Post-depositional history of the Willyama Supergroup in the Broken Hill Block, NSW

Saved in:
Authors:Stevens, B. P. J.
Author Affiliations:Primary:
Geol. Surv. N.S.W., Broken Hill, N.S.W., Australia
Volume Title:Australian Journal of Earth Sciences
Source:Australian Journal of Earth Sciences, 33(1), p.73-98. Publisher: Blackwell, Melbourne, Victoria, Australia. ISSN: 0812-0099
Publication Date:1986
Note:In English. 105 refs.; illus. incl. 7 tables, geol. sketch map
Summary:The Broken Hill Block comprises complexly folded and metamorphosed Proterozoic metasediments and metavolcanics of the Willyama supergroup, and pre- and post-folding intrusives. A precratonic stage commenced with separation of crustal material from the mantle at 2100-2300 m.y. Deposition of the Willyama supergroup occurred at approx 1820 m.y. These rocks were metamorphosed to amphibolite-granulite facies and deformed twice at approx 1660 m.y., in the Olarian orogeny. A third deformation with accompanying retrograde metamorphism occurred soon after. Before 1570 m.y., retrograde schist zones had formed. The transitional tectonic stage commenced at 1490 + or - 20 m.y., with the emplacement of Mundi Mundi type granites and minor muscovite pegmatites. Between 1490 and approx 1100 m.y., the currently exposed rocks were elevated from 13-20 km below the surface, to surface and near-surface levels. During the epicratonic stage (1100-500 m.y.), continental and shallow marine sediments of the Adelaide System plus minor basalt were deposited in grabens or half-grabens on Willyama basement. Substantial erosion of the Willyama supergroup occurred. Alkaline ultramafic plugs and dykes were intruded at 561 + or - 7 m.y., and a thermal pulse at 520 + or - 20 m.y. marks the Delamerian orogeny. This was accompanied by low-grade metamorphism, re-activation of retrograde schist zones, intrusion of dolerite plugs, dykes, and zoned pegmatites, and gentle folding of the Adelaidean rocks. Peridotite stocks and dykes may have accompanied the ultramafic intrusions at 561 + or - 7 m.y., or the dolerite intrusions at 520 + or - 20 m.y. During the cratonic stage (500 m.y. to present) faulting occurred, fluorite-bearing veins and siderite-rich Pb-Ag veins formed. At approx 280 m.y., either a thermal event occurred, or substantial erosion dropped the T of the presently exposed rocks to <100°C. Between 280 m.y. and the present there was substantial weathering, and significant uplift and erosion. Silcrete and ferricrete were formed locally, possibly in the Oligocene (36-23 m.y.). Specific events of uplift and erosion probably occurred in the Palaeocene to Mid-Eocene (65-45 m.y.) and late Miocene to Pliocene (10-2 m.y.). The Willyama supergroup contains the Broken Hill Pb-Zn-Ag orebody and abundant minor deposits. The top of the orebody was removed by erosion and it is possible that other orebodies have been eroded away. [N.C.N.S.]
Subjects:Absolute age; Ar-40/Ar-39; Argon; Block structures; Dates; Delamerian Orogeny; Dikes; Emplacement; Facies; Faults; Granites; Granitic composition; High-grade metamorphism; Igneous rocks; Interpretation; Intrusions; Isotopes; Lower Paleozoic; Metamorphic rocks; Metamorphism; Noble gases; Olarian Orogeny; Orogeny; Paleoproterozoic; Paleozoic; Pegmatite; Plugs; Plutonic rocks; Precambrian; Proterozoic; Radioactive isotopes; Regional metamorphism; Schists; Stable isotopes; Structural geology; Systems; Tectonics; Uplifts; Upper Precambrian; Veins; Willyama Supergroup; Australasia; Australia; Broken Hill Block; New South Wales Australia; Adelaide System; Evolution; Igneous activity
Abstract Numbers:87M/6950
Record ID:1987035807
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
Tags: Add Tag
No Tags, Be the first to tag this record!
Be the first to leave a comment!
You must be logged in first