Download Crustal warpinga possible tectonic control of alkaline magmatism

]OUI•NAI. OX'G•.O?a¾SXCAL
RES•-AI•Ca
VOL. 69, NO. 6
MARCH 15, 1964
Crustal Warping--A Possible Tectonic Control
of Alkaline Magmatism
D.
K.
BAILEY •
Geophysical
Laboratory,CarnegieInstitution o] Washington,Washington,D.C.
Abstract. Alkaline and peralkaline magmatism is peculiar to the epeirogeniccontinental
environment; it is characterizedby unusual concentrationof alkalis, volatiles, and rare elements,the large volumesof alkaline rock contrastingwith the small amountsof suchrocks
in alkali basalt provinces. Probably the most outstanding region of alkaline and related
carbonatite activity is in east and central Africa, where localization of the activity along the
rift valleys has long been recognized.Recent discoveriestend to establishthis relationship.
The restriction of major alkaline provincesto the stable continents has led to the suggestion
that long undisturbedperiods must be necessaryfor slow accretion beneath the crust; the
implicationsare that the continentalplate has a passiverole and that any associationof alkaline magmatismwith rifting is fortuitous.It is here argued that the regional tectonicscontrol
the generationof alkaline magmasand their localization along the rift zones.The African
shield showsa distinct 'basin and swell' pattern, and many of the rifts occupy the axial zones
of the long,narrowupwarps,separatingthe basins.A seriesof erosionsurfaces,dating at least
as far back as the Jurassic,showsincreasingelevation and separationtoward the crestsof the
swells and indicates uplift of these zones in several distinct stages: the continuity of such
erosion surfaces shows that the continental plate behaved competently during these uplifts.
Upwarpingof a rigid continentalplate wouldrelievethe pressurelowerin the crust,allowing
partial melting and the concentrationof fugitive constituentsfrom the underlyingmantle.
In east and central Africa, similar alkaline magmas of salic differentiate character,rich in
volatiles,and oftenwith associated
carbonatites,
wereproducedin differentregionsduringdifferent periodsof rifting; the unusualvolume of suchrockscan more reasonablybe explained
by partial melting of crystallinesourcerocksand collectionof juvenile fugitive constituents
than by the usuallyfavoredprocessof fractionalcrystallization,whichwould requirea vastly
greater volume of parent magma.
Introduction. Many petrologists
havenoted sumablythis tectonicrequirementcouldbe met
that the major alkalineand peralkalineigneous in the oceanbasins.Also, if someprocessother
complexes
appearto be restrictedto the stable than crystal-liquid equilibrium were required
continental or shield areas of the crust, an ar-
for the formation of alkaline rocks,it is difficult
rangementsuccinctlydescribedby Backlund t.o reconcile their concordance with the lowof 'petrogeny'sresidua
[1932] as the epeirodiatresis
relationship,and temperaturecompositions
one which has given rise to the view that t.he system' [Bowen, 1937]. The central questionrespecialtectonicconditionsof these regionsare mains,therefore: Why shouldtheserocksof disnecessaryto the formation of such magmas. tinct salic differentiate character show a marked
There is no generalagreementas to the precise developmenton the continents?
Probably the most spectaculararray of alkarole of the tectonicsin this magma generation;
it has frequentlybeen suggested
that the long line igneousrocks and associatedcarbonatitesis
periodsof quiescencehave favored the accumu- in Africa, particularly in the easternand central
lation of alkali-rich magma by such means as parts of the continent,an excellentsummary of
extensivegastransfer in a magma suchas alkali the information on the activity being provided
basalt. If lack of disturbancewere the criterion, by Kin• and Sutherland [1960]. That this alkawith
however, •he continental associationof these line magmatismshowsa generalcoincidence
magmas would still need explanation,for pre- the rift pattern in this par• of Africa has been
noted by many writers,includingKing and Sutheftand (p. 299), but there has been considerable
• Permanent address: Department of Geology,
divergenceof opinionas to the relationshipbeTrinity College, Dublin, Ireland.
1103
1104
D.K.
