Modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold Belt (east Australia): A Paleozoic forearc terrane
The New England Fold Belt of southeast Australia is divided by the Peel Fault System into two time-equivalent zones showing contrasting depositional, structural, and metamorphic histories. These are inferred to represent a western magmatic arc–frontal arc succession and an eastern subduction complex assemblage. The composition of the early to middle Paleozoic clastic rocks from both east and west of the Peel Fault System indicates derivation from an undissected volcanic arc. Detrital quartz is absent or present only in minor amounts and always as monocrystalline grains. Lithic fragments are almost exclusively volcanic, and normally dominant over feldspar which is invariably plagioclase (now largely albitized). To the west of the fault, the distribution of lathwork, microlitic, felsitic, and vitric rock fragments, as well as variation in the amount of detrital quartz, indicates changes in provenance through time, and two cycles of basaltic to silicic volcanic activity have been identified. The composition of detrital calcic clinopyroxene grains, particularly their low Ti and Al contents, and their Ti:Mn:Na ratios, indicates that they crystallized mainly from subalkaline, probably calc-alkaline, volcanic arc magmas. The dominantly volcanogenic provenance of sandstone from east of the fault and the low Ti content of most detrital clinopyroxene grains indicate that the volcanic chain that lay to the west of the Peel Fault was the most likely source for the major portion of the clastic debris in the eastern rocks. However, the presence of Ti-rich detrital clinopyroxenes, identical in composition to those in the metabasalts of the subduction complex, indicates that uplifted segments of the complex provided an additional sediment source.