Isotopic composition of the felsic rocks of the Mt Read Volcanics, Western Tasmania Pete Hollings1, Tony Crawford1 and Dave Whitford2 1 2 Centre for Ore Deposit Research, University of Tasmania, CSIRO Division of Petroleum Resources CODES SRC Introduction 1000 Suite IA 1.6 Suite IB Crawford et al. (1992) used geochemical criteria to subdivide the Cambrian Mount Suite IC Suite IV Read Volcanics (MRV) into five distinct suites, irrespective of stratigraphy. Suite I, 1.2 100 comprising medium- to high-K calc alkaline rocks, is the most voluminous of the five 0.8 suites and includes rocks from the Eastern sequence, the Central Volcanic Complex Suite III Suite I 0.4 10 (CVC) and the Tyndall Group. Rhyolitic and dacitic samples selected for isotopic analysis Suite II 0 are all from Suite I. 50 55 60 65 70 75 SiO Chondrite and primitive mantle normalised trace element diagrams demonstrate 2 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu that all 30 samples analysed have very similar trace element systematics, comparable 1000 Suite V Suite IA 120 Suite IB to the variably prospective FII suite defined by Lesher et al. (1986) for felsic volcanic Suite IC 100 rocks of the Superior Province, Canada. 100 80 Suite IV Crawford et al. (1992) suggested that the majority of Suite I rocks were erupted 10 60 in a post-collisional setting, as andesites associated with Suite I are typical of orogenic 40 1 Suite III Suite I andesites found in continental margin arcs. The late Tyndall Group lacks rocks of 20 Suite II andesitic composition and consequently was interpreted as being derived entirely from 0 0.1 50 55 60 65 70 75 Ba U La Pr Nd Hf Eu Gd Dy Ho Yb Al Sc SiO melting of Proterozoic lower crust. Isotopic analyses have been undertaken in order to 2 Rb Th Nb Ce Sr Zr Sm Ti Tb Y Er Lu V further evaluate these models and better constrain the tectonic setting of the MRV. Regional geology Hellyer Regional correlations Suite IA Hellyer Winterbrook Suite IB Dacite For the purposes of this study Hellyer Andesite Lava Dove granodiorite The adjacent map illustrates that Suite IC εNd -3.6 Dacite -1.3 εNd -5.2 volcanic rocks of the MRV can be εNd εNd -8.5 -2.4 Beulah the εNd values do not display a strong Tyndall Gp εNd Granite broadly subdivided into the three ignimbrite temporal or geographic correlation εNd -3.5 lithostratigraphic associations: εNd -5.1 across the MRV. However, samples 1) the basal felsic volcanics CV10 C dacite from the younger Tyndall group tend -3.6 and volcaniclastics of the Eastern εNd CVC dacite to have somewhat higher εNd values. 5400 000mN Minnow sequence, interpreted to have εNd -4.6 “Keratophyre” Based on the isotope systematics HELLYER Bond Range QUE RIVER -1.6 erupted through and onto porphyry εNd discussed below the Suite I rocks can Hercules rhyo90 lite Que River εNd -7.3 Precambrian basement εNd -3.2 mine dacite be subdivided into three distinct 2) the CVC comprising feldspar εNd -1.5 Mt Black rhyolite80 suites, which do not appear to display -4.3 TULLAH Murchison porphyritic rhyolitic to dacitic εNd any pronounced geographic or ROSEBERY Granite Rosebery QFP RENISON -4.6 lavas that are interpreted to be 70 εNd stratigraphic correlations. -4.3 coeval with the Eastern sequence εNd HERCULES Mt Read Murchison Samples associated with known HENTY dacite Henty fault 60 ZEEHAN 3) the Tyndall Group which εNd -4.8 deposits (Hellyer & Que River) have wedge andesite overlies both the CVC and the Tyndall Gp higher values of -1.3 to -2.4. This εNd -5.7 εNd 5350 000mN εNd 0.5 Eastern sequence is predominantly Yolande sequence is consistent with a study of the Snow rhyolite Tyndall Gp volcaniclastic but includes some MT. LYELL -4.8 Lake VHMS deposit in Canada, where QUEENSTOWN εNd -7.5 40 εNd Western volcano-sedimentary sequence quartz- and feldspar porphyritic Yolande sequence Pumphouse lava (Dundas Group and equivalents) volcanic rocks in the immediate Tyndall Group dacite STRAHAN εNd -6.6 rhyolitic and dacitic lavas. Andesites and basalts footwall to the deposit are isotopically 30 Mt εNd -9.0 Jukes Eastern quartz-phyric sequence Mt Jukes Rhyolite Granite and related porphyry more primitive than those lower in -6.1 εNd Central Volcanic Complex 20 the stratigraphy (Bailes and Galley, Darwin Granite Sticht Range Beds 0 5 10 km 1999). This was interpreted to reflect εNd -2.6 extension within an arc allowing the sourcing of more primitive magmas. Isotope systematics 4 Suite IV Suite IA Although Suite I samples are characterised by very similar trace element systematics, Sr and Nd isotope 2 Suite IB Suite IC systematics can be used to divide the suite into three distinct subsets, illustrated on the adjacent plots. The three 0 Suites II & III suites provide evidence for at least three isotopically distinct source regions below the MRV. ε -2 Nd 1) The evolved values of the Suite Ib rocks are best explained as the result of mixing with significantly older, -4 εNd -6 isotopically evolved crust, possibly Precambrian basement material. -8 2) Suite Ia rocks appear to lie on a mixing trend between the relatively primitive compositions of Suites II and -10 0.70 0.71 0.72 0.73 0.74 0.75 0.76 III and the highly evolved Suite Ib. This is best interpreted as contamination of the mantle source of Suites II 87Sr/86Sr and III by older crustal material. 4 3) Suite Ic rocks lie on a mixing trend between the evolved crustal component and Suite IV compositions. Suite 2 IV has been interpreted by Crawford et al. (1992) to be typical rift tholeiites erupted in suprasubduction zone Suite IV 0 Suites II & III settings during the early phases of rifting of a magmatic arc. -2 εNd -4 Implications -6 Suite IA -8 Suite IB Isotopic data for the MRV is consistent with the tectonic model of Crawford et al. (1992) which interpreted the MRV Suite IC -10 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15 as post-collisional volcanics in a region of arc-continent collision. There is evidence for the existence of three distinct 147Nd/144Nd mantle source components throughout the relatively brief time span over which the MRV erupted. The association of isotopically primitive compositions with VHMS deposits may offer a useful exploration vector. Acknowledgements References Keith Corbett and Lance Black provided a number of samples utilised in this study. Bailes, A. & Galley, A. (1999). Evolution of the Paleoproterozoic Snow lake arc assemblage and geodynamic setting for Analyses were undertaken at LaTrobe University by Roland Mass. This study was funded associated VHMS deposits, Flin Flon Belt, Manitoba, Canada. Canadian Journal of Earth Sciences, 36, 1789-1805. Crawford, A., Corbett, K. & Everard, J. (1992). Geochemistry of the Cambrian VHMS-rich Mount Read Volcanics, in part by a research grant from the Centre for Ore Deposit Research, an ARC Special Tasmania, and some tectonic implications. Economic Geology, 87, 597-619. Research Centre at the University of Tasmania. Lesher, C., Goodwin, A, Campbell, I. & Gorton, M. (1986). Trace element geochemistry of ore-associated felsic metavolcanic rocks in the Superior Province, Canada. Canadian Journal of Earth Sciences, 23, 227-237..