The Karroo dolerites of southern Malawi and their regional geochemical implications

Transcription

1 MNERALOGCAL MAGAZNE, DECEMBER 97 9, VOL. 43, PP The Karr dlerites f suthern Malawi and their reginal gechemical implicatins A. R. WOOLLEY, J. C. BEVAN AND C. J. ELLOTT Department f Mineralgy, British Museum (Natural Histry), Crmwell Rad, Lndn SW7 5BD SUMMARY. The field relatinships, petrgraphy and mineralgy, including prbe analyses f livines, pyrxcries, and plagiclases, f the Karr dlerite dike swarm f suthern Malawi, are described. Rck analyses are given and define a typical thleiitic trend f irn enrichment. The mst magnesian rcks are thught t represent fairly picritic liquids. A number f authrs have shwn that there are significant gechemical differences between the basaltic rcks f the nrthern and suthern parts f the Karr utcrp in suthern Africa, ntably fr elements such as A1, P, K, Ti, and Ba. The Malawi data presented here, hwever, shw trends that are the reverse f thse fund hithert, the Malawi dlerites being clsely cmparable with the rcks f the central Karr basin f Suth Africa and Basutland. The striking reginal variatin in the gechemistry f the Karr basaltic rcks ver suthern Africa is independent f the tectnics r psitin vis-~t-vis the central Karr basin, and s prbably reflects mantle hetergeneity. ROCKS assignable t the Karr system ccur in nrthern Malawi and in the suth alng the brder with Mzambique. The nrthern utcrp cmprises sedimentary rcks nly, but in the suth the yungest rcks f Karr age are basaltic lavas, and dlerite dikes and sills. The lavas cver abut OO km 2 within Malawi and there is a smewhat greater area, including rhylites, in adjacent areas f Mzambique. Apart frm a brief field and petrgraphic accunt (Habgd, t963) n detailed wrk has been dne n the lavas, r n the dlerite sills which are emplaced in Karr sediments. Brief field and petrgraphical accunts f Karr dterite dikes are given in reginal reprts n the gelgy f suthern Malawi (e.g. Blmfield, 958 and 965; Garsn, 1962; Habgd, 963), and tw rck analyses have been published (Garsn, 962; Blmfield, 965), but there is n accunt f the swarm as a whle, althugh, as can be seen frm fig., it is a cnsiderable gelgical feature. The general trend f the dike swarm is NE-SW, thugh with sme lcal variatins, particularly in the extreme suth where it is N-S (fig. ). The NE-SW trend is parallel t the principal faults which define the rift system in this part f Malawi. Cpyright the Mineralgical Sciety The swarm is mst dense immediately nrth f the Chl fault: in the area just suth f the fault dikes are nt expsed because they are cvered by mre recent sediments. Dlerite was als emplaced in parts f the Chl fault, and in faults parallel t it which extend int a fault system bunding the Zambezi Valley (Wlley and Garsn, 97O, fig. 2). The swarm gradually dies ut t the NE, nly a few dikes being fund in the areas immediately suth and west f Lake Chilwa. Sme 7 km nrth f the Lake a few dlerites again ccur and extend spradically as far as Lake Malawi. n the densest part f the swarm there are six r seven dikes t the mile (Habgd, 963): they usually prduce marked ridges and individual dikes can smetimes be fllwed fr many miles. The 'Brder Dike' just nrth f Nsanje, fr instance, is sme 65 m thick and can be fllwed fr abut 5 km (Blmfield, 958 ). The