Late Miocene coesite-eclogite exhumed in the Woodlark Rift Suzanne L. Baldwin, Laura E. Webb, Brian D. Monteleone* Syracuse University, Department of Earth Sciences, Syracuse, New York 13244, USA ABSTRACT U-Pb ion probe analyses of zircon inclusions Late Miocene–Pliocene eclogites were exhumed in the Woodlark Rift of eastern Papua New in garnet from the sample studied (89321) Guinea, an actively extending region west of the Woodlark Basin seafl oor spreading center. yielded a 238U/206Pb age of 7.9 ± 1.9 Ma (2σ), We report the discovery of coesite in late Miocene eclogite from the lower plate of one of the and, together with in situ ion probe trace ele- D’Entrecasteaux Islands metamorphic core complexes within the Woodlark Rift. Zircon crys- ment and REE chemistry on zircon and garnet tallization temperatures (650–675 °C) and 238U/206Pb age (ca. 8 Ma), and rutile thermometry pairs, indicate zircon growth under eclogite (695–743 °C) combined with garnet-pyroxene thermometry (600–760 °C) and garnet-pyroxene- facies conditions (Monteleone et al., 2007). phengite barometry (18–27 kbar), indicate that the coesite-eclogite was exhumed from mantle The eclogite investigated preserves a peak depths (≥90 km) to the Earth’s surface at plate tectonic rates (cm yr–1). This late Miocene coesite- assemblage of garnet + omphacite + rutile + eclogite is the youngest exhumed ultrahigh-pressure (UHP) rock on Earth, and its preservation phengite + SiO2. Within the matrix, rutile is ahead of the westward-propagating seafl oor spreading center forces reevaluation of models for rimmed by retrograde titanite and is intergrown UHP exhumation, as well as the geologic and tectonic evolution of the Woodlark Rift. with ilmenite. Anhedral garnet contains inclu- sions of omphacite, rutile, and zircon. Petro- Keywords: coesite-eclogite, ultrahigh-pressure exhumation, Woodlark Rift. graphic observations revealed a 150 × 200 μm SiO2 inclusion at the center of a radial fracture INTRODUCTION PRESSURE-TEMPERATURE-TIME pattern in its omphacite host (Fig. 2). Cathodo- One of the most exciting frontiers in the EVOLUTION OF COESITE-ECLOGITE luminescence imaging shows that the SiO2 fi eld of continental dynamics in recent decades IN THE WOODLARK RIFT inclusion is polymineralic with angular, frac- concerns the formation and exhumation of Variably retrogressed eclogite facies rocks tured, darker regions surrounded by rims of ultrahigh-pressure (UHP) rocks. With the dis- have long been recognized (Davies and Ives, polycrystalline quartz exhibiting palisade tex- covery of UHP polymorphs of silica (coesite; 1965) in the lower plates of metamorphic ture (Fig. 2 inset). The SiO2 inclusion also hosts Chopin, 1984; Smith, 1984) and carbon (dia- core complexes (Davies and Warren, 1988, a zircon. Raman spectroscopy of the SiO2 inclu- mond; Nasdala and Massone, 2000; Sobolev 1992; Hill and Baldwin, 1993) exposed in sion confi rms the presence of both coesite and and Shatsky, 1990) in collisional orogens came the D’Entrecasteaux Islands (Fig. 1), in the α-quartz (Fig. 3). Five Raman spectra yielded the realization that buoyant continental crust can active Woodlark Rift of eastern Papua New diagnostic Raman bands (Liu et al., 1997) for be subducted to mantle depths and subsequently Guinea. Structural and fi eld evidence (Hill, coesite at 520, 354–356, 270, and 176 cm–1, and exhumed to the Earth’s surface. The number and 1994; Hill and Baldwin, 1993), combined for quartz, at 463–465 cm–1. volume of known UHP terranes indicate that sub- with U-Pb, trace element, and rare earth ele- The omphacite host of the partially trans- duction and exhumation of continental crust has ment (REE) data (Baldwin and Ireland, 1995; formed coesite inclusion and surrounding garnet had a major impact on Earth’s evolution, includ- Baldwin et al., 2004; Monteleone et al., 2007) and phengite were used to constrain the P-T path ing the recycling of continental crust, and the indicate that mafi c eclogites and their felsic of this sample using garnet-pyroxene-phengite exchange of material between the crust and host gneisses were metamorphosed together barometry (Ravna and Terry, 2004) and garnet- mantle (Chopin, 2003; Liou et al., 2004). under eclogite facies conditions from the late pyroxene thermometry (Ravna, 2000). The Coesite, the high-pressure polymorph of Miocene to Pliocene. assemblage and corresponding mineral com- silica, and the primary indicator mineral of The eclogite studied (89321c; Fig. 1) positions are assumed to refl ect an equilibrium UHP metamorphism, requires high pressure/ was sampled from a locality in which mafi c volume preserved when the coesite-eclogite temperature (P/T) conditions for its formation. eclogites were previously described as xenoliths was at (or near) UHP conditions. These thermo- Coesite inclusions occur in mechanically strong in weakly foliated leucogranite (e.g., Davies and barometers yielded temperatures of 600–760 °C minerals (e.g., garnet, omphacite, zircon; Gillet Warren, 1988; Fig. 2 in Monteleone