|CONTINENTAL FLOOD BASALTS
(and Layered Intrusions)
Layered and/or mineralised Intrusions of CFB type
Flood basalts with the characteristic continental or sialic fingerprint are virtually the only magma type to form massive sills, possibly because of their widespread occurrence in the older cratons which are capped with sandstones and arkoses.
Thick sills of 1000 - 1500ft such as the Peneplain Sill in the Ferrar Dolerites of Antarctica, show extensive "hidden layering" (i.e., progressive variation in Na/Ca, Fe/Mg, Rb/Ni with depth) together with some phase variation. As we have said elsewhere, orthopyroxene is usually not found in the upper half of a sill, while any olivine-bearing rock is invariably near the base, but augite-pigeonite sills are not usually mineralogically layered though curved semicircles of pure plagioclase may be seen.
Layering in the more mafic sills consists of alternating concentrations of light and dark minerals, usually olivine-plagioclase, orthopyroxene-plagioclase, clinopyroxene-plagioclase, but also chromite-olivine, magnetite ilmenite-feldspar or even biotite or arfedsonite-feldspar have all been described. The bands may be sometimes on a scale of a few inches, and sometimes feet and is usually seen only in intrusions of considerable depth, often several km. The Dufek Intrusion in the Horlick Mountains of Antarctica, while of Ferrar Dolerite type, has a least 3000ft of anorthosite and layered rocks (Opx - plag) exposed, capped by 900ft of granophyre, (Ford, A, 1970, Geol.Soc. S.Af. Sp. Pub.1) . The total depth of this and many other intrusions is quite unknown.
Layered intrusions have been extensively studied and described, especially the Skaergaard intrusion of East Greenland. The books of Wager & Deer, (1939), and Wager & Brown, (1967), are the best known, though Wager himself wrote about 20 books and papers on the subject. Wager & Brown also discuss all layered intrusions world wide and are a great source of information on the Bushveldt, Stillwater and the layered subvolcanic intrusions of the British Tertiary, Mull, The Cuillins, Rhum, Ardnamurchan etc.
Layering is not commonly seen in Oceanic ridge formations, though gabbroic intrusions with phase separations and concentrations occur, the continental environment of lighter weight crustal rocks is much more conducive to the formation of deep intrusions. Alkaline rocks may also occasionally demonstrate, inch scale layering is seen in the "leuco-gabbro" of the alkaline Mount Royal Complex in Québec for example.
Layering appears to be always associated with convective overturn of a deep magma body. Cooling and crystallisation takes place mainly at the top, the lower regions are always hotter. The specific gravity of the upper region with a cooler magma, heavily laden with slowly sinking crystals increases until it becomes unstable, and the whole magma body overturns. This brings the crystals close to the base where they will settle out. As olivines are heavy and sink much faster than plagioclase, they will often be overturned some distance above the base, the feldspar settles out, almost pure until the olivine, now sinking from a hotter to a cooler environment, arrives, usually jacketed with a lower temperature, more Fe-rich rim. Large crystals of any type are more stable than tiny ones, and convincing examples are shown of adcumulous growth onto the crystals which have settled out on the more or less solid base of the intrusion until a layer may be almost pure feldspar or olivine. Finally, disequilibrium becomes so great that there is a reversal to another phase, so both convective overturn and disequilibrium growth in which crystals continue to grow even though the magma is supersaturated for another phase, both seem to play a part.
Deep intrusions are also seemingly necessary for the occurrence of sulfide mineralisation. Wager and Brown, (1967) found that when the S content had reached about 400ppm, immiscible pyrite (FeS) separated and proceeded to scavenge up all the chalcophile elements in the magma. In low silica, high Mg basic intrusions an early result will be the formation of the nickel-iron sulphide, pentlandite, or pure niccolite or millerite. Olivine or orthopyroxene reduces the Ni, Co content of a magma rapidly so one would not expect Ni sulfides to be formed below 5-6% MgO. However Cu builds up with fractionation, as does Zn to a less degree, so chalcopyrite, sphalerite may occur later.
The Sudbury Intrusion
Nothing stated here should be taken as engraved on tablets of stone. We have an on-going project of re-examining the Sudbury Intrusion, one of the world's largest producers of Ni, and a significant source of Co, Cu.
