Ali, Asghar (2009) Tectono-metamorphic evolution of the Cambro-Ordovician Balcooma Metamorphic Group, Greenvale Province, north-eastern Australia. PhD thesis, James Cook University.

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Abstract

An identical succession of five foliation intersection/inflection axes preserved in porphyroblasts (FIAs) in a small scale mining pit area and the large scale Balcooma region shows that there were five changes in the direction of bulk shortening during tectonism between ~476±5 and 408.8±8.9 Ma. The E-W trending FIA set 1 in garnet porphyroblasts indicates N-S shortening. The NNW-SSE trending FIA set 2 in staurolite suggests rotation of the direction of shortening to ENE -WSW. The NNE-SSW trending FIA set 3 in staurolite, plagioclase and kyanite porphyroblasts suggests ~30° rotation of the bulk shortening direction to ESE-WNW between 443.2±3.8 Ma and 425.4±3.7 Ma. The E-W trending FIA set 4 in staurolite porphyroblasts indicates further rotation to N-S by 408.8±8.9 Ma. The NESW trending FIA set 5 in andalusite suggests subsequent rotation of the bulk shortening direction to NW-SE. Structural and metamorphic data from porphyroblasts reveal a continuous history of tectonism that is partitioned between samples in the mining pit as well as regionally. The succession of FIA sets and microstructural relationships between garnet, staurolite, plagioclase, kyanite, andalusite, cordierite and fibrolitic sillimanite reveal a regionally consistent pattern of growth during this progression of changes in the direction of bulk shortening. The local coexistence of kyanite, andalusite and sillimanite and a consistent succession in the timing of growth of the different porphyroblastic phases resulted from the effects of deformation partitioning relative to bulk composition and P-T path. Similarly distributed FIA sets 1, 2 and 3 across the pit as well as in the Balcooma region suggest that the partitioning of deformation was relatively pervasive at all scales across the Balcooma Metamorphic Group from ~476±5 to 425.4±3.7 Ma. The preferential development of FIA set 4 in the northern half of the region reveals more localized deformation partitioning effects during this period of bulk shortening.

Electron microprobe monazite dating of a succession of 5 FIA sets (foliation inflection/intersection axes preserved in porphyroblasts) from the Balcooma Metamorphic Group, NE Australia reveals that these Palaeozoic rocks to the east of the Precambrian Georgetown Province have been affected by 5 orogenic cycles of different intensities. Electron microprobe dating of monazite preserved within porphyroblasts containing FIAs 2 through 4 indicates that the Delamerian, Kanimblan and Hunter Bowen Orogenies did not affect this region. The earliest 443.2±3.8 Ma phase of monazite growth within the Balcooma Metavolcanic Group is preserved in porphyroblasts containing FIA 2 whereas 425.4±3.7 Ma and 408.8± 8.9 Ma ages are preserved in those containing FIAs 3 and 4 respectively. This indicates that this region was initially affected by an Early Ordovician Orogeny but was later overprinted by the Benambran (440-420 Ma) and Tabberabberan (410-370 Ma) Orogenies. The lack of younger monazite ages shows that deformation during the Kanimblan Orogeny (360-320 Ma) partitioned such that it did not affect rocks adjacent to the Georgetown Province when it impacted on the Broken River, Hodgkinson and Charters Towers Provinces to the east, northeast and southeast respectively. Sulphide inclusions in porphyroblasts containing FIAs 2, 3, 4, 5 but not in those containing FIA 1 requires that disruption of any primary volcanogenic massive sulphide body took place after FIA 1, or that the mineralization is metamorphogenic.

The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, Foliation Intersection Axes preserved in porphyroblasts (FIAs), P-T conditions of garnet core growth, P-T pseudosection (MnNCKFMASH system) modeling and geothermobarometry provide evidence for a P-T-t- D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at approximately 530°C and 4.6±0.1kbar during the N-S shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnets in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from ~ 443.2Ma to 425.4Ma during the Early Silurian Benambran period of orogeny. The temperature and pressure increased during this time from ~530°C and 4.6±0.1kbar to 623-637°C and 6.2±0.3kbar. The overprinting of garnet, staurolite and kyanite bearing mineral assemblages by low-pressure andalusite and cordierite assemblages implies ~ 4kbar decompression in the belt during Early Devonian exhumation of the Greenvale Province. P-T pseudosection constraints, textural relationships and relative deformation features for rocks containing all three Al2SiO5 polymorphs suggest a kyanite-andalusitesillimanite crystallization sequence in separate tectonic events rather than synchronous nucleation at the aluminosillicate triple point.

Foliation inflection-intersection axes preserved in porphyroblasts (FIAs) reveal the early history of tectonism within the Northern Thomson Fold Belt. A succession of five FIA sets in the Cambro-Ordovician Balcooma Metamorphic Group, which lie due east of the Precambrian Georgetown Block, indicates that five changes in the direction of bulk shortening occurred during Northern Thomson Fold Belt orogenesis. A progressive rotation of the horizontal bulk shortening direction occurred from FIAs 1 through 5. The E-W trending FIA set 1, which is Ordovician in age (476±5 to 454 ±12 Ma), cannot have formed with these rocks in their current geologic setting because the Precambrian rocks to the west are unaffected by this period of N-S bulk shortening. For intense N-S shortening to not affect the adjacent Precambrian to the west, these rocks must have lain approximately 200 km to the south. They were then displaced 200 km north into their current location by the E-W bulk shortening effects of the Silurian Benambran Orogeny (444-420Ma), which controlled the development of FIAs 2 (443.2±3.8 Ma) and 3 (425.4±3.7 Ma). Synchronous N-S bulk shortening orogenesis in the east in eastern Australia with N-S extensional orogenesis to the west in central Australia during the Ordovician can be explained if the Euler pole for relative plate motion was located WSW of the Mt Isa Inlier. A plate containing the southern half of Australia pivoting anticlockwise around this axis of rotation relative to one containing the northern half, would produce crustal shortening orogenesis in the east and extensional orogenesis in the west from 476±5 to 454 ±12 Ma.

Item ID:	10429
Item Type:	Thesis (PhD)
Keywords:	Greenvale, Queensland, tectonic evolution, metamorphic evolution, Cambro-Ordovician Balcooma Metamorphic Group, porphyblasts, FIAs, microstructures, thermodynamics, tectonics, mineral crystallization, orogenies, sulphide mineralization, bulk shortening, Northern Thomson Fold Belt, Ordovician N-S shortening, correlation of deformations, foliation
Date Deposited:	01 Apr 2010 00:53
FoR Codes:	04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 50% 04 EARTH SCIENCES > 0403 Geology > 040306 Mineralogy and Crystallography @ 50%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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