Cañón-Tapia, E., 2010, Origin of Large Igneous Provinces

The Geological Society of America Special Paper 470 2010 Origin of Large Igneous Provinces: The importance of a defi nition E. Cañón-Tapia* Centro de Investigación Científi ca y de Educacíon Superior de Ensenada, Geology Department, P.O. Box 434843, San Diego, California 92143, USA ABSTRACT In the original defi nition of a Large Igneous Province (LIP) much emphasis was put on the “extraordinary” character of these provinces. Such emphasis might have contributed to bias the form in which these provinces are commonly visualized, and consequently has contributed to the selective acceptance of genetic models. To avoid such bias, in this chapter the various available defi nitions are examined, taking into consideration the rules of logic that help us to avoid fallacies. From these defi nitions, the most critical parameters are identifi ed, and an alternative model of formation of LIPs is advanced. The model developed here envisages LIPs as an extreme of a continuum in volcano-magmatic processes that are produced by essentially the same underlying processes. The differences between LIPs and non-LIPs are conceived as the result of different conditions present in a particular region, but that nonetheless have nothing extraordinary. Although the model developed here is one of several possible alternatives, by having identifi ed the most common fallacies surrounding a LIP, even from its very defi nition, it might be possible to assess those alternative models in a more equilibrated form in future works. INTRODUCTION both the extrusive and intrusive components of LIPs is shared by essentially all genetic models, whether they are associated with Large Igneous Provinces, or LIPs, were defi ned more than the arrival of a mantle plume to the surface of the Earth (Eldhom a decade ago as “voluminous emplacements of predominantly and Coffi n, 2000; Hooper, 2000; Richards et al., 1989; White and mafi c extrusive and intrusive rock whose origins lie in processes McKenzie, 1995), or to different processes of global scale (e.g., other than ‘normal’ seafl oor spreading” (Coffi n and Eldholm, Abbott and Isley, 2002; Coltice et al., 2007; Hales et al., 2005; 1992, p. 17). This defi nition has marked much of the research Jones et al., 2002; King and Anderson, 1995; Mutter et al., 1988; done on the subject ever since, as LIPs have been almost unani- Sheth, 1999a; van Wijk et al., 2001). mously considered to represent periods of anomalously high To better appreciate the infl uence of the above defi nition of magma production rates. Thus, the essential interpretation of LIPs in the research made on the subject, I marked with italics LIPs as events of an extraordinary character likely “to record key words that somehow predispose us to look for “extraordi- periods when the outward transfer of material and energy from nary” explanations for these natural phenomena in exactly the the Earth’s interior operated in a signifi cantly different mode same form that saying “do not think of a white elephant” almost than at present” (Mahoney and Coffi n, 1997b, p. ix) has been invariably brings to the mind of the person hearing that message a common feature of the vast majority of papers published on the image of such an animal, even if briefl y. To some extent the this subject over the past 15 yr. Actually, the emphasis put on the problem surrounding any defi nition of a LIP was identifi ed by presumed anomalous magma production rate required to explain Menard (1969) when he said that “the central problem is satis- factorily defi ning normal.” Actually, this is a central problem that *[email protected] we need to face every time that we attempt to organize natural Cañón-Tapia, E., 2010, Origin of Large Igneous Provinces: The importance of a defi nition, in Cañón-Tapia, E., and Szakács, A., eds., What Is a Volcano?: Geologi- cal Society of America Special Paper 470, p. 77–101, doi: 10.1130/2010.2470(06). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved. 77 78 Cañón-Tapia phenomena (not only LIPs) in any sort of classifi cation scheme, tional point of view. Nevertheless, I consider that this model pro- therefore making it necessary to examine very briefl y what the vides an example of the form in which mythical thinking can be purpose of any classifi cation is. avoided in the study of this type of province. Other consequences In the words of Best (1982, p. 20), “Classifi cation is a of having good defi nitions in science, and in particular in the con- human endeavor that attempts to recognize . common or con- text of volcanic activity, are examined in the chapter by Szakács trasting features of related things . [although it should not be] (this volume). an end in itself, but a means of seeing more clearly, simply and unambiguously