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The Palaeoproterozoic Birimian Supergroup of the West African Craton (WAC) consists of volcanic belts composed predominantly of basaltic and andesitic rocks and intervening sedimentary basins composed predominantly of wackes and argillites. Mafic metavolcanic rocks and granitoid-hosted enclaves from the Palaeoproterozoic Lawra Belt of Ghana were analysed for geochemical and Sr–Nd isotopic data to constrain the geological evolution of the southeastern part of the WAC. The metavolcanic rocks display mainly tholeiitic signatures, whereas the enclaves show calc-alkaline signatures. The high SiO2 contents (48.6–68.9 wt%) of the enclaves are suggestive of their evolved character. The high Th/Yb values of the samples relative to that of the mantle array may indicate derivation of their respective magmas from subduction-modified source(s). The rocks show positive εNd values of +0.79 to +2.86 (metavolcanic rocks) and +0.79 to +1.82 (enclaves). These signatures and their Nd model ages (TDM2) of 2.31–2.47 Ga (metavolcanic rocks) and 2.39–2.47 Ga (enclaves) suggest they were probably derived from juvenile mantle-derived protoliths, with possible input of subducted pre-Birimian (Archean?) rocks in their source(s). Their positive Ba–Th and negative Nb–Ta, Zr–Hf and Ti anomalies may indicate their formation through subduction-related magmatism consistent with an arc setting. We propose that the metavolcanic rocks and enclaves from the Lawra Belt formed in a similar island-arc setting. We infer that the granitoids developed through variable degrees of mixing/mingling between basic magma and granitic melt during subduction, when blobs of basic to intermediate parental magma became trapped in the granitic magma to form the enclaves. [ABSTRACT FROM AUTHOR]

Earth's deep crustal granulites, in particular, metamorphosed clastic sedimentary rocks, are important to decode the composition of the pelitic lower crust. General understanding is that granulite-facies metamorphism extensively removes trace elements from the protolith sediments, resulting their high depletion in newly generated granulites. However, there has been no systematic appraisal on the budget of the trace elements in newly-born granulites compared against that of their pre-metamorphic sediments. A demonstrative example taken from the Highland Complex of Sri Lanka, showing the trace element budget including highly incompatibles of pelitic-granulites remain unmodified, attesting that even ultra-high temperature metamorphism does not cause large-scale removal of trace elements from the sedimentary protoliths, is presented here. Our results reveal that a regional-scale chemical modification during granulite generation in some parts of the mid-lower crust does not enhance the mobility of trace elements in whole rock scale. This finding may impart an important message to researchers that high/ultra-high temperature metamorphism of deep crustal granulites does not necessarily disturb inherited trace element characteristics from their source sediments. • No significant alkaline, HFSE and REE fractionations in granulite formation. • Trace element budget of pelitic-granulites remain unmodified at UHT metamorphism. • Pelitic lower crust is chemically analogous to their sedimentary protoliths. [ABSTRACT FROM AUTHOR]

