[S-MP26] Supercontinents and Crustal Evolution

[SMP26-06] Geochemical Characterization of Charnockitic Rocks in the Wanni Complex, Sri Lanka: Insights into Magmatic Evolution

*Pahan Abewardana^1,2、Madhusoodhan Satish-Kumar³、Toshiro Takahashi³、Sajeev Krishnan⁴、Sanjeewa P.K. Malaviarachchi² (1.Graduate School of Science and Technology, Niigata University, Japan、2.Postgraduate Institute of Science, University of Peradeniya, Sri Lanka、3.Department of Geology, Faculty of Science, Niigata University, Japan、4.Centre for Earth Sciences, Indian Institute of Science, Bengaluru, India)

キーワード：Charnockitic rocks, Geochemistry, Petrology, Wanni Complex, Sri Lanka

The geological evolution of Sri Lanka is strongly linked to the assembly and fragmentation of the Gondwana supercontinent. Its position alongside East Antarctica, southern India, and Madagascar, Sri Lanka plays a key role in understanding the tectonothermal processes governing Gondwana formation and dispersal. The Wanni Complex (WC), one of the four major litho-tectonic units in Sri Lanka, is distinct for its widespread exposure of charnockites, which are characterized as ortho-pyroxene-bearing granitic meta-igneous rocks; and originate under granulite facies metamorphic conditions. In addition, minor occurrences of meta-sedimentary rocks and igneous intrusions, contribute to its diverse geological composition. While various studies have been conducted within the WC, a thorough and methodical geochemical investigation is crucial for enhancing the understanding of crustal evolution, the characteristics of its protoliths, and petrogenesis. Samples were gathered from more than ninety localities, predominantly consisting of charnockitic rocks, along with some other meta-igneous and meta-sedimentary rocks from the WC. Petrological observations reveal peak granulite facies metamorphism followed by retrograde conditions in the WC meta-igneous rocks. Major and trace element analyses reveal two distinct trends in the WC: a Mg-enrichment trend (Group 1) in the southern and northern regions, and a Fe-enrichment trend (Group 2) in the central region. Group 1 samples show a calc-alkaline signature, with diorite to granite protoliths indicating fractional crystallization, while Group 2 samples tend towards tholeiitic signature with syenite to granite protoliths. Group 2 samples generally contain higher trace element concentrations, while both trends show LILE enrichment and HFSE depletion. All samples exhibit enriched LREEs and flat HREE patterns, with Group 2 samples having relatively higher HREE concentrations. Furthermore, Group 2 samples exhibit a within-plate granite affinity, while Group 1 samples fall within the volcanic-arc granite field, suggesting two distinct tectonic signatures for the WC meta-igneous rocks. SEM-EDS analyses of the Group 2 samples indicate that the cpx (pigeonite) with exsolution lamellae of opx (ferrosilite) is the major reason for their high Fe content relative to the Group 2 samples. The WC exhibits evidence of two distinct magmatic events, emphasizing the need for further regional geochemical and isotopic studies on charnockitic rocks in Eastern Gondwana.