Geodynamic Implications of Crustal Lithologies from the Southeast GEOSPHERE; V

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 implications 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.

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′

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. 52 Ma. Subduction initiation caused near- east Mariana forearc. A three-dive transect including dives 6K-1232, 6K-1234, trench seafloor spreading and volcanism along the IBM forearc (e.g., Stern and 6K-1236 covered depths from 6425 m to 4646 m at ~143°E latitude south of and Bloomer, 1992), which resulted in an ophiolitic stratigraphy of depleted the Mariana Trough (Fig. 1). The shallower two dives (6K-1234 and 6K-1236) re- peridotites, gabbroic rocks, dolerites, basalts, boninites, and early-arc rocks covered an ophiolitic suite of samples including depleted peridotites, gabbroic from deep to shallow and from the trench toward the arc (Reagan et al., 2010). to tonalitic intrusive rocks, and basalt to andesitic lavas. The deepest dive, The oldest lavas are highly depleted basalts with ages of 51–52 Ma (Ishizuka 6K‑1232, largely recovered variably serpentinized dunites and harzburgites. et al., 2011a; Reagan et al., 2013). Lavas with boninitic compositions erupted Dive 6K-1232 also recovered three loose fragments of metamorphic crustal between 49 and 44 Ma (Cosca et al., 1998; Ishizuka et al., 2006). Normal arc rocks scattered amongst the serpentinized peridotites on the sedimented volcanism began between 44 and 45 Ma along the length of the nascent IBM seafloor. These samples comprise a hornblende garnetite (6K1232R07) and arc (Ishizuka et al., 2006; Reagan et al., 2008). two epidote amphibolites (6K1232R13 and 6K1232R16) collected at depths of About the same time as subduction began, enriched ocean-island basalt 6276 m (11°37.2114′N; 143°0.6132′E), 6046 m (11°37.5336′N; 143°0.4764′E), and (OIB)–like magmas were generated from a mantle plume (Ishizuka et al., 2013) 5934 m (11°37.6291′N; 143°0.4246′E), respectively. Although no direct contact near the West Philippine Basin spreading center. The oldest of these lavas relationships were observed, the close seafloor association of the amphibo- (45–50 Ma) are preserved in the Minami Daito Basin (Hickey-Vargas, 1998), lites, garnetite, and peridotites suggests that these diverse lithologies were and progressively younger lavas are found on the western Oki-Daito Ridge structurally interleaved. (41–45 Ma) and the Urdaneta Plateau (40–36 Ma; Ishizuka et al., 2013). Equiv- An image analysis of the surface cut through the hornblende garnetite alent lavas south of the West Philippine Basin spreading center are found (Figs. 2B, 3A, and 3B) shows that it has ~69% garnet and 31% hornblende. Less on the Benham Rise (ca. 36 Ma, Hickey-Vargas, 1998). Recent ages obtained than 1% of the sample consists of accessory minerals of titanite, magnetite, on lavas from the Kyushu-Palau Ridge (Ishizuka et al., 2011b) suggest that ilmenite, with trace pyrite and chalcopyrite. Garnets are anhedral, essentially its subduction-related volcanism occurred largely after the West Philippine unzoned, and average ~1 cm in diameter. Some grains are crosscut by amphi- basin opened. bole veins (Fig. 4), consistent with retrograde hydration. Under plane-polarized The Kyushu Palau arc began rifting at 25 Ma, eventually forming the Parece- light (PPL), amphiboles are normally pleochroic green to brownish green to Vela–Shikoku Basin by seafloor spreading until ca. 6 Ma (Ishizuka et al., 2011b). tan but can have blue to tan pleochroism near grain boundaries (Fig. 3A). Opening of the Mariana Trough began ca. 6 Ma, leaving the remnant Mariana The epidote amphibolites (Figs. 2C, 2D, 3C, and 3D) have two generations of Ridge to the west (e.g., Fryer, 1995). The modern Mariana Arc is magmatically amphibole totaling 50%–70%. The first generation of amphiboles is millimeter active, as is the Mariana Trough. In the southern Mariana Trough, arc and back- scale, often rounded, and brownish-green hornblende. The second generation arc volcanism converge (Fig. 1) because of the curvature of the Mariana Trench is bluish-green hornblendes intergrown with fine-grained epidote (20%–35%) to a nearly east-west orientation in this area (Martínez et al., 2000). and 5%–10% accessory phases including titanite, ilmenite, and hematite after During the Late Miocene, the Caroline Ridge collided with Mariana Arc, pyrite. These samples have ~5% plagioclase predominantly in veins ± chlorite. effectively pinning the Yap Trench. This stopped backarc extension to the west An intrusive rock (6K1236R14, Figs. 2A, 3E, and 3F) recovered from a depth but caused extension and forearc volcanism adjacent to the Challenger seg- of 4899 m (11°40.6405′N; 143°2.5850′E) as a loose block atop sediment during ment of the Mariana Trench (Martínez et al., 2000). The trench slope of the dive 6K-1236 consists of mingled fine-grained hornblende quartz diorite and

Figure 2. Photographs of cut slab surfaces taken onboard the R/V Yokosuka shortly following Shinkai 6500 diving illustrating the samples studied here. (A) 6K1236R14; t—tonalite; qd—quartz diorite. (B) Horn- blende garnetite 6K1232R7. (C) Epidote amphibolite 6K1232R13. (D) Epidote amphibolite 6K1232R16. C D

coarser tonalite and contains zircons. The tonalite has subhedral zoned plagio METHODS clase with a matrix dominated by graphically intergrown quartz and feldspar. The finer-grained quartz diorite has abundant round inclusions with rims The epidote amphibolite and hornblende garnetite whole rocks were ana outlined by fine opaque minerals. Intergrown euhedral plagioclase crystals lyzed for major-and trace-element compositions, as well as Nd, Hf, and Pb iso- compose the outer zone (10%–20%) of these inclusions. The core of the inclu- topic compositions. In addition, minerals separated from the garnetite were sions consists of anhedral quartz. We speculate that these round inclusions are dated using Sm-Nd, Lu-Hf, and 40Ar/39Ar techniques. Minerals in this sample segregation vesicles where gas-filled bubbles were filled by interstitial late- also were analyzed for major-element compositions by electron probe micro- stage siliceous fluids. The entire sample is heavily altered, with considerable analysis (EPMA) and for trace-element compositions by laser ablation–induc- replacement of sodic plagioclase by fine clays and rare epidote. Secondary tively coupled plasma mass spectrometry (LA-ICPMS). Part of the granitoid chlorite is present in much of the sample. sample (6K1236R14) was divided into tonalite and quartz diorite fractions

A Hornblende B

Garnet

Titanite Hornblende Titanite Hornblende C D Figure 3. Photomicrographs of samples used in this study. (A) 6K1232R7 garnetite plane polarized light (ppl); (B) 6K1232R7 garnetite crossed polars (xpl); (C) 6K1232R16 epidote amphibolite ppl; (D) 6K1232R13 epidote amphibolite ppl; (E) 6K1236R14 tonalite xpl; and (F) 6K1236R14 quartz diorite xpl. Scale bars = 1 mm. The principal mineral phases in each thin section are illustrated. Opaque grains in (A) and (B) are primarily ilmenite with inclusions of magnetite and titanite based on scanning electron microscopy– energy dispersive X-ray spectroscopy (SEM‑EDS) analysis.

Chlorite Plagioclase Quartz Epidote Plagioclase Hornblende E F

LiBO on top of the sample. The crucibles were placed in the furnace at 1050 °C Ilmenite 2 for ~16 min, long enough for the sample to become molten. The molten sam-

ple was quickly poured into prepared bottles of 50 ml 5% HNO3 and sonicated for ~10 min to assure dissolution. Approximately 10 ml aliquot of sample

solution was placed into a 125 ml bottle and diluted with 70 ml HNO3. Major- element data were collected from this solution using a Varian 720-ES ICP-OES with settings and conditions discussed in Reagan et al. (2013). Based on six replicate analyses of standard JA-1, errors on precision and accuracy for all

major-element oxides are less than 2%. Exceptions are accuracy errors for P2O5 (10%) and CaO (3.5%). Hornblende For trace-element analysis, ~0.1 g of sample powders was weighed and Garnet added to Teflon beakers for dissolution using a combination of concentrated

HF acid and concentrated HNO3 acid. After drying, 5 ml of MilliQ water and

5 ml of concentrated HNO3 were added to re-dissolve the samples. Solutions were heated at 90 °C for over 12 h and then transferred to 60 ml high-density polyethy lene (HDPE) bottles and diluted with MilliQ water. An aliquot of 10 ml of this solution was put into a 250 ml HDPE bottle along with 80 ml of dilute

