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(and Layered Intrusions)

Columbia River Basalts (USA)

Thanks mainly to the work of Dr Peter Hooper at WSU and his associates these are perhaps the best known of all the flood basalts. Hooper claims that the Roza centre twice erupted 700 cubic km within a few days 15,000,000 years ago, forming a flow with a length of 190 miles (300km), an area of 15,400 sq miles (40,000 sq km), while the Pomona member flowed an even greater distance of 600km to the Pacific Ocean.

P.Hooper, (2000, G-Cubed, Vol.1, June),
gives a total area for the Columbia River basalts
of approx 77,000 sq miles (=200,000 sq km)

Like the Gondwana flood basalts, the CRBs are similar to MORBs in their major element characteristics but have greatly elevated K, Rb, Ba and LILE with depleted Nb-Ta, and they are divisible into a dozen magma types on their characteristic chemistry. Some of the lava groups, eg the Prineville Basalts and the Umatilla series plot in the Hawaite-Mugearite areas of an alkali-silica diagram but this is due to their enhanced K content. They do not fall into the "Basalt Triangle" on a Al-Mg-Ca diagram but above it, and they are extremely potassic as may be seen in the ternary diagram below.

Their signatures vary from similar to a rather mature island arc andesite type (Picture Gorge, Powder River Group, Imnaha Basalt, Wanapum Basalt) to a more enriched form with Rb at 8-10 times E-type basalt, (Prineville, Grande Ronde, Roza) with the extreme being seen in the Umatilla Basalt which may have only 5-12 times Rb but is very enriched in Ba (50-60 times E-type) as well as in P and Zr.

This means that Umatilla has 2700 to 4000 ppm Ba cf 140-430 Ba for the Imnaha rocks while Zr is at 400-500 ppm cf 120-230. Fortunately some new REE data has become available for Umatilla, these being among the most Ba (and Zr, P) enriched basalts known.

In the western USA, this characteristic andesite-like signature is not confined to Columbia River Basalts. Similar rocks may be seen elsewhere in part of what is termed the Basin and Range province, eg in the Elkhead Vol. Field in SW USA, (Leat, et al, 1988, JGR 102) where the rocks have a typical orogenic signature but have acquired more K (and Ba) than soda and plot in the Hawaiite-mugearite fields, however, Ti as well as Nb remain low. Other continental basalts showing a similar signature include Walton Peak (Thompson, et al, J.Geol.Soc.S.L., 150), the Flattop field, (Gibson et al, 1991, JGR 96). Some of these Basin and Range lavas may therefore also be CFBs.

In the "GEOROC" database, these are included in the Columbia River group, along with the Snake River basalts, the Craters of the Moon and even the Yellowstone-Shoshone Mtn lavas, the latter being a somewhat doubtful association as these are undoubtedly calc-alkaline.

Odd chemical compositions might be expected in basalts of the Washington-Oregon area, where an offshore ocean-spreading ridge (the Gorda Ridge) sends major transcurrent faults under the continent,. This does not however explain the unusually high Ba enrichment or the highly variable P and Zr nor the fact that exactly the same variations are seen in other continental basalts from a wide range of tectonic environment. All are however associated with subductions zones forming under or being underthrust under continents, and the high Ba must be associated with sub-continental mantle enrichment by older subduction processes.

Picture Gorge subgroup of the CRB's

REE normalised diagram of the Picture Gorge basalts. These are the most primitive, least enriched members of the Columbia River series and as can be seen are close to intermediate ORBs in composition.
Variation Diagram for Picture Gorge. Rather more elevated in K, Rb, Ba, U, Th than a typical ORB series and lower in Ti than Kilauea. These compare closely with moderately high-degree melt ORBs.
The metals show steep increase in Zn, Cu and V with fractionation, suggesting they were controlled by Ol or Opx + Plag. Cu reaches a surprising level of > 400ppm, not often seen in ORB basalts suggesting S mineralisation.
Zr > Y > Rb > Nb. If this were Nb > Rb this would be typical ORB. Ba climbs much more steeply than Sr suggesting again Plag + Ol control or possibly Plag + Opx
All Columbia River Groups Combined
(and excluding Steens, Prineville and Snake River Plains)
Columbia River basalts; Na2O - MgO - K2O. A much more potassic trend than seen in ORBs but similar to some continental andesites.
Columbia River basalts variation diagram. This should be compared with ORB basalts of the EPR. As in ORB, fractionation appears to be extensive and a wide range of degree-of-melt is seen, leading to variations in degree of enrichment from the subcontinental enriched mantle.
Columbia River basalts MgO vs metals. Zn increases with fractionation (right to left).
V shows alsmost a complete range of fractionation and partial melt effects cf EPR. Only the very low degree melts are missing. The Roza group have the highest V, with Grande Ronde below and right, Umatilla on extreme left, Picture Gorge is on the extreme right.
Columbia River basalts, Zr vs Nb. Two main trends are visible, the higher Nb coincides with the Parana River Low Ti tend. The suite with the most elevated LILE (Umatilla, on right) oddly, has lower relative Nb.
Columbia River basalts TiO2 vs K2O. The CRB's show one of the widest dispersions of chemical types known of all the major basaltic groups.
Columbia River basalts; MgO vs TiO2 in detail.
Columbia River basalts MgO vs Alkaline earth elements.
The Umatilla basalts with an average of 2500 ppm Ba do not plot on this diagram.
Zr diagram for all CRB's. Data from P. Hooper. Notice that the aberrant "Umatilla Group" is not aberrant at all but stems from a highish Ba parent and extreme fractionation. Again the low Sr suggests plag-ol or plag-opx fractionation and also demonstrates the basic similarities of CFBs to ORBs, except for the great enhancement of the K-group elements and the Nb-Ta depletion.

