- Email: [email protected]
Kimberlites: Their relation to mantle hotspots
260
Earth and Planetary Science Letters, 50 (1980) 260-274 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[31
KIMBERLITES: THEIR RELATION TO MANTLE HOTSPOTS S. THOMAS CROUGH 1 W. JASON MORGAN and ROBERT B. H A R G R A V E S Department of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544 (U.S.A.)
Received April 3, 1980 Revised version received June 10, 1980
Available age data support the hypothesis that kimberlite intrusions are formed by mantle hotspots. The hypothesis has been tested by inverting the volcanic traces formed by three hotspots to determine the post-Triassic motions of Africa, South America, and North America relative to these hotspots. Then, using these motions, the kimberlites intruded on these continents within the last 150 m.y. are relocated to their place of origin in the present hotspot reference frame. The result indicates that a majority of the kimberlites formed within 5 ° of a mantle hotspot. Statistical analysis shows that this kimberlite/hotspot correlation is significant at above the 90% level.
1. Introduction Igneous activity at lithospheric plate margins is simply explained by plate tectonics, but the cause o f intraplate magmatism has remained controversial. Many hypotheses have been advanced, and all are subject to further testing and modification. The two most frequently mentioned theories are that: (1) midplate volcanism is associated with large, deep lithospheric fractures (zones of weakness), and (2) it is related to mantle hotspots (zones of upwelling in the asthenosphere). Of the two main hypotheses, the hotspot hypothesis makes the most specific predictions. The temporal and spatial pattern o f midplate magmatism must be consistent with plate motions over a set of fixed magma sources. Earlier work has demonstrated that the azimuths and volcanic ages o f young seam o u n t chains agree with the Ftxed hotspot concept [1,2]. More recent work suggests that Mesozoic and Tertiary igneous events on several continental margins are also consistent with the theory [3].
1 Present address: Department of Geosciences, Purdue University, West Lafayette, IN 47906, U.S.A.
Support for the zone of weakness theory comes from recent studies showing that cratonic igneous activity, particularly the intrusion o f kimberlites, often occurs onshore from major oceanic fracture zones [4,5]. These studies conclude that lithospheric fractures control cratonic intrusions and that hotspots are not involved. This paper presents evidence implying that kimberlites are produced by mantle hotspots. This possible genetic relation has been suggested previously [ 6 - 9 ] , but never tested. However, there are now enough dated kimberlites to establish that their ages and locations are consistent with known plate motions over hotspots. This demonstration not only provides a reasonable explanation for the occurrence of kimberlite, but also further supports the hotspot hypothesis by extending the theory to include magmatism within cratons.
2. Kimberlite provinces Kimberlites have been discovered on almost every continent and have measured emplacement ages ranging from Tertiary to Pre-Cambrian. Due to limitations in the accuracy of plate reconstructions, we
0012-821X/80/0000-0000/$02.25 O 1980 Elsevier Scientific Publishing Company
261
have restricted our study to post-Triassic age kimberlites which were emplaced on the continents of North America, South America, and Africa (Fig. 1). However, this restriction is not severe, and our compilation of dated intrusions includes representatives from most of the world's kimberlite provinces. The major sources of our data are the recent review by Sykes
'
' X,,,~
'4,)
'L)~
[5] and the new kimberlite ages obtained by Davis [ 1 0 - 1 2 ] . The dated kimberlites are listed in Table 1 and a brief summary of their occurrence follows. There are three post-Paleozoic kimberlite provinces in eastern North America: Labrador Sea, New York-Ontario, and Arkansas-Kansas. The Labrador Sea group includes Mesozoic kimberlite dikes along
,
'
,
I
I
I
t
I
\ o4
° 2 " ~ ~ k l 1WFOUNDLAND O0
o5
~"~)
'~
SMTS
°03 o
3"
RISE
%
0/
~-"
) °I0
o
OLUMBO A SMTS
0
,~,\ 12 ° ,<"
J/
~. L-4"3,14 c~
!
I
!
I
m
!
I
I
!
I
!
I
!
!
I
Fig. 1. Locations of dated, post-Triassic kimberlites (o) intruded within North America, South America, and Africa. Kimberlite provinces are identified by number as given in Table 1. Offshore aseismic ridges and seamount (smt) chains (locations delineated by printing) may mark the subsequent traces of the hotspots which formed the coastal kimberlite provinces.
262 TABLE 1 Dated kimberlites Province
Present location (lat., long.)
Relocated position (lat., long.)
Age (m.y.)
Reference *
Labrador Sea (1)
49.5
-55.4
32.6
-16.9
140
[15]
New York-Ontario (2)
42.5 42.9 48.0
-76.0 -74.5 -79.5
27.7 26.5 30.6
-34.6 -32.2 34.3
136 146 151
[17] [17] [16]
Arkansas-Kansas(3)
40.0 37.7 34.2
-97.0 -96.0 -93.0
29.1 37.3 29.6
-55.5 -64.4 -59.3
115 90 97
[18] [17] [17]
Montana(4)
48.0 -108.5
53.5
-94.7
45
[21]
Colorado Plateau (5)
37.0 - 1 1 0 . 0
41.2 -102.6
31
[20]
Brazil (6)
West Africa (7)
-20.0 -18.4 -13.9
-46.9 -47.2 -54.1
-22.8 -21.9 -21.6
-27.1 -25.9 -26.3
80 86 122
[10] [10] [11]
8.0
11.0
-0.3
-25.6
140
[9]
Zaire (8)
-6.2
23.7
-18.0
13.7
71.3
[12]
Tanzania (9)
-4.2 -3.5
33.2 33.7
-14.1 -11.1
26.4 28.6
53.2 40.6
[10] [11]
Angola (10)
10.0
20.0
-29.6
-3.5
Namibia (11)
-30.8 -30.2 -30.2 -30.2 -26.7
18.1 18.5 18.5 18.5 20.0
-39.0 -40.0 -40.2 -40.5 -37.7
6.7 4.7 4.3 3.7 5.6
54.1 65 66.7 69 71.8
[10] [12] [10] p.c. [12]
Botswana (12)
-26.4 -21.3
22.2 25.4
-38.9 -36.4
6.2 8.2
80 93.1
p.c. [12]
South Africa, Cretaceous (13)
-30.7 -29.5 -29.5 -29.5 -30.0 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -28.5 -30.5
23.0 25.0 25.0 25.0 28.5 25.0 25.0 27.5 25.5 25.0 25.0 25.5 24.0 25.0 24.3 22.7
-43.0 -43.1 -43.2 -43.5 -44.7 43.9 -43.9 -44.5 -44.0 -44.0 -44.1 -44.3 -44.2 -44.5 -45.3 -47.3
6.3 7.4 7.2 6.8 10.6 5.9 5.9 8.8 6.4 5.6 5.4 6.0 3.7 4.6 0.1 -4.2
78.3 84.3 85.3 86.9 87.5 90.3 90.4 90.4 90.4 91.2 92.0 92.2 94.1 95 110 115
p.c. p.c. p.c. [10] [12] [12] [12] [ 10] p.c. [12] [12] [10] [10] p.c. [11] p.c.
South Africa, Jurassic (14)
-25.5 -29.5
26.2 29.0
-47.2 -52.2
-6.9 -7.2
147 150
[22] [11]
* p.c., Gordan Davis, personal communication, 1979.
138
[11]
263 the western margin of Greenland [ 13], an explosive Mesozoic breccia cut by lamprophyre-carbonatite dikes on the coast of Labrador [14], and an ultramafic intrusion associated with breccia pipes in Newfoundland [ 15]. The Newfoundland intrusions, which have a K-Ar age [15], are the only rocks in this province which are dated accurately enough to test the hotspot model. The New York-Ontario group consists of three dated kimberlite dikes, one at Kirkland Lake, Ontario with a K-At age [ 16] and two in upstate New York with Rb-Sr ages [ 17]. The Kansas kimberlites are a group of pipes in Riley County dated by the fission-track method [18] and a possibly related mica peridotite in southeast Kansas dated by the K-Ar technique [ 17]. The Arkansas pipes near Murfreesboro have been stratigraphically dated [ 19]. The two Tertiary provinces in the western United States consist of a group of pipes and dikes in the Colorado Plateau with fission-track ages [20] and kimberlitic intrusions in Montana with K-Ar ages [21]. Southern Brazil contains the only dated kimberlites in South America. The three intrusions parallel a trend of alkalic intrusions, mostly syenites and carbonatites, and have an age progression, from east to west, documented by U-Pb measurements on included kimberlite zircons [10,11]. The West African countries of Guinea, Sierra Leone, Ivory Coast, Ghana, and Nigeria all contain kimberlites, but most intrusions have Pre-Cambrian ages and only the Sierra Leone kimberlites are of Mesozoic age [9]. There are numerous kimberlite provinces throughout central and southern Africa, of which almost all have been dated by U-Pb measurements [10-12]. Tanzania, Zaire, Angola, Botswana, and Namibia all contain groups of pipes with fairly uniform ages. South Africa appears to have at least two provinces, a large group of Cretaceous intrusions near Kimberley and also some older intrusions of Jurassic age. The reliability of kimberlite ages depends on the method of dating. K-Ar ages are the least reliable, because ages of mineral separates are often older than consistent with geological evidence for the time of emplacement. These presumably record the age of lithospheric fragments incorporated into the kimberlite. Rb-Sr ages share this problem, and care must be taken to measure only authigenic minerals. U-Pb
ages of kimberlite zircons are the most reliable. Because lead diffuses rapidly from zircons at mantle temperatures, the technique yields emplacement ages which are reproducible to within a few percent. We would prefer to use only U-Pb dates, but this would greatly limit the number of kimberlite provinces represented. Although we recognize that the K-Ar and Rb-Sr ages may be suspect, for many provinces they are the only dates available. In most cases the consistency of the measured ages within the same province is the best evidence that the ages are reliable. The aseismic ridges and seamount chains in the Atlantic (Fig. 1), which are the characteristic signature of hotspots, suggest that several of the kimberlite provinces were formed by hotspots. As the Atlantic basin opened, North and South America moved westward in the hotspot reference frame and Africa moved eastward [3]. Any hotspot which formed a volcanic ridge intersecting a continental margin must have been beneath that continent at some earlier time, and several of the presumed hotspot lineaments in Fig. 1 are aligned with kimberlite provinces further inland. The Newfoundland Seamounts and New England Seamounts occur immediately offshore from the Labrador Sea and New York-Ontario kimberlite provinces respectively. The Columbia Seamounts appear to connect the Brazilian kimberlites with Trindade hotspot. And the Sierra Leone Rise, Walvis Ridge, and Cape Rise all intersect the African coast near a major kimberlite occurrence. Our test of the hotspot hypothesis is to plot all dated kimberlites at their location of origin in the present hotspot coordinates. From independent evidence, we can determine the plate motions with respect to the hotspots for approximately the past 150 m.y. Thus, for any time instant within this interval, the position of each hotspot relative to the plates can be predicted, and conversely the position of any point on the plates relative to the hotspots can be calculated. Specifically, we can calculate where any intrusion was located relative to the hotspots at the instant when the intrusion was formed. When the known plate motions are used to relocate the kimberlites back to where they originated in the hotspot reference frame, the kimberlites should cluster at the present locations of the hotspots which created them.
