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   Silicic LIPs
Silicic Large Igneous Provinces

Scott Bryan

Department of Geology & Geophysics, Yale University, PO Box 208109 New Haven CT 065208109, USA


Silicic magmatism is an integral part of large volume magmatic events that herald the break-up of continents. The proportion of silicic magmatism, however, appears to be related to the crustal setting of magmatism, and the ability of major thermal and material inputs from the upper mantle to melt continental crust. Several silicic-dominated large igneous provinces, comparable to those of the continental flood basalt provinces, have been generated along continental margins built from Palaeozoic and Mesozoic plate convergence and characterised by fertile, hydrous basaltic to “andesitic” crust that can readily melt. These silicic igneous provinces are major crustal melting events and their eruptive output implies similar mantle processes to those responsible for the continental flood basalt provinces. However, these silicic large igneous provinces place important constraints on mantle dynamics:

  1. the duration of volcanism (up to 40 Myrs) requires sustained thermal and material input from the upper mantle;
  2. mantle material contributions are geochemically similar to younger “hotspot”-style volcanism occurring along the volcanic rifted margins; and
  3. km-scale regional uplift immediately post-dates the main phase of silicic magmatism and is related to the rifting process.
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1. Impetus for this contribution

Large igneous provinces (LIPs), as typified by continental flood basalt (CFB) provinces, are commonly envisaged as short-lived (< 5 Myr), high rate (0.1 – >1 km3/yr), large volume (~106 km3) eruptive events of mafic magma (Figure 1). Although silicic volcanic rocks have long been recognized as being associated with CFB provinces (e.g., the Paraná-Etendeka and North Atlantic igneous provinces), they have often been regarded as volumetrically insignificant and erupted late in the history of the basalt province (e.g., White & McKenzie, 1989; White, 1992). A recent compilation (Bryan et al., 2002a) has demonstrated that silicic volcanic rocks are associated with most, if not all the CFB provinces and volcanic rifted margins where they can form substantial parts of the eruptive stratigraphy and have eruptive volumes >104 km3 (see also the recent work of Ukstins et al., 2002). Large-volume ignimbrites are the dominant silicic volcanic rock type of CFB provinces and the individual silicic eruptive units can have thicknesses, areal extents and volumes that are at least equal to, but more often exceed those of the interbedded flood basalt lavas. The Springbok Quartz Latite of the Etendeka province for example, is arguably the largest volume eruption known from the geologic record (> 6340 km3, Ewart et al., 1998). Placed in the perspective of crustal thickness, this unit would have corresponded to a magma sphere with a diameter of 23 km! It is the silicic eruptive units from the CFB provinces that had the eruptive mechanism (i.e. explosive Plinian-type, ash-generating and stratosphere-penetrating eruptions) and magmatic volume most likely to cause global environmental forcing. The potential aerosol mass associated with such silicic eruptions from LIPs is unknown. This is a fundamental aspect of LIPs that has been overlooked.

Figure 1. Distribution of Mesozoic-Cenozoic large igneous provinces (LIPs) with silicic LIPs in italics. NAIP, North Atlantic Igneous Province; CAMP, Central Atlantic Magmatic Province; Rajm. Rajmahal basalts; TVZ, Taupo Volcanic Zone; NW Aust, Northwest Australian oceanic plateaux; Cuvier, Roo Rise, Scott, Wallaby and Naturaliste. Figure from Bryan et al. (2002a) and modified from Coffin & Eldholm (1994).

It has essentially only been recognised in the last 10 years that silicic-dominated igneous provinces (SLIPs) exist with eruptive volumes comparable to those of the CFB provinces and were associated with continental break-up. Two such SLIPs include the Early Cretaceous volcanic rifted margin of eastern Australia (Bryan et al., 1997; 2000), and the Jurassic Chon Aike Province of South America and the Antarctic Peninsula (e.g., Pankhurst & Rapela, 1995; Pankhurst et al., 1998; 2000; Riley & Leat, 1999). The petrogenesis of these silicic-dominated LIPs is more complex than typical basaltic LIPs because of their wider variety of volcanic and intrusive compositions, and they have low proportions of basalt expressed at the surface. The volcanic rocks typically show calc-alkaline affinities that resemble modern destructive plate margin volcanic rocks rather than bimodal or alkalic volcanism associated with CFBs and continental rifts. The calc-alkaline chemistry of the rhyolites has generated ambiguity when interpreting the tectonic setting of magmatism, and as a consequence, the tectonic setting of the magmatism has been wrongly interpreted in the past (e.g., Eastern Australia; see Ewart et al., 1992; Bryan et al., 1997; Sierra Madre Occidental of Mexico; Bryan et al., submitted). The purpose of this contribution then, is to draw attention to the existence of SLIPs, their relationship to continental break-up, and the implications they have for mantle dynamics and crustal evolution.