HNO3 and was spiked using an internal Rh-In spike. These samples were then Fe-oxyhydroxide run using the Thermal X-series ICP-MS according to methods described in Peate et al. (2010). Based on replicate analyses of standard BRP-1 (Peate et al., Figure 4. Backscatter image of area in garnetite sample 6K1232R7 illustrating hornblende vein- 2010), precision and accuracy errors for most elements are between 1% and ing in garnet. Locations of energy dispersive X-ray spectroscopy (EDS) spot analyses are shown. 3%. Elements with higher precision errors are Sc (7%) and Th (8%). Elements 10 = garnet; 11, 12, 14, 15, 17 = hornblende; 16 = ilmenite; 13 = Fe oxy-hydroxide. with values that diverged from accepted values for BRP-1 by more than 3% are Ni (14%), Nb (7%), Pr (4%), Ta (5%), and Th (5%).

using a rock saw for whole-rock major- and trace-element analyses. The re- Electron Probe Micro-Analysis mainder was kept whole for zircon separation. Weathered rinds were sawed off whole rocks before washing in distilled The chemical compositions of garnet and hornblende were analyzed using water and crushing. Randomized portions of the fresh rock chips were pow- a JEOL electron microscope (JXA733) housed at the Center for Instrumental dered for each sample using a ceramic mill. In preparation for Sm-Nd and Analysis, Shizuoka University, Japan. Analytical conditions were 15 kV acceler- Lu-Hf geochronology, the hornblende garnetite sample (6K1232R07) was ating voltage, 12 nA probe current, and a beam diameter of 20 µm. ground and sieved to ~0.1 mm so that monocrystalline grains of garnet and hornblende could be separated using heavy liquids and handpicking. This 40Ar/39Ar Geochronology process obtained five aliquots: hornblende, whole-rock powder, garnet with visible inclusions, “optically clean” garnet without visible inclusions, and “op- Hornblende from sample 6K1232R07 was incrementally heated with a 25W

tically clean” garnet that underwent partial dissolution procedures. Mineral CO2 laser, and the gas was analyzed using a MAP 215-50 mass spectrome- separates were hand crushed to a powder in a ceramic mortar and pestle. ter at the University of Wisconsin–Madison following methods described in Jicha et al. (2006). Data are calculated relative to a Fish Canyon sanidine stan- Major- and Trace-Element Analysis dard age of 28.201 Ma (Kuiper et al., 2008) using the decay constants of Min et al. (2000). Whole-rock powders of the amphibolites and garnetite were analyzed by X-ray fluorescence (XRF) at Washington State University using methods de- U-Pb Zircon Ages scribed in Johnson et al. (1999). The granitoid was analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES) techniques at the A heavy-mineral separate from a portion of granitoid sample (6K1236R14) University of Iowa. In preparation for major-element analysis by ICP-OES, was obtained by standard pulverizing, magnetic, and heavy-liquid methods.

~0.2 g of research grade LiBO2 flux was added to the bottom of a heat-cleaned Individual zircon grains were handpicked under alcohol, mounted in epoxy graphite crucible, followed by 0.1 g of rock powder and then a layer of 0.2 g resin with natural zircon standards, and polished to expose the grain centers

for analysis by secondary ion microprobe spectrometry (SIMS) at the U.S. collectors with ratios normalized to 203Tl/205Tl = 0.418922 to account for instru- Geological Survey (USGS)–Stanford Microanalytical Center sensitive high- mental mass bias. A seventh Faraday collector was used to monitor mercury. resolution ion microprobe–reverse geometry (SHRIMP-RG) facility. Zircon Lead-isotope ratios are reported relative to NBS-981 values of Thirlwall (2002). grains were imaged by cathodoluminescence (CL) to expose intra-grain zoning Internal precisions for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were ~30 ppm. or complexity and aid in placing SIMS spots. The U-Pb and trace- element Analyses of USGS standard BCR-2 gave 206Pb/204Pb = 18.7631, 207Pb/204Pb = analyses were performed simultaneously following routines outlined in Barth 16.6275, 208Pb/204Pb = 38.7254, in agreement with values reported by Weis et al. and Wooden (2006) and Mazdab and Wooden (2006). Additional trace-element (2006). External reproducibility for BCR-2 and other rock standard analyses data were collected from analytical spots placed adjacent to the initial U-Pb ranges from 200 to 300 ppm. The procedural blank for this analysis was 51 analysis site within the same CL domain. Fractionation corrections were cali picograms (pg), which is insignificant relative to the sample size. brated by replicate analysis of the zircon standard R33 (421 Ma; Black et al., 2004; Mattinson, 2010) with a 2σ calibration error for the R33 206Pb/238U ratio of 0.76% for the analytical session. Ages were calculated from 206Pb/238U ratios cor- Lu-Hf and Sm-Nd Ages; Initial Nd and Hf Isotopic Compositions rected for common Pb using the 207Pb method and 207Pb/206Pb ratios corrected for common Pb using the 204Pb method (see Williams, 1998). Initial common Pb Neodymium and Hf isotopic compositions of whole-rock powders and min- isotopic composition was approximated from Stacey and Kramers (1975). The eral separates were performed at the University of Houston using a Nu Plasma U concentration was calibrated with Madagascar (MAD) green (4196 ppm U, II multiple collector–inductively coupled plasma mass spectrometer. Whole- Barth and Wooden, 2010). Data reduction and plotting were performed using rock, garnet with inclusions, and hornblende samples were powdered, spiked the programs Squid 1.13b (Ludwig, 2001) and Isoplot 3.00 (Ludwig, 2003). with mixed 176Lu-178Hf and 149Sm-150Nd isotope tracers, and then digested in 89 139 140 146 147 153 155 The trace-element routine collected Y, La, Ce, Nd, Sm, Eu, Gd, 8:1 mixtures of distilled HF and HNO3 on a hotplate. One of the picked garnet 163Dy16O, 166Er16O, 172Yb16O, and 180Hf16O at the same time as the U-Pb analysis. fractions was washed in HCl to remove any labile and REE-rich components The following peaks were measured during the second trace-element analysis: such as phosphate leaving a garnet leached residue fraction. All of the garnet 7Li, 9Be, 11 B, 19F, 23Na, 27Al, 30Si, 31P, 39K, 40Ca, 45Sc, 48Ti, 49Ti, 56Fe, 89Y, 93Nb, 94Zr1H, fractions were spiked with mixed 176Lu-178Hf and 149Sm-150Nd isotope tracers 96 139 140 146 147 153 165 157 16 159 16 163 16 166 16 Zr, La, Ce, Nd, Sm, Eu, Ho, Gd O, Tb O, Dy O, Er O, and then digested in 8:1 mixtures of distilled HF and HNO3 on a hotplate. 169 16 172 16 175 16 90 16 180 16 206 232 16 238 16 Tm O, Yb O, Lu O, Zr2 O, Hf O, Pb, Th O, and U O. Concen- After dissolution of all samples, chemical separation of Hf, Nd, Lu, and Sm trations were calibrated against zircon standard MAD (Mazdab and Wooden, was conducted following methods outlined in Lapen et al. (2004, 2017). Mass 2006). The estimated errors based on repeated analysis of MAD are 3%–10% spectrometry, instrumental mass fractionation correction, isotope-dilution cal- for P, Y, Hf, Th, and U and the rare-earth elements (REEs) except for La (20%). culations, and spike subtraction follow methods outlined in Lapen et al. (2004). The 49Ti data, with an estimated error of 4% based on analysis of MAD, were Measured Hf and Nd isotopic compositions of standard BCR-2 analyzed during used to determine the Ti content to avoid interference of 96Zr2+ with the 48Ti the course of this study are 0.282870 ± 6 and 0.512631 ± 13 (2sd), respectively. peak (Watson and Harrison, 2005). Ti-in-zircon temperatures were calculated External precision of the 176Hf/177Hf and 143Nd/144Nd isotope ratio measurements

using Ferry and Watson (2007), assuming the activity of SiO2 is equal to one based on replicate analyses of rock and solution standards during the course

(and activity of TiO2 is ~0.7 for rutile-absent siliceous melts; Hayden and of the study are ± 0.006 and 0.0035%, respectively. Blanks of Hf, Nd, Lu, and Watson, 2007). Sm are <50 pg, <30 pg, <5 pg, and <40 pg, respectively, and are negligible.

