Quentin de Gromard, R. (2011) The Paleozoic tectono-metamorphic evolution of the Charters Towers Province, North Queensland, Australia. PhD thesis, James Cook University.

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Abstract

The main objective of this thesis is to better understand the processes of growth and evolution of continental lithosphere using the example of the Charters Towers Province (North Queensland, Australia) part of the Tasman Orogenic System. This province contains E-W structural and pluton trends that differ in orientation from the N-S trends of the adjacent orogens of the Tasman Orogenic System. The long lasting and complex tectono-metamorphic history of this continental orogenic system is unraveled using a multidisciplinary approach that discriminates discrete events within a seemingly continuous evolution. This involves structural and microstructural investigations, the FIA (Foliation Inflexion/Intersection Axis preserved in porphyroblasts) measurement technique, monazite geochronology and the interpretation of geophysical imagery (aeromagnetic map and seismic section).

The rocks sampled from the Seventy Mile Range Group and the Charters Towers Metamorphics (South Charters Towers Province) consist of lower amphibolite grade pelites containing cordierite +/- andalusite porphyroblasts, which are typical mineral assemblages for low pressure-high temperature terrains. Both porphyroblastic phases are retrogressed into partial to complete micas rich pseudomorphs associated with the appearance of retrograde chlorite.

The first section of the thesis investigates the textural development of mica rich pseudomorphs after cordierite based on two examples: partial pseudomorphs from rocks near the Mooselookmeguntic pluton in western Maine (USA) and complete pseudomorphs from rocks near the Lolworth batholith in the southern Charters Towers Province (North Queensland, Australia). Both regions contain mica rich pseudomorphs after cordierite that show evidence of progressive textural development under an active stress regime. The complete pseudomorphs of the Charters Towers Province consist of preferentially aligned or "randomly oriented" fine grained muscovite, chlorite, ± biotite. The partial pseudomorphs from Maine consist of preferentially aligned coarser grained muscovite and biotite. The preferred orientation of micas is parallel to the orientation of the inclusion trails. The micas grew preferentially along microcracks that developed in the same direction with the inclusion trails and moved progressively from the porphyroblasts margin towards the porphyroblasts core. At least three types of microcracks can be identified with two of them being relevant for the replacement process. The first one formed by direct replacement of quartz inclusions by plagioclase and volume expansion and the second one was caused by high strain rates. A new image analysis based method was developed for mineral amount calculation that allows the production of mineral distribution maps within sections of metamorphic rocks from X-Ray elemental distribution maps. Combined with electron microprobe analyses of the minerals from the pseudomorphs and the surrounding matrix, it is interpreted that the maximum pressures experienced by the rocks from the Charters Towers Province were around 3 kbars and the maximum temperature did not exceed 600°C, most probably lying near the 550°C isotherm. Temperatures obtained via the Ti-in biotite thermometer may reflect retrograde conditions rather than peak conditions. This strongly chemical equilibration dependant thermometer indicates systematically higher temperatures for biotite from the matrix than from the pseudomorphs indicating that equilibration of biotites in the matrix occurred prior to pseudomorphism. This implies that the metamorphic reactions are localized to the volume occupied by the cordierite and the material exchange with the matrix is limited.

The second section of the thesis investigates the anomalous E-W structural and igneous trends in the Charters Towers Province within the overall N-S trending Tasman Orogen. These resulted from a sequence of N-S shortening events. A succession of ~E-W trending FIAs (Foliation Intersection/Inflexion Axis preserved within porphyroblasts) dated at 474.7 ± 7.2, 413 ± 13 and 381.1 ± 8.1 Ma correspond with adjacent granite crystallization ages and E-W trending FIA ages in the Greenvale Province to the NW. The magnetic anomaly map of Australia reveals that this E-W trending portion of the Tasman orogen links with and truncates several earlier formed orogens. The E-W trend of the Charters Towers Province resulted from overprinting of the Tasman Orogen by the Alice Springs Orogeny resolving the E-W pluton shape and distribution of Cambrian to Devonian magmatic activity plus the truncational nature of aeromagnetic data from Central Australia to the East coast.

In the third section of the thesis, strong non-coaxial deformation in the Seventy Mile Range Group in the south of the Charters Towers Province is revealed by the dominance of one inclusion trail asymmetry within porphyroblasts during successive FIA forming events. This asymmetry indicates top to the north thrusting and corresponds with south dipping thrusts on the seismic profile 07GA-GC01 through the Seventy Mile Range Group. Similar thrusts, all of which sole into the Moho, occur across the whole of the Charters Towers Province except where it has been intruded by the Ravenswood batholith. The competency of this coarse feldspar batholith caused partitioning of thrusting deformation around it and this is confirmed by the coaxial character of the inclusion trail geometries present within porphyroblasts within Charters Towers Metamorphics surrounded by the Ravenswood batholith. Long-lived non-coaxial deformation in the Seventy Mile Range Group eventually resulted in thrust sheets being emplaced well to the north of the batholith with a window through a schistose thrust sheet within the Argentine Metamorphics exposing Devonian sediments below.

The last section of the thesis concludes that the tectonic evolution of the Charters Towers Province (northeastern Australia) in the Paleozoic resulted from the interaction between N-S and E-W bulk shortening. The origin of N-S shortening results from horizontal forces acting at the plate boundary that are possibly related to ridge-push effects due to the opening of the Paleotethys at ~450 Ma. This produced the ~450 to 300 Ma intracontinental E-W trending Alice Springs Orogen in central Australia whose effects propagated eastward to create the E-W trending Charters Towers Province. E-W shortening is interpreted to result from the long-lived west dipping subduction along the eastern margin of Australia, which was part of Gondwana at the time. FIA data reveals several discrete deformation events of interspersed ~N-S and ~E-W shortening directions. The FIA ages were obtained through in-situ monazite dating and show N-S shortening events at ~475 Ma and ~415 Ma, followed by ~E-W shortening at ~400 Ma and N-S shortening at ~380 Ma in the Charters Towers Province. The magnetic anomaly map of Australia, used to locate the lateral extensions of shear zones and domain boundaries hidden by overlying sedimentary successions, allowed the linking of deformation structures from central to northeastern Australia. Compressional orogenesis in the Charters Towers Province resulted in sinistral strike-slip extrusion of the Thomson Orogen to the east that was possibly associated with periods of slab roll-back.

Item ID:	22000
Item Type:	Thesis (PhD)
Keywords:	Alice Springs orogeny, Charters Towers Province, cordierite, deformation, extrusion tectonics, FIAs, geological foliation, inclusion trail asymmetry, mica, microcracks, mineral formation, monazite dating, orogenesis, pseudomorphs, strike-slip faults, Tasman Orogenic System, Thomson Orogen
Date Deposited:	13 Aug 2012 00:55
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 34% 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 33% 04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 33%
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