the interrelations of the different properties of TWO RECENT DEFINITIONS OF A LIP different rocks.” According to him (p. 20), the objective of classifi cation is therefore subdividing the continuous spectrum of a Very recently Sheth (2007) suggested the need to reexamine given property “in some meaningful way” that could help us to the currently accepted defi nition of a LIP quoted at the beginning make a genetic interpretation of such diversity. As Best further of this chapter. The motivation for Sheth’s suggestion seemed to recognized, usually there are thousands of possible forms to con- be unrelated to the above considerations concerning the key role ceive a classifi cation scheme, yet not all of the possible classifi - played by a defi nition and its associated classifi cation scheme, but cations are equally helpful for the identifi cation of factors that nevertheless refl ecting some concern for such issues. In any case, are signifi cant in the understanding of the genesis of the object Sheth’s (2007) proposed defi nition of a LIP comprises the area of study. A corollary of such diversity is that some classifi cation covered by the igneous rocks present in a given province, so that schemes might predispose us to make selective judgments based any place where more than a threshold area is covered by igneous in the apparent order artifi cially introduced by the classifi cation rock (he proposed this threshold to be >50,000 km2) should be scheme itself. Consequently, the classifi cation scheme exerts an called automatically a LIP. Evidently, by adopting this defi nition, often overlooked infl uence in the creation of genetic models, and the bulk of the present-day ocean fl oor becomes the largest LIP it is important to keep in mind such infl uence if an unbiased inter- that has ever existed in the geologic history of our planet, which is pretation of observations is really to be made. diametrically opposed to the explicit exclusion of ocean spread- The predisposition for a selective interpretation of observa- ing from the group of LIPs made by the defi nition of Coffi n and tions introduced by a particular defi nition scheme not only repre- Eldhom (1992). Other departure from the original defi nition of a sents a typical example of circular reasoning, but actually it could LIP found in the work by Sheth (2007) is that he devised a hierar- mark the beginning of what Dickinson (2003) has described as chical system in which LIPs are subdivided in different categories the modern mythic style of thinking in geosciences. Using the depending on (1) whether the rocks of the province are extrusive vocabulary of logic (Copi and Cohen, 1994), the characteristic or intrusive and (2) on the predominant composition of the rocks aspect of mythical thinking is the selective assignment of truth found in that province. Thus, at the fi rst hierarchical level LVPs values to some of the premises used in the interpretation of would stand for “Large Volcanic Provinces” and LPPs for “Large observations, sometimes in a very subtle form, but nevertheless Plutonic Provinces” independently of rock composition. At the favoring an a priori accepted conclusion. Consequently, to avoid second hierarchical level, terms such as LRPs standing for “Large mythical thinking it is extremely important to have defi nitions Rhyolitic Provinces,” LGPs for “Large Granitic Provinces,” or leading to classifi cation schemes that are as unbiased as possible, LBPs for “Large Basaltic Provinces” would be required. A most yet at the same time allow us to recognize meaningful aspects appealing aspect of such a classifi cation scheme is that it contains that can be interpreted genetically. In this chapter I examine sev- more subdivisions than a scheme based in a “LIP” versus a “non- eral aspects of the current, and two other more recent, defi nitions LIP” scheme inherent in the defi nition of Coffi n and Eldhom of LIPs, aiming to identify the elements with the largest potential (1992). The increased number of groups with contrasting differ- to yield signifi cant clues that could help us to better understand ences in Sheth’s defi nition might in principle facilitate the task of the genesis of LIPs and their relation to other manifestations of identifying the signifi cant aspects of the formation of each type volcano-magmatic activity in our planet. In the second part of the of province more easily than it could be possible if only two large chapter, I develop an alternative model for the genesis of LIPs groups are defi ned. In turn, such distinction might prove to be an that takes into consideration some of the physical constraints advantage if the mechanisms controlling the genesis of each type identifi ed as more signifi cant in the fi rst part.

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