2. Silicic LIPs

Silicic magmatism on continents can be extensive (> 105 km2), voluminous (104 to > 106 km3) and long-lived (10 – 40 Myr; Table 1). Bryan et al. (2002a) defined the term “silicic LIP” (SLIP) to describe those volcano-plutonic provinces with the following characteristics:

  1. extrusive volumes > 105 km3;
  2. comprising > 75% by volume of dacite-rhyolite, often with mostly calc-alkaline I-type signatures;
  3. rhyodacite-rhyolite compositions near the hydrous granite minimum;
  4. lithologically dominated by ignimbrite;
  5. active over prolonged periods (up to 40 Myr); and
  6. may be spatially and temporally related to other mafic LIPs and plate break-up.

Based on more recent studies, it seems clear that many SLIPs have minimum eruptive volumes of 2.5 x 105 km3. I suggest here that the minimum eruptive volume “cut-off” for identifying a SLIP be revised upwards from the definition of Bryan et al. (2002a). Table 1 illustrates the significant difference in total eruptive volume, eruptive duration, extent and magma flux rates between the largest SLIPs and the more recently formed and better known examples of large silicic volcanic provinces such as Taupo and the Altiplano-Puna. Interestingly, SLIPs (with 10% mafic igneous rocks) and CFB provinces (with 10% silicic igneous rocks) represent end-members, and LIPs with subequal proportions of mafic and silicic igneous rocks are absent from the geological record.


Age (Ma)

Volume (km3)

Dimensions (km)

Magma flux (km3 kyr‑1)*


(Eastern Australia)


>1.5 x 106

>2500 x 200


Bryan et al. (1997; 2000)

(northeast Australia)


>5 x 105

>1900 x 300


Bain & Draper (1997); Bryan et al. (2002b)

Sierra Madre Occidental (Mexico)


>3.9 x 105

>2000 x 2‑500


Ferrari et al. (2002); Aguirre‑Diaz & Labarthe-Hernandez (2003)

Chon Aike
(South America‑Antarctica)


>2.3 x 105

>3000 x 1000


Pankhurst et al., (1998; 2000)

(central Andes)


>3 x 104

~300 x 200


De Silva (1989)

Taupo Volcanic Zone
(New Zealand)**


~2 x 104

300 x 60


Wilson et al. (1995); Houghton et al. (1995)

Table 1. Catalogue of large SLIPs ordered in terms of minimum estimated extrusive volumes (from Bryan et al., submitted). The provinces in italics meet the criteria of Bryan et al. (2002a) for being a SLIP. However, all provinces are dominated by rhyolitic igneous compositions and ignimbrite. *Magma flux rate is averaged eruptive flux, based on known extrusive volumes for the provinces. **Does not include earlier magmatic record of the Coromandel Volcanic Zone (Adams et al., 1994; Carter et al., 2003) that extends the period of silicic volcanism to 12 Ma. The total eruptive volume, province dimensions ± magma flux rate for Taupo are therefore greater than listed.

Such large-scale silicic magmatism must ultimately derive its thermal energy from the mantle (e.g., Hildreth, 1981). A fundamental question then for SLIPs, is whether the large-volume silicic magmatism represents new additions to continents of magmas derived from the mantle, or if instead it mostly reflects melting and recycling of continental crust. By either process, the composition, structure, rheology and other features of the crust would be fundamentally changed by SLIP magmatism sustained over tens of Myr. SLIP magmatism is thus important to understand if we are to address basic tectonic questions regarding the compositional structure and stability of continents, mantle dynamics, and even for the potential of mantle convective instability near the base of the crust.

