Petrogenesis of volcanic rocks from Eastern Manus Basin: indications in mineralogy and geochemistry
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Petrogenesis of volcanic rocks from Eastern Manus Basin: indications in mineralogy and geochemistry* ZHAO Xia1, 2, 3, TIAN Liyan1, SUN Jianhui1, HUANG Peng2, 4, 5, **, LI Yan1, 3, GAO Yue1, 3 1
CAS Key Laboratory for Experimental Study under Deep-sea Extreme Conditions, Institute of Deep-sea Science and Engineering,
2
Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071,
3
University of Chinese Academy of Sciences, Beijing 100049, China
4
Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China
5
Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China
Chinese Academy of Sciences, Sanya 572000, China China
Received Nov. 26, 2019; accepted in principle Mar. 13, 2020; accepted for publication Apr. 17, 2020 © Chinese Society for Oceanology and Limnology, Science Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract To understand the petrogenesis and magma evolution history in the Eastern Manus Basin (EMB), geochemistry of pyroxene and plagioclase mineral phenocrysts in basaltic andesites and dacites were reported. The plagioclase-melt thermometry showed that, plagioclase in dacites crystalized in 1 027.2–1 028.5°C under 3.37–5.08 kbar, whereas in basaltic andesite was 1 181.5–1 187.0°C under 1.79– 4.46 kbar. Pyroxene compositions and invariable La/Sm vs La values of whole rock powders indicate that lavas were erupted in rapid cooling rate and mainly controlled by fractional crystallization (FC). In addition, oscillatory zonings in plagioclase and pyroxene phenocrysts indicated small local perturbations and degassing episodes in the magma chamber. Some high Mg# clinopyroxene antecrysts were found in EMB lavas. The highest Mg# of parental EMB melts is 69, which falls into the range of initial partial melts from upper mantle peridotite source (68–75). In terms of isotopic compositions, the EMB lavas most likely originated from Indian-type MORB mantle which was influenced by subduction components. In details, the subduction components are mainly derived from the dehydration of a subducted altered oceanic crust, and the contribution of sediment influence is minor. The Pb isotopic compositions and end member modeling further suggest that the source of subduction components is more likely from the Pacific Plate instead of the Solomon Plate. Keyword: Eastern Manus Basin; plagioclase; pyroxene; whole rock; Sr-Nd-Pb isotope; magma
1 INTRODUCTION Magmatism in back-arc basins has long been the highlight for studying tectonic and magmatic processes in subduction zones. Back-arc basin basalt (BABB) is compositionally different from mid-ocean ridge basalt (MORB) although both are produced in similar basic process in an extensional environment. In details, the back-arc spreading center is geographically close to active volcanic arc and trench, and therefore the magmatism is inevitably affected by subduction processes. The Eastern Manus Basin (EMB) is situ
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