Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Geodynamic implications of crustal lithologies from the southeast GEOSPHERE; v. 14, no. 1 Mariana forearc doi:10.1130/GES01536.1 Mark K. Reagan1, Luan Heywood1,*, Kathleen Goff1, Katsuyoshi Michibayashi2, C. Thomas Foster Jr.1, Brian Jicha3, Thomas Lapen4, William C. McClelland1, Yasuhiko Ohara5,6, Minako Righter4, Sean Scott7, and Kenneth W.W. Sims7 11 figures; 7 tables 1Department of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa 52242, USA 2Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan CORRESPONDENCE: mark -reagan@ uiowa .edu 3Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA 4Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204, USA 5Hydrographic and Oceanographic Department of Japan, Tokyo 100-8932, Japan CITATION: Reagan, M.K., Heywood, L., Goff, K., 6Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan Michibayashi, K., Foster, C.T., Jr., Jicha, B., Lapen, 7Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA T., McClelland, W.C., Ohara, Y., Righter, M., Scott, S., and Sims, K.W.W., 2018, Geodynamic impli- cations of crustal lithologies from the southeast ABSTRACT can Formation in California, variably metamorphosed lower-plate rocks be- Mariana forearc: Geosphere, v. 14, no. 1, p. 1–22, doi: 10 .1130 /GES01536.1. come interspersed within matrices derived from ultramafic and sedimentary The deep submergence research vehicle Shinkai 6500, diving on the Chal- sources during burial and exhumation processes in subduction zones (e.g., Science Editor: Shanaka de Silva lenger segment of the Mariana forearc, encountered a superstructure of nascent Bebout and Barton, 1989; Sorensen and Grossman, 1989; Wakabayashi, Guest Associate Editor: Robert Stern arc crust atop a younger mantle with entrained fragments of metamorphosed 2012; Penniston-Dorland et al., 2014). The crust-mantle transition zone of crust. A plutonic block from this crust collected at 4900 m depth has a crystalliza- the obducted oceanic arc crust in the Jilal complex in Pakistan (e.g., Dhuime Received 26 March 2017 tion age of 46.1 Ma and mixed boninitic-arc tholeiitic geochemical signatures. A et al., 2007) has a basal crustal sequence of igneous and metamorphic rocks Revision received 10 October 2017 hornblende garnetite and two epidote amphibolites were retrieved from depths that includes hornblende garnetite. Here, we report an association of epi- Accepted 22 November 2017 Published online 28 December 2017 between 5938 m and 6277 m in an area dominated by peridotite. The garnetite dote amphibolite, hornblende garnetite, and partially serpentinized mantle appears to represent a crystal cumulate after melting of deep arc crust, whereas peridotites from the inner trench slope of the southern Mariana forearc west the amphibolites are compositionally similar to enriched mid-ocean ridge basalt of the West Santa Rosa Banks fault (the Challenger segment; Ohara et al., (MORB). The initial isotopic compositions of these crustal fragments are akin to 2012). This “naked” forearc (Stern et al., 2012) lacks accretionary sediments those of Eocene to Cretaceous terranes along the periphery of the Philippine but has a suprasubduction zone stratigraphy reminiscent of ophiolites in- plate. The garnetite achieved pressures of 1.2 GPa or higher and temperatures cluding volcanic rocks as young as Pliocene (Ribeiro et al., 2013), gabbros, above 850 °C and thus could represent a fragment of the delaminated root of and melt-depleted peridotites at its greatest depths (e.g., Ohara et al., 2012). one of these terranes. This sample has coeval Sm-Nd, Lu-Hf, and 40Ar-39Ar ages This segment of the forearc contrasts with that to the north and east of West indicating rapid ascent and cooling at 25 Ma, perhaps in association with rifting Santa Rosa Banks fault (Fig. 1), which has basement lithologies thought of the Kyushu-Palau arc. Peak P-T conditions were lower for the amphibolites, to have formed during subduction initiation and early Izu-Bonin-Mariana and their presence on the ocean floor near the garnetite might have resulted (IBM) arc development in the Eocene (e.g., Stern and Bloomer, 1992; Reagan OLD G from mass wasting or normal faulting. The presence of relatively fusible crustal et al., 2010). blocks in the circulating mantle could have contributed to the isotopic similarity We focus on key lithologies from three Shinkai 6500 dives that tran- of Mariana arc and backarc lavas with Indian Ocean MORB. sected the trench slope of the Challenger segment. The dives recovered associated volcanic and plutonic rocks above