Initiation of arc magmatism in an embryonic continental rifting zone of the Southernmost part of Okinawa Trough

zone of the southernmost part of Okinawa Trough

Sun-Lin Chung

1

_{*, Shing-Lin Wang}

1

_{, Ryuichi Shinjo}

2

_{, Chao-Shing Lee}

3

_{and Cheng-Hong Chen}

1

1_{Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei 106, Taiwan;}2_{Department of Physics and Earth}

Sciences, University of the Ryukyus, Okinawa, Japan;3_{Institute of Applied Geophysics, National Taiwan Ocean University, Keelung,}

Taiwan

Introduction

Backarc basins result from lithospheric extension that occurs behind or, in some cases, within magmatic arcs at the convergent margins. Thus, the for-mation age of backarc basins is gener-ally younger than that of the associated arcs (cf. Taylor, 1995). The Okinawa Trough, extending from SW Kyushu to NE Taiwan (Fig. 1), is widely regarded as an intracontinental backarc basin that is built behind the Ryukyu arc± trench system owing to subduction of the Philippine Sea plate underneath the Eurasian plate (Lee et al., 1980; Letou-zey and Kimura, 1986; Sibuet et al., 1987). In addition to the modern activ-ity (5 0.1 Ma; Sibuet et al., 1995), two phases of crustal extension in the late Miocene (*10±6 Ma) and Pleistocene (*2±0.1 Ma) epochs, with *4 Myr tectonic intermission, have been docu-mented in the middle and northern parts of the Okinawa Trough (see Si-buet et al., 1998, for a review). In its southernmost part, the SPOT (i.e. SW of *1238E; Fig. 1), however, structural (Hsu et al., 1996) and sedimentary (Park et al., 1998) events have been limited to the Quaternary. Thus, in contrast to an apparently simple back-arc environment in the middle±north-ern part of the Trough, the SPOT may have formed through a different, more complicated, kinematic mode that was

probably affiliated with the tectonic evolution of the arc±continent collision in Taiwan (Lee et al., 1998b). Wang et al. (1999) proposed that postcollisional extension in the northern Taiwan mountain belt, which commenced around Plio-Pleistocene time, caused the northern Taiwan volcanic zone ac-tivity (Fig. 1). Moreover, this extension played a role in reactivating the Pleis-tocene rifting in the Middle Okinawa Trough and gave room for its south-westward propagation with associated development of the Ryukyu subduction zone to Taiwan (Shinjo et al., 1999).

Sibuet et al. (1998) presented a de-tailed bathymetric map and seismic reflection profiles of the SPOT, accord-ing to which *70 active submarine volcanoes, are located *100 km above the Wadati±Benioff zone, can be iden-tified (Lee et al., 1998a; unpubl. data). Based on topographic data, Sibuet et al. (1998) proposed that three types of magmatism exist there±represented by arc, backarc and `cross-backarc'-type volcanoes±and ascribed the intense magma production to underthrusting of the Gagua Ridge in the western margin of the Philippine Sea plate (Fig. 1). In order to decode the petro-chemical characteristics of different types of the SPOT magmas and thus understand the tectonic setting for such magmas, new geochemical data are re-ported from volcanic rocks dredged from the SPOT region by the Ocean Research I (Taiwan) and T/S Nagasaki Maru (Japan) cruises. These data indi-cate that all the SPOT lavas show

calc-alkaline affinities and may be explained as products of early arc volcanism caused by the southwestward propaga-tion of the Ryukyu subducpropaga-tion zone. Together with the fact that in the SPOT the rifting event took place broadly synchronously or prior to the volcan-ism, it is suggested that the SPOT is not a backarc basin as often thought. Samples and analytical results Volcanic rocks were dredged from 11 submarine volcanoes in the SPOT cen-tral graben region (Fig. 1b), which in-clude the three types of volcanic do-mains suggested by Sibuet et al. (1998). Among these, 20 fresh samples from five dredging sites were used for geo-chemical analysis and 3 of them (sam-ple # DR2-1, DR4-2 and RN-D9P) subjected to Ar±Ar dating. Whereas the DR samples are too young (5 0.1 Myr age) to date reliably, the last one gives a plateau age of 0.15 + 0.08 (2s) Myr (Chung et al., unpubl. data). Ma-jor and trace element analyses were also carried out for 13 samples from Kueishantao, an emerged islet located at the western end of the SPOT volcanic field that consists dominantly of ande-sitic lava flows (Chen et al., 1995). The main volcanic activity on this islet oc-curred 7.0 + 0.7 ka (Chen et al. 2000). Analytical methods used and represen-tative results of these SPOT lavas are given in Table 1. For comparison, ma-jor and trace element data of a rhyolite (RN-D6P) dredged from the sub-merged volcanic front of southern

