The Genesis of the Gaborone Rapakivi Granite Complex in Southern Africa

Home , Rapakivi granite

J. geof. Soc. London, Vol. 139. 1982, pp. 109-126, 16 figs, 2 tables. Printed in Northern Ireland. The genesis of the Gaborone rapakivi granite complex in southern Africa

R. M. Key & E. P. Wrigbt SUMMARY: The Gaborone Granite is a mushroom-shaped intrusion with a surface area of over 5000 km’. Theintrusion is layered,consisting of a centralcore of rapakivigranite (ThamagaGranite) surrounded bysuccessive shells of anequigranular leucocratic granite (Kgale Granite), a porphyritic granophyre or microgranite (Ntlhantlhe Microgranite) and an outermostzone of massiveEelsite (Kanye Volcanics). Thewhole lithological sequence is deduced to have been derived from a single, highly viscous magma body emplaced into the crust at a high level. The genesis is proposed as follows. The outer felsitesrepresent quenched primary magma with the underlying porphyritic granophyreshaving formed during a subsequenttranquil period after emplacement. The rapakivi granite corewas also of early crystallization abovethe floor but with significant textural characters impressed during updoming in the late crystallization stage. Residual liquid rich in SiO,, K20 and volatiles migrated upwards to form the Kgale Granite. TheGaborone Granite was emplaced in the KaapvaalCraton at about 2400 Ma and its morphology was controlled by pre-existing structures in the crust. The countryrock consists of Archaeangneisses, Lobatse Volcanic Group supracrustals and locally Transvaal Supergroup strata. Chemically the Gaborone Granite is identical to other early Proterozoic non-orogenic granites of theKaapvaal Craton. It is also similar in manyrespects to the Fennoscandian rapakivi granites.

The Gaborone Granite has a surface area exceeding al. (1974) reviewed the various hypotheses in the light 5000 km2 in SE Botswana and adjacent parts of South of results of a series of age determinations. Significant Africa (Figs 1 and 2). The western margins are obs- anomalies were noted but the evidencewas considered cured by Kalahari sands but have been approximately to incline more strongly to the Crockett/Poldervaart delineated by aerial geophysics and drilling. The gra- viewpoint. The presentauthors (Wright 1958, 1961; nite has a complex morphology consisting of a broad Key, in press)consider the entire sequence from sill-like body in the N connectedsouthwards to a rapakivi granite to massive felsitesas the differentiated narrowvertical-sided pluton (Fig. 3). The granite is products of a singlehigh-level intrusive magmatic composed of a centralcore of rapakivigranite sur- body. rounded bysuccessive shells of an equigranular Field exposures are of variable quality but generally leucocratic granite, a porphyritic granophyre or micro- good in the eastern parts (Fig. 2). The authors believe graniteand an outermost zone of massivefelsites. that the Gaborone Granite has intruded the Lobatse Formallithostratigraphic terminology has been ap- Volcanic Group (Ventersdorp)and is alsopost- plied [Key, in press; see also articles 6(h) and 1O(i) in TransvaalSupergroup. Flat-lying Waterberg Super- the Code of Stratigraphic Nomenclature published in groupsandstones unconformably overlie a pene- 1970 by theAmerican Association of Petroleum plained surface on the Gaborone Granite. The central Geologists] and the central rapakivi graniteis referred Thamaga Granite phase intrudes the c. 2600 Ma Mod- to as the Thamaga Granite; the equigranular granite ipeGabbro and is intruded by the c. 2100Ma as the Kgale Granite; the granophyre and microgra- SemaruleSyenite (not shown in Fig. 2). An early nite as the Ntlhantlhe Microgranite; the outer felsites Proterozoic age is therefore indicated for the intrusion as the Kanye Volcanics. All four units have the status of theGaborone Granite (Table 1). Later intrusives of formations. The term ‘Kanye Volcanin’ is retained into the granite include kimberlites and dolerites. fromearlier studies and is well established in the geological literature of South Africa. The surrounding Field occurrence rocks are mainly of the Lobatse Volcanic Group and Archaean basement. The southern (Molapo Wa Bojang) pluton is believed Poldervaart (1952) regarded the rapakivi granite as tointrude the Lobatse Volcanic Group rocks with intrusive into the felsites, with the development of an steeply dipping margins (Figs 3 and 4). The northern aureole of variably migmatized and recrystallized fel- extension(Northern Sill) hasan extensive flat-lying sites. Crockett (1 969) supported thisbasic concept but roof region and is thought to have been emplaced asa extendedthe metamorphic effects toencompass re- sill-like bodyalong the sub-horizontal interface be- crystallization of the entire felsite sequence suppres- tween Archaean rocks andoverlying Lobatse Volcanic sing any features of primary crystallization. Harding et Group rocks (Fig. 5). The layering within the granite

