October 2010 LIP of the Month
Australian LIPs and the Australian Precambrian ‘barcode’
Jon Claoué-Long and Dean Hoatson
Geoscience Australia, GPO Box 378, Canberra, ACT 2601.
Email: Jon.Long@ga.gov.au
© Commonwealth of Australia (Geoscience Australia) 2010.
This material is released under the Creative Commons Attribution 3.0 Australia License.
Introduction
Geoscience Australia has published a series of large-format (A0-size) web-based maps which show the complete record of mafic-ultramafic magmatism across Australia through the Archean and Proterozoic. Five Australian mafic-ultramafic magmatic events have been proposed as Large Igneous Provinces (LIPs) by various authors. In age order, they are: the ~1780 Ma Hart LIP, followed by the ~1210 Ma Marnda Moorn LIP, the ~1070 Ma Warakurna LIP, the ~825 Ma Gairdner LIP, and the ~510 Ma Kalkarindji LIP. Their geometries and characteristics are shown in a Large Igneous Provinces map sheet, with significant updates to the known extents of some LIPs. The early Cambrian Kalkarindji LIP is included in this Precambrian compilation because of its size and global importance.
Figure 1 shows the distributions of the five Australian LIPs. The map series is available for download, in pdf and jpeg form free of charge, from the Geoscience Australia website at:
http://www.ga.gov.au/resources/maps/minerals/geological-maps.jsp
Figure 1: Approximate distributions of the five Australian Large Igneous Provinces.
The compilation includes reference Time–Space–Event Charts of all known Archean and Proterozoic mafic-ultramafic magmatic events in Australia – the Precambrian ‘barcode’ for an entire continent. Thirty events can be recognised in the Proterozoic (Figure 2) with a further twenty-six in the Archean (Figure 3). The solid geology extent of each magmatic event is shown in detailed maps at 1:5,000,000 scale.
Figure 2. Time-Space-Event Chart for Proterozoic mafic-ultramafic magmatic events in Australia
Figure 3. Time-Space-Event Chart for Archean mafic-ultramafic magmatic events in Australia
One map sheet is given to a time series of eight maps of Australia, showing the secular development of time-space relationships for mafic-ultramafic igneous provinces during the Proterozoic. The Australian LIPs are thus presented in three distinct contexts: in spatial context with each other; in spatial context with the major Australian Crustal Elements; and in time-space context with the complete secular evolution of all mafic-ultramafic magmatic events in Australia.
Each map is accompanied by an explanatory guide and the series is available for download, free of charge, at:
http://www.ga.gov.au/resources/maps/minerals/geological-maps.jsp
Map of Australian Proterozoic Mafic-Ultramafic Magmatic Events: Sheet 1 of 2 (Hoatson et al., 2008a): 1:5 000 000 map of thirty Proterozoic mafic-ultramafic magmatic events.
Map of Australian Proterozoic Mafic-Ultramafic Magmatic Events: Sheet 2 of 2 (Hoatson et al., 2008b): large-format Time–Space–Event Chart; 1: 10 000 000 inset map of LIPs; 1: 10 000 000 inset map of associated mineral deposits.
Guide to using the Australian Proterozoic Mafic-Ultramafic Magmatic Events Map: Geoscience Australia Record 2008/15 (Hoatson et al., 2008c): describes the Map of Proterozoic Mafic-Ultramafic Magmatic Events.
Map of Australian Archean Mafic-Ultramafic Magmatic Events, Sheet 1 of 2 (Hoatson et al., 2009a): 1: 5 000 000 map of Archean mafic-ultramafic magmatic events.
Map of Australian Archean Mafic-Ultramafic Magmatic Events, Sheet 2 of 2: Yilgarn Craton (Hoatson et al., 2009b): 1: 3 000 000 maps of the distribution of Archean magmatic events and characterisation of komatiite units in the Yilgarn Craton.
Guide to using the Australian Archean Mafic-Ultramafic Magmatic Events Map: Geoscience Australia Record 2009/41 (Hoatson et al., 2009c): describes the Map of Australian Archean Mafic-Ultramafic Magmatic Events.
