5:15 PM - 6:45 PM
[SGL17-P07] U-Pb ages and Geochemical Characteristics of Zircons in Sedimentary Deposits from the Southwest Highland Complex of Sri Lanka
Keywords:Zircon, Sri Lanka, Highland complex, EPMA, LA-ICP-MS, U-Pb ages
This study conducts the Geochemical characteristics and U-Pb ages of gem-quality zircon in the Kalu and Kuda river sedimentary deposits of the Southwest Highland Complex of Sri Lanka. It primarily investigates variations in REE contents of different colors of zircons and traces the corresponding metamorphic ages of zircons in sedimentary deposits. The Highland Complex extends from the northeast to the southwest of the island of Sri Lanka, traversing the entire island. It encompasses the highest regional metamorphic grade - granulite facies metamorphism. In addition to granulite facies gneisses, this region also includes rocks such as quartzites and marbles. This study classified 50 gem-quality zircon samples into five types based on color: green, purple, orange, yellow, and light yellow. Zircon in this study has a major elemental composition of 30.54 ~ 33.65 wt% SiO2, 62.45 ~ 69.11 wt% ZrO2, and 0.59 ~ 4.13 wt% HfO2. Cathodoluminescence (CL) imaging examination revealed that only the orange group exhibited pronounced oscillatory zoning. However, their dating results from different zoning positions were similar. Other groups did not exhibit zoning characteristics, and the U-Pb ages from different positions were also very similar.
According to previous research, detrital zircons from this region exhibit growth periods during six-time intervals: (1) 3380-3220 Ma. (2) 2430-2660 Ma. (3) 2550-2490 Ma. (4) 2220-2170 Ma. (5) 1900-1700 Ma. and (6) 630-500 Ma. The periods from 3380 to 2170 Ma. are primarily associated with igneous activity, while the interval from 1900 to 1700 Ma. is characterized by both igneous and metamorphic processes. The period from 630 to 500 Ma. represents the final stage of granulite facies metamorphism, marked by high-temperature processes. This study obtained U-Pb age results for five groups as follows: the Green group 559 ± 1.7 Ma, the Purple group 548.8 ± 7.7 Ma, the Orange group 541 ± 12 Ma, the Yellow group 552 ± 12 Ma, and the Light Yellow group 538 ± 12 Ma. The average dating result across all groups was 546 ± 5.2 Ma. The main metamorphic peak of the Highland Complex is concentrated in the period of 610-560 Ma. The dating results of this study are all concentrated in the later stage of the metamorphic period, specifically 560-540 Ma. This suggests that the gem-quality zircons may have rapidly grown during the granulite facies metamorphic period, rather than forming during the period of igneous activity. In Chondrite-normalized REE patterns, aside from the presence of positive Ce and negative Eu anomalies, it is observed that some green zircon samples have negative Eu values lower than 1. The green zircon samples have high uranium content, leading to metamictization, whereby the crystal structure has been damaged, and the fluorescence spectrum has disappeared.
According to previous research, detrital zircons from this region exhibit growth periods during six-time intervals: (1) 3380-3220 Ma. (2) 2430-2660 Ma. (3) 2550-2490 Ma. (4) 2220-2170 Ma. (5) 1900-1700 Ma. and (6) 630-500 Ma. The periods from 3380 to 2170 Ma. are primarily associated with igneous activity, while the interval from 1900 to 1700 Ma. is characterized by both igneous and metamorphic processes. The period from 630 to 500 Ma. represents the final stage of granulite facies metamorphism, marked by high-temperature processes. This study obtained U-Pb age results for five groups as follows: the Green group 559 ± 1.7 Ma, the Purple group 548.8 ± 7.7 Ma, the Orange group 541 ± 12 Ma, the Yellow group 552 ± 12 Ma, and the Light Yellow group 538 ± 12 Ma. The average dating result across all groups was 546 ± 5.2 Ma. The main metamorphic peak of the Highland Complex is concentrated in the period of 610-560 Ma. The dating results of this study are all concentrated in the later stage of the metamorphic period, specifically 560-540 Ma. This suggests that the gem-quality zircons may have rapidly grown during the granulite facies metamorphic period, rather than forming during the period of igneous activity. In Chondrite-normalized REE patterns, aside from the presence of positive Ce and negative Eu anomalies, it is observed that some green zircon samples have negative Eu values lower than 1. The green zircon samples have high uranium content, leading to metamictization, whereby the crystal structure has been damaged, and the fluorescence spectrum has disappeared.