264
kimberlites. Several motion models have previously been determined [3,23], but new age data have negated some of the earlier results and have suggested slightly different reconstructions. As constraints, we have used the Great Meteor, Tristan da Cunha, and Trindade volcanic traces. They are the best-developed and least-controversial traces available; and, because
3. Plate/hotspot motions To relocate the kimberlites accurately and to estimate the errors in these relocations, we have redetermined post-Triassic plate/hotspot motions by using linearized least-squares inversion theory applied to hotspot traces manifest by igneous activity other than
I
I
I
I
!
ROTATION POLE
NDADE
~
"
"_I b O " V
-
(
O
A TR I STAN
i
I
!
I
I
i
I
I
I
I
I
I
I
I
I
Fig. 2. Dated non-kimberlitic intrusions ( . ) on the presumed traces of Great Meteor, Trindade, and Tristan hotspots (TaMe 2), rotated to their location of origin relative to Africa using the k n o w n relative plate motions. These traces are inverted to obtain Africa's past motion with respect to the hotspots in terms of a single pole and a constant rotation rate. The data require a counterclockwise rotation about the pole near Great Meteor.
265 TABLE 2 Dated intrusions used to determine plate motions Hotspot trace
Great Meteor
Tristan da Cunha
Trindade
Present location (lat., long.)
Relocated position (lat., long.)
Age (m.y.)
Reference
[27] [27]
34.4 37.4 44.3 43.5 43.4 44.1 43.9 43.8 43.8 43.5 43.5 43.4 43.2 43.1 43.1 43.0 45.3 45.4
-56.5 -60.1 -73.2 -72.9 -72.4 -70.8 -70.9 -71.2 -71.1 -71.2 -70.7 -70.8 -70.7 -70.6 -71.2 -71.1 -72.6 -74.1
29.0 32.3 31.6 34.8 29.8 31.2 31.8 30.0 31.1 29.9 29.8 29.3 29.3 29.7 29.3 30.2 30.7 32.0
-34.2 -37.4 35.4 -38.2 -33.5 -33.1 -34.1 -32.4 -33.7 -32.7 -32.1 -31.9 32.0 -32.4 -32.5 -33.6 -32.2 -35.0
8O 8O 111 100 120 112 108 121 110 118 120 125 122 116 121 111 131 117
-28.6 -30.0 -35.0 -19.7 -21.7 -20.4 -21.3 -20.3 -21.0 -21.1 -22.0 -22.3 -21.5 -22.0 -24.2 -23.7 -24.2 -23.3 -23.5 -27.5 -24.5 -27.5 -24.1
-30.6 -35.6 -4.5 9.0 14.0 16.3 14.1 14.3 15.2 15.0 15.0 14.4 14.1 -52.0 -48.0 -46.7 -47.7 -47.6 -47.3 -49.2 -48.7 -49.2 -49.0
-31.1 -33.2 -38.6 -32.1 -37.6 -37.7 -36.6 -35.9 -37.1 -37.6 -36.9 -38.1 -37.2 -29.2 -30.6 -29.5 -30.1 -29.1 -29.2 -33.8 30.0 -32.7 -29.2
-14.0 -13.9 -14.6 -12.5 -14.1 -12.0 -11.5 -11.3 -11.4 -13.0 -8.8 -13.2 -12.8 -20.6 -17.2 -15.3 -15.5 -16.8 -16.7 -16.3 -21.0 -22.3 -23.2
72 85 4O 108 132 136 123 125 128 133 115 129 128 127 133 130 127 123 122 129 1i0 105 103
[32] [331 [341 [341
-15.6 -16.2 -17.5 -17.9 -18.3 -18.6 -18.7 -18.5 -19.4
-51.3 -51.3 -50.5 -47.8 -46.5 -46.8 -46.7 -46.0 -47.0
-19.5 -18.2 -20.9 -21.1 -21.0 -21.7 -21.6 -20.3 -23.3
-29.6 -34.7 -29.5 -27.1 -27.5 -26.3 -26.4 -31.1 -24.6
89 72 85 83 78 82 81 68 91
36] 36] 36] 36] 36] [36] [36] [36] [36]
[281 [281 [28] [281 [28] [28] [281 [281 [281 [281
1281 [28] [28]
[28] [29,30]
[31]
[351 [351 351 35] 35] 35] 35] 35] 35] 36] 36] 36] 36] 36] 36] 36] 361 36] 36]
266 TABLE 2 (continued) ttotspot trace
Present location (lat., long.)
Relocated position (lat., long.)
-19.5 -19.3 -21.4 -22.5 -22.0 22.3 -22.3 -22.3 -22.6 -17.6 -17.6 -24.6 -24.5 24.4 -24.3 -27.0
-21.5 -22.4 -24.0 -24.8 -23.8 -24.1 -24.3 -23.8 23.7 -18.8 -18.3 -27.6 - 26.8 -26.6 25.9 28.5
-47.0 -45.5 46.5 -43.5 44.7 -43.5 -42.7 - 4 2.0 -42.1 -39.0 -39.0 -47.9 -48.7 49.3 -49.0 -50.3
-31.2 --25.0 -27.9 -27.0 -30.5 -29.3 -27.4 -29.2 30.9 -27.4 -29.9 27.5 -31.5 32.6 34.9 -36.6
they are on three different plates and have significantly different latitudes, they provide good control on the plate/hotspot motions. The data are listed in Table 2 and summarized as follows. Great Meteor Seamount is situated east of the MidAtlantic Ridge at 30°N, 29°W (Fig. 2). Anomalously shallow seafloor in this vicinity [24] and measured ages o f 1 1 - 1 7 m.y. on dredged volcanics [25] suggest to us that Great Meteor is the site of a recently active mantle hotspot. Progressive closure of the Atlantic basin puts Great Meteor adjacent to Corner Rise, which is on the western side of the ridge, at about 75 m.y.B.P. Corner Rise must be no older than the Late Cretaceous seafloor it rests on, and dredged fossils suggest it is no younger than Eocene [26]. The New England Seamounts, which are of Cretaceous age and are progressively older to the west [27], form an almost continuous trace from near Corner Rise to the coast of North America. The seamounts are aligned with the Cretaceous intrusions of New England [28] and eastern Canada [ 2 9 - 3 1 ] , which are slightly older. Our explanation of these data is that North America and the Mid-Atlantic Ridge have migrated westward over the Great Meteor hotspot, with the ridge passing over the hotspot at 75 m.y.B.P. Since 75 m.y.B.P, the hotspot has remained in approximately its present position relative to Africa. Tristan da Cunha's trace includes both the Walvis
Age (m.y.)
Reference
70 82 77 72 66 66 69 59 51 52 42 82 74 73 66 65
[36] [36] [36] [36] [36] [36} [36] [36] [36] [36] [36] [36] [ 36 ] [36] [36] [36]
Ridge and Rio Grande Rise (which have been sparsely dated by DSDP work [ 3 2 - 3 4 ] ) and the Late Jurassic/Early Cretaceous coastal volcanic fields in Brazil and Africa [35,36], where the oceanic ridges terminate. Trindade is a young volcanic island [36] located off the coast of Brazil. Its postulated trace includes the undated Columbia Seamount chain and the band of Cretaceous alkalic intrusions extending almost 1000 km inland from the Brazilian coast [36]. To simplify the inversion, we have used the known relative motions between North America and Africa ( [ 3 7 ] , Schouten and Klitgord, personal communication, 1977) and between South America and Africa [38] to relocate the dated intrusions (Table 2) from the traces on the North and South American plates to their places of origin relative to Africa (Fig. 2). The effect of this data transformation is to remove the relative plate motions from the hotspot traces. The relocated intrusions indicate how the three hotspot traces would have looked if the Atlantic region had been covered by a single plate rotating with Africa's plate/hotspot motion, rather than by three separate plates rotating with different motions. To calculate plate/hotspot motions, we have solved for the rotation which predicts these relocated traces on the African plate. The concentration of relocated intrusions near Great Meteor suggests that the African plate has been nearly stationary with
267 respect to this hotspot. The spread of points away from Trindade and Tristan imply that, at this latitude, the African plate has moved northwestward with respect to the hotspots. Therefore, to first order, Africa's plate/hotspot motion has been a counterclockwise rotation about a pole near Great Meteor. We have assumed that Africa's motion has been at a uniform rate about a single pole and have solved for the parameters of this rotation. In matrix notation, the present coordinates, XOi, of a dated intrusion should be related to the present coordinates, HSi, of its source hotspot through the rotation matrix, Mij, which is a function of the rotation pole's location, the rotation rate, and the age of the intrusion:
xOi = Mii HS/ Since the rotation matrix is a non-linear function of the unknown rotation parameters, a linearized iterative inversion method is used. An initial estimate of the parameters creates a matrix, Mi/, which is used to calculate predicted coordinates, XPi, for each intrusion. If the differences between the observed and predicted coordinates are small, they are approximately related to the required parameter changes, dpk, by:
XOi - XPi = [(OM~i//Opk)HS/Idpk This over-determined system of linear equations is solved to obtain the least-squared-error estimate of dpk and the corrected parameters are used to create a new rotation matrix. The process is repeated until the calculated changes are small. The results are listed in Table 3, where the confidence limits are twice the square root of the diagonal terms in the covariance matrix for the estimated parameter changes. Whether the traces are inverted in
groups of two or all three together, the estimated rotation parameters are approximately the same. Using the rotation derived from all three traces, the dated intrusions were relocated to their origin in the present hotspot reference frame (Fig. 3 and Table 2). The fit is not perfect, but as shown by the circles in the figure, approximately 70% of the data relocate to within 5 ° of a hotspot center. Other error estimates are possible, but a 5 ° radius circle provides a convenient measure of the scatter and has the physical significance of being the size of the topographic swell usually associated with hotspots. Possible sources of error include: (1) uncertainties in the radiometric ages, (2) errors in the plate relative motion data, (3) coeval magma intrusion over the width of a swell, not just the center, (4) movement between the hotspots, and (5) an absolute motion model which is too simple. Considering these uncertainties, we think the fit is quite good and consider that the hotspot model provides a reasonable explanation of the data. The computed motion describes Trindade's trace particularly well. Combining the South America/ Africa relative motions with the calculated Africa/ hotspot motion yields a predicted South America/ hotspot motion which closely matches the east-west age progression of Brazil's alkalic complexes (Table 2) between 15°S and 24°S (Fig. 4). The present plate/ hotspot velocity at Trindade is computed to be 2 cm/ yr, the same as that derived independently by inverting all recent trace azimuths [2]. 4. Kimberlite sources Using the plate/hotspot motion calculated for Africa and the known relative motions between the
TABLE 3 Calculated absolute motion poles for Africa~ 150-0 m.y.B.P. Traces used
Rotation rate (deg/m.y.)
