A new UHP metamorphic complex in the  1.8 Ga Nagssugtoqidian Orogen of West Greenland

Abstract

The Nagssugtoqidian Orogen is a ca. 1.8 Ga belt of east-west trending, highly deformed rocks that bisects central Greenland. Although a variety of data have suggested this belt marks the location of a continent-continent collision zone, evidence of subduction has been lacking. We report here mineralogical evidence from four samples within a well-defined lithologic unit of metabasic and metasedimentary ocean floor rocks of a previously unrecognized UHP metamorphic episode. The UHP episode is recorded by remnants of orthopyroxene exsolved from majoritic garnet, graphitized diamond, exsolution of rutile from garnet and pyroxenes, exsolution of magnetite from olivine, and complex exsolution textures in ortho-and clinopyroxenes (including omphacite). Associated with these mineralogical features is an unusual occurrence of quartz needles in Mn-rich fayalite. From textural characteristics, we infer that the quartz needles exsolved from the fayalite. To our knowledge, olivine with exsolved silica has not been reported. We note, however, that experimental studies have shown that b-spinel can incorporate excess silica. We therefore speculate these quartz needles may be silica that exsolved from Mn-rich ahrensite, the Fe analog of ringwoodite, upon decompression and inversion to fayalite. If correct, this occurrence would be the first reported sample of naturally occurring olivine (fayalite) that inverted from ahrensite. Corroborating an early UHP history are reaction relationships that delineate a path through high-pressure and high-temperature conditions during decompression. P-T conditions inferred for the UHP episode are ~7 GPa at ~975 °C. The unusually low T for this UHP system at ~1.8 Ga may reflect either very rapid subduction rates at that time, or unexpectedly cool mantle conditions. Preservation of the UHP assemblages probably is due, in large part, to the exceptionally low a H2O during decompression and cooling. These UHP rocks establish that the location of the subduction and suture zones that must have existed prior to and during the collision of continents was along what is now the northern edge of the Nordre Strømfjord shear zone.

On the survival of intergranular coesite inUHPeclogite

“…Indeed, the preservation and exhumation of unexpected relicts of UHP mineral assemblages may be more common that currently realized. We hope that this study, together with that by Glassley et al (2014Glassley et al ( , 2016, will stimulate further careful fieldwork in ancient suture zones in a search for new examples of UHP metamorphism. Such discoveries are critical to the goal of identifying when Earth developed a global plate tectonics regime, how this regime began, what Earth's tectonic style was before plate tectonics, and how to define plate tectonics earlier in Earth history (Sizova, Gerya, & Brown, 2014;Sizova, Gerya, St€ uwe, & Brown, 2015;Stern, Gerya, & Tackley, 2017).…”

“…Estimated P–T conditions are ~7 GPa and ~975°C, comparable with those at Yangkou Bay and consistent with cold subduction (Brown & Johnson, ). These UHP mineral assemblages were preserved in trivial volumes of rock within a high‐strain zone due to the serendipitous combination of strain partitioning, low at the metamorphic peak and an unusual Mn–Fe‐rich protolith composition (Glassley et al., ).…”

“…Recently, Glassley, Korstard, Sorensen, and Platou (2014) reported the preservation of UHP mineral assemblages in the Nordre Strømfjord shear zone along the northern boundary of the Palaeoproterozoic Nagssugtoqidian mobile belt in West Greenland. This occurrence is extraordinary.…”

Metallogeny of Greenland

“…12;Hanmer et al, 1997;van Gool et al, 2002). The northern part of the Nordre Strømfjord Shear Zone is characterized by lenses of Neoarchean metasedimentary and basic metavolcanic rocks, which record ultrahigh pressure metamorphism at ~975°C and 70 kbar followed by isothermal decompression to ~900°C and 10 kbar Glassley et al, 2014 (Manatschal et al, 1998;van Gool et al, 2002). Paragneiss locally has detrital zircons of ca.…”

“…Ultrahigh pressure metamorphism in the NSSZ suggests that this structure represents a suture of the Nagssugtoqidian Orogen just north of the Paleoproterozoic intrusive rocks ( Fig. 12; Glassley et al, 2014). The uniformity of Archean ages for magmatism and metamorphism, the continuity of Paleoproterozoic deformation into the Rinkian Belt further north as well as the occurrence of Palaeoproterozoic passive margin rocks (Karrat Group) in the north were used to propose a suture further north in the Rodebay area ( Fig.…”