3. The biggest silicic LIP - The early Cretaceous Whitsunday Province of eastern Australia

The Early Cretaceous Whitsunday Volcanic Province is the northern extension of a silicic-dominated pyroclastic volcanic belt that extended along the east Australian coast (Figure 2). The northern extension has the dimensions of > 900 km along the strike length, > 1 km thickness, and a minimum extrusive volume of > 105 km3 (Clarke et al., 1971; Bryan et al., 1997). The southern extension and remainder of the SLIP is interpreted to have been eroded and/or rifted from the Australian continent, and to now occur on submerged continental ridges and marginal plateaux following continental break-up and sea-floor spreading in the Late Cretaceous and Tertiary. The original extent of the volcanic belt is thought to have been > 2,500 km along the present eastern Australian plate margin (Figure 3), and igneous rocks of Early Cretaceous age are widespread elsewhere in eastern Gondwana, occurring in New Zealand, on the Lord Howe Rise and in Marie Byrd Land (see references in Bryan et al., 2000). The bulk of the eruptive products of silicic volcanism, however, are preserved as huge volumes of coeval volcanogenic sediment in the adjacent sedimentary basins of eastern Australia (Figure 2) where the volume of the volcanogenic sediment alone (>1.4 x 106 km3; Bryan et al., 1997) exceeds that of several mafic CFB provinces. Such substantial volumes of coeval volcanogenic sediment are not characteristic of other LIPs, and the voluminous pyroclastic eruptions were an important factor in generating fine-grained volcanic material that was rapidly delivered into these sedimentary basin systems (Bryan et al., 1997; 2000).

Figure 2. Location of the silicic-dominated Whitsunday Volcanic Province (132-95 Ma) and Early Cretaceous sedimentary basins of eastern Australia that contain >1.4 x106 km3 of coeval LIP-derived volcanogenic sediment (Bryan et al., 1997). LIP magmatism was followed by:

  1. km-scale uplift of the eastern margin of Australia beginning ~100-95 Ma (e.g., O’Sullivan et al. 1995; 1999);
  2. sea - floor spreading in the Tasman Basin - Cato Trough - Coral Sea Basin occurring 84-56 Ma (e.g., Veevers et al., 1991); and
  3. intraplate alkaline volcanism (80-0 Ma, shown in black) that was partly synchronous with sea-floor spreading, and which defines a broken belt 4,400 km long along the “highlands” of eastern Australia.
Intraplate alkaline volcanism occurred within 500 km of the coastline, and has an extrusive volume of > 20,000 km3 (Johnson, 1989). QLD, Queensland; N.S.W., New South Wales; Vic., Victoria, Tas., Tasmania; S.A., South Australia.

Figure 3. Map of the SW Pacific reconstructed at 100 Ma (Yan & Kroenke, 1993), showing the inferred distribution of the Early Cretaceous silicic pyroclastic volcanic belt along the eastern Australian plate margin: the proposed source of Aptian-Albian volcanogenic sedimentary rocks in the Great Artesian and Otway/Gippsland basin systems. Volcanogenic sediment was shed westwards (arrows) into the basin systems. The location of site 207, DSDP Leg 21 is shown, which bottomed in 96 Ma rhyolites on the Lord Howe Rise (McDougall & van der Lingen, 1974). PNG, Papua New Guinea; QP, Queensland Plateau; LHR, Lord Howe Rise; NR, Norfolk Ridge; NZ, New Zealand.

The main period of volcanic activity occurred between 120 and 105 Ma (Ewart et al., 1992). Lithologically, the volcanic sequences are volumetrically dominated by welded dacitic-rhyolitic lithic-rich ignimbrite, and some interpreted intracaldera ignimbrite units are up to 1 km thick (Clarke et al., 1971; Ewart et al., 1992; Bryan et al., 2000). Coarse lithic lag breccias containing clasts up to 6 m in diameter (Ewart et al., 1992) commonly cap the ignimbrites in proximal sections and record episodes of caldera collapse. The volcanic sequences record a multiple vent, but caldera-dominated, low relief volcanic region (Bryan et al., 2000). Volcanism appears to have evolved from an early explosive phase dominated by intermediate compositions, to a later, bimodal effusive-explosive phase characterized by rhyolitic ignimbrites and primitive basaltic lavas/intrusives (Bryan et al., 2000).

Chemically, the suite ranges continuously from basalt to high-silica rhyolite (Figure 4), with calc-alkali to high-K affinities (Ewart et al., 1992). The range of compositions is interpreted as being generated by two-component magma mixing and fractional crystallization superimposed to produce the rhyolites. The two magma components are:

  1. a volumetrically dominant partial melt of relatively young, non-radiogenic calc-alkaline crust; and
  2. a within-plate tholeiitic basalt of E-MORB affinity, and similar to the Tertiary intraplate basalts of eastern Australia (Ewart et al., 1992; Stephens et al., 1995).

