of LIP events, both mafic and felsic, through time.
Revision to Large Igneous Province Classification
Bryan1, & Richard
of Earth Sciences & Geography, Kingston University,
Penrhyn Rd, Kingston Upon Thames, Surrey KT1 2EE United
Geosciences, 43 Margrave Ave Ottawa, Ontario K1T 3Y2
This webpage has been
stimulated by the paper Sheth,
H.C., ‘Large Igneous Provinces (LIPs)’:
Definition, recommended terminology, and a hierarchical
classification, Earth-Science Reviews 85 (2007) 117–124.
here for a webpage summary of this paper.
Click here to go to
a general Discussion
of this subject.
The term “Large
Igneous Province” (LIP) was initially proposed
by Coffin & Eldholm (1994) to represent
a variety of mafic igneous provinces with areal
>0.1 Mkm2 that were “massive
crustal emplacements of predominantly mafic (Mg and
Fe-rich) extrusive and intrusive rock, and originated
via processes other than ‘normal’ seafloor
spreading”. The initial database upon
which the term LIP was defined relied almost exclusively
on the relatively well-preserved Mesozoic and Cenozoic
record that comprised continental flood basalt provinces,
volcanic passive margins, oceanic plateaus, submarine
ridges, seamount groups and ocean basin flood basalts
(e.g., Coffin & Eldholm, 1994, 2005).
Since the first categorisation of LIPs, substantial
progress has been made in extending the LIP record
back to the Paleozoic, Proterozoic and Archean
Buchan, 1997; 2001, 2003;
Tomlinson & Condie, 2001; Arndt et
al., 2001; Isley & Abbott, 2002).
For many ancient LIPs, where much or all the volcanic
component of the LIP has been lost to erosion, definition
has to be based on the areal extent and volume of
intrusive rock (e.g., giant continental
dyke swarms, sills, layered intrusions), which is
the exposed plumbing system to the province. In addition,
it has additionally been recognised that LIPs are
also massive crustal
emplacements of predominantly silicic
(>65 wt% SiO2) extrusive and intrusive
rock that have originated via processes other than ‘normal’
seafloor spreading or subduction (see Bryan
2002). Consequently, the increasing realisation
that LIPs are more varied in character, age and
composition than first defined has prompted others
to revise and broaden the original definition of
2007; click here for a webpage
summary of this paper).
Our contribution to LIP
classification (Figure 1) has been stimulated by the
newly proposed classification scheme of Sheth (2007).
Like Sheth (2007),
we also consider LIP to be a broad category and it
must encompass a greater variety of igneous provinces
than previously considered by Coffin & Eldholm (1994;
2005). However, in contrast to Sheth (2007) who proposes a minimum areal extent of 0.05 Mkm2,
we argue that the original size definition of at least >0.1
Mkm2 be retained; this is the minimum areal
extent for flood basalt events based on the initial
work of Coffin & Eldholm (1994). Studies
of the areal extents of the exposed plumbing systems
and intrusive provinces of LIPs are also consistent
with this (e.g., Yale & Carpenter,
Marzoli et al., 1999; Ernst
2005). Major regional continental dyke swarms
for example, are >300 km in length (Ernst & Buchan,
1997) and typically have areal extents >90,000 km2;
this approaches the minimum areal extent considered
for the continental flood basalt provinces. Similarly,
Silicic LIPs have dimensions well in excess of 0.1M
km2 (all are >0.5 Mkm2) and
igneous volumes >0.25 Mkm3 (Bryan
2002). The review of several classic Cenozoic-Mesozoic
LIPs by Courtillot
(2003) concluded that the minimum dimensions of
LIPs should be >1 Mkm2. Most LIPs have
areal extents and eruptive and/or intrusive volumes
well in excess of 1 Mkm2 and 1 MKm3,
respectively, and thus are anomalous events emplacing
tremendous volumes of magma throughout the crustal
profile and at the Earth’s surface. We therefore
conclude that LIPs should be defined as having the
minimum dimensions of >0.1Mkm2 and
Figure 1. Proposed
classification of LIPs, based on the initial work of
Coffin & Eldholm (1994), but incorporating recent
advances in the recognition of ancient Mafic LIPs and
Silicic LIPs. Representative examples of the various
types of LIPs are given.
