J OURNAL OF Journal of Petrology, 2018, 1–29 doi: 10.1093/petrology/egy094 P ETROLOGY Advance Access Publication Date: 15 October 2018 Downloaded from https://academic.oup.com/petrology/advance-article-abstract/doi/10.1093/petrology/egy094/5131633 by University of Wisconsin-Madison user on 04 December 2018 Original Article Mechanical Mixing of Garnet Peridotite and Pyroxenite in the Orogenic Peridotite Lenses of the Tvaerdal Complex, Liverpool Land, Greenland Caledonides Hannes K. Brueckner1,2*, L. Gordon Medaris, Jr3, William L. Griffin4, Scott M. Johnston5, Ebbe H. Hartz6, Norman Pearson4, Yue Cai1 and Arild Andresen7 1Lamont-Doherty Earth Observatory of Columbia University, 61 US-9W Palisades, NY 10964, USA; 2The CUNY Graduate Center and School of Earth and Environmental Sciences, Queens College, 65-21 Main St. Flushing, NY 11367, USA; 3Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA; 4Department of Earth and Planetary Sciences, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCF)/(GEMOC), Macquarie University, NSW 2109, Australia; 5Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407, USA; 6Centre for Earth Evolution and Dynamics, Department of Geosciences, Oslo University, P.O. Box 1028 Blindern, 0316 Oslo, Norway; 7Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway *Corresponding author. P.O. Box 1000, 61 Route 9W, Palisades, NY 10964-1000 USA, Telephone: (845) 359-2000. Fax: (845) 365-8150. E-mail: [email protected] Received April 9, 2018; Accepted September 28, 2018 ABSTRACT The Tvaerdal Complex is an eclogite-bearing metamorphic terrane in Liverpool Land at the southern tip of the Greenland Caledonides. It is a Baltic terrane that was transferred to Laurentia during the Scandian orogeny. It exposes a few small garnet dunite and harzburgite lenses, some containing par- allel layers of garnet pyroxenite and peridotite (including lherzolite). Sm–Nd mineral ages from the pyroxenites indicate recrystallization occurred at the same time (405 Ma) as eclogite recrystallization in the enclosing gneiss. Geothermobarometry indicates these eclogites and pyroxenites shared a simi- lar pressure-temperature history. This congruent evolution suggests pyroxenite-bearing peridotite lenses were introduced from a mantle wedge into subducted Baltic continental crust and subsequent- ly shared a common history with this crust and its eclogites during the Scandian orogeny. Some gar- net peridotite samples contain two garnet populations: one Cr-rich (3Á5–6Á2wt% Cr2O3)andtheother Cr-poor (0Á2–1Á4 wt %). Sm–Nd analyses of two such garnet peridotites define two sets of apparent ages: one older (>800 Ma) for Cr-rich garnets and the other younger (<650 Ma) for Cr-poor garnets. We propose that the younger Cr-poor garnets were derived from fractured and disaggregated garnet pyroxenite layers (i.e. are M2) and were mixed mechanically with older (i.e. M1) garnets of the host peridotite during intense Scandian shearing. Mechanical mixing may be an important mantle process. Key words: orogenic peridotite; mechanical mixing; pyroxenite; peridotite, Greenland Caledonides INTRODUCTION rocks in the south (Fig. 1). The metamorphic rocks in- Liverpool Land (LL) at the southern end of the clude numerous eclogites (Augland et al., 2010; Greenland Caledonides is a crystalline terrane com- Johnston et al., 2010; Corfu & Hartz, 2011) and a few, prised of plutonic rocks in the north and metamorphic small (decametre) peridotite lenses that locally are VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] 1 2 Journal of Petrology, 2018, Vol. 0, No. 0 21°30’W Downloaded from https://academic.oup.com/petrology/advance-article-abstract/doi/10.1093/petrology/egy094/5131633 by University of Wisconsin-Madison user on 04 December 2018 4 2 ? 5 1 3 TJ 6 ? 70°30’N 0 1:1000001:100000 5 kmkm Scoresbysund Glaciers and Glacial Deposits Permian Sediments Hurry Inlet Intrusive Suite Hurry Inlet Granite Eclogitized Undifferentiated Gneiss Mafic Body LL Metamorphic Complexes Caledonides Eclogite Hurry Inlet Detachment Ultramafic Body } Jaettedal Sequence Marble Ittoqqortoormiit Shear Zone LL Tvaerdal Sequence 22°W Fig. 1. Map of the Liverpool Land (LL) metamorphic terrane showing the Tvaerdal and Jaettedal metamorphic complexes, major tectonic features (from Johnston et al., 2010) and locations of peridotite lenses (circles) and eclogites (diamonds). Sample numbers refer to peridotite localities in Table 1. Geographic coordinates of most peridotite lenses are given in Table 1. T and J are the valleys called Tvaerdal and Jaettedal. garnetiferous (Augland et al., 2011). Augland et al. Orogeny is generally modeled by the westward (pre- (2010) proposed