GSSL Field Workshop – 2016
Geological structures along the boundary between the Wanni and Highland Complexes in central Sri Lanka
20-21, August 2016
Introduction
Although the subdivision of the Highland Complex (HC) – Vijayn Complex (VC) is discernible in the field with clear evidence from petrology and structures, the HC and WC crustal domains lack obvious petrological and structural geological terrain markers. Thus, the HC-WC boundary is merely an isotopic boundary (i.e. a hypothetical line defined by contrasts in isotopic ratios) based on entirely a regional set of Nd model age ‘numbers’, which is physically unrecognizable in the field. Lithologies along the most part and on either side of this so-called ‘inferred boundary’ are petrologically and structural geologically more or less comparable and therefore serves a poor guide to define an absolute crustal boundary. Kehelpannala (1991, 1997) proposed that the HC-WC boundary should run over the top of the highest marble band of the HC in the north-central Sri Lanka and below the pink granitic gneiss of the Kadugannawa Complex (KC), which marks the lower limit of the KC. Such a modified boundary would incorporate the KC also into the WC and would lie a few hundreds of metres tectonostraigraphically below the KC coinciding with the high-strain shear zone around Digana of the HC. Nevertheless, still there is no strict consensus on whether the KC should be included in the WC due to some geochemical, geochronological and tectonic incompatibilities existing between the two units. Therefore, this workshop would provide a unique opportunity to explore the geological structures at the boundary of the HC and the combined KC-WC boundary in the central Sri Lanka.
Day 01:
Investigating the rocks at the boundary of the combined Kadugannawa – Wanni Complex and deformation-controlled migmatization of mafic rocks around Kandy
The participants will explore the Kadugannawa Complex in detail and how homogeneous mafic-intermediate rocks have become migmatites. They can learn how to distinguish different components of complex migmatites in order to understand their origin. They can also see how mafic rocks respond to high-grade shearing and the role of pink granites in understanding the KC/WC relationship.
Day 02:
Exploring the boundary between the combined Kadugannawa-Wanni Complex and Highland Complex with strongly deformed and mylonitized Highland Complex rocks.
The participants are able to see the folded contact between the combined Kadugannawa-Wanni Complex and the HC, and how the HC rocks have undergone strong high-grade mylonitization and structural fabrics related to the shearing between WC and HC. Also, it will be explained how marble has undergone strain-induced melting during this shearing to form carbonatites locally.
Field Excursion guide: Prof. Wilbert Kehelpannala, Department of Geology, University of Bostwana
Coordinator: Dr. Sanjeewa Malaviarachchi
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2-day field excursion 2014: Geological Society of Sri Lanka (GSSL)
By Dr. L.V. Ranaweera–
September 5th and 6th, 2014
Geological Society of Sri Lanka (GSSL) held a 2-day field excursion on 5th and 6th of September. It was aimed to discuss recent advances on the study of geology of Sri Lanka. Thirty members of the society representing several Universities and Geological Institutes participated the excursion. The excursion route extended from Peradeniya to Tanamalwila through Gampola-Kotmale-Tawalantenna-Nuwara Eliya-Rupaha-Bandarawela-Haputale-Pambahinna-Piyangiriya-Beragala and Wellawaya covering basement rocks of the Wanni Complex (WC), the Highland Complex (HC) and the 
Vijayan Complex (VC). The participants lodged the 1st day night in Bandarawela.
Mr. L.R.K. Perera, Dr. Bernard Prame, Prof. Rohan Fernando, Dr. A. Pitawala, Dr. Tilak Hewawasam and Dr. S. P. K. Malaviarachchi lead the excursion at the field. The excursion was started after an opening address by the coordinator of the excursion (the author of this report) followed by a plenary session. During the plenary sessions, participants were provided details on each stop prior to set out to the field.
Having exposed mainly high-grade rocks with diverse mineralogy and chemical composition yet with contrasting geology among rock units have led numerous interpretations on the basement rocks of Sri Lanka (Cooray, 1994). Available isotopic age data spanning from ~2.0 Ga to 450 Ma has been assigned various geologic events such as sedimentation, crust formation and polymetamorphism (Crawford and Oliver, 1969; Cooray, 1994). A widely accepted interpretation is that the Sri Lankan basement consists of three crustal fragments with distinctly different crust formation ages (Kröner et al., 1991). This hypothesis requires juxtaposition of these units which is referred to the widespread Pan-African metamorphism (Kehelpannala., 1997). The crust formation ages (Nd) have been controversial because those have not been disturbed despite subsequent polymetamorphism. Though some workers argue that isotopic disturbance did not occur, some workers have shown isotopic disturbance in Nd isotope system which questions preservation of crust formation ages (Perera and Kagami, 2011). Thus, the model of differing crustal units have to be discussed with care. Moreover, the island also has a distinct geomorphology. However, early hypothesis on this have not been adequately supported by scientific data.