et al., 2007). and pressures of 18–27 kbar. Given uncertain- et al., 1984). While some coesite inclusions However, a return to this locality in January ties regarding the oxidation state of Fe in garnet are untransformed (Tabata et al., 1998), most 2008 revealed signifi cant new outcrop, inferred and omphacite, and the potential for post-peak exhibit partial transformation to palisade quartz. to result from tsunami waves triggered by the Fe-Mg volume diffusion during retrograde HP The volume increase resulting from the coesite- 1 April 2007 magnitude 8.1 Solomon Islands metamorphism, [Zr] in rutile and [Ti] in zircon quartz transition results in rupture and radial earthquake. Additional observations revealed thermometry was used to further constrain tem- fracturing of the host grain (Van der Molen and that mafi c eclogites occur as boudins within peratures attained by this coesite-eclogite. Both van Roermund, 1986). In cases where partial strongly foliated and isoclinally folded garnet- rutile inclusions (e.g., in omphacite; Fig. 2) and transformation has occurred, petrographic obser- bearing quartzo-feldspathic host gneisses. matrix rutile were analyzed. Rutile tempera- vations target potential relict coesite that can be Amphibolite rinds encapsulate the eclogite tures, determined using Tomkins et al.’s (2007) positively confi rmed by laser Raman spectros- boudins, the protolith of which appears to have calibration that accounts for the pressure effect copy (Boyer et al., 1985; Gillet et al., 1984). In been mafi c dikes. Pegmatite occurs in strain on [Zr] in rutile, yield temperatures from 695 to this paper we present the fi rst evidence, and new shadows surrounding the amphibolite rinds, as 743 °C (assuming P = 28 kbar; Fig. 4). No sys- P-T constraints, for coesite-eclogite exhumed in well as in veins within the host gneiss. tematic temperature differences were observed the Woodlark Rift, and discuss implications for Previous studies of retrogressed eclogite for matrix versus inclusion populations. In com- models of HP-UHP rock exhumation. from this locality reported garnet-pyroxene parison, thermometry based on [Ti] in zircon temperatures ranging from ~700 to 750 °C and (Ferry and Watson, 2007; Watson et al., 2006) *Current address: School of Earth and Space minimum pressures of ~17–19 kbar based on yielded temperatures of 650–675 °C (Fig. 4). Exploration, Arizona State University, Tempe, Ari- the jadeite content of omphacite (Davies and These are interpreted as zircon crystallization zona 85287, USA. Warren, 1992; Hill and Baldwin, 1993). In situ temperatures in this sample. © 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September August 20082008; v. 36; no. 9; p. 735–738; doi: 10.1130/G25144A.1; 4 fi gures; Data Repository item 2008184. 735 146°E 152°E 158° E ^ ^ + + + + + + + +++ + 6°S ^ San Cristobal Trench ^ San Cristobal trench Pacific ^ Plate SI Pacific Plate South Bismark SBP ^ 45 mm/yr v v ^ Plate v + New Britain Trench v WLK Solomon Islands v Trobriand Trough Australian Plate + UHP locality + v + 9°S DI + v G Woodlark Basin 100 km F Woodlark Plate v Fault v v Active volcano v UHP N + 2000 m isobath locality + Rift axis Active subduction zone Inactive subduction zone Australian Eocene intrusive rocks Oceanic crust, < 2 Ma Plate Eclogite–amphibolite-facies meta-sediments Eocene–Oligocene sedimentary rocks and + Oceanic crust, 2−4 Ma + and meta-basalts basalts Oceanic crust, > 4 Ma Blueschist-facies meta-sediments Ophiolite, Mesozoic gabbro and basalt (PUB) Pliocene–Quaternary sedimentary and volcanic rocks and meta-basalts Undifferentiated rocks of the Solomon Islands Miocene–Pliocene intrusive rocks Greenschist-facies Eocene and younger, sediments and basalts Miocene sedimentary and volcanic rocks meta-sediments and meta-basalts Coral Sea of the SBP + + +++ + +++ + + + +12°S 146°E 152°E 158°E Figure 1. Tectonic and geologic map of eastern Papua New Guinea (Baldwin et al., 2004). Asterisk indicates coesite-eclogite locality (9°29′0″S, 150°27′40″E). Red arrows indicate present-day plate motion vectors (Wallace et al., 2004). White dashed line indicates Bruhnes chron. Inset shows microplates of Australian-Pacifi c plate boundary zone. Abbreviations: WLK—Woodlark plate; SBP—South Bismarck plate; UHP—ultrahigh pressure. Map: DI—D’Entrecasteaux Islands; G—Goodenough Island; F—Fergusson Island; N—Normanby Island; PUB—Papuan ultramafi c belt. After Baldwin et al. (2004). The combined textural and mineral composi- et al., 1993) by 3.5 Ma, as indicated by mus- Zircon tion data set is used to assess the HP−UHP his- covite 40Ar/39Ar apparent ages from pegmatite tory preserved in the coesite-eclogite (Figs. 2 in strain shadows surrounding the amphibolite Quartz and 4). At 7.9 ± 1.9 Ma zircon crystallized rind. Apatite fi ssion track data from the quartzo- Coesite under eclogite facies conditions, at temperatures feldspathic host gneiss indicate cooling to below μ of 650–675 °C (i.e., below the closure tempera- ~120 °C by 0.6 Ma (Baldwin et al., 1993).