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“Large Igneous Provinces (LIPs)”: Definition, recommended terminology, and a hierarchical classification

Hetu C. Sheth

Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai, Bombay
(Mumbai) 400 076 India.


hcsheth@iitb.ac.in or hcsheth1@yahoo.co.in

 

This webpage is a condensed and modified version of the paper: Sheth, H.C., ‘Large Igneous Provinces (LIPs)’: Definition, recommended terminology, and a hierarchical classification, Earth-Science Reviews 85 (2007) 117–124.

Click here for Discussion of this page

Click here for a counter proposal "Proposed Revision to Large Igneous Province Classification" by Bryan & Ernst.

Abstract

The term Large Igneous Province (LIP) has been widely applied to large flood basalt provinces (e.g., Deccan), and the term Silicic Large Igneous Province (SLIP) to volcanic provinces of dominantly felsic composition (e.g., Whitsunday, Australia). Neither of these terms has been applied to the large granitic batholiths of the world to which both are perfectly applicable. LIP has also not been applied to broad areas of contemporaneous magmatism and sizeable layered mafic intrusions. I suggest use of the term LIP only in its broad sense and propose a minimum area of 50,000 km2 for a LIP.

I present a simple hierarchical classification of LIPs that is independent of composition, tectonic setting, or emplacement mechanism. I suggest that volcanic provinces such as the Deccan Traps and Whitsunday (Australia) be called Large Volcanic Provinces (LVPs), and large mafic intrusions, dyke swarms, and other intrusive provinces be called Large Plutonic Provinces (LPPs). LVPs and LPPs together cover all large igneous provinces (LIPs), having felsic to ultramafic compositions of sub-alkalic and alkalic lineages, emplaced in continental and oceanic settings.

I subdivide LVPs into four groups:

  1. the dominantly/wholly mafic Large Basaltic Provinces (LBPs) (e.g., Deccan, Ontong Java);
  2. the dominantly felsic Large Rhyolitic Provinces (LRPs) (e.g., Whitsunday, Sierra Madre Occidental);
  3. the dominantly andesitic Large Andesitic Provinces (LAPs) (e.g., Andes, Indonesia), and
  4. the bimodal Large Basaltic-Rhyolitic Provinces (LBRPs) (e.g., Snake River-High Lava Plains; Dongargarh, India).

The intrusive equivalent of LRPs are the Large Granitic Provinces (LGPs) (e.g., the Andean batholiths), but a corresponding term for intrusive equivalents of LBPs is not necessary or warranted. The largest LBP, and LIP, is neither the Ontong Java, the Siberian Traps, or the Central Atlantic Province, but simply the ocean floor.

 

What is a Large Igneous Province (LIP)?

Fig. 1. A view of the spectacular, >1-km-thick exposed section through the Deccan flood basalts at Mahabaleshwar. Photo: Hetu Sheth, December 2005.

Large igneous province (LIP) implies a province of igneous origin that is large. Clearly, the term LIP should cover large volcanic and intrusive igneous provinces, of whatever emplacement mechanism and composition. However, the term LIP has been applied to and used only for “flood basalt” provinces that have been the subject of great interest and extensive research recently (e.g., Macdougall, 1988; Mahoney & Coffin, 1997; Sheth & Pande, 2004; Kerr et al., 2005; Foulger et al., 2005; Saunders, 2005; see www.largeigneousprovinces.org). They represent the eruption of enormous volumes of mantle-derived magma on the Earth’s surface in relatively short time periods. These provinces (e.g., Deccan, Fig. 1, and Siberian Traps) are unquestionably LIPs in the broadest sense of the term. However, large-volume felsic provinces such as the Sierra Madre Occidental in Mexico are also LIPs, as are the huge granodioritic batholiths of the world, such as those of Tibet-Himalaya, western North America, or the Andes (Fig. 2), and the largest mafic layered intrusions such as the Bushveld Intrusion. Indeed, continental rift zones such as the Rio Grande Rift and the Cameroon Line are LIPs. None of these involve flood basalts.

Coffin & Eldholm (1992, 1993, 1994) were amongst the first to use the term LIP in the current restrictive sense. They defined LIPs as “massive crustal emplacements of predominantly mafic (Mg and Fe rich) extrusive and intrusive rock which originate via processes other than “normal” seafloor spreading … [and] include continental flood basalts, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups and ocean basin flood basalts.” This cannot be a suitable definition of LIP since it excludes most types of LIPs. However, the term LIP continues to be used in this restrictive sense (Ernst et al., 2005; Saunders, 2005; www.largeigneousprovinces.org). I suggest that the term LIP be retained but in its broadest sense, but that new, necessary terms be introduced for various categories of LIP.

The definition of “large” is clearly subjective and flexible, and I suggest here 50,000 km2 as the lower limit for LIPs (see below). Volume would be a better size parameter than area, but area is usually easier to measure. Most LIPs are much larger than 50,000 km2. The Deccan, for example, covers 0.5 million km2 of western-central India today and has an estimated original extent of 1.5 million km2 (Wadia, 1975). The Siberian province is vastly larger (Reichow et al., 2002). The smaller of the flood basalt provinces, such as the Emeishan province (~250,000 km2; He et al., 2003) and the Columbia River province (~164,000 km2; Hooper, 1988; Ed: See also comment on Columbia River Province size), are also well above the lower size threshold I suggest.

 

Fig. 2 (at left). A llama overlooks the ruins of Machu Picchu, situated at 2500 m above sea level in the Peruvian batholith, a felsic large igneous province. Photo: Hetu Sheth,  November 2001.

 

Large Volcanic Provinces (LVPs) and Large Plutonic Provinces (LPPs)

Just as there are large mafic volcanic provinces as exemplified by the Deccan and the Columbia River Basalts, there are large felsic volcanic provinces dominated by highly evolved (broadly rhyolitic) lavas. Several such provinces (the so-called SLIPs) are described by Bryan et al. (2000, 2002; SLIPs webpage) and include the early Cretaceous Whitsunday province and the Sierra Madre Occidental. There also are large volcanic provinces dominated by andesite, and others that comprise subequal volumes of basaltic and rhyolitic lavas. For all these volcanic provinces, of whatever composition, I propose the term Large Volcanic Provinces (LVPs). Obviously, all LVPs are LIPs, but all LIPs are not LVPs.

After Pluto, the Roman god of the underworld, I suggest the term Large Plutonic Provinces (LPPs) for all intrusive provinces meeting the size requirements, whatever their composition, emplacement depth, and internal structure. This category includes mafic-ultramafic intrusions such as the Bushveld Complex, large granite batholiths such as the Andean and Tibet-Himalayan batholiths, and giant dyke swarms such as the Mackenzie dyke swarm of Canada (Ernst et al., 1995).

 

Large Rhyolitic Provinces (LRPs) and Large Granitic Provinces (LGPs)

The term “silicic LIP” or “SLIP” used for rhyolite-dominated large volcanic provinces (Bryan et al., 2000, 2002; SLIPs webpage) recognizes the fact that LIPs can be of felsic composition. However, this term should strictly include the big granitic batholiths of the world but does not. I propose that large volcanic provinces dominated by broadly rhyolitic rocks (i.e., the rhyolite-rhyodacite-dacite-trachyte compositional range, both sub-alkalic and alkalic lineages) be called Large Rhyolitic Provinces (LRPs). Besides the Whitsunday and the Sierra Madre Occidental provinces, the Neoproterozoic Malani province of northwestern India (Sharma, 2004, 2005) is a good example.

