Scott Bryan

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

scott.bryan@qut.edu.au

Summary

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.

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: km-scale uplift of the eastern margin of Australia beginning ~100-95 Ma (e.g., O’Sullivan et al. 1995; 1999); sea - floor spreading in the Tasman Basin - Cato Trough - Coral Sea Basin occurring 84-56 Ma (e.g., Veevers et al., 1991); and 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 T_DM 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.

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, http://www.mantleplumes.org/Malani.html).

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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• Yan C.Y., Kroenke, L.W. (1993). A Plate Tectonic Reconstruction of the Southwest Pacific 0 – 100 Ma (animated), Proc. Ocean Drilling Program Scientific Research, 130, 697-709.

Discussion

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, http://www.mantleplumes.org/Malani.html). 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:

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

• Anderson, D.L., 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature, 297, 391-393.
• Bhushan S K and Chandrasekaran V 2002 Geology and Geochemistry of the magmatic rocks of the Malani igneous suite and Tertiary volcanic province of western Rajasthan; Mem. Geol. Surv. Ind., 126 ,179.
• Bryan S.E., Riley T.R., Jerram D.A., Leat P.T., Stephens C.J. 2002a Silicic volcanism: an under-valued component of large igneous provinces and volcanic rifted margins. In: Menzies M.A., Klemperer S.L., Ebinger C.J., Baker J. (eds) Magmatic Rifted Margins. Geological Society of America Special Paper, 362, 99-120.
• Dhar, S., Frei, R., Kramers, J.D., Nagler, T.F., and Kochhar, N., 1996, Sr, Pb and Nd Isotope studies and their bearing onto petrogenesis of the Jalore and Siwana complexes, Rajasthan, India, Journal Geological Society of India, 48, 151-160.
• Pooranchandra Rao S.B., Singh S.B. and Prasanna Lakshmi K.J. 2003 Palaeomagnetic dating of Sankara dyke swarm in the Malani Igneous Suite, Western Rajasthan, India, Current Science, 85, 1486-1492.
• Rathore S.S., Venkatesan T.R. and Srivastava R.K. 1999 Rb­Sr isotope dating of Neoproterozoic (Malani Group) magmatism from south-west Rajasthan, India: evidence of younger Pan-African thermal event ⁴⁰Ar³⁹Ar studies, Gondwana Research, 2, 271-281.
• Roy A.B. and Sharma K.K. 1999 Geology of the region around Sirohi town, western Rajasthan - Story of Neoproterozoic evolution of the Trans Aravalli crust; In: B.S. Paliwal (Ed) Geological Evolution of western Rajasthan, Scientific Publishers (India) Jodhpur, 19-33.
• Sharma K.K. 2004 The Neoproterozoic Malani magmatism of the northwestern Indian shield: implications for crust -building processes Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, 4, 795-807.
• Sharma, K.K., Malani magmatism: An extensional lithospheric tectonic orign, in Plates, Plumes & Paradigms, edited by G.R. Foulger, J.H. Natland, D.C. Presnall and D.L. Anderson, Geological Society of America Special Paper 388, in press 2005.
• Sharma K.K. and Purohit R. 2003 Evolution of “Ur”, an Incipent Continent, to “Seychelles”, a Microcontinent from Northwestern Indian Shield. Proceedings-International Workshop on Earth Ssytem Processes related to Gujarat Earthquake using Space Technology, IIT Kanpur, 85-86.
• Torsvik T.H., Carter L.M., Ashwal L.D., Bhushan S.K., Pandit M.K, and Jamtveit B. 2001 Rodinia refined or obscured; palaeomagnetism of the Malani Igneous Suite (NW India); Precambrian Research, 108, 319-333.
• Tucker R.D., Ashwal L.D. and Torsvik T.H. 2001 U-Pb geochronology of Seychelles granitoids: a Neoproterozoic continental arc fragment; Earth Planet. Sci. Lett., 187, 27-38.

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.