majrity f the specimens n which this accunt is based were kindly selected fr us by Dr R. D. Walshaw frm the cllectin f the Malawi Gelgical Survey Department. They were cllected during reginal mapping by K. Blmfield, M. S. Garsn, R. K. Evans, and A. King. These specimens were supplemented with material frm the cllectin f the British Museum (Natural Histry). Petrgraphy. All the rcks studied are thleiitic dlerites, but sme cntain abundant mdal livine and are perhaps apprpriately termed picritic dlerites. Grain-size varies frm fine t carse with plagiclase laths up t 2-3 mm in length. Prphyritic variants ccur with glmerprphyritic aggregates f plagiclase and dinpyrxene~ and sme livine-rich rcks cntain subhedral livine phencrysts up t 2 mm acrss. Mst f the dlerites have an phitic t sub-phitic texture, but in the carsest rcks pyrxene ccurs as islated angular patches between plagiclase laths. Olivine is usually mantled by clinpyrxene, but in sme f the mre livine-rich rcks which ccur in the extreme suth f the cuntry (fig. i) there is

2 488 A.R. WOOLLEY ET AL. N 3~~ ~t-28 ZOMBA 9 /,'~ // ~47? ~#" / // ii ' ~j / / / '/ /," //" // i 5 1"( 16"5 r ~? tv ' ' ' AREA OF rb';--" " ~ 25~$.._20 / 3PS- B~J1~t~h~~ talian Ocean 20*E 25~ 30~ 35=E r i } f J k i 5 1~ Analysed slecime~ lcality, with numbers crrespnding t Table 1 Karr dlente dykes 1~7 Other Karr d~r~e ~Tus~ns Karr basa~ ~as Karr sediments "~- Majr famts with tick ll dwnthrow side... ntemmi~ bundary

3 KARROO DOLERTES 489 n bvius reactin relatinship, thugh in thers a fine palisade f fibrus rthrhmbic pyrxene surrunds the livine. Olivine is generally fresh but in the mre differentiated rcks it is partly r whlly serpentinized. Clinpyrxene is ubiquitus, and brnzite r pigenite have been identified in abut O ~ f the samples studied. n prphyritic rcks clinpyrxene frms euhedral t subhedral phencrysts and glmerprphyritic aggregates, but in mre equigranular rcks it shws an phitic r interstitial relatinship t plagiclase. Calcium-pr pyrxene frms euhedral phencrysts in sme prphyritic rcks, but in mre equigranular types it is subhedral t euhedral and invariably mantled by clinpyrxene. Plagiclase frms stubby euhedral phencrysts and glmerprphyritic aggregates in prphyritic rcks, but in the mre usual equigranular types develps subhedral laths which, in the livine-rich rcks frm the extreme suth f Malawi, have a length t width rati f up t O:. n these rcks the plagiclases cmmnly have a central cre f pyrxene and paque minerals, which presumably represents trapped liquid. Zning is cmmn frm bytwnite cres, which are invariably sericitized, t labradrite rims. Hrnblende ccurs in abut ne-third f the samples studied, usually in accessry amunts, but in a few rcks is a majr cnstituent. Tw types are present. n mre highly differentiated rcks an live green t pale brwn hrnblende rims and replaces clinpyrxene as well as crystallizing as a primary phase. n ther rcks green, but ccasinally brwn, amphible ccurs as sparse xencrysts up t 3 mm acrss, which in sme instances are rimmed by clinpyrxene. Magnetite is ubiquitus and a little brwn bitite is smetimes present. A micrgraphic intergrwth f quartz and alkali feldspar is abundant in mre highly differentiated types, and needles f apatite are invariably assciated with this texture. Mineralgy. Olivines frm seven rcks have been