that the entire metamorphic terrane be sent coordinates) subduction of Baltica (Scandinavia) called the Liverpool Land Eclogite Terrane (LLET). They beneath Laurentia (Greenland) (Krogh, 1977; Andersen proposed further that the LLET was a Baltic, rather than et al., 1991; see also Brueckner & Van Roermund, 2004; Laurentian, terrane (see also Smith & Cheeney, 1981) Hacker et al., 2010; Gee et al., 2012 for recent reviews). that became stranded on the Greenland side of the If the Tvaerdal Complex were a Laurentian terrane, the Caledonides during the opening of the Atlantic. peridotite bodies within it would have had to move up- Subsequent work has confirmed the Scandian (411–398 ward into the overlying plate. But if the Tvaerdal Ma) evolution of the eclogites and the Baltic origin of Complex was part of Baltica when it subducted beneath the gneisses that host them (Johnston, et al., 2010; Laurentia, it could have picked up the peridotite frag- Augland et al., 2011; Corfu & Hartz, 2011; Brueckner ments from the overlying Laurentian mantle wedge et al., 2016). However, mapping by Johnston et al. (Brueckner & Medaris, 2000). A contrary view to the (2010) showed that the metamorphic rocks are not a sin- mantle subduction model is, however, held by one of gle high pressure/ultrahigh pressure (HP/UHP) complex, us (see Hartz et al., 2005, 2007). Models in which the but rather two complexes, with the eclogites, perido- peridotites were derived from the Baltic rather than tites and their host rocks comprising a Baltican HP/UHP Laurentian mantle have been presented by Terry et al. terrane, the Tvaerdal Complex and the rest of the meta- (1999) and Majka et al. (2014). morphic rocks forming a Laurentian granulite facies ter- To test the possibility that the Tvaerdal Complex is a rane, the Jaettedal Complex. Baltic terrane, and the peridotites within it have a This division into a Baltic Complex (Tvaerdal) and a Laurentian affinity, seven of the known peridotite Laurentian Complex (Jaettedal) resolves the problem of bodies in Liverpool Land, as well as peridotite and pyr- how the peridotite lenses came to be embedded in the oxenite blocks in glacial till, were investigated. The southernmost Greenland Caledonides. The Scandian major, minor and trace element compositions of Journal of Petrology, 2018, Vol. 0, No. 0 3 peridotite and pyroxenite whole-rock samples, as well As noted above, Johnston et al. (2010) concluded as the compositions of their constituent minerals were that the eclogites and peridotites within the meta- Downloaded from https://academic.oup.com/petrology/advance-article-abstract/doi/10.1093/petrology/egy094/5131633 by University of Wisconsin-Madison user on 04 December 2018 determined. Electron microprobe scans were recorded morphic complex are restricted to a tectonically lower across key minerals. Pressure-temperature (P–T) equili- orthogneiss sequence (the Tvaerdal Complex) sepa- bration conditions and Sm–Nd mineral ages were deter- rated by another shear zone (the Ittoqqortoormiit shear mined from selected samples and compared with zone) from a tectonically higher eclogite-free unit con- published pressure-temperature-time (P–T–t) informa- taining predominant orthogneiss and subordinate tion from the associated Tvaerdal eclogites and host granulite, pelitic schist, calc-silicate rocks and marble gneisses. Re–Os isotopic data from sulfides are also (the Jaettedal Complex). Zircon cores from the presented for selected samples. Jaettedal Complex give Archean and Mesoproterozoic The study became complicated when we observed U–Pb ages, while zircon cores from the Tvaerdal that some garnet peridotites contain garnets with vari- Complex give Mesoproterozoic ages only. The able Cr concentrations, which, in at least one case could Jaettedal Complex underwent granulite facies meta- be divided into two distinct populations: Cr-rich and Cr- morphism and melting between 460 and 410 Ma poor. Sm–Nd results from two other peridotites indicate (Johnston et al., 2010, 2015), whereas the peak meta- that their garnets formed or re-equilibrated during two morphic history of the Tvaerdal Complex occurred be- separate metamorphic events. Thus some peridotites tween 412–395 Ma (see below). Thus the two contain two garnet populations and we present evi- complexes had different Precambrian histories and dence that they might have been mixed together mech- were metamorphosed at different grades and at differ- anically. Mechanical mixing may be an under- ent times, providing compelling evidence that the two appreciated process that could play an important role in complexes originated from Laurentia and Baltica, re- mantle