Age and temperature-time evolution of retrogressed eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Constrained from U-Pb dating of zircon, monazite, titanite and rutile

Highlights

• New constraints on Tt-history of eclogite-facies rocks in the Nagssugtoqidian Orogen.

• High-P amphibolite-facies retrogression took place between 1891 ± 10 and 1882 ± 3 Ma.

• Further retrogression at lower-pressure continued until ca. 1740 Ma.

• Eclogite- and high-P amphibolite-facies stages are likely within error of each other.

• Slow, erosion-controlled cooling followed rapid, tectonically-controlled exhumation.

Abstract

LA-ICP-MS U-Pb dating of zircon, monazite, titanite and rutile was carried out to investigate the temperature-time evolution of eclogite-facies rocks in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen in South-East Greenland. The terrane is dominated by Archean TTG gneiss and a variety of supracrustal rocks; basic dykes intruded the gneiss during Paleoproterozoic deformation. Detrital zircon in garnet-kyanite schist gives Archean to Paleoproterozoic dates and confines the maximum deposition of the metasediment precursor to 2107 ± 21 Ma. Intrusion of the metabasic dykes occurred at 2146 ± 63 and 2092 ± 22 Ma, within error of the detrital zircon date, possibly indicating near-contemporaneous dyke emplacement and sedimentation. About 200 m.y. after dyke emplacement, the Kuummiut Terrane underwent a clockwise PT-evolution, involving eclogite-facies metamorphism and subsequent exhumation into the mid crust. The majority of zircon, monazite and titanite give metamorphic dates between 1891 ± 10 and 1882 ± 3 Ma. Although the REE patterns in metamorphic zircon reflect growth at eclogite-facies conditions, the zircons are associated with retrograde mineral assemblages and their dates are indistinguishable from amphibolite-facies titanite. This may be interpreted to indicate that the timing of eclogite- and high-pressure amphibolite-facies metamorphism overlap within error, consistent with rapid and tectonically-controlled exhumation. However, previous studies have shown that the REE and U-Pb systematics in zircon may be decoupled during retrograde metamorphism, and the range in dates is thus best interpreted to reflect mineral growth and recrystallization during high-pressure amphibolite-facies retrogression. Monazite and titanite dates between 1872 ± 70 and 1821 ± 31 Ma reflect regional medium-pressure amphibolite-facies metamorphism and mark the final stages of compressional deformation in the Kuummiut Terrane. The subsequent thermal evolution was associated with titanite growth until 1738 ± 61 Ma, a time where the majority of rutile cooled below its closure temperature. The youngest rutile dates at 1645 ± 63 and 1617 ± 91 Ma correlate with the emplacement of post-tectonic intrusive complexes. Collectively, the data show that after an initial tectonically-controlled exhumation, the Kuummiut Terrane experienced relatively slow, erosion-controlled cooling with only minor thermal perturbations during the waning stages of metamorphic and magmatic activity.