Plutonic provinces with the same compositions can similarly be called Large Granitic Provinces (LGPs). “Granitic” implies the compositional range granitegranodiorite-tonalite-trondhjemite. The Archaean-Proterozoic charnockite (hypersthenegranite) massifs of southern India (Rajesh & Santosh, 2004) are included in the LGP category. Both LRP and LGP are independent of the geodynamic origins and tectonic settings of these rocks.

 

Large Basaltic Provinces (LBPs)

I suggest that large volcanic provinces of dominantly basaltic composition be called Large Basaltic Provinces (LBPs). “Basaltic” implies the compositional range basaltic andesite-basalt-picrobasalt (and their alkalic equivalents). The rock type basaltic andesite belongs in this category because many flood “basalts” are in fact basaltic andesites. Examples include many Deccan lavas from the Western Ghats (see Fig. 4a on the Deccan page) and the Grande Ronde lavas of the Columbia River basalt province that constitute 85% of the volume of that province (Hooper, 1997; Ed: See also comment on Columbia River Province size).

 

Fig. 3 (at right). A thick pile of Palaeogene flood basalts on the Isle of Skye, Scotland. Photo: Hetu Sheth, September 2005.

Fig. 4. The Ofaerufoss in the great Eldgja (A.D. 934) fissure in southern Iceland. Photo: Hetu Sheth, September 2003.

It is to be noted that the term “LBP” refers not only to exclusively basaltic provinces that lack felsic rocks (e.g., the Columbia River province; Hooper, 1997) but equally well to dominantly basaltic provinces with subordinate amounts of more evolved rocks such as rhyolite and trachyte. Most continental flood basalt (CFB) provinces are of this type. Examples of LBPs thus include the Deccan, Columbia River, Rajmahal, Siberian, Emeishan, Madagascar, Karoo, Parana-Etendeka, Yemen-Ethiopia and North Atlantic Tertiary provinces (Fig. 3). The oceanic flood basalt provinces, the so-called oceanic plateaus (e.g., Ontong Java, Iceland, and Kerguelen; Fig. 4), are also LBPs. Large oceanic island-seamount chains such as the Hawaii-Emperor (Fig. 5) and the Ninety East Ridge also belong in the LBP category, as do broad areas of diffuse basaltic volcanism on the continents (e.g., Mongolia; Barry et al., 2003) and in the oceans (e.g., the South Pacific Superswell; Janney et al., 2000). The term “LBP” is independent of tectonic setting.

Fig. 5. Red-hot Kilauea lava in action. Photo: Hetu Sheth, July 2002.

 

Large Basaltic-Rhyolitic Provinces (LBRPs)

When felsic lavas and mafic lavas in an LVP each constitute nearly half of the total volume, they constitute a bimodal LVP with intermediate compositions more or less absent. These, I call Large Basaltic-Rhyolitic Provinces (LBRPs). There are fewer members in this category than the LBPs. Examples are the Snake River Plain-Oregon High Lava Plains province of the western U.S.A. (e.g., Jordan, 2005), and the Palaeoproterozoic (2.5 – 2.2 Ga) Dongargarh Group in central India (Sensarma et al., 2004).

 

Large Andesitic Provinces (LAPs)

I suggest the term Large Andesitic Province (LAP) for any andesite-dominated province that meets the LIP size requirement. No tectonic setting is implied, but it is clear that most andesites are erupted in subduction zones (Gill, 1981). This category includes as members andesitic belts along island arcs (e.g., Indonesia, Japan), active continental margins (e.g., the Ecuadorian-Colombian Andes, Peruvian-Chilean Andes, Cascades, and Mexico-central America), and continental collision zones (e.g., Iran-Turkey).

 

Which is the largest LIP?

The largest LBP and LIP is not the Ontong Java, Siberian Traps, or the Central Atlantic Magmatic Province. It is, of course, the ocean floor. Though Coffin & Eldholm (1992, 1993, 1994) would exclude volcanic provinces formed by “normal” seafloor spreading from their definition of LIPs, this exclusion is not warranted. Though the formation of the ocean floor is gradual and very long-lived, the production rate is high. It takes only a 500-km-long ocean ridge segment spreading with a half-rate of 5 cm/yr to create a 100-km-long (or 50,000 km2 area) expanse of new oceanic lithosphere in only 1 million years. The 50,000-km-long worldwide network of ridges, with this average half-spreading rate, creates 5 million km2 of oceanic lithosphere in just 1 million years. The average thickness of the modern oceanic crust is 7 ± 1 km (e.g., White et al., 1992), of which the basaltic part (pillow basalts and sheeted dykes) makes up ~2 km (Boudier & Nicolas, 1985; Nicolas, 1989). Thus, the lower size limit of 50,000 km2 proposed here for a LIP, and the areas and volumes of most LIPs, are small in relation to the output of the mid-ocean ridge system over comparable time scales.

 

A hierarchical classification of large igneous provinces

The foregoing discussion shows that the term LIP is broad and vague. For this reason I suggest the hierarchical classification of LIPs described above, which is summarised in Table 1. I note the following:

  1. The terms in boldface in Table 1, with the exception of “Large Igneous Province (LIP)” are proposed here for the first time. The terms in italics are either formal terms (e.g., flood basalts, island arcs) currently in vogue for these provinces, or informal (e.g., diffuse provinces).
  2. The classification proposed is independent of tectonic setting. No tectonic setting is excluded a priori, though most of the provinces listed belong to intraplate (continental/oceanic) or rifted continental margin. The LAP category includes members only from subduction zones.
  3. Any LAP may include other rock types – the only criterion to be satisfied by any province for inclusion in the LAP category is dominantly andesitic compositions of volcanic products and an area of at least 50,000 km2. Thus, the andesite-dominated, 1000-km-long and 50-60-km-wide Mexican Volcanic Belt with its considerable amount of alkalic, ocean-island-basalt-type magmatism (e.g., Sheth et al., 2000 and references therein; Verma, 2002; see also Mexico webpages) is included in this category.
  4. Large Basaltic Provinces (LBPs) or Large Volcanic Provinces (LVPs) in general will necessarily include, besides the lavas, their associated dyke swarms and intrusive complexes.
  5. The category of dominantly mafic, intrusive, continental LIPs includes the layered mafic intrusions and the giant dyke swarms. Both types of features may have been feeders to flood basalt provinces now lost to erosion. The famous Skaergaard Intrusion is quite small (100 km2) and forms part of the East Greenland flood basalt province, and thus does not feature in Table 1. The Dufek and Forrestal Intrusions of Antarctica that cover ~6,600 km2 (Ferris et al., 2003) similarly belong to the Jurassic Ferrar flood basalt province. Worldwide, only the Precambrian Bushveld Complex of South Africa (60,000 km2; Winter, 2001) reaches the lower proposed size limit for a LIP. Other well-known layered mafic intrusions (Duluth, Stillwater, Muscox, Kiglapait) are well below this size.
  6. Whereas “Large Granitic Provinces (LGPs)” is an apt term for broadly granitic (granite-granodiorite-tonalite) batholiths, a corresponding short and single term for dominantly mafic, intrusive, continental category (e.g., “Large Gabbroic Provinces”) does not seem possible. This is because many layered mafic intrusions have significant volumes of ultramafic rocks, and associated giant dyke swarms (e.g., Ernst et al., 1995) are mostly dolerite.

