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Research Article in Situ Geochemical Compositions GeoScienceWorld Lithosphere Volume 2020, Article ID 8878501, 24 pages https://doi.org/10.2113/2020/8878501 Research Article In Situ Geochemical Compositions of the Minerals in Basaltic Rocks from the West Philippine Basin: Constraints on Source Lithology and Magmatic Processes 1,2,3 1,2,3 1,2 1 1 Long Yuan, Quanshu Yan , Yanguang Liu, Shiying Wu, Ruirui Wang, 1,2 and Xuefa Shi 1Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China 2Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China 3College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China Correspondence should be addressed to Quanshu Yan; [email protected] Received 27 July 2020; Accepted 28 September 2020; Published 21 October 2020 Academic Editor: Songjian Ao Copyright © 2020 Long Yuan et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). Since the early Cenozoic, the West Philippine Basin (WPB) and the whole Philippine Sea Plate (PSP) has undergone a complex geological evolution. In this study, we presented K-Ar ages, in situ trace element, and major element compositions of minerals of basalts collected from the Benham Rise and the Central Basin Fault (CBF) in the WPB, to constrain their magmatic process and regional geological evolution. Olivine phenocrysts and microlites in the alkali basalts (20.9 Ma) from the Benham Rise have forsterite (Fo) contents of 56.90%–76.10% and 53.13%-66.41%, respectively. The clinopyroxenes in the tholeiites (29.1 Ma) from : – : the CBF is predominantly diopside and augite, and it is depleted in light rare earth elements (LREEs) (LaN/YbN = 0 13 3 40) and large-ion lithophile elements (LILEs). The plagioclases in the basalts from both of the Benham Rise and the CBF are predominantly labradorite and andesine, with a minor amount of bytownite, and it is enriched in LREEs, Ba, Sr, and Pb and exhibits strong positive Eu anomalies. However, there exist obvious differences in plagioclase compositions between these two tectonic sites. The source lithology of the Benham Rise basaltic rocks could be garnet pyroxenite, and yet that of the CBF could be spinel-lherzolite. The calculated mantle potential temperature beneath the Benham Rise is 1439°C–1473°C, which is significantly higher than that beneath the CBF (1345°C–1381°C), suggesting there existed thermal anomaly beneath the Rise during basaltic magmatism. This study also calculated the temperature and pressure of the clinopyroxenes and plagioclases, which have been used to indicate magmatic processes. Finally, we suggest that the Benham Rise basaltic rocks may be related to a mantle plume (e.g., the Oki-Daito mantle plume), and the CBF was once located in a back-arc spreading center behind an active subduction zone. The extinction of the Oki-Daito mantle plume activity might be at about 20.9 Ma, and cessation of the back-arc spreading of WPB was at about 29.1 Ma or younger. 1. Introduction trench to the north, the Mariana and Ogasawara Trench to the east, the Yap and Palau trenches to the south, and the The Philippine Sea is one of the largest marginal seas in the Philippine Trench to the west. The Philippine Sea Plate con- world, and it has been studied extensively. The Philippine sists of the West Philippine Basin (WPB), the Shikoku Basin, Sea Plate (PSP) lies at the intersection of the Eurasian Plate, the Parece Vela Basin, the Mariana Trough, and two remnant the Pacific Plate, and the Indo-Australian Plate, and it is sur- arcs (the Kyushu-Palau Ridge and the West Mariana arc) [1– rounded by a series of subduction zones, with the Ryukyu 5]. Tectonic reconstructions of the Philippine Sea Plate Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8878501/5293321/8878501.pdf by guest on 29 September 2021 2 Lithosphere suggest that the plate was located near the equator before rane in the north, including Gagua Ridge (123-105 Ma), the 50 Ma, and it migrated northward to its present position, Amami Plateau (~115 Ma), the Daito Ridge, and the Oki- with a nearly 90° clockwise rotation [1]. Daito Ridge (44.4–40.5 Ma); the Benham Rise (39.8– At present, there are three models for the origin of the 35.9 Ma) and the Urdaneta Plateau (41.6–37.2 Ma) in the West Philippine Basin. (1) The WPB is a segment of trapped west; and the CBF (49.0–35.0 Ma) in the middle (Figure 1) oceanic crust [3, 6]. In this model, the WPB is a marginal [3, 13, 16]. Since 50 Ma, there has been widespread magma- basin formed along the Kula-Pacific accretionary plate tism in the West Philippine Basin (e.g., [2, 16, 17]; Haraguchi boundary. (2) The