Elsevier

Chemical Geology

Volume 497, 10 October 2018, Pages 146-161
Chemical Geology

The dynamics of the southern Okinawa Trough magmatic system: New insights from the microanalysis of the An contents, trace element concentrations and Sr isotopic compositions of plagioclase hosted in basalts and silicic rocks

https://doi.org/10.1016/j.chemgeo.2018.09.002Get rights and content

Abstract

The Okinawa Trough (OT) is a back-arc, initial marginal sea basin located behind the Ryukyu arc–trench system. Analysis of the plagioclase phenocrysts from two basalts and two silicic rocks dredged from different locations in the southern Okinawa Trough (SOT) provides a detailed record of the nature and composition of and changes in the magma within the magmatic system. Plagioclase phenocrysts from the basalt record 87Sr/86Sr values of 0.704015–0.704360 and 0.704686–0.704863, respectively, which overlap with previously reported whole-rock values and suggest that the parent magma of the plagioclase represents upwelling mantle materials influenced by a subducted component. Plagioclase-melt thermobarometry records the crystallization of plagioclase phenocrysts in the basalt at the bottom of the lower crust near the Moho (depths of ~20 ± 7.5 km and ~25 ± 7.5 km). The temporal record of the Sr and Ba contents in the melt recorded by these crystals, which were calculated using partition coefficients and the Sr isotopic compositions of the plagioclases from the basalts, demonstrates the existence of two distinct mafic magmas within the SOT due to the presence of different subducted components (i.e., the subduction of the Gagua Ridge).

Significant textural variations and wide ranges in the An35-89 and trace element concentrations and in situ Sr isotopic compositions (0.705748–0.707520) are observed in plagioclase crystals from silicic rocks in the SOT. The barometry results for the plagioclases from silicic rocks yield a wide range of crystallization depths ranging from 6.6 to 17.8 km. The Sr isotopic compositions and trace element concentrations of high-An80 plagioclases in basalt and silicic rocks indicate that there is no direct relationship between these two magma chambers. The negative correlation between the An contents and the Sr isotopic compositions in the plagioclases from silicic rocks indicates that more upper continental crust materials joined the magma source as its evolution proceeded. The presence of core-sieved plagioclase in the silicic rock indicates that decompression and/or magma mixing occurred. Thus, we propose a multilayer magma chamber system to explain the complex plagioclase crystals in silicic rocks. Two potential mush zones were recognized; their generation may have been controlled by the adiabatic decompression melting of lower crustal materials resulting from crustal extension or the post-collisional extension of the Northern Taiwan Mountain Belt. The two potential mush zones at greater depths are relatively basic, whereas the shallow magma reservoirs, which are the host magmas of the plagioclase, are relatively silicic.

Introduction

The lavas in back-arc basins exhibit distinct compositional diversity due to the addition of a subduction component (Pearce and Stern, 2006); thus, they record significantly different compositions than mid-ocean ridge basalt (MORB) does (Taylor and Martinez, 2003). Differences in lava compositions can be generated by the presence of a heterogeneous mantle source (Stern et al., 1990; Pearce et al., 2005), differences in the crystal fractionation of the magma (Coogan et al., 2001), the reactions of melts with the crust during their ascent towards the surface (Browne et al., 2006) and the mixing of different magmas (Lai et al., 2016). Usually, a considerable proportion of magma evolution occurs under conditions of relatively high crystallinity in the crust, where the mineralogy of the solid phases (e.g., plagioclase) is significantly more mafic than that in erupted lavas (Ridley et al., 2006; Zellmer et al., 2011). However, interactions with the crust can also occur at shallower levels in upper crustal reservoirs. The details of these processes, including the location and degree of fractional crystallization, the size and location of magma chambers and open- or closed-system behavior, are complex and variable (Ginibre and Wörner, 2007). Plagioclase is a useful phase with which to study these processes.

