,Internal Textures and Geochronology of Zircon and Monazite as Indicators of Multithermal History of Southwestern Highland Complex, Sri Lanka

Abstract

Constituent rocks of the Southwestern Highland Complex (SWHC) of Sri Lanka is thought to have experienced multithermal events during the Gondwana amalgamation. However the lack of evidence of these events has motivated this study. Behavior of internal textures of zircon and monazite is considered as an important factor before the chronological analyses. Therefore, interanl textures of zircon and monazite in garnet-biotite gneiss, garnet-biotite-cordierite gneiss, hornblende-bearing charnockitic gneiss and charnockitic gneiss were studied in detail. U-Pb ages of zircons and Chemical U-Th-total Pb Isochron MEthod (CHIME) dating of monazites were used as conventional methods of geochronology. Zircons show detrital cores and overgrowths with two to five growth stages. Detrital zircon cores are rounded or euhedral to subhedral in shape and show transgressive internal textures or oscillatory zoning. In minor cases, zircons consist of rounded or skeletal cores containing inclusions and/or voids and overgrowths showing two or three generations. Ages of detrital cores are in the range of 3.31.7 Ga, implying source ages of 3.31.7 Ma. Most of the overgrowths gave ages from 2730500 Ma. Especially, zircons with ages in the ranges of 19001700 and 630500 Ma have Th/U-ratios less than 0.1, implying formation by metamorphic events. Zircons with ages in the range of 630500 Ma imply the generation at the latest metamorphic event stage. The monazites have core-rim zoned, inherited core-bearing, complexly zoned and oscillatory zoned type internal textures. The determined isochron ages are grouped into four clusters: group I of 1830–1648 Ma, 1766±140 Ma, 1788±30 Ma; group II of 803±99 Ma, 679±99 Ma; group III of ages with 550–485 Ma, 533±22 Ma, and481±42 Ma; and group IV of ages with 470–430 Ma, 470±45 Ma and 433±14 Ma. The ages of the group I may imply either magma emplacement ages or depositional ages of sediments. The ages of the group II correspond to the stage of the most prominent thermal event recorded in the region. The groups III and IV can be identified as post-peak thermal events. However, the groups III to IV can be considered as one group or event within the error ranges of the ages. The internal textures and the age data of the zircons and monazites prove the repeated thermal events which signify the complex evolution process of the SWHC. Several growth stages of overgrowths observed in some zircons and Group I ages of monazite suggest much more complex thermal events than those have been considered in the recently published simplified models and seem to be consistent with the previous published crustal model.