analysed and they vary frm Fa12 t Fa2s. The rims f crystals are cmmnly richer in fayalite than the cres, in cntrast t the bservatins f Walker and Pldervaart (1949, p. 633) wh fund that in Suth African dlerites the livines are usually f unifrm cmpsitin. The livines are pltted n fig. 2a and tied t c-existing calciumrich pyrxenes. Calcium-rich pyrxenes frm eleven rcks were analysed (fig. 2a), and cmpsitins f bth cres and rims were btained fr seven f these. They prved t be augites in all samples except fr ne in which the cres were endipside. These data are pltted n fig. 2b frm which the increase in Fe and decrease in Ca in the rims is apparent fr the mre magnesian pyrxenes, but in the tw mst irnrich pyrxenes irn enrichment in the rims is accmpanied by a slight Ca increase als. There are surprisingly few chemical analyses f pyrxenes frm Karr dlerites r basalts: Walker and Pldervaart (949), fr instance, relied n ptical determinatins. Hwever, there are prbe data fr pyrxenes f the Birds River Gabbr, Suth Africa (Eales and Bth, 1974); they frm a Ca-rich series (fig. 2a), and, unlike the Malawi dlerites, shw extreme irn enrichment, thugh cmpsitins are clearly cmparable fr the augitic pyrxenes. Three Ca-pr pyrxenes were analysed. The mst magnesian, a brnzite, frms phencrysts cexisting with phencrysts f augite and plagiclase in a markedly prphyritic rck. The ther Ca-pr pyrxenes are a magnesian pigenite and an intermediate pigenite. They are pltted n fig. 2a, tied t c-existing augites, and shw a typical irnenrichment trend. A pyrxene pair frm the Birds River Gabbr (Eales and Bth, 1974), als pltted n fig. 2a, shws mre marked irn enrichment than is fund in the Malawi dlerites. Amphibles frm tw rcks were analysed. n ne rck (Table, n. 24) the amphible frms phencrysts r xencrysts, and in the ther (Table, n. 9) it ccurs as rims n pyrxene, as well as frming subhedral prisms and interstitial patches. Bth are ferr-hrnblendes, accrding t the nmenclature f Leake (968), and the phencrysts are the mre magnesian. They are pltted n fig. 2a and tied t c-existing pyrxenes. t is apparent that as the pyrxenes becme unstable the amphibles cntinue the irn-enrichment trend, and in terms f Ca cntent are intermediate between the tw pyrxene series. The relatively flat amphible-pyrxene tie-lines prbably reflect the fact that, at the time f crystallizatin f the amphibles, the pyrxenes were in a strng disequilibrium with the melt. The relative abundance f amphible in the mre differentiated dlerites indicates that water pressures were cmparatively high, and it may have been the crystallizatin f FG.. Map f suthern Malawi and sme adjacent parts f Mzambique t shw the distributin f the Karr dlerite dike swarm (Malawi nly) and Karr lavas and sediments. The lcatin f analysed specimens is indicated, the numbers crrespnding t thse in Table. The inset map indicates the areas f utcrp f igneus rcks in suth and suth-east Africa f Karr age.