Ryu-*C_{2000 Blackwell Science Ltd} 225

ABSTRACT

The Okinawa Trough is a young, intracontinental backarc basin that has formed behind the Ryukyu arc±trench system since late Miocene time.In the Southernmost Part of the Okinawa Trough (SPOT), a cluster of active submarine volcanoes delineates a volcanic belt, which is located only *100 km above the Wadati± Benioff zone.We report herein new major and trace element data for the SPOT volcanic rocks.These rocks show a compositional range from medium-K andesite to rhyolite.Their geochemical characteristics are similar to those of pre-backarc rifting volcanic

rocks from the central Ryukyu arc, and different from those of backarc basin lavas from the Middle Okinawa Trough and the post-backarc rifting Ryukyu arc volcanics.Therefore, despite being topographically contiguous with the rest of the Trough, the SPOT that developed in the Quaternary is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction.

Terra Nova, 12, 225±230, 2000 Ahed Bhed Ched Dhed Fig marker Table marker Ref end Correspondence: Tel.: +886/2-83691242; Fax: +886/2-23636095; E-mail: sunlin@ ccms.ntu.edu.tw

kyus (Shinjo etal.,1998), in the east ofthe SPOT (Fig. 1b), are also listed.

These results indicate that the SPOT magmas possess a rather wide range of composition from andesite to rhyolite (SiO2& 58±73 wt%). Although mafic

rocks have not been recovered, this compositional range differs from that delineated by the remainder of the Oki-nawa Trough, where magmas show a bimodal composition (Shinjo et al., 1999). The SPOT rocks plot in the high-er portion of the median-K field in the SiO2vs. K2O diagram (Fig. 2a), similar

to early to middle Miocene (*21±13 Myr old) volcanic rocks from the cen-tral Ryukyu arc, which were emplaced before the Okinawa Trough opening (Shinjo et al., 1999). The incompatible

element variation patterns of the SPOT magmas are marked by relative enrich-ment in the large ion lithophile eleenrich-ments (LILE; Cs, Rb and Ba) and Th, U and Pb, and depletion in the high field-strength elements (HFSE; Nb, Ta and Ti) (Fig. 2b), a feature typical of arc magmas from the Ryukyu subduc-tion zone (Shinjo et al., 1999) as well as other convergent plate margins (Tat-sumi and Eggins, 1995). The SPOT magmas, moreover, show remarkable geochemical variations from contem-poraneous arc volcanics obtained from an adjacent region in southern Ryukyu. In the K2O vs. SiO2plots (Fig. 2a), the

southern Ryukyu arc lavas (those from RN-D6 and RN-D8; Fig. 1b), which are also of intermediate to acidic

com-positions, have lower potassium and thus straddle the median-K and low-K fields. These lavas reveal different chondrite-normalized rare earth ele-ment (REE) patterns (Fig. 3), with less enriched light REE (e.g. La and Ce) but apparently higher heavy REE concen-trations, from the SPOT magmas. Hence, it can be inferred that, to a certain exten several scenarios of mag-ma production took place between the SPOT and its eastern counterpart under a similar Ryukyu subduction context. Comparison with the central Ryukyu arc±backarc system Shinjo et al. (1999) documented a secu-lar change in magma compositions in

