Geochemical and Sr‐Nd isotopic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: Implications for the evolution of a young, intracontinental back arc basin

Abstract

Abstract. The Okinawa Trough is an incipient intracontinental back arc basin that has developed behind the Ryukyu arc-trench system. To explore its magmatic evolution and the nature of the mantle source, we present major and trace element and Sr-Nd isotopic data for mafic volcanic rocks from the Middle and Southern Okinawa Trough and the Central Ryukyu Arc. Two episodes of activity formed the latter: older (-21-13 Ma) and younger (-6-4 Ma). Although Quaternary basalts from the Middle Okinawa Trough (MOT)have major element compositions comparable to basalts from intra-oceanic back arc basins, they are characterized by relative enrichments of large ion lithophile elements and Pb and by depletions of high field strength elements analogous to those observed in Ryukyu Arc volcanics. Two components have been involved in the generation of MOT basalt, which we identify as an E-MORB type (or oceanic island basalt type) upwelling asthenospheric mantle and a"subduction component." Quaternary basalts from the Southern Okinawa Trough (SOT) have uniform Nd but heterogeneous Sr isotopic ratios and incompatible trace element compositions. This may be ascribed to more complicated tectonic and magmatic processes in the SOT compared with the MOT, such as oblique subduction of the Philippine Sea Plate and interaction with postcollisional extension in the northern Taiwan orogenic belt. Integrating geological information available from nearby regions, we emphasize that the SOT is an "atypical" back arc basin because its development essentially occurred synchronous with or even prior to development of the arc-trench system.

Geochronological, geochemical and Nd–Hf–Os isotopic fingerprinting of an early Neoproterozoic arc–back-arc system in South China and its accretionary assembly along the margin of Rodinia

“…Thus, the two endmembers should be represented by E-MORB and newly subducted component, respectively. The Group 3 samples show high (La/Yb)cn (1.53-5.90) and low Sm/Nd ratios (0.23-0.31), comparable to those of the BABB with continental basement (e.g., Gribble et al, 1998;Shinjo et al, 1999;Sandeman et al, 2006), which is also evidenced by the discrimination results in Figs. 13a-d and 14c-d.…”

“…Their multi-elemental patterns resemble those of average E-MORB, and are also similar to those of the Okinawa Trough with the exception of insignificant Nb-Ta anomalies ( Fig. 10b; e.g., Shinjo et al, 1999;Fan et al, 2010). The measured ( 143 Nd/ 144 Nd) ratios range from 0.512747 to 0.512880 and initial ε Nd (t = 980 Ma) values from +3.1 to +4.2.…”

“…The majority of the Group 1 samples show similar multi-elemental patterns to average N-MORB and New Caledonia fore-arc basin basalt ( Fig. 10a) with the exception of weakly negative P and Ti anomalies, but several samples display the patterns similar to average Lesser Caucasus back-arc basin basalt due to relatively high (Th/Nb) ratios of 1.42-2.64 (e.g., Shinjo et al, 1999;Rolland et al, 2009;Hässig et al, 2012). The measured Nd isotopic ratios of the samples range from 0.512856 to 0.512954, and the corresponding ε Nd (t = 980 Ma) values from +4.3 to +5.2 (Table 1; Li et al, 1999a,b).…”

“…N-MORB and E-MORB are after Sun and McDonough (1989). The Pickle Nb-enriched basalts and ∼1.0 Ga Nb-enriched basalts in the Yunkai domain are from Hollings and Kerrich (2004) and Zhang et al (2012a), respectively. samples shared an E-MORB-like source with an addition of a new subduction-related component (e.g., Pearce and Peate, 1995;Shinjo et al, 1999). The following observations suggest that the addition should be characterized by subduction-related metasomatism: (1) Group 2 has (Ta/La)n ratios of 0.57-0.99 and (Hf/Sm)n of 0.94-1.28 and the majority defines the trend related to fluid-related metasomatism (e.g., LaFlèche et al, 1998); (2) their Nd/Pb ratios are lower and Nb/Y ratios are higher relative to those of depleted mantle, and the Nb/Zr ratios are mildly variable relative to Th/Zr (e.g., Fig.…”

Geochemical and isotopic evidence for Carboniferous rifting: mafic dykes in the central Sanandaj-Sirjan zone (Dorud-Azna, West Iran)

“…The E-MORB and OIB-type affinities of the mafic dykes are typically found in tectonic settings including oceanicisland/seamounts, continental-rifts and some flood-basalt provinces (e.g., Hochstedter et al 1990;van Staal et al 1991;Goodfellow et al 1995;Shinjo et al 1999;Leitch & Davies 2001). Their genesis has mostly been attributed to mantle plumes, however, there are also occurrences where no plume has been involved (e.g., Haase & Devey 1994).…”