J. R. Richards

Potassium‐argon ages in eastern Australia

Abstract From the results of this preliminary survey it would appear that the bathyliths of the southern belt of the Tasman Geosyncline represent a history of continuous tectonic activity from Middle Silurian through to Middle Devonian, with a gradual movement eastwards of the axis of the intrusions. The gneissic “Ordovician‐type” granite is dominant in the Upper Silurian, just preceding the foliated “Silurian‐type” plus massive types which are dominant in the Lower Devonian, but with significant overlap in time between them to the extent that “rock type” is not a safe determinant of age. It is therefore recommended that this terminology be no longer used. A few cases of Carboniferous intrusion have been observed. These generally are transgressive to the main trend. Later intrusions occurred to the east and north, ranging from Permian in New England to Mesozoic further north, in good agreement with expectation. Smaller Mesozoic instrusions in the southern coastal region have ages different from those anti...

Palaeozoic Victoria, Australia: Igneous rocks, ages and their interpretation

Abstract A reconnaissance traverse across Victoria yields 160 K‐Ar dates on igneous rocks from 94 localities. These are supplemented by Rb‐Sr dating in critical cases, and major‐element analyses (some new) on a proportion of the samples. All dates quoted in text and tables, new and previously‐published, have been revised in terms of the latest decay‐constant conventions. The dates range from Early Ordovician (480 Ma) in the west to Late Devonian (360 Ma) among the high‐level intrusives of Central Victoria. The relatively complex age pattern, and the petrochemical character of the rocks, are compared with the published chronology of neighbouring States, and are interpreted in terms of a long‐duration regime of westwards compression, which began in the deformations of the Adelaide System of South Australia, and continued until the Mid‐Devonian Tabberabberan Movement.

Isotopic age determinations on precambrian rocks of the carpentaria region, Northern territory, Australia

Abstract A sequence up to 40,000 ft thick of unmetamorphosed and only slightly deformed sedimentary and volcanic rocks occurs in the Carpentaria Province of Northern Australia. Metamorphic and granitic rocks form the basement to this sequence, and K‐Ar and Rb‐Sr age measurements show that the basement granites are about 1,800 ± 50 m.y. old. Associated in space and time with the granitic rocks are acid volcanics which form the basal unit in the overlying sequence. Glauconites in sedimentary rocks from this succession yield dates ranging from 1,600 m.y. in the Tawallah Group, the second lowest unit, to about 1,390 m.y. in the Roper Group, the uppermost unit. Plagioclase and pyroxene from dolerites intrusive into the Roper Group give K‐Ar dates ranging from 1,100 to 1,280 m.y.; the older date provides a younger limit to the age of the Roper Group. Following slight folding the Wessel Group was deposited unconformably on the Roper Group; a single glauconite from the topmost formation of the Wessel Group yields...

Some potassium‐argon ages in New England, New South Wales

Abstract The results of potassium‐argon measurements are reported. Three samples from the southern end of the New England bathylith confirm its Permian age (240–245 m.y.). Two samples of the “pre‐Permian” granites are not younger than Lower Permian (Hillgrove, 270 m.y.; Barrington Tops, 260 m.y.). A sample of the analcite basalt from Spring Mountain gave an Oligocene age (34 m.y.) by measurements on two separate minerals.

Some age measurements on micas from broken hill, Australia

Abstract K‐Ar measurements were carried out on 24 mineral concentrates, mostly biotite, from rocks in the Broken Hill district of New South Wales; Rb‐Sr measurements were carried out for comparison on eight of these. The results range from 430 to 1360 m.y. for K‐Ar, and from 460 to 550 m.y. for Rb‐Sr. They indicate that the last event of sufficient intensity to affect the biotites of this area occurred in the Lower Palaeozoic. This may be correlated, as a first hypothesis, with the event which caused emplacement of the secondary “Thackaringa‐type” lead deposits.

