Nature and geodynamics of Grenville-age orogens in Norway and Dronning Maud Land (East Antarctica) during the assembly of Rodinia: insights into the lateral terminations of the greater Grenville Orogen

Abstract

The Grenville Orogeny represents a globally pervasive tectonothermal event resulting in the amalgamation of the supercontinent Rodinia between ca. 1.3 and 1.0 Ga. The Grenville Orogen appears to be a continuous collisional orogen connecting Laurentia, Amazonia, Baltica and Kalahari and is generally compared to the modern Himalaya-Alpine orogenic system. Its eastern lateral termination is seen in the Sveconorwegian orogen in Baltica, while on the extreme other end, the Namaqua–Natal–Maud orogenic system, on the periphery of the Proto-Kalahari Craton, represents the western lateral termination along the conjugate Kalahari-Laurentia margin. This study focuses on the Grenville-age orogenic history of the Maud Belt, East Antarctica, and the Sveconorwegian orogen exposed in Caledonian nappe windows, West Norway, with an aim to provide a potentially comprehensive understanding of spatial terminations of the Grenville Orogen with comparison to its modern analogues. The specific goals are (1) to figure out the tectonic evolution and develop a refined tectonic model of the Maud Belt, and thus shed light on the orogenic style of the Grenville-age orogenic system along the margin of the Proto-Kalahari Craton; (2) to determine the extent and distribution of the Sveconorwegian crust, and to test a continental collision vs. accretionary model for the formation and evolution of the Sveconorwegian orogen. These aims are addressed by conducting large-scale regional investigations on geochronology and isotopic (radiogenic and stable) compositions of magmatism and high-grade metamorphism. It has been proposed that the formation of the Namaqua-Natal belt is related to the process of continental collision between Laurentia and the Proto-Kalahari Craton, however, the tectonics of the Maud Belt, which is located perpendicular to the Namaqua-Natal belt, were poorly understood so far. In this study, new zircon U–Pb–Hf–O isotopic data along a regional profile from western to eastern Dronning Maud Land (DML) and the comparison to the Natal belt provide new insights into the tectonic interpretation of the Maud Belt and the geodynamics of the western termination of the Grenville Orogen. Isotopic results indicate that older crustal components were involved in the generation of Grenville-age magmas and the spatial variation of Nd–Hf isotopes indicates an increasing juvenile input from west to east, i.e. away from the interior of the craton. The Maud Belt is interpreted to have developed on the substrate of the Proto-Kalahari Craton as a long-lived continental arc, and periodic magmatic influx is attributed to the switching between advancing and retreating subduction zone systems. The Maud and Natal belts differ clearly in their subduction polarity, geochronology and spatial-temporal trend of isotopic compositions. It is thus speculated that the Namaqua-Natal to Maud Belt may represent a changed tectonic environment from arc/continent-continent collision to slightly younger continental margin orogenesis at the westernmost termination of this part of the global Grenville Orogen. The Kalahari-Laurentia collision may have caused tectonic extrusions of a number of blocks including the Coats Land block, Tasmania and the Southern Tasman Ridge, which were detached from Laurentia and subsequently joined Kalahari and Australo-Antarctica respectively during Rodinia break-up. The Maud Belt appears to be the temporal starting point for a protracted accretionary tectonic cycle along the eastern margin of Kalahari, and Grenville-age arc magmatism probably evolved to form island arc chains (Tonian Oceanic Arc Super Terrane, TOAST) at ca. 1000–900 Ma. Subsequent to a period of quiescence during the stabilization of Rodinia between ca. 900–800 Ma, this continental margin of Kalahari was reactivated during ca. 780–630 Ma, following the break-up of Rodinia and subduction of the Mozambique Ocean, which is recorded by voluminous arc magmatism and high-grade metamorphism in central DML. The subduction process terminated when a series of terranes/blocks including Indo-Antarctica, TOAST, and Kalahari were welded together during Gondwana assembly at ca. 570–550 Ma, followed by post-collisional delamination during ca. 530–485 Ma. The Sveconorwegian orogenic evolution remains a hot topic for debate, and the debate centers on whether the Sveconorwegian orogen represents an accretionary orogen with long-term subduction, or a continental collision orogen that can be regarded as an extension of the Grenville Orogen in Laurentia. Our study traces the Sveconorwegian records from the well-established Sveconorwegian Province in southwest Norway into basement windows underneath the Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending profile reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding with the gradual transition from Telemarkian to Gothian crust. The Hf–O isotopic patterns show that ca. 1050–950 Ma Sveconorwegian magmas are different from the typical arc magmas and Gothian-Telemarkian magmas by much less addition of juvenile material and sedimentary components, which makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The southwest margin of Baltica was most likely transformed from a long-lived Mesoproterozoic accretionary margin into the collisional Sveconorwegian orogen by collisional interactions of Baltica with Laurentia and Amazonia in the context of Rodinia. Based on combined geological, isotopic and paleomagnetic results from the present and previous studies, it is suggested that the Grenville Orogen probably terminated on its western end by a tectonic transition from collisional orogeny in the Namaqua-Natal belt to accretionary orogeny in the Maud Belt, and Kalahari-Laurentia collision probably caused lateral extrusions of a number of crustal terranes. On its eastern side, the Sveconorwegian Orogen most likely formed by collisional interactions of Baltica with Amazonia, and represented the extension of the Grenville Orogen into southwest Baltica.