GEMOC ARC National Key Centre

Charlotte M. Allen^1,2, Doone Wyborn¹ and Bruce W. Chappell¹

1. GEMOC National Key Centre, Geology Department, Australian National University, Canberra ACT 0200
2. Research School of Earth Sciences, Australian National University, Canberra ACT 0200

Whereas the first order petrogenesis of granites in the Lachlan Fold Belt is still being debated, the New England Orogen (NEO) granites are generally accepted to be subduction zone related. In the northernmost part of the NEO (that section north of Rockhampton), this model can be tested on plutons ranging in age from Devonian to Cretaceous by comparing their compositions to batholithic rocks known to have formed above subduction zones, like the Mesozoic Peninsular Ranges batholith of California and Mexico.

The NEO, in structural terms, includes those rocks affected by the Late Permian-Triassic Hunter-Bowen Orogeny. The style of the deformation in the northernmost NEO was thin- to thick- skinned westward thrusting. In terrane terms, the basic element of the Orogen, found along its entire length, is Late Devonian-Early Carboniferous forearc deposits. Backarc deposits of the same age are preserved in the Drummond Basin west of the NEO. Separating the Drummond Basin from the NEO is the Permian-Triassic Bowen Basin. Its eastern margin was deformed by Hunter-Bowen thrusting which helped expose the Connors Arch, a middle crustal block of mostly granite that defines the western margin of the NEO at its northern end.

LATE DEVONIAN-EARLY CARBONIFEROUS

Although sedimentary rocks of this age are widespread, felsic to intermediate igneous rocks of this age are rare. Those found in the forearc deposits are dominantly mafic. The remainder lie in the Drummond Basin (Henderson et al., 1998) and relatively little geochemical work has been published on these. Taking inheritance into account, these mostly volcanic tuffs crystallised between 350 and 340 Ma. This is a compositionally restricted suite. Plutons of somewhat greater age (385-366 Ma; Crouch et al., 1995) occur in an uplifted basement block in the Drummond Basin (Anakie Inlier).

LATE CARBONIFEROUS-PERMIAN

A large portion of the northern NEO are igneous rocks of this age. Plutonic and volcanic rocks occur in the Connors and Auburn arches, and in the Drummond Basin. Volcanic rocks of this age also floor the Bowen Basin. Before orogeny, volcanic rocks blanketed the northern NEO. Compositions are diverse; plutonic rocks are dominated by granodioritic-tonalitic compositions. A suite of similar rocks of Late Permian age, called the Clarence River Suite, occurs at the southwest edge of the Clarence Morton Basin (Bryant et al., 1997) in the southern NEO.

TRIASSIC

Triassic plutonic and volcanic rocks occur in the Whitsunday Province and near Bowen (Ewart et al., 1992) and in the Gayndah region (Stephens, 1991). Stephens has subdivided this age group into older and younger series, where the older tends to be intermediate, and the younger, felsic and/or bimodal.

CRETACEOUS

Cretaceous igneous rocks occur in the Whitsunday Province, and in the Connors Arch. Rocks 125-100 Ma rocks occur in both areas. Plutonic rocks are dominantly true granites; volcanic rocks are bimodal. In contrast, Early Cretaceous rocks (>125 Ma) are restricted to the Connors Arch and these are intermediate (Allen et al., 1997). Cretaceous plutons also occur near Noosa, and at Mount Dromedary (southern coastal NSW).

The occurrence of fore- and backarc deposits of Late Devonian-Early Carboniferous age leaves little doubt that subduction was locally active, however it is interesting how little evidence of "an arc" can be found. As presently understood, igneous activity at this time was dominated by dispersed felsic volcanism. The bulk of plutonic rocks in the northern NEO, are distinctly younger (322-278 Ma; Allen et al., 1998) and comprise three suites: the Urannah, the Bulgonunna, and the more minor Thunderbolt. Because of Hunter-Bowen thrusting, two structural levels of the same igneous pile are exposed. The Urannah Suite in the Connors Arch is mid-crustal based on Al-in-hornblende geobarometry, and the Bulgonunna Suite is subvolcanic. Bulgonunna igneous rocks overlie Drummond Basin sedimentary rocks and are essentially undeformed. When rock compositions of the two suites are compared, there are only subtle differences. The more oceanward and deeper Urannah is marginally more mafic on average. The Bulgonunna Suite is more continental in character having higher average concentrations of K, Ba, Rb, Pb, Th, U, Nb, Zr, Y La, Ce, Cr and Ga, and lower concentrations of Ca, Sr, Mn and Ni. The suites are almost indistinguishable with respect to Rb-Sr, Sm-Nd and Pb isotopes. For instance, the Urannah and Bulgonunna suites contain rocks with initial 87Sr/86Sr as calculated at 280 Ma of 0.704-0.707 (n=33 and 20, respectively) but the Urannah contains two samples more radiogenic than this (0.7085 and 0.7088).