Latitude (°N)
Longitude (°W)
Great Meteor, Trindade, and Tristan Great Meteor, Trindade Great Meteor, Tristan Trindade, Tristan
0.23±0.01 0.23±0.02 0.23±0.02 0.26±0.03
29.3±2.8 33.1±2.0 29.1±3.2 18.2±7.5
35.7±2.6 29.6±2.0 36.5±3.3 34.0±5.2
268 r...j \ ,
,
,
,
I
I
I
l
I,..
/i
/ I
TRINDADE
RISTAN
!
!
I
I
I
I
I
I
[
I
I
I
I
I
I
Fig. 3. Dated intrusions in Fig. 2 rotated to their origin (Table 2) relative to the present hotspots, using the calculated African motion. With no errors in the age m e a s u r e m e n t s or in the rotations used, the fixed h o t s p o t model predicts that the points should overlie the hotspots. The 5 ° circles drawn around the h o t s p o t s enclose a b o u t 70% of the data.
plates, all the kimberlites in Fig. 1 were rotated back to their predicted location of origin relative to the present hotspots (Fig. 5 and Table 1). Because the calculated African m o t i o n is only well-constrained back to 130 m.y.B.P., the age of the oldest trace data used, we have had to assume the motion prior to that time. The relocations in Fig. 5 use the assumption that Africa's motion was uniform from 150 m.y. B~P.
to the present. Later we will use an alternate assumption and show that different assumptions do not change the conclusion regarding the hotspot hypothesis. The hotspot population in Fig. 5 is taken from earlier work [24] and is meant to include all island or seamount groups which have been volcanically active within the past 25 m.y. or which are situated on large
269 -100
"~kx~" ~."I
80 1>
oe e
°o
60
m
3
•
•
TRI"i~
!
TRACE
5'OoW
4() ° LONGITUDE
40
~
v
20
30 °
0
Fig. 4. Observed ages and locations of non-kimberlitic intrusions on Trindade's trace across Brazil compared to the calculated plate/hotspot motion (solid line). A more detailed motion model could improve the fit only slightly; most of the scatter must be due to age errors or to coeval intrusion in a zone several hundred kilometers wide.
topographic swells. There is a general consensus as to the number and location of Atlantic hotspots, so different compilations would give similar results. For example, Burke and Wilson [39] have not identified Great Meteor Seamount as a hotspot, but they infer a hotspot at Colorado Seamount which is immediately west of Great Meteor. Their hotspot location correlates even better with the trace data in Fig. 3 and the relocated kimberlites in Fig. 5. Almost all kimberlites relocate near a hotspot. Within the Atlantic Ocean basin, we identify the relocated kiinberlite provinces according to the hotspot they are closest to. For example, the one dated intrusion of the Labrador Sea province is predicted to have originated at 33°N, 16°W, so the observed association is Labrador Sea/Madeira. The other associations are: New York-Ontario/Great Meteor, ArkansasKansas/Bermuda, Sierra Leone/Ferna~do de Noronha, Brazil/Trindade, Angola/Tristan, Namibia/ Discovery, Botswana/Vema, Cretaceous South Africa/ Meteor, and Jurassic South Africa/Bouvet. On the continents, the kimberlite/hotspot associations are less certain. Estimates of the number of hotspots presently beneath Africa range from 5 [24] to 34 [40]. The low estimate has no hotspots beneath the two kimberlite provinces relocated within Africa;
the high estimate has hotspots (highspots) beneath both. The late Tertiary kimberlites relocated in Colorado may be related to the Raton hotspot [41 ], but the Montana intrusions relocated in Canada have no obvious hotspot association and may be subduction-related [42] instead. Even assuming that none of the kimberlites relocated onto the continents are associated with hotspots, 67% of the relocated kimberlites lie within 5 ° of a hotspot, which is the same quality of fit obtained with the non-kimberlite traces in Fig. 3. Given the inaccuracies in the data, we consider that the hotspot model satisfactorily predicts the locations and ages of most of the kimberlites considered. Trindade's track provides the best example of the proposed hotspot origin for kimberlites. The predicted path not only matches the ages and locations of the three dated kimberlites but also coincides with the location of most of the alluvial diamond deposits in Brazil (Fig. 6). The ages of these alluvial deposits are still uncertain. Some geological evidence suggests a Cretaceous age [43], but other deposits are considered to be Pre-Cambrian. Different ages in the same locality are not necessarily in conflict with the hotspot hypothesis, because Brazil has had a long history of movement over hotspots and kimberlites of many ages should be found. However, the alignment of alluvial diamond deposits with Trindade's track is remarkable. It suggests that the area contains many presently unlocated and undated kimberlites which could be used for further testing of the hotspot concept.
5. Discussion The motion of Africa in the interval 1 2 5 - 1 5 0 m.y.B.P, is the major uncertainty in this study. Only the Tristan trace has ages in this range, so the absolute motion is poorly constrained. The relocations in Fig. 5 assume that Africa's motion relative to the hotspots has been uniform from 150 m.y.B.P. to present, but other evidence suggests that Africa and South America may have moved westward, rather than eastward, prior to 125 m.y.B.P. [3]. This alternative motion would change the relocated positions of all the kimberlites older than 125 m.y. To estimate the effect of this uncertainty, we have
270 I
%d\'
I
,
I
,
,
o5
©t £'o sioN 0
°8
ADE ~ A
10o
°9
/
o~o
14o~ i n
I
I
I
I
I
I
I
I
I
I
t
I
I
I
]
I
Fig. 5. Dated kimberlites in Fig. l rotated to their origin (Table 1) relative to the present hotspots using the plate/hotspot motion determined from the three traces in Fig. 2. The kimberlite data match the hotspot prediction with the same accuracy as the trace data in Fig. 3 - approximately 67% of the relocated intrusions lie within the 5 radms circles surrounding the hotspots. No continental hotspots are shown; estimates of the population of continental hotspots vary considerably and it is uncertain whether the kimberlites relocated on the continents are associated with hotspots. o
also r e l o c a t e d t h e k i m b e r l i t e s using a p r e v i o u s m o d e l [3] for A f r i c a ' s r o t a t i o n f r o m 150 to 125 m . y . B . P . a n d t h e p r e s e n t m o d e l f r o m 125 t o 0 m . y . B . P . F o r t h e k i m b e r l i t e s in Fig. 5 w h i c h are 150 m.y. old, t h e l o c a t i o n c h a n g e is given b y a c o u n t e r c l o c k w i s e rotat i o n o f 12 ° a b o u t a pole j u s t n o r t h o f Madeira. Since
.
,
t h e k i m b e r l i t e s near Madeira a n d G r e a t M e t e o r are very close t o this pole, t h e i r l o c a t i o n s are n o t c h a n g e d b y m o r e t h a n a few degrees. T h e k i m b e r l i t e to the n o r t h e a s t o f T r i s t a n m o v e s a d j a c e n t to t h e circle a r o u n d V e m a , a n d t h e t w o k i m b e r l i t e s west o f B o u v e t ' s c e n t e r m o v e east o f B o u v e t a n d closer to
271 by the binomial distribution: +
/7 70 ~
V o`~
,r,n0aOe 3~00
Fig. 6. Locations of dated kimberlites (o) and major alluvial diamond deposits (*) [43] in Brazil compared to the predicted track of Trindade hotspot, ages in m.y.B.P. The hotspot model matches the ages and locations of the known kimberlites and may explain the observed concentration of diamond deposits in southern Brazil.
Meteor. The overall fit to the hotspot model is therefore unaffected, but the specific kimberlite/hotspot associations change slightly. Because the fit of the relocated kimberlites to the present hotspot positions is not perfect, we have sought to evaluate the statistical significance of the observed associations. We have done this by supposing that kimberlites are not related to mantle hotspots and then calculating the probability that the observed associations were obtained by chance. Because individual kimberlite pipes are not independent of one another but occur in provinces, each province has been treated as a single datum. We have utilized the following probability model. When a kimberlite province is relocated, there are only two possible outcomes; it lies within a 5 ° circle surrounding a hotspot or it does not. If provinces are independent of hotspots, then some will be relocated into hotspot circles just by chance alone. The probability, p, of an individual province being relocated within a circle is given by the ratio of the area within the circles and the total area where the province might have been relocated. The probability of observing k provinces within the hotspot circles is given
pk(1 _ p ) n
k [n!/k[(n - k)!]