1 2 0 1 2 5 1 3 0 E 2 5 3 0 3 4 N

Philippine Sea

Plate

Eurasian

Plate

Northern Taiwan Volcanic Zone

Taiw

an

7 0 k m / m y 2000 Volcanic front

Asia

Japan Okina wa Philippine Sea

Kyushu

Trench

Ryukyu

1 2 2 E 1 2 3 E 1 2 4 E 2 5 2 4 N DR2 1 0 0 0 1 0 0 0 1 0 0 0 3 000 2000 _{1 0 0 0} 2 0 0 2 0 0 2 00 0 2 0 0

Taiwan

Yaeyama Graben Yonaguni Kueishantao RN-D6 RN-D9 DR3 DR4

Okinawa Trough

a

b

SPOT

Volcanic front 200 km 100 km

Luzon

Arc

Gagua Ridge

Fault A

RN-D8

(NTVZ)

500 1000 Iriomote Ishigaki o o o o o o o o ο ο ο

Fig.1 Map showing the tectonic framework of the Ryukyu arc-backarc system. Asterisks indicate Quaternary volcanoes that comprise the present-day Ryukyu volcanic front and the northern Taiwan volcanic zone (NTVZ) in the north of southern Okinawa Trough. Bathymetric contours are in metres. Modified from Shinjo et al. (1999) and Wang et al. (1999). Inset (a), major tectonic units surrounding the Philippine Sea; inset (b), locations of Kueishantao islet and dredging sites for submarine volcanoes are indicated. A broad line marks the southern Ryukyu volcanic front (Sibuet et al., 1998) and dashed line shows the trajectory of Fault A (Hsu et al., 1996).

the central Ryukyu arc±Middle Okina-wa Trough (backarc) system that may offer clues to understanding the geo-chemical variation between the SPOT and remaining southern Ryukyu volca-nic rocks. In central Ryukyu, arc vol-canism occurred in two phases, in the early middle Miocene (*21±13 Ma) and the latest Miocene (*6±4 Ma), respectively, i.e. before and after the

first stage (*10±6 Ma) of the Okinawa Trough opening. Arc volcanism ceased when rifting in the backarc basin was active. With a shared compositional range from basalt to rhyolite, the two volcanic phases define two distinct trends in the K2O vs. SiO2plots (Fig.

2a). The post-backarc-opening arc la-vas are less enriched in the incompati-ble elements (LILE and LREE) and

possess higher concentrations of Y and HREE than the prebackarc open-ing lavas. This is illustrated in the REE diagram (Fig. 3a), in which arc volca-nics of the early phase display `steeper' REE patterns [(La/Yb)N & 5.8±3.7]

than those of the later phase [(La/ Yb)N& 3.3±1.5] (Shinjo et al., 1999).

Contribution by shallow-level pro-cesses, e.g. crystal fractionation and crustal contamination, has been con-sidered insignificant for such a secular variation because only mafic rocks with rather primitive compositions were used for comparison (Wang, 1998; Shinjo et al., 1999). Given that Sr±Nd isotope compositions of the two volca-nic phases are similar (Shinjo et al., 1999), the secular variation may be interpreted as a result of a change of thermal structure in the mantle wedge resulting from opening of the Okinawa Trough that could have caused at-tenuation of the lithosphere and upwel-ling of the asthenosphere. Therefore, the later phases of the central Ryukyu arc lavas were generated by increasing degrees of melting and shallower depths of magma segregation (Shinjo et al., 1999).

Although magmatism synchronous with the first stage of Okinawa Trough opening is yet to be established, the second stage of opening was associated with intense volcanic activity commen-cing *1.5Ma (Wang, 1998) in the axial zone of the Middle Okinawa Trough (Shinjo et al., 1999). These Quaternary magmas show bimodal compositions dominated by basalt and rhyolite. As reported by Shinjo et al. (1999), whereas most of these basalts are low-K tholeiites (Fig. 2a), which have major element systematics comparable to those from mature backarc basins, these magmas show arc-like trace ele-ment signatures (e.g. relatively enriched in LILE and Pb and depleted in HFSE). These signatures are explained by the Trough being an incipient backarc ba-sin with a prolonged early rifting stage (Sibuet et al., 1998). Two mantle source components, an upwelling astheno-sphere and a `subduction component', have been proposed for the magma generation (Shinjo et al., 1999). The associated rhyolites are sodium-rich (Na2O/K2O & 2.8±1.5) and possess