Pb Isotopes RESULTS

Lead isotopic compositions were measured at the University of Wyoming Tonalite–Quartz Diorite High-Precision Isotope Laboratory using a ThermoFinnigan™ NeptunePlus multicollector–inductively coupled plasma mass spectrometer (MC-ICPMS). The mingled tonalite–quartz diorite sample (6K1236R14) yielded a small

Rock powders were dissolved using a mixture of concentrated HF-HNO3 fol- population of euhedral, oscillatory zoned zircon (Fig. 5A). Sixteen analy- lowed by 6M HCl. Lead was purified using standard anion exchange chroma- ses give a weighted-mean 206Pb/238U age of 46.1 ± 0.7 Ma (mean square of tography in HBr after Strelow and Toerien (1966). Samples were analyzed in 5% weighted deviates [MSWD] = 1.6; Table 1; Fig. 5B) that is interpreted as the

HNO3 solutions and introduced into the plasma using an Aridus II desolvating crystallization age of the sample. The trace-element data show low light-REE nebulizer. Thallium (National Bureau of Standards [NBS]-997) was added to and heavy-REE enrichment characteristic of magmatic zircon (Fig. 5C). The U each sample to correct for mass fractionation with Pb/Tl ratio target of ~3/1. and Th concentrations are generally low (U = 20–192 ppm; Th = 4–91 ppm) Lead and thallium isotopes were analyzed in static mode using six Faraday with one analysis of CL-dark zircon reaching higher concentrations of 954 ppm

elements that are commonly enriched in subduction-related lavas, such as K, A 1.2t 6.1 B 0.09 46.1 ± 0.7 Ma 6K1436R14 Rb, and Ba (Fig. 6), although these enrichments might have been affected by (MSWD= 1.6) alteration. The quartz diorite portion of this sample has higher REE concentra- 6.2t tions, a flatter REE pattern, and a more pronounced depletion in Nb compared Pb with the tonalite. Overall, the compositions are consistent with mingling be- 206 tween a differentiated boninitic magma and a more mafic but still differenti-

Pb / 52 ated arc tholeiite. 20 7 54 38 1.1 0.03 46 9.1 Amphibolites and Garnetite 10.1 120 238 206 160 U/ Pb The two epidote amphibolite samples have major-element compositions resembling those of enriched mid-ocean ridge basalts (Table 4), with flat REE C patterns, with a low Ba/La and no Nb anomaly (Fig. 6B). Uranium is somewhat 9.2t 3 10 enriched in both samples, and Ba is relatively enriched in sample 6K1232R16, 10.2t 2 e but these isolated enrichments most likely occurred during metamorphism 5.1 10 11.1 and hydration. In contrast, the hornblende garnetite has relatively low concen- 1 10 trations of SiO2, Na2O, and K2O compared to basaltic lavas and has relatively 0 10 high concentrations of Al2O3, FeO, and CaO (Table 4). The enrichment in the middle rare-earth elements (REEs) compared to heavy and light REE and low -1

zircon/chondrit 10 5.2t Trace element only Zr/Sm in this sample would be highly unusual for a basalt from any tectonic -2 Trace element with U/Pb setting. All of these compositional traits suggest that this sample consists of 11.2t 10 accumulated garnet and amphibole or pyroxene after extraction of melt either 50 µm La Ce Pr Nd PmSmEu Gd Tb Dy Ho Er Tm Yb Lu during melting of mafic crust at high pressure (e.g., Garrido et al., 2006) or crystal fractionation of a mafic lava in the deep crust (e.g., Jagoutz et al., 2009). Figure 5. Zircon geochemistry and geochronology for bulk granitic sample 6K1436R14. Data are from The relatively low Mg # suggests that the associated melt and bulk initial Tables 1 and 3. (A) Cathodoluminescence images of representative zircon grains showing location of rock or bulk initial magma were relatively differentiated. The relatively high analytical spots. (B) Tera-Wasserburg plot (Tera and Wasserburg, 1972) illustrating U-Pb ages for spots on zircons extracted from 6K1236R14. (C) Chondrite-normalized plot of rare-earth element (REE) con- concentrations of Rb, Ba, Sr, and Pb in this sample are consistent with the centrations. Chrondrite REE abundances are from Anders and Grevesse (1989) multiplied by a factor addition of a hydrous fluid from a subducting plate to its parental magma of 1.36 (Korotev, 1996). Chondrite-normalized values for Pr were calculated by interpolation (Pr(N) = source. La(N)0.33 × Nd(N)0.67). The amphiboles in the hornblende garnetite are pargasites (Table 5) using the classification scheme of Hawthorne and Oberti (2007). The garnets are dominated by pyrope, almandine, and grossular components and classify as and 1005 ppm for U and Th, respectively. Ti-in-zircon crystallization tempera- G3 (Grütter et al., 2004) and eclogitic (Schulze, 2003). Al-in-hornblende geo- tures calculated using the equation of Ferry and Watson (2007) range from barometry (Schmidt, 1992) gives a pressure of ~1.2 GPa, which we interpret 734 to 824 °C. Excluding one estimate due to high Fe and Al concentrations as a pressure along the retrograde path because of the textural evidence that that indicate contamination from inclusions, the remaining data define mean hornblende grew after garnet. A pseudosection was drawn using a whole- temperature estimate of 768 °C. rock composition constructed from mineral compositions and proportions The trace-element data collected on the separate tonalite and diorite por- in this sample but without Mn, Ti, and P using Theriak/Domino (de Capitani, tions by solution ICP-MS have lower Zr concentrations than the data collected and Petrakakis, 2010). At 1.2 GPa, an assemblage lacking chlorite and spinel by ICP-OES after flux fusion (Table 2). We attribute this to incomplete zircon dis- and dominated by garnet, hornblende, and melt is produced at temperatures solution before the ICP-MS analyses. Therefore, to determine the incompatible between 850 and 1000 °C (Fig. 7). Approximately 4% clinopyroxene appears trace-element compositions of these portions, the trace-element composition in the pseudosection under these conditions, although it is not present in the of the average zircon (Table 3) for this sample was added until Zr determined garnetite sample. Doubling the Fe3+/Fe used in the modeling halves the clino- by the ICP-MS and ICP-OES matched. The resulting composition for the tonalite pyroxene proportion but does not remove it entirely. Either the appearance of is boninitic, with low overall REE concentrations, a light-enriched and some- clinopyroxene is an artifact of errors in mineral solution models, or clinopyrox- what U-shaped REE pattern, and elevated concentrations of some fluid soluble ene once existed in this sample but was replaced by hornblende during retro-

TABLE 1. SECONDARY ION MICROPROBE SPECTROMETRY (SIMS) U-Pb GEOCHRONOLOGIC ZIRCON DATA AND APPARENT AGES FOR SAMPLE 6K1236R14 U Th 206Pb*b 1σ 1σ 206Pb/238Ud a 206 b238 206 c 207 206 c Spot (ppm) (ppm) Th/U (ppm)fPbc U/ Pb (%) Pb/ Pb (%) (Ma)1σ 1.1 c2860.2 0.2 1.92 134.97(3.4).0621 (13.7) 46.7(1.7) 2.1 c2450.2 0.2 1.17 132.43(3.9) .0562(16.4)47.9(1.9) 3.1 c5117 0.3 0.3 <0.01137.13(3.0) .0438 (12.9) 47.0(1.4) 4.1 c7217 0.3 0.4 1.77 137.54 (2.5).0610 (9.8)45.9(1.2) 5.1 c 188 84 0.5 1.1 0.46142.81 (1.8).0505 (6.5)44.8 (0.8) 6.1 c44 12 0.3 0.3 0.67 147.21 (3.1).0522 (13.3) 43.3(1.4) 7.1 c2060.3 0.1 1.04 141.95 (4.3).0551 (22.3) 44.8 (2.0) 8.1 c 954 1005 1.1 6.0 <0.01136.29(1.3) .0456 (2.8)47.2(0.6) 9.1 c 137 48 0.4 0.8 0.06 140.68 (2.0).0474 (9.0)45.6(0.9) 10.1 c2240.2 0.1 <0.01156.52(4.2).0420 (29.1) 41.3(1.9) 11.1 c 172 91 0.5 1.1 <0.01138.74 (1.8).0465 (6.5)46.3(0.8) 12.1 c5816 0.3 0.4 0.46129.99(2.7) .0506(11.2)49.2(1.4) 13.1 c 119 37 0.3 0.7 0.77 140.50 (2.1).0530 (7.7)45.4 (1.0) 14.1 c 124 37 0.3 0.8 <0.01138.43(2.0) .0443(8.5) 46.6(0.9) 15.1 c5717 0.3 0.4 0.15 137.26 (3.1).0482 (13.9) 46.7(1.5) 16.1 c 192 80 0.4 1.2 <0.01140.77 (2.8).0466 (7.7)45.7(1.3) Note: All analyses were performed on the sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) at the U.S. Geological Survey–Stanford Microanalytical Center at Stanford University. The analytical routine followed Barth and Wooden (2006). Data reduction utilized the Squid 1.13b program of Ludwig (2001). aAbbreviations and cathodoluminescence designations: c—core. b 206 206 206 Pb* denotes radiogenic Pb; Pbc denotes common Pb; f Pbc = 100*( Pbc/ Pbtotal). cCalibration concentrations and isotopic compositions were based on replicate analyses of R33 (421 Ma, Black et al., 2004; Mattinson, 2010) and Madagascar green (MAD; 4196 ppm U, Barth and Wooden, 2010). Reported ratios are not corrected for common Pb. Errors are reported in parentheses as percent at the 1σ level. dAges were calculated from 206Pb/238U ratios corrected for common Pb using the 207Pb method (see Williams, 1998). Initial common Pb isotopic composition approximated from Stacey and Kramers (1975). Uncertainties in millions of years reported as 1σ.

