Metamorphic evolution of Highland Complex-

1. Metamorphic evolution of Highland Complex through PT paths ESH 32212 ESH 32212 Geology of Sri Lanka Geology of Sri Lanka 1

2. Content  Introduction  PT path evolution  Conclusion 2

3. 1. Introduction 1.1 General Geology of Sri Lanka The metamorphic basement of Sri Lanka is subdivided into 4 major geotectonic units based on stratigraphic correlation and metamorphic grade figure 1. (a) Geological subdivision of the Sri Lankan basement 3

4.  The HC contains granulite facies metasedimentary and metaigneous rocks including quartzites, marbles, calcsilicates, pelitic gneisses, charnockites, and orthogneisses. In the central and northern parts of the HC, hundreds of meters thick marble and quartzite units are traceable for more than 40 km.  In contrast, in the southwestern part of the HC, marble, and quartzite are scarce, and cordierite-bearing metapelitic gneisses, orthogneisses, and thin bands of wollastonite- and scapolite-bearing calcsilicates is the prominent rock types meta igneous.  4

5.  The reconstruction of the PT path for the HC granulites is still a matter of debate and no general consensus exists on the PT trajectory followed by the HC. 5

6.  Suggested PT trajectories for Sri Lankan basement rocks.  The PT path described in following steps. • Prograde Evolution. • Peak Metamorphism • Post - Peak Metamorphism • Late stage of Retrograde Evolution. 6

7.  Conventional thermobarometric calculations and petrogenetic grids, the HC has been classically interpreted as a tilted crustal section with a peak metamorphic gradient increasing from 4.5 - 6 k.bar and 700 - 750 C in the southwest up to 8- 9 k.bar and 800 - 900 C in the east and southeast.  In addition granulites that formed at extreme crustal conditions of 925 - 1150 C and 9 - 12.5 k.bar have been reported from a few localities in the central and southwestern HC (UHT)  As per the observations, garnet porphyroblasts and their textures were taken into consideration as garnet is abundant in high-grade metamorphic rocks.  Various chemical reactions in Khondalite rocks and their special mineral textures were also considered when estimating the direction of the PT path.  With lots of research the geologists are suggesting a clockwise PT path for the HC 7

8. 2.P-T path of the khondalite in relation to the HC evolution.  The detailed microtextural observations, pseudosectionsand relevant experimentally calibrated reactions can be used ,  To evaluvate the metamorphic history of the studied khondalite  It allows to determine a possible PT path for the HC.  The four steps of P-T path of HC can be identified. • Prograde Evolution. • Peak Metamorphism • Post - Peak Metamorphism • Late stage of Retrograde Evolution. 8

9. 2.1.Prograde Evolution.  The Prograde evolution can be devided in to  Prograde Compression  Prograde Decompression. Prograde Compression.  The initial stage of the prograde path that was started in the Silimanite stability field.  The interpretation was based on “the Early sillimanite needle pre-dating kyanite inclusions in garnet propyroblasts which were observed in pelitic granulite in Eastern Highland Complex.”( Raise and Schenk.1994). 9

10. The Peak Pressure Prograde Compression Figure 2:P-T diagram outlining the overall shape of the inferred P-T path of the khondalite 10

11.  Then the pressure was increased up to peak pressure when has minor heating in the P-T path .  IT will be indicated by the transition of the sillimanite to the Kyanite +Staurolite in garnet stability field.  It would be assumed a this process when ocuured due to setting of crustal subduction related of continenet –continenet collision. The Peak Pressure condition.  The occurrence of rutile in garnet (minimal peak P at around 10 kbar)  By using pseudo section calculation of melt –intergration modeling wet solidus and calculated mineral modes.(maximum pressure >10 kbar ,T – 780˚C.)  Peak Pressure around the 10.5 -12kbar,A similar Peak Pressure ~11-12kbar at T ~820- 850˚C.( Dharmapriya et al.2015a,b) 11

12. Progade Decompression.  After the peak pressue condition ,the Silimanite was appeared by replacing the kyanite . Ti Rich Bt+Sil+Qtz = Grt+Kfs+Ilm+Melt Ti Rich Bt+Sil =Grt +Spl+Kfs+Ilm+Melt 12