4 49 ~ / Ca/ ay A. R. WOOLLEY ET AL. -\ 3E 10 _1 v Y v Y Y Fe C v v 15,,/ '~--Na ~19 40 J ' -~k9 c ~ 14 : Ca FG. 2a. Cmpsitins f pyrxenes (filled circles), livines (pen circles), and amphibles (filled squares) pltted in terms f atmic Ca, Mg, and Fe 2 Tie-lines jin c-existing phases. Als shwn are the crystallizatin trends f Skaergaard pyrxenes (Brwn and Vincent, 963)--dashed lines, and f the Birds River Gabbr pyrxenes (Eales and Bth, 974)--dtted line, filled triangles, and dt-dash tie-line. Numbers crrespnd t hst rcks given in Table. b, Part f the Ca-Mg-Fe 2+ plt shwing cmpsitins f cres (slid circles) and rims (pen circles) f Malawi dlerite clinpyrxenes. Nte the change f trend with increasing irn enrichment, c. Cre and rim cmpsitins f plagiclases pltted in terms f atms f Ca, Na, and K. amphible, tgether with the cntinuing separatin f pigenite, that caused the early Ca-enrichment in the Ca-rich pyrxene trend. The plagiclases are strngly zned frm cres ranging frm An91 t An67 thrugh rims up t An4a (fig. 2c). Rck chemistry. Specimens frm twenty-ne dikes, frm a wide area f suthern Malawi (fig. ), were analysed fr majr and sme trace elements using the methds described in Vartiainen and Wlley (1976, p. 7)- These data are given in Table. All the rcks are hypersthene nrmative and abut half are livine- and the ther half quartznrmative. Three f the rcks have ntably high cntents f MgO (t ~) and shuld, therefre, be called picritic dlerites. The data are pltted n an AFM diagram (fig. 3) n which is als shwn, fr cmparisn, the trend f the strngly differentiated Karr Elephant's Head dike and the New Amalfi sheet f Suth Africa (Pldervaart, t944), and it is apparent that the Malawi dlerites have a similar general trend, althugh ver a mre restricted cmpsitin range. On fig. 4 the majr and sme trace elements are pltted against weight per cent MgO, the three picritic dlerites being readily apparent. Als pltted n fig. 4 are the values fr SiO2, FeO, and CaO f livines frm tw f the picritic dlerites, t which they are tied in the diagram, and an average f c!inpyrxene frm the same tw rcks. t is apparent that initial differentiatin f the suite is prbably cntrlled by livine, plus pssibly chrme spinel, and nt by clinpyrxene, and the series 9, 32,, O, 33, and 3 (Table )frms such

5 KARROO DOLERTES 49 t T~BLE i CHEMCAL ANALYSES AND NORMS OF MAJ~AW KARROO DOLERTE DYKE ROCKS 9 32 ii io Si t Ti O AzO O3 15.O O Fe2Oz O O O2 2.O FeO CaO io.96 io.31 io.ol 9.4O MgO MnO.2O O O.19 O.19 O O.20 O.19 O.21 O.17 O.18 O O.19 O.21 O O Na O O K20 O.37 O.35 O.42 O.18 O.15 O O O.96 O.78 1.O8 i.ii 1.13 i.oo P20~ O.12 O.ii O O.O O O.O8 O.iO O.18 O.i 0.22 O.14 O O.14 O O.21 H O O O.71 O O i.oo O H=O- O.24 O.ii O.35 O.17 O O.18 O.28 O O O5 O O.04 O.33 CO= O O.61 O O O O O O.44 F O.16 O O O.i O O1.O S O.ii O.ii O.iO O5 0.O9 O.O O6 O.O8 O O.iO O iO O.O4 O.O9 Others 0.28 O.23 O.21 O.17 O O O.ii O.19 O.14 O.17 O.14 O O O Sub tlal 101.1Z OO.18 1OO ioo.17 1OO.31 1OO.31 1OO OO O OO.35 OO.24 1OO O~F.S O O O O.O9 O.O3 O.O O O O.O9 Ttal io1.oo *34 1OO.80 1OO.16 1OO ioo.12 1OO.24 1OO.22 1OO.64 O1.25 ioo.79 io1.o OO.19 1OO Trace elements (ppm) Cr Li Ni OO 2O ii iio c~ i0 l Zn OO 3OO Sr ii 4O O 23O ii ioo 1OO iio Ba < ioo ioo ioo 125 OO OO Rb iio S O Q 0.t r ii ii ii O ab an di ii hv O l i.o mt i.i 0.4 i.i O.9 O il i.i 1.0 i.o O Specimen numbers all refer t British Museum (Natural ~istry) cllectin 1970,P23) except fr specimen 90 which is frm ellectln 1911,331. Lcalities are indicated n fig. 1. Rck types are as fllws: 9, 32 and 11, picritic livine dlerite; 40, 28, 14, 10, 33, 31 and 18, livine dlerite; 47 and 29, dlerlte; and 19. quartz dlerite; 15, feldsparphyrie dlerlte; 5 and 46 prphyritic dlerite. S.. = ioo ~O MgO + FeO + Fe20~ + Na20 + K=O a trend almst perfectly. Because the mre basic rcks d nt appear t cntain livine phencrysts in sufficient abundance t accunt fr this by accumulatin, this is presumably a fractinal crystallizatin path. Therefre, specimen 9 represents a smewhat picritic liquid, and Dr K. G. Cx (pers. cmm.) pints ut that these are prbably the first Karr rcks derived frm picritic magmas t have been fund utside the Lebmb. The three rcks 40, 28, and 4, althugh very similar t, 33, and 3 in terms f MgO, are higher in CaO and lwer in incmpatible elements. Their petrgenesis must be rather different, and it is ntewrthy that these three rcks derive frm the mre nrtherly part f suthern Malawi (fig. 0, whereas the rcks in the series 9, 32,, O, 33, and 31 are all frm the extreme suth f the cuntry (fig- 0- Fr the rest f the suite it appears that plagiclase and clinpyrxene, as well as livine, were imprtant in determining the path f crystallizatin. Specimen 15 with high A1203 and Sr and lw ttal irn prbably invlved sme degree f plagiclase accumulatin. Reginal gechemical variatin. Walker and Pldervaart (949) shwed that there are significant gechemical differences between the rcks f the nrthern and suthern parts f the Karr utcrp and mre detailed studies were made by Cx et el. (965), Cx and Hrnung (i966), Cx et al. (1967) and Vail et al. (1969). The suthern prvince, cmprising essentially the basalts f Lesth and Swaziland and the dlerites f Suth Africa, has a typical thleiite chemistry, whereas the nrthern prvince, cnsisting principally f the Karr lavas f Rhdesia, is abnrmal by virtue f high values fr K, Ti, P, Ba, Sr, and Zr, and prbably Nb and Y, and lw values fr Ca and A1 (Cx et al., 967). Rhdes and Krhn (197 z) cnfirmed statistically that there is cnsiderable reginal variatin in the chemistry f Karr lavas. Hwever, they interpreted their data as indicating the presence f a central Karr basin (utlined n fig. x, inset map) and a chemically distinct area marginal t the

6 492 A. R. WOOLLEY ET AL. leo +Fe203 TABLE. Average analyses f Karr basalts Nrthern Suthern Malawi prvince prvince dlerites wt. 9/ Na20+K20 m px MgO FG. 3. AFM diagram (wt. per cent) f Malawi Karr dlerites (slid circles). The pen squares are fr rcks frm the differentiated Elephant's Head dike and New Amalfi sheet (Pldervaart, 1944). The cmpsitins f livines (average f six mst magnesian values--see fig. 2), rthpyrxene, and average clinpyrxene f the Malawi dlerites are indicated. basin. A glance at the map (fig., inset) shws that the area t the suth f the basin is very much smaller than the area t the nrth, and as analyses frm the suthern margin are relatively few their similarity t analyses f rcks frm the nrthern marginal area was missed by earlier wrkers. Hwever, because the available analyses cme verwhelmingly frm the central basin and areas t the nrth f the basin, it is simpler and mre practicable t divide the Karr utcrp int nrthern and suthern areas at a latitude f 26 ~ S (see fig. i, inset map). Because f lack f data all previusly published reginal gechemical studies have nly extended nrthwards as far as the Victria Falls t Featherstne line f utcrps f central Rhdesia (fig., inset diagram), but there was extensive Karr vlcanic activity in the central Zambezi Valley, in Mzambique, east and suth f Tete, and in suthern Malawi. The