It is probably the most argued over intrusion in the world and the theories propounded range from the serious to the ridiculous. It lies NE of Lake Ontario and WNW of Ottawa, in a mainly Archaean terrane. The intrusion itself has been dated at about 1.7 billion years and is given a thickness now of greater than 3km. It is unlayered but has a basic orthopyroxne-bearing gabbro or norite, a more felsic norite, a micropegmatite zone, and some quartz diorites which appear to be related in most element trends though the diorites have double the Nb of the norites. Above lies 1400m of the Onaping Tuff, a fine black welded? rock, also rather altered as is the whole intrusion. It has a similar fingerprint to the underlying norite.
The norites have up to several percent of pentlandite, niccolite, millerite etc, and we will later append a list of the reported sulfides which is extensive.
These data show normal amounts of Ni, Cu, Co to which have been added variable amounts of up to 10,000 ppm as sulfides as the excess metals correlate with the S and Te contents. There is little As data and less Se, Te than one might wish for.
One of the more fanciful theories long expounded is that Sudbury is the site of a nickel-iron meteorite impact, which shock-melted the country rock into norite and vaporised leaving behind the excess nickel. Quite how this was transposed from nickel-iron metal to sulfide is not revealed, but on a visit once I was shown peripheral shock cones in the country rock, as found in meteorite impact craters. Later theories suggest that a meteorite caused both the formation of shatter cones and the shock-melting of the norite, though the similarity of the latter to Stillwater, the Bushveldt etc demands an impossible series of coincdences
Origins and Associations of the Sudbury Intrusion
The fingerprints above appear at first glance more potassic calc-alkaline than Flood Basalt. It is a low Ti type but with unusually elevated Cs, Rb, Ba, more so than the Ferrar Dolerites. The relative depletions of Zr and P are very similar to those seen in the island of Salina and others in the Aeolian Arc, though no Zr data is available for the quartz diorites.
However, flood basalts and orogenic rocks have very similar fingerprints both originating in the enriched sub-continental mantle and a perfect discriminant has yet to be discovered. The more basic rocks of Sudbury have only 49-51% silica and include ferro-gabbros of 14-15% Fe2O3(t), and apart from the Columbia River CFB's, no ferro-basalts are found associated with andesites between the Canadian Border and Tierra del Fuego or indeed in any other island-arc or continental andesite.
Preliminary calculations suggest the average amount of Cu-Ni present is greater than that in the average unmineralised rock but we do not at this point have a good estimate of the parental magma.
The Onaping data is remarkable in view of the obvious alteration in hand sample, though each line represents an average.
Sept. 2005. Dr Tony Beswick of Sudbury has recently sent us a pre-publication data set of almost 1000 new analyses. These show very clear CFB trends including OPX accumulation all with similar Zr/Nb and La/Sm but with an odd break into high and low Fe zones. Our understanding of Sudbury is about to undergo a major advance as soon as this data is released
Shown below is the normalised diagram for a mix of E-type MORB with 10-15% of the standard obsidian NIST 278 from Newberry Crater. The result is somewhat similar to a typical flood basalt with the low Ti, and Nb-Ta and elevated Cs, Rb, Ba etc. Lead was not included for the obsidian (Gladney,1992) but Pb is usually high in rhyolites, 25-30ppm, and would give the usual high Pb seen.
Recently theories on the origin of Flood basalts tend to assume an origin at great depth, near the Earth's metallic core, forming a great mushroom shaped diapir. One might assume that any basaltic rock generated from such depths would be highly alkaline.
One can say somewhat tentatively that flood basalts are mantle-generated in subcontinental environments where the mantle retains some enrichment from an older subduction phase. Because of the overlying continental roof of low specific gravity, large volumes of magma are generated without penetrating high in the crust. Tensional fractures in the crust opening up fissures toward the surface followed by a short compressive phase forcing magma sometimes onto the continental surface would seem to be ideal. An incipient spreading centre developing under continental crust or a spreading centre being forced under a continent seems to be requisite.
Large scale contamination by crustal material is unlikely, The remarkable homogeneity of enormous volumes precludes it. Inspection of contacts once deep in the crust shows that granite and sediment are sealed off by a chilled barrier of basalt glass. Xenoliths are not seen to be even partly fused, though often recrystallised.
Copyright © 1998-2006 Dr B.M.Gunn