[S-MP22] Supercontinents and Crustal Evolution

[SMP22-04] Petrological and Geochemical Study of Charnockites from the Wanni Complex, Sri Lanka: Implications for Correlations with Southern India

*Pahan Abewardana^1,2, Madhusoodhan Satish-Kumar¹, P.L. Dharmapriya², S.P.K. Malaviarachchi², Zhao Lei³ (1.GRADUATE SCHOOL OF SCIENCE AND TECHNOLOGY, NIIGATA UNIVERSITY, 2.POSTGRADUATE INSTITUTE OF SCIENCE, UNIVERSITY OF PERADENIYA, SRI LANKA, 3.INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCE, CHINA)

Keywords:Charnockites, Wanni Complex, Madurai block, Geochemistry, Petrology

The geology of Sri Lanka is associated with the assembly of East Gondwana because of its critical position between India and East Antarctica. Among the four major litho-tectonic units, Wanni Complex (WC) is the westernmost crustal segment in Sri Lanka, which consists of mainly orthogneisses metamorphosed in upper amphibolite to granulite facies conditions. Felsics to mafic compositional orthopyroxene-bearing gneisses (Charnockites) are distributed throughout both WC and Highland Complex (HC) as one of the important components of the mid to lower continental crustal domains. Petrological and geochemical investigations on charnockites will provide clues on their petrogenesis, nature of source rock and tectonic environment, which will further aid in understanding the correlations between neighboring Gondwana fragments. Recent petrological studies carried out for the WC suggest the presence of important metamorphic textures indicative of prograde mineral inclusions, peak metamorphic assemblages, decompression reactions (symplectites), hornblende/pyroxene breaking down reactions, secondary garnet/biotite formations, perthitic-antiperthitic and myrmeckitic textures represent the WC rocks are found to be in retrograde metamorphic conditions in a clockwise P-T path. The major element composition of WC charnockites reveals they have an igneous origin with calc-alkaline signature and granitic to granodioritic compositional protoliths of subalkaline affinity. Trace element geochemistry indicates volcanic arc granite to within plate granite signature whereas the protolith rocks have originated from intermediate to felsic arc magma by subduction-related magmatism. Recent geochemical studies on Southern Granulite Terrane (SGT) of Madurai block, India represent some similarities with the WC charnockitic rocks whereas they indicate granitic to dioritic in composition, tholeiitic to calc-alkaline affinity and origin of continental margin magmatism in a subduction-related environment. In addition, both WC and SGT granulites show relatively higher concentrations of LREE than HREE with the majority of lack of Eu anomalies. In this presentation, we highlight in detail the geochemical affinities of metaigneous rocks and discuss on their tectonic setting.