The Geochemistry of Tasmanian Devonian–Carboniferous Granites and Implications for the Composition of Their Source Rocks
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Mineral Resources Tasmania Tasmanian Geological Survey Tasmania DEPARTMENT of INFRASTRUCTURE, Record 2006/06 ENERGY and RESOURCES The geochemistry of Tasmanian Devonian–Carboniferous granites and implications for the composition of their source rocks by M. P. McClenaghan CONTENTS Introduction ……………………………………………………………………………… 3 Recent work ……………………………………………………………………………… 3 Field relationships and petrographic features of the granites ………………………………… 5 Western Tasmania ……………………………………………………………………… 5 Grassy, Bold Head and Sea Elephant adamellites …………………………………………… 5 Three Hummock Island Granite ………………………………………………………… 5 Housetop Batholith …………………………………………………………………… 5 Dolcoath Granite ……………………………………………………………………… 5 Meredith Batholith …………………………………………………………………… 5 Mt Bischoff porphyry dykes …………………………………………………………… 6 Birthday Granite ……………………………………………………………………… 6 Granite Tor Granite …………………………………………………………………… 6 Renison Complex ……………………………………………………………………… 6 Heemskirk Batholith …………………………………………………………………… 6 Pieman Granite ……………………………………………………………………… 7 Interview and Sandy Cape granites ……………………………………………………… 7 Grandfathers Granite ………………………………………………………………… 7 Cox Bight Granite …………………………………………………………………… 7 Eastern Tasmania ……………………………………………………………………… 7 Introduction ………………………………………………………………………… 7 Hornblende-biotite granodiorites ………………………………………………………… 9 Biotite adamellites/granites……………………………………………………………… 9 Garnet-cordierite-biotite adamellite/granites ……………………………………………… 10 Alkali-feldspar granites ………………………………………………………………… 10 Classification and geochemistry of the granites ……………………………………………… 11 Age of the granites ………………………………………………………………………… 19 Isotopic variation of the granites …………………………………………………………… 22 Metallogenic potential of the granites ……………………………………………………… 24 Petrogenesis of the granites ………………………………………………………………… 25 Inferences for crustal composition …………………………………………………………… 25 Synthesis ………………………………………………………………………………… 28 Acknowledgements………………………………………………………………………… 28 References ………………………………………………………………………………… 29 Tasmanian Geological Survey Record 2006/06 1 TABLES 1. Suite assignments for Tasmanian mid-Palaeozoic granite bodies …………………………… 12 FIGURES 1. Distribution of mid-Palaeozoic Tasmanian granites ……………………………………… 4 2. Age range for Rb-Sr, K-Ar and U-Pb SHRIMP for Tasmanian granite bodies ………………… 8 3. Variation diagrams for western Tasmanian mid-Palaeozoic granites………………………… 13 4. Variation diagrams for eastern Tasmanian mid-Palaeozoic I-type granites…………………… 15 5. FeO*–Na2O/K2O diagram for eastern Tasmanian I-type granites …………………………… 16 6. Variation diagrams for moderately crystal-fractionated eastern Tasmanian mid-Palaeozoic granites and Mathinna Supergroup rocks ………………………………… 17 7. Variation diagrams for strongly crystal-fractionated eastern Tasmanian mid-Palaeozoic granites ………………………………………………………………… 20 8. Initial 87Sr/86Sr ratios for western Tasmanian granite bodies and 87Sr/86Sr ratios for possible source rock terrains ………………………………………………………… 22 9. Initial 87Sr/86Sr ratios for eastern Tasmanian granite bodies and 87Sr/86Sr ratios for possible source rock terrains ………………………………………………………… 23 While every care has been taken in the preparation of this report, no warranty is given as to the correctness of the information and no liability is accepted for any statement or opinion or for any error or omission. No reader should act or fail to act on the basis of any material contained herein. Readers should consult professional advisers. As a result the Crown in Right of the State of Tasmania and its employees, contractors and agents expressly disclaim all and any liability (including all liability from or attributable to any negligent or wrongful act or omission) to any persons whatsoever in respect of anything done or omitted to be done by any such person in reliance whether in whole or in part upon any of the material in this report. Tasmanian Geological Survey Record 2006/06 2 Introduction the Three Hummock Island body (Everard et al., 1997). Magnetic data from the Western Tasmanian Regional The Tasmanian mid-Palaeozoic granites (fig. 1) were Minerals Program indicated that the Three Hummock emplaced mostly after the main Tabberabberan Island Granite was part of a much larger submarine deformation at high crustal level with narrow contact body. aureoles. In eastern Tasmania, the granites intruded an Ordovician to Early Devonian (Banks and Smith, 1968; In eastern Tasmania, the various granite bodies in the Rickards and Banks, 1979; Rickards et al., 1993) St Helens area were more fully characterised by quartzwacke turbidite sequence (Mathinna McClenaghan et al. (1992). Supergroup) whereas in western Tasmania they Leaman and Richardson (1992) more