Lower crust-mantle interactions in the massif-type anorthosite formation: New evidence from zircon U-Pb-Hf-O isotopes of the Neoproterozoic Kadavur Complex, southern India

Abstract

The Kadavur massif-type anorthosite, which intruded the basement of the Madurai Block in southern India, comprises anorthosite, leucogabbro, and (noritic) gabbro with minor Fe-Ti oxide ores. The U-Pb zircon SIMS dating indicates that the magmatism occurred during the Neoproterozoic period (Tonian), i.e., noritic gabbro (790.4 ± 5.1 Ma), gabbro (793.0 ± 4.1 Ma), leucogabbro (784.9 ± 4.1 Ma), and anorthosite (790.7 ± 4.3 Ma). Combined with the published age data from the coeval charnockite and A-type granite, we conclude that the anorthosite-charnockite-rapakivi granite suite was emplaced during ca. 785–805 Ma. In-situ zircon Hf-O isotopic data of the Kadavur gabbros and anorthosite provide insights on the source and evolution of the parental magmas of the massif-type anorthosites. Zircons from the anorthosite are characterized by ancient crustal εHf(T) values (−11.4 to −6.5) and higher δ18O values (5.92 to 6.4‰). In contrast, zircons from the coarse-grained gabbro have relatively primitive εHf(T) values (−6.3 to −2.3) and mantle-like δ18O values (4.86–5.73‰). Moreover, zircons from the noritic gabbro and leucogabbro have εHf(T) values (−11.1 to −3.7) and δ18O values (5.35 to 6.77‰) distributed between those of the anorthosite and coarse-grained gabbro. The new zircon Hf-O data demonstrate that the parental melt of the anorthosite was subjected to crustal contamination during the early evolution stages, producing an evolved magma with crustal isotope and trace element signatures. In contrast, the gabbros show less crustal influence and like to represent original magma source of the Kadavur Complex. The zircon Hf-O isotope compositional array from the primitive gabbros to the more-evolved anorthosite demonstrates that the parental magmas were derived from partial melting of the upper mantle with varying crustal input, which can be up to 30–40% for the anorthosite formation. Contamination of the ponded basaltic magmas by the felsic crust can effectively increase the SiO2, Al2O3, Na2O, and Sr contents in the magmas, which was likely essential for enormous plagioclase fractionation in the massif-type anorthosites.