Plagioclase is one of the major rock-forming minerals that constitute the crust of the Earth. As a low-pressure liquidus phase that crystallizes over a wide range of temperatures, plagioclase is often present as a phenocryst in volcanic rocks or as a cumulus phase in mafic to felsic intrusions (Sun et al., 2017). Many studies have used combinations of compositional data and the textural characteristics of plagioclase crystals from basaltic rocks to investigate the open- or closed-system processes that occur within volcanic plumbing systems throughout the history of a volcano (Barca and Trua, 2012), such as the residence times of crystals (Zellmer et al., 1999, Zellmer et al., 2003, Zellmer et al., 2011; Costa et al., 2003; Turner et al., 2003), the composition and evolution of magmas (Kuritani, 1998; Coogan et al., 2000; Ginibre et al., 2002; Browne et al., 2006; Ginibre and Wörner, 2007; Nakai et al., 2008; Kinman et al., 2009; Barca and Trua, 2012; Lange et al., 2013; Togashi et al., 2017), and the physicochemical conditions of magma chamber systems (Putirka, 2008; Benisek et al., 2010; Samaniego et al., 2011; Geiger et al., 2016; Sheehan and Barclay, 2016; Erdmann et al., 2016; An et al., 2017). Plagioclase is a useful phase to analyze because the compositional and textural features that develop during primary growth may be preserved under certain conditions due to the slow diffusion of CaAl–NaSi (Smith et al., 2009) and because the composition of plagioclase is sensitive to the chemical and physical conditions of the magma (Putirka, 2005, Putirka, 2008), including its temperature, pressure and H2O content (Blundy and Wood, 1991; Bindeman et al., 1998).

The use of in situ analytical techniques to examine the products of volcanic systems at the mineral grain scale can provide additional insight into magmatic processes beyond that yielded by whole-rock studies alone, provided that all due considerations to diffusion, partition coefficients, and the physical processes behind the observed compositions and textures are taken into account (Blundy and Shimizu, 1991; Blundy and Wood, 1994; Wood and Blundy, 1997; Bédard, 2006; Tepley et al., 2010; Nielsen et al., 2017; Sun et al., 2017). Furthermore, Ca-rich minerals, particularly plagioclase, are typically rich in Sr and are ideal for Sr isotopic analysis, and numerous studies of volcanic rocks have indicated that the Sr isotopic compositions of plagioclase phenocrysts may differ significantly from those of microlitic plagioclase and whole-rock samples (Knesel et al., 1999; Charlier et al., 2006; Davidson et al., 2007; Lange et al., 2013). Thus, in situ Sr isotopic analyses of plagioclase, in conjunction with Sr isotopic analyses of whole-rock samples, can help to determine the petrogenetic processes by which specific rocks are formed (Gao et al., 2015; Wilson et al., 2017).

The Okinawa Trough (OT) is a young back-arc basin located in the Ryukyu arc–trench system (Shinjo, 1998; Sibuet et al., 1998) and represents an ideal area for studying the magmatic characteristics of back-arc basins during their early spreading stages (Guo et al., 2016). Most petrographic studies have focused on the magma source (Ishizuka et al., 1990; Honma et al., 1991; Chen et al., 1995; Shinjo et al., 1999; Hoang and Uto, 2006), the magmatic evolution of lavas (Zhai et al., 1994; Shinjo and Kato, 2000; Guo et al., 2018a), and the influence of the subduction of the Philippine Sea Plate (Pi et al., 2016; Guo et al., 2017; Shu et al., 2017), but fewer have investigated the magma chamber processes (Lai et al., 2016). The Northern Taiwan Volcanic Zone (NTVZ), which resulted from the post-collisional extension of the Northern Taiwan Mountain Belt (NTMB) (Chen, 1997; Wang et al., 1999), is located near the southern end of the OT and has also been influenced by the subduction of the Philippine Sea Plate (Wang et al., 2004; Pi et al., 2016). However, the NTVZ is indistinct in whether or not the magma formation at the end of the southern OT (SOT), which was similarly influenced by the subduction of the Philippine Sea Plate (Guo et al., 2017), is related to the post-collisional extension of the NTMB. In addition, the subduction of the Gagua Ridge and the slab tear at 123°E (Lin et al., 2004, Lin et al., 2007) may impact the composition of the lavas in the SOT.