Karr dlerites f suthern Malawi described here are perhaps the mst nrtherly manifestatin f the Karr igneus episde, and extend cnsiderably the area which must nw be included in any reginal gechemical study. Table (cl. 5) shws average analyses fr the twenty-ne Malawi Karr dlerites analysed during this wrk and, fr cmparisn, averages are als given fr representative areas f the nrthern and suthern (essentially central Karr basin f Rhdes and Krhn, 972 ) prvinces. The dif- SiO TiO A Fe FeO 8.18 MnO.16 MgO 7. 4 CaO 8.42 Na KzO 1.68 P ppm Ba 757 Sr 737 Cr 400 V 258 Ni 2 7 Rb O O ' i. Average f twenty-nine basalts frm the nrthern part f the Karr utcrp including Nyamandhlvu (8), Featherstne (6), Wankie (2), and Nuanetsi (i 3) (Cx et al, 967, Table 3)- Fr lcalities see fig. 1, inset diagram. 2. Average basalt (seventy-ne analyses frm twentytw lcalities) frm margin f the Karr basin nrth f latitude 26 ~ S. (Rhdes and Krhn, 972, Table ). 3. Average f seventy-three basalts frm the suthern part f the Karr utcrp, including Swaziland (Cx et al, 1967, Table 3), Basutland (Cx and Hrnung, 1966, Table 2), and Suth Africa (Walker and Pldervaart, 1949, Table 15). 4. Average basalt (119 analyses frm thirty-eight lcalities) frm the central Karr basin (Rhdes and Krhn, 972, Table ). 5. Average f the twenty-ne Karr dlerites f suthern Malawi. ferences between nrthern and suthern prvince areas identified by earlier wrkers are readily apparent frm Table, ntably fr TiO2, A1203, CaO, K20, P205, Ba, and Sr. t is als apparent that the Malawi data fr all these xides and elements match clsely thse f the suthern, nt the nrthern, areas. n fig. 5 values fr TiO2, A1203, K20, P205, and Ba are pltted against MgO, fr a number f nrthern and suthern prvince areas, tgether with the Malawi dlerite data. These plts indicate variable degrees f verlap between the prvinces, fr all five elements, but the distinctin between the tw prvinces is clearly significant. n each case the Malawi data plt with the suthern prvince

7 distributins, a fact which radically changes the reginal gechemical pattern described hithert. Discussin. The Karr dlerites f suthern Malawi are chemically and mineralgically typical thleiites and, therefre, unremarkable. Hwever, when cnsidered in the cntext f the reginal gechemistry f the Karr basaltic prvince they are imprtant because they differ frm reginal chemi- KARROO DOLERTES 493 cal trends which have been recgnized and interpreted by a number f earlier wrkers. The simple divisin f the Karr utcrp int chemically cntrasting nrthern and suthern prvinces nw n lnger hlds, and neither des the divisin int a central Karr basin with cntrasting basin marginal areas. Nne f the many mechanisms which have been Ti02 MnO P205 K20 Na q~ m,au~ 9 ~ 9 a. ~176 9 qjlj~ 9 90 "'ao,b. ~ 9.. " u r~a ~ e t t " b 9 Oe ~ Sr Ba (p.p.m.) Cr A : 9 " " i i r f r i r MgO ( Wt ~) Ni (Wt?.} CaO qu~ 9 % 2- FeO + Fe203 Si O Z 9 r r l l r 6 10 _.._. ~ MgO(Wt ~.) FG. 4. Plt f wt. per cent xides, and ppm fr five elements, against wt. per cent MgO fr the Malawi Karr dlerites (see Table ). Als pltted are livine cmpsitins (pen circles)fr the tw mst magnesian rcks, t which they are tied, and an average augite (pen triangle) frm the same tw rcks. t is apparent that initial differentiatin f the suite was strngly cntrlled by livine, nt pyrxene.

