Petrochemical evidence for off‐ridge magmatism in a back‐arc setting from the Yakuno ophiolite, Japan

Abstract

The Permian ophiolite emplaced in the Yakuno area, Kyoto Prefecture, consists of metavolcanic sequences, metagabbro and a troctolitic intrusion. The metavolcanics are associated with thick mudstone through a contact that shows the flowage of lava over unconsolidated mud layers on the sea floor. The metavolcanics and metagabbro have rare earth element (REE) patterns that are similar to enriched (E)-and transitional (T)-types ([La/Yb] N = 0.77-11.2) of mid-oceanic ridge basalts (MORB), whereas their Nb/La ratios (0.40-1.20) are as low as those of back-arc basin basalts (BABB). Cr-spinels in the metavolcanic rocks have Cr# of 40-73 and an Fe 3+ # of 9-24, numbers which are comparable to the values of BABB. These lines of evidence suggest that the Yakuno ophiolite originated more likely from an early stage back-arc basin rather than from an oceanic plateau, as has been suggested by some researchers. The troctolitic body that intrudes as a 0.5-km long lens in the metagabbro is composed of troctolite, olivine gabbro and microgabbro. The troctolite is marked by an olivine-plagioclase crystallization sequence, different from the commonly observed olivine-clinopyroxene sequence in other mafic/ultramafic cumulates of the Yakuno ophiolite. The microgabbro, with a composition close to that of the parental magma of the troctolite, is depleted in light REE ([La/Yb] N = 0.18-0.55) so that it has an REE pattern that mimics normal (N)-type MORB. The interstitial clinopyroxene of the troctolite has highly variable TiO 2 contents (0.2-1.4 wt%), which is interpreted to result from postcumulus crystallization of heterogeneous intercumulus melts. The troctolitic intrusion may represent a late stage intrusion that formed in an off-ridge environment during sea floor spreading of the back-arc basin. The geochemical variation observed in the Yakuno ophiolite, ranging from N-to E-MORB affinities, reflects the changes in both mantle source compositions and processes involved in magma generation during the evolution of the back-arc basin.

Olivine‐spinifex basalt from the Tamba Belt, southwest Japan: Evidence for Fe‐ and high field strength element‐rich ultramafic volcanism in Permian Ocean

“…The Tamba Belt was thrust by the Permian Ultra‐Tamba accretionary complex, which was in turn thrust by the Permian Yakuno ophiolite (Maizuru Belt) originated in an island arc – marginal basin system (e.g. Ichiyama & Ishiwatari 2004; Osozawa et al . 2004; Suda 2004).…”

Crustal Evolution of a Paleozoic Intra-oceanic Island-Arc-Back-Arc Basin System Constrained by the Geochemistry and Geochronology of the Yakuno Ophiolite, Southwest Japan

“…Ichiyama and Ishiwatari [11] suggested that the Yakuno ophiolite in the Yakuno-cho area (Figure 1) originated from a BAB crust, consisting of metabasalt and metagabbro of BAB affinities and a minor amount of troctolite.…”

“…Data sources for the mafic rocks in Yakuno ophiolite are excerpted from Suda and Hayasaka [7] and Ichiyama and Ishiwatari [11]. The metagabbros in Yakuno ophiolite plotted in this diagram were evaluated to be the melt origin which do not possess the cumulite compositions by Suda and Hayasaka [7].…”

“…Geochronological data for the Yakuno ophiolite [11] and the noncumulitic metagabbro in the Asago area [7]. Also plotted for comparative purposes are the compositions of Cenozoic back-arc basin basalts (BABBs) in various areas [31], MORBs (N-MORB [34]; T-and E-MORBs [35]), and island-arc basalt (IAB) [36].…”

Crustal anatexis and evolution of granitoid magma in Permian intra-oceanic island arc, the Asago body of the Yakuno ophiolite, Southwest Japan

“…1a). At least three different segments have been recognized according to their petrogenetic features: (1) an unusually thick oceanic crust mantle section; (2) island arc type crust; and, (3) back arc basin oceanic crust as a basement of the Permian Maizuru Group (Ishiwatari, 1985(Ishiwatari, , 1999Koide, 1986;Hayasaka, 1990;Ishiwatari et al, 1990;Ichiyama and Ishiwatari, 2003). The Asago body, 10 13 km in size, occurs as a fault bounded slice that tectonically overlays the Late Permian accretionary complex of the Ultra Tanba Terrane (Fig.…”