Map of Australian Proterozoic Large Igneous Provinces, Sheet 1 of 2 (Claoué-Long and Hoatson, 2009a): 1: 5 000 000 map of the five Proterozoic LIPs; 1:7 500 000 inset maps annotated with features of the three youngest LIPs.
Map of Australian Proterozoic Large Igneous Provinces, Sheet 2 of 2: Time Series of Proterozoic Mafic-Ultramafic Magmatic Events (Claoué-Long and Hoatson, 2009b): secular evolution of LIPs and other mafic-ultramafic Magmatic Events related to the Australian Crustal Elements.
Guide to using the Map of Australian Proterozoic Large Igneous Provinces: Geoscience Australia Record 2009/44 (Claoué-Long and Hoatson, 2009c): describes the Map of Australian Proterozoic Large Igneous Provinces.
The Mafic-Ultramafic Magmatic Events Series
The compilation recognises discrete ‘Magmatic Events’ in the sense of “probable geological incidents of significance that are suggested by geological, isotopic or other evidence” (modified after Neuendorf et al. (2005). This all-encompassing definition has allowed the capture of all known Precambrian mafic-ultramafic magmatic events, large and small, across the Australian continent.
The recognition of magmatic events and their correlation has been made possible by the vastly increased coverage of geochronology in Precambrian Australia over the last twenty years. Published ages were assessed for their geochronological methods, the interpretation confidence, and consistency with other temporal-related evidence, e.g., field relationships of associated rock units. In context with the available resolution of Precambrian geochronology, the magmatic event series is defined at a maximum resolvable duration of 20 Ma (i.e., ±10 Ma).
The map series shows all known Precambrian mafic-ultramafic magmatic events across the Australian continent. Some are known only as a single outcrop or drillcore occurrence. Others have geographic extents that may be drawn as igneous provinces. A very few meet the extent, duration, and magma volume criteria for recognition as LIPs. Precambrian mafic-ultramafic igneous rocks of unknown age are included in the coverage, as ‘Undefined Event’. Each event in the newly-defined series has been given a Series Number and an informal name, together with a letter code recording the presence of mafic rocks only (m), or both mafic and ultramafic rock units (mu).
The Precambrian record consists mainly of eroded or disrupted remnants of magmatic events, sometimes at deep erosion levels that do not preserve near-surface expressions such as lavas. Related remnants may now reside in other continents – recognition of which can lead to reconstructions and expand the assessment of original magma volumes. Our purpose in making the compilation widely available is to facilitate comparisons of other continents with the reference Australian series.
Delineation of Igneous Provinces
An ‘igneous province’ may be defined as the extent in space and time of igneous rock units that relate to a single overall magmatic (or thermal) process. This is broader than the narrow definition of a ‘magmatic event’ because an ‘igneous province’ may encompass multiple ‘events’ and a much longer period of time than the ±10 Ma resolution of the Precambrian magmatic events series. An Australian example is the period 1870-1750 Ma, in which six magmatic events shared a common spatial extent over a 120 Ma time period.
What is a LIP?
The published literature debating the meaning of ‘Large Igneous Province’ is enormous (e.g., Coffin and Eldholm, 1994, 2005; Courtillot and Renne, 2003; Ernst et al., 2005; Bryan and Ernst, 2008; and references therein). Recent definitions require not merely wide areal extent in an igneous province, but also demonstrably huge volumes of magma, and a narrow time dimension to distinguish transient magmatic ‘events’ from the long-lived magmatic systems associated with plate boundary environments.
LIPs are usually considered as being dominated by mafic (±ultramafic) igneous rock units, but there are important examples of association with silicic magmatism, and some authors refer to silicic-dominated LIPs as a related class. The question of definition is related to divergent views of the mantle plume hypothesis of LIP formation, on which the literature is also enormous (c.f. Campbell, 2007 and references therein).
The Australian compilation does not take a position on definitions, accepting instead the five Australian LIP definitions, or proposals, already made by other authors in the context of the continuing debate.
Two aspects of the Australian context are important. First, it is clearly a precondition that the broad definition of an igneous province must first be met, before the size and time dimension of the province can be tested for adherence to definition as a LIP. Only the youngest two of the named Australian LIPs (~510 Ma Kalkarindji LIP; ~825 Ma Gairdner LIP) have clearly been demonstrated to be both time equivalent and geochemically comagmatic systems throughout their extent (Glass and Philips, 2006; Zhao et al., 1994). The older proposed Proterozoic LIPs refer to igneous provinces recognised only by time equivalence of correlated igneous rock units: their geochemical coherence as igneous provinces is yet to be established.
