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Eocene volcanism during the incipient stage of I zu– O gasawara A rc: Geology and petrology of the M ukojima I sland G roup, the O gasawara I slands
Author(s) -
Kanayama Kyoko,
Umino Susumu,
Ishizuka Osamu
Publication year - 2012
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12000
Subject(s) - geology , dacite , pyroclastic rock , andesite , lava , geochemistry , volcanic rock , rhyolite , andesites , volcano , petrography , petrology , pillow lava , volcanism , seismology , tectonics
The O gasawara I slands mainly comprise E ocene volcanic strata formed when the I zu– O gasawara– M ariana Arc began. We present the first detailed volcanic geology, petrography and geochemistry of the M ukojima I sland G roup, northernmost of the O gasawara I slands, 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 M aruberiwan, A sahiyama and M ikazukiyama F ormations on the C hichijima I sland G roup to the south. On C hichijima, a short hiatus is identified between the M aruberiwan (boninite, bronzite andesite, and dacite) and A sahiyama F ormation (quartz dacite and rhyolite). In contrast, these lithologies are interbedded on N akodojima of the M ukojima I sland G roup. The stratigraphically lower portion of M ukojima 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 M ukojima I sland G roup 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 O gasawara R idge during the E ocene. Boninites from the M ukojima I sland G roup are divided into three types on the basis of geochemistry. Type 1 boninites have high SiO 2 (>57.0 wt.%) and Z r/ T i (>0.022) and are the most abundant type in both M ukojima and C hichijima I sland G roups. Type 2 boninites have low SiO 2 (<57.1 wt.%) and Z r/ T i (<0.014). Type 3 boninites have 57.6–60.7 wt.% SiO 2 and are characterized by high CaO / Al 2 O 3 (0.9–1.1). Both type 2 and 3 boninites are common on Mukojima but are rare in the C hichijima I sland G roup.