Tectono-metamorphic evolution of the high-temperature low-pressure (HTLP) Cooma Metamorphic Complex, Lachlan Fold Belt, S. E. Australia

Munro, Mark A. (2013) Tectono-metamorphic evolution of the high-temperature low-pressure (HTLP) Cooma Metamorphic Complex, Lachlan Fold Belt, S. E. Australia. PhD thesis, James Cook University.

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Abstract

[Section A] MARD 1.0 is a computer program for generating smoothed rose diagrams by using a moving average, which is designed for use across the wide range of disciplines encompassed within the Earth Sciences. Available in MATLAB®, Microsoft® Excel and GNU Octave formats, the program is fully compatible with both Microsoft® Windows and Macintosh operating systems. Each version has been implemented in a user-friendly way that requires no prior experience in programming with the software. MARD conducts a moving average smoothing, a form of signal processing low-pass filter, upon the raw circular data according to a set of pre-defined conditions selected by the user. This form of signal processing filter smoothes the angular dataset, emphasising significant circular trends whilst reducing background noise. Customisable parameters include whether the data is uni- or bi-directional, the angular range (or aperture) over which the data is averaged, and whether an unweighted or weighted moving average is to be applied. In addition to the uni- and bi-directional options, the MATLAB® and Octave versions also possess a function for plotting 2-dimensional dips/pitches in a single, lower, hemisphere. The rose diagrams from each version are exportable as one of a selection of common graphical formats. Frequently employed statistical measures that determine the vector mean, mean resultant (or length), circular standard deviation and circular variance are also included. MARD's scope is demonstrated via its application to a variety of datasets within the Earth Sciences.

[Section B] Porphyroblastic phases preserve a wealth of information pertaining to the P-T conditions experienced by the hosting metamorphic rocks. However, accurately interpreting the timing of the growth of porphyroblasts relative to one another and to matrix structures is not simple. Porphyroblast inclusion trail geometries range from straight, straight with inflected margins, to sigmoidal and spiral. Accordingly, this has required the development of a number of strategies for the examination, and qualitative and quantitative documentation of inclusion trails, each targeting different aspects of these geometries. This includes 'P-N' sectioning, pitch measurement, strike measurement, radial serial sectioning, parallel serial sectioning, best-fit plane assignment ('FitPitch') and High-Resolution X-ray Computed Tomography (HRXCT). Each technique has its own associated merits and restrictions, and is most effective when applied to different scenarios. Cross-referencing the findings of HRXCT, radial asymmetry, inclusion trail strike measurement and FitPitch with one another conveys that their results are in excellent agreement. Investigators must be mindful in the selection of their techniques as mounting evidence suggests that they risk potentially omitting key observations if utilizing a restricted number of perspectives based upon the orientations of matrix structures. A comprehensive documentation of the tectonometamorphic history preserved within porphyroblastic rocks may require the implementation of multiple techniques, even within a single sample.

[Section C] Detailed investigation within the classic Slacks Creek Schists of the Cooma Complex of S. E. Australia identifies two previously unrecognized stages of cordierite growth and reconciles persisting ambiguities regarding the timing of andalusite growth relative to matrix structures (e.g. Johnson, 1992; Johnson et al., 1994; Johnson & Vernon, 1995). The implementation of multiple microstructural techniques in conjunction with high-resolution field mapping in a small section of differentiated schists supports the interpretation that in some scenarios the vast majority of porphyroblasts may remain representative of their initial orientations despite multiple post-dating deformations. The tectono-metamorphic history of the schists also involved the development of more generations of foliation than distinguished in preceding studies. These observations were possible due to the range of thin section orientations used to examine the rocks. Two detailed studies of the complex have yielded different suggestions regarding the relative proportions of cordierite inclusion trails in the Slacks Creek schists, showing that representative sampling of porphyroblastic rocks is difficult even within relatively localized areas. Many microstructural studies of orogens are conducted over regions up to hundreds of km² and employ far lower spatial sampling densities than those used here. Therefore, the relative modal maxima of inclusion trail orientations in data from porphyroblastic studies may not reflect that of the entire population.