gression. An eclogite-melt assemblage is calculated for this bulk composition 25 Ma, their initial εHf values (13.3–13.4) are similar to that of the garnetite, but at 1.2 GPa and above 1000 °C. their εNd values are significantly more radiogenic (8.3–8.5; Fig. 10A). The epidote amphibolites have complex textures implying three gener- The 206Pb/204Pb values for the epidote amphibolites are moderate for IBM ations of mineral growth: the early coarse brownish hornblende, the finer- crustal rocks (18.748 and 18.832) with small positive Δ7/4 and Δ8/4 values (Table grained intergrown epidote and blue-green amphibole, and the plagioclase- 4; Fig. 10B), whereas the hornblende garnetite has a lower 206Pb/204Pb value chlorite veins. Theriak/Domino modeling, using whole-rock compositions (18.345) and a higher Δ8/4 value. This range of isotopic compositions is shared and assuming water concentrations of 1–2 wt%, Fe3+/ΣFe = 0–0.2, no garnet, by early arc boninites and pre-arc terranes such as the Amami Plateau and the and no melting, suggests that the early-formed brownish amphibole grew at Oki-Daito Ridge. However, they differ from later plume-related lavas from the pressures <1.3 GPa and temperatures <800 °C. The second-generation amphi- Benham Rise, the Oki-Daito Rise, the Urdaneta Plateau, and the Minami-Daito bole-epidote intergrowth is limited to temperatures between ~650 and 400 °C Basin (cf. Ishizuka et al., 2013). All of these comparisons remain valid after adjust- and pressures between 1.3 and 0.8 GPa. The plagioclase-chlorite assemblage ing the Pb isotopic compositions for the 25 Ma age of the hornblende garnetite. likely grew at temperatures below 475 °C and pressures below 0.7 GPa. Mineral separates from hornblende garnetite 6K1232R07 produced a Sm-Nd isochron age of 24.8 Ma ± 2.5 Ma and a Lu-Hf isochron age of 25.21± DISCUSSION 0.47 Ma (Table 6; Fig. 8). Both ages are concordant with this sample’s 40Ar/39Ar plateau age of 25.20 ± 0.86 Ma (Table 7; Fig. 9). The initial 143Nd/144Nd ratio for Tonalite-Quartz Diorite this sample was 0.512817 ± 0.000014 (2σ), and its initial 176Hf/177Hf ratio was 0.2831322 ± 0.0000096 (εNd = 4.1; εHf = 13.4). Sample 6K1236R14 consists of mingled boninitic tonalite and arc tholeiitic The epidote amphibolites have 176Hf/177Hf values of 0.283168 and 0.283170 quartz diorite, illustrating that it is a magma generated in association with and 143Nd/144Nd values of 0.513065 and 0.513073 (Table 6). Assuming an age of subduction. Its age of 46.1 Ma is during the time when the nascent IBM arc

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 9 by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/1/4034910/1.pdf Research Paper f Y Lu Yb Er Ho Dy Tb Ti Gd Eu Sm Hf Zr Nd Pr Ce La Ta Nb U Th Pb Sr Ba K Rb Cs ICP Ba Zr Y Sr Zn Cu Ni Cr V Sc XRF trace elements SUM P K Na Ca MgO MnO Fe Al Ti SiO luor 2 2 O ¶ § # Not 2 O O 2 St Pe Be fo O O 2 O 2 -MS (measur O 2 5 escence 3 oichiometr # 3 rc e: * ent zir re Major nor . malization. con added. -element data ar ic concentration. ed) 3175 200 131.2 127 189 180 101.40 Dior 25.7 10.55 14.91 18.1 50 26.5 40 17 21.0 15.86 63.16 11 4.60 0.38 2.53 2.6 0.9 4.13 0.63 3.62 0.74 3.09 0.67 2.19 5.62 0.097 1.51 0.153 0.429 1.42 0.16 72 0.08 2.15 0.13 0.52 1.12 5.30 7.08 b .6 .d. e nor it eT maliz T ABLE 2. ed. A bbre MAJOR- (WT%) viations: 1890 171 152.3 145 165 100.81 128 14.5 21 13.21 73.87 57 onalit 4.03 3.2 0.14 0.92 0.91 0.3 1.44 0.23 1.39 0.55 1.24 0.19 1.19 4.07 0.083 1.62 0.085 0.437 1.24 0.106 0.05 0.82 0.06 0.32 1.14 3.41 9 5.09 9.2 9.2 9 6.2 3.08 b bd—belo .d. e AND w de te TRA ction limit ; ICP CE-ELEMENT (PPM) D 430000 10033 1509 121. 80 603. 22 281. 32 122. 84 Zir 54.83 23. 53 96 34. 80 9.50 0.56 2.00 0.01 2. 11 0. 91 00 0.64 8.42 0 0 0 9.31 0 co n0 ¶ -MS—inductiv ATA el FOR 6K1236R14 Co y coupled plasma–mass spectr rr ection fo 3175 9463 200 131 25. 10.55 14.91 18.10 50 Dio ri .01% 0.90 0.16 0.63 0.74 3.09 1.57 5.62 .6 .4 .3 .5 .6 .1 .5 .4 .1 89 r zir 90 81 20 41 21 11 30 21 60 te § con loss omet ry 1889 9546 171 152 128 T 0.03% 14.50 onalit ; XRF—X-r 0.32 0.11 0.23 0.55 1.24 3.10 4.07 5.09 9.20 .4 .1 8 .0 9 .9 9 .4 8 .4 0 .6 2 .4 5 .2 4 .1 1 e § ay