 

Table 1. Proposed terminology and hierarchical classification of the large igneous provinces (LIPs), with examples (Sheth,2007)

LARGE IGNEOUS PROVINCES (LIPs)

Extrusive/intrusive provinces of any composition and tectonic setting with a minimum area of 50,000 km2

EXTRUSIVE
(lavas: pyroclastics = 100:0 to 0:100, sub-alkalic: alkalic = 100:0 to 0:100)

LARGE VOLCANIC PROVINCES (LVPs)

INTRUSIVE

LARGE PLUTONIC PROVINCES (LPPs)

Dominantly or wholly felsic:

Large Rhyolitic Provinces (LRPs)

Dominantly or wholly andesitic:

Large Andesitic Provinces (LAPs)

Dominantly or wholly mafic:

Large Basaltic Provinces (LBPs)

Bimodal:

Large Basaltic-Rhyolitic Provinces (LBRPs)

Dominantly or wholly felsic:

Large Granitic Provinces (LGPs)

Dominantly or wholly mafic:

Continental only

Usually continental

Both continental & oceanic

Continental only

Continental only

Both continental & oceanic

“Silicic” LIPs:
Whitsunday,
Sierra Madre Occidental,
Malani

Island arcs:
Indonesia,
Japan

Active continental margins:
Ecuadorian-Colombian Andes,
Peruvian-Chilean Andes,
Cascades,
Mexico

Continental collision zones:
Iran-Turkey

Continental flood basalts:
Deccan,
Rajmahal,
Madagascar,
Karoo,
Ferrar,
Siberia,
Emeishan,
Columbia River,
Parana-Etendeka, Yemen-Ethiopia,
North Atlantic Tertiary,
Central Atlantic (CAMP)

Diffuse provinces:
Mongolia

The ocean floor

Oceanic plateaus:
Ontong Java,
Iceland,
Kerguelen,
Shatsky Rise,
Manihiki,
Caribbean

Oceanic island-seamount chains:
Hawaii-Emperor,
Ninety East

Diffuse provinces:
South Pacific Superswell

Snake River Plain-Oregon High Lava Plains,
Dongargarh,
Ethiopia (in part)

Orogenic/Anorogenic granitic batholiths:
Tibet-Himalaya,
Patagonia,
Peru-Chile Coastal Batholith,
Coast Range Batholith NW USA

Charnockite massifs:
Southern India

Layered mafic intrusions:
Bushveld

Giant dyke swarms:
Mackenzie,
Red Sea,
CAMP

Anorthosite massifs (size permitting)

Deeper portions of oceanic plateaus

 

Conclusions

The term Large Igneous Province (LIP) is loosely defined (Saunders, 2005), and no attempts have hitherto been made to evolve a more appropriate and accurate classification. In my opinion, LIP covers a very broad category of provinces larger than 50,000 km2 in area. I suggest separate terms for each of the several categories of LIPs and define them here on the basis of dominantly extrusive or intrusive emplacement and rock compositions. Correct terminology is important as it influences the way we approach scientific problems. The classification proposed here is simple and easy to use. I hope that it will be adopted and lead to more accurate and effective communication among scientists working in this field.

 

Acknowledgements

I am grateful to Ian Skilling, James White, and Thor Thordarson for valuable and thought-provoking reviews, and to Ninad Bondre, Kamal Sharma, Sarajit Sensarma and Gillian Foulger for several helpful comments.

References

last updated 3rd December, 2007

Discussion

9th March, 2006, Andy Saunders
Without getting into a protracted debate, I find the new classification interesting but too broad.  It seems to encompass all large provinces that have protracted magmatism and, technically, should also include the entire ocean floor, as Hetu says.  An important feature of LIPs, and one which makes their defintion rather more restrictive, is the period of emplacement which I have always assumed to be geologically short [here, 'short' is as fairly loosely defined as is 'large'].
 
Any useful classification scheme needs to give a least a glance towards process.  Thus LIPs are produced by unusual processes (plume or not), and are thus distinct from subduction – and MOR-related systems (but can overlap geographically and tectonically with these (viz Iceland and CRB).
 
I think Hetu has hit on an interesting topic; existing definitions are vague, as a number of us have indicated (see recent issue of Elements).  I'm not sure that this is necessarily a bad thing – we've all tried to define the terms 'plume' or 'lithosphere' - but the definition should not be too all-embracing.  If it does, it becmes unwieldy and won't, I believe, help further our understanding.
 
9th March, 2006, Mike Coffin
Here are two relatively recent encyclopedia articles of relevance:
Coffin, M.F., and Eldholm, O., 2001. Large igneous provinces, in Steele, J.H., Thorpe, S.A., and Turekian, K.K., eds., Encyclopedia of Ocean Sciences, Academic Press, London: 1290-1298.
Coffin, M.F., and Eldholm, O., 2005. Large igneous provinces, in Selley, R.C., Cocks, R., and Plimer, I.R., eds., Encyclopedia of Geology, Elsevier, Oxford: 315-323.

9th March, 2006, Don Anderson
I always thought that the duration and rapidity had to be controlled by the lithosphere (stress-valve) rather than mantle temperature, and that large-scale ponding prior to release was implied. I don't know if this glance toward process can be built into a definition. LIPs differ from non LIPs in that the process is finite in duration (the rates and volumes are not exceptional). This could, of course, reflect the size of the fertile blobs, or the duration of extension. The tectonic context is also necessary.

An updated definition of LIP may be in order ( and plume! ). Lavoisier said that you cannot pretend to have a science unless you have a language and definitions. Things have moved along since the Encyclopedia artiicles were published. The Elements volume is a fantastic update of the situation. But the ideas that mantle convection controls the plates rather than vice versa , and that magma volume and rate is a proxy for high T are still around. You have to decide whether volcanism, anomalous or not, and uplift and rifting, is cause or effect. The main changes in thinking are papers on melting instabilities and on the possibility that high homologous temperature, rather than high absolute temperature, is the key variable. Papers by Tackley, Stevenson, Raddick, Parmentier...on melt instabilities, and Abt, Rudnick, Kays, Cin-Ty Lee, Humphreys etc. on delamination, are important in the LIP discussion.

Although LIPs MAY require very large absolute temperature (rather than ponding, fertility, shallow recycling, delamination, homologous T, melting instabilities and lithospheric control), this should not be part of the definition. If short duration is part of the definition it should not then be stated that this is a characteristic of a plume, as Richards and Courtillot did; this is circular reasoning. Stress and fertility can change rapidly; temperature cannot.

9th March, 2006, Andy Saunders
I agree with you that process (source T or fertility, fundamental cause such as plume, EDGE convection, delamination etc) should not play a role in the definition or classification of a LIP. My initial comment was intended to mean that a definition or classification, to be useful, should bear the processes in mind. A similar approach is taken with the classification of most rocks, especially igneous (e.g, the broad division into volcanic and plutonic nods towards a recognition of emplacement style, but is not in itself dependent upon that process).

9th March, 2006, Richard Ernst
Here is my contribution to the current discussion of LIP definition. There are three points relevant to a definition of LIPs:
1) size
2) duration
3) setting

To these Hetu has proposed subclassifications based on magmatic composition (mafic, felsic intermediate) and whether the main magmatism is volcanic or intrusive (plutonic).

SIZE OF LIPS: See a proposal for a new classification of magmatic events (including LIPs) based on size which is extracted from the attached paper by Bleeker & Ernst (2006). Note that we retain the conventional minimum size of 100,000 km2, in disagreement with Hetu's suggestions of 50,000 km2
-----------------------------------------------------------------
Bleeker & Ernst (2006) "We suggest the following classification of event sizes, compatible with common usage in large igneous province terminology (e.g. Coffin & Eldholm 1994, 2001):
 
Giant (LIP):      >107 km3
Major (LIP):      106 - 107 km3
Substantial (LIP):      105 - 106 km3
Moderate:         103 - 105 km3
Small:               ≤103 km3
 
The first three categories (giant, major, and substantial) qualify the size of what are generally considered large igneous provinces (LIPs), with (eruptive) volume estimates on the order of one to several million cubic kilometres[1]. When intrusive and underplated volumes are considered as well, some of the largest LIPs would classify as true giants, e.g. the Ontong Java plateau at ca. 45x106 km3. The two smaller categories (moderate to small) describe sub-LIP scale events.

