Links within this page: Gondwana Flood Basalts and Sills Formation of Dolerite Sills Ferrar Dolerites (Antarctica) Tasmanian Dolerites Deccan Traps (India) Karoo Dolerites (South Africa) Parana Flood Basalt types (Southern Brazil)

Throughout geological history and on both continents and seafloor there have periodically been erupted enormous amounts of basalt in very short periods of time, often apparently thousands of cubic Km in a matter of a few hundred years, or even days. These may cover areas of as much as 100,000 sq. km or more as both flood basalts and as intruded sills, the magmas of each major pulse often being remarkably homogenous. At the same time, different units in the same region can differ widely especially in their Ti, P, K, Zr, in fact in all residual elements. They are in many respects similar to but are seldom as basic as ORBs usually having 52 - 56% SO2, have high and variable Ti, Fe as have both ORBs and OIBs (but usually showing marked Fe-Mg fractionation) and have a typical continental signatures with high K-group elements and relatively low Nb-Ta, indistinguishable from the andesites. Late stage alkali-basalts, melilotites and carbonatites are sometimes associated. Small volumes of residual iron-rich granophyres or micro-granites are fairly common. Usually tholeiitic, the typical cumulate is an orthpyroxenite, but sometimes picrite, rarely anorthosite.

Gondwana Flood Basalts and Sills

When the Gondwana supercontinent fragmented into Europe, Asia, the Americas, Africa, Australia and Antarctica in the Jurassic era (about 165Myr bp), the fractures resulted in enormous outpourings of basalt, not only along the spreading axes now seen as mid-oceanic ridges but along transverse faults extending into the continents where we now see the continental flood basalts. These are now termed the Ferrar Dolerites and Volcanics in Antarctica, the Tasmanian Dolerites in Australia, the Deccan Traps in India, (mainly basalts), the Parana Basalts in southern Brazil, the Karoo Dolerites in South Africa, the Hebridean Province of the British Tertiary in western Scotland and northern Ireland, the Palisades Sill and related intrusions found in eastern USA., and the Flood basalts of East and West Greenland. There are older Flood basalts, mainly found in the Archaean and Proterozoic, for example the Coppermine Basalts of the north-western Canadian Shield, and the Keewenawan Basalts of Southern Canada-Northern Minnesota. The Siberian Traps are another extensive series of flood-basalts but of Permo-Triassic age. The only younger members seem to be the 15 myr Columbia River basalts of the state of Oregon, Western USA, and the related, partly younger, flood basalts of the Snake River Plains in Idaho.

Variation diagram for all available Jurassic flood basalts of Antarctica. Some sills included, seen especially in those samples of more than 10% MgO which include orthopyroxenites and with some higher fractionates, esp ferro-basalts. Note wide range in TiO2 in different magmas, the Kirwan basalts being the lowest.

Originating in the enriched sub-continental mantle, continental Flood Basalts have a very different fingerprint for the LIL elements to the Oceanic Ridge basalts. No depleted members are known, the least enriched being close to EMORB composition, but usually with even more elevated Cs, Rb, Ba, U, Th, K but depleted Nb-Ta. Some form the most LILE-enriched quartz saturated basalts known,in fact they have a typical continental signature, very similar, as stated bove, to Andean andesites, though lower in silica and alumina and much more iron-rich in fractionated members.

Because they were extruded through very thick continental plates of lower specific gravity, which are in most areas covered with a up to 5-6000ft of detrital sandstone-arkose in the old Gondwana terranes, the basalts often intruded as sills, many of 1000-1200ft in thickness. The volumes of sill in Antarctica are of at least 200,000 cubic km, though possibly as much more is buried under ice.