WPB was developed by a back-arc spread- et al. 2011; [5];Yan and Shi 2013; [13]). The igneous rocks of ing system [7–11]. Hall [1] detailed the tectonic evolution of the Amami-Daito province are basalts, dacites, and tonalites. the western Pacific, including the PSP since 55 Ma. The PSP They have the geochemical characteristics of intraoceanic was originally located on the equator, and it has gradually island arc rocks, and they may have been affected by a mantle migrated northward since the Early Cenozoic. During its plume [4, 5, 16]. The igneous rocks in the Huatung Basin are northward migration, the West Philippine Basin formed basalts and gabbros, with E-MORB trace element signatures (50–30 Ma) and the proto-Izu-Bonin-Mariana (IBM) (35– and Indian MORB Sr, Nd, and Hf isotopic signatures [18]. 30 Ma) was rifted in turn. The rifting of the proto-IBM leads The igneous rocks of the Kyushu-Palau Ridge are tholeiitic, to the formation of the Shikoku Basin (25–17 Ma) and the and they have the geochemical characteristics of oceanic arc Parece Vela Basin (28–23 Ma), and the subduction region tholeiites (Haraguchi et al. 2011). The igneous rocks of the of the Pacific Plate is backward to the east of the Izu- Benham Rise and the Urdaneta Plateau are basalts, and they Bonin-Mariana arc [1–5, 7, 12]. In addition, a clockwise rota- have the geochemical characteristics of OIBs [2, 13, 16]. The tion of nearby 90° occurred as the PSP migrated northward igneous rocks of the CBF are basalts, and they have the geo- [1, 4]. (3) The development of the WPB is influenced by both chemical characteristics of MORBs [2, 17]. back-arc spreading and a mantle plume [1, 7, 13, 14]. In the The eleven basaltic rock samples in this study were col- West Philippine Basin, scientists have collected various rock lected from two dredge stations (100DS-Vinogradov Sea- types, including N-MORBs (Normal Mid-Ocean Ridge mount in the Benham Rise and 94DS in the CBF) during Basalts) from the Central Basin Fault (CBF); OIBs (Ocean China-Germany joint leg no. SO-57 cruise of R/V Sonne Island Basalts) from the Benham Rise, the Urdaneta Plateau, (Figure 1) [19]. Samples from 100DS are alkali basalts and the Amami Plateau, the Daito Ridge, and the Oki-Daito are of oceanic island basalt- (OIB-) like characteristics, and Ridge; and arc volcanic rocks from the Kyushu-Palau Ridge, those from 94DS are tholeiites and similar to back-arc basin and their geodynamics settings appear to be related to back- basalts (BABBs) [20]. The six samples from site 100DS arc spreading, a mantle plume, and volcanic arc processes, exhibit porphyritic textures and contain plagioclase and oliv- respectively [2, 4, 5, 13, 15–17]. The basalts erupted from ine phenocrysts, with some microlites (olivine and plagio- the spreading centers on the Philippine Sea Plate have the clase, <0.1 mm grain size) in the groundmass (Figures 2(a) isotopic characteristics of Indian Mid-Ocean Ridge basalts, and 2(b)). The phenocryst content of the samples from site rather than Pacific Mid-Ocean Ridge basalts [2]. Previous 100DS is 5%–10%, and the plagioclase content accounts for studies have shown that the Amami Plateau, the Daito Ridge, 70%–90% of the total phenocrysts. The grain size of the oliv- and the Oki-Daito Ridge are related to arc-plume interaction. ine phenocrysts in the basalts from site 100DS is mostly 0.2– The Benham Rise and the Urdaneta Plateau may have been 0.4 mm, and some are even as large as 1.5 mm (sample affected by a mantle plume (e.g., the Oki-Daito mantle 100DS7). The olivine phenocrysts are idiomorphic and hypi- plume) [5, 13, 16]. Until now, many studies have focused diomorphic, and some have been altered to iddingsite. The on the WPB, but few basement rock samples have been col- grain size of the plagioclase phenocrysts ranges from 0:04 × lected from the Benham Rise, and the CBF and the magmatic 0:20 to 0:2×2:00 mm2. The five samples from site 94DS processes of the WPB have not been studied in detail, which exhibit porphyritic textures and contain plagioclase and clin- hinders our understanding of the tectonic evolution of the opyroxene phenocrysts, with some microlites (clinopyroxene WPB and the PSP. and plagioclase) in the groundmass (Figures 2(c) and 2(d)). In this study, we obtained in situ trace element (using The phenocryst content of the samples from site 94DS is laser ablation inductively coupled plasma mass spectrometry 10%–20%, and the plagioclase content accounts for 50%– (LA-ICP-MS)) and major element (using EMPA) composi- 70% of the total phenocrysts.
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