[1] Coffin & Eldholm (1994, 2001) and others (e.g. Ernst et al. 2005) have generally used a surface area of >105 km2, rather than volume estimates, to define LIPs.
-------------------------------------------------------------------------------------
 
DURATION OF LIPS:
Some LIPs are clearly very short duration events, Deccan, Siberian Traps, Mackenzie (1270 Ma). However, other events are clearly more protracted; Keweenawan (1116-1085 Ma), probably Parana, Iapetus-margin (615-550 Ma), Matachewan (2500-2450 Ma), and many potential Archean analogues. Many of the protracted events are actually characterized by multiple pulses (e.g. the NAIP, Blekinge Dalarna (975-948 Ma; main pulse at 948 Ma), and Central Scandinavian Dolerite Group event (1270-1248; 3 pulses)).  So a short duration is not a required component of the LIP definition.
 
SETTING
Ideally, LIP definition and classification should be based on observable/measurable characteristics, not be based on interpreted origin, plume, rifting, delamination. So Hetu's classification based on magmatic type (dominantly mafic, felsic intermediate, extrusive, intrusive) is interesting in that regard.  I certainly agree that discussion of the LIP definition is required. Do we go toward something similar to Hetu's, or do we stay with the traditional definition (e.g. Coffin & Eldholm definition) which links LIPs to process other than normal sea floor spreading, and which implicitly excludes normal subduction-related magmatism.  I look forward to further discussion on this.
 
But I disagree strongly on at least two points in Hetu's article:
1) It is a mistake to group the 1270 Ma Mackenzie with the Tibet-Himalayan batholiths  as LPPs. There is no insight or understanding gained by grouping a classic mafic-ultramafic event (rifting/plume related) with a classic felsic event (collision/subduction related). 
2) I think the 100,000 km2 minimum size for LIPs should be retained (see my point above).

10th March, 2006, Don Anderson
Size should not be in the definition if it is arbitrary. Is there a natural cut off?   Likewise for duration and rate and thickness. If these merge into non LIPs then there may be a continuum and artificial limits would not be helpful. LIPs occur when continents are converging and diverging. They also overlap MORB, back arc basin and arc basalts in chemistry.  The temperatures of the magmas or parents or source mantle would be helpful.

10th March, 2006, Gillian Foulger
Clearly size, duration, rate and thickness are all a continuum from very small to very large. Maybe instead of proliferating categories we should just dump the term "LIP" – it might be misleading us.

10th March, 2006, Hetu Sheth
I am pleased to see the amount of discussion my contribution has generated. In this common Reply, I provide some clarifications on the issues raised by the readers above.

First, there seems to be complete (and fortunate) agreement that LIP is a broad and vague term. Dr. Saunders’ words, “the definition should not be too all-embracing. If it does, it becmes unwieldy and won't help further our understanding” are perfectly true of LIP. LIP IS an all-embracing term, though it has always been used in the incorrect, restrictive sense. By itself, LIP conveys very little to me.
I would disagree with Dr. Saunders’ implication that “LIPs” are unusual because they form in plume/rift settings, whereas subduction and MOR are “normal” settings. I see no reason why they should be separated, or discussed separately. What is the basis to exclude this or that province? And surely there is no requirement in the Earth machine that “LIPs” cannot be formed by subduction. As he notes, and so does Dr. Anderson, there is a real continuum in tectonic settings and size. In fact, LIPs may not be all that unusual.
In contrast to LIP, the new term I have coined, “Large Basaltic Province”, is specific and accurate. When one says LBP, one can’t mean the Andean granite batholith. Similarly for LRP, and so on. When one says LVP, one is aware that the term includes all volcanic provinces and excludes all plutonic provinces. My contribution goes further than a mere criticism of the current inappropriate usage of the basket term LIP, by providing new terminology and a classification, and is therefore, I believe, a real contribution.

Regarding proposals to incorporate a tectonic context or geodynamic origin into the classification, I think that this is not only unnecessary in a viable classification, but that the classification should be free of these features, themselves often the subject of heated debate. The classification is particularly meant to be independent of the plume/non-plume debate. An LIP is not an LIP because someone believes it to have come from a plume, and does not cease to be an LIP if someone rejects the plume model.
I strongly believe that an LIP definition should be based on size (how “large”) and not eruptive duration or tectonic setting. An LIP classification should be based on observable, non-contentious characteristics of igneous provinces. Chemical composition (felsic, mafic…) and emplacement depth (volcanic or plutonic) are two such parameters and so form the bases of my scheme. Eruptive durations for many LBPs are debated (e.g., Deccan, whose total duration is no less than 8-9 m.y., Sheth et al., GRL, 2001, and Ivanov et al. Terra Nova 2005 reach similar conclusions for the Siberian Traps). The tectonic setting and geodynamic origin are even more debated and by no means well known. This is the reason why such subjective parameters should not be the guiding factors while constructing a classification.

Regarding the minimum LIP size: A good classification scheme should be able to encompass as many examples worldwide as possible. I chose 50,000 km2 as the minimum size limit because with a higher limit (say, 100,000 km2, to which I did give a thought), provinces like the Mexican Volcanic Belt (50-60,000 km2 area) would not find a place in the classification and no “Large Andesitic Province” category could be formulated. Why such exclusion? 100,000 km2 is not “better” than 50,000 km2, but as arbitrary, and because there IS a continuum in igneous provinces in terms of size or tectonic setting, such bounds are necessarily artificial. I chose 50,000 km2 as the limit merely because it is by all means “large”, in absolute terms, and any higher limit would result in exclusion of several important provinces. With MY classification, 50,000 km2 is a much more suitable cut-off. I will be happy to discuss this aspect more.

Regarding LPPs: I have by no means equated or compared mafic dyke swarms with granite-granodiorite batholiths. They are very different things, as Dr. Ernst points out. However, by saying that they should not be together because the former are plume/rifting-related and the letter subduction-related, we are again creating all sorts of nightmares by going into geodynamic origins/tectonic settings. There are Proterozoic granite batholiths in India, for example, that are (or appear to be) anorogenic. They still are batholiths and SLIPs in every sense of the term. Do we place them in a different category from the (presumably) subduction-related batholiths and SLIPs? No, because descriptively, physically, they are all granite-granodiorite batholiths and SLIPs. In the same way, giant mafic dyke swarms and SLIPs are plutonic (in the sense of “intrusive”) and so come under the same LPP category, just as two contrasting types of volcanic provinces – LBPs and LRPs – come under the LVP category.

Finally, I am not recommending dropping the term LIP. I merely indicated its inappropriate and thoughtless use, and recommended proper use and newly coined acronyms for many LIP types. Acronyms are good as long as they are accurate and used correctly. If LIP is dropped, then so be it, but there is no reason to abandon the new ones, specific and accurate as they are. For the first time, we have a classification and accurate terms that are specific, easy, and unambiguous. The contribution was meant as a constructive one, not a criticism of the term “LIP”. My efforts will be well spent if the new terms and classification goes into use.

10th March, 2006, Richard Ernst
With respect, size MUST be part of a definition of LARGE Igneous Provinces. Some possible natural cutoffs are:
100,000 km2 – which seems to be the minimum areal extent for flood basalt events based on the initial work of Coffin & Eldholm. Furthermore, our work on dyke swarms suggests that there is a natural break in dyke swarm length. Those less than about 100 km in length have more local origin (e.g. individual volcano), while those more than 300 km in length have regional origin (rift zone, plume, etc.). Furthermore, a swarm 300 by 300 km is 90,000 km2, which is similar to the flood basalt value – this is a reason that we have also endorsed the 100,000 km2 minimum for LIPs..