BAILEY
I000 Miles
Scale
]Fig. 1. The East and Central African rift zone.
tween the t.wo, if any exists;King and Suther- can Rift zone, has focused attention on the•n
land, for instance,wereclearlyperturbedby the and made them a subject of controversy since
lack of detailed correlation between the igneous the time of the classicdescriptionby Gregory
centers and the rift faults. But the African rift.
[1921]. But, as clearly shown by Dixey [1956]
zonesare one of the world's major tectonic fea- in his survey of the East African Rift, much of
tures and if they are in any way a locusof alka- the Tertiary movement was on the site of lateline magmatism they undoubtedly hold the key or post-Karroo (Jurassic) movements,and as
to the relationshipof suchmagmatismwith epei- the Western Rif• is traced from Lake Rukwa
rogenicearth movements.The following discus- into the Nyasa trough, and farther south, the
sion will be confinedto this one region,but it age of the major movement is predominantly
is felt that the basicrelationshipthat is deduced, post-Karroo. New work is showingthat south of
namely, partial melting by relief of lit,hostatic LakesTanganyikaand Rukwa a pat.ternof post-
load, may have a more general application to
the problem of the alkaline rocks.
Karroo rifting emergesthat is comparablein
ex•tentand scalewith the Tertiary rifting to the
The rift zones. The impressivetopogra.
phic
expressionof the Tertiary rifts in east,and northeast Africa, commonlyknown as the East Afri-
north, and only lessstrikingbecauseits topographicexpressionhas been subduedby longer
erosion:the mid-Zambezi-Luangwa
rift, for in-
CRUSTAL WARPING
1105
EXPLANATION
TERTIARY-RECENT
•..........Volcanic
fields
o Alkaline centres
and
volcanoes
PRE-TERTIARY
+ Alkaline
centres
IOOO Miles
Scale
Fig. 2. Distribution of alkaline centers and volcanic fields.
stance,is merely a p,•r• o.f a much longer rift
zoneof post-Karrooage [Bailey, 1961]. Almost
certainlythere was pre-Karroo rifting too, and
Dixey [1956] has alsostressed
the tendencyof
the rift zonesto parallel the major structural
trendsin the Precambrianrocks; the latter are
largely orogenicstructures,however,and, as
hintedby Shackleton[1956], this may be essentially accommodation
of the rift pattern to a
previouslyestablished'grain' in the crust. The
presentlyknown pattern of rifting in ea.
stern
and southernAfrica is shownin Figure1.
activity around rift intersections[Dixey, 1955,
p. 28; Bailey, 1961] and in the strongcorrelation
of Tertiary and post-Karroo activities with
those sections of the rift in which movements of
these periods respectivelypredominate,makes
it. hard to avoid the conclusion that the mag-
matism and the rifting are both expressionsof
the same major process.
The type and characterof the alkaline igneous
activity have alreadybeenreviewedby King and
Sutherland [1960], with a comprehensivebibliography,so that only a brief survey is in order
The alkaline magmatism. When the distri- here. The extensive Tertiary to Recent lava
bution of alkalineigneousactivity is superim- fieldsin Ethiopia are generallyrecordedas being
posedon the rift pattern, as shownin Figure2, predominantly basaltic, but this may well be
there is a remarkablecorrespondence.
Certainly a convenientgeneralizationbecauseto the south,
the activity is not solely confinedto the actual in Kenya, although basalts (often basanites
rift faults; it. would seemnaive to expectthis, according to Kent [1944]) are widespread,
but the generalconcurrence
of the two patterns, phonolite is very commonand is perhaps the
especially noticeable in the concentration of predominantlava throughoutthe Eastern Rift
1106
D.K.
BAILEY
and in adjacent areas suchas the Uasin Gishu
plateau, the Aberdare Mountains and Mount
Kenya. Trachytes,nephelinites,
and alkali rhyolites are also well representedin these fields.