Secondly, this comprehensive documentation of all Australian Precambrian mafic-ultramafic rocks highlights other large and transient magmatic events which have not yet attracted a proposal as candidate LIPs. A prominent example is the ~2420 Ma Widgiemooltha Event preserved as an extensive dyke swarm across the Yilgarn province (identified in the global large mafic magmatic event compilation of Ernst and Buchan, 2001). Another remnant of the same event may be the Sebanga Poort dyke swarm in the Zimbabwe craton of southern Africa (Söderlund et al., in press; December 2009 LIP of the Month, http://www.largeigneousprovinces.org/09dec).
Spatial Representation of Events, Provinces and LIPs
There exists no single solid-geology map for all Australia. Base maps used are the most current solid-geology coverages available from the State and Territory geological surveys. These maps vary in their mapping approaches, coverage, scale and detail. Variations in the mapping approaches are evident, especially where maps join at State borders.
Despite these limitations, the advantage of solid-geology coverage, over outcrop-based maps, is insight into the total areal extent of rock units under younger cover. This is a primary criterion when assessing the overall volume of magmatic systems and the identification of Large Igneous Provinces. Areal extent cannot fully constrain the volume of a magmatic system, however, owing to subsequent erosion and the difficulty in assessing original thicknesses of rock units.
Each ‘Magmatic Event’ is therefore mapped by its available solid geology coverage. Each ‘igneous province’ is then drawn as the inferred spatial extent of an event, or a related series of events. Interpolation is always required to create an inferred province boundary. The boundaries of Precambrian igneous provinces today reflect variable erosion, preservation and burial of evidence under cover.
The Large Igneous Provinces are drawn on the same basis as the smaller igneous provinces, using boundaries of the Australian Crustal Elements (Shaw et al., 1995) as province boundaries wherever possible and appropriate. The Australian Crustal Elements (Fig. 4) comprise a geophysical domain map of Australia emphasising major crustal domains under the cover of later basins. The approximate easternmost limit of the Precambrian crustal elements, and of the five Australian LIPs, is defined by the Tasman Line (c.f. Shaw et al., 1996; Direen and Crawford, 2003).
The overlay of all five Australian LIPs in Figure 1 highlights shared spatial characteristics. Regions of central Australia preserve remnants of four major LIPs (excluding the geographically distinct Marnda Moorn LIP). These zones of the Australian continent have repeatedly channelled the passage of huge volumes of mafic-ultramafic magma through the lithosphere and may be a pointer to primary lithosphere-scale controls.
The annotated Figures that follow are extracts from the Geoscience Australia publications. Figures 5-9 show the five major Australian LIPs. The following Figures 10-17 place the five LIPs within the evolution of all Australian mafic-ultramafic Magmatic Events through the Proterozoic. Large-scale and detailed coverage, with explanatory notes, is available for download at the Geoscience Australia website at:
http://www.ga.gov.au/resources/maps/minerals/geological-maps.jsp
The Australian LIPs
Figure 4 shows a division of the Australian continent into Major Crustal Elements (West, North, South, Central and Tasman) and constituent informal provinces, after Shaw et al. (1995, 1996). These are geophysically defined domains intended to portray shallow crustal elements under the cover of later basins. The Australian LIPs, and other mafic-ultramafic igneous provinces, are mapped within this crustal framework in the following Figures 5-17.
Figure 4. Major Crustal Elements of Australia, after Shaw et al. (1995)
Figure 5: The Hart LIP (~1780 Ma) comprises disconnected igneous provinces across all the Major Crustal Elements west of the Tasman Line. The original LIP proposal was confined to Kimberley province where the Carson Volcanics and Hart Dolerite represent an estimated volume of 250,000 km3 of mafic magma. Tyler et al. (2006) point out that the geology of these units is not known in detail. Large time-equivalent belts of mafic magmatism exist in four additional zones across the continent. Two are north-south belts in the east of the North and South Australian Elements. Two are east-west belts in the West Australian Element, and the south margin of the North Australian Element. Detailed geochemical and other work is needed to establish the extent to which they may represent comagmatic systems. The pairs of magmatic belts would be linked by closure of the northeast-trending crustal zone that now separates them.
