Structural and metamorphic evolution of the Robertson River metamorphics with pressure-temperature-deformation-time (P-T-D-t) path

Cihan, Mustafa (2004) Structural and metamorphic evolution of the Robertson River metamorphics with pressure-temperature-deformation-time (P-T-D-t) path. PhD thesis, James Cook University.

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Abstract

SECTION A. Understanding the relationships of inclusion trail geometries in porphyroblasts relative to matrix foliations is vital for unravelling complex deformation and metamorphic histories in highly tectonized terranes and the approach used to thin sectioning rocks is critically important for this. Two approaches have been used by structural and metamorphic geologists. One is based on fabric orientations with sections cut perpendicular to the foliation both parallel (P) and normal (N) to the lineation, whereas the other uses geographic orientations and a series of vertical thin sections. Studies using P and N sections reveal a simple history in comparison with studies using multiple-vertical thin sections. The reason for this is that inclusion trails exiting the porphyroblasts into the strain shadows in P and N sections commonly appear continuous with the matrix foliation whereas multiple vertical thin sections with different strikes reveal that they are actually truncated. Such truncations or textural unconformities are apparent from microstructures, textural relationships, compositional variations and FIA (Foliation Intersection Axis) trends. A succession of four FIA trends from ENE-WSW, E-W, N-S to NE-SW in the Robertson River Metamorphics, northern Queensland, Australia, suggests that these truncations were formed because of the overprint of successive generations of orthogonal foliations preserved within porphyroblasts from growth during multiple deformation events. At least four periods involving multiple phases of porphyroblast growth can be delineated instead of just the one previously suggested from an N and P section approach. SECTION B. A succession of four foliation intersection/inflection axes preserved in porphyroblasts (FIAs) trending ENE-WSW, E-W, N-S and NE-SW has been distinguished in the Proterozoic Robertson River Metamorphics (Georgetown Inlier, Queensland, Australia) based upon relative timing plus inclusion texture and orientation. The successions of asymmetries of inclusion trails defining these FIAs document the geometry of deformation associated with folding and fabric development during discreet episodes of bulk shortening. The successions of asymmetries bear no relationship to the geometry of macroscale folds present in the area suggesting that these folds predate porphyroblast growth, the widespread metamorphism and matrix fabric development. The onset of regional macro-scale folding may have begun soon after the deposition at around 1655 Ma in Georgetown Inlier. These folds were then amplified, overturned and refolded during NNWSSE, N-S, E-W and NW-SE regional bulk shortening. Earlier deformations were erased from the matrix because of bedding-induced shearing (reactivation) on the limbs of pre-existing macro-scale folds. Four foliations, S1 to S4, identified in the matrix provided information about the youngest deformations preserved in these rocks. SECTION C. Contouring XMn, XFe and XCa for garnet porphyroblasts and XAn for plagioclase inclusions in the MnNCKFMASH system provides an estimation of the P-T variation during the growth history of these porphyroblasts. Integration of this approach with relative timing constraints obtained from successions of Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) reveals a more extensive P-T-D history than previously recognised in the Robertson River Metamorphics, Georgetown Inlier (NE Australia). A succession of four FIA trends (ENE-WSW, E-W, N-S, NE-SW) reveals three extended periods of garnet porphyroblast growth and two of staurolite growth in this region. Chemically zoned garnet porphyroblasts were selected based on successively formed FIAs in their cores from four representative rock samples. The intersection of XMn, XFe and XCa isopleths for the cores of the successively generated garnet porphyroblasts plus that of XCa and XAn isopleths for garnet and plagioclase inclusions suggests that pressures progressively increased from 3.2 to 5.8 kb and at temperatures from 530º to 560º C. This accompanied an orogenic progression from NNWSSE (O1), N-S (O2) to E-W (O3) shortening. The maximum pressures and temperatures achieved, around 6-7 kb at 590º-610º C, were followed by decompression and retrograde metamorphism with andalusite replacing an early formed generation of staurolite. These rocks were overprinted by NW SE shortening (O4) occurring synchronously with low pressure – high temperature metamorphism, resulting in the overprint of early minerals by sillimanite and prograde muscovite. This last event was attributed to widespread granitic intrusion in all NE Australian Craton at around 1550 Ma. SECTION D. Electron microprobe dating of monazite confirms the relative timing of a succession of Foliation Intersection Axis trends in porphyroblasts (FIAs) and two extended periods of metamorphism, revealing a lengthy history of orogenesis in the Robertson River Metamorphics (NE Australia). A complete pressure-temperature-deformation-time (P-T-D-t) path has been deciphered involving an early clockwise P-T loop (not previously recognized in the NE Australian Craton) followed by an anticlockwise P-T trajectory. Metamorphism continued episodically throughout orogenesis changing from medium pressures and temperatures to lower pressures and higher temperatures after an intervening retrogressive phase. Successive generations of fine-grained monazite (5μm-20μm) were identified using microstructure and FIA trends and then dated. The succession of four FIAs trending ENE-WSW (FIA1), E-W (FIA2), N-S (FIA3), NE-SW (FIA4) plus four matrix structures, S1-S4 reveal three periods of garnet and two of staurolite growth, and suggest that monazite grains were episodically grown, dissolved and regrown due to successive periods of foliation development or reactivation. Isopleth thermobarometery, using P-T pseudosections in the MnNCKFMASH system for garnet porphyroblasts selected based on FIA trends in their core, reveal that the pressure increased progressively from 3- 4kb at 530º-550ºC to 6-7kb at 600º-620ºC during medium pressure temperature metamorphism. This was accompanied by changing bulk shortening directions from NNW-SSE to N-S to E-W (perpendicular to FIAs 1,2 and 3). The development of FIA1 and FIA2 occurred between ca.1655 (depositional age) and 1592 Ma. FIA3 developed over a 30 Ma period from 1592 Ma to 1559 Ma. Retrogressive metamorphism, decompression and exhumation occurred over a 10 Ma period. Further metamorphism accompanied regional granite intrusion around ca. 1550 Ma and generated lower pressure - higher temperature metamorphism during the development of FIA4 (NW-SE bulk shortening). This period of orogenesis possibly extended for another 30-50 Ma based on the ages obtained from the youngest foliations in the matrix. Correlation of these data with other NE Australian Proterozoic Inliers suggests that they were all once part of a single orogen that developed from 1655 to 1500 Ma.

Item ID: 1069
Item Type: Thesis (PhD)
Keywords: Robertson River Metamorphics, Truncation, Inclusion trails, Porphyroblast growth, Reactivation, Georgetown Inlier, FIA, Porphyroblasts, MnNCKFMASH, P-T pseudosections, Monazite, Isopleth thermobarometry
Date Deposited: 13 Oct 2006
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 0%
04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 0%
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