The Gondwana basalts are mainly quartz saturated rocks with plagioclase, clinopyroxene, orthopyroxene, pigeonite and, rarely, olivine except in South Africa where olivine-bearing rocks are more common. There may be half a dozen distinct magma types in one area usually but not always obviously related by orthopyroxene fractionation. Sills may form in the basement granites, at the peneplaned surface on which rest the sandstones (at least in Antarctica and Tasmania) and at intervals of a few thousand feet up through the flat-lying sandstones above. Usually the heavier, orthopyroxene or olivine-rich sills are found low in the sequence, higher up may be ferrobasalts with up to as much as 56% silica.

The sills are quite differentiated whenever the thickness exceeds a few hundred feet with the most magnesian compositions being found in the lower third (often with crystal cumulates of orthopyroxene) with more feldspathic rock in the upper third and lenses rich in quartz and potash-group elements near the top.

High in the sequence and resting on the old Jurassic landsurface are remnants of lavas, tuffs and laharic mudflows. Some flows and tuffs near the upper Mawson Glacier (South Victoria Land) have granophyric glass altered to coarse (up to 3 in) zeolites especially mesolite, scolecite, heulandite, okenite, stilbite etc, with quartz geodes while in the Parana basalts of South America tropical weathering and percolation of warm acid water causes silica solution at the surface resulting in cavities deeper in the flows being partly filled with gorgeous secondary amethyst quartz.

The Finger Mountain Sill of Jurassic Ferrar Dolerite, Upper Taylor Glacier, Antarctica is about 600ft thick and is intruded into Beacon Sandstone. The younger inclined sheet on the skyline cuts across it diagonally. Finger Mtn is an easy climb from higher up the glacier, not so easy straight up! For futher information on the Beacon Sandstones, visit www.RossSea.info (Photo B.Gunn 1959)

Formation of Dolerite Sills

A basaltic sill of perhaps 1000ft thickness being intruded at depth and being well insulated may take up to 10,000 years to solidify. In this time a good deal of differentiation may take place. Even a small lava lake such as 1959 Kilauea Iki in Hawaii took more than 10 years to solidify and as heavy olivine was present, it showed quite large variations in Cr, Mg, Ni with depth when later drilled, (Helz, 1987, Geochem. Soc.Sp.Pub., 241-258). Virtually all sills more than a few hundred feet in thickness will show chemical variations due to two reasons, both of which may be sometimes found in a single sill.

1. Gravitational Fractionation
   Early formed minerals especially chromite, olivine, orthopyroxene, clinopyroxene and calcic plagioclase tend to settle into the lower half of the sill. The residual liquid, lower in MgO, Cr, Ni, Co etc but richer in Si, Al, Na, Sr, solidifies in the upper half of the sill. The results in an "S" shaped distribution with height for Mg, Cr, Ni, Ca and a reverse "S" for Na, Al, Sr, Na. The last few percent of the basalt to solidfy is greatly enriched in the elements found in low temperature minerals. These include include K, Ba, Rb, Cs, U, Th, Si, Fe, Ti, P, Y, V, REE all of which percolate upward to form lenses of residual pegmatite a few inches to feet thick at levels a few feet to perhaps as much as 100ft below the top of the sill. Graphic intergrowths of quartz and a potash feldspar are typical.
   Variation diagrams of such sills are usually elliptical or form a figure "8". In a plot of say Na against Mg, the Na content at 7% Mg on the lower limb of the "S" will be much less than the Na at 7% Mg on the middle limb of the "S". Smooth variation diagrams are therefore impossible in sills, a fact that baffled me for years.
   The end product of sill fractionation is an Fe-rich granophyre which may be very coarse-grained and look almost granitic. There are a few cases of an iron poor rhyo-dacitic rock being formed.
   
