Geochemical and isotopic study of a plutonic suite and related early volcanic sequences in the southern Mariana forearc

Abstract

The forearc of the southern Mariana arc preserves igneous suites formed during the initiation of subduction between the Pacific and Philippine Sea plates about 50 Ma ago. We have studied rare suites of gabbroic to tonalitic plutonic rocks dredged from two locations in the Mariana forearc by cruise by University of Hawai'i cruise KK81-06-26. Comparison of the chemical and isotopic (Sr, Nd, Pb, and Hf) characteristics of these rocks with well-studied volcanics from the forearc reveals that the plutonics from dredge RD63 and RD64 are chemically related to boninites erupted at 48-43 Ma. This is the first report of boninitelike plutonics in the southern Mariana trench. These suites have trace element characteristics consistent with island arc settings (U/Th: 0.58-1.44, Nb/La: 0.18-0.79) and other features uniquely connected with boninites: TiO 2 < 0.15 wt % and Zr/Sm > 25. RD63 plutonics resemble nearby boninite volcanics and were likely derived from differentiated boninite magma with 58% SiO 2 , forming gabbro by crystal accumulation, diorite and quartz diorite by crystallization, and tonalite by crystallization and/or partial melting. The RD64 suite (gabbro through tonalite) may have had a more depleted magma source and formed by accumulation and crystallization only. Although the physical dimensions of the plutonic body are unknown, the relationship with boninites indicates that felsic intrusives can form during early stages of island arc development. Such rocks could form part of midcrustal low-velocity layers detected in arc crust by seismic studies. Tonalites similar to those studied here are also found in some ophiolites.

Philippine Sea Plate inception, evolution, and consumption with special emphasis on the early stages of Izu-Bonin-Mariana subduction

“…4). Johnson et al (2014) analyzed plutonic rocks from two dredge sites along a normal fault scarp off Guam very close to the trench. These rocks were created from boninite parental magmas that were modified into tonalites and are now part of a midcrustal layer.…”

Geochronology, geochemistry, and petrogenesis of the Kezijiaer gabbros, southern Chinese Altai: Evidence for ridge subduction

“…Obviously, such a tectonic scenario is inconsistent with the geochemical compositions of the Kezijiaer gabbros, which are dominated by LREE enrichment and high Ti/V (Figures and 9b). Moreover, the fore‐arc basalt is commonly formed during the subduction initiation accompanied by asthenospheric upwelling and fore‐arc spreading, as manifested in the Mariana fore‐arc basin (Johnson, Hickey‐Vargas, Fryer, Salters, & Reagan, ; Whattam & Stern, ). In the Chinese Altai, arc‐related magmatism occurred at least in the Early Cambrian (Wong, Sun, Zhao, Yuan, & Xiao, ) and subsequently lasted until in the Late Carboniferous and even the Early Permian (Ao, Xiao, Han, Mao, & Zhang, ; Cai et al, ).…”

The Record of the Transition From an Oceanic Arc to a Young Continent in the Talamanca Cordillera

“…Kagalaska pluton from Fraser and Barrett (1959), Citron (1980), Kay et al (1983), Kay et al (1986Kay et al ( , 1990, Romick et al (1990), Tsvetkov (1991, Yogodzinski et al (1995), and Cai et al (2015). Mariana forearc data from Johnson et al (2014). Mariana forearc data from Johnson et al (2014).…”

“…The Mariana islands forearc plutonic rocks (48-43 Ma) compared with the Talamanca suite are characterized by overall depletions in incompatible elements (Figures 11 and 12). These rocks are thought to belong to the initiation of subduction in the Western Pacific, possibly representing derivative magmas that fractionated from contemporaneous boninitic parental magmas (Johnson et al, 2014). Below the Mariana frontal arc seismic velocities range from 5.0 to 6.5 km/s above 12-km depth and increase to 6.6-7.4 km/s between 12 and 20 km and are >7.6 km/s between 20 and 30 km (Calvert et al, 2008).…”

“…Delamination of dense eclogite into the less dense asthenosphere at the base of the lithosphere resulted in upwelling of hotter asthenosphere that subsequently decompressed to generate melt (e.g., Condie et al, 2011;van Thienen et al, 2004;Van Kranendonk, 2010;Zegers & van Keken, 2001). The foundering of denser material would have enhanced decompression melting and catalyzed multiple melting events that ultimately generated suites of tonalite, trondhjemite, and granodiorite (TTGs) that rose to Earth's surface to form felsic continental crust (Bédard, 2006;Johnson et al, 2014;Smithies et al, 2009). Although the "stagnant lid" model is numerically plausible, melting an anhydrous basaltic protolith produces small melt fractions that differ in composition from Archean TTGs (Moyen & Stevens, 2006).…”