In this study, we investigated the textural and compositional characteristics of plagioclase crystals in four volcanic samples dredged from the SOT, examined back-scattered electron (BSE) images and analyzed the major and trace element concentrations and Sr isotopic compositions of plagioclase phenocrysts in basalt and silicic rock samples to study the magma source and petrogenesis of the basalt and silicic rocks and the evolution of the silicic magma. We also examined the influences of the post-collisional extension of the NTMB and the subduction of the Gagua Ridge on the SOT.

Section snippets

Geological setting and petrology of the southern Okinawa Trough (SOT)

The OT is located on the eastern margin of the East China Sea, which extends from Kyushu Island in the north to Taiwan Island in the south (Fig. 1). The OT opened behind the Ryukyu arc–trench system, where the Philippine Sea Plate is currently subducting beneath the Eurasia Plate (Sibuet et al., 1998; Shinjo, 1999). Both the eastern and western margins of the trough are delimited by high-angle normal faults that dip towards the trough axis. A central graben is present in the trough axis, which

Sampling and petrography

The samples analyzed in this study were dredged from the SOT during China's State Oceanic Administration Okinawa Trough Geological and Geophysics Investigation (OTGGI) cruise (R/V KEXUE YIHAO, 1995) and the HOBAB (hydrothermal activity of back-arc basin) cruises 3 (R/V KEXUE HAO, 2014) and 4 (R/V KEXUE HAO, 2016). The samples were collected from 4 dredge sites, as shown in Table 1 and Fig. 1.

Based on their chemical compositions, the four samples include basalt (X-2 and TVG9-2; Guo et al., 2018a

Textural and compositional features of plagioclase crystals

The plagioclase crystals in the samples were divided into 5 main textural types, including homogeneous-core plagioclase, fine oscillatory-zoned plagioclase, core-sieved plagioclase, glomerocrystic plagioclase and microlitic plagioclase. The simple interpretations and ranges of An contents of these different types plagioclase are shown in Table 2. The plagioclases in basalt X-2 sample included glomerocrystic (~5%, based on the number of plagioclase grains; hereinafter inclusive) and microlitic

Impacts of kinetic and diffusive processes

In this paper, the combined analyses of the An contents and trace element concentrations (e.g., Fe, Mg, Sr, Ba, Y and some REEs) of the plagioclase grains were used to study their magma chamber systems. These parameters can commonly vary due to closed-system effects (i.e., fluctuations in intensive parameters, decompression) and open-system effects (i.e., the recharge of new magma) (Barca and Trua, 2012) as well as kinetic or diffusive processes, which are discussed in this section.

Kinetic

Conclusions

The analysis of the plagioclase from basalts and silicic rocks of the SOT suggests that this region features complex magma chamber systems. Plagioclase-melt thermobarometry indicates the crystallization of plagioclase phenocrysts in the basalt at the bottom of the lower crust near the Moho (depths of ~20 ± 7.5 km and ~25 ± 7.5 km) at temperatures of 1218–1239 ± 36 °C. In situ measurements of the Sr isotopic compositions of the plagioclase in the basalts indicate that their parental magma was

Acknowledgments

We thank the two anonymous reviewers for their thorough revisions of the manuscript and their insightful reviews, and we are grateful to Guo-liang Zhang for his insightful comments. This work was supported by the National Natural Science Foundation of China (Grant No. 41476044, 41472155, and 41706052), the National Basic Research Program of China (No. 2013CB429702), the Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology grant (No. MGQNLM-KF201707), the

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