GEOSPHERE | Volume 14 | Number 1 Reagan et al. | Crustal lithologies from the southeast Mariana forearc 10 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/1/4034910/1.pdf Research Paper W Micr 13.2t 12.2t 11.2t 10.2t Spot 13.2t 12.2t 11.2t 10.2t Spot 13.2t 12.2t 11.2t 10.2t Spot SIMS trace-element data f 16.1 15.1 14.1 13.1 12.1 11.1 10.1 Spot 9.2t 7.2t 6.2t 5.2t 9.2t 7.2t 4.2t 6.2t 3.2t 5.2t 2.2t 4.2t 1.2t 6.2t 4.2t 3.2t 2.2t 7.1 6.1 5.1 9.1 4.1 9.2t 7.2t 3.2t 1.1 2.1 5.2t 2.2t 8.1 3.1 1.2t 1.2t a Not atson (2007). Spot labeled as grain number oanal a a a a e: All ana ly ses we re ytical C (ppm) (ppm) (ppm) (ppm) 1002 2991 2654 1184 4208 1176 1723 2257 1199 1890 5910 16 18 29 14 179 116 101 178 202 20 64 40 81 92 14 65 12 523 498 583 407 789 Tb Sc Th 4.3 2.9 3.2 8.7 2.6 5.1 5.5 4.7 77 82 75 79 72 52 86 Y 73 83 83 73 ent er at Stanf perf (ppm) (ppm) (ppm) (ppm) 0.011 0.034 0.002 0.056 0.020 0.006 0.036 0.006 0.010 0.012 0.015 0.010 0.022 0.007 0.141 0.221 15.2 11.9 10.8 11.7 219 109 224 215 172 222 170 356 175 or zir T Dy 7.8 8.8 7.9 9.0 7.7 7.7 7.0 6.2 60 43 47 58 36 64 80 61 80 61 31 La i49 U 6 9 7 1 T ABLE 3. or or con (separat med on the sensitiv .spot number d Univ TRA er (ppm) (ppm) (ppm) (ppm) 104 106 145 0.02 0.02 0.03 0.06 0.03 0.05 0.01 0.02 0.03 0.03 0.04 0.02 16 11 22 10 14 71 0.7 0.2 0.3 0.3 0.3 0.3 2.5 0.2 0.2 0.7 0.1 0.4 Ho Ce sit 27 20 22 46 19 29 38 28 74 Fe Li 2 3 4 5 5 9 6 3 3 5 CE-ELEMENT D y, e fr f ; ollo t—trace-element anal om U-Pb ana ION MA wing pr e high-r 1304 3641 1054 1777 3536 2817 (ppm) (ppm) (ppm) (ppm) 0.004 0.003 0.004 0.019 0.011 0.036 0.004 0.035 0.003 0.025 0.006 146 108 123 530 240 480 153 221 141 740 399 626 848 715 533 452 805 0.2 0.2 0.5 1.7 1.8 0.5 3.4 0.4 0.7 1.2 0.5 0.8 0.2 0.1 4.6 0.4 Nd Be Er Y SS SPECTR 94 – ocedur esolution ion micr ATA ly sis) FOR ZIRCON FR e outlined in Mazdab and (ppm) (ppm) (ppm) (ppm) 0.005 0.028 0.031 0.046 0.021 0.130 0.011 0.038 0.044 0.032 0.010 0.031 13.8 124 106 168 y Sm OMETR Tm 1.5 1.3 1.2 2.4 1.5 1.7 3.5 2.1 1.0 5.8 1.1 2.1 Nb 0.6 0.7 1.6 4.6 4.9 1.6 9.2 1.3 2.3 3.7 1.8 2.6 0.7 0.4 1.1 36 27 30 56 36 23 54 34 93 B sis on ly Y (SIMS) DURING U-Pb . opr obe– reve (ppm) (ppm) (ppm) (ppm) 114 42 146 42 0.005 0.008 0.003 0.013 0.004 0.004 0.033 0.003 0.006 0.021 0.004 0.026 0.2 27 0.2 57 0.5 21 1.2 65 1.3 84 0.4 41 2.4 38 0.5 21 0.5 92 1.0 84 0.5 72 0.6 03 0.2 58 0.1 55 3.0 81 0.3 51 11 OM 6K1236R14 COLLECTED BY 34 77 26 45 30 06 51 92 71 23 04 34 07 52 11 31 76 86 91 Yb Eu La 57 55 45 67 52 47 72 56 54 34 61 F rs Wo e geomet ry oden (2006) . (ppm ) (ppm ) (ppm ) (ppm ) 83 61 18 21 19 Gd Na Ce 30 04 77 11 84 74 31 Lu 70 30 30 50 91 50 30 30 50 49 61 79 81 19 45 81 51 78 22 92 99 63 79 61 01 09 11 72 36 ANAL (SHRIMP Te Y 10,297 10,616 10,019 (ppm ) (ppm ) (ppm ) (ppm ) 0,639 SIS 0,802 0,221 mperatur 53 01 95 61 9664 9508 9416 2. 46 1. 54 0. 62 Nd Dy 43 32 68 37 92 50 41 39 43 30 Hf Al 10 22 11 18 17 11 14 17 88 07 84 67 62 47 .7 .2 .3 .2 .0 .2 .1 .1 .4 41 62 32 82 61 13 51 12 -RG) at the U. SECOND e estima te (ppm ) (ppm ) (ppm ) 1.98 0.61 1.14 0.92 2.92 0.78 0.44 0.43 1.30 (°C) 25 02 35 63 25 81 68 14 12 31 30 22 34 12 53 91 16 68 22 91 55 21 48 71 74 61 32 06 42 89 34 17 10 02 11 32 02 61 74 6 78 8 75 6 79 8 73 4 82 4 75 7 77 0 75 4 Sm Er .6 91 .3 31 .4 90 P T 47 95 91 29 25 29 80 55 53 55 68 97 AR S. s calculat Geologica Y 43 10 (ppm ) (ppm ) (ppm ) 30.1 40 71 52 31 50 41 50 41 21 91 12 01 01 49 61 33 20 71 91 0. 0. 0. 0. 0. 0. 0. 0. 0. ed fr Yb Eu K 71 08 96 15 05 39 63 .9 .0 .6 31 83 41 31 26 26 52 16 41 l Su rv .4 .1 .6 .1 .1 .8 .0 .9 .4 19 39 om Fe ey –Stanf 10,003 10,677 10,376 rr 1,398 0,707 0,910 1,429 0,538 0,585 0,336 0,627 1,221 (ppm ) (ppm ) (ppm ) y and 9978 8209 74 49 35 Gd Ca 3. 4 3. 6 2. 9 3. 1 4. 7 7. 2 4. 4 3. 0 2. 6 2. 6 4 0. 2 69 6 40 4 Hf 3 2 3 1 2 3 .5 .8 .5 or d

GEOSPHERE | Volume 14 | Number 1 Reagan et al. | Crustal lithologies from the southeast Mariana forearc 11 Research Paper

Modern Mariana A andesite Sample 6K1236R14

Quartz diorite Tonalite

Mariana fore-arc boninite

B 6K1232R16 Epidote amphibolites Primitive mantle normalized concentrations

6K1232R13 Garnetite (6K1232R7)

Figure 6. Primitive mantle (Sun and McDonough, 1989) normalized concentrations of incompatible trace elements or- dered with “fluid-mobile” elements Cs, Rb, Ba, Sr, Th, and U to the left and rare-earth and high-field strength elements in order of partition coefficient during melting to the right. (A) The reconstructed compositions of the quartz diorite and tonalite portions of 6K1236R14 from Table 2. Other data from Reagan et al. (2010). (B) The compositions of the epidote amphibolites 6K1232R13, 6K1232R16, and garnetite 6K1232R7 are from Table 4.

transitioned from boninite genesis during mantle upwelling and shallow hy- have similar mixed boninite-arc tholeiite affinities (Johnson et al., 2014). This drous mantle melting to normal arc volcanism melting related to mantle coun- portion of the Challenger segment with its elevated seafloor, therefore, likely terflow (see Ishizuka et al., 2006; Reagan et al., 2010; Ishuzika et al., 2011a). represents a fragment of the Mariana early-arc terrane that is present along Plutonic rocks collected by dredging between 8150 m and 7200 m depth in the much of the length of the IBM forearc (Reagan et al., 2013) and is subaerially Mariana forearc southeast of Guam and east of the Santa Rosa boundary fault exposed nearby on the forearc island of Guam (Reagan and Meijer, 1984).

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 12 by guest on 29 September 2021 on 29 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/1/4034910/1.pdf Research Paper 208 207 206 ± ± ± Ta Hf Y5 Lu Yb Tm Er Ho Dy Tb Gd Eu Sm Zr Nd Pr Ce La Nb Ub Th Pb Sr Ba Rb Cs Sn Mo Ga Zn Cu Ni Co Cr V Sc Be Li Mg# Sum P K Na Ca MgO MnO F Al Ti SiO eO* 2 2 O Abbr 2 Pb/ Pb/ Pb/ O O O O 2 2 O 2 5 3 204 204 204 e Pb Pb Pb viations: n.a.—not anal Hor T ABLE 4. nb 6K1232R7 113 270 lende ga yz 11.22 17.46 19 59.30 59 35 19 21 26.8 99.80 19.18 39.72 38.273 43 15.496 21 18.353 81 0.34 0.0013 0.0005 0.0006 0.01 0.29 43 0.53 3.91 n.a. 4.99 1.84 9.41 1.16 4.86 1.12 2.27 5.20 0.82 4.70 1.30 0.8 0.51 1.10 n.a. n.a. n.a. 72 n.a. n.a. 1.2 0.50 0.02 9.63 0.50 0.77 0.96 b ed; .d. .d. MAJOR- (WT%), b .d.—belo r netit e w det ection limit. TRA CE-ELEMENT (PPM), Epidot 6K1232R1 3 120 102 321 124 392 12.6 14.4 54 13.8 13.5 12.8 91 45 54.3 97.2 14.6 42.5 e amphibolit 0.29 0.17 73 12 0.0014 0.0005 0.0007 0.34 2.50 0.58 3.99 0.59 4.06 1.41 6.60 0.99 5.58 1.61 4.27 2.39 5.04 4. 0.39 0.99 0.01 1.02 2.37 0.50 0.55 0.17 8.94 0.20 1.87 0.26 9.56 2.12 8.430 5.538 8.832 74 81 51 99 71 81 71 84 80 40 40 23 51 31 AND LEAD-ISO e T OPE DA Epidot TA 6K1232R1 6 19 6 19 9 14 4 39 3 10 0 17 8 62 81 58.2 96.5 5 e amphibolit 8.449 6 5.556 6 8.747 8 0.57 2.94 7.97 1.57 3.03 4.57 4.77 0.41 8. 5 0.0013 0.0004 0.0006 0.35 2.66 0.58 3.84 0.62 3.86 1.31 5.91 0.87 4.80 1.34 3.58 2.01 5.27 0.90 1.48 1.06 0.93 0.84 0.55 0.14 8.94 0.23 1.66 4 2 2.66 .5 .208 .300 .040 e

GEOSPHERE | Volume 14 | Number 1 Reagan et al. | Crustal lithologies from the southeast Mariana forearc 13 Research Paper

TABLE 5. ELECTRON MICROPROBE ANALYSES OF GARNETS AND AMPHIBOLES FOR SAMPLE 6K1232R7 Amphibole Amphibole Phase Garnet Garnet Garnet Garnet Garnet Garnet Garnet Core RimCoreRim

SiO2 38.31 38.71 39.09 38.6438.49 38.9239.22 39.3239.44 39.5839.86

TiO2 0.14 0.16 0.12 0.10 0.12 0.10 0.08 1.37 1.39 1.44 1.40

Al2O3 21.27 21.29 21.79 21.5721.59 21.4621.51 16.2416.18 16.2716.43 FeO* 19.59 18.99 19.54 19.5019.24 19.6119.15 12.9713.25 13.2213.18 MnO 0.93 0.86 0.93 0.81 0.82 0.80 0.89 0.15 0.16 0.11 0.14 MgO 7.36 7.58 7.55 7.62 7.26 7.61 7.72 11.1611.11 11.3011.33 CaO 11.02 11.15 11.15 11.1311.08 11.0211.34 12.1711.68 11.7511.86