001 6-7649/82/0300-0109$02.00 @ 1982 The Geological Society

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20 S

'RT

-\ 0 l 1 , km 1 1 1 25 E 30 E

FIG. 1. The regional geological setting of the Gaborone Granite.

in the apical region is horizontal but the roof country In the Sephatlhaphatlha Hills, a strong vertical folia- rocks no longerexist. The floorhas, however,been tion exists striking approximately NE-ENE. The area identified in a basement inlier to the W of the Sephat- is regarded as a feeder zone and a continuation of the lhaphatlha Hills. The occurrence of the Northern Sill southern pluton. Thicknesses of the outer layers sur- asa relatively thin (< 5000 m)horizontal sheet is rounding the rapakivi granite varycore (see Key, in press confirmed by geophysical dataand, indeed, by the for details). The felsitesmay attain 1300m, whereas nature of the magmatic differentiation. the granophyres and leucocratic granite are appreci-

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A / A / 7*

M Moshaneng Llneament L LobatseFault System S Sephatlhaphatlha H~lls A’ FIG. 3. Three-dimensional reconstruction of the Gaborone Granite.

ably less (under 100 and 200 m, respectively). A drill Kanye Volcanics and Ntlhantlhe hole in the vicinity of thebasement inlier demon- Microgranite strated the existence of a very thin layer of felsites and granophyres (c. 3 m) below the rapakivi granite, indi- The so-calledKanye Volcanics are felsites consisting cating a possibly local and thinner occurrence of these predominantly of homogeneous fine-grained to aphani- peripheralrocks below theNorthern Sill. tic rocks,strongly jointed and with occasionalfeldspar

TABLE1. Outline of the Archaean and Proterozoic stratigraphy

Stratigraphic Unit IntrusiueAge activity

Kimberlites Dolerites Waterberg Assorted braided stream Middle Supergroup deposits-red clastic Proterozoic sedimentary rocks Unconformity Bushveld Igneous c. 2050 Ma Complex Semarule Syenite c. 2100 Ma Interlayered arenaceous and argillaceous sedimentary Transvaalrocks with a majorvolcanic Supergroup unitSupergroup Gaborone Granite c. 2400 Ma Unconformity Interlayered cherts and dolomites. Black Reef Quartzite Unconformity Lobatse Various volcanic and Volcanic sedimentary rocks incl- ModipeGabbro c. 2600 Ma Group uding the Plantation (Ventersdorp) Porphyry Unconformity Metamorphic ‘basement’ > 2700 Ma

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Wst Motsenekatse East Hill

0 km

West Sephatlhaphatlha €ad Hdls present erosion swfacs 4BO _A------.~ . . -,_-- L=---3500' ?

t 140km 4 b NORTHERN SILL

FIG.4. Cross-sections of the Gaborone Granite (see Fig. 2 for key).

