Petrographic characterisation and zircon geochronology of the Cobungra granite and and low-P high-T Omeo Metamorphic Complex , Lachlan fold belt
thesisposted on 28.03.2022, 21:52 authored by Nicholas Tefay
The low-P high-T Omeo Metamorphic Complex, located in the Lachlan Fold Belt, formed in the Silurian by amphibolite-grade metamorphism of quartz-rich Ordovician turbidites. I-type and S-type granites intruded the complex in the Silurian and Devonian. The study area is a 15 km by 15 km area of this terrane located near the town of Omeo in the Snowy Mountains of Victoria. I-type and S-type granites (predominantly the S-type Cobungra Granite) make up the western part of the study area with the metasedimentary rocks of the Omeo Metamorphic Complex decreasing in grade toward the east away from the Cobungra Granite, forming a regional aureole. This study involved the sampling and characterisation of rocks in the study area, focusing on petrography, mineral chemistry and whole-rock chemistry. Zircon morphology, U/Pb geochronology and major and trace element chemistry was characterised for four samples to determine whether the high-T metasedimentary rocks of the Omeo Metamorphic Complex are the source for the adjacent S-type Cobungra Granite. A crystallisation age of 423.6 ± 4.8 Ma was resolved for one Cobungra Granite sample which marks the first time this granite has been dated using U/Pb in zircon. Unfortunately, the U/Pb zircon data from the other samples was too disturbed to be useful. However, zircon trace element data showed that the metasedimentary sample was distinct from the granite samples suggesting the source of the S-type magmatism is different to the exposed high-T metasedimentary rocks. Zircon trace element data was also useful in making links between geochronological and morphological data.