Figure 4. A) Total alkalis vs. silica diagram showing the range and continuity of volcanic and intrusive compositions of the Whitsunday Volcanic Province, which is represented at the small scale by individual island sequences (South Molle). The diagram contains 315 X-ray fluorescence analyses. B) Hf-Ta-Th relationships for the Whitsunday Volcanic Province from Ewart et al. (1992). Note the projection of the mafic compositions into the E-MORB or within-plate field, whereas the rhyolites plot in the destructive plate margin field.

This magmatic event heralded the onset of continental break-up in eastern Gondwana, and the formation of the eastern Australian passive margin in the Late Cretaceous/Tertiary (Bryan et al., 1997). Apatite fission track thermochronology has shown that kilometre-scale uplift and erosion began along the length of the eastern Australian highlands following the cessation of magmatism at 100-95 Ma (O’Sullivan et al., 1995; 1999). Regional uplift of 1-2 km, as predicted in large plume models did not occur prior to Whitsunday volcanism; in fact much of eastern Australia (the Great Artesian Basin in Figure 2) was a shallow sea during the Early Cretaceous. Sea-floor-spreading in the Tasman Basin began at ~80 Ma and continued into the Early Tertiary until ~60 Ma (Veevers et al., 1991).

The Whitsunday SLIP is like many other LIPs in being followed by asthenospheric-derived or “hotspot”-style mafic volcanism (see review in Johnson, 1989; Figure 2). The onset and widespread eruption of within-plate alkali basalts in eastern Australia (Johnson, 1989) began at ~80 Ma, and thus was coincident with sea-floor-spreading. However, there was a 15 Myr hiatus between the terminal phases of Whitsunday LIP magmatism and the first expressions of “hotspot” volcanism. Although time-space patterns in intraplate volcanism can be explained by the northward plate motion of Australia, several short-lived hotspots are required to explain the width and space-time patterns of intraplate volcanism. Of interest is that some of the youngest “hotspot” continental volcanism occurs in northern and southern Australia and at the extremities of the intraplate volcanic belt (Figure 2).

Sea-floor-spreading patterns in the Tasman Basin occurred in a “zipper” fashion, propagating northwards with time. Symonds et al. (1987) infer that seafloor-spreading ceased along the length of the Tasman Basin system (Figure 2) by about 56 Ma. In conclusion, these time-space relationships between magmatism, highlands uplift and sea-floor-spreading are most readily explained by detachment models where eastern Australia is interpreted as an upper-plate passive margin (e.g., Lister & Etheridge, 1989).

4. Discussion - The Generation of Large Silicic LIPs

Although rhyolites can occur in a variety of tectonic settings, both oceanic and continental, large volume (> 104 km3) silicic volcanism is restricted to continental margin settings and, to a lesser extent, to continental interiors when associated with CFB provinces. The silicic volcanic rocks associated with the CFB provinces are widely believed to be the end-result of varying amounts of assimilation by basaltic magmas of partial melts of either anhydrous granulitic lower crust or mafic underplate at high temperatures, followed by extended fractional crystallisation. For SLIPs where the volume of silicic magma generated is at least an order of magnitude larger, partial melting of lower crust is essential, with the most suitable source materials being hydrated, calc-alkaline and high-K calc-alkaline andesites and basaltic andesites/amphibolites (e.g., Roberts & Clemens, 1993).

Basement to the Whitsunday Volcanic, Chon Aike, and most of the Sierra Madre Occidental provinces (Table 1) comprises Palaeozoic-Mesozoic volcanic and sedimentary rocks accreted and/or deposited along the continental margin. The involvement of Mesozoic to Palaeozoic crust in magma genesis is supported by Nd model TDM ages for the Whitsunday Volcanic Province (see Ewart et al., 1992), whereas mid-late Proterozoic (“Grenvillian”) model ages are indicated for the crustal source in the eastern (interior) part of the Chon Aike Province (Pankhurst & Rapela, 1995; Riley et al., 2001). These older depleted model ages may reflect either that of the sedimentary provenance or formation of the crust (Pankhurst et al., 1998). Nevertheless, the long history of subduction and intrusion of hydrous melts into the lower crust along the proto-Pacific margin is considered crucial for the generation of the large volume rhyolites of the Chon Aike Province (Riley et al., 2001). This difference in lower crustal materials between mafic and SLIPs (i.e., the presence of anhydrous or hydrous crust) led Stephens et al. (1995) to coin the term “wet” LIP to describe SLIPs such as the Whitsunday Volcanic and Chon Aike provinces.