That being said, we are really at early days in understanding the size of LIP – even the youngest ones: The Siberian traps nearly doubled in known size a few years ago, from about 2.6 million km3 to >3.9 million km3 with the dating of basalts under the West Siberian Basin (Reichow & Saunders and others; see March 2004 LIP of the Month on our website). Furthermore, there are estimates of 16 million km3 (i.e. by scientists in Novosibirsk, Dobretsov and Vernikovsky) if all the possible Siberian Trap equivalents are included, in the Ural mountains, and in the Central Asia fold belt. The same 'changing size' is true for Ontong Java. Brian Taylor's recent EPSL paper (see also his February LIP of the Month) indicates that Ontong Java, Manihiki Plateau and Hikurangi plateaus were originally formed as a SINGLE LIP that was subsequently fragmented.

If events at 250 Ma and 120 Ma can be so uncertain in size, then the many Paleozoic and Proterozoic LIPs dominated by dykes and sills are even more uncertain in their extents. So the order of magnitude size estimates for LIPs and other rmagmatic provinces, that Wouter Bleeker and I have adopted (and contributed earlier in this discussion) seem the best way to go for now until the database is more robust and natural breaks become more apparent.

10th March, 2006, Andy Saunders
I agree with Richard.  Size matters.

10th March, 2006, Gillian Foulger
I would expect the sizes of magmatic provinces to be a continuum and I think it would be most ill advised to make assumptions regarding origin, let alone incorporate them in definitions. If we ever become 100% sure of the origins of all of them, it'll be a sign we are not doing our jobs properly.

10th March, 2006, Scott Bryan
Sheth has erected an unnecessary subdivision with regards to Large Rhyolitic Provinces and Large Granitic Provinces. The term "igneous" includes plutonic and volcanic rocks; there is absolutely no requirement to coin terms for separate plutonic and volcanic provinces. As noted by the other commentators, there are spectrums in many aspects of LIPs; there certainly is between the proportion of volcanic rocks (preserved) and plutonic rocks (exposed) in mafic and silicic LIPs. Any LIP with a volcanic expression, will also have a plutonic component; therefore trying to make a subdivision into volcanic or plutonic is unnecessary splitting. The first-level division of Sheth's nomenclature reflects more on the exhumation history of a LIP rather than any fundamental differences in terms of emplacement, relative proportions of magma emplaced at depth or erupted, areal extents, volume, duration etc.

Sheth has misrepresented the definition of Silicic Large Igneous Provinces (SLIPs) as given in Bryan et al. (2002) and on the SLIPs webpage.To reiterate the definition, the term “Silicic Large Igneous Province”  was used to describe silicic volcanic-plutonic provinces with voluminous magmatism >105 km3. The original definition did specify extrusive volumes, and it should be revised to be "igneous" volumes to account for provinces where silicic intrusive rocks may be voluminous (e.g., Kennedy-Connors-AuburnSilicic LIP; Table 1 of SLIPs webpage). All the listed examples of Silicic LIPs (Whitsunday, Chon Aike, Sierra Madre, Kennedy-Connors-Auburn) have both volcanic and intrusive rocks exposed, with the proportion largely dependent on the depth of erosion. Therefore, any extensive granitic batholiths that have areal extents, emplacement rates and intrusive volumes comparable to LIPs may be just deeper level features or plumbing/chamber systems of silicic LIPs. This is no different to the argument Ernst et al. have made for ancient mafic LIPs where dyke swarms, sills and layered intrusions are the deeper level plumbing structures to mafic LIPs (eg, continental flood basalt provinces).

 It is a very dangerous approach to add confusion to the term LIP by simply using it to refer to a large mass of similar composition igneous rock or lithology (e.g. oceanic crust, large granitic batholith). Using a tectonic-based term like "igneous or volcanic terrane" would be much better for this purpose. It is well-known that many granitic batholiths (that can be on the scale of LIPs) have intrusive phases that can span 100's of Myr but which were emplaced during completely different tectonothermal regimes. To me, LIPs are discrete, huge volume igneous events - they are the sites of the largest individual eruptions (both mafic and silicic) and cumulative erupted volumes in Earth history that are the result of very high melt production and eruption rates.

 There are fundamental differences between the Silicic LIPs and other large silicic volcanic systems such as the SRP, Altiplano-Puna, TVZ, and although the volcanism has been extensive, and erupted volumes prolific, these latter provinces are still an order of magnitude smaller in just about every respect to the Silicic LIPs (see Table 1 of SLIPs webpage).

 A critical part of the original LIP definition is that they "originate via process other than ‘normal’ seafloor-spreading or those associated with ‘normal’ subduction processes". This reflects the fact LIPs form independently of plate setting and fundamentally differ in style and geochemical characteristics from MOR and subduction zone magmatism. Therefore, the statement that subduction-related arc igneous accumulations or MORB are LIPs is contradictory and adds confusion. Magma flux rates at subduction zones do not come close to those of LIPs.

 I also agree with Ernst & Saunders that "size does matter". For Silicic LIPs, a natural cut-off is apparent at present; eruptive/intrusive volume is more useful than Area and the Whitsunday, Sierra Madre Occidental, Chon Aike & Kennedy-Connors-Auburn provinces all have volumes >250,000 km3. Other provinces with large volume silicic volcanism (eg TVZ, Altiplano-Puna, SRP) all have (extrusive) volumes <50,000 km3. The 100,000 km2 area cut-off used for mafic LIPs also seems currently applicable for Silicic LIPs.

11th March, 2006, Don Anderson
Better define 'production rate' and 'eruption rate'.  This implies a very specific mechanism, steady state and equilibrium ("melt upon demand").  If there is already melt in the asthenosphere, or ponding, then the eruption (and intrusion) rate need not equal the rate at which the mantle melts. In the plume hypothesis and in the adiabatic ascent hypothesis for melting, there is a steady state, but this is not general. Even in the more general case, eruption + intrusion + underplating need not equal the rate at which melt is produced although it is likely to be close. For example, in the delmaination case, dense eclogite can bottom out at 400-km, warm up and partially melt, become buoyant, melt more upon ascent, and then underplate, pond and start to intrude when stress condtions are right, and then extrude (erupt) at a later time, but 100% efficiency is not required, or a steady-state. This may be nit picking but some current theories assume steady-state, or assume that ALL melting has to be due to adiabatic ascent. I suspect that no current theory can produce melt from scratch at the rates at which it is erupted, except possibly at ridges.

Some LIPs form at RRR triple junctions, and in back-arc settings and at incipient  plate boundaries or at new ridges. Most LIPs are NOW midplate but this is because ridges migrate.

11th March, 2006, Sami Mikhail
I think the recent page by Hetu Sheth entitled Large Igneous Provinces (LIPs)”: Definition, recommended terminology, and a hierarchical classification is long overdue.

I have been quite upset by the lack of constraints used by authors regarding the umbrella term (LIPs); it’s like the whole plume thing part 2. We as a scientific community must be specific and concise to avoid confusion and misinterpretation. Its not complicated science, but utilising our ability to communicate by setting up definitive parameters based on terminology, which are governed by definition, otherwise know as words (‘Large’ & ‘Igneous’ are pre-existing words with definitions). The term LIP does incorporate all Sheth was talking about (LBP, LRP, LPP, LVP, LBP…) but Sheth’s paper is exhibiting that the term LIP (by definition) is Igneous Geology as a whole. The use of the term ‘LIP’ was set up with basaltic flood lavas in mind. These have large volumes extruded over short periods of time, which is why I think the bulk of research is biased towards CFB & oceanic plateaus. This is probably because they are quite odd, interesting and with regard to biota-DANGEROUS as we all know are linked to extinction.