Introduction

In the last few decades, a growing number of studies reported the presence of eclogite- to high-pressure granulite-facies mineral assemblages in Paleoproterozoic collisional belts. The high-pressure assemblages are particularly common in metabasic dykes intrusive into Archean TTG gneiss, e.g. in the Nagssugtoqidian Orogen in South-East Greenland (Nutman and Friend, 1989, Nutman et al., 2008, Müller et al., 2018), the Aldan Shield of Siberia (Nutman et al., 1992, Smelov and Beryozkin, 1993), the North China Craton (Zhao et al., 2001, Guo et al., 2002, Tam et al., 2012), the Snowbird tectonic zone in the western Canadian Shield (Baldwin et al., 2004) and the Belomorian Mobile Belt in the Kola Peninsula (Skublov et al., 2011, Imayama et al., 2017, Liu et al., 2017, Yu et al., 2017). In other Paleoproterozoic collisional belts, the high-pressure assemblages occur in metabasic lithologies of MORB affinity, e.g. in the Usagaran and Ubende belts of Tanzania (Möller et al., 1995, Sklyarov et al., 1998, Collins et al., 2004, Boniface et al., 2012) and the Eburnian-Transamazonian orogen of southern Cameroon (Loose and Schenk, 2018). The Paleoproterozoic high-pressure mineral assemblages document an era of global subduction-related orogenic activity during the formation of the supercontinent Nuna or Columbia (Hoffman, 1988, Rogers and Santosh, 2002, Zhao et al., 2002, Brown, 2008, Reddy and Evans, 2009, St-Onge et al., 2009, Mertanen and Pesonen, 2012, Müller et al., 2018). The accurate and precise dating of the high-pressure metamorphic stage and other stages in the PT-evolution of these rocks is not straightforward and remains a challenge due to an often strong retrograde overprint or the lack of datable minerals. Bulk-rock or multigrain dating methods such as Rb-Sr, Sm-Nd and K-Ar may be complicated by the low closure temperatures of the geochronometers in question (e.g. Harrison, 1981, Mezger et al., 1992, Christensen et al., 1994), low element concentrations (Thöni, 2006) or incomplete trace element and isotopic equilibration during metamorphism (Rubatto and Hermann, 2003, Xie et al., 2004, Thöni, 2006). A more promising method to gain robust age data may be U-Pb dating of accessory phases with relatively high closure temperatures for Pb diffusion, such as zircon, monazite, titanite and rutile (Möller et al., 2000, Parrish, 2001, Ayers et al., 2002, Corfu et al., 2003a, Brewer et al., 2003, Timmermann et al., 2004). Zircon is the most commonly dated accessory phase and a robust tool for determining metamorphic ages due to its high U concentrations and low-Pb diffusivity over a large range of crustal conditions (Cherniak et al., 1997, Rubatto et al., 1999). In addition, it commonly preserves complex growth histories, with inherited magmatic and prograde, peak and retrograde metamorphic ages (Rubatto et al., 1999, Hermann et al., 2001, Ayers et al., 2002, Rubatto and Hermann, 2003, Zhang et al., 2005, Zheng et al., 2005, Nutman et al., 2008, Liu et al., 2017). Closure temperatures are estimated at >900 °C (Cherniak et al., 1997, Lee et al., 1997, Cherniak and Watson, 2000). Monazite incorporates larger amounts of actinides than zircon (Cherniak, 2010) but shows similar characteristics, in that it has very low Pb diffusivity (Cherniak et al., 2004) and is able to record complex P-T-t histories (Ayers et al., 2002, Mahan et al., 2006, Williams et al., 2007, Sindern et al., 2012). Closure temperatures have been estimated between 500 and 750 °C (Black et al., 1984, Copeland et al., 1988, Parrish, 1990, Suzuki et al., 1994, Smith and Giletti, 1997) and 900 °C (Bingen and Van Breemen, 1998, Braun et al., 1998, Kalt et al., 2000, Cherniak et al., 2004, Cherniak and Pyle, 2008). However, due to their accessory nature, an unequivocal correlation of zircon and monazite dates to a certain metamorphic stage on the PT-path is only possible via trace element data or inclusion content (Lanzirotti and Hanson, 1996, Hermann et al., 2001, Hermann and Rubatto, 2003, Rubatto and Hermann, 2007, Nutman et al., 2008). In contrast to monazite and zircon, titanite and rutile take part in metamorphic reactions, and their stability in PT-space can be determined via phase diagram modelling; however, ambiguity exists in the closure temperature estimates for both minerals. Early field-based studies indicated closure temperatures of around 450–650 °C for titanite (Mattinson, 1978, Heaman and Parrish, 1991, Mezger et al., 1991), consistent with subsequent experimental results suggesting closure around ca. 600 °C for slow cooling rates of ∼5 °C/Ma and diffusion radii <500 μm (Cherniak, 1993). Other studies estimated higher closure temperatures of ∼700 °C based on inheritance in titanite (Schärer et al., 1994, Scott and St-Onge, 1995, Pidgeon et al., 1996, Zhang and Schärer, 1996). Following from more recent studies (Kohn and Corrie, 2011, Gao et al., 2012, Spencer et al., 2013), Kohn (2017) suggested a closure temperature of around 800 °C, even at diameters of 10 μm. U-Pb dating of rutile has generally received less attention due to the often extremely low U content of rutile (Zack et al., 2011). Closure temperatures for Pb diffusion are estimated between 400 and 700 °C, depending on grain size and cooling rate (Mezger et al., 1989, Cherniak, 2000, Vry and Baker, 2006, Kooijman et al., 2010, Warren et al., 2011). Where successfully measured, rutile usually gives cooling ages (Connely et al., 2000, Li et al., 2003, Baldwin et al., 2004, Zack et al., 2011).

Based on U-Pb isotope data of zircon, monazite, titanite and rutile, we examine the geochronology of supracrustal deposition, dyke emplacement and metamorphism in a variety of retrogressed eclogite-facies rocks from the mainly high-pressure amphibolite-facies Kuummiut Terrane in the Paleoproterozoic Nagssugtoqidian Orogen of South-East Greenland (Wright et al., 1973, Chadwick et al., 1989, Dawes et al., 1989, Nutman and Friend, 1989, Messiga et al., 1990, Nutman et al., 2008, Kolb, 2014, Müller et al., 2018, Dziggel and Müller, 2018). These rocks were recently investigated in terms of their mineral textural evolution and PT-history, yielding four metamorphic stages on a clockwise PT-path (Müller et al., 2018). In order to provide constraints on the thermal evolution of the Kuummiut Terrane, we combine the PT-evolution with new and previously published age data (Bridgwater et al., 1990, Kalsbeek et al., 1993, Nutman et al., 2008, Thrane et al., 2016, Nicoli et al., 2018). Although the age of eclogite-facies metamorphism could not be constrained unequivocally, our results are consistent with an initially rapid, tectonically-controlled exhumation followed by slow, erosion-controlled cooling.