[Section D] Localized High-Temperature, Low-Pressure (HTLP) metamorphic zones are a hallmark of many orogens worldwide. Thermal softening and an enhanced potential for dynamic recrystallization in these areas serve to promote the development of foliations and associated folds. Associated porphyroblast growth over a number of these foliations is essential in their preservation, preventing their concealment during subsequent deformation. Consequently, HTLP metamorphic complexes may preserve more intricate tectono-metamorphic evolutions than those recorded by lower-grade regional rocks. The Cooma Metamorphic Complex and Murrumbidgee aureole of the Palaeozoic Lachlan Fold Belt, S. E. Australia, preserve evidence of switching between approximately horizontal and vertical bulk shortening. Interpretations suggest that these alternations in principal shortening orientation reflect regional-scale tectonism in the fold belt. An analogous history of tectonic mode switching is also evidenced by episodic extensional basin development and inversion during the Late Ordovician to Mid Devonian, suggesting that near orthogonal fabric development in the complexes may represent a mid-crustal manifestation of these processes. Porphyroblast growth over a number of these foliations is essential in their preservation, such detailed evidence on fold belt evolution will be incomplete where porphyroblasts are not preserved.

[Section E] Geochemical bulk rock analysis of the undifferentiated Spring Creek pelitic gneisses and strongly differentiated Slacks Creek schists of the high-temperature low-pressure Cooma Metamorphic Complex of S. E. Australia demonstrates them to possess highly similar major and trace element compositions. Isocon analysis of the chemical components indicates that the schists and gneisses are not related purely through isochemical metamorphic transition, as there is evidence of a small degree of protolith variation. Panchromatic scanning electron microscope and wavelength spectral cathodoluminescence (SEM-CL and W-CL) imaging of the schists shows quartz and feldspar (albite) inhabiting the mica-rich (M-) and quartz/feldspar-rich (QF-) domains of the prominent S₄ differentiated crenulation cleavage to exhibit identical signatures and lack any textural evidence of subsequent overgrowths or significant variations in (CL-activating) trace element concentrations. Lower intensities across the entire WCL spectra in M-domain quartz may reflect subjection to higher strain relative to that experienced by their counterparts in the QF-domains. Quartz and albite precipitated in syn-D₄ veins show indistinguishable SEM-CL and W-CL signatures to that throughout the host rock. Therefore, all quartz in the schists records either syn-D₄ (~peak) or post-D₄ metamorphic conditions and detrital signatures have been overprinted. Comparison with previously documented quartz behaviour with varying metamorphic grade strongly suggests it to have re-equilibrated at low-temperature (retrograde) conditions. Electron probe microanalyzer (EPMA) compositional mapping demonstrates that albite precipitated syn-D₄ also has indistinguishable major element composition to pre-existing grains. W-CL signatures in quartz inclusions preserved in cordierite and andalusite porphyroblasts of different timing preserve the same metamorphic conditions, having continued to evolve in response to changing pressure-temperature conditions along with that in the bulk matrix. Therefore, W-CL of quartz inclusions cannot be utilized to distinguish between different generations of porphyroblast. Coarse recrystallization of a number of porphyroblast strain shadows syn-D₄ has resulted in a relative depletion in albite with respect to the surrounding bulk matrix. Therefore, strain shadows in the Slacks Creek schists are not necessarily representative windows of pre-porphyroblast matrix composition.

Item ID: 42129
Item Type: Thesis (PhD)
Keywords: cathodoluminescence; circular statistics; Cooma Complex; crenulation cleavage; Gondwana; gravitational collapse; high-temperature low-pressure (HTLP) metamorphism; inclusion trails; mass transfer; metamorphic differentiation; microstructures; moving average; non-rotation; pluton emplacement; porphyroblasts; P-T-t-deformation path; rose diagram; rotation; strain shadow; tectonic mode switching; vector mean
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 1: Munro, Mark A., and Blenkinsop, Thomas G. (2012) MARD: a moving average rose diagram application for the geosciences. Computers and Geosciences, 49. pp. 112-120.

Date Deposited: 17 Dec 2015 01:51
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040313 Tectonics @ 34%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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