Localities visiting during the 1st day covered outcrops in the Kaduagannawa Complex (or part of the Wanni Complex) and the Highland Complex. where outcrops were characterised by well-layered, high-grade gneisses. The participants were able to observe spectacularly developed arrested charnockitization of some orthogneisses at first two localities. Isotope systematic of retrogressed host and arrested charnockite patches yield two contrasting ages which are shown as evidence for isotopic disturbance (Perera and Kagami, 2011). Some argues that the arrested charnockitization is fluid-controlled and related to late Pan-African extensional tectonics (Kehelpannala, 1999).
The participants were also able to observe the conspicuous pelitic gneiss-quartz-charnockite-marble rock associations widespread in the Highland Complex. An outcrop of serpentinized rock with olivine and pyroxene relics received a great interest during the excursion since these rocks have been centred for several discussions. Most recent study shows that parent rocks have an ultramafic composition and could be a mantle peace (Fernando et al., 2013).
Localities visited during the 2nd day continued to the outcrops in the Highland Complex and covered the Vijayan Complex. Calcite deposits occurring in the Piyangiriya area in close vicinity to the isotopically demarcated HC-VC boundary are distinctly different from the marble on their textural and chemical characteristics (Pitawala and Madugalla, 2014). They also contain crustal xenoliths with sharp contacts with the host. Determination of origin of such deposits will contribute for the lower crustal processes and crustal evolution of Sri Lanka.
The morphology of the Sri Lankan landmass can be divided in to three altitude zones or peneplains (I, II and III). A steep gap between the III and II peneplain (southern escarpment) was observed near Haputale. The origin and evolution of these landforms were discussed in terms of processes and quantitative estimations (Hewawasam et al., 2013). In the VC, participants examined the nature of the migmatized rocks that is very characteristics for the Complex and observed the differences of rock associations between the VC and the HC.
During the excursion, discussions were mainly centred on the isotopic demarcation of the basement rocks, petrological evolution of the HC, geochemical characteristics of the VC, mineralization in the lower crust and possible Mantle rocks exposed within the Sri Lankan lower crust. Through out the excursion, geological meaning of available isotopic ages received wide attention. Overall, participants perceived the importance of poly metamorphic processes in interpreting isotopic ages of the basement rocks and agreed upon the need of more fundamental geological research. Moreover, it was realised that geomorphology of the Island is also one of the key and interesting area available for further studies. The contribution received from the resource persons throughout the excursion was commendable.
References
Fernando, G.W.A.R., Baumgartner, L.P. and Hofmeister, W. (2013) High-Temperature Metasomatism in Ultramafic Granulites of Highland Complex, Sri Lanka. J. of the Geol. Soc. of Sri Lanka, Vol. 15: 163-181.
Hewawasam, T., Friedhelm von, B., Julien, B., Jean, D., Jan, S. and Ricarda, B. 2013 Slow advance of the weathering front during deep, supply-limited saprolite formation in the tropical Highlands of Sri Lanka, Geochimica et Cosmochimica Acta, 1: 202–230.
Kehelpannala, K.V.W. (1997) Deformation of a high-grade Gondwana fragment, Sri Lanka. Gondwana Research, Vol. 1: 47-68.
Kehelpannala, K.V.W. (1999) Shear zone-controlled charnockitization, retrogression and metasomatism of high-grade rocks. Gondwana Research, Vol. 2: 573-577.
Kröner, A., Cooray, P.G. and Vitanage, P.W. (1991) Lithotectonic sub division of the Precambrian basement in Sri Lanka. Geol. Surv. Dept. Sri Lanka, Prof. pap.5: 5-21.
Milisenda, C.C., Liew, T.C., Hofmann, A.W. and Kröner, A. (1988) Isotopic mapping of age provinces in Precambrian high-grade terrains, Sri Lanka. J. Geol. 96; 608-615.
Perera, L.R.K. and Kagami, H. (2011) Centimetre- and Metre-scale Nd and Sr Isotopic Homogenization in Kadugannawa Complex, Sri Lanka. J. of the Geol. Soc. of Sri Lanka, Vol. 14: 135-142.
Pitawala, H.M.T.G.A., Madugalla, T.B.N.S. (2014). Calcite Deposits associated with Marble in Balangoda Area, Sri Lanka, Excursion Guide (Geological Field Trip 5th and 6th September 2014), unpublished.