(1)
where n is the total number of provinces relocated. From this relation, the number of provinces expected within the circles is rip. In other words, if the area covered by hotspots is very large, then a large number of kimberlite provinces will appear to be associated with hotspots even if kimberlites and hotspots are unrelated. If the area covered by hotspots is relatively small, then very few chance associations are expected. First, we take the worst case by considering all the relocated provinces and by assuming that none of the continental provinces are associated with hotspots. The provinces might have been formed anywhere on the continents and over any part of the mantle now covered by the Atlantic, except for a small portion of the North Atlantic basin which is over 150 m.y. old. From 50°N to 60°S, the combined area of the three continents and the Atlantic basin older than 150 m.y. is 3.08 R 2, where R is the earth's radius. The area enclosed b y 5 ° circles around the oceanic hotspots in Fig. 5 (Cape Verde is excluded because it has not passed beneath a continent in the past 150 m.y.) and the 6 continental hotspots in the population used [24] is 0.47 R 2. With these areas the probability of relocating a single kimberlite province over a hotspot is 0.17 and the expected number of hotspot-related provinces is only 2.1 out of the total of 14 which have been relocated. Because kimberlite provinces often relocate only partially within the circles, kimberlite/hotspot associations are counted according to the fraction of intrusions which lies within a hotspot region. For example, the Arkansas-Kansas/Bermuda association counts as 1/3 of a province correlated with a hotspot. With this method, there are 5.3 provinces within the circles and, from equation (1), the probability of finding more than 5 of the 14 provinces within 5 ° of a hotspot center is only about 0.02. Thus, although in this worst case the hotspot hypothesis explains less than half of the kimberlite provinces, even this poor fit is probably too good to be due to chance. Second, we consider only those provinces which relocate within the ocean basin. Because there is more agreement as to the hotspot population within oceans, this may be a fairer test of the hotspot con-
272 cept. The above calculations are repeated using tile ocean area and ocean hotspots alone. The probability of relocating a single province within a circle is 0.21, which is slightly higher, but the expected number of hotspot-related provinces is 2.1 out of 10, which is still much lower than the observed 5.3. The probability of observing more than 5 of the 10 provinces within the circles is 0.01, so the observed correlation between hotspots and kimberlite provinces is statistically significant at about the 99% confidence level. These probabilities are clearly a function of the area assumed to be covered by each hotspot, but dilL ferent values give similar results. Making the hotspot circles larger increases the pi-obability of fortuituous associations but also increases the number of provinces explained by hotspots. The significance of the fit remains about the same. Reducing the circles to 3° reduces the number of observed associations, especially since many kimberlites relocate almost exactly 5° from hotspot centers. However, the probability of a chance association also decreases and the close associations observed for Madeira and Trindade keep tire overall correlation significant at above the 90% level. If the circles are reduced to points, then the correlation disappears; but it is inappropriate to expect greater accuracy than the data allow. The remaining test of the validity of the kimberlite/hotspot associations observed in Fig. 5 would be to examine if there are continuous volcanic traces (with the predicted age relations) linking these kimberlite provinces to the present hotspot locations. Continuous traces are not required by the hotspot hypothesis; but if such traces exist, they provide additional support for the theory. Unfortunately, most of the aseismic ridges in Fig. 1 are not dated adequately enough for this test. The Cape Rise is the worst example; this seamount chain appears to link the South African kimberlites with the hotspots at Discovery and Meteor Seamounts, yet it is completely undated. Coastal alkalic intrusions in South Africa, located to the southwest of the Cretaceous kimberlites, have Late Cretaceous/Early Tertiary ages [8,44] as predicted by the hotspot model, but samples from the Cape Rise are needed to confirm the remainder of the predicted trace. We see no fundamental objection to this proposed hotspot origin for kimberlites. Available evidence suggests that kimberlite magma is produced by partial
melting of the mantle at depths of 100 km or more [45]. As suggested by Eggler and Wendt [46], plumes rising beneath cratons may be constrained by the thick overlying lithosphere to melt at great depths and produce kimberlites. Plumes beneath younger regions may rise shallower and generate less alkalic melts. Perhaps if cratons are thick enough, plumes may be unable to melt at all and hotspots, although active in the mantle, will not generate a surface volcanic trace. This possibility might explain why continental hotspot traces are so difficult to find and why there is presently no volcanism near the relocated kimberlite provinces in Africa. Reviewing the evidence for both the zone of weakness and the hotspot hypotheses, we find that the hotspot concept explains more of the available kimberlite data. The zone of weakness idea is consistent with the observed alignment of kimberlite intrusions with neighbouring volcanic provinces and seamount chains, but these observations are equally compatible with the hotspot theory. Many kimberlites are located near the presumed onshore extensions of oceanic fracture zones [5]; but, because hotspot tracks in the South Atlantic and western North Atlantic are predicted to be approximately parallel to fracture zones, this is also equally well-explained by the hotspot idea. In our opinion, the deciding factor in favor of the hotspots concept is that it predicts the ages of the kimberlites so well, whereas the zone of weakness theory makes no prediction. 6. Conclusion Available data from North America, South America, and Africa imply that many kimberlites have been formed by mantle hotspots. A majority of dated kimberlites have the locations and ages predicted by a simple model of plate/hotspot motions. Trindade's predicted hotspot track explains not only the dated kimberlites in Brazil but also most of that country's diamond deposits. Probability calculations indicate that the observed correlation between hotspots and kimberlites has only a 1% chance of occurring if kimberlites are unrelated to hotspots. This hypothesis can be further tested by additional age measurements in kimberlite provinces already examined and by a compilation and analysis of kimberlite data from other continents.
273 Acknowledgements We t h a n k G o r d o n Davis for furnishing us additional details o n the ages and l o c a t i o n s o f his d a t e d k i m b e r l i t e zircons. R e s e a r c h s u p p o r t e d b y NSF grant EAR7904033.
References 1 W.J. Morgan, Plate motions and deep mantle convection, Geol. Soc. Am. Mem. 132 (1972) 7. 2 J.B. Minster, T.H. Jordan, P. Molnar and E. Haines, Numerical modelling of instantaneous plate tectonics, Geophys. J. R. Astron. Soc. 36 (1974) 541. 3 W.J. Morgan, Hotspot tracks and the opening of the Atlantic and Indian Oceans, in: The Sea, 10, C. Emiliani, ed. (1980). 4 H.R. Williams and R.A. Williams, Kimberlites and plate tectonics in West Africa, Nature 270 (1977) 507. 5 L.R. Sykes, lntraplate seismicity, reactivation of preexisting zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation, Rev. Geophys. Space Phys. 16 (1978) 621. 6 H.W. Green and Y. Gueguen, Origin of kimberlite pipes by diapiric upwelling in the upper mantle, Nature 249 (1974) 617. 7 D.L. Anderson, Chemical plumes in the mantle, Geol. Soc. Am. Bull. 86 (1975) 1593. 8 R.A. Duncan, R.B. Hargraves and G.P. Brey, Age, paleomagnetism and chemistry of melilite basalts in the Southern Cape, South Africa, Geol. Mag. 115 (1978) 371. 9 D.A. Hastings and W.E. Sharp, An alternative hypothesis for the origin of West African kimberlites, Nature 277 (1979) 152. 10 G.L. Davis, The ages and uranium contents of zircons from kimberlites and associated rocks, Carnegie Inst. Washington Yearb. 76 (1977) 631. 11 G.L. Davis, Zircons from the mantle, Carnegie Inst. Washington Yearb. 77 (1978) 895. 12 G.L. Davis, T.E. Krogh and A.J. Erlank, The ages of zircons from kimberlites from South Africa, Carnegie Inst. Washington Yearb. 75 (1976) 821. 13 J.R. Andrews and C.H. Emeleus, Structural aspects of kimberlite dyke and sheet intrusion in South-West Greenland, Phys. Chem. Earth 9 (1975) 43. 14 A.F. King and N.J. McMiUan, A mid-Mesozoic breccia from the coast of Labrador, Can. J. Earth Sci. 12 (1975) 44. 15 J. Helwig, J. Aronson and D.S. Day, A Late Jurassic mafic pluton in Newfoundland, Can. J. Earth Sci. 11 (1974) 1314. 16 W.H. Poole, B.V. Sanford, H. Williams and D.G. Kelley, Geology of southeastern Canada, in: Geology and
Economic Minerals of Canada, R.J.W. Douglas, ed. (Geological Survey of Canada, 1970) 228. 17 R.E. Zartman, M.R. Brock, A.V. Heyl and H.H. Thomas, K-At and Rb-Sr ages of some alkalic intrusive rocks from central and eastern United States, Am. J. Sci. 265 (1967) 848. 18 D.G. Brookins and C.W. Naeser, Age of emplacement of Riley County, Kansas, kimberlites and a possible minimum age for the Dakota Sandstone, Geol. Soc. Am. Bull. 82 (1971) 1723. 19 H.O.A, Meyer, Kimberlites of the continental United States: a review, J. Geol. 84 (1976) 377. 20 C.W. Naeser, Geochronology of the Navajo-Hopi diatremes, Four Corners area, J. Geophys. Res. 76 (1971) 4978. 21 B.C. Hearn and F.R. Boyd, Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite, Phys. Chem. Earth 9 (1975) 247. 22 H.L. Allsopp and D.R. Barrett, Rb-Sr age determinations on South African kimberlite pipes, Phys. Chem. Earth 9 (1975) 605. 23- K. Burke, W.S.F. Kidd and J.T. Wilson, Relative and latitudinal motion of Atlantic hot spots, Nature 245 (1973) 133. 24 S.T. Crough, Hotspot epeirogeny, Tectonophysics 61 (1979) 321. 25 I. Wendt, H. Kreuger, P. Muller, U. yon Rad and H. Raschka, K-Ar age of basalts from Great Meteor and Josephine Seamounts (eastern North Atlantic), Deep-Sea Res. 23 (1976) 849. 26 B.A. McGregor and D.C. Krause, Evolution of the sea floor in the Corner Seamounts area, J. Geophys. Res. 77 (1972) 2526. 27 B.E. Tucholke et al., Initial Reports of the Deep Sea Drilling Project 43 (1979) 847. 28 K.A. Foland and H. Faul, Ages of the White Mountain intrusives - New Hampshire, Vermont, and Maine, U.S.A., Am. J. Sci. 277 (1977) 888. 29 H.W. Fairbairn, G. Faure, W.H. Pinson, P.M. Hurley and J.L. Powell, Initial ratio of strontium 87 to strontium 86, whole-rock age, and discordant biotite in the Monteregian igneous province, Quebec, J. Geophys. Res. 68 (1963) 6515. 30 R.M. Macintyre, Apparent periodicity of carbonatite emplacement in Canada, Nature Plays. Sci. 230 (1971) 79. 31 M. Shafiqullah, W.M. Tupper and T.J.S. Cole, K-Ar age of the carbonatite complex, Oka, Quebec, Can. Mineral. 10 (1970) 541. 32 A.E. Maxwell et al., Initial Reports of the Deep Sea Drilling Project 3 (1970). 33 P.R. Supko et al., Initial Reports of the Deep Sea Drilling Project 39 (1977). 34 H.M. Bolli et al., Initial Reports of the Deep Sea Drilling Project 40 (1978). 35 G. Siedner and J.A. Miller, K-Ar age determinations on basaltic rocks from South-West Africa and their bearing
274
36
37 38
39 40
41
on continental drift, Earth Planet. Sci. Lett. 4 (1968) 451. N. Herz, Timing of spreading in the South Atlantic: information from Brazilian alkalic rocks, Geol. Soc. Am. Bull. 88 (1977) 101. W.C. Pitman and M. Talwani, Sea-floor spreading in the North Atlantic, Geol. Soc. Am. Bull. 83 (1972) 619. J.W. Ladd, Relative motion of South America with respect to North America and Caribbean tectonics, Geol. Soc. Am. Bull. 87 (1976) 969. K. Burke and J.T. Wilson, Hot spots on the earth's surface, Sci. Am, 235 (1976) 46. R. Thiessen, K. Burke and W.S.F. Kidd, African hotspots and their relation to the underlying mantle, Geology 7 (1979) 263. J. Suppe, C. Powell and R. Berry, Regional topography, seismicity, Quaternary volcanism, and the present-day
42 43
44
45 46
tectonics of the western United States, Am. J. Sci. 275-A (1975) 397. W.E. Sharp, A plate tectonic origin for diamond-bearing kimberlites, Earth Planet. Sci. Lett. 2l (1974) 351. D.P. Sviscro, H.O.A. Meyer and H. Tsai, Kimberlites in Brazil: an initial report, in: Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology and Geochemistry, F.R. Boyd and H.O.A. Meyer, ed. (American Geophysical Union, Washington, D.C., 1979) 92. R.V. Dingle and R.I. Gentle, Early Tertiary volcanic rocks on the Agulhas Bank, South African continental shelf, Geol. Mag. (1972) 127. J.J. Gurney, The origin of kimberlite: modern concepts, Trans. Geol. Soc. S. Afr. 77 (1974) 353. D.t[. Eggler and R.f". Wendt, Phase relations of a kimbcrlite composition, Carnegie Inst, Washington Yearb. 77 (1978) 751.