   
2. Multiple Injection.
   Lava rising through the continental crust forms a sill when the "Hydrostatic" (magmastatic??) head of pressure exceeds the confining pressure of the country rock. For basalt of sg 3.5 or more rising through quartzites and arkose of sg 2.4 or less, this point is reached well below the surface, as much as 8000ft below, possibly even more.
   As it may take several hundreds to several thousands of years to solidify there is a high probability that a second pulse of magma will occur before final solidification. This new injection follows the path of least resistance into the already-formed sill and makes it thicker. If the sill has solidified the new pulse will be injected above it, in some cases actually between it and the overlying sandstone, sometimes a few hundred feet higher. If the new pulse has a different composition the variation with height in a doubly injected sill may look very odd. In some cases what follows as a second injection is a thick mush of olivine (Palisades Sill, NY) or orthopyroxene (Vanda Sill, Basement Sill, Solitary Rocks Sill (Antarctica), or the Ben Lomond Sill, Tasmania) which has settled out in some vast magma chamber below. Perhaps the added weight of the sill above renders it unstable and it is set in motion upwards to pour into the partly formed sill above. Some times the contact between mush and new sill is gradational, but if the sill is 90% solidified it may be quite abrupt. In the Solitary Rocks Sill in the Taylor Valley, (Gunn, 1966) a sill of about 7-8% MgO within a few inches grades into a crystal cumulate with an average of 18-20% MgO. This mush in turn may gravitationally settle so the cumulate of 25% MgO may decline to only 10-12% a few hundred feet above but becoming richer in bytownitic plagioclase. Near the top is the granophyre which came out of the original sill, orthopyroxene cumulate having little or no granophyric residue.
   The result of the cumulate injection is to produce an average bulk compostion with perhaps 150% or more Cr, Ni, Mg etc than the average found in the upper and lower chilled margins. Few sills anywhere have an average bulk composition the same as that of the margins. so repeated injection into sills must be common. The Palisades Sill has much higher bulk Ca-Al for example.
   Fingerprints of sills of variable orthopyroxene content will, like lavas of variable olivine content, show a parallel pattern. What will be the effect on light REE vs heavy REE in an REE diagram?? What ought to be the effect on REE in sills like the New Mtn Sill (Ferrar Dolerites) where only Cpx has accumulated? I do not believe anyone has yet analysed REE from a sill known to be clinopyroxene-fractionated rather than olivine or orthopyroxene-fractionated.

Ferrar Dolerites (Antarctica)

	Variation in MgO% and ppm Cr, Ni, Zn in the ~1000ft Vanda sill on the southern lip of the Wright valley above Salina Pond, half a dozen miles west of Lake Vanda. The average Cr and Ni especially show that the bulk volume greatly exceeds the average chilled margins. This sill shows pre-emplacement differentiation and multiple injection, and only minor post emplacment differentiation seenn in the greater concentration of Al, Na (not shown) and the K-rich pegmatites in the upper sill.