Workers must see that using pre-existing words in any order to define something previously undefined is proven by the use of the term plume to be hazardous and a constant nuisance. The idea of inventing words for new discoveries is not sensible, but is useful. For example we didn’t call electrons ‘little moon like things’! But where do we draw the line? Who decides?

We do not really need the term as we use localised names such as Siberian Traps, Emeishan flood basalts, NAIP etc. The term (LIP) is set up to classify these large igneous episodes as a whole. Sheth has just exposed a problem with this system in this field of research (LIPs). Similar to Scott Bryan (& others) who put an S in front of LIP and simply said what about SLIPs, but unlike Scott, Sheth gave an accelerated expansion of the term that future workers would ‘inevitably’ do using the new hierarchical classification and terminology one by one (first LGP then LRP etc…) he just said them all at once. I feel it’s a clear sign that the field is alive and growing with each publication, which is fascinating.

As an undergrad seeking a PhD place I feel this Sheth system is exciting. It opens doors for future research projects like identification and petrogenesis of LBP, LRP, LPP, LVP, LBP… comparisons, histories and expansion of the database & website www.largeigneousprovinces.org.

11th March, 2006, Hetu Sheth
It is good to be able to provide some further clarifications. My apologies to Dr. Bryan if I misread or misquoted his work and definitions.

I maintain that LRP and LGP should be separate categories, just as LBP and large mafic-ultramafic intrusive complexes are separate categories. It is true that a flood basalt province/LBP does include the associated intrusive complexes WHEN EXPOSED (see my explanation to Table 1), and this is similarly true of LRPs. What I had in mind, however, was situations when the intrusive counterparts of an LBP or LRP are not yet exposed by erosion, or when only the intrusive counterparts (mafic dyke swarms or granite-granodiorite batholiths) remain and the upper levels have been lost to erosion. Ample examples exist of each. Of the latter, take the many giant dyke swarms (the Mackenzie and many others) that may well have fed flood basalts, now eroded, or the felsic batholiths that may simply be the root zones of rhyolitic stratovolcanoes/flood provinces (many examples in the southern Indian shield, e.g., the charnockite massifs I have mentioned). The latter is indeed due to exhumation – it is immaterial, and the classification addresses what is seen at the surface.

I have tried to define or coin terms for what I see, without any conjectures or guess-work. Whether a granite batholith exposed today at the surface fed rhyolitic volcanoes at the top, or did not, is immaterial, and should not affect the classification. Similarly, giant dyke swarms in all probability fed surface flood basalts, but whether they actually did or not we may never know. In any case, it is immaterial, in this classification, and should be. I emphasize once again that the classification is purely descriptive, based on what is at the surface and what can readily be seen and what is not contentious. This is nothing but the emplacement depth and the compositions. Tectonic setting, geodynamic origin, even the age and eruptive duration, are sufficiently contentious, and likely to remain so. In whichever direction our ideas and knowledge evolve regarding these issues, ten years, twenty years from now, the basic characteristics (volcanic/intrusive, chemical composition) are not going to change.

Whether the community will adopt this classification, or a modified version of it, or none of it, the future that’s not too far off will tell, but I really think that if the term LIP is to continue (and there is no reason why it shouldn’t), then the various categories defined and named here are the best thing available. If the classification is basically faulty, or deemed unappealing despite the explanations and clarifications, then the option we are left with perhaps is to blow LIP and all other acronyms up and end up with no terms and nothing to work with. Just use the plain “flood basalt provinces”, “granite batholiths”, and so on. This may be counter-productive however. After all a classification is meant to simplify things, and this one does it.

I still fail to see how sea-floor spreading and subduction are “normal”, whereas plume/rift/rifted margin are not “normal”. How do we define “normal”? As DeLaughter et al. in the P3 book note, “the main problem is satisfactorily defining normal”. Today may be “normal” with a hundred active volcanoes worldwide, or we may be living in a particularly “abnormal” time in Earth history with only a hundred volcanoes active. Even notions of “normal” and “not normal” may be prejudices rather than real. I see no basis to separate subduction and MOR from the other features if there is no physical basis for this. And as I note in my article, the crustal production is not in any way less along a moderately fast-spreading ridge than it is in a flood basalt, over time scales of 5 m.y. or 10 m.y. Many flood basalts do have such durations, as Dr. Ernst notes. Is 5 m.y. short or long? Is 10 m.y. short or long? These things are bound to remain subjective and debatable.

In any case, the ocean floor does satisfy perfectly the LIP (and LVP and LBP) definitions. It is the largest igneous, volcanic, and basaltic province, in every sense of these English terms. Including it in the LIP definition does not create confusion. It is the thoughtless use of the term LIP for over a decade that is the cause of ample confusion. Not including the ocean floor in the LIP definition basically means we are not prepared for clear and accurate terminology. There’s no escaping the fact that the ocean floor is an LIP, and the largest one. If this is not appealing, then one should stop using LIP for the other things as well. It is somewhat ironic that some of us who think that “size does matter” are recommending exclusion of the ocean floor, a true LIP and in fact the largest LIP, from the LIP definition itself.

12th March, 2006, Scott Bryan
Eruption rate (Note: we made comments on this in our 2002 GSA Spec Publ 362 paper, see conclusions on p. 115) - this varies depending on if it is a mafic or silicic eruption. We now know the mafic flood basalt eruptions from the CFBPs are very large volume (1 to ~5000 km3, may be larger if Steve Self et al are correct for some eruptions from the Deccan), long-lived (yrs to 10s of yrs) and although the effusion rates are high (~4000 m3/s), they may not be too dissimilar to recent eruptions (eg comparisons between Laki and Columbia River flood basalt eruptions by Self, Thordarson et al). The big silicic eruptions (eg Parana-Etendeka & Afro-Arabia) represent similar and much larger erupted volumes (up to 6340 km3 and possibly much larger, up to 10,000 km3). Although no detailed work has been done on their eruption rates, they are no doubt going to be similar or greater to other well-constrained large plinian/explosive eruptions. Probably a key point in both (mafic & silicic) cases is that the individual eruptions will have been far more sustained (primarily by virtue of the tremendous volume that had to have been erupted) than any other non-LIP eruption. Therefore, if we argue or interpret that eruption rates are not dissimilar to those observed in recent times, then the eruptions had to be far more sustained. If we interpret much higher eruption rates, then eruption duration will be less. There is currently, insufficient age resolution to constrain recurrence rates of both mafic and silicic eruptions in LIPs. This may well be the crucial distinctor for LIPs compared to other igneous provinces (arcs, MORs etc).

Melt production rates - I am coming at this from the silicic LIP point of view. We know that they require massive thermal inputs into the crust to drive crustal melting, and if you look at Table 1 on the Whitsunday LIP of the month web page, you'll see that the magma flux rate is substantially higher for the Whitsunday LIP when compared to what is the most productive silicic volcanic province on the Earth today (TVZ). There does seem to be a basic difference here between the silicic LIPs when compared with other volumetrically significant and hyperactive silicic volcanic provinces (TVZ, Altiplano-puna, SRP etc).

 I note your points, and agree on the potential complexities on how melt is generated. That is why I think silicic LIPs have an important contribution to make here in understanding the origin of mafic LIPs.