Na2O 0.00 0.01 0.01 0.00 0.00 0.00 0.00 2.04 2.06 2.16 2.08

K2O 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.73 0.68 0.74 0.72

V2O3 0.00 0.04 0.03 0.03 0.00 0.01 0.02 0.00 0.00 0.00 0.00 Total 98.64 98.78 100.21 99.4098.60 99.5399.93 96.1695.95 96.5697.00 Amphibole Amphibole Amphibole Amphibole Amphibole Phase Core RimCoreRim Core RimCoreRim Core Rim

SiO2 39.13 39.54 39.72 39.2739.66 39.5239.28 39.3038.98 38.68

TiO2 1.44 1.23 1.49 1.34 1.43 1.33 1.29 1.22 1.11 1.06

Al2O3 16.28 16.48 16.30 16.0916.40 16.9216.42 16.7717.69 17.97 FeO* 13.36 13.12 13.33 13.9413.27 13.2913.01 13.3813.19 13.14 MnO 0.15 0.23 0.20 0.21 0.15 0.19 0.13 0.14 0.17 0.14 MgO 10.92 11.24 11.19 10.8111.18 10.7210.93 11.0311.06 11.04 CaO 11.81 11.82 11.59 11.6911.76 11.9111.74 11.7811.74 12.03

Na2O 2.14 2.06 2.03 2.05 2.08 2.03 2.05 2.07 2.14 2.24

K2O 0.78 0.62 0.68 0.75 0.70 0.69 0.75 0.62 0.57 0.54

V2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.03 Total 96.01 96.35 96.54 96.1596.63 96.6095.62 96.3096.69 96.87 Amphibole Amphibole Amphibole Phase Core RimCoreRim Core Rim

SiO2 39.31 39.01 39.97 39.9039.74 39.45

TiO2 1.12 1.03 1.14 1.20 1.19 1.14

Al2O3 17.33 17.64 17.07 17.2017.76 18.02 FeO* 13.01 13.37 12.85 13.3512.92 13.02 MnO 0.16 0.13 0.14 0.11 0.13 0.11 MgO 10.93 10.78 11.36 11.1010.74 10.74 CaO 12.09 12.07 12.05 12.1612.18 12.18

Na2O 2.04 1.78 1.71 1.79 1.81 1.80

K2O 0.57 0.58 0.60 0.56 0.56 0.56

V2O3 0.09 0.05 0.04 0.04 0.06 0.06 Total 96.66 96.42 96.92 97.4297.09 97.06 Note: FeO*—total Fe as FeO.

Amphibolites and Garnetite garnetite has a significantly lower initial Nd isotopic composition, placing it firmly in the Indian MORB field like most arc and backarc lavas on the Phil- The Pb, Nd, and Hf isotopic compositions (Fig. 10) of the epidote amphib- ippine plate (see Pearce et al., 1999; Hickey-Vargas, et al., 2008; Reagan et al., olites overlap with those of pre-arc and early-arc lavas in the IBM system and 2008). Similarly, the Δ8/4 Pb value for the hornblende garnetite is high, and are near the boundary between mid-ocean ridge basalt (MORB) domains from its Pb isotopic compositions plot in the Indian Ocean MORB field. Siliciclas- the Pacific and Indian oceans as defined by Pearce et al. (1999). The hornblende tic sediments atop the Pacific plate (Meijer, 1976; Plank and Langmuir, 1998)

Figure 7. Pseudosection for CNFMASH 1.5 composition indicates that it is a crystal cumulate either produced by anatexis system calculated using Theriak/ Domino (de Capitani, and Petrakakis, and melt removal (Garrido et al., 2006) or crystal cumulate of high-pressure –Sp. 2010) and a bulk composition based basalt fractionation (Alonso-Perez et al., 2009; Jagoutz et al., 2009). Its enrich- on average mineral compositions in 1.3 ment in fluid-soluble elements is consistent with this basaltic crust being the Table 5 and estimated phase propor- 2 1 tions in hornblende garnetite sam- roots of an island arc. The Eocene forearc basalts found on the nearby Mariana ple 6K1232R7. Dashed lines are for a 1.1 trench-slope are too depleted in incompatible trace elements, and most have composition lacking Fe3+. Solid lines +Plag. 208 204 Pb/ Pb values (Reagan et al., 2010) that are too radiogenic for these lavas to assume the amount of Fe3+ needed to +Chlorite +Melt be the precursors to any of the crustal samples analyzed here. Similarly, West charge balance the hornblende com- H2 O ende P GP a l position (molecular Fe3+/ΣFe = 0.19) 0.9 Philippine basin basalts have Pb isotopic compositions that differ from the am- and enough water to have 2 H for phibolites and garnetite and are unlikely protoliths. every 24 O in the hornblende compo- –Hornb More likely equivalents of the metamorphic lithologies from dive sites are sition (2.01 wt%). Na2O and K2O were 0.7 combined for the modeling because 3 4 the pre-IBM arc terranes of the Philippine plate. These terranes are currently rnet an appropriate solution model for K a situated in the far backarc, to the west of the Kyushu-Palau Ridge (Fig. 1). Prior –G in amphiboles is lacking. Phase solu- +Hornblende to the opening of the Parece-Vela Basin and the Mariana Trough at 25 Ma, tion models used are from Holland 0.5 and Powell (1998), except: plagioclase 400600 800 1000 the present forearc would have been situated much closer to these terranes (Holland and Powell, 2003), garnet, Temperature °C (Fig. 1). The best matches in terms of spread in Pb and Nd isotopic composi- melt (White et al., 2007), spinel, ortho- tions (Fig. 10) are the lavas from the eastern Oki-Daito Ridge and Amami Pla- pyroxene (White et al., 2002), amphibole, and clinopyroxene (Diener and Powell, 2012). Calculated phase proportions for the Fe3+-bearing composition at point 1: 0.71 garnet, 0.23 hornblende, 0.04 clinopyrox- teau, which have affinities to both ocean-island and arc settings (Hickey-Vargas ene, 0.02 melt; point 2: 0.68 garnet, 0.25 hornblende, 0.05 clinopyroxene, 0.01 spinel, 0.01 chlorite; point et al., 2008; Ishizuka et al., 2013). The unusually high Hf isotopic composition 3: 0.60 garnet, 0.30 hornblende, 0.07 clinopyroxene, 0.02 spinel; point 4: 0.22 clinopyroxene, 0.21 melt, for the garnetite sample with respect to its Nd isotopic composition is consis- 0.19 plagioclase, 0.18 garnet, 0.08 spinel. The melt-in reaction is illustrated with the red line. The calculated field for a water-rich fluid is bounded by pale-blue lines. The gray box illustrates the approximate tent with a pre-arc history for this sample. P-T conditions for crystallization of rock 6K1232R7 (see text for details). The chemical association of the hornblende garnetite with older Philippine plate terranes suggests that it was a portion of deep and dense crustal root of a similar terrane that delaminated (e.g., Tatsumi, 2000), melted and disaggre- also are characterized by high Δ8/4 Pb, but these sediments have much higher gated, and became entrained in asthenospheric mantle. Currently, we can only 206Pb/204Pb values compared with the hornblende garnetite. These data suggest speculate at the trigger for deep crust delamination. One possibility is that man- that the amphibolites and garnetite are more likely affiliated with the Philippine tle upwelling associated with subduction initiation at 52 Ma (Stern and Bloomer, plate rather than the subducting Pacific plate. 1992; Ishizuka et al., 2011a) heated the deep roots of one or more pre-arc terranes, The enriched (E)–MORB-like trace-element compositions of the epidote causing them to melt and shed their dense residues (Fig. 11). Thermobarometric amphibolites are most consistent with basaltic protoliths that erupted at a constraints from the hornblende garnetite suggest it achieved pressures of at spreading center or in an intraplate setting. The tectonic setting for genesis least 1.2 GPa (>40 km) and temperatures exceeding 850 °C (Fig. 7). The lack of of the hornblende garnetite is more ambiguous. Its major- and trace-element zonation for its cm-scale garnets suggests these high-grade conditions lasted

