Special Publication No 11 launched at the GSSL Field Workshop 2016
Participants of the GSSL Field Workshop 2016
Prof. Wilbert Kehelpannala delivering the introductory lecture before leaving for the field
Some of the members heading for field Workshop with the Resource Person, Prof. Wilbert Kehelpannala
Participants investigating a dioritic rock in the Wanni/Kadugannawa Complex, at Elugoda
Prof. Kehelpannala explaining the structural features associated with the Wanni/Kadugannawa Complex, at Elugoda
Prof. Kehelpannala explaining the structural features at the WC-HC boundary in Peradeniya
Prof. Kehelpannala explaining the structural features at the WC-HC boundary in Elugoda Quarry
Prof. Kehelpannala answering questions from the participants
Prof. Kehelpannala explaining the competency difference of rocks and multiple veining at the Nugawela Quarry
Prof. Kehelpannala explaining the participants regarding viscosity contrast in WC rocks
Student participants at Getambe quarry listening to the structural interpretations
Travelling from Dumbara Synformal area in the Wanni Complex into Highland Complex in Randenigala Road
Road Outcrop at Rambukwella in Mahiyangana road. participants witnessing extensive milonitization
Sheared rocks at Randenigala road
Sheared fabrics in rocks at Randenigala road witnessing severe shearing and ductile deformation
Participants are looking better structural planes in least exposed rocks along the HC-WC boundary in Haragama
Evidence of melting of impure marble forming carbonatitic marble
Extending gratitude by the participants to the resource person Prof. kehelpannala
GSSL Field Excursions: Recent advances on the study of geology of Sri Lanka
2-day field excursion 2014: Geological Society of Sri Lanka (GSSL)
September 5th and 6th, 2014
by L.V. Ranaweera, Department of Natural Resources, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka
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.
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.
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.