Current work (Bryan et al., submitted) focuses on the Sierra Madre Occidental province of Mexico (see also Mexico pages). Voluminous (~3.9 x 105 km3), prolonged (~18 Myr) explosive silicic volcanism makes the mid-Tertiary Sierra Madre Occidental of Mexico one of the largest intact silicic volcanic provinces known. We used zircon isotopic systematics (via laser ablation ICP-MS) as probes to assess crustal involvement in Sierra Madre Occidental silicic magmatism. Zircon xenocrysts in some of the oldest rhyolite ignimbrites provide direct evidence for some involvement of Proterozoic crustal materials, and potentially of more importance, the recycling of Mesozoic and Eocene age, isotopically primitive, subduction-related igneous basement. Some of the youngest rhyolitic ignimbrites show even stronger evidence for inheritance in the age spectra but lack old inherited zircon (i.e., Eocene or older). Instead, inherited grain ages in these young Sierra Madre Occidental ignimbrites (with eruptive ages of 20-23 Ma) lie in the range ~23-32 Ma.

Reworking of igneous rock formed during earlier phases of Sierra Madre Occidental magmatism is clearly apparent from the U/Pb age spectra in the youngest rhyolite ignimbrites. This would be predicted if continued basalt injection leads to remelting of formerly intruded magma (Annen & Sparks, 2002), aided by an elevated geotherm due to the prolonged history of basaltic flux. It is worthwhile noting that rhyolites in the Deccan and Karoo flood basalt provinces (see also Deccan pages) have been interpreted as remelts of earlier formed mafic igneous underplate, based on isotopic similarities to the associated flood basalts (Cleverly et al., 1984; Lightfoot et al., 1987). The recycling of Sierra Madre Occidental-age zircons into the youngest rhyolite ignimbrites may be a record of a similar process.

An important implication of the U/Pb age data for the Sierra Madre Occidental (and other SLIPs) is that the xenocrystic zircons suggest that remelting of young crustal materials may have been important in producing the geochemical and isotopic signatures of the rhyolites. The involvement of young, non-radiogenic, mafic-to-intermediate and calc-alkaline crust has been fundamental to the generation of other large-volume silicic igneous provinces (e.g., Whitsunday Volcanic Province, Ewart et al., 1992; Bryan et al., 2002a; Chon Aike Province, Pankhurst et al., 1998; Riley et al., 2001). Most recently, new studies on the ages of inherited zircons occurring in Taupo Volcanic Zone rhyolites (Brown & Smith, 2004; Charlier et al., 2004) suggest that melting of Early Cretaceous volcanogenic sedimentary rock was a contributor to rhyolite generation. These volcanogenic sediments, regionally extensive in eastern Gondwana during the Early Cretaceous, were themselves sourced from coeval, isotopically primitive, calc-alkaline intermediate-to-silicic explosive volcanism (Bryan et al., 1997). Such data have important implications for how we interpret mantle-like isotopic compositions in rhyolites.

Table 2 summarises the key processes that lead to the development of SLIPs or CFB provinces with associated large volume rhyolites. The presence of a fertile crustal source appears to be the main difference between silicic and mafic LIP formation. Large degrees of crustal partial melting, essential to produce the large volumes of rhyolitic magma, are controlled by:

  1. the water content and composition of the crust, and
  2. a large thermal input from the mantle.

Although the thermal budget for mafic and SLIPs is considered the same, hydrous crustal material will be more receptive to melting, and will begin to melt at lower temperatures. In contrast, melting of a refractory dry crust will be limited by prior depletions in “minimum melt” components and pre-existing low geothermal gradients. Subsequent melting events will not only require higher temperatures, but will produce less silicic (rhyodacitic) compositions.


Mafic LIP

Silicic LIP

Crustal Setting

Craton interior

Accreted orogenic margin

Crustal Composition and age

Refractory Archean - Proterozoic, dry mafic/silicic, brittle crust

Fertile Proterozoic - Phanerozoic, hydrous crust with a large I‑type (calc‑alkaline) meta‑igneous component

Driving Processes

Thermal and mass transfer into crust caused by hot mantle upwelling, and lithospheric extension

Nature of crust/magma interaction

Crust with low pre‑existing geothermal gradient, melts to produce low volume, high temperature (dry) ternary granite minimum magma

Widespread partial melting of crust (~20%) to produce large volumes of hydrous, ternary granite minimum magma.