Farther south, in Tanganyika, the lava fields
give way to central volcanoesbuilt largely of
fragmental materials, mainly nephelinitesand
phonolitesalong the Eastern Rift and trachytes
and phonolitesto the east in the Kilimanjaro
range. In eastern Uganda a chain of Tertiary
volcanoes,largely composedof fragrnental nephelinites, stretches northward from Mount
Elgon [Davies, 1952], one of these, Napak
[King, 1949], having an exposedijolite-carbona-
ment in rare elements.Implicit in nearly all discussions of such alkaline rocks is the view that
they are differentiatesof a primary magma,such
as alkali basalt: Harkin, however,clearly recognizes that vast quantities of basalt would be
requiredto yield the observedvolumesof salic
rocks.
In addition to the Panda Hill carbonatite in
the Rungwevicinity there are other old carbonafire and alkaline complexessuchas the Nkumbwa Hill carbonatite and the Mivula nepheline
syenite in Northern Rhodesia and the Ilomba
nephelinesyeniteand anothergroup of carbonatires in Nyasaland. A most impressive concenrite core. Farther to the south and west lie the
tration of pre-Tertiary carbonatites,nepheline
pre-Tertiary carbonatite centers of Tororo, syenites,and syenitesis found in southern Nyasalandand adjacentparts of Mozambique.The
Sekulu,and Bukusu.•
The potash-rich lavas of Toro-Ankole and majority of theseare of Chilwa age, i.e., immeBirunga in the Western Rift are well known diate post-Karroo,but some, such as those of
[Holmes and Harwood, 1932; Combe and Port Herald, are pre-Karroo. All theseare deepHolmes, 1945], but not all the voleanieity of seated complexes, and the large masses of
this rift is strongly potassie,as indicated by nepheline syenite, and especially syenite (or
Bowen [1938], although it is characteristically perthosite),suchas Zomba and Mlange, which
alkaline, basaltsbeing well representedonly in distinguishthis region,are especiallysignificant.
South Kivu.
Such rocks are older, plutohie equivalentsof
Farther south, where the Eastern and Western rifts intersect,is the more isolatedTertiaryRecentvolcanicfield of Rungwe,whichhasbeen
studied in detail by Harkin [1960]. Alkali basalt is the predominantlava, and in degreeof
alkalirfityandscopeof differentiation
the magma
seriesis comparedby Harkin to that of Tahiti;
the widespreadphonolitesand trachytes of the
Tertiary rifts to the north, and they indicate
that the sametype of magmatismcharacterized
the post-Karroorifts. Theseplutonsalsotend to
negate any suggestionthat the Tertiary volcanics are products of differentiation in intracrustal magma chambers.
Similarconsiderations
apply to the morescatbut the volumeof salielavas,mainly trachytes
and phonolites,with somenephelinites,
is about tered groups of post-Karroo and pre-Karroo
half the total eruptive materiM, and Harkin is complexesto the south and west of Nyasaland
forced to conclude that the alkali basalt has
(Figure 2). The only exceptionwould seemto
been contaminatedby earbonatiteand alkaline be the group of pre-Karroo carbonatitesand
material,suchas formsthe neighboring
but dis- alkaline complexesfalling in a roughly E-W
tinctly oldercomplexof PandaHill. Essentially, zonein the Transvaal,South Africa. These are
this problem is the same as that of t,he whole not related to any well-definedrift, but when
of the volcanism of the East African Rift--a
it is rememberedthat even the post-Karroo
preponderanceof alkaline salie rocks,in many rift pattern is blurredby erosionthis is perhaps
easesfragrnental,indicatingvolatile-richexplo- not surprising; they may be related to Presive activity, and frequenfiy showingsigns of
assoeated carbonatite with its attendant enrich-
2 In a summary of the igneousactivity in Kenya,
just published, Wright [1963] has drawn the additional distinction that within the rift valley the
younger lavas (post-Miocene) are largely nepheline-free. This may have important implications in
the sequence and level of magma generation, but
it does not greatly modify the outline given here.
cambrian rifting which .is now almost undecipherable.
The large-scalecorrelationof the rifting and
igneousactivity in time and space,as indicated
above,seemsindisputable.