"Some LIPs form at RRR triple junctions, and in back arc settings and at incipient  plate boundaries or at new ridges. Most LIPs are NOW midplate but this is because ridges migrate." Agreed, but the point I was making and that I think Mike Coffin was making was that LIPs can occur anywhere and everywhere - ie are independent of plate setting. Supra-subduction-zone arcs or MORs are not. "Back-arc" is vague - do you mean back-arc setting (could be anywhere from 0 to >1000 km inboard of the volcanic arc), or do you mean it has a tectonic connection to the subduction zone and plate boundary processes? Yes, the CRFBG are back-arc (in terms of setting), but it is a big call to say they are subduction-related. The Sierra Madre Occidental can also be argued to be erupted in a back-arc setting; many have argued (wrongly) that the volcanism was arc- or back-arc related - this is primarily due to the erroneous assumption that calc-alkaline = arc/subduction signature (a similar story for the Whitsunday Silicic LIP), but this is a whole other topic of discussion. I think these LIPs emplaced near active continental margins have a lot to offer on how LIPs are formed - if from a CMB-derived plume, then how does such a plume navigate through an actively subsiding slab of oceanic crust?

12th March, 2006, Don Anderson
Scott – Thanks for the clarification. My point is that 'melting rate' should not be in the definition, or the assumption that eruption rate equals the rate of latent heat supply. My terse comment was just to point out that melting rate does not have to equal eruption rate, and that high eruption does not imply the need to melt as fast as it erupts, i.e. high melt production rate is a prerequisite only  in the plume theory. Some plume advocates argue that high production is required, and therefore high vertical plume velocities, and that no non plume theory can give such high rates of melting. But when calculations are attempted (Cordery et al., 1997) even a plume cannot give the implied melting rate and even a plume head salted with eclogite and made hot instead of warm (ala Campbell) cannot produce much melt under thick lithosphere. Some plume modifictions accept ponding and lateral flow but many current papers demand high melting rates, rather than the ponding/stress valve release mechanisms. The SLIP situation is particularly instructive since the sources of heat and material are clearly different and  something else controls when and where the eruption/intrusion occurs and eruption rates have little to do with intrusion rates and ponding volumes. The opposite is usually assumed in LIPs.

"recurrence rates of both mafic and silicic eruptions in LIPs. This may well be the crucial distinctor for LIPs compared to other igneous provinces (arcs, MORs etc)." Good point. And this may be controlled by stress and litosphere, rather than mantle and temperature.
Any criteria that distinguishes LIPs from ridges and arcs would be useful.

"Melt production rates - I am coming at this from the silicic LIP point of view. We know that they require massive thermal inputs into the crust to drive crustal melting...you'll see that the magma flux rate is substantially higher for the Whitsunday LIP when compared to what is the most productive silicic volcanic province on the Earth today (TVZ). " Thermal input into the crust is the key, and this is likely due to basalt. In the delamination/reheating model the infinite heat source of the manltle heats and melts the eclogite blobs but no superheat or high temperature is needed. In the plume model it is the relatively more modest core heat that is used; ambient mantle is considered to be cool and isothermal, as well as having a pyrolite-like solidus throughout. Geophysics implies mantle potential temperatures well above the standard 1300 C adiabats and bringing basalt into the crust at these kind of temperatures may be adequate for the SLIPs and probably for the LIPs. But, in any case, the melting rate is not the eruption rate. Eruption rate and intrusion rate may be controlled by the lithospheric situation.

"I note your points below, and agree on the potential complexities on how melt is generated. That is why I think silicic LIPs have an important contribution to make here in understanding the origin of mafic LIPs." AGREED

"....the point I was making and that I think Mike Coffin was making was that LIPs can occur anywhere and every where - ie are independent of plate setting." I would contest this. Hawaii is a still being worked on example, but as far as I know ALL LIPs  have a fairly well understood tectonic and stress relationship... e.g to mobile belts, suture zones, craton edges, new triple junctions, extension. A long time ago Cox pointed out that Karooo was in general terms 'back arc' from the SAMFRAU belt.  CRB, Keweenawen, Deccan are also in convergent situations with a slab somewhere below and the possibility of arc delamination. You are right, back arc may be too vague; convergence, slabs, arc, sutures...and maybe, delamination, plus extensional stress, are usually involved somehow.  But we definitely do not see a full fledged LIP  popping up in the middle of Canada or Australia or the Pacific plate. Detailed plate reconstructions show that LIPs have a tectonic context, often between two cratons, or an evolving Pacific plate, or ~1000-km offshore of a fragmenting continent, or at a new ridge (but there is cause and effect for this one; plumologists have plumes causing breakup bu this leaves out Karoo, Deccan, CRB etc.)

"Yes, the CRFBG are back-arc (in terms of setting), but it is a big call to say they are subduction-related." Delamination is similar to subduction and most candidates for delamination are in arcs or convergent mountain belts. The B&R and the Sierras seem to have uplift and delamination somehow related to the Farallon slab, and CRB are somehow related to the only active subduction in this part of the world. Humphreys wants all of western N America to be related to dewatering of dead slabs. The Changbei volcanics are 2000 km form a trench but a slab has been imaged beneath it.  Tomography does show that slabs can bottom out and extend for thousands of km horizontally. Most of China seems to be underlain by a flat slab. I prefer to think that the current subduction is secondary but the stresses and arcs and thick crust may be involved.

"I think these LIPs emplaced near active continental margins have a lot to offer on how LIPs are formed." I AGREE,  but the past tectonic context is also important. Atlantic and Indian ocean LIPs seem to be related to supercontinent fragmentation  (delamination in my view)  but the previous convergent situation may also be critical.

13th March, 2006, Gillian Foulger
Persons interested in this debate might be interested in a new Powerpoint presentation I just posted: Self-organized breakup of Gondwana: An argument against the deep mantle plume paradigm by James Sears. He just gave this at Durham, and he presents a novel model for the location of LIPs.

13th March, 2006, Don Anderson
The words 'normal', as in normal MORB or NMORB, and 'anomalous', as in anomalous seafloor bathymetry or a swell, seem innoculous but are largely responsible for the present confusion and debate about plumes. The terms imply that the mantle is isothermal and homogeneous and two-dimensional. One does not expect magmatism to be steady in time or uniform in space, or ridges to be constant depth. In 3D convective simulations upwellings may start linear but then become concentrated, as in salt domes from salt beds. The upwelllings do not need a special theory and they are not anomalies. Plate tectonics makes the mantle heterogeneous and not isothermal and this is normal. Some places will make more basalts than other places. The distinctions between NMORB, TMORB, EMORB, PMORB and OIB are arbitrary and have led to the situation where when EMORB is found along a ridge it means a plume (and lower mantle), even though the N- and E-MORB distinction is arbitrary. Likewise, 'high 3He/4He' is arbitrarily defined as >9.5 Ra, and 'therefore'  'high 3He/4He' is plume (anything greater than 9-10 Ra found at ridges is excluded from the normal average and when found, by definition, is a plume).

Every parameter has a statistical range and a standard deviation. To arbitrarily make a cutoff and then say that a different theory is needed for things outside the cutoff is not to appreciate statistics or sampling theory. If  igneous volumes or areas are bimodal or have many peaks, then different theories may be needed.  But even this can be due to sampling differences. The standard deviation is sometimes more important than the parameter. Likewise, if the chemistry (or eruption rate, or temperature ) does not overlap MORB, or BABB, IAV... then it should be in the definition. If there is overlap and if there is uncertainty in the definitions or cutoffs then this probably tells us a great deal about the mechanism. LIPs may just be the endgames of 'normal' plate tectonic and seafloor spreading processes; the transients that occur as spreading starts or as ocean basins close or when a TJ jumps.