Nd isotope ratios (eNd & +5.1 to +2.5) similar to those of the basalts. This suggests a cognate magma origin, i.e. both the basalts and rhyolites are Table 1 Major and trace element data of representative volcanic rocks from the SPOT area

Sample No.KST-3 KST-10 DR2±1 DR3±2 DR4±2 DR4±3 RN-D9P{ RN-D6P{ Majorelements (wt.%)* SiO2 58.34 61.43 72.91 72.25 60.63 64.96 65.16 74.82 TiO2 0.71 0.53 0.30 0.30 0.57 0.60 0.40 0.19 Al2O3 17.01 14.29 13.84 13.67 15.72 15.29 19.53 13.08 Fe2O3 7.70 6.60 2.62 2.60 6.96 5.64 4.12 3.13 MnO 0.12 0.11 0.05 0.07 0.12 0.09 0.08 0.12 MgO 3.78 5.01 0.54 0.47 3.60 2.05 1.23 0.21 CaO 6.66 6.04 2.26 2.17 6.86 4.93 3.88 2.34 Na2O 2.69 2.28 3.64 3.74 2.44 2.88 2.84 4.90 K2O 1.78 2.18 2.86 3.04 1.65 2.18 2.42 1.08 P2O5 0.14 0.09 0.06 0.06 0.10 0.11 0.09 0.03 LOI 0.75 0.10 0.80 0.85 1.22 1.05 1.27 2.54 Total 99.68 98.64 99.90 99.22 99.86 99.77 99.75 99.90 Trace elements (ppm)* Sc 27 24 8.8 5.3 19 13 15 15 V 213 149 27 21 161 103 94 2 Cr32 211 8 6 48 15 18 7 Co 24 22 3 3 18 10 8 1 Ni 12 37 4 6 9 6 5 1 Rb 59.5 72.9 130 93.3 51.6 68.2 106 31.8 Sr305 200 90 83 132 126 180 113 Y 28.1 23.3 30.6 19.4 13.9 16.5 20.2 43.2 Zr141 129 111 130 105 135 85 154 Nb 10.2 9.53 8.69 7.37 5.46 7.30 6.90 3.70 Cs 1.71 3.65 5.73 5.44 3.14 4.18 5.73 1.71 Ba 388 377 515 502 275 364 387 225 La 21.5 21.6 28.0 28.1 15.3 20.8 22.4 11.6 Ce 45.2 45.8 56.3 56.5 33.4 43.9 43.9 26.6 Pr5.42 5.30 6.77 6.09 3.85 7.98 4.63 3.43 Nd 21.7 20.4 24.0 20.7 13.9 17.9 17.1 15.7 Sm 4.70 4.34 4.83 4.46 3.07 3.88 3.39 4.42 Eu 1.02 0.83 0.73 0.49 0.50 0.56 0.75 1.15 Gd 4.43 3.93 4.51 3.11 2.25 2.75 3.29 5.28 Tb 0.75 0.66 0.81 0.53 0.37 0.45 0.52 0.98 Dy 4.54 4.00 5.08 3.12 2.26 2.62 3.05 6.25 Ho 0.97 0.84 0.97 0.64 0.45 0.54 0.64 1.39 Er2.77 2.39 2.87 1.88 1.35 1.59 1.90 4.23 Tm 0.41 0.35 0.45 0.29 0.20 0.25 0.30 0.69 Yb 2.63 2.30 2.99 1.89 1.28 1.55 1.97 4.51 Lu 0.39 0.34 0.47 0.30 0.20 0.24 0.31 0.71 Hf 3.68 3.40 3.80 4.25 2.93 3.81 2.25 3.63 Ta 0.78 0.79 0.76 0.72 0.46 0.60 0.56 0.26 Pb 6.24 8.63 13.9 13.1 9.14 11.5 15.5 5.62 Th 7.82 8.06 8.84 12.5 6.47 9.25 9.22 3.25 U 1.69 1.75 1.84 2.70 1.39 1.86 1.82 0.81

