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Alkalic magmatism in the Lyra Basin: A missing link in the late-stage evolution of the Ontong Java Plateau

By
Kenji Shimizu
Kenji Shimizu
Department of Solid Earth Geochemistry and Submarine Resources Research Project, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
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Takashi Sano
Takashi Sano
Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
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Maria Luisa G. Tejada
Maria Luisa G. Tejada
Department of Solid Earth Geochemistry and Submarine Resources Research Project, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
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Hironobu Hyodo
Hironobu Hyodo
Research Institute of Natural Sciences, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan
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Keiko Sato
Keiko Sato
Submarine Resources Research Project, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan, and Research Institute of Natural Sciences, Okayama University of Science, Ridai-cho, Okayama 700-0005, Japan
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Katsuhiko Suzuki
Katsuhiko Suzuki
Submarine Resources Research Project, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
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Qing Chang
Qing Chang
Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
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Masao Nakanishi
Masao Nakanishi
Graduate School of Science, Chiba University, Chiba 263-8522, Japan
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Published:
May 01, 2015

The Lyra Basin is believed to be a contiguous part of the Ontong Java Plateau (OJP), based on geophysical studies. Volcaniclastic rocks dredged at two sites in the Lyra Basin document another post-plateau episode of magmatism on the OJP; they are olivine-titanaugite-phyric alkali basalts with as much as ~30% modal phenocrysts. Lyra Basin basalts have compositions that vary from picritic (MgO ~22 wt%) to more evolved (MgO ~5 wt%) and have low SiO2 (41–46 wt%), high TiO2(2–4 wt%), and high Na2O + K2O (1–5 wt%) contents that are distinctly different from tholeiites that compose the main OJP. The 40Ar-39Ar weighted mean age of Lyra Basin basalts is 65.3 ± 1.1 Ma, determined using a single-grain laser fusion method of the ground-mass from the least altered alkali basalt and of biotite separates from differentiated samples. This age is interesting because it is much younger than the main stage of OJP formation (122 Ma) and no ca. 65 Ma alkaline basalts have been found previously near or on the OJP. Incompatible trace element modeling suggests that the volcanic rocks of the Lyra Basin may have been formed by a low degree of partial melting (~3%), predominantly at the garnet-lherzolite stability field from the same OJP mantle source preserved in its thick lithospheric root. However, major and trace elements and isotopic compositions can be better explained by magma mixing of Rarotongan alkali magma and magma derived from OJP-source mantle melting (12% partial melting at garnet stability field) in the ratio of 1:2. Although the trace element compositions of Lyra basalts can be reproduced by OJP-source mantle melting with or without contribution from the Rarotongan hotspot, the lower potassium content of the calculated Rarotongan hotspot-influenced melt is more compatible with that of an average composition of Lyra basalt. These results are consistent with previous reconstruction of the OJP path from 120 Ma to its present position, indicating that it may have passed over the Rarotongan hotspot at 65 Ma. In either case, the petrogenesis of Lyra Basin basalts highlights the role of the thick lithospheric root of the OJP in the late-stage development of the plateau. Additional evidence for episodic late-stage magmatic activity on the OJP helps to elucidate the magmatic evolution of the plateau and may provide insights into the origins of other large igneous provinces.

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GSA Special Papers

The Origin, Evolution, and Environmental Impact of Oceanic Large Igneous Provinces

Clive R. Neal
Clive R. Neal
Department of Civil & Environmental Engineering & Earth Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, Indiana 46556, USA
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William W. Sager
William W. Sager
Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA
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Takashi Sano
Takashi Sano
Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
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Elisabetta Erba
Elisabetta Erba
Department of Earth Sciences, Università degli Studi di Milano, 20133 Milan, Italy
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Geological Society of America
Volume
511
ISBN print:
9780813725116
Publication date:
May 01, 2015

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