These chronological and timing parameters may be more important than size, if you eventually want the definition to help in determining the mechanism. Has a LIP ever happened in the middle of a plate, away from the current or future plate boundaries or craton boundaries and not related to slabs, incipient boundaries etc? Are LIPs really independent of tectonics and are they ever midplate? Are they always atop thick sedimentary basins? Are emplacements always followed by uplift? Is it clear that oceanic plateaus and CFB are the same thing (LIPs)? How does a  (bounded) small ocean basin or BAB differ from a LIP?

13th March, 2006, Sami Mikhail
LOGIC, LANGUAGE & LIPs.

I have a few questions if anyone would be so kind to hazard some suggestions, assumptions or propose answers, but first some points:

LARGE is defined as big in size or amount relative to a known comparable X which is small (or large is only large in comparison to small as it is a relative term, Large > Small). IGNEOUS is defined as rocks formed from magma. As Anderson quoted from Lavoisier on 9/3/2006 ‘you cannot pretend to have a science unless you have a language and definitions!’ So to have a large igneous province one must classify such a province relative to a small igneous province. The umbrella term ‘LIPs’ like it or not does include all igneous provinces which are ‘anomalously’ large compared to ‘normal’ igneous activity regardless of tectonic setting, chemical signature and/or process of formation. This includes almost all orogenic settings such as Cadomia (N Armorican massif), the Andean SZ, the plutons of Himalaya (along strike to the collision zone and back arc) and all ocean floors. Whether or not this means it is harder to say the term LIP and inflict a simple mental picture is irrespective of the terms’ definition using the English language.  

Ernst (9/3/06) has put up three constraints on LIP definition. 1- Size, without a doubt, the term states a relative description of size using the word LARGE. 2- Duration, well does it? Maybe for the conceptional idea it plays a part, but what has that got to do with anything? If it is large (>small) & igneous it is a LIP by definition of the words in the term. I understand how it may be helpful to estimate or assume the petrogenesis of areas such as NAIP, Deccan Traps, Siberian Traps ect from just being told it’s a LIP which gives the recipient the idea that the petrogenesis was fast (rapid eruption rates) and big. But does that constrain a LIP? Also 3- Setting, how? Recently we have seen many people publish ideas on LIP petrogenesis with no indicative conclusion, i.e. OJP. Ingle & Coffin (2003) proposed impact of an extra terrestrial bolide, then plume activity was proposed in a geochemical sense. The OJP basalts contain enough concentrations of Platinum Group Elements (PGE) to indicate an outer-core origin transported by a deep mantle plume from D’’ according to Ely & Neal (2003). Also lithospheric delamination seems plausible as Anderson (2005) states ‘the possibility of delamination is increased at ridge-ridge-ridge triple junctions’ like the one surrounding the OJP. Burke & Torsvik (2004) concluded no relation to continental break-up unlike most LIPs! So when classifying the OJP as a LIP which setting was used?

Saunders states (9/3/06) ‘Thus LIPs are produced by unusual processes’, but are they? According to Burke & Torsvik (2004) and Ernst et al (2005) and the LIP data base LIPs appear almost 1 per 10 Ma over the last 200 Ma (LIPs >200 Ma may be destroyed by surface processes) which is not a geologically unusual process. It is more like a steady occurrence through time which isn’t anomalous and therefore is not unusual.

A few questions K0228562@kingston.ac.uk:

  1. 1 Bryan has done a good job of solidifying the term SLIP, so now we have two ‘agreed’ types of LIP (SLIP & ‘normal-mafic’ LIP). But the proposal put forward in this article by Sheth is not going to disappear. Now the two terms (LIP & SLIP) are readily used they will be broken down. Just like simplifying an algebraic equation. Thus the possibility of more sub-divisions/umbrella terms. For example, a SLIP may have both rhyolitic and andesitic components, but the ratio of andesite:rhyolite may be (hypothetically) 2:8. To increase clarity one may call it a Rhyolitic Large Igneous Province (RLIP). Does this not improve classification?  Example (LIP) = ('normal basaltic' LIP + SLIP) & (SLIP) = (RLIP  + ALIP[andesitic] + DLIP[dacitic]).
  2. Are subduction-related LIPs not considered by workers as our knowledge of the magma generation at subduction zones is relatively well known & documented compared to MOR & intra-plate where there is less clarity in understanding?
  3. Why is process of formation even considered when there is no agreement towards either end-member of the argument with regard to plume vs. non-plume (tectonics, fertile mantle…)?
  4. With regard to Ernsts’ (9/3/06) comments and constraints as to LIP definition on this page there is no mention of geochemistry. So are SLIPs not SLIPs but LIPs?  
  5. Foulger stated on 10/3/06 ‘Maybe instead of proliferating categories we should just dump the term "LIP" – it might be misleading us’. Saunders (2005) makes a point that LIPs have no strict definition. So as there is no strict definition and no set constraints in nature for strict definitions with regard to Earth sciences. We could just agree to use the term ‘unofficially’ and off the record. So, why use the term LIP in published literature?       

The term LIP is very useful to spread interest of geoscientists of the future, Scientists and non-geo/scientists. It also helps current workers to easily identify with each others field (for example the formation of the large igneous province commission). The only problem appears to be in agreeing constraints for the definition. I put forward a suggestion that a more specific name or umbrella term may be more useful. Such as ‘Areas of Rapid Magma Generation’. I am not proposing this title but simply implying that a more representative term could be used to replace LIP and better suit the generally agreed definition (such as the definition given by Coffin and Eldholm (2005) in the Encyclopaedia of Geology).    

  • ANDERSON, D.L. 2005. Large Igneous Provinces, Delamination, and Fertile Mantle. Elements. 1 (271-275)
  • BURKE, K., TORSVIK, T.H. 2004. Derivation of Large Igneous Provinces of the past 200 million years from long-term heterogeneities in the deep mantle. Earth and Planetary Science Letters. 227 (531-538)
  • Coffin, M.F., and Eldholm, O., 2005. Large igneous provinces, in Selley, R.C., Cocks, R., and Plimer, I.R., eds., Encyclopedia of Geology, Elsevier, Oxford: 315-323.
  • ELY, J.C., NEAL, C.R. 2003. Using Platinum-group elements to investigate the origin of the Ontong Java Plateau, SW Pacific. Chemical Geology. 196 (235-257)
  • ERNST, R.E., BUCHAN, K.L., CAMPBELL, I.H. 2004. Frontiers in Large Igneous Province research. Lithos. 79 (271-297)
  • INGLE, S., COFFIN, M.F. 2004. Impact origin for the greater Ontong Java Plateau. Earth and Planetary Science Letters. 218 (123-134)
  • SAUNDERS, A. D. 2005. Large Igneous Provinces: Origin and Environmental consequence. Elements. 1

15th March, 2006, Kamal Sharma
Hetu described various categories of LIPs on the basis of extrusive or intrusive emplacement and rock compositions. The terminology is a scientific one and gives an idea of occurrence and rock type of the igneous body. Adhering to 100, 000 km2 minimum size for a LIP seems inappropriate. A minimum size of 50, 000 km2 to characterize the LIP is not improper. The size parameter should be applied loosely, while defining LIPs.

LIPs developed on Earth in different tectonic settings during the geological past. If a significant area is presently preserved, than it can be categorized as an LIP. The south Indian charnockite massif formation is a large scale Precambrian igneous province. Similarly, the Tibet-Himalayan batholith also formed during Precambrian time. Categorizing both as LIPs will give a better understanding of the formation of the Large Igneous Provinces on the Earth through time. In my view, the message will be more specific through the use of standard LIP terminology in scientific descriptions.

20th March, 2006
Please visit the webpage "Proposed Revision to Large Igneous Province Classification" by Scott Bryan & Richard Ernst where the authors present a comprehensive proposal for LIP classification.

... for continuation of this discussion, click here.

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