We subdivide LIPs into
two compositional groupings: “Mafic LIPs”
and “Silicic LIPs”. We note that it is
possible to subdivide LIPs in other ways, such as
by age (e.g.,
Archean, Proterozoic, Phanerozoic), size (e.g,
& Ernst, 2006), crustal setting (continental
vs. oceanic) and whether primarily intrusive or extrusive
2007). The review of Bryan
(2002) concluded that there is a clear compositional
demarcation for LIPs where Silicic LIPs have ≤10%
by volume of mafic igneous rocks, and mafic LIPs,
as represented by continental flood basalt provinces,
≤10% by volume of silicic igneous rock. Importantly,
in the current LIP database (Ernst & Buchan,
2001), there is an absence of LIPs with subequal
proportions of mafic and silicic igneous rocks. Silicic
igneous rocks do occur independently of crustal setting
and as demonstrated by the recent ODP results from
the Kerguelen oceanic plateau (Frey
2003), should not be assumed to be absent from
LIPs in oceanic settings. It is important to stress
that Mafic and Silicic LIPs are compositionally
bimodal, and may also show a spectrum of compositions
from basalt to high-silica rhyolite, but the prefix
modifier reflects the overwhelming volumetric dominance
of that composition. Whereas the presence of silicic
igneous rocks is an additional characteristic of
Mafic LIPs, and vice versa for the Silicic LIPs,
they are not required for categorization. We emphasise,
however, that ultramafic magmatism is an important
but usually minor component of Mafic LIPs.
Mafic and Silicic LIPs
vary in terms of the proportional volumes of volcanic
and intrusive rock preserved, which reflects the degree
of exhumation and age of the LIP, and potentially,
lithospheric conditions at time of LIP emplacement
that may have promoted eruption or magma underplating/intrusion.
The extrusive and intrusive components of LIPs are
fundamentally related. For example, giant continental
radiating dyke swarms are part of the intrusive architecture
of a continental flood basalt province. For these
reasons, we have not followed the approach of Sheth
(2007) of subdividing LIPs into primarily volcanic
or plutonic types.
Mafic LIPs can simply
be subdivided into those occurring on the continents
or in oceanic settings. Silicic LIPs are exclusively
continental as they are produced by large-scale and
prolonged crustal anatexis (Bryan
2002). The products of continental Mafic and Silicic
LIPs can, however, be deposited in oceanic settings
Peate et al.,
2003), but their primary sites of eruption and accumulation
are on the continents.
Mafic LIPs are then further
subdivided into different types based partly on the
various provinces recognised and discussed by Coffin
& Eldholm (1994), but updated and expanded
to include those major ancient LIPs in which the dyke
swarms and mafic-ultramafic intrusive provinces are
dominant. A separate grouping is also made for greenstone
belts of tholeiitic and komatiitic rocks that may be
mafic LIPs of Archean age.
The subgroupings of Mafic
LIPs also have temporal significance, as the character
of mafic LIPs varies through time:
- Greenstone belts of the tholeiite-komatiite
association in the Archean
- Proterozoic-Palaeozoic Mafic LIPs occurring
as eroded flood basalts with exposed plumbing systems
of dykes, sills, layered intrusions, and
- Continental flood basalts, volcanic
rifted margins and oceanic plateaus in the Mesozoic
Silicic LIPs were defined
(2002), but have not been subdivided further. The
Mesozoic-Cainozoic examples are the best preserved,
and their general architecture is as an extensive (>0.5
Mkm2), relatively flat-lying ignimbrite plateau
of ≥1 km thickness (e.g., Sierra Madre
Occidental). More ancient examples occur as continental
caldera systems and major batholiths (e.g.,
Kennedy-Connors-Auburn province). Silicic LIPs are expected
to have similarly extensive mid- to upper-crustal granitic
batholith underpinnings and dyke swarms, and more mafic
igneous underplating at lower crustal depths. The identification
of deeply exhumed Silicic LIPs that are comparable to
the giant continental dyke swarms and mafic-ultramafic
intrusive provinces of the Mafic LIPs awaits further
discovery. A provisional subdivision of Silicic LIPs
is made here into those that occur on or form volcanic
rifted margins, and those that likely represent failed
rifts. The Permo-Carboniferous Kennedy-Connors-Auburn
province of eastern Australia (Bain & Draper,
2003) is a possible example of this latter type.
Volcanic rifted margins can therefore be characterised
as Mafic or Silicic LIPs.
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last updated 3rd