Genesis and evolutional processes of the Paleozoic oceanic island arc crust, Asago body of the Yakuno Ophiolite, Southwest Japan

Abstract

A middle to lower crustal section of Paleozoic oceanic island arc is exposed in the Asago body of the Yakuno ophiolite. Based on field occurrence, petrography, and geochemical modeling, we investigated the evolution of the Asago body and relevant magmatic processes. The Asago body consists of two stages of rocks. The first-stage rocks consist of metagabbro and schistose amphibolite that represent the basement to the Permian Yakuno paleo-island arc. The second-stage rocks are mainly arc granitoids that intrude the first-stage rocks. Mafic migmatites occur in the lower crustal section of the Asago body. Field occurrences and petrographic data suggest that the migmatites formed by the anatexis of first-stage rocks, and that segregation and accumulation of the anatectic melt resulted in morphological changes in the migmatites toward the middle crustal section. Geochemical data indicate that the first-stage rocks were derived from a basaltic magma of back-arc affinity, suggesting in turn that the Yakuno paleo-island arc was developed within a back-arc basin. Moreover, a low-K series hornblende gabbro, quartz diorite, and tonalite and high-K series quartz monzodiorite and granodiorite within the second-stage rocks were generated by high and low degrees of partial melting of first-stage rocks, respectively. We conclude that the Asago body is an example of the transition from oceanic to continental crust, related to the anatexis of mafic lower crust in an oceanic island arc setting. Abstract

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

“…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].…”

“…Suda [8] and Suda and Hayasaka [7] suggested that the amphibolite and metagabbro in the Asago area possess geochemical affinities of BAB basalt (BABB), which are derived from a tholeiitic BABB source. These mafic rocks of BABB affinities are intruded by tonalite, quartz-diorite, and granodiorite of arc affinities.…”

“…The variation diagram of the (Nb/La) ratio with respect to the (La/Y) ratio ( Figure 5) characterizes the geochemical signatures in the range between MORB and IAB [7]. Data source for the MORBs are excerpted after Pearce and Parkinson [34], and Kelemen et al [35].…”

“…This field evidence suggests that the mafic rocks in this area originally formed as a BAB crust, which was subsequently brought to an OIA setting. In this regard, the Yakuno ophiolite in this area records various lines of evidence to help understand the tectonic evolution of an intra-OIA-BAB system [7,8].…”

“…The Yakuno ophiolite in the Asago area is composed of metagabbro and amphibolite of BAB origin ( Figure 1) [7]. These mafic rocks are intruded by the granitoid of arc affinities.…”