Note the significant expansion of the Hart LIP correlation from the original proposal by Tyler et al. (2006), which was restricted to mafic magmatism in the Kimberley province.
Figure 6: The Marnda Moorn LIP (~1210 Ma) is constructed from what were once thought to be disconnected dyke swarms of widely different orientations around the west, south and east margins of the Yilgarn province. Wingate and Pidgeon (2005) assembled dating information which shows them all sharing a common emplacement age of ~1210 Ma. So far, the only distinguishing feature of these dykes is their age. The highly varied emplacement orientations are similar, in places, to otherwise separate Yilgarn province dyke swarms emplaced during different magmatic events, including the earlier east-west trending ~2420 Ma Widgiemooltha Event, and a later suite of east-west dykes identified at the northwest margin of the Yilgarn province as part of the ~1070 Ma Warakurna Event. Most of the dykes are poorly exposed, weathered, or buried under later cover. Thus, the extent of the swarm is established mainly from aeromagnetic data which delineate the dykes under cover. Geochemical data, which would associate the swarm as one comagmatic system, is lacking. Only small hypabyssal intrusions are known: evidence for larger intrusions and erupted lavas is absent.
Figure 7: The Warakurna LIP (~1070 Ma) exhibits the problem of preservation inherent in older magmatic suites. The LIP extent is drawn following Wingate et al. (2004), with the province boundaries of the Tasman Line and the Pinjarra province indicated as the LIP boundaries in the east and west. This LIP is over 1 Ga old and subsequent changes in the continent mean that its paleogeographic context is difficult to reconstruct. The same tectonism has also exposed deeper crustal levels of the magmatic plumbing, including very large mafic-ultramafic intrusions emplaced in the deep crust of the Musgrave province. Differential tectonism means that correlation of units that potentially define the LIP is not always clear. For example, the overall extent of the LIP is evident as a broad east-west trending zone: but coeval dykes in the south margin of the North Australian Element are orthogonal to this direction. The correlation of the Musgrave province Giles Complex is based on time equivalence only, and this age evidence is being questioned (Smithies et al., 2009). Detailed geochronology is required to substantiate the age relationships of this important mineralised province.
Figure 8: The Gairdner LIP (~825 Ma) was proposed by Zhao et al. (1994) from the age and geochemical correlation of NW-trending dykes in South and Central Australia with comagmatic lavas and sills in the basal stratigraphy of the Amadeus and Adelaide provinces. The LIP distribution is extended here, beyond the original definition, to include coeval mafic sills in the basal stratigraphy of the West Officer Basin (Williams, 1992; Scotnicki et al., 2008), which is another remnant of the Centralian Superbasin. Evidence that the LIP also extends into the Paterson province, under the younger cover of the Canning Basin, includes recent ages in drillcore (Maidment et al., 2008) and there is a swarm of undated NW-trending dolerite dykes at the southwest margin of the Kimberley province (Griffin et al., 1993) with a location and orientation consistent with the Gairdner LIP. Further work is required to test the inference that they form part of the same magmatic event. Components of this LIP have undergone more tectonic disruption than the younger ~510 Ma Kalkarindji LIP and deeper, hypabyssal, components of the crustal feeder plumbing are exposed. The distribution drawn here defines a NW-trending linear zone (later reactivated for the Kalkarindji LIP and the Larapinta Seaway) as the dominant control.
Note the recognition of the Gairdner LIP as a continent-wide linear belt is significantly expanded from the original proposal by Zhao et al. (1994) and recent correlations (c.f. Wang et al., 2010; August 2010 LIP of the Month, http://www.largeigneousprovinces.org/10aug.html).
