The Petrogenesis of the Mesozoic Rocks, Bowen Region, Queensland, and Changes of Tectonic Setting from 145 to 100 Ma

Charlotte M. Allen1 and B.W. Chappell1

Key Centre for the Geochemical Evolution and Metallogeny of Continents, Department of Geology, Australian National Univerisity, Canberra 0200

INTRODUCTION

Cretaceous igneous rocks occur sparsely along the entire length of Australia's eastern seaboard, from as far south as Mount Dromedary to as far north as Cape Upstart. These Cretaceous rocks have been modelled as the product of rifting associated with formation of the Coral and Tasman seas although the oldest documented sea floor off Queensland is ~85 Ma (Veevers et al., 1991; Ewart et al., 1992). In the Bowen Region we have identified three main age groups of intrusions: 145-135, 135-125, and 125-100 Ma. We describe here the time-temperature-emplacement level characteristics of the age groups and how these characteristics might relate to tectonic setting. Furthermore, we emphasise the isotopic differences of the Cretaceous (and Triassic) plutons from the Carboniferous-Permian ones that many of them intrude.

AGE DISTRIBUTION AND DEFINITION OF SUITES

Figure 1 shows geochemistry sample locations divided by age. On this map the structure called the Connors Arch is roughly coincident with the Urannah Suite and the plutons that intrude it. The adjacent Permian and older rocks east of the Arch comprise the Midgeton Block.

The oldest group of Cretaceous rocks is restricted to the Don River area and these range in age from 145-135 Ma as determined by K-Ar ages from hornblende and biotite (Table 1). Although we call these rocks the Don River suite, the suite includes very diverse rock types including quartz monzodiorite, monzonite, and granodiorite. Besides for age, these rocks share the features of being more mafic than granite, and having calculated Al-in-hornblende pressures of 250-215 MPa using the Al-in-hornblende method of Anderson and Smith (1995) (Table 1). We have applied the technique strictly to those rocks containing the assemblage Pl+Ksp+Qtz+Hb+Bt+Tnt+Mt (Hollister et al., 1987).

The most extensive age group of Cretaceous granites in the Bowen area is 135-125 Ma as determined by K-Ar mineral ages and SHRIMP zircon geochronology (Webb and McDougall, 1968; Allen et al., 1994). We divide this age group into two suites based on location. Those intruding the Permo-Triassic sedimentary rocks of the Bowen Basin are called the Bowen Suite, and those in the Connors Arch are called the Hecate-Eungella Suite due to their distribution between the Hecate Homestead and Eungella Dam. The Bowen Suite is diverse compositionally (Pattison, 1984). The samples for this study comprise three quartz diorite samples from the eastern part of the basin. These rocks have textures suggesting very shallow levels of intrusion. The Hecate-Eungella Suite includes granodiorite and granite that have many textural and compositional similarities. The granodiorite samples yield Al-in-hornblende pressures of 156-76 MPa, with the rocks near Eungella giving the lowest pressures.

Several areas of Cretaceous rocks younger than 125 Ma have been identified in the Connors Arch. These include: Mount Pring, Mount Abbot, and Cape Upstart. These rock groups are dominated by granite (s.s.) but also include the quartz syenite at Mount Abbot, and gabbro at Cape Upstart. All granites are very fine grained, vesicular, pink granites that most likely crystallised at very shallow depths. The Cretaceous igneous rocks of the Airlie Beach-Proserpine-Whitsunday area are <125 Ma with important exceptions. There are samples as old as 135 Ma in the eastern Whitsunday Islands. Ewart et al. (1992) suggested that the age distribution of granites from the mainland to the outer Whitsunday Islands, indicated rifting as igneous rocks <125 Ma occur between areas where 125-135 Ma intrusions are found. Thus the Cretaceous age groups overlap in geographical distribution.

GEOCHEMISTRY AND MAGMA TEMPERATURE

The age groups defined above have distinct geochemical characteristics, especially if rocks with SiO2>65% are examined. We use analyses of the Proserpine Volcanics (Ewart and Parianos, unpublished; Parianos, 1993; and two samples from this study) as representative of the overall Airlie-Proserpine-Whitsunday area. The trace element Harker diagrams of Figure 2 show two ways in which the rocks greater or less than 125 Ma differ. First, the older rocks average granodiorite in composition, and include few granites, whereas most of the younger plutonic rocks sampled in this study are granites. Second, the younger rocks contain greater abundances of Zr and Y, and lesser abundances of Sr than the older rocks as emphasised by the diagonal in Figure 2. As fractionation among granitic rocks tends to decrease, not increase, Zr, we attribute the high to very high Zr contents of the <125 Ma granites to higher temperatures at the source after 125 Ma (Table 2).

Although the age groups have distinctive trace element characteristics, there is little isotopic contrast across all Cretaceous samples (Figure 3). The entire range of initial Sr ratio is 0.7033-0.7039. This range of values is relatively low for continental marginal rocks, and much lower than those of the Urannah Suite, especially if those ratios are calculated to ~120 Ma. Note that Triassic samples have isotopic compositions like the Cretaceous ones. We attribute the relatively high initial Sr ratios of the Bowen Suite to contamination of the intermediate to mafic, shallow intrusions by shallow crust.