TABLE 6. CONCENTRATIONS OF Lu, Hf, Sm, AND Nd AND ISOTOPIC COMPOSITIONS OF Hf AND Nd FOR WHOLE ROCKS AND MINERAL SEPARATES Hf Lu ±2σ ±1 se Nd Sm ±2σ ±1 se Sample (ppm) (ppm) 176Lu/176Hf (%) 176Hf/177Hf (int) (ppm) (ppm) 147Sm/144Nd (%) 143Nd/144Nd (int) 6K1232 R13, whole rock 1.301 0.727 0.0793 0.50.283168 513.44 4.4090.1984 0.20.513065 5 6K1232 R16, whole rock 1.078 0.605 0.0797 0.50.283170 68.719 2.9110.2019 0.20.513073 4 6K1232 R07, whole rock 0.511 0.731 0.2037 0.50.283234 65.284 2.5470.2914 0.20.512873 4 Hornblende 0.839 0.163 0.0275 0.50.283139 711.72 3.9150.2021 0.20.512845 5 Hornblende 0.786 0.151 0.0274 0.50.283145 611.53 3.8540.2021 0.20.512848 5 Garnet with inclusions 0.304 1.87 0.8996 20.283555 71.143 2.0791.1001 0.20.512983 3 Garnet no inclusions 0.250 3.64 2.123 0.50.284132 61.087 2.1511.1963 0.20.513011 4 Garnet leached residue 0.245 4.01 2.452 40.284279 10 1.1132.272 1.2339 0.20.513026 4 Note: Internal (int)1-standard error (se) measurement uncertainties in the 176Hf/177Hf and 143Nd/144Nd ratios are reported as times 10–6. External precisions of the 176Hf/177Hf and 143Nd/144Nd ratios used for the age and source calculations are ±0.006 and 0.0035%, respectively. Uncertainties in the 176Lu/177Hf and 147Sm/144Nd ratios are based on replicate analyses of standards and uncertainty propagation of the isotope-dilution calculations.

long enough to allow for diffusion to homogenize original compositional variation within the garnet (Woodsworth, 1977). The mineral assemblages in the A epidote amphibolites indicate that they were metamorphosed at stages along a 0.2844 Garnetite 6K1232R7 retrograde path at lower temperatures, and probably, at lower pressures. Leached Garnet The consistent 25 Ma age of the hornblende garnetite determined by multi 40 39 Inclusion-free Garnet ple techniques (i.e., Lu-Hf, Sm-Nd, and Ar- Ar) with differing closure tem- 0.2840 peratures suggests rapid cooling at that time. This age is coeval with the ini-

hF tial opening of the Parece-Vela Basin (Ishizuka et al., 2011b). We posit that this rifting at 25 Ma (Fig. 11) caused mantle with entrained hornblende garnetite 177 and related metamorphic blocks to upwell into the rifted Kyushu-Palau arc. This 0.2836 assemblage of rocks then cooled and eventually became part of the mantle hF/ Garnet with Inclusions beneath the crust sampled by dive 6K-1236. The epidote amphibolites could

176 also have been entrained in the mantle associated with rifting at 25 Ma, but the close association of rocks with different P-T histories and compositions needs 0.2832 Whole Rock additional explanation. Possibilities include: (1) faulting associated with the col- Amphibole (2 analyses) Age = 25.21±0.47 Ma 176 177 Hf/ Hfi = 0.2831322 ± 0.0000096 lision of the Caroline Ridge with the Mariana Trench in the Late Miocene tecton- MSWD = 0.28 ically shuffled rocks in the southern Mariana forearc as has been postulated for 0.2828 interleaved metamorphic rocks and peridotites on Yap Island (see Hawkins and 0123 Batiza, 1977; Ohara et al., 2002); (2) rocks with differing histories were chaoti- 176 177 cally mixed during serpentine diapirism (e.g., Maekawa et al., 1993); (3) epidote Lu/ Hf amphibolites were mass wasted from crustal outcrops farther upslope onto garnetite-bearing mantle; or (4) normal faulting associated with subduction erosion and exposure of the mantle in the forearc (Reagan et al., 2017) juxtaposed deep crustal amphibolites and garnetites embedded in the upper mantle. Direct B evidence for (1) and (2) in the dive area is lacking, whereas evidence for nor- Leached Garnet 0.51305 mal faulting and mass wasting in the Mariana forearc is widespread (Martínez Inclusion-free Garnet et al., 2000; Chapp et al., 2008), suggesting that some combination of (3) and (4) moved rocks from the crust section or uppermost mantle downslope to where they were collected during Shinkai 6500 dive 6K-1232.

Nd Dispersed fragments of crust in the mantle would be relatively fusible, and 0.51295 Garnet with Inclusions their presence could affect Nd and Hf isotope values for modern-day lavas 144 from Mariana Arc and Mariana Trough. However, these crustal materials are rare, with only one garnetite collected during all of the recent Shinkai 6500 div- Nd / ing in the Mariana and Bonin forearcs (e.g., Reagan et al., 2010; Ishizuka et al.,

143 2011a; Ohara et al., 2012). Thus, it is unclear both how widespread these frag- 0.51285 Whole Rock ments are and what their bulk isotope contributions to mantle sources of lavas Age = 24.8±2.5 Ma Amphibole (2 analyses) 143 144 would be. Nevertheless, the relatively unradiogenic Nd isotopic compositions Nd/ Ndi = 0.512817 ± 0.000014 MSWD = 0.92 compared with Hf for the garnetite indicate that the presence of such materials in the mantle might contribute to its overall Indian Ocean–like isotopic nature 0.51275 (cf. Hickey-Vargas, 1998; Pearce et al., 1999; Woodhead et al., 2012). 0.0 0.2 0.4 0.60.8 1.01.2 1.4 147 144 Sm/ Nd CONCLUSIONS Figure 8. Isochron plots for separated constituents from garnetite sample 6K1232R7. Data are from Table 6. (A) Plot of 176Hf/177Hf versus 176Lu/177Hf. (B) Plot of 143Nd/14Nd versus 147Sm/144Nd. MSWD—mean square of Shinkai 6500 diving on the Challenger segment of the Mariana forearc weighted deviates. during the YK10-12 expedition of the R/V Yokosuka collected a wide variety of mantle and crustal lithologies. A plutonic rock collected at relatively shallow

TABLE 7. COMPLETE 40Ar/39Ar RESULTS FOR AMPHIBOLE FROM SAMPLE 6K1232R07

40 40 39 38 37 36 Laser power Ar Ar ±1σ40 Ar ±1σ39 Ar ±1σ38 Ar ±1σ37 Ar ±1σ36 Age ±2σ 40 40 39 File (W) (moles) (volts) (volts) (volts) (volts) (volts) (volts) (volts) (volts) (volts) (volts) % Ar* Ar*/ ArK ±2σ (Ma) (Ma) K/Ca BI1039 7.5 3.060E-16 0.051855 ± 0.000132 0.001746 ± 0.000030 0.000036 ± 0.000023 0.009630 ± 0.0001410.000166 ± 0.0000075.631.679257 ± 1.14516946.10 ± 62.09 0.078 BI1040 8.2 3.472E-16 0.058826 ± 0.000107 0.003152 ± 0.000031 0.000092 ± 0.000016 0.021683 ± 0.0003610.000187 ± 0.0000077.811.463998 ± 0.62068940.25 ± 33.76 0.062 BI1052 8.9 5.957E-16 0.100940 ± 0.000237 0.007044 ± 0.000040 0.000148 ± 0.000024 0.049807 ± 0.0006230.000324 ± 0.0000078.191.178663 ± 0.28260832.48 ± 15.44 0.061 BI1053 9.5 8.345E-16 0.141399 ± 0.000212 0.010983 ± 0.000056 0.000282 ± 0.000032 0.079697 ± 0.0008810.000458 ± 0.0000077.660.991208 ± 0.19941027.35 ± 10.92 0.059 BI1055 10.0 6.965E-16 0.118018 ± 0.000215 0.018175 ± 0.000047 0.000338 ± 0.000021 0.130227 ± 0.0014560.000371 ± 0.00000914.93 0.974291 ± 0.14548926.89 ± 7.970.060 BI1056 10.5 4.891E-16 0.082873 ± 0.000157 0.029051 ± 0.000092 0.000516 ± 0.000024 0.207782 ± 0.0022950.000240 ± 0.00000633.19 0.951426 ± 0.06457126.26 ± 3.540.060 BI1058 11.0 5.925E-16 0.100390 ± 0.000175 0.078976 ± 0.000148 0.001488 ± 0.000049 0.572891 ± 0.0062350.000248 ± 0.00000671.45 0.912878 ± 0.02576325.20 ± 1.410.059 BI1061 12.0 3.663E-16 0.062057 ± 0.000144 0.050585 ± 0.000115 0.000893 ± 0.000040 0.364074 ± 0.0040190.000151 ± 0.00000673.66 0.908144 ± 0.03756325.07 ± 2.060.059 BI1062 12.5 3.195E-16 0.054129 ± 0.000132 0.045770 ± 0.000102 0.000803 ± 0.000044 0.331206 ± 0.0036320.000133 ± 0.00000674.86 0.889777 ± 0.03724424.57 ± 2.040.059 BI1064 13.0 2.722E-16 0.046116 ± 0.000132 0.032268 ± 0.000076 0.000593 ± 0.000026 0.236626 ± 0.0026330.000119 ± 0.00000663.68 0.914714 ± 0.05412025.25 ± 2.970.058 BI1065 13.5 2.697E-16 0.045701 ± 0.000140 0.036607 ± 0.000097 0.000689 ± 0.000047 0.271119 ± 0.0030700.000117 ± 0.00000770.36 0.882896 ± 0.05797624.38 ± 3.180.058 BI1067 14.0 2.507E-16 0.042475 ± 0.000170 0.023450 ± 0.000079 0.000404 ± 0.000023 0.178900 ± 0.0020140.000113 ± 0.00000753.98 0.982937 ± 0.08863527.12 ± 4.860.056 BI1068 15.0 6.857E-17 0.011619 ± 0.000116 0.005752 ± 0.000035 0.000114 ± 0.000014 0.043454 ± 0.0005340.000033 ± 0.00000545.46 0.923102 ± 0.28337525.48 ± 15.54 0.057 Weighted-mean age (13 of 13):25.20 ± 0.86 J-value: 0.0151870 ± 0.0000046 (1σ) D/amu: 1.00890 ± 0.00071 (1σ) The values in this table represent blank, discrimination, and decay (37Ar and 39Ar) corrected values. Instrument: MAP 215-50 Standard:Fish Canyon sanidine Standard age (Ma): 28.201 ± 0.046Kuiper et al. (2008) Atmospheric argon ratios: 40Ar/36Ar 298.56 ± 0.31 Lee et al. (2006) 38Ar/36Ar 0.1885 ± 0.0003 Lee et al. (2006) Interfering isotope production ratios: 40 39 ( Ar/ Ar)K 0.00054 ± 0.00014 38 39 ( Ar/ Ar)K 0.01210 ± 0.00002 39 37 ( Ar/ Ar)Ca 0.000695 ± 0.00001 38 37 ( Ar/ Ar)Ca 0.0000196 ± 0.000001 36 37 ( Ar/ Ar)Ca 0.000265 ± 0.00002 Decay constants: λ40Ar (0.580 ± 0.014) × 10–10 a–1 Min et al. (2000) λB– (4.884 ± 0.099) × 10–10 a–1 Min et al. (2000) 39Ar (2.58 ± 0.03) × 10–3 a–1 37Ar (8.23 ± 0.042) × 10–4 h–1 36Cl (2.303 ± 0.046) × 10–6 a–1