Earth and Planetary Science Letters, 50 (1980) 260-274 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
[31
KIMBERLITES: THEIR RELATION TO MANTLE HOTSPOTS S. THOMAS CROUGH 1 W. JASON MORGAN and ROBERT B. H A R G R A V E S Department of Geological and Geophysical Sciences, Princeton University, Princeton, NJ 08544 (U.S.A.)
Received April 3, 1980 Revised version received June 10, 1980
Available age data support the hypothesis that kimberlite intrusions are formed by mantle hotspots. The hypothesis has been tested by inverting the volcanic traces formed by three hotspots to determine the post-Triassic motions of Africa, South America, and North America relative to these hotspots. Then, using these motions, the kimberlites intruded on these continents within the last 150 m.y. are relocated to their place of origin in the present hotspot reference frame. The result indicates that a majority of the kimberlites formed within 5 ° of a mantle hotspot. Statistical analysis shows that this kimberlite/hotspot correlation is significant at above the 90% level.
1. Introduction Igneous activity at lithospheric plate margins is simply explained by plate tectonics, but the cause o f intraplate magmatism has remained controversial. Many hypotheses have been advanced, and all are subject to further testing and modification. The two most frequently mentioned theories are that: (1) midplate volcanism is associated with large, deep lithospheric fractures (zones of weakness), and (2) it is related to mantle hotspots (zones of upwelling in the asthenosphere). Of the two main hypotheses, the hotspot hypothesis makes the most specific predictions. The temporal and spatial pattern o f midplate magmatism must be consistent with plate motions over a set of fixed magma sources. Earlier work has demonstrated that the azimuths and volcanic ages o f young seam o u n t chains agree with the Ftxed hotspot concept [1,2]. More recent work suggests that Mesozoic and Tertiary igneous events on several continental margins are also consistent with the theory [3].
1 Present address: Department of Geosciences, Purdue University, West Lafayette, IN 47906, U.S.A.
Support for the zone of weakness theory comes from recent studies showing that cratonic igneous activity, particularly the intrusion o f kimberlites, often occurs onshore from major oceanic fracture zones [4,5]. These studies conclude that lithospheric fractures control cratonic intrusions and that hotspots are not involved. This paper presents evidence implying that kimberlites are produced by mantle hotspots. This possible genetic relation has been suggested previously [ 6 - 9 ] , but never tested. However, there are now enough dated kimberlites to establish that their ages and locations are consistent with known plate motions over hotspots. This demonstration not only provides a reasonable explanation for the occurrence of kimberlite, but also further supports the hotspot hypothesis by extending the theory to include magmatism within cratons.
2. Kimberlite provinces Kimberlites have been discovered on almost every continent and have measured emplacement ages ranging from Tertiary to Pre-Cambrian. Due to limitations in the accuracy of plate reconstructions, we
0012-821X/80/0000-0000/$02.25 O 1980 Elsevier Scientific Publishing Company
261
have restricted our study to post-Triassic age kimberlites which were emplaced on the continents of North America, South America, and Africa (Fig. 1). However, this restriction is not severe, and our compilation of dated intrusions includes representatives from most of the world's kimberlite provinces. The major sources of our data are the recent review by Sykes
'
' X,,,~
'4,)
'L)~
[5] and the new kimberlite ages obtained by Davis [ 1 0 - 1 2 ] . The dated kimberlites are listed in Table 1 and a brief summary of their occurrence follows. There are three post-Paleozoic kimberlite provinces in eastern North America: Labrador Sea, New York-Ontario, and Arkansas-Kansas. The Labrador Sea group includes Mesozoic kimberlite dikes along
,
'
,
I
I
I
t
I
\ o4
° 2 " ~ ~ k l 1WFOUNDLAND O0
o5
~"~)
'~
SMTS
°03 o
3"
RISE
%
0/
~-"
) °I0
o
OLUMBO A SMTS
0
,~,\ 12 ° ,<"
J/
~. L-4"3,14 c~
!
I
!
I
m
!
I
I
!
I
!
I
!
!
I
Fig. 1. Locations of dated, post-Triassic kimberlites (o) intruded within North America, South America, and Africa. Kimberlite provinces are identified by number as given in Table 1. Offshore aseismic ridges and seamount (smt) chains (locations delineated by printing) may mark the subsequent traces of the hotspots which formed the coastal kimberlite provinces.
262 TABLE 1 Dated kimberlites Province
Present location (lat., long.)
Relocated position (lat., long.)
Age (m.y.)
Reference *
Labrador Sea (1)
49.5
-55.4
32.6
-16.9
140
[15]
New York-Ontario (2)
42.5 42.9 48.0
-76.0 -74.5 -79.5
27.7 26.5 30.6
-34.6 -32.2 34.3
136 146 151
[17] [17] [16]
Arkansas-Kansas(3)
40.0 37.7 34.2
-97.0 -96.0 -93.0
29.1 37.3 29.6
-55.5 -64.4 -59.3
115 90 97
[18] [17] [17]
Montana(4)
48.0 -108.5
53.5
-94.7
45
[21]
Colorado Plateau (5)
37.0 - 1 1 0 . 0
41.2 -102.6
31
[20]
Brazil (6)
West Africa (7)
-20.0 -18.4 -13.9
-46.9 -47.2 -54.1
-22.8 -21.9 -21.6
-27.1 -25.9 -26.3
80 86 122
[10] [10] [11]
8.0
11.0
-0.3
-25.6
140
[9]
Zaire (8)
-6.2
23.7
-18.0
13.7
71.3
[12]
Tanzania (9)
-4.2 -3.5
33.2 33.7
-14.1 -11.1
26.4 28.6
53.2 40.6
[10] [11]
Angola (10)
10.0
20.0
-29.6
-3.5
Namibia (11)
-30.8 -30.2 -30.2 -30.2 -26.7
18.1 18.5 18.5 18.5 20.0
-39.0 -40.0 -40.2 -40.5 -37.7
6.7 4.7 4.3 3.7 5.6
54.1 65 66.7 69 71.8
[10] [12] [10] p.c. [12]
Botswana (12)
-26.4 -21.3
22.2 25.4
-38.9 -36.4
6.2 8.2
80 93.1
p.c. [12]
South Africa, Cretaceous (13)
-30.7 -29.5 -29.5 -29.5 -30.0 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -29.5 -28.5 -30.5
23.0 25.0 25.0 25.0 28.5 25.0 25.0 27.5 25.5 25.0 25.0 25.5 24.0 25.0 24.3 22.7
-43.0 -43.1 -43.2 -43.5 -44.7 43.9 -43.9 -44.5 -44.0 -44.0 -44.1 -44.3 -44.2 -44.5 -45.3 -47.3
6.3 7.4 7.2 6.8 10.6 5.9 5.9 8.8 6.4 5.6 5.4 6.0 3.7 4.6 0.1 -4.2
78.3 84.3 85.3 86.9 87.5 90.3 90.4 90.4 90.4 91.2 92.0 92.2 94.1 95 110 115
p.c. p.c. p.c. [10] [12] [12] [12] [ 10] p.c. [12] [12] [10] [10] p.c. [11] p.c.
South Africa, Jurassic (14)
-25.5 -29.5
26.2 29.0
-47.2 -52.2
-6.9 -7.2
147 150
[22] [11]
* p.c., Gordan Davis, personal communication, 1979.
138
[11]
263 the western margin of Greenland [ 13], an explosive Mesozoic breccia cut by lamprophyre-carbonatite dikes on the coast of Labrador [14], and an ultramafic intrusion associated with breccia pipes in Newfoundland [ 15]. The Newfoundland intrusions, which have a K-Ar age [15], are the only rocks in this province which are dated accurately enough to test the hotspot model. The New York-Ontario group consists of three dated kimberlite dikes, one at Kirkland Lake, Ontario with a K-At age [ 16] and two in upstate New York with Rb-Sr ages [ 17]. The Kansas kimberlites are a group of pipes in Riley County dated by the fission-track method [18] and a possibly related mica peridotite in southeast Kansas dated by the K-Ar technique [ 17]. The Arkansas pipes near Murfreesboro have been stratigraphically dated [ 19]. The two Tertiary provinces in the western United States consist of a group of pipes and dikes in the Colorado Plateau with fission-track ages [20] and kimberlitic intrusions in Montana with K-Ar ages [21]. Southern Brazil contains the only dated kimberlites in South America. The three intrusions parallel a trend of alkalic intrusions, mostly syenites and carbonatites, and have an age progression, from east to west, documented by U-Pb measurements on included kimberlite zircons [10,11]. The West African countries of Guinea, Sierra Leone, Ivory Coast, Ghana, and Nigeria all contain kimberlites, but most intrusions have Pre-Cambrian ages and only the Sierra Leone kimberlites are of Mesozoic age [9]. There are numerous kimberlite provinces throughout central and southern Africa, of which almost all have been dated by U-Pb measurements [10-12]. Tanzania, Zaire, Angola, Botswana, and Namibia all contain groups of pipes with fairly uniform ages. South Africa appears to have at least two provinces, a large group of Cretaceous intrusions near Kimberley and also some older intrusions of Jurassic age. The reliability of kimberlite ages depends on the method of dating. K-Ar ages are the least reliable, because ages of mineral separates are often older than consistent with geological evidence for the time of emplacement. These presumably record the age of lithospheric fragments incorporated into the kimberlite. Rb-Sr ages share this problem, and care must be taken to measure only authigenic minerals. U-Pb
ages of kimberlite zircons are the most reliable. Because lead diffuses rapidly from zircons at mantle temperatures, the technique yields emplacement ages which are reproducible to within a few percent. We would prefer to use only U-Pb dates, but this would greatly limit the number of kimberlite provinces represented. Although we recognize that the K-Ar and Rb-Sr ages may be suspect, for many provinces they are the only dates available. In most cases the consistency of the measured ages within the same province is the best evidence that the ages are reliable. The aseismic ridges and seamount chains in the Atlantic (Fig. 1), which are the characteristic signature of hotspots, suggest that several of the kimberlite provinces were formed by hotspots. As the Atlantic basin opened, North and South America moved westward in the hotspot reference frame and Africa moved eastward [3]. Any hotspot which formed a volcanic ridge intersecting a continental margin must have been beneath that continent at some earlier time, and several of the presumed hotspot lineaments in Fig. 1 are aligned with kimberlite provinces further inland. The Newfoundland Seamounts and New England Seamounts occur immediately offshore from the Labrador Sea and New York-Ontario kimberlite provinces respectively. The Columbia Seamounts appear to connect the Brazilian kimberlites with Trindade hotspot. And the Sierra Leone Rise, Walvis Ridge, and Cape Rise all intersect the African coast near a major kimberlite occurrence. Our test of the hotspot hypothesis is to plot all dated kimberlites at their location of origin in the present hotspot coordinates. From independent evidence, we can determine the plate motions with respect to the hotspots for approximately the past 150 m.y. Thus, for any time instant within this interval, the position of each hotspot relative to the plates can be predicted, and conversely the position of any point on the plates relative to the hotspots can be calculated. Specifically, we can calculate where any intrusion was located relative to the hotspots at the instant when the intrusion was formed. When the known plate motions are used to relocate the kimberlites back to where they originated in the hotspot reference frame, the kimberlites should cluster at the present locations of the hotspots which created them.