8 494 A. R. WOOLLEY ET AL q, m OO Ba (p.p.m.) j9 Q Q 9 D )'5 )'4 P205 )'3 400 =.t ~ "2 0"1 Ti02 2O0.1. " ',; 9 t" :,... ~ ilb" ~ ~176 ~ ~ ~ ~ $, ~~ O, : O ~,-,~ t.,~.: t A ~176 " ~ ~.~ ~ " " MgO(wtX) ~,~0 (wt 7=) FG. 5. Plt f selected xides and barium against MgO (wt. per cent) t illustrate sme chemical differences between the nrthern and suthern parts f the Karr utcrp. Filled circles, Karr basalts and dlerites frm areas nrth f latitude 26 ~ S (see fig., inset map); pen circles, Karr basalts and dlerites frm suth f latitude 26 ~ S; filled triangles, Malawi dlerites. The Malawi dlerites have a strng tendency t crrelate with rcks frm the suthern part f the Karr. Data frm Cx and Hrnung (i966), Cx et al. (967), Vail et al. (969), and this paper. suggested t explain the Karr reginal chemical variatin, and which are summarized in Rhdes and Krhn (972), is whlly satisfactry. The variatin seems t be generally independent f mst majr tectnic and structural features, and the Malawi data d nt fit the idea f a thermal regime peripheral t a central Karr basin, unless a similar basin existed in the area f NE Rhdesia, central Mzambique, and suthern Malawi, and there is n evidence fr this. The Malawi data are cnsistent with the hypthesis f Cx (972) that Karr magmatism was due t a single cmplex cycle, initiated by the breakup f Gndwanaland, and invlving the rise f a ptassium-rich magma bdy in the apprximate area f Rhdesia. Magmatism peripheral t this area 'was characterized by mre nrmal magmatism derived frm nly mderate depths in the mantle and caused nly by increased heat flw' (p. cir. p. 330). Althugh the depth f rigin f magmas was prbably an imprtant factr in accunting fr sme f the variatin amng Karr igneus rcks, particularly f sme f the mre extreme alkaline types (Wlley and Garsn, 97O), the present authrs feel that the general reginal chemical variatin acrss the Karr prvince is best explained by hetergeneity in the mantle, and rather than ne particularly large mantle plume beneath Rhdesia suggest that there were numerus mantle perturbatins the vlcanic prducts f which reflect the cmpsitin f that particular area f mantle. The large scale f the variatin and the fact that the Karr prvince was truncated alng the line f the present SE African cast by plate mvements suggest that the same pattern f reginal variatin shuld be present in the equivalent prvinces f Antarctica, and pssibly ndia. Acknwledgements. Dr R. D. Walshaw kindly made available a selectin f dlerites frm the cllectin f the Gelgical Survey f Malawi, and Mr S. Mwabile was very helpful in prviding backgrund infrmatin n these samples. We are mst grateful t Dr K. G. Cx wh made a number f very significant suggestins fr imprving the paper, and we are als grateful t him and Dr P. Hendersn fr critically reading an earlier versin f the manuscript. REFERENCES Blmfield (K.), Bull. Gel. Surv. Nyasaland, 9, 76 pp Bull. Gel. Surv. Malawi, 16, 93 pp. Brwn (G. M.) and Vincent (E. A.), J. Petrl. 4,

9 KARROO Cx (K. G.), Q. Jl gel. Sc. Lnd. 128, Jhnsn (R. L.), Mnkman (L. J.), Stillman (C. J.), Vail (J. R.), and Wd (D. N.), 965. Phil. Trans. Ry. Sc. Lnd., Ser. A. 257, and Hrnung (G.), 966. Am. Mineral. 51, Macdnald (R.), and Hrnung (G.), Am. Mineral. 52, Eales (H. V.) and Bth (P. W. K.), 974. Trans. Gel. Sc. S. Africa, 77, Garsn (M. S.), 962. Mere. Gel. Surv. Nyasaland, 2' 248 pp. Habgd (F.), i963. Bull. Gel. Surv. Nyasaland, 14, 60 pp. Leake (B. E.), i968. Gel. Sc. America, Special Paper, 98, 2O pp. Pldervaart (A.), Trans. Ry. Sc. S. Africa, 30, DOLERTES 495 Rhdes (R. C.) and Krhn (D. H.), 972. Trans. Gel. Sc. S. Africa, 75, 1-2t. Vail (J. R.), Hrnung (G.), and Cx (K. G.), 969. Bull. Vlcan. 33, Vartiainen (H.) and Wlley (A. R.), 976. Bull. Gel. Surv. Finland, 280, Walker (F.) and Pldervaart (A.), 949. Bull. Gel. Sc. Amer. 60, 59-7O6. Wlley (A. R.) and Garsn (M. S.), 97. n African Magmatism and Tectnics, eds. T. N. Cliffrd and. G. Gass (Oliver and Byd, Edinburgh), [Manuscript received 7 March 1979; revised 2 August 1979]