Australia: The Land Where Time Began
Large Igneous Provinces
There have been a number of periods in the history of the Earth during which there was increased magmatic activity, with the emplacement of especially large amounts of magma, mainly of mafic material, in a pulse or pulses of short duration that were not linked to processes occurring at plate boundaries. These Large Igneous Provinces (LIPS) can occur in continental or oceanic settings or even a mixture of the two. The volumes of magma erupted during these events can vary widely, from <0.1 Mkm3 to 80 Mkm3, and the duration of these events can range from a single pulse that takes 0.5 Ma to complete to multiple pulses that extend over 10s of millions of years. Associated with LIP events there can be huge silicic provinces, as can carbonatites and kimberlites. There are notable associations of LIPS with global or environmental changes that include extinction events, or regional topographic changes such as domal uplift or continental breakup or abortive breakup. They have also been associated with the formation of a wide range of ore deposits, as well as implications for the oil/gas industry and the flow of aquifers. A number of origins are considered but for many LIPs the evidence favours the involvement of mantle plumes. In this book Ernst provides an overview of all aspects of LIPs, beginning with the history of the term and a review of definitions.
Types of LIPs
Continental + Oceanic
· Archaean Greenstone Belts, Extensive Tholelite-Komatiite [continental]
· ± Rhyolite Volcanic Sequences and Sill Complexes [oceanic]
– Superior, Slave, Yilgarn Cratons
This LIP classification is based on initial work (Coffin & Eldholm, 1994) but incorporating recent advances in the recognition of ancient LIPs and SLIPs (Bryan & Ernst, 2008). Modified after Bryan & Ernst (2008) and Bell (2010)
An overview of LIPs over time
According to Ernst most of the research that has been carried out on LIPS has focused on dramatic flood basalts characterising Mesozoic-Cainozoic events, continental flood basalts and oceanic flood basalts – oceanic plateaus and oceanic basin flood basalts. In the LIP record these are parts that are generally well preserved and have been critical in the development of many key LIP concepts, of which their large size and short duration (or short duration pulses) are the most important.
Emplacement events that take place over short periods of time are evident from the main stage of flood-basalt magmatism consisting of monotonous sequences that can be up to several kilometres thick, comprised of large tubular flow commonly lacking any interlayered sediments that are of significant thickness; emplacement of flood basalts over time periods of 1-5 Ma is confirmed by dating that is of high-precision (Courtillot & Renne, 2003; Jerram & Widdowson, 2005).
Continental flood basalts are composed dominantly of mafic material (tholeiitic) with a minor component of ultramafic (picritic) material and transitional-alkaline components that are lower in the sequence, and silicic magmatism becoming increasingly more significant in the levels higher in the sequence. LIP-related rifted margins are associated with bimodal magmatism. Oceanic plateaus and oceanic basin flood basalts originate from variable sources in the mantle, and the same sublithospheric mantle sources are represented by continental flood basalts, together with the added aspect of interaction with lithospheric mantle and crust (e.g. Hofmann, 1997; Condie, 2003; Hawkesworth & Scherstén, 2007; Kerr & Mahoney, 2007).
The best preserved and best studied flood basalts are typically those from the Mesozoic and Cainozoic. The record from the pre-Mesozoic is more deeply eroded and therefore LIPs from the Palaeozoic and Proterozoic, in contrast, are typically recognised by the remnants of flood basalts and plumbing systems that are exposed represented by giant dyke swarms, sill provinces, and layered intrusions. During the closure of oceans oceanic LIPs are preserved incompletely and can occur as obducted deformed sequences in orogenic belts. The most promising candidates for LIPs are greenstone belts that contain sequences of tholeiite-komatiite.