Thermal and mass transfer characteristics

Crust‑penetrating structures readily transfer mafic melt to surface. Mafic magma can be thermally and chemically insulated from crust by chilling along reservoir margins limiting further crustal melting.

Density/buoyancy filter caused by silicic melt zone, and lack of well‑defined crust‑penetrating structures, suppresses rise/transfer of mafic magma. Containment of mafic melt promotes further increase in temperature and degree of crustal partial melting.

Magmatic processes and geochemical signature

Magma processes dominated by FC/AFC producing large volumes of variably contaminated within-plate basalt. Volumetrically minor silicic magma generated by AFC/PM. Melting of mafic underplate may occur.

Magma processes dominated by mixing and AFC producing large volume, volatile‑rich rhyolitic‑ rhyodacitic melt with calc‑alkaline signature and highly contaminated mafic‑intermediate magmas.

Eruption characteristics

Effusive, flood basalt lava‑dominated volcanism. Variable proportions of silicic pyroclastic rocks and lesser lavas from calderas, central igneous complexes ± fissures.

Explosive silicic‑dominated volcanism erupted from multiple caldera complexes with minor mafic‑intermediate lavas. Highly variable, upper crustal structure/ rheology controls character of upper crustal magma reservoirs & eruptive centres (plutons, calderas, rifts)

Table 2. Summary of the important crustal preconditions, magmatic processes and erupted products that lead to the development of mafic and silicic LIPs (from Bryan et al., 2002a).

Palaeo and active convergent margins tend to be characterized by a fertile, hydrous lower crust that can readily melt. Long-lived subduction promotes the development of a hydrated lower crust and lithospheric mantle that can extend for several hundred kilometres from the active margin (e.g., Karoo, western USA; Fitton et al., 1988; Davis et al., 1993), particularly if significant lateral accretion has occurred over time. Previous subduction episodes may also have been important in the development of low-Ti source regions for some CFBs (e.g., Hawkesworth et al., 1988). Heating and partial melting of a hydrous, mafic crust will generate intermediate to silicic composition melts (55-75% SiO2; Rapp & Watson, 1995). The silicic melts can act as a “density barrier”, preventing the mafic magmas from reaching the surface (c.f. Huppert & Sparks, 1988), as will a lack of deep, crust-penetrating structures that can transfer mafic magma to the surface. Note that SLIPs can form in regions where juvenile crust generated by earlier subduction is melted by a major thermal input from the upper mantle. It might be expected therefore, to find SLIPs on young Proterozoic crust prior to, and associated with, Rodinia break-up. The Proterozoic Gawler Range-Hiltaba igneous province (e.g., Fanning et al., 1988; Giles, 1988; Creaser & White, 1991) of southern Australia may be one such Precambrian remnant of a SLIP. The Neoproterozoic (~750 Ma), Malani anorgenic magmatic province of NW India is another example of SLIP development at a time of Rodinia break-up (Roy & Sharma, 1999, Sharma, 2004, Sharma, 2005,

By contrast, Mesozoic-Cenozoic CFBs are emplaced on or adjacent to Archean cratons (Anderson, 1999), where the crust is relatively old (Proterozoic-Archean) and refractory, and any lower crustal melting would occur only at very high temperatures. Extensive mafic dyke swarms (e.g., the Central Atlantic Magmatic Province) imply a brittle crust with deep-penetrating structures that can channel mafic melt to the surface. In the cases of CFBs that have significant volumes of silicic volcanism, crustal melting and assimilation is generated by achieving such high temperatures at the base of the crust, caused by sustained thermal and material input of mafic magma. The Paraná-Etendeka rhyolites for example, are anhydrous and had an eruption temperature in excess of 1050°C (Harris & Milner, 1997), consistent with partial melting and assimilation of anhydrous crustal material at very high temperatures.