Figure 9: The Kalkarindji LIP (~510 Ma) encompasses the original definition within Northern Australia by Glass and Philips (2006) and extension to the West Officer Basin (Evins et al., 2009; October 2005 and October 2006 LIP of the Months; http://www.largeigneousprovinces.org/05oct, http://www.largeigneousprovinces.org/06oct), but excludes the Musgrave and Amadeus provinces which contain no evidence of this LIP. The Irindina province at the southern margin of the North Australian Element (see Fig. 4) is included from the evidence of ~510 Ma mafic igneous rocks (Maidment, 2005). The Early Paleozoic Larapinta Seaway developed over this period (Cook, 1988; Cook and Totterdell, 1991). The development of this linear marine feature suggests that the Kalkarindji LIP coincided with the development of a shallow-marine rift environment across the continent, with the volcanic sequences now preserved to either side. The youngest Australian LIP has undergone the least tectonic disruption. The lava sequence is still preserved in many areas and subvolcanic feeder intrusions are still buried. Dolerite dykes in northwest Australia lie at the margin of the Larapinta Seaway, supporting identification of this zone as the primary location control. At the southeast end of the same corridor, deformed and metamorphosed ~510 Ma mafic rock units in the Irindina province are coincident with the Kalkarindji LIP; these little-studied igneous units are in juxtaposition with Centralian Basin sedimentary rocks.
Note the expansion of the Kalkarindji LIP from the original definition in northern Australia, to include important correlatives in Western Australia. Time Series of Mafic-Ultramafic Magmatic Events
A time series construction shows the secular evolution of the five named LIPs in time-space context with smaller mafic-ultramafic magmatic events, and in spatial relation to the Australian Crustal Elements. The evolution falls naturally into eight Proterozoic time periods. In each of the maps an inferred extent is drawn for all the mafic-ultramafic magmatic events within the relevant time period. This is distinct from the extents inferred for LIPs because the inferred igneous provinces may refer to magmatic activity over long periods of time.
Figure 10: Australian Mafic-Ultramafic Magmatic Events ~2500–1900 Ma. Prior to ~1870 Ma, the Australian mafic-ultramafic magmatic record is confined to the western half of the continent. The pattern of east-west magmatic trends in this part of the continent remains a feature to the end of the Proterozoic. The ~2420 Ma Widgiemooltha Event is of wide extent, comprising an east-west mafic and ultramafic dyke swarm that traverses the Yilgarn province. The very similar Sebanga dyke swarm in the Zimbabwe craton of southern Africa has been proposed as another remnant of the same magmatic event (Söderlund et al., in press). Other magmatic events in this period comprise a series of east-west belts at the southern margin of the Pilbara province and the northern margin of the Yilgarn province. The ~2015 Ma Stag Creek Event, and the ~1910 Ma Ding Dong Downs Event, are the first to have expression within the North Australian Element, in the Pine Creek and Halls Creek provinces.
Figure 11: Australian Mafic-Ultramafic Magmatic Events ~1870–1750 Ma. Starting with the ~1870 Ma Bow River Event, an intense and geologically continuous period of mafic-ultramafic magmatic activity commenced in both the North and South Australian Elements. Activity is confined to a distinctive north-south belt in the South Australian Element, and this has parallels with the evolution of a north-south belt on the Queensland side of the North Australian Element. The continuity of magmatism in this period invites consideration of plate boundary tectonic processes. Only two of the six magmatic events of this period are correlated in the West Australian Element. The ~1780 Ma Hart LIP is preserved in five disconnected magmatic regions across all the Major Crustal Elements: a component within the Kimberley province; a separate east-west magmatic belt in the West Australian Element (south Pilbara and Paterson provinces); another east-west belt at the south margin of the North Australian Element (part of Aileron province), and within two north-south belts in the South Australian Element and the Queensland area of the North Australian Element. Time equivalence alone does not support comagmatic relations and detailed geochemistry and other work will be required to test the equivalence of the disconnected ~1780 Ma magmatic belts. It is notable that the two north-south belts, and the two east-west belts, would be joined by closure of the later NW-trending Central Australian Element that now separates them.
