Australia: The Land Where Time Began
Large Igneous Provinces – Intraplate Tectonic Setting
In the original definition (Coffin & Eldholm, 1992; 1993a, 1994) an integral part was that the processes involved in the formation of LIPs are not characteristic of modern plate boundaries, i.e. mid-ocean ridges and subduction zones. This independence was also emphasised (Halls et al., 2008) in which LIPs were considered to result from large-scale transient magmatic processes that were rooted in the mantle and they are not predicted by plate tectonic theory. Therefore, LIP events show the hallmarks of intraplate magmatism, though they involve extremely high volumes and extend over large areas.
In the tectonic and petrological and geochemical sense, the term “intraplate” has been used widely (Bryan & Ernst, 2008). The term “intraplate magmatism” generally implies that the generation of magma is so remote from the existing plate boundaries that it is not possible to associate it with the release of energy and tectonic processes that are located at mid-ocean ridges, subduction zones, and leaky transform boundaries, though the latter is less common (Johnson & Taylor, 1989; Neuendorf et al., 2005).
An intraplate tectonic setting for most LIP events has been demonstrated in the reconstructions of plates from the Mesozoic-Cainozoic that the initial emplacement of many continental LIPs was in the interiors of tectonic plates, as well as on or along the current edges of Archaean cratons in areas where incipient rifting was taking place. This also applies to dyke swarms which radiate into the interiors of plates from areas that eventually become the margins of continents following the associated rifting and breakup. Also, it is typical to find that these dyke swarms are linked to associated breakup events that resulted in the formation of the margins of new continents (e.g. Courtillot et al., 1999; Ernest & Bleeker, 2010), though at the time of the initial emplacement of the LIP (the initial pulse) it was at an intraplate location.
According to Ernst there can be a second pulse of magmatism by LIPs that is linked to continental breakup and ocean opening (e.g. NAIP and Kerguelen Bunbury-Comei LIP). It is, however, important to recognise that in such cases intraplate settings, that were in existence before the onset of rifting and opening of an ocean, were the sites of the initial pulse of the LIP. Also, complete rifting and ocean basin formation, does not accompany all LIPs. As exemplified by the Emeishan, Siberian Trap, and Columbia River flood-basalt provinces, which have provided the most recent examples, with an older example of this type of setting being provided by the Keweenawan LIP.
Most continental LIPs were emplaced into stable regions of continents with a long history, commonly of hundreds of millions of years, during which there was no prior magmatism or deformation by contraction. As a consequence, the abrupt emplacement of voluminous magmas in regions of stable continental regions is what makes LIP events so distinctive and anomalous, underlining the intraplate character of these events.
It is also apparent that oceanic plateaus and flood-basalts in ocean basins have been emplaced in interoceanic (intraplate) settings, and many of these LIPs have remained undeformed, apart from edges that have subsequently entered subduction zones (Mann & Taira, 2004; Petterson, 2004). It appears the Shatsky Rise is anomalous as it was sited along an active ridge triple junction, being emplaced into a region of oceanic crust that was affected by multiple spreading ridge jumps (Nakanishi et al., 1999; Sager, 2005), though importantly, subsequent to ridge migrations it is now in a mid-plate setting.
Many oceanic LIPs share the same relationships between breakup and magmatism timing that has been observed in multi-pulse continental LIPs. Though the initial pulse of an oceanic LIP may be intraplate, subsequent pulses of such LIPs can be associated with new spreading centres, such as the Ontong Java-Manihiki-Hikurangi oceanic plateaus (Taylor, 2006); Kerguelen-Broken Ridge plateaus (Frey et al., 2003); Agulhas-Maud Rise plateaus (Jokat et al., 2004; Gohl et al., 2011). In cases where the development of a new mid-ocean ridge system is predated by the initiation of such LIPs, or where it post-dates the cessation of seafloor spreading (e.g. flood basalts in ocean basins), these LIP events can therefore be classified as “intraplate” in a tectonic sense.
It is less applicable to characterise those LIP events that initiate close to active plate boundaries, such as the Columbia River and Shatsky Rise LIPS, as intraplate in the tectonic sense. For LIP events that occur in regions that are tectonically active, such as mobile belts between cratons or along continental margins where an ocean is closing as continents approach a collision (e.g. Circum-Superior LIP at about 1,800 Ma; Heaman et al., 2009; Ernst & Bell, 2010; Minifie et al., 2013) an intraplate setting may be less evident.