The Grand Ronde

The Grande Ronde Basalts are one of the major early subdivisions of the CRB's. By analogy with the ORBs, we would judge by the low constant Al2O3 etc that these are high degree partial melts showing a range of fractionation effects, but not extending to the TiMt point.

>> The Grande Ronde gorge. Each of the 20 flows is 15-20m thick.

Photo by Steve Self.

Variation Diagram for 270 Grande Ronde basalts. A small range of partial melts and pronounced fractionation.
Metals for Grande Ronde. Note Zn steadily increasing with fractionation.
Zr, Nb, Y, Rb, Sr, Ba. Note elevated Rb compared to ORB and to Nb.
REE for Grande Ronde. These most voluminous lavas are more LILE enriched than the Picture Gorge but not as much as Saddle Mtn and Umatilla.
Zr group diagram for the Grande Ronde basalts. Though of limited fractionation, Ba and Rb climb steeply with increasing Zr (and decreasing MgO) while Sr remains constant. An anomaly is seen in that the highest Zr sample has quite low Ba, Rb, possibly alteration and leaching.
The Grande Ronde Basalts; EMORB-normalised diagram. The data are mainly pre-ICPMS and are done on whole rocks, not glasses, but the fingerprint is typical for the CRBs.

The Grand Ronde field can be identified on the combined CRB diagram above.

Steens Mountain, Oregon

Click to enlarge
The block-faulted scarp of Steen's Mts, Southern Or. as seen from the Alvord Desert. The thick stacked flows of continental flood basalt make this the best cross section of an old shield volcano known.
Photo courtesy of Nick Jarboe who is beginning new work. He promises close-ups of the feldspar cumulates!

Steens Mountain is a centralised Flood basalt subprovince of 16.6M year age closely allied to the Columbia River Basalts and lying in southern Oregon. An excellent cross-section is on view due to the centre being cut by a Basin and Range fault which has created a NNE-trending, east-facing scarp about 3200ft (1000m) high, one of the greatest vertical exposures of Tertiary basalt known. Related flows may have once covered 25,000 to 50,000 km2 but effusion finally centred near the crest of the present Steens Mtn, forming a shield volcano.

In the early 1970's the late N.D.Watkins and I began a study, in the hope that such a large lava pile would reveal mantle depletion effects in late compared to early stages. Watkins sampled by diamond drill some 70 lava flows of a total of perhaps 100 ranging from 1-10 m thick, taking four samples per flow.

Steens forms the surface expression of a presumed large fractionating magma chamber, which periodically expelled highly feldspar-phyric magmas. Plagioclase is the main phenocrysts phase (as in most ORBs) followed by much finer-grained olivine, at least no coarse olivine has been expelled. The plagioclase probably floated and was largely extruded with some flows including up to 50% feldspar phenocrysts sometimes up to 4cm long. The Sc content remains constant showing little or no clinopyroxene formed. Other flows are quite aphyric. Extensive olivine fractionation is indicated by the trend of increasing Fe/Mg from bottom to top and the strongly decreasing Ni and Cr. No olivine cumulates or picrites were extruded however, the most magnesian rocks in our study having 7% MgO and in a later study by J. Johnstone of WSU, 12%. The magma chamber below must therefore now include extensive peridotite. Residual elements not included in either olivine or plagioclase, eg Cs, Rb, Ba, Th, U, Nb, La, K and light REE increase steadily towards the top of the lava pile while feldspars become more sodic and ferro-magnesians more iron-rich, an excellent example of "hidden layering" . Nickel variation is wide, 5 - 345ppm, yet the correlation with MgO is unusually poor which may indicate sulfide mineralisation. Olivines in section are limited to a few grains and normatively to only 8.25% in the most magnesian rock.

A little below the centre of the series there occurs an odd "mafic zone" of 15 flows of higher MgO, averaging about 8% compared with 7% near the base and 2.3% at the top of the mountain. In our original paper (Gunn & Watkins, 1970, GSAB 81) this is termed the "E" group. This horizon may be seen in other sections, eg between 7200ft and 7600ft in Johnstone, (1999).

