Field Trip to the Base of the Bega Batholith - An Overview

Simon W. Richards, William J. Collins, R.A. Weibe, and B. Healy

On this trip we will be examining granite plutons, layered mafic complexes and related country rocks. Why? Hopefully to generate an understanding of granites, which, in turn will help us to decipher the tectonic regime of the region (SE LFB) during their emplacement. Granite plutons and their associated rocks should be viewed as tectonic indicators, just like faults and folds. However, when studied in conjunction with often intensively studied faults and folds and with accurate dating, plutons (including their geochemistry and internal structure) provide a very powerful tool indeed.

These rocks are slightly younger (~422 Ma) than those at Cooma (~435 Ma), which lie approximately 70 km to west. At Cooma we viewed some classic metamorphic rocks with well-developed structures and obvious migmatitic leucosomes. In contrast, the structures and rocktypes observed on this trip are unusual. The granite is a classic granodiorite but the migmatites are schlieren migmatites and are quite distinct from the classic layered migmatites (which are metatexites) which are common in high-temperature metamorphic complexes around the world. The migmatites in the Wog Wog River are diatexites: they have lost all their original structure (be it bedding or a tectonic layering) and therefore resemble sedimentary breccias. Hornfelsic xenoliths (mm-m-size) are suspended in a fine-grained, typically foliated leucosome matrix.

On this trip we will be principally examining, discussing and evaluating geological processes. These include the origin of microgranitoid (or microgranular) enclaves, the significance of granitic textures, magmatic versus solid-state deformation (e.g., Paterson et al., 1989), stratigraphy and sedimentary-type structures in plutons, granite emplacement mechanisms (e.g. Paterson & Vernon, 1995), evaluating the relations between deformation and metamorphism and determining the origin of migmatites of low-P, high-T metamorphic complexes in general.

To address these issues, we will be walking an E-W trending section of the Wog Wog River. The section traverses layered gabbros, migmatites, granite sheets, megacryst-rich and megacryst-poor granodiorite, layered mafic bodies within granite and house-sized stoped blocks of countryrock which have come to rest on the pluton floor. Based on internal magmatic and countryrock structures and general characteristics, we have suggested that the creek section provides a section across the basal base of the Kameruka pluton and some of the underlying rocks.

Many of the magmatic structures and features observed over the two days are comparable to sedimentary structures so much of the trip will be focused on identification and interpretation of these structures and how the recognition of these features influences our view of mechanisms of pluton construction. In turn, we can discuss the role that plutons can play as a significant tectonic indicator, just like classic structures such as folds and faults