partially serpentinized dunites OPEN ACCESS INTRODUCTION and harzburgites that host epidote amphibolite and hornblende garnetite. In this study, we explore the petrology, geochemistry, and geochronol- Close association of amphibolites, garnet-rich crustal lithologies, and ogy of the amphibolites and garnetite to constrain their origin and path serpentinized peridotites is found in mélanges and the deep roots of over- to the collection site, which bears on the tectonic development of the IBM thrust oceanic terranes. In mélanges, such as the Catalina schist and Francis- system and the fate of crust in oceanic terranes. We also determine the age and composition of one complex granitoid to determine an age for This paper is published under the terms of the *Now at Department of Geology, Western Washington University, MS 9080, Bellingham, Wash- magmatic activity associated with construction of the forearc crust in CC-BY-NC license. ington 98225, USA this area. © 2017 The Authors GEOSPHERE | Volume 14 | Number 1 Reagan et al. | Crustal lithologies from the southeast Mariana forearc Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/1/4034910/1.pdf 1 by guest on 29 September 2021 Research Paper 14° e B gh Arc Trou Guam iana Ridg iana Mariana Parece Vela Basin r 13° Ma West Mar 12° Fig. 1C WSRBF 35° Challenger Deep A 11° Caroline Ridge 10°N 140°E 141° 142° 143°144° 145° 146° 30° 11°45′ Shikoku Basin C 4 0 Amami Plateau 10 Okinawa 11°44′ Daito Ridge 25° Minami -Daito Oki-Daito RidgeBasin 11°43′ 00 44 0 0 0 3 50 4 4 Urdaneta e 0 20 Plateau 4 0 70 4 4 8 00 4 6 20° 0 11°42′ 0 0 0 8 4 P a l a u R i d g Parece Vela Basin h West Philippine Basin Mariana h c c 4 Trough 800 Benham n y u s h u - 11°41′ e Rise K 4 9 00 r 0 15° T 47 0 Saipan a 6K-1236 00 n 50 a i 00 Fig. 1B r 51 0 Guam a 520 M 11°40′ 00 53 00 Challenger Deep 54 0 0 60 0 5 SSF 10° 55 Yap Caroli h 11°39′ c 0 ne Ridg 0 0 7 0 6K-1234 n 5 8 e 5 r T e Palau p a 00 Palau Y 59 00 Basin 60 0 0 Depth (m) 0 61 11°38′ 0 20 5°N 6 125°E130° 135 140° 145° 150° 0 30 6 0 40 6K-12326 00 11°37′ 65 00 66 0 Figure 1. (A, B) Location maps for geographic features in the Izu-Bonin-Mariana sub- 70 6 0 90 0 6 0 duction system and Philippine plate mentioned in the text. (C) Locations for the 2010 68 Shinkai 6500 dive sites. WSRBF—West Santa Rosa boundary fault; SSF—Shinkai Seep 11°36′ Field (Ohara et al., 2012). 00 70 7 1 00 11°35′N 143°00′E 143°01′143°02′143°03′143°04′143°05′ GEOSPHERE | Volume 14 | Number 1 Reagan et al. | Crustal lithologies from the southeast Mariana forearc Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/1/4034910/1.pdf 2 by guest on 29 September 2021 Research Paper GEOLOGICAL BACKGROUND Challenger segment has large tracks of exposed serpentinized peridotite, with numerous normal faults and isolated areas of rifting and associated younger The IBM arc-trench system is a complex suite of arc, backarc, and oceanic magmatism (Ohara et al., 2012; Ribeiro et al., 2013). Tectonic stresses on the terranes formed along the eastern plate boundary of Eurasia. The oldest ter- forearc largely result from arc-parallel extension associated with slab rollback ranes include the Cretaceous to Paleocene remnant ocean islands and island (Martínez et al., 2000), possibly caused by a tear in the subducting Pacific arcs that make up the Oki-Daito and Daito ridges and the Amami Plateau in plate (Ribeiro et al., 2013). This area is near where the spreading axis of the the northern West Philippine Basin (Hickey-Vargas, 1998; Hickey-Vargas, 2005; South Mariana Trough and the active volcanic front are closest to each other Ishizuka et al., 2011a; Fig. 1). The Huatung Basin abutting Taiwan on the west (Martínez et al., 2000). Serpentine mud volcanoes, which are common north side of the West Philippine Basin is Cretaceous oceanic crust with an enriched of Guam in the IBM forearc, are not evident in the Challenger segment of the mid-ocean ridge basalt (E-MORB) affinity (Hickey-Vargas et al., 2008). The Mariana Trench slope (Ohara et al., 2012). southern end of the Philippine plate west of Palau, the Palau Basin, is thought to be Mesozoic oceanic crust (Taylor and Goodliffe, 2004). SHINKAI 6500 DIVES AND ANALYZED SAMPLES The West Philippine Basin opened between ca. 54 and 30 Ma (Deschamps and Lallemand, 2002) and has a basement of MORB-like basaltic lavas (Savov Samples for this study were collected by the Shinkai 6500 deep-submer- et al., 2006). Subduction of the Pacific plate beneath the West Philippine Basin gence vehicle (DSV) during cruise YK10-12 of the R/V Yokosuka in the south- and older terranes began at ca.