Calculated CIPW Norms show the lower lavas to have an An51 plagioclase compared with An57 in the mafic zone and An27 at the top. Ferro-magnesian ratios for minerals are 78.5 in the mafic zone and 61 and 50. The most plagioclase-rich sample recorded came from above the mafic layer and had An57.5 with a total plagioclase content of 76%. However, with 1-inch drill cores and 4 cm plagioclases, such a mode has little meaning.

The REE show a divergence toward the light REE end. La/Lu ratio varies from 28.5 in the mafic zone to 41.7 at the base to 63 at the top. Johnstone (1999) therefore erred when it was stated "Rare Earth and trace element patterns throughout the sequence are similar"!

Though not shown in our original paper, a Ni/Ba diagram shows Ni decreasing from 260 ppm to 5 while Ba increases from a low of 300 ppm to almost 1000 ppm in some upper flows. If we note that the plagioclase is considerably more sodic and the ferromagnesians more Fe-rich than is seen in layered flood basalts of the British Tertiary, we might be tempted to guess that a considerable volume of more calcic and magnesian magma was not erupted. Such a basic gabbroic intrusion might well be heavily Cu-Ni mineralised.

MORB- normalised fingerprint for Steens lavas.
Data from Johnstone, (1999). Compare with Imnaha and Grand Ronde.
Variation in FM ratio and Na/(Na+Ca) for the Steens lava pile. Data from Gunn and Watkins (1971) Both increase upwards, and in general at the secondary variation level show some similarity, but it is difficult to explain the apparent variability. The magma chamber may have been periodically refilled, or magmatic overturn might have brought variable compositions to the top.
Are the first part of each flow to be extruded the most feldspar rich? Why is the feldspar content often cyclic? (Watkins & Gunn, 1969, Nature, 224, (5217), 360-361). Steens is still somewhat enigmatic.
Variation Diagram for a cross section of Steens Mtn in Southern Oregon, all available data. See the high alumina plagioclase cumulate and the low alumina, high Ti,Fe plag- depleted samples. As all tholeiitic basalts of mantle origin crystallise in the order olivine - ol+plag - ol + plag + cpx it is perhaps surprising we do not see plagioclase cumulates more frequently.
Metals for Steens Mtn. There appears to be some mineralisation of Cu and Ni. V appear to be unusually irregular, reason not known.
Rare Earths diagram for Steens, unpublished data by Hooper and Johnstone. Note the Pm data has been interpolated. Only a trace of a Eu anomaly is seen suggesting that only fine grained samples were analysed which is rather unfortunate.

Further Work

Steens has perhaps been already subject to undue attention, but as it has the greatest number of exposed flood basalt lavas known together with the fact it is perhaps the only exposed "layered extrusion" known and has the same "allivalite" fractionation patterns (Ol + Plag) seen in the British Tertiary, still more could be learnt. What is the range in composition of single flow members both vertically and in length? What an unparalleled opportunity to study feldspar compositions and their effects on fractionation! Why are some flows described as "highly altered"? This usually suggests different chemistry. How deep down does the now solidified peridotitic magma chamber lie?
Have other members of the CRBs undergone similar fractionation?
There are many questions yet to be answered.

A late stage feldspar-phyric plateau basalt from Table Legs Butte,
Snake River Plains.
Drill cores of the Eastern SRP show, as in Steens Mtn, periodic cyclic pulsing with highly feldspar-phyric lavas at the beginning, grading into Al-depleted, Ti and LILE enriched lavas at the top of a group of half a dozen flows. These flows are not as coarse grained as those of Steens where crystals of plag 2 cm x 0.5 cm may be seen.
Photo: Scott Hughes

The Snake River Basalts

The Snake River Plains lie in a boomerang-shaped fault depression extending east from the eastern edge of the  Columbia River Basalts  for 400 miles to the edge of the rhyolitic caldera of the Yellowstone thermal area. The rocks are mainly very flat-lying basalts with occasional associated rhyolites while the very youngest cinder cones at the “Craters of the Moon” national park fall in the  hawaiite-mugearite fields on an alkali/silica diagram.  The ages range from 15myr at the west, (approximately the same as the youngest CRBs) and reportedly young to the east with the 2 ka Craters of the  Moon lie in the middle of the eastern sector.

Map of the Snake River Plains Basalts (USGS)

Drilling and seismic work show, according to the USGS, the maximum thickness of the older flows to be in excess of  4000ft, so that very roughly, about 60,000 cub.km.of lava must be present. Flows have mainly pahoehoe  surfaces though the glass “skin” has been etched away by wind-blown dust. Fissures lie open, one of the youngest shows a levee enclosing an area of a square mile from which most of the lava has drained back.  Down in one of the fissures at a depth of 200ft ice lies in the hottest summer.

The Snake River Plains, satellite view. Basin and Range structure is seen both to the north and South. The Craters of the Moon are seen right of middle and numerous late stage vent cones are seen.

