Understanding Zircon Geochronology - Constraints from Imaging and Trace Elements

Sammanfattning: Formation of incipient charnockite in Söndrum, SW Sweden and the Kerala Khondalite Belt (KKB), S. India is accompanied by zircon growth. The age of the dehydration event is constrained by dating newly formed zircon to 1397±4 Ma (2?, MSWD=1.7) in Söndrum and ca. 520-510 Ma in S. India by secondary ionization mass spectrometry (SIMS). The formation of incipient charnockites correlates with regional magmatic or metamorphic events. In Söndrum, SW Sweden, charnockitisation is simultaneous with the formation of the Varberg Charnockite- Granite Association (CGA). Identical ages for incipient charnockite formation in studied localities in the KKB, S. India imply that charnockitisation is a major regional event related to the latest stage of Pan-African orogeny. Commonly observed bulk rock HREE depletion in incipient charnockites is not caused by zircon dissolution but by involvement of garnet as a reactant in the dehydration reactions. The Kerala Khondalite Belt, S. India, previously believed to be an entirely supracrustal belt, in fact contains magmatic rocks as a major lithological component. Ages (SIMS U-Pb on zircon) of magmatic garnet-biotite gneiss (1877±24; 2?, MSWD=0.6), augen gneiss (1891±36; 2?, MSWD=1.04) and massive charnockite (1865±16; 2?, MSWD=0.41) identify a previously unknown, Paleoproterozoic magmatic major crust forming event in the Kerala Khondalite Belt, S.India. Two distinct metamorphic events at 580-550 Ma and 520-510 Ma, identical in age in three studied localities of garnet biotite ortho- and paragneiss and augen gneiss, have been identified. The pervasive 580-550 Ma regional metamorphic event dates deformation and migmatisation of the magmatic and sedimentary rocks in the Kerala Khondalite Belt during Pan-African orogeny. The second metamorphic event is related to formation of the incipient charnockites. The entire population of magmatic oscillatory zoned zircons in a migmatised granitic sample from the Tjärnesjö intrusion, SW Sweden, experienced brittle hydraulic fracturing and rapid healing. The oscillatory zoned zircon fragments are rotated but not dispersed. Fractures between individual fragments are sealed by newly formed CL-bright zircon. Both the internal structure of fractured zircons and the LREE-enriched, low-Th character of CL-bright zircon suggest that cracks between oscillatory zoned zircon fragments were rapidly sealed after fracturing by hydrothermal CL-bright zircon. Zircon fracturing and crack-sealing has been dated by SIMS U-Pb on zircon to 920 ± 51 Ma with a limit for the youngest possible age of 960 ± 16 Ma provided by sector zoned rims that form overgrowths on the fractured cores.