Nature and tectonic setting of accreted basalts from the Mino terrane, central Japan

Abstract

The Mino tectono-stratigraphic terrane of central Japan is dominated by thrust-deformed Jurassic terrigenous clastic sediments, thought to have formed as a subduction–accretion complex at an active margin. However, in the central Mino terrane, basic volcanic rocks, together with stratigraphically overlying Permian shallow-water platform and slope carbonates and radiolarian cherts (Okumino Group), are present as tectonic intercalations. These Permian units have previously been interpreted as representing a large oceanic seamount-type body, formed in a distal oceanic setting. In addition, basaltic volcanic and volcaniclastic rocks of unknown age and affinities are also found as tectonic slivers within the adjacent Jurassic trench clastic sequences (Neo Formation). New petrological and geochemical investigations of all these basalts show that two main compositional groups are present: (i) alkali basalts (Mid-Permian) which are highly enriched in incompatible trace elements and the light REE (Group 1); (ii) tholeiitic basalts (mainly Lower Permian) which are moderately enriched in incompatible trace elements and light REE, relative to N-type MORB (Group 2). These characteristics suggest derivation from an enriched mantle source, such as a mantle plume. The compositions of Neo Formation basalts closely resemble those of the Okumino Group, suggesting that they were probably derived from the latter by both tectonic and sedimentary processes. Geochemical comparison with modern oceanic plume-influenced basalts, notably using La/Sm n –Sm/Yb n ratios, suggests that the older Group 2 basalts are more akin to volcanics erupted along a constructive plate boundary (P- or E-type MORBs), than to tholeiitic basalts of intraplate oceanic islands (OIB). The sedimentary evidence for bathymetric elevation further indicates that the setting for this volcanism was most likely a spreading axis-centered oceanic plateau or ridge (e.g. Iceland, Ninety-East Ridge). The younger alkali basalts probably formed in an off-axis position, either due to normal lateral spreading processes, or perhaps more complex spreading centre–plume interactions. Accretion of this plateau onto the southern margin of the Asian landmass in the Mid–Late Jurassic apparently involved significant subduction-erosion of the plateau, evidenced by reworking of the Permian units into adjacent trench clastic sequences.