Insights for intraplate extensional settings revealed from geochemistry of metamorphosed magmatic rocks of the central Gondwana supercontinent

The extensional episodes related to the shaping of the Gondwanan margins with the collapse of the Cambrian Orogens have not been deeply investigated despite extensive studies on compressional tectonics. The rift-setting considered here in Sri Lanka at the boundary zone of eastern parts of Kadugannawa and Highland Complexes (KC and HC, respectively) may correspond to the central Gondwanan segments in the vicinity of India, Antarctica, and Madagascar. Hence this study offers a pathway to geochemical characterization of intra-continental extensional margins adding a new value for global tectono-geochemical knowledge. Analysed meta-mafic rocks of the KC display smooth Chondrite normalized rare earth element (REE) patterns with most of them having distinct heavy REE (HREE) depletions, with (La/Yb)_N ratios in the range of 2–20, and slightly negative to positive Eu anomaly. The majority of felsic-rocks also show HREE depletion with high light REE (LREE) enrichment accompanying variably-positive to slightly-negative Eu anomalies. In contrast, meta-mafic rocks of the HC mostly show flat middle REE (MREE) to HREE patterns with variably-enriched LREE with the majority showing discrete-negative or no Eu anomalies. All the meta-mafic rocks show that their protoliths were derived from enriched mantle/OIB sources. Characteristically, none of the meta-felsic rocks studied exhibits arc magmatic signatures, while all of them were consistent with the origin of A2-type anorogenic characteristics. Hence, the overall geochemistry of the protolith melts of both meta-mafic and meta-felsic rocks studied here mostly suggests their genesis at an intra-plate extensional setting, contrary to arc-tectonic settting observed in predominant previous studies. Our U-Pb age data correspond to magmatic pulses from ca. 2250 to ca. 550 Ma (ca. 2250–1750 Ma, 1050–850 Ma, 700–627 Ma, and 550–575 Ma) followed by a significant interaction with crustal components at ∼580––530 Ma which is consistent with the escalation of the Th/U and Ce/Ce* values in Zircon. The predominant age peaks match well with both collisional and rifting events associated with the dispersal and amalgamation of global supercratons (e.g. Superia), supercontinents (e.g. Columbia, Rodinia, and Gondwana) and regional micro-continents (e.g. Azania), spanning from early to late global geodynamic history from ca. 2250 to ca. 500 Ma. Hence, this study provides new insights into intra-continental rifting/extensional tectonic settings hitherto-unknown from the inner-central Gondwana.