264
kimberlites. Several motion models have previously been determined [3,23], but new age data have negated some of the earlier results and have suggested slightly different reconstructions. As constraints, we have used the Great Meteor, Tristan da Cunha, and Trindade volcanic traces. They are the best-developed and least-controversial traces available; and, because
3. Plate/hotspot motions To relocate the kimberlites accurately and to estimate the errors in these relocations, we have redetermined post-Triassic plate/hotspot motions by using linearized least-squares inversion theory applied to hotspot traces manifest by igneous activity other than
I
I
I
I
!
ROTATION POLE
NDADE
~
"
"_I b O " V
-
(
O
A TR I STAN
i
I
!
I
I
i
I
I
I
I
I
I
I
I
I
Fig. 2. Dated non-kimberlitic intrusions ( . ) on the presumed traces of Great Meteor, Trindade, and Tristan hotspots (TaMe 2), rotated to their location of origin relative to Africa using the k n o w n relative plate motions. These traces are inverted to obtain Africa's past motion with respect to the hotspots in terms of a single pole and a constant rotation rate. The data require a counterclockwise rotation about the pole near Great Meteor.
265 TABLE 2 Dated intrusions used to determine plate motions Hotspot trace
Great Meteor
Tristan da Cunha
Trindade
Present location (lat., long.)
Relocated position (lat., long.)
Age (m.y.)
Reference
[27] [27]
34.4 37.4 44.3 43.5 43.4 44.1 43.9 43.8 43.8 43.5 43.5 43.4 43.2 43.1 43.1 43.0 45.3 45.4
-56.5 -60.1 -73.2 -72.9 -72.4 -70.8 -70.9 -71.2 -71.1 -71.2 -70.7 -70.8 -70.7 -70.6 -71.2 -71.1 -72.6 -74.1
29.0 32.3 31.6 34.8 29.8 31.2 31.8 30.0 31.1 29.9 29.8 29.3 29.3 29.7 29.3 30.2 30.7 32.0
-34.2 -37.4 35.4 -38.2 -33.5 -33.1 -34.1 -32.4 -33.7 -32.7 -32.1 -31.9 32.0 -32.4 -32.5 -33.6 -32.2 -35.0
8O 8O 111 100 120 112 108 121 110 118 120 125 122 116 121 111 131 117
-28.6 -30.0 -35.0 -19.7 -21.7 -20.4 -21.3 -20.3 -21.0 -21.1 -22.0 -22.3 -21.5 -22.0 -24.2 -23.7 -24.2 -23.3 -23.5 -27.5 -24.5 -27.5 -24.1
-30.6 -35.6 -4.5 9.0 14.0 16.3 14.1 14.3 15.2 15.0 15.0 14.4 14.1 -52.0 -48.0 -46.7 -47.7 -47.6 -47.3 -49.2 -48.7 -49.2 -49.0
-31.1 -33.2 -38.6 -32.1 -37.6 -37.7 -36.6 -35.9 -37.1 -37.6 -36.9 -38.1 -37.2 -29.2 -30.6 -29.5 -30.1 -29.1 -29.2 -33.8 30.0 -32.7 -29.2
-14.0 -13.9 -14.6 -12.5 -14.1 -12.0 -11.5 -11.3 -11.4 -13.0 -8.8 -13.2 -12.8 -20.6 -17.2 -15.3 -15.5 -16.8 -16.7 -16.3 -21.0 -22.3 -23.2
72 85 4O 108 132 136 123 125 128 133 115 129 128 127 133 130 127 123 122 129 1i0 105 103
[32] [331 [341 [341
-15.6 -16.2 -17.5 -17.9 -18.3 -18.6 -18.7 -18.5 -19.4
-51.3 -51.3 -50.5 -47.8 -46.5 -46.8 -46.7 -46.0 -47.0
-19.5 -18.2 -20.9 -21.1 -21.0 -21.7 -21.6 -20.3 -23.3
-29.6 -34.7 -29.5 -27.1 -27.5 -26.3 -26.4 -31.1 -24.6
89 72 85 83 78 82 81 68 91
36] 36] 36] 36] 36] [36] [36] [36] [36]
[281 [281 [28] [281 [28] [28] [281 [281 [281 [281
1281 [28] [28]
[28] [29,30]
[31]
[351 [351 351 35] 35] 35] 35] 35] 35] 36] 36] 36] 36] 36] 36] 36] 361 36] 36]
266 TABLE 2 (continued) ttotspot trace
Present location (lat., long.)
Relocated position (lat., long.)
-19.5 -19.3 -21.4 -22.5 -22.0 22.3 -22.3 -22.3 -22.6 -17.6 -17.6 -24.6 -24.5 24.4 -24.3 -27.0
-21.5 -22.4 -24.0 -24.8 -23.8 -24.1 -24.3 -23.8 23.7 -18.8 -18.3 -27.6 - 26.8 -26.6 25.9 28.5
-47.0 -45.5 46.5 -43.5 44.7 -43.5 -42.7 - 4 2.0 -42.1 -39.0 -39.0 -47.9 -48.7 49.3 -49.0 -50.3
-31.2 --25.0 -27.9 -27.0 -30.5 -29.3 -27.4 -29.2 30.9 -27.4 -29.9 27.5 -31.5 32.6 34.9 -36.6
they are on three different plates and have significantly different latitudes, they provide good control on the plate/hotspot motions. The data are listed in Table 2 and summarized as follows. Great Meteor Seamount is situated east of the MidAtlantic Ridge at 30°N, 29°W (Fig. 2). Anomalously shallow seafloor in this vicinity [24] and measured ages o f 1 1 - 1 7 m.y. on dredged volcanics [25] suggest to us that Great Meteor is the site of a recently active mantle hotspot. Progressive closure of the Atlantic basin puts Great Meteor adjacent to Corner Rise, which is on the western side of the ridge, at about 75 m.y.B.P. Corner Rise must be no older than the Late Cretaceous seafloor it rests on, and dredged fossils suggest it is no younger than Eocene [26]. The New England Seamounts, which are of Cretaceous age and are progressively older to the west [27], form an almost continuous trace from near Corner Rise to the coast of North America. The seamounts are aligned with the Cretaceous intrusions of New England [28] and eastern Canada [ 2 9 - 3 1 ] , which are slightly older. Our explanation of these data is that North America and the Mid-Atlantic Ridge have migrated westward over the Great Meteor hotspot, with the ridge passing over the hotspot at 75 m.y.B.P. Since 75 m.y.B.P, the hotspot has remained in approximately its present position relative to Africa. Tristan da Cunha's trace includes both the Walvis
Age (m.y.)
Reference
70 82 77 72 66 66 69 59 51 52 42 82 74 73 66 65
[36] [36] [36] [36] [36] [36} [36] [36] [36] [36] [36] [36] [ 36 ] [36] [36] [36]
Ridge and Rio Grande Rise (which have been sparsely dated by DSDP work [ 3 2 - 3 4 ] ) and the Late Jurassic/Early Cretaceous coastal volcanic fields in Brazil and Africa [35,36], where the oceanic ridges terminate. Trindade is a young volcanic island [36] located off the coast of Brazil. Its postulated trace includes the undated Columbia Seamount chain and the band of Cretaceous alkalic intrusions extending almost 1000 km inland from the Brazilian coast [36]. To simplify the inversion, we have used the known relative motions between North America and Africa ( [ 3 7 ] , Schouten and Klitgord, personal communication, 1977) and between South America and Africa [38] to relocate the dated intrusions (Table 2) from the traces on the North and South American plates to their places of origin relative to Africa (Fig. 2). The effect of this data transformation is to remove the relative plate motions from the hotspot traces. The relocated intrusions indicate how the three hotspot traces would have looked if the Atlantic region had been covered by a single plate rotating with Africa's plate/hotspot motion, rather than by three separate plates rotating with different motions. To calculate plate/hotspot motions, we have solved for the rotation which predicts these relocated traces on the African plate. The concentration of relocated intrusions near Great Meteor suggests that the African plate has been nearly stationary with
267 respect to this hotspot. The spread of points away from Trindade and Tristan imply that, at this latitude, the African plate has moved northwestward with respect to the hotspots. Therefore, to first order, Africa's plate/hotspot motion has been a counterclockwise rotation about a pole near Great Meteor. We have assumed that Africa's motion has been at a uniform rate about a single pole and have solved for the parameters of this rotation. In matrix notation, the present coordinates, XOi, of a dated intrusion should be related to the present coordinates, HSi, of its source hotspot through the rotation matrix, Mij, which is a function of the rotation pole's location, the rotation rate, and the age of the intrusion:
xOi = Mii HS/ Since the rotation matrix is a non-linear function of the unknown rotation parameters, a linearized iterative inversion method is used. An initial estimate of the parameters creates a matrix, Mi/, which is used to calculate predicted coordinates, XPi, for each intrusion. If the differences between the observed and predicted coordinates are small, they are approximately related to the required parameter changes, dpk, by:
XOi - XPi = [(OM~i//Opk)HS/Idpk This over-determined system of linear equations is solved to obtain the least-squared-error estimate of dpk and the corrected parameters are used to create a new rotation matrix. The process is repeated until the calculated changes are small. The results are listed in Table 3, where the confidence limits are twice the square root of the diagonal terms in the covariance matrix for the estimated parameter changes. Whether the traces are inverted in
groups of two or all three together, the estimated rotation parameters are approximately the same. Using the rotation derived from all three traces, the dated intrusions were relocated to their origin in the present hotspot reference frame (Fig. 3 and Table 2). The fit is not perfect, but as shown by the circles in the figure, approximately 70% of the data relocate to within 5 ° of a hotspot center. Other error estimates are possible, but a 5 ° radius circle provides a convenient measure of the scatter and has the physical significance of being the size of the topographic swell usually associated with hotspots. Possible sources of error include: (1) uncertainties in the radiometric ages, (2) errors in the plate relative motion data, (3) coeval magma intrusion over the width of a swell, not just the center, (4) movement between the hotspots, and (5) an absolute motion model which is too simple. Considering these uncertainties, we think the fit is quite good and consider that the hotspot model provides a reasonable explanation of the data. The computed motion describes Trindade's trace particularly well. Combining the South America/ Africa relative motions with the calculated Africa/ hotspot motion yields a predicted South America/ hotspot motion which closely matches the east-west age progression of Brazil's alkalic complexes (Table 2) between 15°S and 24°S (Fig. 4). The present plate/ hotspot velocity at Trindade is computed to be 2 cm/ yr, the same as that derived independently by inverting all recent trace azimuths [2]. 4. Kimberlite sources Using the plate/hotspot motion calculated for Africa and the known relative motions between the
TABLE 3 Calculated absolute motion poles for Africa~ 150-0 m.y.B.P. Traces used
Rotation rate (deg/m.y.)
