![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Essential
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: admin@austhrutime.com Sources & Further reading |