depths was derived from a proto-arc terrane with boninite and arc tholeiite af- that mantle flow brought the garnetite to the trench, where it was uplifted and finities and an age of 46.1 Ma, illustrating that shallow portions of the Chal- cooled during rifting of the Kyushu-Palau Ridge. The lower-grade epidote am- lenger segment are fragments of the IBM nascent arc terrane that has been dis- phibolites also might have been transported in the mantle or could represent sected by subsequent rifting. Epidote amphibolites and a hornblende garnetite fragments of the basement of the overlying crust. In either case, the amphib- collected at greater depth from an area dominated by serpentinized dunite and olites were likely juxtaposed with the garnetite by normal faulting and mass harzburgite have isotopic affinities with the Oki-Daito Ridge and similar pre- wasting related to forearc extension and erosion. Thus, the forearc south of the IBM arc terranes. Coincident Hf, Nd, and 40Ar/39Ar ages of 25 Ma indicate that Mariana Trough has Eocene arc crust underlain by younger mantle with em- the garnetite was uplifted rapidly and cooled at this time. We conclude that the bedded crust-derived delamination of the deep crustal root of a pre-arc terrane. garnetite is a delaminated fragment of the deep arc crust of a Late Cretaceous The similarity between the Nd and Hf isotopic compositions for Mariana area to Early Eocene terrane in the West Philippine basin. The epidote amphibo- lavas and Indian Ocean MORB might at least partly result from the presence of lites are fragments of shallower crust with an E-MORB affinity. We speculate fusible crustal blocks within the asthenospheric mantle in the Mariana region.

80 0.0040

Garnetite 6K1232R7 ) 60 0.0030 25.20 + 0.86 Ma (2σ) Ar 40 40 0.0020 Ar/ 36

Apparent age (Ma 20 0.0010 Isochron age: 24.90 + 0.96 Ma (2σ) 40 36 Ar/ Ari = 303.7 + 6.6 MSWD = 0.15 0 0.0000 0 20 40 60 80 100 0.00.2 0.40.6 0.81.0 39 39 40 % ArK released Ar/ Ar

Figure 9. 40Ar/39Ar age spectrum and inverse isochron diagram for hornblende separated from sample 6K1232R7. Data are from Table 7.

A B KPR

ODMP εHf i 8/4 WPB Garnetite Δ Amphibolites Pre-IBM arc FAB terranes NHRL Boninite

206 204 εNdi Pb/ Pb Figure 10. (A) Plot of initial εHf versus εNd values for the epidote amphibolites and garnetite calculated at 25 Ma. Data are from Table 6. (B) Plot of Δ8/4 versus 206Pb/204Pb for the epidote amphibolites and hornblende garnetite. Data are from Table 4. Delta values were calculated using equations from Hart (1984). The red line is the location of the boundary between the Indian Ocean mid-ocean ridge basalt (MORB) field with highΔ 8/4 and Δ7/4 and Pacific MORB field with lower Δ8/4 and Δ7/4 as defined in Pearce et al. (1999). The dashed line labeled “NHRL” represents the Northern hemisphere regression line of Hart (1984). The green fields represent lavas from pre-arc terranes including the Oki-Daito Ridge, Amami Plateau, and Huatung Basin. The yellow fields labeled “ODMP” are lavas associated with the Oki-Daito Mantle Plume. The orange fields labeled “WPB” are seafloor basalts from the West Philippine Basin. The blue fields labeled “KPR” represent lavas from the Kyushu-Palau Ridge. The gray fields labeled “FAB” represent forearc basalts, and the patterned fields represent boninites from forearc dive sites, Deep Sea Drilling Project sites 458 and 459, and Chichijima (data sources: Hickey-Vargas, 1998; Pearce et al., 1999; Hickey-Vargas 2005; Savov et al., 2006; Hickey-Vargas et al., 2008; Reagan et al., 2010; Ishizuka et al., 2011a, 2011b; Ishizuka et al., 2013; Hickey-Vargas et al., 2013).

A Ca. 50 Ma B Ca. 50 Ma KPR W E AP ODR DR Bon ODR E W

WPB Figure 11. Schematic diagrams illustrating the potential origin of the crustal rocks discussed in this paper. (A), (C), and (E) represent cross sections along lines illustrated Mesozoic on schematic maps (B), (D), and (F). Brown areas illustrate arc crust; basaltic crust orig- Oceanic inating in a subduction initiation, backarc, Crust or mid-ocean ridge setting is gray; mantle is illustrated in light green. The dashed line in (F) is the West Santa Rosa bound- Pacific plate ary fault. AP—Amami Plateau, DR—Daito Ridge, ODR—Oki-Daito Ridge, WPB—West 25 Ma D 25 Ma Philippine Basin, MOC—Mesozoic oceanic C KPR W E crust, KPR—Kyushu Palau Ridge, WMR— AP West Mariana Ridge, PVSB—Parece-Vela DR and Shikoku basins, Bon—early-arc boni- WPB nitic crust. The red star illustrates the loca- ODR tion of the Shinkai 6500 dive sites 6K1232 and 6K1236, which were on a lithospheric block consisting of Eocene crust underlain by more recently emplaced mantle. The cooled AP-DR-ODR area consists of Mesozoic arc West terranes, plume-related ocean islands, and garnetite basins of late Mesozoic and Eocene age Philippine W E (see Ishizuka et al., 2013; Arculus et al., Basin 2015). MOC probably consisted of oceanic lithosphere with minor arc and ocean- island terranes. Izu-Bonin-Mariana (IBM) plate rotations over time are not illustrated. (A, B) Early-arc phase shortly after initiation > 50Ma crust fragments PVSB of subduction when upwelling mantle might have triggered delamination of deep segments of a crustal terrane, such as the E Today F Today WMR Guam Oki-Daito Ridge. (C, D) Rifting at 25 Ma to NSParece Mariana create the Parece-Vela and Shikoku basins, WMR causing disaggregated fragments of de- Vela N Trough laminated crust to rise to shallow levels in Basin the mantle. The dots with varied colors represent fragments of the crust, which could have varying metamorphic grade. (E, F) The situation today, where mantle with embedded crustal blocks crop out near the Mariana Trench.

ACKNOWLEDGMENTS of island arcs and generation of continental crust: Journal of Petrology, v. 47, p. 1873–1914, Rasmus Andreasen, David Peate, and Guillaume Girard are thanked for analytical assistance. We https://doi.org/10.1093/petrology/egl030. thank the Japan Agency for Marine-Earth Science and Technology for funding the cruises of the Grütter, H.S., Gurney, J.J., Manzies, A.H., and Winter, F., 2004, An updated classification scheme R/V Yokosuka and the Shinkai 6500 diving. We also thank the Shinkai 6500 and R/V Yokosuka crews for mantle-derived garnet for use by diamond explorers: Lithos, v. 77, p. 841–857, https://doi for their outstanding work. 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