COMPARISON TO HOST ROCKS

Late Carboniferous-Early Permian intrusions probably underlie most of the study area. This conclusion is based on the following: 1) The Urannah Suite, those rocks that make up most of the Connors Arch, are compositionally very similar to the Bulgunonna Suite that occurs west of the northern tip of the Bowen Basin (Black, 1994; Oversby et al., 1994). Urannah Suite rocks are found in the Midgeton Block near Mackay as well as in the Connors Arch. The Connors Arch is thought to be a basement block uplifted during the westward thrusting the Hunter-Bowen Orogeny at ~230 Ma such that the Arch now interrupts the original extent of the Bowen Basin (Fergusson, 1991; Fielding et al., 1994 and 1996). The Urannah Suite is more radiogenic than the Cretaceous ones with respect to initial Sr, Nd and Pb isotopic composition. We argue elsewhere that the structure of the crust was fundamentally changed during the long period of extension that produced the Late Carbonifeorus-Early Permian Urannah Suite, the dike swarm that cuts the Urannah Suite, the Permian Thunderbolt Granite that intrudes the Urannah Suite, and the Permian volcanic rocks that floor the Bowen Basin (Fig. 1; Allen and Chappell, JGR, provisionally accepted). That period of extension ended at 230 Ma with the Hunter-Bowen Orogeny but not before severe crustal extension had effectively removed Urannah Suite like rocks or their sources as sources for Cretaceous intrusions.

DISCUSSION OF TECTONIC SETTING

Cretaceous intrusions ranging in age from 145 to 103 Ma are found in the Bowen area. Although rocks greater or less than 125 Ma have distinctive trace element signatures, they are indistinguishable with respect to radiogenic isotopes. Furthermore, the age groups overlap in geographic distribution. Ewart et al. (1992) have made a strong case for generation of igneous rocks <125 Ma in an extensional environment based on the age distribution of intrusions, and the bimodal distribution of rock types. They argue for emplacement of E-MORB into the crust, crustal melting, and variable mixing between the two. We concur that an extensional setting is likely. What setting generated the older Cretaceous rocks is unclear. Could the first evidence of crustal rifting that culminated in the formation of new seafloor at ~85 Ma have started 60 Ma before? Falvey and Mutter (1981) have suggested that magmatic activity can predate seafloor formation by as much as 50 Ma. No matter the tectonic setting, it is likely that the sources for all felsic Cretaceous igneous rocks were very similar given the small ranges of isotopic composition. Since t these rocks differ chiefly in trace element abundances, we argue that sources were similar but that the temperature at the source was significantly higher for igneous rocks <125 Ma. It is possible that as rifting progressed during the whole of the Early Cretaceous, progressive heating and isotherms rising in the crust gave rise to granites emplaced at shallow levels in the crust after 125 Ma. Moreover progressive uplift of the Connors Arch over the same period can be modelled as uplift on the margin of a rift zone.

ACKNOWLEDGEMENTS

Special thanks to Ian McDougall and Joe Wooden for access to their labs. This is publication number ** in the Key Centre for the Geochemical Evolution and Metallogeny of Continents.

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Figure 1 Bedrock map of study area near the town of Bowen (B). Geochemistry samples are divided by age. Places indicated by squares. C=Collinsville, E=Eungella, H=Hecate homestead, U=Urannah homestead. Triassic rocks are restricted to the coastal area and Whitsunday Islands. Cretaceous volcanic and plutonic rocks are distributed from the Bowen Basin to the Whitsunday Islands. The dikes are mostly Permian in age except the ring dikes associated with the Cretaceous Mount Abbot complex. Unpatterned area at "C.H." are Tertiary deposits at Cape Hillsborough.

Figure 2 These trace element Harker Diagrams show the relatively enriched Y and Zr and relatively depleted Sr compositions of the rocks with ages < 125 Ma as compared to the older groups.. Diagonals divide the younger and older samples at SiO2>65%. Granite from Ball Bay (18) has Y=112 ppm. Asterisks=Don River Suite (145-135 Ma), dark diamonds=Hecate-Eungella Suite (135-125 Ma), light diamonds=Bowen Suite (135-125 Ma), dark stars=Mount Abbot (~120 Ma), light stars=Mount Pring (~120 Ma), dots=Cape Upstart (~100 Ma), cirlces=Proserpine Volcanics (<120 Ma).

Figure 3 Stacked-bar diagram showing numbers of samples of a given initial 87Sr/86Sr (Sri). Mesozoic samples are less radiogenic than Late Carboniferous-Early Permian ones from the Urannah Batholith. Symbols as in Fig. 2. Data sources: this work, Ewart et al. (1992), and Parianos (1993). Inset shows non-systematic relationship of SiO2 and Sri among Cretaceous rocks. The Bowen Basin samples are more radiogenic than the rest.

Figures available from Charlotte Allen.