The base of the Fortescue Group, Western Australia: further galena lead isotope evidence on its age

Traces of galena have been found within amygdales of the Kylena Basalt, third unit of the Fortescue Group which is, by definition, the oldest sequence within the Proterozoic Hamersley Basin of the NW of Western Australia. Mutually-supporting Pb-isotope data from two laboratories yield a model age 2.76 ± 0.03 Ga, in good agreement with Rb-Sr and U-Pb datings of stratigraphically equivalent material. The low indicated source U/Pb is in accord with the observed basalt association, and contrasts with some other, fluorite-associated, samples of comparable model age, for which a ‘granite’ association is postulated. Lead ratios from a stratiform deposit within the Whim Creek Group, defined as of Late Archaean age, yield a model age 2.95 ± 0.01 Ga, in accord with a published Rb-Sr mineral age. Hence the base of the Fortescue Group, generally regarded as initiating the Early Proterozoic Era in Australia, cannot be younger than 2.8 Ga. This date has implications for future definitions of the Archaean-Proterozoic bo...

Isotopic and lead‐alpha ages of some Australian zircons

Abstract Isotope dilution Pb‐U ages have been measured on zircons from three areas in the Australian Precambrian. Three samples, from the Rum Jungle Complex, basement to the Pine Creek Geosyncline, yield an extrapolated age of 2,550 m.y. This result, in conjunction with published evidence concerning the age of intrusive granites, enables preliminary time‐limits, 2,550 m.y. to about 1,700 m.y., to be assigned to the period of sedimentation in this Geosyncline. Results from the Mount Isa area support geological deductions that the oldest phases of the Sybella Granite are remnants of basement rocks, approximating in age to the Ewen Granite at around 1,780 m.y.; that the Wonga Granite may well be derived from sediments older than the tectonic event reflected in the 1,300–1,450 m.y. K‐Ar ages observed in this region; and that the Wimberu Granite may belong to a later event, but older than that indicated by the biotite ages. Zircon from metasedimentary gneiss underlying the Ravens‐thorpe System in Western Austr...

Potassium‐argon ages on micas from the Precambrian region of North‐Western Queensland

Abstract Potassium‐argon measurements have been carried out on the separated micas of 27 samples, principally granites, from the Mount Isa‐Cloncurry region of north‐western Queensland. There is evidence for at least two tectonic periods within the “Lower Proterozoic” of the area. The first is represented only in the north‐western portion, with ages greater than 1,770 m.y. on the Ewen Granite, and on the granites of the Nicholson River area to the far north‐west. The second at 1,400–1,450 m.y. is manifested only to the south and east of the Kalkadoon‐Leichhardt complex, and including the Sybella Granite. The results may be further interpreted as lending support to the concept of a possible “metamorphic discontinuity” along the western flank of the Kalkadoon‐Leichhardt complex, postulated by Carter, Brooks and Walker; as suggesting possible contemporaneity of the Cliffdale Volcanics and the Argylla Formation; and as giving further evidence for the antiquity of stromatolites. Comparison with earlier work sug...

Rock lead isotopes in northeast Queensland

Abstract U‐Pb and Th‐Pb studies of rocks from an extensive Palaeozoic volcano‐plutonic province in northeast Queensland support conclusions derived from a previous Rb‐Sr study. The data oppose an earlier hypothesis, that two widespread and fundamentally different magma types are present. Interpretation of the lead data, however, must be treated with some caution since uranium movement apparently begins at a very early stage of weathering; X‐ray fluorescence data are displayed in a manner which suggests a simple rejection criterion.

Rb-Sr study of some igneous rocks near Chillagoe and Herberton, Northeastern Queensland

Abstract Sr isotope ratios indicate that the Herbert River and Elizabeth Creek Granites at the northeastern margin of the Georgetown Inlier of northeast Queensland are closely related to each other and to the spatially-associated Featherbed Volcanics. Although there is field evidence for a range of granite ages, the isotopic evidence is that a large proportion of the granite was formed within the Late Carboniferous (328 ± 5 m.y.). The Featherbed Volcanics are about 30 m.y. younger, at 299 ± 13 m.y. These are post-dated by other granites of the region.