5. Afterword

It is important to note that SLIPs comprise similar erupted volumes of magma to CFB provinces, but are produced over much longer time periods (e.g., up to 40 Myr). Although CFB provinces were once considered to have been emplaced over short periods (~1 Myr), recent geochronological studies indicate that they have minimum eruptive histories of 5 Myr (see summary in Bryan et al., 2002a), and the Kerguelen and Ontong-Java oceanic plateaux were emplaced over similar timespans as SLIPs (30-40 Myr). Therefore, the generation of both SLIPs and oceanic plateaux requires the sustained upwelling and melting of mantle material rather than the transient impact of a large plume head as in the plume models commonly applied to explain mafic LIPs (e.g., Campbell & Griffiths, 1990; White & McKenzie, 1989).


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Friday March 4th, 2005: Hetu Sheth
Scott Bryan's page on silicic LIPs is very good and attractively produced. However, there is no mention of the Malani province, which is one of the largest and best examples of these. Indian geology is often overlooked, to the detriment of the science, as it has much to offer. There is also no mention the Madagascar LIP. This is a mafic LIP, but there are huge volumes of associated rhyolite in it, just as in the Karoo. The related rhyodacites of the
Madagascar LIP are, of course, the St. Mary's Islands volcanics of SW India. [Ed: see also Malani page].

Saturday March 5th, 2005: Kamal Sharma
Scott Bryan's page on silicic LIPs (SLIPs) is a nice endeavor. However, there is a strange omission of the Malani volcanic province on the NW Indian shield. The Malani volcanism is the largest Precambrian SLIP on Earth. The inclusion of Malani in the Figure 1 of the page will improve understanding of the Earth’s SLIPs through space and time. The Malani activity was caused by the extensional tectonic regime that resulted after splitting of the Rodinia supercontinent at ~750 Ma (Roy & Sharma, 1999, Sharma, 2004, Sharma, 2005, This silicic event is recorded on the NW Indian shield, Pakistan and the Seychelles, which were part of the Rodinia supercontinent during Neoproterozoic.

The salient characteristics of Malani igneous activity are:

  1. The Malani volcano-plutonic province is spread over approximately 50,000 km2 in western Rajasthan. Besides this, Malani activity is reported from Sind Province of Pakistan (Bhushan & Chandrasekaran, 2002), Kutch, Madagascar and the Seychelles (Tucker et al., 2001; Torsvik et al., 2001; Sharma & Purohit, 2003). Continental fragmentation caused the spread of the large Malani province onto detached outcrops on different landmasses. The largest one is the Malani province in NW India.
  2. The Malani volcanism is dominantly silicic in nature. The rhyolites and rhyodacites spread over 31,000 km2. The maximum stack of 45 flows, constituting a thickness of 3.5 km, is identified in the Siwana caldera, which has a volume of more than 1,00,000 km3.
  3. The Malani magmatism is generally of terrestrial in origin. Aqueous conditions are observed locally or in the initial stage of volcanism. The volcanism resulted in ignimbrite eruptions, rhyolite flows, hot avalanches and ash fall eruptions through multiple fissure/rift systems that developed in the intraplate tectonic setting. This was followed by granite plutonism and terminal felsic/silicic dykes.
  4. Due to lack of well-constrained geochronological data on Malani rocks, there are different opinions about the age and duration of the magmatism. The volcanism commenced with initial basaltic eruptions which were followed by large-scale rhyolite and other silicic flows. The felsic volcanics have been dated at 779 ± 10 Ma (Rb-Sr) and the ultrapotassic high-silica rhyolites gave a Rb-Sr age of 681 ± 2 Ma (Rathore et al., 1999). Dhar et al. (1996) reported 723 ± 6 Ma as the average Rb-Sr age of the cogenetic and comagmatic rhyolites and the granitoids of Jalore and Siwana. Torsvik et al. (2001) described 771-751 Ma U/Pb ages from Malani igneous rocks. Generally, there is unanimity on the ~750 Ma age of the Malani igneous rocks.
  5. The Malani SLIP is the best example that accompanies continental break-up during Precambrian time. Torsvik et al. (2001) and Poornchadra Rao et al. (2003) report that the Malani Rhyolite magnetic direction and its pole position produce a good fit of the Seychelles Islands with the Indian subcontinent at 750 Ma, from the palaeomagnetic data on granitoids from the Seychelles Islands having the same age, later fragmenting due to Rodinia continental break-up.
  6. Rodinia splitting resulted in widespread Neoproterozoic anorogenic, commonly bimodal, magmatism on most of the continents under extension (Sharma, 2004). After the assembly of Rodinia (~1000 Ma), the crust became thick and remained thermally insulated for a long time. Anderson (1982) suggests that a stalled supercontinent should insulate the mantle, and that the resulting accumulation of heat should partially melt and expand the asthenosphere. Prolonged heat build-up in the silicic crust led to extensional tectonics and intraplate anorogenic magmatism. The splitting of the Rodinia supercontinent led to the development of new ocean floor and cratonic fragmentation, which ultimately led to the development of intraplate anorogenic rift magmatism on the northwestern Indian shield, Madagascar, Seychelles and other landmasses.