Lava which flowed into pre-existing stream and lake beds formed porous pillow lava and tuff which now act as channels by which aquifers drain into the Snake River Canyon.  We had a rather amusing experience, too long a story to relate in detail, when the Idaho Nuclear Research Station (located about 20 miles from Pocatello) chief asked me to give a talk to some of the technicians on how to locate aquifers for which they were drilling, without a very good success rate.  It turned out they saw an aquifer as a good way to dispose of unwanted radioactive nuclear waste. They were not geologists and it had not occurred to them that water at a depth of several hundred feet could ever come to the surface. It had not occurred to them either that the water-falls cascading from between the flows into the Snake were  those same streams and they were proposing to pour radioactive waste into the drinking-water supply for Lewiston, Clarkston, Portland, Sanfrancisco and in fact a good proportion of the western US.

Challenged to suggest an alternative I could only suggest the opposite, a desert area in a tectonic depression which was seismically stable such as Death Valley where there could be no fear of concrete enclosed waste ever getting below a water table.  The waste disposal unit for the USA is in fact now located on the edge of Death Valley, so I hope it pans out.

TiO2 / P2O5 diagram for eastern Columbia River Basalts, Snake River Basalts and Craters of the Moon. In general the youngest rocks are the most enriched in P. The uppermost mainly but not all belong to the Craters of the Moon.
Detailed FeOT, TiO2 and P2O5 for Eastern Columbia River, Snake River and COM. Again the extreme high Ti, P ferrobasalts are mainly COM. Zr and Ba peak at about 1% MgO, near the commendites.

One of the many cinder cones that lie on the Snake River Basalts, older than the "Craters of the Moon", but not much, perhaps 10-50,000 yrs.

Taken in 1963 but so far no one has positively identified it. Some of these late basalts are also enriched in P, K, Fe but not to the extent the COM are.

Late stage spatter cones at Craters of the Moon, age probably less than 2000 yr. The COM range from ferro-basalts with the highest P2O5 and TiO2 known in basaltic differentiates, down to commendites with the highest Zr and Ba known. Only the Skaergaard ferro-basalts approach it in composition. It is curious that only Continental Flood Basalts exhibit marked fractionation effect. Occasional high P, Ti ferrobasalts are scattered throughout the CRBs. The entrapment of huge pools of magma beneath continental crust for long periods enable the extreme fractionation processes.

Lava Compositions of the Snake River Basalts

Pahoehoe ferrobasalt at Craters of the Moon. The fragile glass surface has weathered away leaving the underlying complexly folded lava skin.

As yet (year 2000) there is (was) surprisingly little data.  Leeman and Vitaliano (1976, BGSA 87, 1777-92) show 16 ME analyses of a compact group of high K basalts  some showing marked Fe enrichment. Tilley and Thompson (1970) and George Stone of the USGS (unpublished data, circa 1967) showed a wider range varying from 44-48% SiO2 and I analysed some hawaiites from Craters of the Moon (unpublished).

Lum and Leeman (1989, JGR, B94, 7871-84) and Honjo and Leeman (1987, C.Min.Pet. 96, 163-167) added trace elements and some REE.   The data is now rather old and there is a good deal of scatter.  In general they show the high K and Ba with REE equal to or greater than EMORB levels as have most CFB’s, but except possibly for the most basic, no negative Nb-Ta anomaly is seen. As we have seen, this is an invariable characteristic of CFBs, there being no exceptions. Two more detailed though old analyses by Thompson et al, 1983, ("Continental Flood Basalts”) also give a low Zr/Nb of 11. However the presumably related Steens Mountain and Columbia River Basalts are notoriously variable and at least one characteristic, the very high P seen especially in  the  Umatilla series  is also seen in  Snake River where it reaches 2%.

Variation diagram for some Snake River Basalts. Note the high Fe trend.
(Data up to mid-2003)

Geist et al, (2001, Geol.Soc.Amer.Sp. Pap. 353) presented data from drill cores from the north-eastern extremity of the group and are also puzzled by the high P2O5 content and suggest the possibility of crustal contamination, though their highest P2O5 is only 1%. Their MORB normalised diagrams again show a more  OIB-type fingerprint, not a typical CFB one.

This late stage collapsed vent or possibly phreatic explosion crater forms "Cottrel's Blowout" of the surface of the Snake River flood basalts.
Photo: Susan Sakimoto & Scott Hughes

Iron is high at 16% and TiO2  is unusually high even for CFBs at 4% with some indication of two levels being present. Again this matches the Umatilla rocks of the CRB. Rhyolites are very potassic (6% K2O) but seem to be related to the basalts and intermediate rocks though the extensive ignimbrites of the Yellowstone region are almost certainly of refused crustal keel material and there is a mineralogical suggestion of hybridisation.