Erosion affects LIPs of pre-Mesozoic more greatly, which largely removes their flood basalts and exposes their plumbing systems. As a result of this continental LIPs from the Palaeozoic and Proterozoic typically consist of giant dyke swarms, which are those >300 km long, large layered intrusions, and remnants of flood basalts (Ernst & Buchan, 1997, 2001a). Like their flood-basalt equivalents from the Mesozoic-Cainozoic, this class of LIPs, which are intrusive-dominated, has large areal extents and volumes, exhibits pulses that are of short duration, and that has “intraplate” character, which is consistent with definition as a LIP (Coffin & Eldholm, 1994, 2001, 2005; Ernst et al., 2005; Bryan & Ernst, 2008; Bryan & Ferrari, 2013).
The Fortescue sequence of the Pilbara Craton, Western Australia, and the Ventersdorp sequence of the Kaapsvaal Craton, South Africa, are among the erosional remnants of typical flood-basalt provinces from the Archaean. Most volcanic rocks from the Archaean occur, however, as deformed, fault-fragmented packages that have been referred to a greenstone belts. There is 1 class of greenstone belt that contains igneous rocks that are mafic to silicic with geochemical signatures of calc-alkaline and is interpreted to be arc-related. The other major class of greenstone belt is comprised of tholeiite-komatiite sequences, which are considered to be the best candidates for being the remnants of LIPs from the Archaean, In terms of settings, these Archaean LIPS include accreted oceanic plateaus as well as those emplaced in a platform setting.
LIPs on other planets
There is evidence of intraplate magmatism, which includes LIPs, that has been found on other planets. When the large-volume intraplate magmatism on the Earth is compared with those of other planets insights are provided to the record of LIPs on Earth. Between ~2,600 Ma and 180 Ma the frequency of production of LIPs, which are mainly of the continental type, is relatively constant back to 2.6 Ga, at an average frequency of about 1 per 20 Myr. Ernst says the record of pre-Mesozoic LIP production underestimates the actual rate, as most oceanic plateaus do not survive the subduction process and are difficult to recognise in orogenic belts from the Mesozoic. If the frequency of the oceanic LIP record observed in the Mesozoic-Cainozoic continues back through time the average rate of LIP production, including oceanic and continental LIPs, back to the Archaean, may be closer to 1 event in 10 Myr. However, multiple independent LIPs can occur at the same time (plume clusters) which would skew the average rate of LIP production to a smaller value, as has been noted (N. Dobretsov, pers. Comm. to Ernst, 2007). The average rate of LIP production is closer to 1 per 30 Myr, by his estimate, therefore the combined production of oceanic and continents LIPs would be closer to 1 per15 Myr.
A number of models have been proposed to explain the origin of LIPs. Among these are mantle plumes that originate at the core-mantle boundary (e.g. Richards et al., 1989; Campbell & Griffiths, 1990; Campbell, 2005, 2007; Dobretsov, 2005). There are other models proposed that include:
1. Impact-induced decompression (e.g. Jones et al., 2002; Ingle & Coffin, 2004);
2. Lithospheric delamination (Elkins-Tanton & Hager, 2000; Elkins-Tanton, 2005, 2007; Hales et al., 2005);
3. Decompression melting during rifting (White & McKenzie, 1989) or following the heating of the mantle beneath supercontinents (Coltice et al., 2007);
4. Edge-driven convection (King & Anderson, 1998);
5. Melting of fertile mantle without excess heat (Anderson, 2005) or
6. Shallow-melting anomalies generated by processes related to plate tectonics (“Plate” model of Foulger, 2007);
7. Stress-induced lithospheric fracturing and rapid drainage of a sublithospheric basaltic magma reservoir that accumulated relatively slowly (Silver et al., 2006);
8. Back-arc rifting (e.g. Smith, 1992; Rivers & Corrigan, 2000); or
9. Overriding by a continent of a spreading ridge (Gower & Krogh, 2002).
There is strong evidence favouring a dominant role for mantle plumes for many LIPs, with contributions from other mechanisms (especially decompression melting that is rift-related) which causes an additional pulse or pulses of LIP activity.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|