Recent tectonic models of Sri Lankan basement suggest that the Highland Complex (HC), which is dominantly composed of metasedimentary rocks, forms part of an accretionary complex developed during Neoproterozoic subduction-collision tectonics between the Wanni and Vijayan Complexes. Here we present new U–Pb geochronological data on detrital zircon in metasediments from the HC and adjacent regions to investigate the source characteristics, deposition and regional correlations with similar rocks in adjacent Gondwana fragments. Cores of detrital zircon grains in twelve metasediment samples (quartzite, pelitic gneiss, leptynite, and metamorphosed banded iron formation) characterized by higher Th/U ratios indicating derivation from magmatic protoliths yield Neoarchean to Paleoproterozoic ages (ca. 2700–1700 Ma) with minor Paleoarchean (ca. 3500 Ma), Mesoproterozoic (ca. 1200 Ma), and Early to Middle Neoproterozoic (ca. 800–600 Ma) components. The rims of most grains as well as homogeneous grains show Late Neoproterozoic ages (582–533 Ma) with lower Th/U ratios that can be interpreted as metamorphic ages. Our results constrain the depositional age of the HC sediments to be Late Neoproterozoic (ca. 600–550 Ma) based on the ages of the youngest detrital zircon grains and metamorphism. Although exposed Archean magmatic units are absent in Sri Lanka, several crustal blocks including the Dharwar Craton, Coorg Block, and Salem Block in the adjacent Gondwana fragment of India carry Archean (ca. 3400–2500 Ma) basement, which we consider to be the major source for the Sri Lankan detrital zircons. The Paleoproterozoic zircons might have been derived from the Congo–Tanzania–Bangweulu Block in East Africa that includes Archean to Paleoproterozoic blocks (e.g., ca. ∼2700 Ma Tanzania Craton; ca. 2300–1800 Ma Usagaran–Ubendian belt). The Kibaran belt (ca. 1400–1000 Ma) in the Congo–Tanzania–Bangweulu Block might have served as the source of minor Mesoproterozoic zircons. Early to Mid Neoproterozoic grains might have come from the adjacent Wanni, Vijayan, and Kadugannawa Complexes in Sri Lanka (ca. 1100–750 Ma). The similarity in detrital age spectra of the metasediments in the HC and those of the northern Madurai Block in southern India suggests that they could have similar provenances (Archean cratons in India and/or East Africa). Similar age distributions of detrital zircon cores reported from the Itremo Group (central Madagascar) indicate that the metasedimentary terranes in central Madagascar, southern India, and Sri Lanka could have been juxtaposed, and with sediment source including those in the East African cratons. Similar magmatic features in central Madagascar, southern India, Sri Lanka, and eastern Antarctica also suggest a series of volcanic arcs developed through a complex subduction-accretion process in these regions before the final amalgamation of Gondwana. [ABSTRACT FROM AUTHOR]

Abstract: The Dunhuang Block, located in the eastern Tarim Craton, widely exposes Precambrian TTG gneisses and metamorphosed supracrustal rocks. A suite of metasedimentary rocks of the Dunhuang Group in the Hongliuxia area on the eastern margin of the Tarim Craton are characterized by moderate contents of quartz, plagioclase, and biotite, with minor amounts of garnet or kyanite, most of which have experienced strong deformation, resulting in well-developed amphibolite-facies foliations. The examined samples are characterized by moderate Fe2O3 +MgO and TiO2 contents, and Al2O3/SiO2 and K2O/Na2O ratios with low Chemical Index of Alteration (CIA) and high Index of Compositional Variability (ICV) values, suggesting that the protolith sediments originated from continental island arc environment. The U–Pb ages of magmatic-type detrital zircons from the Hongliuxia metasedimentary rocks range between 2476Ma and 1950Ma, with a peak at 2417Ma. Detrital zircons of metamorphic origin yielded an age peak at 1832Ma, which may represent a tectono-thermal event in the Paleoproterozoic. Considering that the main age populations of the analyzed samples are consistent with the tectono-thermal events in the Tarim Craton during the early Paleoproterozoic, we propose that the Tarim Craton was possibly the major source for the protolith sediments of the metasedimentary rocks in the Hongliuxia area. Zircon Hf isotopic compositions (εHf(t)=−5.8 to +2.9) suggest voluminous addition of juvenile materials to the crust and minor reworking of old materials in the eastern Tarim Craton during the Mesoarchean (TDM C =2.8–2.9Ga). The metasedimentary rocks of the Hongliuxia area provide clear constraints to the evolution of the Tarim Craton. A possible tectonic model is that multiple thermal events (~2.4Ga, 2.2Ga, 2.0Ga and 1.8Ga) occurred in the study area, and the time period of Mesoarchean was important for crustal growth in the eastern Tarim Craton, whereas the ~1.83Ga metamorphism might represent a thermal event associated with the Tarim Craton within the supercontinent Columbia. [Copyright &y& Elsevier]