A group from the University of Utah headed by Dr John Shervais are making a new study of the Snake River and we may hope to see new data by early 2004. The Idaho Geological Survey also have 400 new analyses in the publication process.
Other groups from  Idaho State University as well are also investigating the series. Dr Scott Hughes has recently (Dec/03) forwarded a data file of the SRB with a total of nearly 1000 new and old analyses from the eastern Snake River plains. Two fairly distinct series are present, the tholeiitic flood basalts which, though lacking Nb, appear to be CFB type and a series of alkali basalt, hawaiite, mugearite, trachyte, commendite, rhyolite which seem to be confined to the monogenetic superficial scoria and lava cones as seen at Craters of the Moon.

In general, the high iron enrichment  of the tholeiitic type and the high Ba and P suggests a CFB affiliation  in the older rocks. The reversion to an alkaline type in the closing stages seems similar to that seen in Mauna Kea (Hawaii) where after the cessation of the shield-building tholeiitic stage and a considerable time hiatus, superficial monogenetic alkaline rocks are extruded. No fingerprints are as yet available for the  olivine tholeiite-type rocks of the 9 – 7 ma period (Bonnishchen et al, see NET page).

Ternary diagram of combined Snake River data, show the very high K trend.
(updated 31-March-2004 with 10 times more data)
Variation diagram for 914 Snake River Basalts.(2004)
Thanks to members of the Idaho Geological Survey and ISU we have at last got really adequate data, (Jan 2004). While the pattern does not differ from earlier work it is now much better defined. Note the range from low P, Ti tholeiitic rocks and the transition to the very high P, Ti more alkaline rocks. The alumina while variable does not however show a classic alkaline trend. The TiO2 become very high at 4% maximum and P phenomenally high at 2.9%, greater than usually seen in classical alkaline basalts of the OIB series. Ferro-basalts are present as in all CFB's. The high K is also seen in almost all CFBs and exceeds soda in both rhyolites derived from tholeiites and in the "trachytes". Samples were recovered mainly from drill cores from the eastern SRP.
Variation diagram for the plateau basalts only. They form a compact cluster about unusually low silica, most lying between 46 and 49%. They are normatively transitional, with a high percentage of normative iron-rich olivine.
Compared to Grand Ronde or Esmeralda, they are markedly more magnesian with unusally high Fe, Ti and P.
How do they compare with the ferro-picrites of the Etendeka or West Greenland? A good class excercise! Or the Ferro-picrites of the Shiant Sill (BT)?
Alkaline Earth elements for plateau basalts only. Note the flattening of Sr in rocks with more than intermediate Zr. This usually indicates the entry of plagioclase in fractionation, and MAY correspond with the beginnings of the Eu anomaly shown below. The sudden increase in Sm/Eu is easily seen in samples that have been normalised, not so easy in the original data. All that needs to be done is normalise 700 Sm/La in the original file!!! Some day perhaps!
MgO vs P2O5, shown on an enlarged scale. The late stage alkaline centres show the unusually high P in the intermediate "hawaiite - mugearite" rocks.
Metals of the Snake River Plains CFB's.
The planes of partial melt lie quite flat (between about 6 and 10% MgO, as for the Oceanic Ridge basalts. We can see especially for Co that the plane of partial melt and the fractionation path are very near to parallel by the lack of variation. Zn is hgh at 250 ppm suggesting higher amounts on low degree, alkaline melts. There are only a few V values mainly for tholeiitic basalts.
Alkaline Earths.
Note that here we see on average, Ba > Sr > Zr the reverse of what we see in depleted ORBS. Nb has been estimated as = Ta * 17, in the absence of Nb data. There is no Y data given. A very great difference in Zr values for the more evolved tholeiite vs low degree melt rocks is seen.
The Snake River Plains rocks are like no other CFBs in some respects. At least three fractionation trends are present all leading to rhyolite of 70 - 80% silica. The basalts are of low silica, 43-49% and are of average ol-hy normative transitional type. However some have higher P and may be ne normative. The more "alkaline" peak in transitional ferro-basalts of > 16% FeOT and up to 3% P2O5.
In the diagram above the three trends can be seen with Zr peaking at about 1% MgO and 400, 2,250 and 3300 ppm. Ba peaks at < 0.5% MgO at about 100, 2,200 and 3,300 ppm while Sr stays abnormally low at < 400 ppm.
The ferro-basalts have a normative feldspar of An34 and so have been called "Hawaiites" but are not at all like the Hawaiites of Hawaii. Whether the more sodic rocks are at all like the "mugearites" of Mugeary on the Isle of Skye, we cannot say as there are few good TE data for Skye.
Typically enriched CFB range, with a spread of Ce/La etc in the more enriched members between tholeiite and low degree melts. The most "alkaline" Craters of the Moon have even more REE and a higher La/Nd.
REE normalised diagram for Snake River - Craters of the Moon fractionates have been separated at the point where a Eu anomaly becomes apparent. Though there are at least two main factionation trends, see above, the REE do not show it very plainly.
REE for basalts only before incoming of plagioclase. Pm and some others interpolated.
When time permits we will try and fix the exact MgO content at this point.
REFS References to papers contributing to the University of Idaho database on the Snake River Basalts and fractionates.