13. Prograde Decompression. Figure:P-T diagram outlining the overall shape of the inferred P-T path of the khondalite 13

14.  The textural observations in Grt 2 indicated the rocks are evolved from the Garnet –Corundum stability field to the Spinel –Sillimanite field at this satage. Grt+Crn=Spl+Sil  Due to the lateral extrusion of the crust and contienet contienet collision ,the prograde metamorphism was occurred. 14

15. Arguments .  Ellis (1987) : “The temperature dependent rheology of the lower crust, extension could be expected after convergence if the temperature exceeds some critical value.”  Harley (1991) : “The extension could result in increasing temperature during slight decompression and could be followed by isobaric cooling .  ( Hopper, 1996): “The isobaric cooling process could be accelerated by the presence of melt. The incorporation of curved quartz inclusions of core and inner mantle areas in Grt3 via reaction could provide evidence for increasing temperature and melt production towards peak metamorphism under non-coaxial strain. Such melt-assisted non-coaxial strain at lower crustal levels is consistent with lower crustal extension .” 15

16.  Dharmapriya : Also containing of rotated crenulation lineation demarcated by biotite and tiny sillimanite needles from prograde garnets (has formed at near peak conditions via melting reactions) of UHT granulites in the HC. These crenulation lineation is oblique to the major matrix lineation of the UHT granulites indicating the rotation of garnet during their growth . 16

17. Peak metamorphism Khondalite reaches a peak temperature of around 900 c at a pressure of 9-9.5 kbr It represents the HT/ UHT boundaries 17

18. Post-peak evolution  Meta sedimentary granulites show that after peak T the HC experienced a period of near isobaric cooling followed by isothermal decompression.  Even though a phase of isobaric cooling after peak T in the HC is poorly constrained in meta pelitic rock, Cal -silicate rock.  It has been largely documented from reaction texture in intermediate and mafic granulites IBC after peak T was also inferred from corundum-bearing khondalite.  Granulites inferred a stage of ITD after IBC decompression is very strong in interlayered meta basites. After IBC & IDT events here no fluids and melts so it's formed dry meta pelitic granulites. 18

19. Late stage of the retrograde evolution  Several geologists suggested that the latest part of the retrograde path in the HC occurred in the andalusite stability field.  Generally most of the andalusite forming reactions were observed in the South western part of the HC where low pressure conditions prevailed.  However Hiroi (1994) reported andalusite in the central HC towards the north eastern part close to the present sampling locality. 19

20. Peak P Peak T 20

21. Conclusion  The metamorphism process of Khondalite began in the sillimanite field at 575°C and 4.5 kbar, transitioning to the kyanite stability field at around 660°C and 6.5 kbar.  The rock experienced peak pressure (>10 kbar) at 780°C, displaying a garnet-corundum- kyanite-staurolite assemblage. Subsequent prograde decompression led to re-entry into the sillimanite field at 825°C and 10.5 kbar.  The highest temperature peaked at 900°C around 9-9.5 kbar, followed by near-isobaric cooling, resulting in the growth of Grt4 and rutile at approximately 880°C.  Unlike the Highland Complex, no evidence of isothermal decompression was recorded, possibly due to the lack of fluids/melts infiltrating the rock after the isobaric cooling event. 21

22.  Highland conditions in the sillimanite stability field during prograde development. Complex metasediments experienced high-temperature, low-pressure  During crustal thickening associated with continent-continent collision for Gondwana assembly, the metasediments entered the kyanite stability field.  Prograde compression persisted, leading to peak pressure conditions at high-pressure granulite facies.  Subsequently, the rocks re-entered the sillimanite stability field. Peak metamorphism occurred with ductile deformation at high-temperature/ultra-high-temperature granulite facies. 22

23.  A subsequent stage involved near-isobaric cooling at lower crustal levels.  The rocks were rapidly uplifted to upper crustal levels.  Uplift occurred along an isothermal decompression path.  The process was marked by folding and thrusting, shaping the final geological structure of the Highland Complex metasediments. 23