Divergenceof opinion
seemsto arise when the picture is examinedin
detail; e.g., the nephelinitevolcanoesof eastern
Ugandaare not on a recognizablerift line [King
and Sutherland,1960], the volcanicphasesof
CRUSTAL
WARPING
1107
uplifts of some thousandsof feet along zones
300 to 400 miles in width and several thousand
milesin length.'Sucherosionsurfaceshave their
maximumvertical separationalongthe flanksof
the rifts (Figure 3) and convergeand overlap
as they are traced into the adjacent basinal
areas; they give preciseexpressionto the longknown fact that the rifts are located in the more
uplifted portionsof the continent (most clearly
seen in the Tertiary East African Rift zone,
which lies along the continental divide). This
'basin and swell' structure
Fig. 3. Vertical section illustrating the 'rise to
the rift' with increasing separation of several erosion surfaces; the upper surface may be taken to
represent a Jurassicpeneplane,the lowest an endTertiary peneplane. The heavy broken line marks
a Jurassicdatum plane within the crust,at an arbitrary depth. The vertical attitude of the rift fracture is diagrammatic.
activity in Kenya appearto be terminatedby
rift movements [Kent, 1944], and there is no
correlation
between
the
volumes
of the
rift
troughsand the effusivematerials.But theseare
real anomaliesonly if a restrictedview is taken
of the relationshipbetweenthe magmatismand
rifting; they refer to the premise that any relationshipmust be simply oneof causeand effect,
whereasit is just as likely that the rifting and
magmatism are both expressionsof a more fundamental process.Some such processbecomes
of Africa
has been
well illustrated by Holmes [1944], whosemap
is reproducedas Figure 4. Uplift of the flanks
of the rifts in many placesis also suggestedby
the increasein metamorphic grade of the Precambrian crystalline rocks as the rift is approached,but this evidenceis not always unequivocal.In Rhodesia,for instance,there is an
increasein metamorphicgrade in the Lomagundi/Katanga rocks (625 _ 20 m.y.) as the
Jurassicmid-Zambezirift is approached[Macgregor, 1947]; pegmatites in this high-grade
belt give agesof 430 to 460 m.y. [Nicolaysen,
1962], showingthat this is not simply exposure
by uplift of originallydeeperKatanga rocksbut
probably also indicates localization of the rift
alonga distinctmetamorphicbelt which formed
after the main Katanga orogeny.
The riseof erosionlevelsof differentagesover
the rift zonesshowsthat the continentalplate
has beenwarpedin severaldistinctphases,at
least sincethe Jurassic,and that the rifts are
apparent as soon as the rifts themselves are
relatively subordinatefeatures on the crests of
lookedat in a regionalcontext.
Many people have noted that the uplift of
parts of the African continent is far more impressivethan the rifting, this being expressed
most forcibly by Willis [1936], who considered
that the major problem connected
With the rifts
was the uplift of the neighboringplateaus.Uplift of the plateaus is a misleadingterm, however, becauseit has becomeincreasinglyclear
that 'the rise to the rift,' to which Wayland
[1930] called attention, is a generalfeature, the
rifts beingessentiallylongitudinalfracture zones
along the crestsof broad geanticlines.The geomorphologicalevidencehas been reviewedby
the crustal swells.Obviouslythe causeof the
warping is as subjectto contentionas the cause
of rifting, but it is possibleto clarify the situa-
Dixey [1956], and the riseof erosionsurfacesof
different ages (Jurassicto Quaternary) toward
of uplift couldlead to partial melting by relief
of lithostatic load, but compressionwill be
adopted as a working hypothesisbecauseit is
tion a little. Dixey [1956], for instance,favors
the conceptthat the basinshave saggedand
that the intervening
swellsare largelylag aress,
but, otherconsiderations
aside,the arrangement
of erosionlevels,with the maximum separation
on the swellsand the hingelinesin the basins,
requirespositiveuplift of the swellswith respect
to the base levels. This observation is critical
because
positiveuplift canbe accomplished
only
by lateral compressionor direct vertical movement below the uplifted area. Either mechanism
the rift shouldersis striking evidence of 'the
archingthat accompaniedthe rifting, involving more consistentwith present observations.
1108
D.K.
BAILEY
IRAN
Pladeira
DESE
EL-JUF
CHAD
•
ß
....
5•::•. .' .:.-.-
..
..'-.?.•
..-..
.....
