Lithium isotopes in kimberlites, lamproites and lamprophyres as tracers of source components and processes related to supercontinent cycles

Our pilot study reveals potential fingerprints of Li isotopes recorded in the Mesoproterozoic (∼1.4-1.1 Ga) kimberlites, lamproites and lamprophyres from the Eastern Dharwar Craton and Paleocene (62 Ma) orangeite from the Bastar Craton in India. The new data are interpreted in the context of available Li isotope composition of lamproitic to lamprophyric rocks occurring in Variscan (Bohemian Massif) and Alpine-Himalayan (SW Tibet) orogenic belts formed in response to Gondwana-Pangea amalgamation and break-up. As a result of supercontinents development, kimberlites from the Eastern Dharwar Craton and ‘orangeite’ from the Bastar Craton show clear presence of a component with a heavy Li isotope signature (δ7Li up to 9.7‰) similar to an ancient altered oceanic crust, whereas the Eastern Dharwar Craton lamproites (2.3-6.3‰) and lamprophyres (3.3-6.7‰) show Li isotope signatures indicative of a dominant contribution from heterogeneous lithospheric mantle. Variscan lamprophyric to lamproitic rocks and post-collisional mantle-derived (ultra)potassic volcanic rocks from SW Tibet, i.e., rocks from the orogenic belts outside the cratonic areas, are characterized by a clear Li isotope shift towards isotopically lighter component (δ7Li as low as -9.5‰) comparable with the involvement of an evolved continental crust and high-pressure metamorphic rocks in their orogenic mantle source. Such components with isotopically light Li are strikingly missing in the source of cratonic kimberlites, lamproites and lamprophyres.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5495097

Petrogenesis of a calc-alkaline lamprophyre (minette) from Thanewasna, western Bastar craton, Central India: Insights from mineral, bulk rock and in situ trace element geochemistry

The lamproites and kimberlites are well known from the eastern Bastar craton, Central India. However, a Proterozoic lamprophyre dyke is discussed here, from the Western Bastar Craton (WBC). The field geology, petrographic, mineralogical, and whole-rock, and in situ trace element geochemistry of biotite were described for understanding its petrogenesis and lithospheric evolution in the WBC. The Thanewasna lamprophyre (TL) is undeformed and unmetamorphosed, intruded into ca.2.5 Ga charnockite and metagabbro but closely associated with ca. 1.62 Ga undeformed Mul granite. The TL has a characteristic porphyritic texture, dominated by phenocrysts of biotite, microphenocryst of amphibole, clinopyroxene, and a groundmass controlled by feldspar. Mineral chemistry of biotite and amphibole suggest a calc-alkaline (CAL) type, and pyroxene chemistry reveals an orogenic setting. The TL is characterized by high SiO2 and low TiO2, MgO, Ni, and Cr, consistent with its subcontinental lithospheric origin. The presence of crustal xenolith and ocelli texture followed by observed variations in Th/Yb, Hf/Sm, La/Nb, Ta/La, Nb/Yb, Ba/Nb indicate substantial crustal contamination. Whole-rock and in situ biotite analysis by LA-ICPMS show low concentrations of Ni (30-50 ppm) and Cr (70-150 ppm), point to the parental magma evolved nature. Enrichment in H2O, reflected in magmatic mica dominance, combined with high LILE, Th/Yb ratios, and striking negative Nb-Ta anomalies in trace element patterns, is consistent with a source that was metasomatized by hydrous fluids corresponding to those generated by subduction-related processes. Significant Zr-Hf and Ti anomalies in the primitive mantle normalized multi-element plots and the REE pattern of the TL, similar to the global calc-alkaline average trend, including eastern Dharwar craton lamprophyres. Our findings provide substantial petrological and geochemical constraints on petrogenesis and geodynamics. However, the geodynamic trigger that generated CAL magmatism and its role in Cu-Au metallogeny in the WBC, central India, is presently indistinct in the absence of isotopic studies. Nevertheless, the lamprophyre dyke is emplaced close to the Cu-(Au) deposit at Thanewasna.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5361958