	Variation in metals for all Ferrar dolerites lavas and some sills.
	K/Rb with a ratio of near 240, usually only found in potassic granites, and the beginning of the realisation that CFBs were a very odd basalts series.
	The REE relative dispersion is typical of flood basalts. being more LREE enriched than standard EMORB, with Ce/La = 2 and Nd = La. The Kirwan Basalts from the Kirwan Mts, on the South African side of the continent are relatively the lowest in both LILE and LREE as well as TiO2.
	Alkaline earths / Zr. Note that Y far exceeds Nb (and Ta) as it does in all NMORBs. Rb is variable but usually also in excess of Nb, while Ba which is enriched steeply with smaller degrees of melt and also with fractionation is much greater than Y (unlike NMORB) and very variable. The fact that few of these samples are chilled or glasses increaces the Ba scatter especially.
	Type Ferrar Dolerite sills, Ferrar Glacier region, South Victoria Land, (Gunn, 1962, 1963, 1965, 1966) Note low but regular TiO2, moderately high K, regular CaO but erratic Al2O3 due to sill effect, moderated in the case of CaO by clinopyroxene compensating for decrease in calcic plagioclase. Cumulates are all orthopyroxene, but with some enrichment in Cpx + pigeonite at intermediate MgO levels. Some low MgO pegmatites are iron enriched.
	Composite diagram of hypersthene dolerite type from Portal Peak, near Beardmore Glacier, Hergt et al, (1989). These are akin to the Lake Vanda Sill type of the Wright Valley near McMurdo Sound. Included also are "high Ti" lavas from Scarab Peak, in Northern Victoria Land (Fleming et al, 1992). These are "pigeonite dolerite" type (actually lavas), fractionated from the Vanda type by removal of orthopyroxene-plagioclase. Sc is not depleted so no clinopyroxene is involved. The La/Lu remains the same so the difference is due to fractionation only.
	Two variants of Ferrar Dolerite from Coates Land on the Weddell Sea coast.
	Vestfjella lavas and dolerites, Dronning Maud Land, Indian Ocean coast. Typical Ferrar type but low Th-U. Luttinen et al (2001)
	Lutinen et al have divided the VestFjella lavas into three chemical types, this being type 1. However there appears to be more than one type present so we have redivided them on the basis of the normalised La/Lu ratio.
	These have REE La/Lu of <2. This strongly domed type seems to have no equivilents elsewhere.
	La/Lu of 2-3, an almost homogenous group.
	La/Lu >3, including some unusually light REE enriched. It is next to impossible to type perhaps 200,000 Km³ of basalt by a few dozen widely separated samples, but the CFB's of Dronning Maud Land seem to be quite different to those of Victoria Land, especialy when we include the very depleted Kirwan Basalts.
	Lavas of Ferrar Volcanics, Prince Albert Mts, North Victoria Land. Again, a typical Ferrar signature. (Antonini et al, 1999)
	Kirwan basalts from Dronning Maud Land, on South African side of continent. Note similarity to low LILE Flood Basalts of Picture Gorge, Columbia River, the Low-Ti basalts of the Deccan and the Lesotho Basalts of the Karoo.