::;'-&'
..,.. •;: '•.... :.::"
•,,.•:
.. •• •;::
.....
:.,,
....
:.•',,-- -;¾,•1
ß:•
R•n,on
Fig. 4. Generalized map of tectonic basins,plateaus, and swells of Africa. (Reproduced from
Principleso• Physical Geology by Arthur Holmes, 1944,with the author'spermission.)
Compression,uplift, and melting. Lateral
compressionon a large scaleis suggestedeven
in the simplebasin and swellstructureof Africa,
whichhasa tessellatedpattern suchas might be
expectedfrom compression
of a rigid but anisotropic plate that has tended to yield along
'grains'establishedby earlier metamorphicbelts.
In an isotropicplate of uniform thicknessthe
compressionrequired for initial warping would
be much greater than that necessaryto increase
the flexure,and it is to be expected,therefore,
that in a continental plate the initial flexures
would appear where the plate was already
thickened or already uplifted. Such conditions
would obtain in old orogenicbelts, especiallyif
in a state of over-recovery from previous isostatic imbalance,which would accord with the
tendency of swells and rifts to parallel earlier
orogenictrends. Present lack of knowledgeof
the mechanismof major earth processesmakes
it fruitlessto dwell long on the possiblecauseof
this compression,
but if the southerncontinents
were once welded together and have since been
driven apart, then Africa, the Gondwananucleus,has presumablybeen under compression
from
all sides.
Upwarping of the rigid continental plate
would producerelief of lithostatic load below
the crest of the flexure. Where the bulk com-
positionand temperaturelower in the crustwere
favorable, this would lead to partial melting
[Yoder, 1952],3 and the zone of pressurerelief
would act alsoas a trap for volatticsand fugitive
constituentsfrom the underlying mantle even
if melting in the mantle were not possible.It
was noted before that the alkaline
rocks were
a Since the original presentation of this paper
(Trans. Am. Geophys. Union, •, 115, 1963) my
attention has been directed to a lecture, 'The Teetonic Significance of Alkaline Igneous Activity,'
bv R. M. Shackleton, at the first (British) InterUniversity Geological Congress, 1953. In the
printed summary he states,'Subcrustalmelting was
due to the lifting of part of the load on the layers
under the sxvells,and eruption was facilitated by
the fractures.'This is an essentialpart of the process elaborated in this paper.
CRUSTAL WARPING
1109
generally regardedas derivative magmas,but if
it is possiblefor them to form as residualliquids
from the crystallizationof a magma such as
several times since the Jurassic, and •his implies that they were in isostatic equilibrium at
the end of each erosion cycle. To achieve such a
alkali basalt, then before any crystallinebasalt condition lighter material would have to be
above the eclogite-basalttransition zone can be added at depth to compensateeach succeeding
rendered completely molten it should partly uplift. The relevanceof this to the presentsugmelt to yield alkaline liquids. This mechanism gestionof partial melting below the rigid layer
produces'derivative' magmaswithout requiring of the crust and withdrawal of light materials
the vast quantities of heat necessaryto com- from the underlyingmantle needsno emphasis.
pletelymelt all the parent material,and at the But this doesintroduce,too, the alternative that
same time it disposesof the volume problem addition of lighter material at the ba•seof the
wherebythe volume of 'derivative'magma is crust might, in itself, be responsiblefor the
far t,oo large in comparisonwith that of the uplift. The main objectionto sucha mechanism
'parent.' It may be that only belowthe cover is the difficulty in explainingthe rifting in the
of the continentsare all the conditionsappro- crestof the upwarp, for this would requirewithpriate for partial melting of basaltic materials drawal of material from belowthe rifts and long
at the baseof the crust,hencethe restrictionof sections of these have little or no associated
major alkalinemagmatismto this environment. volcanism.Expressedin another way, accretion
The cause of the rifting in the crest of the
arch is, of course,a thorny subject of debate,
but there is litt.le reason to supposethat the
rifts representsubsidence
due to removalof underlyingmagmato the surface;e.g.,there is no
volcanismassociatedwith the deepestrift, that
of Lake Tanganyika, its floor being 2500 feet
below sea level. Strong argumentshave been
made that the rifts are compression
structures
of light material to producethe upwarpsrequiresthat rifting is a consequence
of magmatism, whereasthe alternative preferred here is
that the accretionof light material,the magmatism, and the rifting are all consequences
of
lateral compression
and upwarpingof the crust.