Figure 12: Australian Mafic-Ultramafic Magmatic Events ~1720–1530 Ma. Following the ~1780 Ma Hart LIP, the West Australian Element entered a ~300 Ma hiatus in mafic-ultramafic magmatic activity Mafic-ultramafic magmatic activity in the North and South Australian Crustal Elements in the same period is geologically continuous. There is no hiatus between the last magmatic event on the previous map (~1750 Ma) and the first magmatic event on this map (~1720 Ma). Within the North Australian Element the record is confined to the south and northeast margins and there is diachroneity over time. Regions that preserve the earlier (~1850–1750 Ma) events became quiescent, and the focus of magmatism shifted to the margins. The continuity and diachroneity invite consideration of plate boundary tectonic processes. The same series of events is correlated across the South Australian Element, now separated into the Curnamona and Gawler provinces by later development of the Adelaide province. The ~1590 Ma Curramulka Event is notable for correlating across a very wide extent of both the North and South Australian Elements, and as the first magmatic event recorded in protoliths of the Musgrave province in Central Australia. Following the ~1590 Ma Curramulka Event, both the North and South Australian Elements entered a 300 Ma hiatus in magmatic activity.
Figure 13: Australian Mafic-Ultramafic Magmatic Events ~1470–1130 Ma. The ~1415 Ma Loongana Event commences a series of poorly known mafic-ultramafic Magmatic Events which form an apparent northeast belt extending from the Albany–Fraser province in Western Australia. Much of the province is buried beneath later cover and its evolution is yet to be established in detail; it is probable that further mafic-ultramafic rock units, and events, remain to be discovered. A sill complex in far north Australia is time-equivalent with, and along strike from, the ~1310 Ma Fraser Event in the Albany–Fraser province. The ~1210 Ma Marnda Moorn LIP is also focussed on this northeast-trending margin of the Yilgarn province. Two later events extend the apparent NE-trending corridor. The ~1180 Ma Pitjantjatjara Event, known only within the Musgrave province; and the ~1135 Ma Mordor Event which includes an alkaline-ultramafic intrusion and coeval ENE-trending dolerite dykes in the Mount Isa province.
Figure 14: Australian Mafic-Ultramafic Magmatic Events ~1070 Ma. The ~1070 Ma Warakurna LIP is an isolated magmatic event whose implied extent is a correlation of three distinct mafic-ultramafic igneous provinces across the West, Central and North Australian Crustal Elements. As with the earlier ~1780 Ma Hart LIP and ~1210 Ma Marnda Moorn LIP, the proposal that this correlation is a single LIP is based only on time equivalence of rock units: the detailed geochemistry, and other attributes that would substantiate comagmatic relationships, are yet to be established. The ~1070 Ma mafic-ultramafic rock units define a broad east-west belt across the south of the Pilbara province, the northern margin of the Yilgarn province, and the Capricorn province between them. There is no evidence that it extends into the Pinjarra province so the western LIP extent is drawn at that boundary. In Central Australia the correlation includes the poorly-dated Giles Complex mafic and ultramafic intrusions in the Musgrave province. The age of this important series of large and mineralised intrusions is being questioned (Smithies et al., 2009). A coeval dolerite dyke swarm at the southern margin of the North Australian Element has a north-south orientation, orthogonal to the overall east-west trend of the Warakurna correlation.
Figure 15: Australian Mafic-Ultramafic Magmatic Events ~975–825 Ma. The ~825 Ma Gairdner LIP was originally proposed from the correlation of northwest-trending dyke swarms in South Australia and the Musgrave province with coeval lavas and sills in the basal stratigraphy of the Centralian Superbasin. In the construction mapped here, the distribution is widened to include similar coeval sills in the basal stratigraphy of the West Officer Basin, and evidence that the LIP extends under cover in Paterson province. This northwest-trending belt is the initiation of a new pathway for mantle-derived magma through the Australian lithosphere, coinciding with the initiation of the Centralian Basin system. It contrasts with earlier magmatic events in the time series which clearly re-use pre-existing magmatic belts oriented either east-west or north-south. As a relatively young (Neoproterozoic) magmatic event, shallow erosion levels of this LIP are exposed – hypabyssal dykes and sills, with some lavas also preserved.
Figure 16: Australian Mafic-Ultramafic Magmatic Events ~775–575 Ma. Commencing at ~775 Ma, a series of mafic-ultramafic Magmatic Events is recorded at the west and east margins of the Proterozoic continent. The ~775 Ma Boucaut Event is known only from a single occurrence at the eastern margin of the South Australian Element. It was followed 20 Ma later by the ~755 Ma Mundine Well dolerite dyke swarm at the opposite margin of the continent. The ~575 Ma Skipworth Event includes mafic-ultramafic rock units on King Island and adjacent Tasmania, and isolated occurrences of both mafic and ultramafic rock units forming an apparent belt approximately parallel with the present Australian coastline.
