The deformational and metamorphic history of the Georgetown Inlier, North Queensland: implications for the 1.7 to 1.5 Ga tectonic evolution of northeastern Proterozoic Australia

2017-02-09T02:23:46Z (GMT) by Quinton Hills
This investigation presents a structural, metamorphic, geochronological and stratigraphic analysis of the Proterozoic rocks of the Georgetown Inlier of northern Queensland. The Georgetown Inlier provides important information that helps to constrain the tectonic evolution of the Australian continent during the Proterozoic. <br><br>The majority of the Georgetown Inlier is composed of two Palaeoproterozoic metasedimentary rock packages, the Einasleigh Metamorphics and the Etheridge Group, which is subdivided into the Robertson River Subgroup and the overlying Upper Etheridge Subgroup. Structural mapping indicates that the Einasleigh Metamorphics and Robertson River Subgroup contain an early tectonic fabric (Sl) that is not present in the overlying Upper Etheridge Subgroup. S 1 is parallel to relict bedding in upper greenschist to amphibolite facies rocks of the Robertson River Subgroup and parallel to gneissic and migmatitic layering in the upper amphibolite to granulite facies rocks of the Einasleigh Metamorphics. S1 is not associated with folding and is cross-cut by mafic intrusives interpreted to be emplaced at ca 1655-1675 Ma. The feathery, interfingering contact between one of these mafic intrusives and the rocks of the Einasleigh Metamorphics suggests that these mafic magmas were intruded in the late stages of the first deformation and metamorphism (D1-M1, referred to by previous authors as the Ewamin Orogeny). <br><br>The lack of an angular unconformity between the Upper Etheridge Subgroup and the Robertson River Subgroup suggests three scenarios for the formation of S1: 1) S1 formed through strain partitioning into the lower parts of the stratigraphy; 2) the Upper Etheridge Subgroup does not contain S1 because it was tectonically emplaced above the Robertson River Subgroup during or subsequent to the formation of S1; or 3) the deposition of the overlying Upper Etheridge Subgroup post-dates the formation of S1. Poor geochronological resolution on the depositional age of the Upper Etheridge Subgroup does not allow these scenarios to be distinguished. However, when the stratigraphic record is analysed, I believe that a case for the Robertson River Subgroup and the Upper Etheridge Subgroup representing two tectonostratigraphic cycles that were deposited in response to two discrete episodes of lithospheric extension can be argued. I speculate that the metasedimentary components of the Robertson River Subgroup and the Einasleigh Metamorphics were deposited into an extending basin in which mafic rocks were intruded and extruded (Dead Horse Metabasalt) at ca 1675 Ma. Subsequently, the Robertson River Subgroup and the Einasleigh Metamorphics were deformed, metamorphosed and intruded by mafic sills (ca 1655 Ma) in the lower plate of an extensional system. The Upper Etheridge Subgroup (≤ ca 1655 Ma) was deposited during or subsequent to ductile extension in the middle and lower crust that is now preserved as S1 in the Robertson River Subgroup and the Einasleigh Metamorphics. I speculate that lithospheric extension was asymmetric with lower crustal attenuation or thinning recorded in the Georgetown Inlier and the brittle upper crustal expression of extension being offset and preserved elsewhere. <br><br>The early, compositional layering parallel fabric in the Georgetown Inlier is overprinted by folds associated with the Jana (D2), Tagalag (D3) and the Waruna (D4) orogenies. Folds associated with the Jana Orogeny (F2) are east-west to northeast-southwest trending, consistent with a north-south to northwest-southeast shortening direction. F2 folds range from tight to isoclinal recumbent folds in the strongly deformed Einasleigh Metamorphics in the east, to upright to slightly inclined in the least deformed Upper Etheridge Subgroup in the west. Metamorphism associated with this orogeny has been dated previously at ca 1553 Ma. The Jana Orogeny was followed by period of erosional exhumation and metamorphic decompression, correlated with the relative timing of deposition of the Langlovale Group, unconformably overlying the Etheridge Group. In this context, the Langlovale Group is best interpreted as a foreland basin sequence. The Langlovale, Etheridge Group and the Einasleigh Metamorphics were then overprinted by northwest-southeast to east-west trending folds associated with the Tagalag Orogeny (F3). The orientation of F3 folds is consistent with a northeast-southwest to north-south shortening direction. F3 folds range from tight to isoclinal, upright to slightly inclined folds in the Einasleigh Metamorphics in the east, to open, upright folds in the Upper Etheridge Subgroup in the west. U-Pb (SHRIMP) analyses of zircon from a pegmatite body that cross-cuts an F3 fold indicates that the Tagalag Orogeny occurred sometime before ca 1545 Ma but prior to the ca 1553 Ma metamorphism associated with the Jana Orogeny. D3 structures were in tum overprinted by north-south trending, open to gentle folds associated with the Waruna Orogeny (D4). The trend of these folds is consistent with an east-west shortening direction. U-Pb (SHRIMP) analyses of zircon from pegmatite within a post-D3 semi-brittle fault/shear zone constrains the age of the Waruna Orogeny at ca 1530 Ma. <br><br>The orogenic events recorded in the Georgetown Inlier are correlated with similarly aged terranes of the northeastern Proterozoic Australia, such as the Cumamona Craton and the Mount Isa, Dargalong, Yambo and Coen inliers. Analysis of the available geochronological data from all of these terranes are interpreted to indicate that orogenesis occurred in two stages aged ca 1600-1550 Ma and ca 1550-1500 Ma. The first stage of orogenesis (including Jana and Tagalag orogenies as well as the early Isan and early Olarian orogenies) is interpreted to be the result of north-south to northwest-southeast shortening related to subduction along the southern margin of Proterozoic Australia. During this orogenic stage, the Mount Isa, Dargalong, Yambo and Coen inliers and the Cumamona Craton record anticlockwise P-T-t paths and peak metamorphism at ca 1600-1580 Ma, whereas the Georgetown Inlier records a clockwise P-Tt path and peak metamorphism at ca 1553 Ma. The anticlockwise P-T-t path and earlier metamorphic ages in the Mount Isa, Dargalong, Yambo and Coen inliers and the Cumamona Craton are explained by crustal shortening that was superimposed on a pre-existing extensional architecture consisting of heterogeneously thinned, thermally and mechanically weakened crust. Whereas, the clockwise P-T-t path and later metamorphic age in the Georgetown Inlier is explained by crustal thickening followed by metamorphism, in a region that was not thermally pre-conditioned by extension. The clockwise P-T-t path, isothermal decompression, erosional exhumation and deposition of the foreland basin sequence of the Langlovale Group is consistent with many modern orogenic zones, where crustal thickening generates metamorphism, late orogenic magmatism and erosional exhumation. The second stage of orogenesis (including Waruna Orogeny in the Georgetown Inlier and the late Isan and late Olarian orogenies) is interpreted to be the result of approximately east-west shortening related to subduction along the eastern margin of Proterozoic Australia. <br><br>This investigation has shown that evolution of the Georgetown Inlier broadly falls within a ca 1700-I 500 Ma period of extension and orogenesis recorded in the other Proterozoic terranes of northeastern Australia. However, specific aspects of the Georgetown Inlier's evolution ( eg. extensional basin architecture, timing and nature of metamorphism) highlight spatial and temporal heterogeneities in both the extensional and orogenic history. These difference can be used to help constrain the tectonic evolution of the Australia continent during the Proterozoic.