Late Miocene mafic volcanic rocks of southwest Japan. Such gross heterogeneity developed on a local scale has implications for models that deal with regional chemical variations of mafic volcanic rocks in southwest Honshu.

Production of High‐Sr Andesite and Dacite Magmas by Melting of Subducting Oceanic Lithosphere at Propagating Slab Tears

“…We infer that those older ages represent nonadakitic intermediate rocks from the basement of Wakurayama. The middle to late Miocene mafic to felsic rocks, termed the Matsue Formation, are distributed widely around Wakurayama (Kano et al, ; Morris & Itaya, ).…”

Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

“…Mafic rocks of this period are classified into either alkaline or sub‐alkaline series; the former type includes volumetrically minor, highly alkaline rocks (lamprophyre and nephelinite; Nagao & Fujibayashi, 1989; Tatsumi et al, 1999). Intermediate to felsic eruptions in this stage are volumetrically minor and mainly occur in the Sanin and offshore islands (Matsue and Oki; Morris et al, 1990; Morris & Itaya, 1997). At the end of Stage III (4–3 Ma), the volcanic activity ceased in the Chugoku district, except for that in Oki Islands in the Sea of Japan (Kimura et al, 2003, 2005).…”

“Volcanism in the Chugoku region: a review for the first UNESCO field school on Geoenvironmental disaster reduction”

“…-I-Type vs. S-Type intrusiva distinguished on the basis of igneous (I) and sedimentary (S) source rocks (Chappell and White 1992) et al 1999). The majority of volcanic rocks on the backarc side (Matsue Formation) also show hybrid characteristics between alkaline and subalkaline compositions with highly variable trace element compositions (Morris and Itaya 1997). No simple geochemical classification is sufficient to confidently identify a single magma source and two general scenarios have been discussed (which may work alone or in concert): One source could be an effect related to subduction of the leading edge of the Philippine Sea Plate (Valentine and Connor 2015), the possible effect of the downward propagating slab is further discussed in the next paragraph.…”