Organic,mineralogic and magnetic indications of metamorphism in the Tapley Hill Formation,Adelaide Geosyncline |
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| Authors: | DM McKirdy J Sumartojo DH Tucker V Gostin |
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| Institution: | 1. Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T. Australia;2. Geology Department, University of Adelaide, Adelaide, S.A. Australia |
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| Abstract: | The dispersed carbonaceous matter (kerogen), illite, and magnetic response of the Tindelpina Shale Member in the lower part of the thick, extensive Tapley Hill Formation provide three complementary methods for zoning the incipient metamorphic character of rocks comprising the late Precambrian Adelaide System where they crop out between Adelaide, Olary, and Marree in the Adelaide Geosyncline. The methods are based on the following parameters: kerogen structure (as determined by X-ray diffraction) and composition (percentage carbon, hydrogen to carbon atomic ratio, δ 13CPDB); illite crystallinity; and amplitude and type of aeromagnetic anomalies.Kerogen is the most definitive indicator of metamorphic change in the Tindelpina Shale. It has been used to delineate a western subgraphitic zone (85–91%C, H/C > 0.10) which is separated from an eastern graphitic zone (91–98%C, H/C < 0.10) by a north-trending line through Adelaide, Mintaro, Orroroo, and Baratta. A similar two-fold zonation appears to exist in the Mouth Painter—Copley—Marree area. Metamorphic adjustment of the stable carbon isotopic composition of the kerogen is also evident. Kerogen rank correlates well with illite crystallinity. Illites in the western zone have Weaver indices of less than six. Crystallinity increases to the east where 2M illite becomes the dominant illite polymorph. The eastern graphitic zone largely coincides in location and extent with a zone of linear aeromagnetic anomalies of amplitude exceeding 100 gammas. In the lower Tapley Hill Formation the anomaly is attributed to remanent magnetism, probably associted with metamorphic growth of magnetite.All three indicators suggest an increase in metamorphic grade from west to east across the geosyncline, in agreement with published observations based on conventional petrographic analysis of pelitic (and to a lesser extent, carbonate) rocks. Illite and chlorite, characteristic of the anchizone of burial diagenesis, are the dominant sheet silicates in the shales studied, although incipient metamorphic alteration of chlorite to biotite has occurred in some specimens from the graphitic zone. The subgraphitic and graphitic facies of the Tindelpina Shale correspond with the chlorite and biotite (and higher) zones, respectively, of low-pressure intermediate-type metamorphism previously established for the Mount Lofty and Flinders Ranges.Greater depth of burial of the lower Tapley Hill Formation in the eastern half of the geosyncline would account for the metamorphic trend observed in its organic matter and clay mineral content. Differential burial does not, however, adequately explain the magnetics, nor the absence of biotite-grade rocks in the central Flinders Ranges. Other magnetically anomalous beds are found throughout the Adelaide System and in overlying Cambrian strata. For the magnetism in these different stratigraphic intervals to be of metamorphic derivation, a regional thermal event, perhaps related to the Cambro-Ordovician Delamerian Orogeny, must be postulated. |
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