Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt, Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating

Abstract

Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre...

From sink to source: Using offshore thermochronometric data to extract onshore erosion signals in Namibia

“…On top of the basement are the siliciclasticcarbonate successions of the Damara Supergroup, which experienced high P-low T metamorphism along the southern branch of the intracontinental Damara metamorphic belt during Pan-African (600-480 Ma) orogenesis (Miller, 1979). The central Damara orogen experienced low P-high T contact metamorphism associated with the emplacement of voluminous granitic plutons at 540-500 Ma (Jung et al, 2019(Jung et al, , 2020.…”

“…On top of the basement are the siliciclastic-carbonate successions of the Damara Supergroup, which experienced high P-low T metamorphism along the southern branch of the intracontinental Damara metamorphic belt during Pan-African (600-480 Ma) orogenesis (Miller, 1979). The central Damara orogen experienced low P-high T contact metamorphism associated with the emplacement of voluminous granitic plutons at 540-500 Ma (Jung et al, 2019(Jung et al, , 2020. The tectonic structure of the Damara Orogen is characterised by NNW-trending transpressional faults of the northern Kaoka and southern Gariep Belts and the ENE-trending structures in the main central Damara Belt that formed during Pan-African convergence between the Congo and Kalahari cratons (Frimmel, 1995;Miller, 1983;Passchier et al, 2002).…”

Petrogenesis of a late-stage calc-alkaline granite in a giant S-type batholith: geochronology and Sr–Nd–Pb isotopes from the Nomatsaus granite (Donkerhoek batholith), Namibia

“…The metamorphic grade of the Central Zone increases from east to west with partial melting and granulite-facies conditions (Masberg et al 1992). Peak conditions reached ~ 835 °C at ~ 0.5-0.6 GPa and occurred between ~ 520 and ~ 500 Ma (Jung et al 2018;Longridge et al 2017).…”

“…The conditions necessary to generate the diverse igneous complexes in the Damara orogen are not clear and existing Nomatsaus granite ) Donkerhoek batholith (Clemens et al 2017Schwark et al 2018) Donkerhoek Onanis ( Conrad et al (1988) reywacke Holtz & Johannes (1991) (Barnes and Sawyer 1980) to ridge subduction and opening of an asthenospheric window (Meneghini et al 2014), to intracrustal heat production due to elevated K, Th and U contents (Haack et al 1983;Jung et al 2018;Longridge et al 2017), and to intra-crustal heat transfer (Hartmann et al 1983;Kröner 1982). Features that are considered to be essential in classical subduction zone settings are absent in the Damara orogen.…”

Metasedimentary Metatexites with Trondhjemitic Leucosomes from NE Sicily: Another Example of Prograde Water-fluxed Melting in Collisional Belts

“…Trondhjemitic leucosomes in metasedimentary migmatites are mainly considered to represent cumulates of early crystallizing phases left behind after the segregation of K 2 O-rich granitic melts [8][9][10][11]72], or products of water-fluxed melting of metapelites and metagreywackes [2,4,19,[67][68][69][73][74][75][76][77][78][79].…”