Latitude (°N)
Longitude (°W)
Great Meteor, Trindade, and Tristan Great Meteor, Trindade Great Meteor, Tristan Trindade, Tristan
0.23±0.01 0.23±0.02 0.23±0.02 0.26±0.03
29.3±2.8 33.1±2.0 29.1±3.2 18.2±7.5
35.7±2.6 29.6±2.0 36.5±3.3 34.0±5.2
268 r...j \ ,
,
,
,
I
I
I
l
I,..
/i
/ I
TRINDADE
RISTAN
!
!
I
I
I
I
I
I
[
I
I
I
I
I
I
Fig. 3. Dated intrusions in Fig. 2 rotated to their origin (Table 2) relative to the present hotspots, using the calculated African motion. With no errors in the age m e a s u r e m e n t s or in the rotations used, the fixed h o t s p o t model predicts that the points should overlie the hotspots. The 5 ° circles drawn around the h o t s p o t s enclose a b o u t 70% of the data.
plates, all the kimberlites in Fig. 1 were rotated back to their predicted location of origin relative to the present hotspots (Fig. 5 and Table 1). Because the calculated African m o t i o n is only well-constrained back to 130 m.y.B.P., the age of the oldest trace data used, we have had to assume the motion prior to that time. The relocations in Fig. 5 use the assumption that Africa's motion was uniform from 150 m.y. B~P.
to the present. Later we will use an alternate assumption and show that different assumptions do not change the conclusion regarding the hotspot hypothesis. The hotspot population in Fig. 5 is taken from earlier work [24] and is meant to include all island or seamount groups which have been volcanically active within the past 25 m.y. or which are situated on large
269 -100
"~kx~" ~."I
80 1>
oe e
°o
60
m
3
•
•
TRI"i~
!
TRACE
5'OoW
4() ° LONGITUDE
40
~
v
20
30 °
0
Fig. 4. Observed ages and locations of non-kimberlitic intrusions on Trindade's trace across Brazil compared to the calculated plate/hotspot motion (solid line). A more detailed motion model could improve the fit only slightly; most of the scatter must be due to age errors or to coeval intrusion in a zone several hundred kilometers wide.
topographic swells. There is a general consensus as to the number and location of Atlantic hotspots, so different compilations would give similar results. For example, Burke and Wilson [39] have not identified Great Meteor Seamount as a hotspot, but they infer a hotspot at Colorado Seamount which is immediately west of Great Meteor. Their hotspot location correlates even better with the trace data in Fig. 3 and the relocated kimberlites in Fig. 5. Almost all kimberlites relocate near a hotspot. Within the Atlantic Ocean basin, we identify the relocated kiinberlite provinces according to the hotspot they are closest to. For example, the one dated intrusion of the Labrador Sea province is predicted to have originated at 33°N, 16°W, so the observed association is Labrador Sea/Madeira. The other associations are: New York-Ontario/Great Meteor, ArkansasKansas/Bermuda, Sierra Leone/Ferna~do de Noronha, Brazil/Trindade, Angola/Tristan, Namibia/ Discovery, Botswana/Vema, Cretaceous South Africa/ Meteor, and Jurassic South Africa/Bouvet. On the continents, the kimberlite/hotspot associations are less certain. Estimates of the number of hotspots presently beneath Africa range from 5 [24] to 34 [40]. The low estimate has no hotspots beneath the two kimberlite provinces relocated within Africa;
the high estimate has hotspots (highspots) beneath both. The late Tertiary kimberlites relocated in Colorado may be related to the Raton hotspot [41 ], but the Montana intrusions relocated in Canada have no obvious hotspot association and may be subduction-related [42] instead. Even assuming that none of the kimberlites relocated onto the continents are associated with hotspots, 67% of the relocated kimberlites lie within 5 ° of a hotspot, which is the same quality of fit obtained with the non-kimberlite traces in Fig. 3. Given the inaccuracies in the data, we consider that the hotspot model satisfactorily predicts the locations and ages of most of the kimberlites considered. Trindade's track provides the best example of the proposed hotspot origin for kimberlites. The predicted path not only matches the ages and locations of the three dated kimberlites but also coincides with the location of most of the alluvial diamond deposits in Brazil (Fig. 6). The ages of these alluvial deposits are still uncertain. Some geological evidence suggests a Cretaceous age [43], but other deposits are considered to be Pre-Cambrian. Different ages in the same locality are not necessarily in conflict with the hotspot hypothesis, because Brazil has had a long history of movement over hotspots and kimberlites of many ages should be found. However, the alignment of alluvial diamond deposits with Trindade's track is remarkable. It suggests that the area contains many presently unlocated and undated kimberlites which could be used for further testing of the hotspot concept.
5. Discussion The motion of Africa in the interval 1 2 5 - 1 5 0 m.y.B.P, is the major uncertainty in this study. Only the Tristan trace has ages in this range, so the absolute motion is poorly constrained. The relocations in Fig. 5 assume that Africa's motion relative to the hotspots has been uniform from 150 m.y.B.P. to present, but other evidence suggests that Africa and South America may have moved westward, rather than eastward, prior to 125 m.y.B.P. [3]. This alternative motion would change the relocated positions of all the kimberlites older than 125 m.y. To estimate the effect of this uncertainty, we have
270 I
%d\'
I
,
I
,
,
o5
©t £'o sioN 0
°8
ADE ~ A
10o
°9
/
o~o
14o~ i n
I
I
I
I
I
I
I
I
I
I
t
I
I
I
]
I
Fig. 5. Dated kimberlites in Fig. l rotated to their origin (Table 1) relative to the present hotspots using the plate/hotspot motion determined from the three traces in Fig. 2. The kimberlite data match the hotspot prediction with the same accuracy as the trace data in Fig. 3 - approximately 67% of the relocated intrusions lie within the 5 radms circles surrounding the hotspots. No continental hotspots are shown; estimates of the population of continental hotspots vary considerably and it is uncertain whether the kimberlites relocated on the continents are associated with hotspots. o
also r e l o c a t e d t h e k i m b e r l i t e s using a p r e v i o u s m o d e l [3] for A f r i c a ' s r o t a t i o n f r o m 150 to 125 m . y . B . P . a n d t h e p r e s e n t m o d e l f r o m 125 t o 0 m . y . B . P . F o r t h e k i m b e r l i t e s in Fig. 5 w h i c h are 150 m.y. old, t h e l o c a t i o n c h a n g e is given b y a c o u n t e r c l o c k w i s e rotat i o n o f 12 ° a b o u t a pole j u s t n o r t h o f Madeira. Since
.
,
t h e k i m b e r l i t e s near Madeira a n d G r e a t M e t e o r are very close t o this pole, t h e i r l o c a t i o n s are n o t c h a n g e d b y m o r e t h a n a few degrees. T h e k i m b e r l i t e to the n o r t h e a s t o f T r i s t a n m o v e s a d j a c e n t to t h e circle a r o u n d V e m a , a n d t h e t w o k i m b e r l i t e s west o f B o u v e t ' s c e n t e r m o v e east o f B o u v e t a n d closer to
271 by the binomial distribution: +
/7 70 ~
V o`~
,r,n0aOe 3~00
Fig. 6. Locations of dated kimberlites (o) and major alluvial diamond deposits (*) [43] in Brazil compared to the predicted track of Trindade hotspot, ages in m.y.B.P. The hotspot model matches the ages and locations of the known kimberlites and may explain the observed concentration of diamond deposits in southern Brazil.
Meteor. The overall fit to the hotspot model is therefore unaffected, but the specific kimberlite/hotspot associations change slightly. Because the fit of the relocated kimberlites to the present hotspot positions is not perfect, we have sought to evaluate the statistical significance of the observed associations. We have done this by supposing that kimberlites are not related to mantle hotspots and then calculating the probability that the observed associations were obtained by chance. Because individual kimberlite pipes are not independent of one another but occur in provinces, each province has been treated as a single datum. We have utilized the following probability model. When a kimberlite province is relocated, there are only two possible outcomes; it lies within a 5 ° circle surrounding a hotspot or it does not. If provinces are independent of hotspots, then some will be relocated into hotspot circles just by chance alone. The probability, p, of an individual province being relocated within a circle is given by the ratio of the area within the circles and the total area where the province might have been relocated. The probability of observing k provinces within the hotspot circles is given
pk(1 _ p ) n
k [n!/k[(n - k)!]
