Eocene volcanism during the incipient stage of Izu–Ogasawara Arc: Geology and petrology of the Mukojima Island Group, the Ogasawara Islands

Abstract

The Ogasawara Islands mainly comprise Eocene volcanic strata formed when the Izu–Ogasawara–Mariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the Mukojima Island Group, northernmost of the Ogasawara Islands, and show that the volcanic stratigraphy consists of arc tholeiitic rocks, ultra‐depleted boninite‐series rocks, and less‐depleted boninitic andesites, which are correlatable to the Maruberiwan, Asahiyama and Mikazukiyama Formations on the Chichijima Island Group to the south. On Chichijima, a short hiatus is identified between the Maruberiwan (boninite, bronzite andesite, and dacite) and Asahiyama Formation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on Nakodojima of the Mukojima Island Group. The stratigraphically lower portion of Mukojima is mainly composed of pillow lava, which is overlain by reworked volcaniclastic rocks in the middle, whereas the upper portion is dominated by pyroclastic rocks. This suggests that volcanic activity now preserved in the Mukojima Island Group records growth of one or more volcanoes, beginning with quiet extrusion of lava under relatively deep water followed by volcaniclastic deposition. These then changed into moderately explosive eruptions that took place in shallow water or above sea level. This is consistent with the uplift of the entire Ogasawara Ridge during the Eocene. Boninites from the Mukojima Island Group are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO2 (>57.0 wt.%) and Zr/Ti (>0.022) and are the most abundant type in both Mukojima and Chichijima Island Groups. Type 2 boninites have low SiO2 (<57.1 wt.%) and Zr/Ti (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO2 and are characterized by high CaO/Al2O3 (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the Chichijima Island Group.

Did boninite originate from the heterogeneous mantle with recycled ancient slab?

“…These resulted from the hydrous melting of hot depleted mantle wedge fluxed by the slab‐derived fluid. Kanayama et al () divided boninites into high‐MgO and high‐SiO 2 type (high‐Si boninite), which is severely depleted in fluid‐immobile incompatible elements with U‐shaped chondrite‐normalized rare earth element (REE) patterns, and low‐MgO and low‐SiO 2 type (low‐Si boninite), which is less depleted and has a flat to U‐shaped REE pattern. By analyzing primitive melt inclusions hosted in chrome spinels, Umino et al () demonstrated the coexistence of the high‐Si and low‐Si boninitic magmas.…”

Boninite and boninite-series volcanics in northern Zambales ophiolite: Doubly-vergent subduction initiation along Philippine Sea Plate margins

“…Both the Barlo extrusive section and Holes U1439 and U1442 are marked by abundance of LSB relative to HSB (Reagan et al, 2015). In contrast, the onshore boninite sequence at Ogasawara is dominated by 48-46 Ma HSB and HSB series volcanics interbedded with minor LSB (Maruberiwan Formation), which are overlain by 45 Ma LSB and calc-alkaline andesite-dacite (Mikazukiyama Formation) (Kanayama et al, 2012;Taylor et al, 1994;Umino, 1986). Several dikes in the Subic crustal section in the southern San Antonio massif have compositions like Barlo boninitic basalts (Yumul et al, 2000), albeit their petrographic characteristics have not been described.…”

“…Figure 6 shows oxide vs. SiO2 variation diagrams of Zambales boninite and boninite series volcanics. Also shown are boninite and boninite series volcanics from Ogasawara (Kanayama et al, 2012;Nagaishi, 2008;Taylor et al, 1994;Umino & Nakano, 2007;Yajima & Fujimaki, 2001) and Holes U1439 and U1442 of IODP Expedition 352 (Haugen, 2017;Reagan et al, 2015). MgO, TiO2, CaO and Al2O3 contents of HSB and LSB 20 form subparallel trends suggesting distinct parental magma compositions and fractionation paths.…”

“…After the subduction of Pacific Plate beneath the Philippine Sea Plate commenced at 52 Ma, magmatism progressed from 52-48 Ma mid-ocean ridge basalt (MORB)-like proto-arc basalts (or forearc basalts) to 48-44 Ma boninite and boninite-series volcanics followed by 45-35 Ma arc tholeiites and calc-alkaline lavas (Ishizuka et al, 2011;Kanayama et al, 2012;Reagan et al, 2010). The widespread occurrence of subduction initiation rock suites along the eastern margin of the Philippine Sea Plate is summarized in Fig.1a.…”

“…The subsequent late Oligocene opening of the Shikoku-Parace Vela Basin resulted in its present configuration with Site U1438 in the backarc side of KPR. Based on dredging and dive observations in the forearc 20 trench slope, proto-arc basalts are found at deeper depths relative to boninites which occur upslope and subaerially at Chichijima (type locality) and Mukojima island groups (Ishizuka et al, 2011;Kanayama et al, 2012;Umino & Nakano, 2007). The hypothesis that proto-arc basalt lies stratigraphically below boninite has been verified by IODP Expedition 352 based on four sites drilled in the trench-side slope of the Bonin Ridge (Reagan et al, 2015).…”