The Siberian Traps (Russia)

Lightfoot et al (1990, 1993) show the Siberian Traps to be similar to the CRB's. Unfortunately Ba was not determined, and the wide range of Rb may suggest some alteration. Six magma types are present ranging from tholeiitic to alkali-basalt. Carbonatites are also found associated with these rocks. At minimum the Siberian Traps of the Norils'k area cover 1,000,000 sq km but related areas have either been buried or removed by erosion. These rocks are now a major source of nickel and also of PGE's.
The Siberian Traps.

The Palisades Sill (USA)

The Palisades form an extensive 1000ft-thick dolerite sill lying along the western side of the Hudson River, west and north of New York. Being so prominent it has been the focus of a good deal of primary study. It is of typical Flood Basalt composition being rather high in K, Ba (x2 - x4 times MORB) and what data are available suggest it is of similar composition to the geochemical standard rock W-1, the Centerville Diabase from near Washington DC which is I believe of the same age.

Walker (1940, BGSA 51) of Walker & Poldervaart fame, decided that the variations in the sill, including the well-known olivine ledge near the base, was due to olivine-clinopyroxene fractionation with the heavy olivines settling to the bottom and attributed the lack of any smooth compositional variation diagrams to the fact that clinopyroxene had about the same silica content as the average rock. Pearce (1969, J.Pet 11) ratioed the major oxides to Al2O3 which then showed fair agreement in diagrammatic slope with olivine and clinopyroxene. In the same era or a little earlier (1955-65) I was also somewhat baffled by the erratic compositional nature of the Antarctic Ferrar Dolerite sills, but found that element variation diagrams described ellipses, not straight lines or simple curves, the reason being that two points of similar Mg contents in the lower and upper parts of a sill, will have different LILE/HFSE ratios. Plotting several overlapping ellipses from different locations does indeed produce a profusion of apparently scattered points Plotting element concentration against height also showed that all the larger Ferrar Dolerite sills were multiple injections. I tested the same theory on the Palisades sill and found that the bulk average composition of the sill was much higher in Ca, Al etc that the average of the upper and lower chilled margins, the magnesian olivine ledge being only ten feet thick. Like the Ferrar Sills it must have been multiply injected by magmas of different composition, on top of which was superposed fractionation effects, the lower parts of the sills always being more magnesian that the upper parts.

As the Palisades Sill was by this time regarded as the classic example of insitu fractionation, I had considerable trouble defending my dissertation! In about 1970 I spoke to Dr Ken Walker who, using an electron microprobe, found two distinct generations of olivines, which indicated multiple injection again.

In about 1990, Mathew Gorring of Columbia University, did his doctoral thesis on the sill, and sent me his results which included some trace elements, (Ni, Co, Cr, Ba, Cu, Zr etc)

Mathew did two sections, but only of the lower 100ft across the olivine layer, at Fort Lee and at Alpine, N.J. The latter is somewhat more magnesian but the trends are the same. An oddity lies in the fact that which Ni/MgO and Co/MgO diagrams show the usual gentle curve, but Cr/MgO shows an inverted "V", the highest magnesian rocks having about half the Cr of the intermediate rocks(450 ppm vs 900), Clinopyroxene of course contains much higher Cr than olivine, but this is usually masked by the inclusion of Cr spinels. It seems any chrome spinel must have been dropped out before the intrusion of the olivine ledge.

Another PhD student, Shirley by name, sent me data in 1987 of a large number of ferro-basalts and granophyres terminating in highly fractionated rocks of FM#96, though only 62% SiO2. Unfortunately he did not send any location data, but we now have a compositional range from 25% MgO down to about 0.2% and can produce quite repectable variation diagrams. As the only granophyre previously reported was a single thin late-stage dike, one would love to know where he found them.

Mathew Gorring also analysed some olivines, all very iron rich at Fo72 to Fo55, but the picrite trend in the sill does not 'point' to these olivines but to a more magnesian but still unusually iron-rich one of about Fo80. They must be the most iron rich 'picrites' known except for the Shiant Sill. One can only assume MG did not find the most magnesian olivines.

So it seems there are still some mysteries to be cleared up about the Palisades Sill. I have never seen the data of Gorring or Shirley in print or referred to, but presumably it rests in some archive at Columbia U.

Recently we found that Shirley's data did include REE which had been overlooked and though no Zr, Nb or Y, it does include Ta; so using the constant Nb/Ta = 15, we have added some Nb values.

The results show that the Palisades are formed of a typical Flood Basalt, rather like Grande Ronde in the CRB's, but less enriched in LILE than are the Ferrar Dolerites.

It is moreover a virtual match for the standard Centreville Diabase rocks, W-1 and W-2 even to the rather high Cs. Pb is not included in Shirley's data but is oddly high in W-1 accepted levels.

