Source composition, fractional crystallization and magma mixing processes in the 3.48–3.43Ga Tsawela tonalite suite (Ancient Gneiss Complex, Swaziland) – Implications for Palaeoarchaean geodynamics
Abstract The ca. 3480–3430 Ma Tsawela Gneiss (TG) is a well-preserved coarse-grained diorite to tonalite suite within the Ancient Gneiss Complex of Swaziland, eastern Kaapvaal craton. These gneisses are texturally and compositionally distinct from the hosting oldest components of the ca. 3200–3660 Ma TTG-type Ngwane Gneisses (NG). Major and trace elements, in combination with whole-rock hafnium-neodymium isotopic data, were analyzed in the TG and in three samples of ca. 3450 Ma grey NG to constrain sources and magmatic processes. High-field-strength element data (HFSE) were combined with U-Pb SHRIMP II ages and Hf-in-zircon data for key samples to constrain their ages and petrogenesis. In c…
Palaeoproterozoic ancestry of Pan-African high-grade granitoids in southernmost India: Implications for Gondwana reconstructions
Abstract SHRIMP dating of magmatic zircons from granitoid gneisses and charnockites of the Trivandrum and Nagercoil Blocks in the granulite terrane of southernmost India yielded well-defined protolith emplacement ages between 1765 and ca. 2100 Ma and also document variable recrystallization and/or lead-loss during the late Neoproterozoic Pan-African event at around 540 Ma. Hf-in-zircon and whole rock Nd isotopic data suggest that the granitoid host rocks were derived from mixed crustal sources, and Hf–Nd model ages vary between 2.2 and 2.8 Ga. A gabbroic dyke, emplaced into a charnockite protolith and deformed together with it, only contained metamorphic zircon whose mean age of 542.3 ± 4.0…
Archaean Crystalline Rocks of the Eastern Kaapvaal Craton
The eastern part of the Kaapvaal Craton represents a classical granitoid-greenstone terrain and contains the oldest rocks of the African continent, exhibiting about 1000 Ma of crustal evolution from 3.66 to 2.67 Ga. The granitoid rocks predominantly consist of the tonalite–trondhjemite–granodiorite (TTG) association with true granites becoming abundant at about 3 Ga. Greenstones are represented by the well-preserved and well-studied 3.54–3.2 Ga Barberton Greenstone Belt and smaller ca. 3.45 Ga greenstone belt remnants infolded in TTG gneisses around the BGB as well as in the Ancient Gneiss Complex in Swaziland. The origin of both the TTGs and greenstone units is still debated as strong defo…
Middle to Late Ordovician arc system in the Kyrgyz Middle Tianshan: From arc-continent collision to subsequent evolution of a Palaeozoic continental margin
New geological, geochronological and isotopic data reveal a previously unknown arc system that evolved south of the Kyrgyz Middle Tianshan (MTS) microcontinent during the Middle and Late Ordovician, 467-444 Ma ago. The two fragments of this magmatic arc are located within the Bozbutau Mountains and the northern Atbashi Range, and a marginal part of the arc, with mixed volcanic and sedimentary rocks, extends north to the Semizsai metamorphic unit of the southern Chatkal Range. A continental basement of the arc, indicated by predominantly felsic volcanic rocks in Bozbutau and Atbashi, is supported by whole-rock Nd- and Hf-in-zircon isotopic data. epsilon(Nd(t)) of + 0.9 to -2.6 and epsilon(Hf…
Detrital and igneous zircon ages for supracrustal rocks of the Kyrgyz Tianshan and palaeogeographic implications
Abstract We report detrital zircon ages for Precambrian and early Palaeozoic metasediments from the Tianshan orogen in Kyrgyzstan, an important component of the Central Asian Orogenic Belt and compare these with published ages from the Chinese Tianshan and the Tarim craton. These data provide information on possible source terrains and suggest that Precambrian basement is widespread in the Tianshan and may, at least in part, represent continental fragments rifted off the Tarim craton during the early history of the Central Asian Orogenic Belt. Distinct differences in the Precambrian zircon age distribution between the North and Middle Tianshan of Kyrgyzstan support earlier ideas that these …
Age, Nd–Hf isotopes, and geochemistry of the Vijayan Complex of eastern and southern Sri Lanka: A Grenville-age magmatic arc of unknown derivation
Abstract The ca. 1.0–1.1 Ga Vijayan Complex (VC) of eastern and southeastern Sri Lanka is one of three high-grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent, older Highland Complex. It consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite, with a distinct calc-alkaline geochemical signature, and is interpreted as a magmatic arc. Strong ductile deformation has obliterated almost all original intrusive relationships. High-grade metamorphism during the Pan-African event at ca. 610–520 Ma has produced widespread granulite-facies assemblages that are now largely retrogressed and were affected b…
Zircon ages and Nd–Hf isotopes in UHT granulites of the Ider Complex: A cratonic terrane within the Central Asian Orogenic Belt in NW Mongolia
Abstract The Ider Complex of the Tarbagatai Block in northwestern Mongolia is part of a Precambrian microcontinental terrane in the Central Asian Orogen Belt and has experienced a polymetamorphic tectono-metamorphic evolution. We have investigated an enderbitic gneiss, derived from a quartz diorite and a charnockite, derived from a leucogranite, and zircon SHRIMP data reveal late Archaean protolith ages of 2520–2546 Ma for these rocks. Metamorphic overgrowth on these zircons as well as newly-formed metamorphic zircons document a high-temperature metamorphic event (T = 930–950 °C) at about 1855–1860 Ma. Nd whole-rock isotopic systematics show these and other gneisses of the Ider Complex stra…
Zircon ages defining deposition of the Palaeoproterozoic Soutpansberg Group and further evidence for Eoarchaean crust in South Africa
Abstract The precise age of the volcano-sedimentary Soutpansberg Group, which was deposited upon the Palala shear belt separating the Kaapvaal Craton from the Central Zone of the Limpopo Belt, has long been debated. The Soutpansberg Group is subdivided into a lower and upper succession, which are separated from each other by a prominent regional unconformity. Zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the Soutpansberg Group rocks. The zircon grains of the investigated samples from both successions yield a wide range of ages, spanning from 1831 to 3937 Ma. Most of the zircon grains have rounded shapes, …
Chronology of the oldest supracrustal sequences in the Palaeoarchaean Barberton Greenstone Belt, South Africa and Swaziland
Abstract Zircon age data for felsic metavolcanic rocks of the Sandspruit and Theespruit formations, the two oldest supracrustal components in the Palaeoarchaean Barberton Greenstone Belt, show that these two successions are time-equivalent and constitute one single volcanic event at ca. 3530 Ma. The Sandspruit felsic rocks are ubiquitously metasomatized, intensely deformed and intruded by, and tectonically interlayered with, ca. 3450 Ma granitoid sills that are probably part of the Theespruit Pluton. One metasomatized Sandspruit sample contains abundant metamorphic zircons with a weighted mean 207Pb/206Pb age of 3220.1 ± 1.6 Ma, reflecting a widespread metamorphic event in parts of the east…