In summary, Neoproterozoic Malani igneous activity can be ascribed to the status of a SLIP (Bryan et al., 2002a).

Sunday March 6th, 2005: Scott Bryan
Yes, the Malani igneous province is an omission from the text section where I suggest/predict that SLIPs should also be present during Rodinia break-up and suggest that the Gawler Range-Hiltaba igneous province of south Australia may be one example of this. Note however, based on the volumes you present, the Malani is not one of the best and largest examples of SLIPs as suggested by Hetu Sheth (cf. Table 1 of SLIPs web page). I have added a sentence to Section 4 for completeness.

Figure 1 is from Bryan et al. (2002) which focused exclusively on the Mesozoc-Cainozoic LIPs and silicic volcanism associated with them. It was beyond the scope of that paper to delve into the Precambrian and discuss potential examples associated with Rodinia break-up. This web page is essentially an extension of that paper, with some more recent thoughts of mine on the topic. There are many big silicic igneous provinces formed through time, and it is beyond the scope of Figure 1 of the web page to show all these. However, what will be useful in the future is to collate all these examples and compare/contrast them in a review paper, similar to some of the work that Richard Ernst has been doing.

Although the Malani may be classified as a SLIP, it only just meets the volume criteria of Bryan et al. (2002) of 100,000 km3, but not the revised minimum volume criteria of 250,000 km3 suggested on this web page. This may be because substantial erosion of the sequences since 750 Ma has greatly reduced the preserved extrusive volume. But the basis for the extrusive volume calculations for the Malani province is unclear. Kamal Sharma states that the volcanics cover 31,000 km2, but an intracaldera section thickness of 3.5 km has been used to estimate eruptive volume for the entire province. Use of an average thickness across the province would be more appropriate, and not the maximum thickness produced by "local" volcanic collapse structure(s). There is some mention that the Malani led to break-up, but the rifted margin that followed this SLIP magmatism is not specified in the discussion above. The Whitsundays, Chon Aike and Sierra Madre Occidental all led to continental rifting.

Based on the limited age data, it is difficult to compare the Malani with the other SLIPs in terms of magma flux rate, but based on the volume estimates and duration of 20 Myr (771-751 Ma) given by Sharma above, the Malani only had a magma flux rate of 5 km3/kyr that is most similar to the TVZ and Altiplano silicic volcanic provinces rather than the big SLIPS listed in Table 1. This rate (and total volume estimate) may be even less if the thickness across the province of volcanics is less than the 3.5 km as discussed above. Some clarification on the proportion of rhyolites to basalt, and the proportion of rhyolitic lavas to ignimbrite for the Malani province would be useful. It seems there may be a lot of lava. The Gawler Range Volcanics have very large-volume lavas, with ignimbrite minor. Was there something peculiar about the Proterozoic silicic igneous provinces that produced more lavas than ignimbrites? I'd be interested in comparative comments from Sharma and Sheth with the recent work on the Gawler Range lavas [e.g., Allen SR, McPhie J (2002) Geological Society of America Bulletin, 114: 1592-1609; Allen SR, Simpson CJ, McPhie J, Daly SJ (2003) Australian Journal of Earth Sciences, 50: 97-112].

Thanks for your feedback.

Monday March 6th, 2005: Kamal Sharma
Many thanks to Scott et al. for classifying Malani as a SLIP. I quoted the reported case of Siwana caldera for thickness. Malani is related to the Precambrian and after the closing of Malani in the NW Indian shield, sediments of the Marwar supergroup were deposited on it. Also, the crust was eroded after the 750-Ma Malani event. So it is difficult to get precise data on the thickness as for intact LIPs. OK, Figure 1 is exclusively for Mesozoic-Cenozoic LIPs. Actually, the title does not mention this fact, which is why I have raised the question [Ed: Scott has now amended this oversight]. Malani silicic volcanism represents ignimrite, ash fall and other pyroclsts in significant amounts. Volumetrically, basalt occurs in small amounts in comparison with the rhyolites.

last updated 7th March, 2005