For the Cainozoic LIPs of North America that were emplaced along the margin of the continent, that had a history of magmatism and deformation related to subduction immediately before and after the LIP events, an intraplate tectonic setting is particularly problematic. Specifically, geographically, the Columbia River flood-basalt province and the Sierra Madre Occidental SLIP overlap with earlier magmatism related with subduction. It has been argued, given this regional setting, that the Columbia River LIP may result from back-arc spreading (e.g. Carlson & Hart, 1088; Smith, 1992).
LIP events at plate margins are, however, distinctive from magmatism that is spatially associated and synchronous with plate margins in terms of their extent (both in terms of total area and extent inboard from the margin of the plate), volume, eruption rapidity and rates of melt production, association with extension, and composition. The largest and earliest to be emplaced flood-basalt formations of the Columbia River LIP have geochemical similarities with oceanic island tholeiites, as well as other intraplate compositions of magma and therefore display a stark contrast to neighbouring magmatism that is related to subduction (e.g. Hooper, 1997; Hooper et al., 1997). According to Ernst the intraplate characteristics of the LIP event in these cases are based more on petrological and geochemical features, and other features such as the production rates of melts if they these are well constrained, where the LIP has distinctly different compositions from those that have formed at mid-ocean ridges and subduction zones. For continental LIPs, however, the frequent addition to flood basalts and rhyolites of lithospheric geochemical signatures makes it difficult to recognise an intraplate signature. In the case of SLIPs such as the Sierra Madre Occidental, in which both rhyolites and basalts have geochemical signatures that are transitional between within-plate convergent margin fields on trace-element discrimination diagrams (Bryan, 2007).
Finally, even LIPs spatially associated with plate boundaries can have an “intraplate setting” in the following sense. Ernst suggests that it is reasonable that ascending plumes will not have a preference for a particular tectonic setting in the upper mantle if it is accepted that LIPs represent magmatism that is derived from deep plumes that are ascending (see Section 14.3 in Arndt et al., 2008). Therefore a percentage of mantle plumes that are deep-sourced will ascend into an existing plate boundary setting in the upper mantle in spite of not having a tectonic link with such a setting. Ernst suggests another plausible scenario in which a plume ascends beneath a craton that has a thick lithospheric root in which case it slides sideways and upwards along the craton root towards thin spots in the interior or along an edge (or edges) of the craton (e.g. Thompson & Gibson, 1991; Sleep, 2003; Begg et al., 2010; Bright et al., 2014), and such a scenario can potentially be applied to the Circum-Superior LIP (e.g. Ernst & Bell, 2010).
The petrological, geochemical, and isotopic characteristics are important indications of an intraplate setting, which can be used to discriminate between tholeiites related to a LIP within a plate and magmatism at the margins of a plate (mid-ocean ridge basalt (MORB) and basalts that are related to subduction. Also distinctive are the volume and magma generation rate in LIP events, and when combined with geochemical LIP characteristics have led many workers to regard these features as being related to hot mantle upwellings with magmas that have been sourced from the asthenospheric mantle or plume, lithospheric mantle below continents, and the depleted asthenospheric mantle (Carlson, 1991; Turner & Hawkesworth, 1995; Hofmann, 1997; Condie, 2003; Ewart et al., 2004a). Major causes of inter- and intra-LIP variation are crustal contamination and heterogeneity of the source, which can in some cases, lead to signatures in flood basalts and rhyolites that indicate they are apparently related to subduction. The least contaminated basalts and picrites in many continental LIPs have isotope and geochemical compositions that are similar to basalts from island arcs, i.e. magma suites that are high in Ti, that reinforce the intraplate characteristics of LIP magmatism.
The intraplate criterion in the LIP definition after Bryan and Ernst (2008) includes any of these:
Magmatism that is remote from any plate boundaries that are concurrently active;
Magmatism that occurs in stable crustal regions that have long histories that don’t include prior magmatism or constructional deformation;
Magmatism that occurs in the interiors of plates that are undergoing extension;
Magmatism that has been initiated in an intraplate setting (as defined in the first 2 points), but where there may be subsequent pulses of the LIP in association with ocean-ridge spreading systems that are newly forming;
Characteristics of composition (“intraplate” or “within-plate”) that are distinct from magmatism related to plate boundaries;
Magmatism that is linked to a mantle plume that originated from the deep mantle and which ascends randomly into tectonic settings of the upper mantle.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|