Low-grade metamorphism in the Palaeozoic sequences of the Townsville hinterland, northeastern Australia

The transition from diagenesis to low-grade metamorphism for the Ordovician - Early Carboniferous sequences of the Burdekin and Clarke River Basins and the Broken River - Camel Creek region is characterised by b-cell dimension and Kübler Index (KI = illite crystallinity) of K-white micas, clay-mineral assemblages and conodont colour alteration index (CAI). Data indicate that: (i) there is a regional metamorphic pattern characterised by anchizonal conditions, with consistently higher grade in the Camel Creek region (or Subprovince); (ii) anomalies are explicable in terms of location relative to areas of differing intensity of deformation (e.g. the Ruxton Formation) and inferred greater depth of burial (e.g. southern versus northern outcrop tracts of Quinton Formation); (iii) the regional pattern in the Broken River - Gray Creek area shows general alignment with the regional grain; (iv) there is a generalised north to south increase in CAI for the Broken River region ('Top Hut' to Broken River, <4 to >6), with higher values towards the Montgomery Range intrusions; (v) in the Burdekin Basin, there are higher CAI values north of Mt Podge and 'Star', and lower values (<4) in the southern part of the basin ('Fletcher View' to 'Fanning River'); (vi) variation of both KI and CAI mean values through the stratigraphical sequence illustrates the existence of three metamorphic sequences, all possibly connected with interludes of orogenic activity: during the interval end-Ordovician - earliest Silurian (early Llandovery), earliest Devonian (mid-Lochkovian, broadly eurekaensis-delta Zones) and Middle Devonian (mid/late Givetian, broadly between the Middle varcus Subzone and the hermanni Zone); (vii) the relationship of KI data to CAI data from the Townsville hinterland indicates that the range of CAI values inferred for the anchizone should be enlarged to include CAI values of up to 6; and (viii) the average values of the K-white mica b-cell dimension is consistent with low-intermediate pressure-temperature metamorphic facies conditions. Distribution of the cumulative curves for the three areas indicates that metamorphism in the Camel Creek Region pelites took place under higher pressure conditions than in Broken River region or the Burdekin Basin.