A.L. Pickard

SHRIMP U–Pb zircon ages of tuffaceous mudrocks in the Brockman Iron Formation of the Hamersley Range, Western Australia*

Tuffaceous mudrocks are common in the banded iron‐formations (BIF) of the Brockman Iron Formation. These tuffaceous mudrocks are either stilpnomelane‐rich or siliceous. Their compositions reflect bimodal volcanic activity in the vicinity of the Hamersley BIF depositional site. They also contain complex zircon populations that record resedimentation, syndepositional volcanism and post‐depositional isotopic disturbance. The best estimates of depositional age are obtained from siliceous tuffaceous mudrocks in the Joffre Member that contain 2459 ± 3 Ma and 2454 ± 3 Ma zircon populations most likely derived from felsic volcanism coeval with BIF deposition. These dates constrain the sedimentation rates for the ∼370 m‐thick Joffre Member BIF to >15 m per million years. Siliceous tuffaceous mudrocks are not present in the underlying ∼120 m‐thick Dales Gorge Member and it is uncertain whether previously reported ages of ca 2479–2470 Ma for this unit reflect detrital/xenocrystic or syndepositional zircon populations in resedimented stilpnomelane‐rich tuffaceous mudrocks. The increased abundance of tuffaceous mudrocks in the Joffre Member suggests that a pulse of enhanced igneous and hydrothermal activity accompanied deposition of the bulk of the Brockman Iron Formation BIF after ca 2460 Ma. This preceded and culminated in the emplacement of the 2449 ± 3 Ma large igneous province represented by BIF and igneous rocks of the Weeli Wolli Formation and Woongarra Rhyolite.

Southeast Asian sediments not from Asia: Provenance and geochronology of north Borneo sandstones

Eocene–lower Miocene sandstones of the Crocker turbidite fan of north Borneo were derived from nearby Borneo and southeastern Asian sources, rather than distant Asian sources eroded after India-Eurasia collision. They are compositionally mature due to tropical weathering, but are mostly first-cycle sandstones derived from granitic rocks and subordinate metamorphic, sedimentary, and ophiolitic rocks. Detrital zircon ages range from Archean to Eocene, and the majority are Mesozoic. The most important source areas were Cretaceous granites of the Schwaner Mountains in southwest Borneo during the Eocene, and Permian–Triassic granites and Proterozoic basement of the Malay-Thai Tin Belt during the Oligocene.

Australian provenance for Upper Permian to Cretaceous rocks forming accretionary complexes on the New Zealand sector of the Gondwanaland margin

U–Pb (SHRIMP) detrital zircon age patterns are reported for 12 samples of Permian to Cretaceous turbiditic quartzo‐feldspathic sandstone from the Torlesse and Waipapa suspect terranes of New Zealand. Their major Permian to Triassic, and minor Early Palaeozoic and Mesoproterozoic, age components indicate that most sediment was probably derived from the Carboniferous to Triassic New England Orogen in northeastern Australia. Rapid deposition of voluminous Torlesse/Waipapa turbidite fans during the Late Permian to Late Triassic appears to have been directly linked to uplift and exhumation of the magmatically active orogen during the 265–230 Ma Hunter‐Bowen event. This period of cordilleran‐type orogeny allowed transport of large volumes of quartzo‐feldspathic sediment across the convergent Gondwanaland margin. Post‐Triassic depocentres also received (recycled?) sediment from the relict orogen as well as from Jurassic and Cretaceous volcanic provinces now offshore from southern Queensland and northern New South Wales. The detailed provenance‐age fingerprints provided by the detrital zircon data are also consistent with progressive southward derivation of sediment: from northeastern Queensland during the Permian, southeastern Queensland during the Triassic, and northeastern New South Wales — Lord Howe Rise — Norfolk Ridge during the Jurassic to Cretaceous. Although the dextral sense of displacement is consistent with the tectonic regime during this period, detailed characterisation of source terranes at this scale is hindered by the scarcity of published zircon age data for igneous and sedimentary rocks in Queensland and northern New South Wales. Mesoproterozoic and Neoproterozoic age components cannot be adequately matched with likely source terranes in the Australian‐Antarctic Precambrian craton, and it is possible they originated in the Proterozoic cores of the Cathaysia and Yangtze Blocks of southeast China.

1170 Ma SHRIMP age for Koras Group bimodal volcanism, Northern Cape Province

The bimodal volcanic and coarse siliciclastic sedimentary rocks of the Koras Group are virtually undeformed and weakly metamorphosed, and overlie the highly metamorphosed and strongly deformed rocks of the Kheis subprovince of the Namaqua—Natal Metamorphic Province with a sharp erosional unconformity. Zircons from the Swartkopsleegte quartz porphyry, close to the base of the Koras Subgroup, have been dated at 1171 ±7 Ma by SHRIMP. This age is in good agreement with, but far more precise than, previously published ages for the Koras Group. The new age defines the onset of bimodal volcanism in the Koras Group and also indicates that the compressional Kibaran Orogeny in the Namaqua—Natal Province ended before or at ~1170 Ma. The main phase of Kibaran deformation, uplift, and erosion in the Namaqualand sector of the orogen took place in the period between the extrusion of the Wilgenhoutsdriflavas at ~1330 Ma and deposition of the Koras Group at 1171 Ma. Younger ages for peak metamorphism and post-tectonic intrusions (1060 to 1030 Ma) in the Namaqua—Natal Province clearly post-date Kibaran compressional tectonics and magmatic activity by more than 100 Maand are related to a subsequent magmatic event in the orogen. The Koras Group in Namaqualand is widely regarded as a typical representative of the ~1105 Ma Umkondo igneous province, a group of volcano—sedimentary successions and igneous bodies which post-date the Kibaran Orogeny in southern Africa. However, extrusion of the Koras lavas predates the Umkondo suite in Zimbabwe and coeval lavas in Botswana andNamibia by 60 Ma.