The Palisades Sill, (Shirley, 1987).
Variation diagram for the Palisades sill. Note high Fe extending towards Fe-rich olivine. Olivine is joined by cpx not by plag, with a sudden drop in Ca and Sc (not shown).
Metals for Palisades Sill. Note flatter slope in Ni and decline in Cr with increasing MgO due to Fe-rich olivine.
In spite of multiple injections, diagrams are quite coherent.


The Skaergaard Intrusion is composed mainly of cumulates and chilled margins vary within inches. Producing a good chilled margin fingerprint is therefore difficult to impossible. This is probably the best developed and best known layered intrusion world-wide of peridotite, troctoliite, anorthosite, pyroxenite etc. The chemisry of any layered rocks is wildly variable, there is no recent TE data and chemically speaking we do not learn much from Skaergaard in spite of it's wonderful mineralagy and petrographic texture.

Inch scale banding in the Skaergaard Intrusion with olivine-clinopyroxene grading upwards into plagioclase-rich rock.
Photo courtesy of Kurt Hollocher, (2001)

The Hebridean (British Tertiary) Basalts

From recent data these are certainly Continental Flood basalts and should be included here,

The scene from Sligachan Pub, Isle of Skye. On left is Mt Marsco, an odd high Fe microgranite (Marscoite) associated with the British Tertiary. On right is Ben Gillean, part of the Cuillins layered Gabbro intrsuion. Under the heather in the foreground are plateau basalts. A grand wee place!
Through the gap is Loch Scavaig, where peridotite is exposed. There is a nice walking track to it across the heather,
"Will ye gang to the Highlands with me?"

Unfortunately, while not metamorphosed as the Archaean flood basalts have been, there has been deep glacial erosion and more than a million years of rain, so that really fresh sample in the British Tertiary is the exception rather than the rule.

Originally grouped in the "Brito-Arctic" province including Greenland, Iceland, the Faeroes and Jan Mayen, (see Turner & Verhoogen, 1960, McGraw Hill, p222), we now know that while some of the basalts of East Greenland are rather similar to OIBs such as Mauna Loa, (see Thirwall et al, 1994, J.Pet. 35 for the Hold-With-Hope basalts), the bulk of the East Greenland rocks appear to be Flood basalts, see Hald et al, 2000, Lithos 54, for the Jameson Land Basalts) though the chilled margins of the Skaergaard Intrusion, (Hoover, et al. 1989, (J.Pet.30) are highly variable and not distinctive. Iceland of course is composed of simple MORBs, differing only in the presence of higher fractionates such as rhyolite which may be due the much greater local crustal thickness, allowing extensive fractionation to take place in sub-crustal chambers. Jan Mayen is a potassic ankaramite-basanite-trachy-phonolite platform while the Faeroe basalts are a repeat of Iceland. So what are the Hebridean lavas?

Many people have claimed the presence of picrites in the Hebridean rocks but they prove to be of erratic composition with a few samples of 25-30% MgO but with a wild association of Mg, Cr and Ni and again seem to be simply occasional random Ol+Cpx cumulates. (eg, see Gibson et al, 2000, E.P.S.L. 174, 366-376). Fergus Gibb et al (2006, J.Pet. 47, 191-230) have shown that the Shiant Sill on an island midway between Skye and Harris-Lewis is a multiply injected ferro-picrite sill. In spite of the iron-rich olivines the Zr/Nb is > 30 and the La/Sm in the range 1-2 suggesting extensive pre-injection fractionation.

County Antrim, Northern Ireland
British Tertiary lavas at the Giant's Causeway
County Antrim, Northern Ireland. (Rather Altered)

Some of the Hebridean lavas are tholeiites, some olivine basalts, some alkali basalts and even trachytes. Unfortunately there is no recent data for the high-soda mugearites of Mugeary on Skye. The general fingerprint resembles an EMORB in the many cases where little or no trace element data is available and Nb especially is lacking. We can say definitely they are not MORBs, rarely are they truly alkaline (the Nb for example is too low and the Nb/Zr far too low for such enriched rocks. All show considerable fractionation and they are not a partial melt series, their trend often being at right angles to a partial melt trend.
Reasonably complete data has been presented by Preston et al, 1998, (J.Pet.39), by Kent and Fitton, 2000, (J.Pet.41) and by Geldmacher 1998, (C.Min.Pet. 131). All these show a typical Continental Flood Basalt fingerprint, though there is some scatter in the data. Scarrow et al, 2000, (J.Pet.41) gives data for Skye, Mull, Eigg and Canna. While it includes few trace elements the data overlaps that by the other authors and is almost certainly of flood basalt also.
The Hebridean Province is well inside the continental margin and we would expect flood basalts here, not MORBs.