(1)
where n is the total number of provinces relocated. From this relation, the number of provinces expected within the circles is rip. In other words, if the area covered by hotspots is very large, then a large number of kimberlite provinces will appear to be associated with hotspots even if kimberlites and hotspots are unrelated. If the area covered by hotspots is relatively small, then very few chance associations are expected. First, we take the worst case by considering all the relocated provinces and by assuming that none of the continental provinces are associated with hotspots. The provinces might have been formed anywhere on the continents and over any part of the mantle now covered by the Atlantic, except for a small portion of the North Atlantic basin which is over 150 m.y. old. From 50°N to 60°S, the combined area of the three continents and the Atlantic basin older than 150 m.y. is 3.08 R 2, where R is the earth's radius. The area enclosed b y 5 ° circles around the oceanic hotspots in Fig. 5 (Cape Verde is excluded because it has not passed beneath a continent in the past 150 m.y.) and the 6 continental hotspots in the population used [24] is 0.47 R 2. With these areas the probability of relocating a single kimberlite province over a hotspot is 0.17 and the expected number of hotspot-related provinces is only 2.1 out of the total of 14 which have been relocated. Because kimberlite provinces often relocate only partially within the circles, kimberlite/hotspot associations are counted according to the fraction of intrusions which lies within a hotspot region. For example, the Arkansas-Kansas/Bermuda association counts as 1/3 of a province correlated with a hotspot. With this method, there are 5.3 provinces within the circles and, from equation (1), the probability of finding more than 5 of the 14 provinces within 5 ° of a hotspot center is only about 0.02. Thus, although in this worst case the hotspot hypothesis explains less than half of the kimberlite provinces, even this poor fit is probably too good to be due to chance. Second, we consider only those provinces which relocate within the ocean basin. Because there is more agreement as to the hotspot population within oceans, this may be a fairer test of the hotspot con-
272 cept. The above calculations are repeated using tile ocean area and ocean hotspots alone. The probability of relocating a single province within a circle is 0.21, which is slightly higher, but the expected number of hotspot-related provinces is 2.1 out of 10, which is still much lower than the observed 5.3. The probability of observing more than 5 of the 10 provinces within the circles is 0.01, so the observed correlation between hotspots and kimberlite provinces is statistically significant at about the 99% confidence level. These probabilities are clearly a function of the area assumed to be covered by each hotspot, but dilL ferent values give similar results. Making the hotspot circles larger increases the pi-obability of fortuituous associations but also increases the number of provinces explained by hotspots. The significance of the fit remains about the same. Reducing the circles to 3° reduces the number of observed associations, especially since many kimberlites relocate almost exactly 5° from hotspot centers. However, the probability of a chance association also decreases and the close associations observed for Madeira and Trindade keep tire overall correlation significant at above the 90% level. If the circles are reduced to points, then the correlation disappears; but it is inappropriate to expect greater accuracy than the data allow. The remaining test of the validity of the kimberlite/hotspot associations observed in Fig. 5 would be to examine if there are continuous volcanic traces (with the predicted age relations) linking these kimberlite provinces to the present hotspot locations. Continuous traces are not required by the hotspot hypothesis; but if such traces exist, they provide additional support for the theory. Unfortunately, most of the aseismic ridges in Fig. 1 are not dated adequately enough for this test. The Cape Rise is the worst example; this seamount chain appears to link the South African kimberlites with the hotspots at Discovery and Meteor Seamounts, yet it is completely undated. Coastal alkalic intrusions in South Africa, located to the southwest of the Cretaceous kimberlites, have Late Cretaceous/Early Tertiary ages [8,44] as predicted by the hotspot model, but samples from the Cape Rise are needed to confirm the remainder of the predicted trace. We see no fundamental objection to this proposed hotspot origin for kimberlites. Available evidence suggests that kimberlite magma is produced by partial
melting of the mantle at depths of 100 km or more [45]. As suggested by Eggler and Wendt [46], plumes rising beneath cratons may be constrained by the thick overlying lithosphere to melt at great depths and produce kimberlites. Plumes beneath younger regions may rise shallower and generate less alkalic melts. Perhaps if cratons are thick enough, plumes may be unable to melt at all and hotspots, although active in the mantle, will not generate a surface volcanic trace. This possibility might explain why continental hotspot traces are so difficult to find and why there is presently no volcanism near the relocated kimberlite provinces in Africa. Reviewing the evidence for both the zone of weakness and the hotspot hypotheses, we find that the hotspot concept explains more of the available kimberlite data. The zone of weakness idea is consistent with the observed alignment of kimberlite intrusions with neighbouring volcanic provinces and seamount chains, but these observations are equally compatible with the hotspot theory. Many kimberlites are located near the presumed onshore extensions of oceanic fracture zones [5]; but, because hotspot tracks in the South Atlantic and western North Atlantic are predicted to be approximately parallel to fracture zones, this is also equally well-explained by the hotspot idea. In our opinion, the deciding factor in favor of the hotspots concept is that it predicts the ages of the kimberlites so well, whereas the zone of weakness theory makes no prediction. 6. Conclusion Available data from North America, South America, and Africa imply that many kimberlites have been formed by mantle hotspots. A majority of dated kimberlites have the locations and ages predicted by a simple model of plate/hotspot motions. Trindade's predicted hotspot track explains not only the dated kimberlites in Brazil but also most of that country's diamond deposits. Probability calculations indicate that the observed correlation between hotspots and kimberlites has only a 1% chance of occurring if kimberlites are unrelated to hotspots. This hypothesis can be further tested by additional age measurements in kimberlite provinces already examined and by a compilation and analysis of kimberlite data from other continents.
273 Acknowledgements We t h a n k G o r d o n Davis for furnishing us additional details o n the ages and l o c a t i o n s o f his d a t e d k i m b e r l i t e zircons. R e s e a r c h s u p p o r t e d b y NSF grant EAR7904033.
References 1 W.J. Morgan, Plate motions and deep mantle convection, Geol. Soc. Am. Mem. 132 (1972) 7. 2 J.B. Minster, T.H. Jordan, P. Molnar and E. Haines, Numerical modelling of instantaneous plate tectonics, Geophys. J. R. Astron. Soc. 36 (1974) 541. 3 W.J. Morgan, Hotspot tracks and the opening of the Atlantic and Indian Oceans, in: The Sea, 10, C. Emiliani, ed. (1980). 4 H.R. Williams and R.A. Williams, Kimberlites and plate tectonics in West Africa, Nature 270 (1977) 507. 5 L.R. Sykes, lntraplate seismicity, reactivation of preexisting zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation, Rev. Geophys. Space Phys. 16 (1978) 621. 6 H.W. Green and Y. Gueguen, Origin of kimberlite pipes by diapiric upwelling in the upper mantle, Nature 249 (1974) 617. 7 D.L. Anderson, Chemical plumes in the mantle, Geol. Soc. Am. Bull. 86 (1975) 1593. 8 R.A. Duncan, R.B. Hargraves and G.P. Brey, Age, paleomagnetism and chemistry of melilite basalts in the Southern Cape, South Africa, Geol. Mag. 115 (1978) 371. 9 D.A. Hastings and W.E. Sharp, An alternative hypothesis for the origin of West African kimberlites, Nature 277 (1979) 152. 10 G.L. Davis, The ages and uranium contents of zircons from kimberlites and associated rocks, Carnegie Inst. Washington Yearb. 76 (1977) 631. 11 G.L. Davis, Zircons from the mantle, Carnegie Inst. Washington Yearb. 77 (1978) 895. 12 G.L. Davis, T.E. Krogh and A.J. Erlank, The ages of zircons from kimberlites from South Africa, Carnegie Inst. Washington Yearb. 75 (1976) 821. 13 J.R. Andrews and C.H. Emeleus, Structural aspects of kimberlite dyke and sheet intrusion in South-West Greenland, Phys. Chem. Earth 9 (1975) 43. 14 A.F. King and N.J. McMiUan, A mid-Mesozoic breccia from the coast of Labrador, Can. J. Earth Sci. 12 (1975) 44. 15 J. Helwig, J. Aronson and D.S. Day, A Late Jurassic mafic pluton in Newfoundland, Can. J. Earth Sci. 11 (1974) 1314. 16 W.H. Poole, B.V. Sanford, H. Williams and D.G. Kelley, Geology of southeastern Canada, in: Geology and
Economic Minerals of Canada, R.J.W. Douglas, ed. (Geological Survey of Canada, 1970) 228. 17 R.E. Zartman, M.R. Brock, A.V. Heyl and H.H. Thomas, K-At and Rb-Sr ages of some alkalic intrusive rocks from central and eastern United States, Am. J. Sci. 265 (1967) 848. 18 D.G. Brookins and C.W. Naeser, Age of emplacement of Riley County, Kansas, kimberlites and a possible minimum age for the Dakota Sandstone, Geol. Soc. Am. Bull. 82 (1971) 1723. 19 H.O.A, Meyer, Kimberlites of the continental United States: a review, J. Geol. 84 (1976) 377. 20 C.W. Naeser, Geochronology of the Navajo-Hopi diatremes, Four Corners area, J. Geophys. Res. 76 (1971) 4978. 21 B.C. Hearn and F.R. Boyd, Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite, Phys. Chem. Earth 9 (1975) 247. 22 H.L. Allsopp and D.R. Barrett, Rb-Sr age determinations on South African kimberlite pipes, Phys. Chem. Earth 9 (1975) 605. 23- K. Burke, W.S.F. Kidd and J.T. Wilson, Relative and latitudinal motion of Atlantic hot spots, Nature 245 (1973) 133. 24 S.T. Crough, Hotspot epeirogeny, Tectonophysics 61 (1979) 321. 25 I. Wendt, H. Kreuger, P. Muller, U. yon Rad and H. Raschka, K-Ar age of basalts from Great Meteor and Josephine Seamounts (eastern North Atlantic), Deep-Sea Res. 23 (1976) 849. 26 B.A. McGregor and D.C. Krause, Evolution of the sea floor in the Corner Seamounts area, J. Geophys. Res. 77 (1972) 2526. 27 B.E. Tucholke et al., Initial Reports of the Deep Sea Drilling Project 43 (1979) 847. 28 K.A. Foland and H. Faul, Ages of the White Mountain intrusives - New Hampshire, Vermont, and Maine, U.S.A., Am. J. Sci. 277 (1977) 888. 29 H.W. Fairbairn, G. Faure, W.H. Pinson, P.M. Hurley and J.L. Powell, Initial ratio of strontium 87 to strontium 86, whole-rock age, and discordant biotite in the Monteregian igneous province, Quebec, J. Geophys. Res. 68 (1963) 6515. 30 R.M. Macintyre, Apparent periodicity of carbonatite emplacement in Canada, Nature Plays. Sci. 230 (1971) 79. 31 M. Shafiqullah, W.M. Tupper and T.J.S. Cole, K-Ar age of the carbonatite complex, Oka, Quebec, Can. Mineral. 10 (1970) 541. 32 A.E. Maxwell et al., Initial Reports of the Deep Sea Drilling Project 3 (1970). 33 P.R. Supko et al., Initial Reports of the Deep Sea Drilling Project 39 (1977). 34 H.M. Bolli et al., Initial Reports of the Deep Sea Drilling Project 40 (1978). 35 G. Siedner and J.A. Miller, K-Ar age determinations on basaltic rocks from South-West Africa and their bearing
274
36
37 38
39 40
41
on continental drift, Earth Planet. Sci. Lett. 4 (1968) 451. N. Herz, Timing of spreading in the South Atlantic: information from Brazilian alkalic rocks, Geol. Soc. Am. Bull. 88 (1977) 101. W.C. Pitman and M. Talwani, Sea-floor spreading in the North Atlantic, Geol. Soc. Am. Bull. 83 (1972) 619. J.W. Ladd, Relative motion of South America with respect to North America and Caribbean tectonics, Geol. Soc. Am. Bull. 87 (1976) 969. K. Burke and J.T. Wilson, Hot spots on the earth's surface, Sci. Am, 235 (1976) 46. R. Thiessen, K. Burke and W.S.F. Kidd, African hotspots and their relation to the underlying mantle, Geology 7 (1979) 263. J. Suppe, C. Powell and R. Berry, Regional topography, seismicity, Quaternary volcanism, and the present-day
42 43
44
45 46
tectonics of the western United States, Am. J. Sci. 275-A (1975) 397. W.E. Sharp, A plate tectonic origin for diamond-bearing kimberlites, Earth Planet. Sci. Lett. 2l (1974) 351. D.P. Sviscro, H.O.A. Meyer and H. Tsai, Kimberlites in Brazil: an initial report, in: Kimberlites, Diatremes, and Diamonds: Their Geology, Petrology and Geochemistry, F.R. Boyd and H.O.A. Meyer, ed. (American Geophysical Union, Washington, D.C., 1979) 92. R.V. Dingle and R.I. Gentle, Early Tertiary volcanic rocks on the Agulhas Bank, South African continental shelf, Geol. Mag. (1972) 127. J.J. Gurney, The origin of kimberlite: modern concepts, Trans. Geol. Soc. S. Afr. 77 (1974) 353. D.t[. Eggler and R.f". Wendt, Phase relations of a kimbcrlite composition, Carnegie Inst, Washington Yearb. 77 (1978) 751.