The Ferrar Dolerites show low HFSE and high LILE and are of the kind sometimes called "Low Ti-type". However as we shall see in the Parana and Columbia River, a range of type exists between "High Ti" and "Low Ti" The "Pigeonite Dolerites" found in the Ferrar seem to be crystal fractionates and have high silica (54-55%) elevated LILE and Fe, but low Mg (<3%).

Tasmanian Dolerites (Australia)

The Tasmanian Dolerites have a similar high LILE to the Ferrar Dolerite type but are of fairly constant composition in chilled marginal rocks. Orthopyroxene cumulates are common. Drill cores of thick intrusions reveal thick black glassy granophyres in the upper sections of, eg, the Great Lakes Sill. The Mt Wellington Sill, above Hobart, the capital city, is the most studied, and appears to be a multiple-injection type sill with excess orthopyroxene. Ian McDougal of A.N.U. Canberra, argues that the sill dips west, and sampling up a sloping road has falsified the apparent heights in the sill. I do not believe this controversy has been settled as yet.

	Variation diagram for Tasmanian Dolerites. Many years ago Dr Janet Hergt sent me her PhD data on Tasmanian Dolerites. As this is not on the GEOROC database it may not have been published, but together with Dr Ian MacDougals' PhD data, it makes up a useful file. This diagram throws new light on all the CFB's. The cluster of points between 6 and 7% MgO are the chilled margins or primary rocks and the whole series are quite linear until the separation of the granophyres and pegmatites at MgO <2%. Obviously both orthopyroxne and clinopyroxene (mainly latestage pigeonite) have settled, alumina is expelled but CaO increases with MgO. Sc also shows that CPX goes along with OPX. There is obviously only one magma type the small differences in chilled margins being related by difference in orthopyroxene, as in the different chilled margins seen in South Victoria Land Ferrar Dolerites, and if there is any late stage autointrusion of orthopyroxenite it is directly related. The Ben Lomond sill in NE Tasmania appeared in thin section to have unusually pure Opx concentrate which in fact shows at the high MgO end of the diagram. Ni-Mg distribution (see below ) is unusually flat and quite linear from 10% MgO to 2% as is Zn, Cu, etc. This series gives us definite fractionation paths for CFB's in high level sills. We can envision pools of several hundred thousand cubic km of magma trapped beneath the continental mass possibly for times measured in millions of years, slowly separating out orthopyroxene, (and in early stages , olivine) concentrates, until fracturing of the crust and earth movment allow sudden outpouring, which in regions of dense crust, may actually mainly reach the surface. The crustal signature appears to be inherited from from the subcrustal mantle previously enriched by subduction. The Columbia River Basalts seem to indicate sometimes separate olivine, OPX and CPX trends. Neither seem at this point to offer an explanation of the very different Ti, P levels seen in other CFB's, which, while never greater than NMORB levels, would demand unacceptable levels of crustal contamination to reduce the TiO2 to the 0.75% levels seen. P2O5 is also remarkably low, compared with P2O5 in the Umatilla Basalts of the Columbia River Group where it may exceed 1%. However, the same scale of variation is seen in all arc andesites.
	Mg vs Cr, Co, Ni, Cu, Zn, V for Tasmanian Dolerites. The unusually flat Ni trend points toward orthopyroxene. Zn declines with increasing MgO more steeply than in olivine controlled series such as seen in Kilauea. Cu is unusually regular. Is the Cr concentrated in Cpx or in sulfides? We do not know at this point, one does get kicks on the SEM when traversing OPX grains, probably Cr spinels are present.
	The alkaline earths plotted against K2O for Tasmania, Note the very high Rb, greater than both Y and Nb (the latter being low in all CFBs. Ba is very high, from 210 to 1400 ppm. Sr declines in the granophyres which have the highest K2O.
	The Tasmanian Dolerites.
	Two types of magma from Tasmania. Pigeonite type from Red Hill and the more magnesian Ben Lomond and Mt Wellington type both associated with orthopyroxene-rich cores to sills, approximating to Peneplain Sill type of Antarctica. The great similarity to Ferrar type is obvious.(Brauns & Hergt, 2000)
	Na-Mg-K diagram for the Tasmanian dolerites. As these are all sills, it shows that within sill fractionation is no different to the trends seen elsewhere for lavas.
	Zr-group diagram for Tasmanian Dolerites Sill. The high Ba and Sr samples are from pegmatite pods. Flattening of Ba at higher Zr suggests that fractionation is Opx + Plag, not Opx only. Note also high Rb and low Nb characteristic of the CFB's. These seem to differ from the Ferrar Dolerites having Ba>Sr>Zr>Rb>Y>Nb but remember this is a single magma type all of opx-fractionated sills, with no lavas. The Ferrar Dolerites included lavas of many parentages and relatively small volume and we have little Zr etc data on the massive sills as yet.

Deccan Traps (India)

Mambai, India

By Hetu Sheth

Karoo Dolerites (South Africa)

The Karoo Series.
Prof Tony A.J. Erlank of Cape Town University, now deceased, over ten years ago put together a file of about 950 analyses of Karoo rocks. While now somewhat outdated, these show perhaps 2/3 of the samples to be tholeiitic basalts with numerous picrites with, after a considerable andesite gap, numerous potassic rhyolites, with some alkaline rocks, basanites and even carbonatites. Karoo sediments intruded by scattered sills and plugs extend from near Cape Town to about the Limpopo River. Flood basalts are found at Lesotho, west of Durban and along the Lebombo Mts west of Maputo, near Kruger National Park. A third area occurs in the west in the Angolan desert at Etendeka.

