Zircon ion microprobe dating of high-grade rocks in Sri Lanka
The high-grade gneisses of Sri Lanka display spectacular in-situ granulitization phenomena similar to those observed in southern India and of current interest for evolutionary models of the lower continental crust. The absolute ages of these rocks are poorly constrained and so, using the SHRIMP ion microprobe, the authors have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments. Detrital grains from a metaquartzite of the Highland Group preserve premetamorphic U-Pb ages of between 3.17 and 2.4 Ga and indicate derivation of the sediment from an unidentified Archean source terrain. The Pb-loss patterns of these zircons and the other samples suggest severe disturbance at ca 1100 Ma ago, which the authors attribute to high-grade regional metamorphism. Two pelitic gneisses contain detrital zircons with ages up to 2.04 Ga and also record an approx. = 1100 Ma event that is also apparent from metamorphic rims around old cores and new zircon growth. A granite intrusive into the Highland Group granulites records an emplacement age of 1000-1100 Ma as well as metamorphic disturbance some 550 Ma ago but also contains older, crustally derived xenocrysts. Zircons from a metaquartzite xenolith within the granitoid Vijayan Complex are not older than approx. 1100 Ma; therefore the Vijayan is neither Archean in age nor acted as basement to the Highland Group, as previously proposed. The authors suggest that the Vijayan Complex formed significantly later than the Highland Group and that the two units were brought into contact through post-1.1 Ga thrusting. Although the granulitization phenomena in India and Sri Lanka are similar, the granulite event in Sri Lanka is not Archean in age but took place in the late Proterozoic.
- Research Organization:
- Universitaet Mainz, West Germany
- OSTI ID:
- 5718360
- Journal Information:
- J. Geol.; (United States), Vol. 95:6
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GEOLOGIC DEPOSITS
AGE ESTIMATION
SRI LANKA
CONTINENTAL CRUST
GEOLOGIC HISTORY
GEOLOGIC MODELS
GNEISSES
ION MICROPROBE ANALYSIS
ISOTOPE DATING
LEAD ISOTOPES
METAMORPHISM
ORIGIN
URANIUM ISOTOPES
ZIRCON
ACTINIDE ISOTOPES
ASIA
CHEMICAL ANALYSIS
DEVELOPING COUNTRIES
EARTH CRUST
ISLANDS
ISOTOPES
METAMORPHIC ROCKS
MICROANALYSIS
MINERALS
NONDESTRUCTIVE ANALYSIS
OXYGEN COMPOUNDS
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES
580100* - Geology & Hydrology- (-1989)