Prolonged Neoproterozoic high‐grade metamorphism of the Wanni Complex, Sri Lanka: New insights from petrology, phase equilibria modelling, and zircon U–Pb geochronology of partially melted cordierite gneiss from Walpita

Abstract

We report new petrological and geochronological data of garnet‐cordierite (Grt‐Crd) gneiss as well as two types of leucocratic rocks (garnet‐bearing and cordierite‐bearing leucosomes) crystallized during prograde and near‐peak metamorphisms of the Wanni Complex (WC) in Sri Lanka, and investigate the pressure–temperature–time evolution of granulite‐facies metamorphism for unravelling the timing and tectonics of the Neoproterozoic collisional processes related to Gondwana amalgamation. Grt‐Crd gneiss, garnet‐biotite (Grt‐Bt) gneiss, garnet‐bearing leucocratic rock (Grt‐L), and cordierite‐bearing leucocratic rock (Crd‐L) discussed in this study were collected from Walpita in the margin of the WC along the boundary with the Highland Complex (HC). Grt‐L occurs as thin layers parallel to the foliation of matrix Grt‐Bt and Grt‐Crd gneisses. The occurrences of porphyroblastic garnet in quartzo‐feldspathic matrix and crystallised melt inclusions within the garnet suggest that the Grt‐L could have formed by partial melting of host Grt‐Bt gneiss during prograde to peak metamorphism. Crd‐L is coarse‐grained and massive, lacks obvious foliation, and crosscuts the foliation of host Grt‐Bt gneiss. These observations suggest that the Crd‐L could have intruded after the crystallization of the Grt‐L. Application of phase equilibria modelling in the system NCKFMASHTO for the rocks yielded three discrete thermal events: 600–800°C/4.0–6.0 kbar (Stage 1 from Grt‐L), 700–840°C/5.5–7.0 kbar (Stage 2 from Grt‐Crd gneiss), and 840–880°C/4.5–5.5 kbar (Stage 3 from Crd‐L), suggesting a clockwise P–T evolution. LA‐ICP‐MS zircon U–Pb dating of the leucosomes yielded several metamorphic events: >577 Ma (partial melting of Grt‐L), ~562 Ma (crystallization of Grt‐L), 537 ± 14 Ma (peak metamorphism and crystallization of Crd‐L), and 507–503 Ma (post‐peak cooling event). The results of this study therefore confirmed prolonged (>70 Myr) high‐grade metamorphism of the WC, although the duration might be shorter than that of the HC (>100 Ma). The continuous heat supply necessary for such long‐lived high‐grade metamorphism is explained by a recent tectonic model of Sri Lanka. The model suggests double‐sided subduction and continent‐continent collision, which could have given rise to subsequent slab delamination and differential heating of lower to middle crust. Such complex subduction–collision processes possibly controlled the difference in the style of P–T path and the duration of high‐grade metamorphism between the HC and the WC.

Metamorphic Evolution of Charnockites Across the Inferred Highland-Wanni Boundary in Northeastern Sri Lanka

“…Raase and Shenk, 1994;Hiroi et al, 1994;Hirayama et al, 2020;Dharmapriya et al, 2017Dharmapriya et al, , 2020 The calculated using two-pyroxene Fe-Mg exchanged thermometer after Brey and Kohler (1990) for coexisting clinopyroxene and orthopyroxene pairs in the rocks HC-3 (CHCs) and WC-8 (CWCs) yielded average temperatures around 693, 694, 696, 697 °C and 599, 601, 604, 606 °C respectively for the considered nominal pressures. The above-calculated temperatures are considerably lower than the previously reported general metamorphic temperatures in both Highland and Wanni Complexes (Rass and Shenk, 1994;Hiroi et al, 1994;Hirayama et al, 2020;Dharmapriya et al, 2017Dharmapriya et al, , 2020. The two pyroxene thermometer introduced by Putirka (2008) gives reliable minimum peak metamorphic temperatures and for nominal pressure 7.5, 8, 8.5 and 9 kbars these values are ranged from 762 to 776 °C (average 769 °C), 763-777 °C (average 770 °C), 765-779 °C (average 772 °C) and 766-780 °C (average 773 °C) for HC-3s.…”

“…The western part of WC consists of less deformed granites including unmetamorphosed posttectonic K-feldspar-rich granite of Thonigala (Hölzl et al, 1991;Cooray 1994). The peak metamorphic conditions of the WC have been suggested to be 700-830 ⁰C and 5-7kbar (Raase and Schenk, 1994;Santosh et al, 2014;Hirayama et al, 2020). The intrusion ages of Wanni gneisses mainly are around 1100 -920 Ma, 800 -750 Ma and around 580 Ma Hölzl et al, 1994;Santosh et al, 2014;He et al, 2016).…”

“…The temperatures were calculations at nominal pressures of 7.5, 8, 8.5 and 9 kbars after considering the obtained pressure of the HC and WC by the previous researchers (e.g. Raase and Shenk, 1994;Hiroi et al, 1994;Hirayama et al, 2020;Dharmapriya et al, 2017Dharmapriya et al, , 2020 The calculated using two-pyroxene Fe-Mg exchanged thermometer after Brey and Kohler (1990) for coexisting clinopyroxene and orthopyroxene pairs in the rocks HC-3 (CHCs) and WC-8 (CWCs) yielded average temperatures around 693, 694, 696, 697 °C and 599, 601, 604, 606 °C respectively for the considered nominal pressures. The above-calculated temperatures are considerably lower than the previously reported general metamorphic temperatures in both Highland and Wanni Complexes (Rass and Shenk, 1994;Hiroi et al, 1994;Hirayama et al, 2020;Dharmapriya et al, 2017Dharmapriya et al, , 2020.…”

“…In contrast, there is no attempt made to study the metamorphic evolution of rocks in the northeastern part of the HC. The P-T studies on WC is also mainly confined to the central part and the southern-western part of the Complex (Rasse and Schenk, 1994;Hirayama et al, 2020). Charnockite is one of the dominant lithologies across the northeastern part of the inferred HC-WC tectonic boundary.…”

Hybrid phase equilibria modelling with conventional and trace element thermobarometry to assess the P–T evolution of UHT granulites: An example from the Highland Complex, Sri Lanka

“…The rocks in these three complexes were metamorphosed under upper amphibolite to granulite facies conditions simultaneous with the assembly of Gondwana (e.g. Cooray, 1994; He et al., 2015, 2016; Hirayama et al., 2020 Kehelpannala, 1997; Kröner et al., 2013; Malaviarachchi et al., 2019; Mathavan et al., 1999; Perera & Kagami, 2011; Ratheesh‐Kumar, Dharmapriya, Windley, Xiao, & Jeevan, 2020).…”