Australia: The Land Where Time Began
Attributes of LIPs -
A defining characteristic of LIPs is the emplacement of large volumes of magma in a geologically short, finite period of time in a focused area. This allows LIPs to be distinguished from normal processes at plate boundaries where magmas can be generated on a LIP-scale, i.e. mid ocean ridges and subduction zones, if they are considered over a period of time and areal extent. E.g., it has been pointed out (Sheth, 2007) that the world-wide network of mid-ocean ridges, that is 50,000 km long, which has an average half-spreading rate of 5 cm/year, generates 5 Mkm2 of oceanic lithosphere that is about 7 km thick in as little as 1 million years, though Ernst points out that it should be noted that the production rates for LIPS have been estimated to be about 10 to more than 100 % greater that emplacement rates at mid-ocean ridges.
The determination of timing and duration of LIP events depends strongly on the quality of the age data, sample quality and the degree of weathering/alteration, the resolution that can be achieved by the dating technique used, and the availability of data, and the U-Pb method has proven to be of particular usefulness (Heaman & LeCheminant, 1993; Courtillot & Renne, 2003; Söderlund et al., 2013). The study of LIPs has been at variable levels of detail, and in terms of the reliability, technique, and quantity, age data are extremely variable. It appears that there is a maximum possible duration of LIPs of up to about 50 Myr (Ernst & Buchan, 2001a). There are, however, many LIPs that have durations of less than 10-15 Myr, and in many cases emplacement is completed in only a few million years or less (Hofmann et al., 2000; Courtillot & Renne, 2003; Jerram & Widdowson, 2005; Blackburn et al., 2013). This applies to the flood-basalt component (e.g. Hofmann et al., 2000) and the dyke-swarm components (Heaman & LeCheminant, 1989, 1991). Also emplaced rapidly were layered intrusions (Cawthorn & Walraven, 1998) (see also Cawthorn & Webb, 2013), such as the Bushveld Complex, which is part of the Bushveld LIP, from the Palaeoproterozoic, that is 9 km thick, and is estimated to have been emplaced in as little as about 75,000 years.
Though dating is really only available for the top few hundred metres of oceanic plateaus sampled at Ocean Drilling Program (ODP) drill sites, these plateaus can also have been emplaced rapidly in as little as 3 Myr. In some cases, such as the Kerguelen Plateau, it is indicated by the ODP results to span a minimum of 25 Myr of volcanic activity, though Ernst suggests a number of magmatic pulses and hiatuses are likely to be represented by this, instead of continuous magmatism (Tejada et al., 2002).
Pulsed nature of Magmatism
A complex igneous history of LIPs has been revealed by volcanological and detailed stratigraphic studies combined with systematic dating. It has been shown by geochronological studies that LIPs with ages spanning more than 20 Ma were emplaced by multiple pulses of shorter duration of about 1-5 Myr rather than as a continuous, longer-lasting magmatic event (e.g. Tolan et al., 1989; Saunders et al., 1997; Courtillot & Renne, 2003; Jerram & Widdowson, 2005; Storey et al., 2007a; Ernst et al., 2008). Evidence for a single short pulse, in comparison, and rapid emplacement of a large volume of magma is expressed well in the Columbia River flood-basalt province, the youngest and smallest LIP where more than 90 % of the total volume, about 0.123 Mkm3 was erupted between 16.6 and 15.3 Ma, though basaltic eruptions occurred between 17 and 6 Ma (Tolan et al., 1989; Camp et al., 2003; Hooper et al., 2007; Reidel et al., 2013).
In some continental LIPs 2 distinct pulses may correspond to pre- and syn-rift magmatic events. The North Atlantic Igneous Province (NAIP) exemplified this, where the first pulse occurred from 62-58 Ma, magmatism corresponded to the emplacement of sequences of terrestrial continental flood-basalt, though the bulk of the volcanic sequences along the continental shelves forming the so-called “seaward-dipping reflector series” (SDRS), were emplaced during the 2nd syn-rift pulse that occurred from 56-52 Ma (e.g. Saunders et al., 1997; Jerram & Widdowson, 2005; Storey et al., 2007a). It is also believed that zones of high-velocity lower crust were produced during continental breakup associated with rifting (Menzies et al., 2002a). The hiatus between distinct pulses of magmatism is variable, though it can be a few to 10s of millions of years. The relative extrusive volumes of pulses can vary, and the volume of the 2nd pulse may be greater than the 1st (Campbell, 1998; Courtillot et al., 1999; Storey et al., 2007a). During such pulses the igneous volumes emplaced represent a substantial proportion, more than 75 %, of the total volume of a LIP (Bryan & Ernst, 2008).
In detail, it has been indicated by comparative studies that the formation of several flood-basalt provinces took place during at least 3 phases of eruptive activity varying in duration from 0.1 to more than 5 Myr:
1. An initial phase of transitional-alkaline basaltic eruptions at very low volume;
2. The main phase of flood volcanism during which the bulk of volcanic stratigraphy was emplaced rapidly in repeated eruptions of large volume of tholeiitic basalt magmas, which were in some cases associated with silicic magma;
3. A more protracted phase during which volcanism was waning and the volumes of erupted material decreased rapidly, possibly becoming distributed more widely, or focused when rifting is taking place (Bryan et al., 2002; Jerram & Widdowson, 2005).
In the majority of SLIPs there is a lack of systematic dating combined with stratigraphic studies, as a result of which it is not known whether these events have well-defined, pulsed magmatic characters. The Chon-Aike Province of South America-Antarctica, from 188-153 Ma, has been divided into 3 main pulses of silicic volcanic activity, each of 5-10 Myr duration, based on U-Pb zircon and 40Ar-39Ar dating, that were associated with the emplacement of rhyolite magma with a volume of 0.05 to more than 0.1 Mkm3 (Pankhurst et al., 1998, 2000). For the Whitsunday SLIP pulses in volcanism occurred at about 118-113 Ma and 110-105 Ma, though it is indicated by age data that there was a main active period from about 120-105 Ma (Ewart et al., 1992; Fig. 3 of Bryan et al., 1997). Also, it has been shown by recent work that at the Sierra Madre Occidental (SMO) SLIP in Mexico, that the bulk of the rhyolite ignimbrites was emplaced in 2 pulses each of which were of about 4 Myr duration (about 38-20 Ma). The age, about 1-3 Ma, ranges for many exposed ignimbrite sections > 1km thick also emphasise a rapid emplacement (Ferrari et al., 2002, 2007; Swanson et al., 2006).
Ernst summarises LIPs as finite events that are dominantly mafic and no longer than about 50 Myr duration, and are characterised by a pulse or pulses of magmatic activity ≤ 5 Myr duration during which a large proportion, > 75 %, of the total igneous volume of the LIP was emplaced. Associated SLIPs have similar characteristics.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|