Direct vertical movementbelow the swells,
the alternativemechanismt• lateral compression, might be supposedto result from uplift
helddownbv the overridingflanks,but the sup-
above a rising convectioncurrent. The latter
porting evidenceof negativegravity anomalies
in the rifts [Bullard, 1936] has beenweakened
by drilling evidenceof a considerablethickness
of light sedimentsin the best example,the Albert Rift [Dixey, 1956]. The wholesubjectof
possibility
willundoubtedly
findadvocates
among
rifting, like most geologicproblems,probably
suffersfrom our naturaltendencyto fit a simple
explanationto a complexphenomenon.
If the
swellswere producedby a seriesof compres-
Atlantic rise with its median 'rift valley'; he
t.hose who believe tha•t such a mechanism is re-
sponsiblefor the midoceanrises,for Heezen
[1962] has shown that the form of the African
swells and rifts is similar to that of the midalso shows the East African Rift zone as a con-
tinuation of the Indian Oceanrise, but the detailed pattern of the Gulf of Aden and Red Sea
sions,as indicatedby the seriesof erosionsur- rifts requiresthat this be an extremelytortuous
faces,then in a periodof relief of compressiontransition. If, however,the African rift zones
a rift zone might be initiated as an elastic-release fracture zone, which, once established,
wouldbe perpetuatedand aggravatedby subsequent compressions
and releases.An explanation
of this kind might resolvethe conflict between
the opposingtensionand compression
hypothesesof rifting.
Perhaps the most interesting result of Bullard's [1936] gravity survey of the rift zones,
albeit overshadowedby the discoveryof the
negative anomaliesof the rifts, is that the adjacent plateaus are in isostatic equilibrium.
Theseareashave beenuplifted and peneplaned
are the continental analogs of the midocean
rises, they pose difficulties for a convection
origin, becauseit is postulatedthat the rising
convectioncurrent causesthe uplift., splits to
give the median'rift,' and then spreads,giving
ocean-floorspreadingand separationof the continents. There is so far no evidence of similar
spreading of the African continent around the
rift zones; in fact, it is held by some that the
rifts are compression
features,and it is difficult
to see why a tensionalprocessshouldhave the
same expressionthrough the different thicknesses and structures of the oceanic and con-
11 I0
D.K.
BAILEY
tinental crusts(perhapsthe similar wavelengt.
hs
are merely a function of the physicalstate of
the outer shellsof the earth). Oceanspreading
and concomitantcontinental drift are also sup-
posedto have started in the Cretaceous,
but
there is evidenceof rifting in the Jurassic,and
probably much earlier, in Africa. Other digculties are that the African rift pattern would
require the operationof numeroussmall convection cells (one at least as small as the Lake
Victoria basin!) and the situation of rifts along
earlier metamorphic belts requires that there
are now rising currentswhere previouslythere
were descendingones. These remarks are intended chieflyas a warning agains•too facile an
application of the convectionhypothesisto all
major crustalprocesses,
for even if it couldbe
demonstrated
that the African
swells were in-
termittently uplifted by rising convectioncurrents the pressurewould be relieved during the
intervals and the main thesis of this paper
would remain applicable.
Conclusion. The positionof the East African
Tertiary Rift zone along the continental divide,
the generaloff-lap of youngerformationsaway
from the rifts, and the rise of the seriesof old
erosion bevels to the rift shoulders indicate the
is to say that vast quantitiesof parent maqma,
such as alkaline o!ivine basalt magma, were
availablebut only a fraction of this reachedthe
surfaceor even the plutonic levelsthat are exposedtoday in the post-Karroo rifts; moreover,
such a state of affairs must have prevailed in
many different placesand in severalgeological
periods.It would seemmore logicalto regard
these salle rocks as products of partial melting
of parent crystalline materials, such as alkali
basalt, in the lower crustal region, with contributions of juvenile material from the upper
mantle, and relief of pressurewould be a suitable mechanism for this.