*Major elements were determined by X-ray fluorescene spectrometry at National Taiwan University and trace elements were determined by inductively induced plasma-mass spectromotry at Guangzhou Institute of Geochemistry. Analytical details were reported by Wanget al. (1999).

essentially mantle-derived, and crustal contamination, if any, played only a minor role during magma ascent and differentiation (Wang, 1998; Shinjo and Kato, 2000). In comparison to the Ryukyu arc lavas, these backarc basin magmas are marked by com-position bimodality and significantly higher Nd and lower Sr isotope ratios. The backarc basin basalts are less en-riched in LILE and LREE than the post-backarc opening lavas from cen-tral Ryukyu (Shinjo et al., 1999). Con-sequently, decompression melting of the ascending asthenosphere caused by substantial lithospheric thinning is a likely process governing the magma generation in the middle part of Okina-wa Trough.

Mechanism for initiating the SPOT volcanism

The present data indicate that no mat-ter where the SPOT lavas were recov-ered [from the arc (DR-3 and RN-D9), backarc (DR-4) or abnormal `cross-backarc' (DR-2) types of volcanoes proposed by Sibuet et al. (1998)], they demonstrate coherent geochemical characteristics that are comparable to the pre-backarc rifting arc volcanics from central Ryukyu. Coexistence of arc and backarc volcanism in a small area like the SPOT is most unlikely. Whereas magma generation in the backarc basin requires upward convec-tion of the asthenosphere, which would trigger decompression melting, in the

arc setting it is essentially controlled by dehydration from the subduction zone, so that the solidus of the mantle wedge is depressed to cause melting. Thus, the central Ryukyu arc and backarc lavas display different isotope compositions because of the different magma source components involved (Shinjo et al., 1999; 2000). According to Chen et al. (1995; unpubl. data), the SPOT mag-mas possess Sr (87_Sr/86_{Sr 4 0.705) and}

Nd (eNd &±2 to ±5) isotope ratios, which can be discriminated from those of backarc magmas from the Middle Okinawa Trough (with eNd values up to + 5) and correspond to those of certain arc volcanics from central and northern Ryukyus (Shinjo et al. 2000). Hence, it is proposed that Ryukyu sub-duction, rather than intracontinental (backarc) rifting, serves as the govern-ing process for the SPOT magmatism.

This argument is consistent with the tectonic configuration of volcanoes in the SPOT comprising a volcanic belt *100 km above the Wadati±Benioff zone (Fig. 1), the location where the volcanic fronts are observed in the northern Ryukyu subduction zone (Fig. 1) and other convergent margins (Tatsumi and Eggins, 1995). Recent studies (Hsu et al., 1996; Ujiie et al., 1997; Park et al., 1998; Sibuet et al., 1998) have repeatedly shown that the SPOT is an embryonic geological fea-ture and its development was restricted to the Quaternary epoch. The continen-tal crust hence remains intact (*30-km thick) beneath the SPOT area, in con-trast to an apparently thinner crust (*15km) in the Middle Okinawa Trough (Sibuet et al., 1987). It is thus reasonable to regard the SPOT as an incipient extension zone created by the southwestward propagation of the Oki-nawa Trough rifting that has little ef-fect on the magma generation. Such a mode of rift propagation, relating to the Plio±Pleistocene extensional col-lapse of the northern Taiwan mountain belt (Wang et al., 1999), was accompa-nied by southwestward migration of the Ryukyu subduction system, which resulted in the SPOT volcanism. In this sense, volcanoes within the SPOT are products of early arc magmatism. De-spite its topographic continuation with the rest part of the Trough, the SPOT is unlikely, by definition, to be a simple backarc basin because it developed broadly synchronously or even prior to the early arc volcanism.

