7.1 Conclusions
Based on the results from previous chapters, data from the literature and discussion above, the following conclusions have been drawn:
i. The basalt dyke at Bombo Headland intruded through the Permian Bumbo latite during a continuous period of volcanic activity throughout the Jurassic. Igneous activity was fed from a single large magma chamber below the Illawarra. Jurassic magmatism may have been a precursor to the opening of the Tasman Sea.
ii. The gravity high anomaly under the Illawarra may be a result of the gabbroic pluton.
This pluton is traced back to the initial sedimentation within the Sydney Basin back-arc at the Carboniferous-Permian boundary. The mafic composition of the pluton is the result of mafic intrusives within the basement of the Gunnedah Volcanic Basin.
This is confirmed by widely variable gabbroic xenoliths documented within the thin sections.
iii. The gabbro and diorite samples comprise of mafic and ultramafic xenoliths of
gabbro/syenite, gabbro-peridotite, mantle dunite and monzonite, with minor amounts of eclogite, metasedimentary rocks and granitoid clasts plus individual xenocrysts.
Crystals of olivine, alkali-feldspar and hornblende-rich cognate inclusions were interpreted as large phenocrysts.
iv. The first discovery of a relict eclogite is recorded at Bombo Headland. This was detected in an exploratory thin section of the dyke, near the start of the project.
Textural evidence indicates the single euhedral garnet crystal had been extensively pseudomorphed by chlorite. The altered garnet is in association with an intergrown network of clinopyroxene and altered plagioclase. Texturally this resembles eclogitic decompressed omphacitic clinopyroxene. Upon high temperature the omphacite underwent exsolution into low Na-Al clinopyroxene and plagioclase. The high temperature metamorphism is indicative of the passive arc collision and subduction zone within the Hunter-Bowen Orogen during the Carboniferous-Permian boundary.
v. In course of EDS exploration of the selected areas subjected to SEM study, chemical analysis of three types of olivine were collected; euhedral olivines in textural
equilibrium with the dyke matrix, olivine within gabbro xenoliths and olivine within
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polycrystalline aggregates showing evidence of reaction with the dyke magma. Most magnesian are olivine as aggregates, which show textural evidence of reaction with dyke magma. These are interpreted as small fragments of mantle dunite. Intermediate
#Mg olivines are most abundant in the dataset and occur in the gabbro xenoliths.
Lowest #Mg olivines are euhedral phenocrysts within the dyke that are in textural equilibrium with their matrix.
vi. The U-Pb dating of zircons from the gabbro and diorite samples produced the first definitive age on the basalt dyke. The leucocratic gabbro/syenite xenolith produced a Jurassic age of 204.6 ± 4.6 Ma. This gives the absolute maximum age of the dyke.
Given the rounded cognate appearance of this xenolith the age might coincide with the age of the dyke itself. The dyke is significantly younger (> 50 million years) than the Bumbo latite it cuts, and is clearly younger than the Permian volcanic activity.
vii. The U-Pb dating of zircons from a ‘random’ composite sample of the gabbro and diorite samples are interpreted to be derived from the gabbroic-peridotite xenoliths that dominate the sample. They have an age of 303.8 ± 5.5 Ma. This indicates that this xenolith suite is dominated by Upper Carboniferous-Lower Permian gabbro derived from a layered pluton beneath the Sydney Basin at Bombo. The three older rounded detrital grains of Carboniferous age are likely derived from Lachlan Fold Belt rocks in the deep crust.
7.2 Recommendations
In addition to the preliminary investigations provided in this project, there is still further work that could be carried out to enhance the datasets of various mineral assemblages within the gabbro and diorite samples. As this project is the first investigation on the in-situ dykes and profiling the age and provenance of xenoliths, more analytical work can be done in order to understand the evolution of the igneous textures within the samples. In order to enhance the dataset of this project it is recommended to perform Laser Ablation-ICP-MS analysis for trace elements on the zircons from the dyke material in order to find grains with mafic REE patterns as these will correspond to rock protolith.
Further petrographic analysis is recommended with more thin sections to be made. The analysis in this project provided detailed textural evidence of multiple xenolith assemblages
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including the first discovery of the relict eclogite amongst various gabbros, monzonites and magmatic kaersutite within olivine-bearing lithologies of olivine + pyroxene + plagioclase.
With further analysis a more comprehensive investigation on the xenoliths would enhance the potential of discovering more eclogite as well as gabbros and other igneous textural
assemblages.
Further SEM-EDS analyses performed on all thin sections (only twelve spot analyses on six thin sections were conducted for this project). With more petrographic thin sections used for SEM-EDS a comprehensive microstructural examination of features within rocks and minerals (Goldstein et al. 1992). As this project is the first analytical investigation of the Bombo in-situ dykes and profile of the age and provenance of xenoliths the recommendations provided would enhance the dataset carried out in this investigation to assess the continuity of Lachlan zones and deep crustal-upper mantle structure under the Sydney Basin.
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