Indications of older Mesoproterozoic metamorphism

The first major metamorphic event affecting the Maud Belt is reported from zircon rim overgrowths from various Grenville–age basement rocks, mainly between 1090–1030 Ma. The U–Pb dataset for the samples herein does not show any evidence of this metamorphic event.

However, similar findings are reported from earlier studies conducted in Gjelsvikfjella, except for a few gneisses investigated by Bisnath et al. (2006) dated at 1090–1070 Ma.

One zircon from sample JT10 indicates a metamorphic rim age of 1128±5 Ma. The rim is CL–

dark and structureless with a moderate Th/U ratio of 0.3. The age is slightly reversely discordance and is only supported by this single analysis. The age is suspicious as no similar metamorphic age has been recorded in previous studies. However, as discussed in the sections above, the Maud Belt experienced a peak in magmatic activity around 1140–1130 Ma and this peak is also prominent in Gjelsvikfjella (study area). Thus, the metamorphic age could be a result of metamorphism related to the emplacement of igneous rocks. However, this age is not highly reliable as it is only supported by a single zircon analysis.

Sample JT10 also indicates metamorphism prior to the constrained Mesoproterozoic metamorphic event related to the collision of the Proto–Kalahari Craton during the Rodinia assembly. Two zircon rims yield a concordant age of 1105±8 Ma. The rims show low Th/U ratios of 0.09 and 0.1, indicating a metamorphic origin. U–Pb metamorphic zircon overgrowths of ca. 1100 Ma are rarely detected within the Maud Belt. However, a metaquartzite from Vardeklettane, Heimefrontfjella, is suggested to record granulite–facies metamorphism at 1104±5 Ma (Arndt et al., 1991). In addition, a metamorphic age of 1095±14 Ma is obtained by zircon rims from an adjacent area, Sivorgfjella (Heimefrontfjella) (Jacobs et al., 2003c).

Furthermore, a magmatic pulse overlapping the ca 1105 Ma metamorphism is commonly reported within the Maud mountains, e.g. in Heimefrontfjella (1107±11 Ma, 1098±11 Ma), Kirwanveggen (1103±13 Ma), Gjelsvikfjella (1104±8 Ma), Orvinfjella (1107±8 Ma) (Jackson, 1999; Jacobs et al., 2003c; Bisnath et al., 2006; Wang et al., 2020).

The well–constrained magmatic activity around 1100 Ma within the Maud Belt and the poorly defined ca. 1105 Ma metamorphic event is intriguingly comparable with a large–scale intra–

plate magmatic event (~1110 Ma) recognized across the Proto–Kalahari Craton (Fig. 6.3). This magmatic event was first detected from mafic sill intrusions representing the Umkondo LIP in Africa. However, later work discovered coeval intrusions throughout the Proto–Kalahari Craton, such as the Borgmassivet Suits (~1107 Ma) within the Grunehogna Province (western Dronning Maud Land) (reviewed by Hanson et al., 2006). This major anorogenic magmatic event was rapidly emplaced between ca. 1112–1106 Ma as a result of a plume center beneath the Kalahari Craton during the Rodinia assembly (Hanson et al., 1998; Hanson et al., 2004).

Some authors (e.g. Grosch et al., 2007), suggest the older metamorphic imprints detected within the Maud Belt, such as ca. 1104 Ma (Arndt et al., 1991) and ca. 1095 Ma (Jacobs et al., 2003c) (comparable with this study), to possibly date this large–scale thermal Umkondo–Borgmassivet magmatic event. Furthermore, Grosch et al. (2007) presented geochemical data examined on Mesoproterozoic amphibolites from the western part of the Maud Belt (H.U. Sverdrupfjella–

Gjelsvikfjella). Their Sm–Nd model age and εNd value revealed a similar signature as the Borgmassivet Sills, implying a possible relationship to the Umkondo–Borgmassivet thermal event. Another thermal imprint of the Umkondo–Borgmassivet Suits could be reflected by leucosome development in Kirwanveggen, dated at 1098±5 Ma (Harris, 1999).

Figure 6.3: Illustration of the extend of the Umkondo Igneous Province within southern Africa and Antarctica.

The generalized map shows the location for some of the intrusions that have been correlated with Umkondo magmatism. The U–Pb baddeleyite and zircon ages presented are representative igneous intrusion ages, from the work of Hanson et al. (2004) and Hanson et al. (2006), inferred to be emplaced during the intraplate magmatic event within the Kalahari Craton (~1110 Ma). Modified from Moabi et al. (2015) (after Hanson et al., 2004).

Other studies opine the short–lived magmatism to be a consequence of subduction–related magmatism and asthenospheric upwelling, resulting in mafic intrusions within the margin of the Grunehogna Craton, such as the Borgmassivet intrusions and volcanic activity in the Maud arc, synchronous to the Umkondo LIP magmatism (Grosch et al., 2015; Hokada et al., 2019).

Wang et al. (2020) reported more juvenile Hf isotope compositions for the rocks crystallizing at ca. 1100 Ma in the Orvin–Wohlthat Mountains compared to the older dated rocks. These

findings may reflect a tectonic switching of the inboard subduction underneath the eastern margin of the Proto–Kalahari Craton. They infer the subduction to transform from advancing to retreating subduction–process, generating a significant magmatic pulse around 1100 Ma. The widespread ca. 1100 Ma magmatic activity within the Maud arc could then explain the older recorded metamorphic ages, such as the ca. 1105 Ma metamorphic age detected in this study, as well as the ca. 1098 Ma leucosome development reported in Kirwanveggen (Harris, 1999).

Following, the Maud Belt was affected by a major metamorphic event and granitic magmatism around 1090–1030 Ma.

To conclude, most of the Grenville–age rocks within the Maud arc were emplaced through several episodes of magmatic activity, with the most prominent pulses peaking at ca 1140–1130 Ma, 1100 Ma, and 1090–1050 Ma (Fig. 6.2). Sample JT10 records two Mesoproterozoic metamorphic imprints at ca. 1128 Ma and ca. 1105 Ma. These ages indicate that some parts of the mountains underwent metamorphism prior to the so far major metamorphic event around 1090–1030 Ma. There are probably several episodes of metamorphism related to increased magmatic activity within the Maud Belt. The metamorphic ages presented in this study are coeval with magmatic pulses and could thus represent thermal imprints related to the igneous activity in the area. However, JT10 is a complex sample with a high number of discordant analyses. Further, the metamorphic overprints are constrained from a very limited number of rims overgrowths. This makes the metamorphic age data less reliable and needs to be interpreted with care. Further work is required to verify if these magmatic pulses triggered thermal imprints within some of the Mesoproterozoic basement rocks of the Maud Belt.