The odd composition has been explained by some authors as being due to ingestion of Lewisian gneiss and shale by MORB basalt, but the low Nb, frequently less than 1ppm at 9% MgO cannot be explained by the addition of a few percent of shale when we regard that these rocks are in general quite enriched in LILE. Their similarity to other Flood Basalts, eg of Antarctica, is marked.
Two small groups of lavas, one highly magnesian, analysed by Kerr, (1995, Chem.Geol. 122); (1999, J.Pet.40) for the Plateau Lavas of Mull, do not have a distinct flood basalt fingerprint but have high Ba, variable K and a slightly -ve Nb, and moderately high silica (46-48%). Their IUGG classification is "Tholeiitic basalt" with exception of a single odd rock of very high soda (6.00%) with double the Zr and 3-4 times the Sr of other rocks and classified as a "latite basalt", or "trachybasalt" on the IUGG formula. The less magnesian rocks have a slight -ve Nb anomaly, rather like the Picture Gorge basalts of the Columbia River. What kind of fingerprint do the type "Mugearites" have? I wish we knew! We have also seen a transition from tholeiitic flood basalt to late stage alkali basalt in the Snake River basalts.
Rhyolites, also of Mull, (Preston, 1998) are of low Nb-Ta type, similar to calc-alkaline rhyolites and quite unlike those of Iceland. Rhyolites are sometimes produced from Continental flood basalts by fractionation but there seems to be no good data on them. Latites are found in the Etendeka Province of the Karroo flood basalts in South Africa and have a very similar fingerprint to the general Hebridean "Plateau type" shown here.

The eroded roots of old shield volcanoes are seen in Skye, Rhum, Canna, Ardnamurchan etc with strongly layered intrusive gabbros with peridotite and anorthosite (troctolite-allivalite) banding.

Fingerprint of Mull Plateau Lavas. (Preston, J.et al, 1998, J.Pet.39)
Note typical flood basalt low Nb, high LILE.
Fingerprint of a sill containing xenoliths, but retaining the regional fingerprint.
Zr/Nb for the Mull Plateau. Two series, both low Nb.
High and low Mg tholeiites (including one latite basalt) also of Mull.

The North Atlantic CFB Province

    All the basalts of the British Tertiary,  the Faeroes, Iceland, the MAR, East Greenland and Eastern USA were once looked on as a single basaltic province. We now know that CFB’s of the British Tertiary and East Greenland are quite different to the OIB’s of Iceland which are different, (though in some cases very slightly) from the MORBs and NMORBs of the Neo-volcanic zone of Iceland as well as the Mid Atlantic Ridge.

   Being all generated by partial melting of the upper mantle there are similarities, but the British-Tertiary and East Greenland basalts are generated in the sub-continental Upper Mantle and are so contaminated and enriched by the products of old and no-longer active subduction zones. GEOROC have combined the basalts and Dike swarms of East Greenland  with the British Tertiary so we will show them here together, but at some time in the future we may show East Greenland separately.

Variation Diagram for the North Atlantic CFB’s. GEOROC after some urging no longer includes Iceland or ORB rocks in this province, but the very high TiO2 shown by some (over 6%) identify them as being the melilite nephelinites of the East Greenland nunataks. These are allied to Bermudites and are ultra-alkaline, uncontaminated by andesitic residue, and almost certainly do not belong here.

There are many allivallite-troctolite etc cumulates in the central volcanoes of the British Tertiary,  and some plateaux lavas are quite alkaline as in Snake River Plains ( though not as P-rich). However the general pattern is typically CFB, with high K2O (+Rb, Ba, Cs, LREE) and the fingerprints are classic Nb-Ta depleted. Low grade alteration is also unfortunately a common feature in British Tertiary rocks.

Although 1400 samples have been analysed for REE, there is a wide scatter between tholeiitic and ultra-alkaline members  so these are not shown though they reach from 2 - 200 ppm La in the more extreme cases.

The metallic elements are show by their variance with MgO. As in most CFB’s the Zn is fairly constant from 100 to about 140 ppm  while Co remain below 100ppm in the most magnesian rocks. Ni, Cr, and Cu are more variable and often high enough, eg, >500 ppm Cu, to show that some sulphide mineralization is present.

V shows the usual  MORB pattern peaking at 5-6% MgO at 450 ppm, but is unusually high (300 ppm) in the picritic rocks.

The alkaline earth group, show Nb low except in the highly alkaline rocks and Ba, Sr and Zr show the usual variability seen in all rocks associated with the crust. Both Ba and Sr may exceed 1000 ppm over a wide range of rock type.

Isotopes are extremely variable as could be expected of rocks of such mixed parentage, with Sr87/86 ranging from 0.702  to 0.716, while leads show a greater range than all the oceanic rocks combined.

When combined as the North Atlantic Province, the CFB's of both sides of the Atlantic look decidely messy due to inclusion of many cumulative rocks and some sodic alkaline rocks, however the main trend is definitely CFB.

Copyright © 1998-2006 Dr B.M.Gunn