As compared to the eastern and western Peninsular Ranges Batholith (PRB), the compositional difference between the Urannah and Bulgonunna rocks is less than between the wPRB and ePRB. Only CaO and Zr show more variability in the Queensland rocks. The wPRB contains very high Ba contents. Overall the Queensland rocks appear more continental in character with higher K2O, Rb, Pb, U, Th and REE than PRB averages (using only rocks with 62-68 wt% SiO2), but note they are generally less radiogenic with respect to Pb isotopes. The ePRB contains 1.5 times more Sr and half as much Y as the wPRB, but the Bulgonunna Suite contains somewhat lower average Sr, and 1.5 times as much Y as Urannah rocks. This is the critical difference between the two systems. The ePRB has been interpreted as derived from rocks where garnet was stable and involved continental lithosphere; whereas the wPRB is interpreted as derived from lithosphere relatively free of continental influence and where plagioclase was stable. The wPRB has initial 87Sr/86Sr of 0.7035-0.7060. The Urannah-Bulgonunna Supersuite shares many features with the wPRB. Modelling suggests that the dominant sources of these Queensland rocks were mafic lower crust of early Paleozoic to Neoproterozic age. Nd model ages range from 1270 to 800 Ma minus 2 extraordinary samples.

Unlike igneous rocks older than ~250 Ma, those younger are distinctly less radiogenic with respect to Rb-Sr, Sm-Nd and Pb isotopes (initial 87Sr/86Sr of 0.7030-0.7390). Their Nd model ages are 560-365 Ma. Moreover, their tectonic origins are less certain. Cretaceous rocks from the Whitsunday Province have been suggested to relate to opening of the Tasman Sea. For those rocks less than 125 Ma, this explains their bimodality, however the 145-125 Ma rocks tend to be intermediate and are more easily related to subduction. Note that the oldest seafloor in the Coral Sea is 65 Ma. We suggest that underplating related to formation of Australia's passive margin in the Neoproterozoic provided the sources for the Urannah and Bulgonunna suites. Sources of igneous rocks < 250 Ma were distinctly younger and may include basalts about the age of the crystallization events themselves.

REFERENCES

Allen, C M, Wooden, J L, and Chappell, B W, 1997. Late Paleozoic crustal history of central coastal Queensland interpreted from geochemistry of Mesozoic plutons: the effects of continental rifting. Lithos 42, 67-88.

Allen, C M, Williams, I S, Stephens, C J, and Fielding, C R, 1998. Granite genesis and basin formation in an extensional setting: the magmatic history of the northernmost New England Orogen. Australian Journal of Earth Sciences 45, in press.

Bryant, C J, Arculus, R J, Chappell, B W, 1997. Clarence River Supersuite: 250 Ma Cordilleran tonalitic I-type intrusions in eastern Australia, Journal of Petrology 38, 975-1001.

Crouch, S B S, Withnall, I W, Tenison Woods, K, and Hayward, M A, 1995. Plutonic rocks of the southern Anakie Inlier. In: Withnall, I.W., Blake, P.R., et al. (eds). Geology of the southern part of the Anakie Inlier, central Queensland, pp. 95-115. Queensland Geology 7.

Ewart, A, Schon, R W, Chappell, B W, 1992. The Cretaceous volcanic-plutonic province of central Queensland (Australia) coast--a rift related 'calc-alkaline' province. Transactions of the Royal Society of Edinburgh 83, 327-345.

Henderson, R A, Davis, B K, and Fanning, C M, 1998. Stratigraphy, age relationships and tectonic setting of rift-phase infill in the Drummond Basin, central Queensland. Australian Journal of Earth Sciences 45, 579-595.

Stephens, C J, 1991. The Mungore Cauldron and Gayndah Centre. Late Triassic large-scale silicic volcanism in the New England Fold Belt near Gayndah, southeast Queensland; Ph.D. thesis, University of Queensland, Brisbane (unpublished).