Any discussioninvolving the origin of carbonarite must touch on the origin of kimberlite,
for rocks of kimberlitic compositionare associated with carbonatites,and vice versa, and
somepeople [e.g., yon Eckermann, 1961] hold
that carbonatite is derived from a magma of
kimberlite composition.But while the distribution of carbonatite centers shows a strong
correlation with the rift pattern, the major
kimberlite provincesdo not, althoughthey do
appear in uplifted areas; the associatedalkaline
igneous activity of carbonatites and their unusualsuite of trace elementsalsoset them apart
from kimberlite activity, which is typically expressed as breccia pipes and intrusions with
closespatial and temporal relationshipbetween little or no associatedmagmatism.Apparently
the rifting and alkaline, peralkaline,and car- the two kinds of activity, in their typical debonatiticmagmatism,characterized
by abundant velopment,reflect differing tectonic conditions,
volatiles,both in the Tertiary rifts and the older as inferredby James[!956], and thereis a vital
rifts such as those of the post-Karroo. Essen- need for more information on the regional
tially the questionis, can the generationof these tectonicsof the major kimberlite provincesas
vast quantitiesof uncommonrocksbe related •o well as gravity and seismic data from both
the warping and rifting of the crust? The con- tectonicenvironments.It seemslikely that bot.h
tinuity of the variouserosionsurfacesacrossthe kimberlite and carbonatite are derived from the
upwarps indicates that the crust has behaved upper mantle, though not necessarilyrelated
competently,and present evidencefavors the through somesimple processsuch as fractionaview that it has warped in responseto lateral tion from a commonparent. The differencein
compression.
Regardlessof the cause,however, the expressionsof the two activities may lie in
it is to be expectedthat there would be relief the fact that below the rift zones the PT condiof lithostatic load below the crest of a rigid tions are such as to produce melting in the
arch due to support in the limbs, giving rise, lower crust, as indicated by the igneousactivunder favorable conditions,to partial melting ity; therefore, any kimberlitic material from
lower in the crust and perhaps in the upper the mantle would generallyhave to be erupted
mantle and withdrawal of fugitive constituents through a crustal zone of melting and magma
from the uppermantle.The bulk of the magmas generationand would thus be expectedto reach
in the rift zonesare of salic differentiate type, the surfaceonly in such form as melilite basalts
e.g. phonolites,trachytes, and nephelinites;to and kindredlavassuchas t,hoseof Toro-Ankole,
call them productsof fractional crystallization which show affinities with both k'nuberlite
position of the rifts as median fracture zonesin
the crests of broad crustal arches. There is a
CRUSTAL
WARPING
1111
[HolmesandHarwo.od,
1932]and carbonatite central and southernAfrica, Trans. Geol. Soc. S.
A•rica, 58, Annexure, 1955.
[Holmes,1956; von Knottingand Du Bois, Dixey,
F., The East African rift system,Colcmi•l
1961].Awayfromthe riftsthe conditions
in the
Geol. Mineral Resources(Gr. Brit.), Suppl. Set.,
crust are not suitablefor extensivemelting, and
Bull. Suppl. 1, 1956.
kimberliteis allowedto passfrom the mantle Gregory,J. W., Riit Valleys and Geology o• East
Seeley, Service and Co., Ltd., London,
to the surfacein its typical form of a gas-trans- A•rica,
1921.
ported breccia.
Harkin, D. A., The Rungwevolcanicsat the north-
Bowen,in 1937,usedthe compositions
of the
ern end of Lake Nyasa, Mem. Geol. Surv. Tanganyika, 2, 1960.
rift valleylavasto test 'petrogeny's
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the productsof crystal-liquidequilibrium,
which
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for him, it seems,was most often synonymous Holmes, A., Principles o• Physical Geology, Ron-
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partial melting,and, whenthe volumesof such
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Acknowledgments. I wish to thank Professor
(190, 191, and 192), 1960.
C. E. Tilley and Dr. H. S. Yoder for their helpful
Macgregor, A.M., An outline of the geological
discussionsand review of the manuscript.
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