Figure 17: Australian Mafic-Ultramafic Magmatic Events ~510 Ma. The ~510 Ma Kalkarindji LIP was originally defined by the correlation of early Cambrian basalt lavas across northern Australia. Sills in the West Officer basin are coeval and geochemically comagmatic, and this broadens the extent to include parts of the West Australian Crustal Element. The preservation reaches its thickest development adjacent to the Kimberley province. A contemporaneous northwest-trending rift zone is indicated by the incursion of the Cambrian–Ordovician Larapinta Seaway across Central Australia. Much of this zone is now buried beneath marine sedimentary rocks, but uplift has exposed deeper crustal correlatives: dolerite dykes in the Kimberley province, and at the south margin of the North Australian Element where dated ~510 Ma mafic rocks, subsequently deformed and metamorphosed at deep-crustal levels during the Ordovician, are exposed (Maidment, 2005). The inference that the Larapinta Seaway zone was the focus of magma passage for the Kalkarindi LIP requires further detailed research.
A reference for global LIP correlations
While the focus of LIPs is on large magmatic systems, we suggest that it is vital to consider them in time-space context with every known mafic-ultramafic magmatic event, large and small. This is because the many smaller and local occurrences of mantle-derived magmatism share temporal and spatial characteristics with the larger systems, and so they can express important elements of the shared geodynamic controls.
Precambrian mafic-ultramafic magmatism in Australia has been resolved into a series of fifty-six magmatic events, each defined as a short period of less than 20 million years in keeping with the resolution of current geochronology. Correlations across Australian provinces have been constrained, and this is a reference ‘barcode’ series for correlation with other continents (cf. Bleeker and Ernst, 2006, Ernst and Bleeker, 2010).
The Australian Precambrian record includes protracted periods of mafic-dominated tholeiitic magmatism in the form of flood basalts, mafic dyke swarms and sills, and mafic±ultramafic intrusions. It also includes long periods of relative quiescence punctuated by widely separated magmatic events. Approximately one-third of all the Precambrian magmatic events took place during two intensely magmatic periods: in the Archean between ~2750 Ma to ~2650 Ma, and in the Paleoproterozoic from ~1870 Ma to ~1590 Ma.
Five major LIPs, formed by the rapid and voluminous emplacement of mafic-dominated magmas, have left a record of intrusions, dyke swarms and lavas across extensive regions of the continent. The new maps show some of them to be more extensive than previously thought. Their geometries share important aspects of spatial location, with controlling lithospheric-scale directions meeting in the centre of the continent: potentially, this is a pointer to the locations of repeatedly re-used crustal-scale feeder zones for mafic-ultramafic magmatism.
The five LIPs are shown in context with the geophysically-defined Australian Crustal Elements. It is probable that some provinces were not in their current configuration at the time of emplacement of some of the igneous events. Some provinces may not yet have formed when earlier Precambrian magmatic events occurred. The numerous small magmatic events provide new context on the secular evolution of the continent and the place of mafic-ultramafic magmatism in its development.
The comprehensive Australian map series is made available as a basis for new studies of Large Igneous Provinces: their geochemistries, origins, and potential for mineralisation. The physical geometries of LIPs and the smaller igneous provinces are significant in themselves, because mafic-ultramafic magmatic rocks are the result of physical anomalies within the mantle that give rise to partial melting, and of physical passage through the overlying lithosphere to surface and near-surface emplacement. The spatial expressions are a framework for geochemical studies which may not otherwise be diagnostic of different origins, unless the magmas happen to have sampled a segment of mantle or crust that is distinctive. Such studies will be essential to confirming the proposed LIP status of the Warakurna, Marnda Moorn and Hart Magmatic Events, which in each case is currently based only upon age equivalence of mafic-ultramafic rock units. It may also lead to the definition of other Australian LIPs, from among the larger Magmatic Events that are mapped in the compilation.
References
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