accurately intruded more variable successions ranging in age delineated the subsurface extent of the from Precambrian to Early Devonian. In eastern Devonian–Carboniferous granite bodies by modelling Tasmania, intrusion took place in the Devonian based on gravity data. This is useful for assessing whereas in western Tasmania intrusion was generally prospectivity for granite-related mineralisation. later and took place in the Devonian and Early An increase in geochemical data (B. W. Chappell, pers. Carboniferous. Intrusion appears to have occurred comm., 1995; J. L. Everard, pers. comm., 2005; mainly by upward displacement or crustal rifting McClenaghan et al., 1992; McClenaghan, 1994; M. P. although there is local, marginal evidence for McClenaghan, pers. comm., 2005) has improved the ‘shouldering-aside’ of the country rock (Gee and characterisation of the granites and their assignment to Groves, 1971). The granites generally range from suites. Poorly-known minor and remote granite bodies granodiorite to alkali-feldspar granite, with have been sampled and analysed in conjunction with a granodiorite being much more abundant in eastern granite dating program (Black et al., 2005; Black, in Tasmania. The classification used is that of Streckeisen prep.). Additional geochemical data have been (1973) with the modification that the term adamellite is obtained from granites in eastern Tasmania and the used for part of the ‘granite’ field with over 35% modal Furneaux Group. Changes have been made to the suite plagioclase. The term granite will be used to refer to assignments in both west and east Tasmania based on granitic rocks in the wide sense except where these data. New isotope data (Sawka et al., 1990; Sun specifically discussing composition. Rock types used and Higgins, 1996) have also been used to characterise in body names will be used rather broadly to remain the granite bodies and make suite assignments. consistent with previous usage. Many of the Tasmanian granite bodies are moderately Recent work or strongly fractionated and Chappell (1999) showed how strongly fractionated granites can be Since the last review of the Devonian–Carboniferous characterised as I-type or S-type based on chemical granites (McClenaghan et al., 1989) there have been a composition, using two strongly fractionated number of developments relating to the granites, Tasmanian granite suites as examples. This which are reflected in this report. methodology has been applied to other strongly Various small changes have taken place to the mapped fractionated Tasmanian granite suites. Mineralogical, distribution and improvements made to the isotopic and geochemical characterisation of the characterisation of the granites. In western Tasmania granites and their grouping into suites has a bearing on Brown et al. (1994) provided the first published map of their metallogenic potential and mode of genesis. the distribution of granite types within the Heemskirk Blevin and Chappell (1992) have pointed to the Batholith. This built on earlier mapping by Klominsky importance of magma sources, oxidation states and (1972) who had recognised the division of the degree of fractionation of granites in determining the Heemskirk Batholith into red and white granite. potential for granite-related mineralisation. McClenaghan (1994) described the field characteristics Recent U-Pb SHRIMP dating of zircon, both of the Heemskirk Batholith granite types. Mapping of crystallisation and inherited, from many of the the Meredith Batholith (Turner et al., 1991) established Devonian–Carboniferous granites (Black et al., 2005; that it consisted of two main bodies. The distribution of Black, in prep.) has increased understanding of their these bodies, in an inaccessible part of the batholith, intrusive history and has provided data that may assist was more accurately delineated by Everard (1999, understanding of their genesis. 2003) based on new Western Tasmanian Regional Developments in petrogenetic models proposed for Minerals Program radiometric data. This radiometric Lachlan Fold Belt granites (e.g. Collins, 1998; Chappell data, together with geological mapping, also allowed et al., 1998; Chappell et al., 1999) and isotopic evidence refinement of the boundary of the remote Granite Tor on the provenance of turbidites from the belt (Gray and body (McClenaghan, 2003). Webb, 1995) since the last review (McClenaghan et al., A minor granite body was recorded at Penguin Islet in 1989) may be applicable to genetic models for the southwest Bass Strait which may be an apophysis of Tasmanian Devonian–Carboniferous granites. Tasmanian Geological Survey Record 2006/06 3 E E E E m m m m 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 4 5 75 34 6 65 6 43 79 32 36 33 66 77 44 5 76 57 37 62 58 38 64 13 41 35 5500000 mN 78 63 61 69 59 74 68 18 24 46 82 60 20 56 83 25 55