Volcanics from Central Ryukyu Arc

(Pre-OT Opening) Med.-K 80 70 60 50 0 1 2 3 4 SiO₂ (wt.%) Volcanics from Central Ryukyu Arc

(Post-OT Opening) MOT Rhyolites High-K series Low-K series MOT Basalts a Cs Rb Ba Th U K Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Y Yb 1 10 100 1000

SPOT volcanic rocks (Data from Table 1)

b

SPOT dredged volcanics Kueishantao volcanics Southern Ryukyu arc lavas

Volcanics from S. Ryukyu Arc (east of SPOT) Sample/Primitive Mantle K 2 O (wt. %)

Fig.2 (a) K2O vs. SiO2plots and (b) primitive mantle-normalized elemental variation

patterns of the SPOT volcanic rocks. In (a), data of dredged volcanic rocks from the southern Ryukyu arc, east of the SPOT (Shinjo et al., 1998; unpubl. data), are also plotted. Fields for magmas from the central Ryukyu arc (pre-and post-Okinawa Trough opening) and the middle Okinawa Trough (MOT) are based on Shinjo et al. (1999; references therein). Normalizing values for the N-type mid-ocean ridge basalt (MORB) are from Sun and McDonough (1989).

The spatial variation of magma com-positions between the SPOT and its eastern region implies a difference be-tween the lithospheric and/or thermal structure underneath the SPOT and that of its eastern counterpart. Re-markably, lavas from the eastern part of the southern Ryukyu volcanic front (RN-D6 and RN-D8) exhibit SiO2±

K2O correlations (Fig. 2b) and REE

patterns (Fig. 3) comparable with the latest Miocene, post-backarc opening arc volcanics from central Ryukyu. Thus, in this part of southern Ryukyu, arc lavas could have formed in a setting akin to central Ryukyu, i.e. with a well-developed rift zone in the backarc side under which the continental litho-sphere has been stretched rather sub-stantially. Such an interpretation is in-consistent with the observation, east of the SPOT area, that the southern Ryu-kyu arc±backarc system has existed since middle Miocene times (Shinjo et al., 1999). It is speculated herein that the `boundary' of variation in the mag-ma composition, or inferred change of the lithospheric structure, exists *123± 1248E in the Trough (Fig. 1b), corre-sponding to the strike-slip Fault A documented by Hsu et al. (1996). From here, the second phase of the Okinawa Trough rifting may have propagated rapidly southwestward (at a rate of *126 mm yr71_{; Liu, 1995) with a}

clockwise rotation of *158 (Miki,

1995), to form the SPOT and the com-plex collision/extension/subduction tectonic context off Taiwan (Lee et al., 1998b; Wang et al., 1999).

Concluding remarks

Geochemical data reported herein suggest that early arc volcanism occurs in the SPOT, an embryonic continen-tal extension zone formed by rift pro-pagation from the Okinawa Trough in the north-east. This is analogous to the case observed in the southern Harve Trough where the active Kermadec volcanic front lies in the axial rift grabens and interaction of migrating arc volcanism with backarc extension have been documented by Wright et al. (1996). The present results provide important clues to under-standing not only the regional tecto-no-magmatic evolution, but also the geochemical characteristics of early arc magmatism in the intracontinental convergent margins, which, marked by the median-K calc-alkaline nature, differ from those observed in the intraoceanic setting, where early arc lavas display more depleted boninitic or tholeiitic compositions (Bloomer et al., 1995). In the SPOT, intense sub-marine volcanoes associated with ac-tive hydrothermal venting, a deep-water biological community (Lee et al., 1998a; unpubl. data) and shallow

(*10±40 km) earthquakes (Sibuet et al., 1998), deserve further detailed in-vestigations to study how early arc magmatism occurs and continental rift begins.

Acknowledgements

We thank Y.G. Chen, S.K. Hsu, T.Y. Lee, C.S. Liu, C.H. Lo and K.L. Wang for useful discussion and/or sharing unpublished in-formation, and J.-C. Sibuet and Y. Tatsumi for their insightful reviews, which signifi-cantly improved the paper. This study benefited from financial supports by the National Science Council, Taiwan, ROC and the Ministry of Education, Science, Sports and Culture, Japan.

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