Prolonged (>100 Ma) ultrahigh temperature metamorphism in the Napier Complex, East Antarctica: A petrochronological investigation of Earth's hottest crust

Abstract

The Napier Complex in East Antarctica preserves a record of ultrahigh temperature (UHT) metamorphism during the late Archean to early Palaeoproterozoic. While there is little argument that the UHT metamorphic event began at c. 2,580 Ma, the duration over which the rocks resided at UHT has been the subject of intense debate, with estimates for the end of metamorphism ranging from 2,545 to 2,440 Ma—a discrepancy of some 105 Ma. To resolve the time‐scale of UHT metamorphism, a zircon and garnet petrochronological (U–Pb, REE and Ti) data set from a suite of rocks from the Tula Mountains region of the Napier Complex was analysed. Individual concordant populations define zircon U–Pb ages for (a) reset zircon cores of 2,502–2,439 Ma; (b) zircon rims of 2,491–2,454 Ma; and (c) neocrystallized sector‐zoned zircon from 2,492 to 2,443 Ma. Ti‐in‐zircon thermometry places a minimum estimate of 830°C for zircon crystallization, with the majority of concordant populations yielding temperatures >900°C. Zircon–garnet partitioning (DYb vs. DYb/Gd) arrays reveal that the bulk of metamorphic zircon defines an equilibrium relationship with the garnet that forms part of the peak assemblage. Combined with existing geochronological constraints, the new petrochronological data demonstrate that the Napier Complex remained at UHT from c. 2,585 Ma until at least 2,450 Ma, a residence time of 135 Ma. In the absence of evidence for contemporaneous emplacement of large volumes of igneous rocks, a number of factors likely combined to drive and maintain these extreme temperatures. We propose that the P–T conditions experienced by the Napier Complex were achieved through a combination of orogenic plateau formation, preconditioning of the crust by a high‐T magmatic and UHT metamorphic event at c. 2,850 Ma, inefficient removal of heat‐producing elements during partial melting and slow exhumation. This style of long duration, regional, extreme metamorphism is becoming more commonly identified in the rock record as larger and more robust data sets are collected (e.g. the Eastern Ghats of India and the Gondwanan East African Orogen) and is commonly associated with the amalgamation phases of supercontinents/cratons.

Persistence of melt-bearing Archean lower crust for >200 m.y.—An example from the Lewisian Complex, northwest Scotland

“…Such prolonged metamorphism is increasingly being recognized in the geological record through careful interpretation of coupled U-Pb and trace element data (e.g. Kelly and Harley, 2005;Korhonen et al, 2013;Clark et al, 2015Clark et al, , 2018Harley, 2016;Taylor et al, 2016;Cavalcante et al, 2018). The extremely slow rate of cooling implied by our data have implications for mechanisms of heat loss, and require that the lower crust remained melt present and close to isostatic equilibrium for more than 200 Myr.…”

“…For example, continental crust subducted to ultrahigh pressure during collisional orogenesis is generally exhumed rapidly and cools fast (Hermann et al, 2001;Rubatto and Hermann, 2001;. By contrast, cooling of some granulite terranes may be slow (< or <<5 C/Myr) and close-to-isobaric (Harley, 1985;Harley and Black, 1987;Harley, 1989;Mezger et al, 1991;Ashwal et al, 1999;Möller et al, 2000;Vry and Baker, 2006;Korhonen et al, 2013;Clark et al, 2018;Mitchell et al, 2019), implying formation near the base of continental crust that was never significantly overthickened and which experienced sustained high mantle heat flow (Sandiford and Powell, 1986;Bohlen, 1991).…”

“…In addition, Archean granulite terrains commonly record a complex polyphase tectonothermal history that may obscure their early evolution (e.g. Clark et al, 2018).…”

Testing the fidelity of thermometers at ultrahigh temperatures

“…In this case the ‘rim’ specifically refers to garnet in contact with the quartz‐cordierite symplectite. These array plot parameters are key for identifying partitioning of REE between zircon and garnet during metamorphic events (Clark, Taylor, Kylander‐Clark, & Hacker, ; Taylor et al, , ). As can be seen in Figure , when the zircon grains are plotted against the garnet core they are offset from the experimental data set of Taylor et al (); however, when plotted against the garnet rim they fall closer to the experimental values.…”

Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

“…The thermal history of the Napier Complex is important for unraveling the Earth's crustal evolution, including deep crust; however, geochronological constraints, such as the timing and duration of the metamorphic events, are still debated. Two hypotheses for the timing are proposed in previous studies: (i) the UHT metamorphism occurred no earlier than 2840 Ma and possibly from 2590 to 2550 Ma [4][5][6][7][8], (ii) it occurred from around 2500 to 2450 Ma [9][10][11][12][13][14].…”