	Karoo basalts variation diagram. n=950
	Karoo basalts MgO vs Metals. n=950
	Karoo basalts, La vs REE
	Lesotho Fm. Basalts. These are the familiar "Low Ti" flood basalt similar to the Kirwan Basalts of Antarctica, or the Picture Gorge type of the Columbia River. The Kraai River Basalts and the Brosterlei Basalts are similar. The data for the Lebombo rocks, is not complete but they appear to be considerably more enriched, in fact the bulk of samples are very potassic, but not enough information is available at this point to delineate potassic areas.
	Etendeka basalts- Quartz latites. (Erlank File). These occur in the western Angolan desert and range from potassic basalts with again an andesitic gap to what are termed "latites". They appear related to the basalts and appear to be potassic granophyres. The La/Yb ratio changes continuously and there seem to be a series of lineages. With up to 5% K2O these latites are probably the most potassic rocks associated with Flood Basalts.
	Etendeka picrites. Thompson et al, 2001 shows similar, probably in part the same rocks, without the "latite" but including picrites up to 25% MgO. These are similarly highly potassic.
	Etendeka ferro-picrites. Gibson et al, (2000) show a series of HFSE depleted ferro-picrite from the same region with elevated titanium and only a minor negative Nb anomaly.
	Na-Mg-K diagram for the Karoo, Tony Erlank's file. The Karoo is more variable than perhaps any other CRB province and in includes the Etendeka dacites. Later data which includes carbonatites is even more variable.

No data is yet available on the Dicker Villem carbonatites which occurs in the Etendeka area.

The Gramado type Parana Basalts on the opposite shore of the Atlantic are similarly potassic.

Parana Flood Basalt types (Southern Brazil)

The Parana Basalts extend through southern Brazil, Paraguay and Uraguay and are reputed to cover over a million sq km. At Iguazu Falls on the Parana R,, the river cascades over a series of massive flat-lying basalts flows in a series of the most spectacular falls in the world. The different magma types range from high Ti to low Ti (about the same as E-Type MORB), but show only very slight fractionation effects.

Lower TiO2		Higher TiO2
	Variation diagram for the Esmeralda and Gramado low Ti Parana magmas. Fractionation is more obvious in this group, but note how, apart from the high and variable K, the similarity to MORBs, showing the same spread of degrees of partial melt. In this series the P levels are quite low.		Parana variation diagram with 980 analyses including the Pitanga type, the Paranapamena, Ribeira, and Urubici flows with the Ponta Grossa Dykes but no sills. Thanks are cordially extended to D. Peate of Open University who sent us his entire database. The Parana series have a mode between 4 - 4.5% MgO. A poorly defined break can be seen between the high and low TiO2 members. The Parana show the characteristics of most Flood Basalts, low alumina averaging 14%, higher than usual Fe and Ti but without any peak which would be seen had fractionated sills been included, though a few samples with > 7% MgO suggests some fractionation in the Ponta Grossa Dykes.
	Variation in metallic trace elements in the Gramado and Esmeralda types with MgO. Notice the erratically high Co, Cu and Cr which suggests the inclusion of sulfides, yet the Zn remains constant!		Trace metals for High Ti Parana CFB variants. The V follows entirely the ORB fractionation - partial melt constraints as does Cu and Zn. Cu has occasional straying values, Cr and Ni again seems to have rather more variance than they should but the plane of partial melt is known to be at a broad angle to the fractionation direction but is not yet well defined.
	The alkaline earth elements for the low-Ti Gramado and Esmeralda types of the Parana Basalts. Note the domination of fractionation effects, the low Nb vs Y as in all CFBs and the minimal scatter in Zr.		The alkaline earths. In this more enriched, higher Ti group the Nb is almost = to Y and to Rb. Ba & Sr lie mainly within the fractionation - partial melt limits but about 10% are undoubtedly outside these limits. It may be that these late stage elements tend to puddle in the flows. We can see the absence of high magnesian members and that CFBs such as these are far from being all high-degree melts.
	Esmeralda REE normalised to EMORB (~=average ORB). The range of normalised La/Lu is quite small, from 0.98 to ~2. Other variants have higher but overlapping REE.
	Low Ti Parana basalt		Paraparanema Magma Type, Parana Basalts
	Esmeralda Type		Pitanga Type.
	Gramado Type.		Urubici type Parana basalt
			Parana Rhyolites.