1-30 1-30 km I

FIG. 5. Schematic petrogenesis of the Gaborone Granite. Key as in Fig. 2, although in stages 2-4 the undifferen- tiated granite is in the form of melt. Section line in Northern Sill and SE of Sephatlhaphatlha Hills (see Figs 2 and 3).

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phenocrysts. In very rare occurrences foliation, possi- dle-shaped or streakyin the foliated ones.Locally they bly indicative of flow, may be present, notably in the mayoccur aggregated into elongatedschlieren or extreme E on theborder with South Africa(Tyler rounded bodiesresembling basic xenoliths. Both oc- 1979). Certain agglomeratic rocks in this area could be currences are regarded as segregations. contemporaneous and therefore indicative of local ex- Directional structures vary from weakly-developed trusion. The felsites show a rapid transition over a few trends, distinguishable only on large surfaces of appar- metresinto the Ntlhantlhe Microgranite(porphyritic ently massive rocks, to the very pronounced structures granophyre or microgranites) with the texture becom- of the granites of theSephatlhaphatlha Hills,which ing more distinctly granular and with usually an accom- include banded augen gneisses with the streaked-out panying development of numerous, randomly-oriented schlieren of mineral aggregateswinding roundor phenocrysts of feldspar up to 5 mm in length. Neither through the porphyroclastic feldspar ovoids. The most felsites nor granophyres show any feature indicative of widespread structures include a platy foliation of the a break in continuity or suggestive of actual extrusion. tabularfeldspars and a vertical or steeplyplunging The external contacts of the felsites with the Lobatse lineation of major axes of feldspar and quartz pheno- Volcanics are very poorly exposed and detailed inter- crysts and dark mineral aggregates. relations uncertain. The peripheral rapakivi granites are mostly of massive, less markedly porphyritic rocks generally showing some degree of mild brecciation. In appearance they Kgale Granite are transitional into the Kgale Granite and the com- The Kgale Granite is typicallyan equigranular, parison is reflected also in the chemistry and petrog- homogeneous, medium- to coarse-grained, leucocratic raphy. The innergranites include mostly massive to granite.Contacts are mainlywith theporphyritic weakly foliated rocks with the strongly foliated rocks granophyres but locally extend into the felsites. The largely restricted to an axial zone between Moshupa boundaries are sometimes abrupt,with peripheral fine- andthe Sephatlhaphatlha Hills. The foliation in the grained granite showing a rapid coarsening away from granites trends parallel to this general direction and is thecontact. More commonly the boundary with the either vertical or steeply dipping. Concordant features granophyres is atransitional passage (a few metres) include thetrends of microgranitedykes, mineral with thedevelopment of variablytextured fine- to schlierenand major joint and fracture planes. The medium-grainedgranites. Phenocrystal quartz occurs same joint and fracture trends are also continued into commonly in peripheralthe granites. Minor the outer members of the complex along with those of metamorphic effects observedare in the certain major faults and quartz reefs. felsites/granophyresadjacent to abrupt contacts. A An abrupt horizontal contact between the Thamaga contactalso occurs with the basal Black Reef and Kgale Granites is exposed in a stream section in Quartzite(Transvaal Supergroup) with some indica- thecentre of theMolopa Wa BojangPluton. tionsthat the latter has been metamorphosed (Key, Elsewhere the actual contact interface is not seen but in press). it would appearthat the contact is abrupt with a general absence of contact effects such as chilled mar- Thamaga Granite gins,invasive dykes, xenolithic inclusions or metasomatic mineralization. The trends of the minor The Thamaga Granite includes porphyritic rapakivi deformational structures are concordant with the reg- granitesand subordinate microgranites. Thepheno- ional pattern and are not generally parallel to the line crystsinclude feldspar andsometimes quartz. The of contact and cannot therefore be related to differen- feldsparphenocrysts varyconsiderably in sizebut tial movement along this plane. Microgranites form an rarely exceed 2 cm in length. The shapes range from extensive, but variable, and frequently indistinct and tabular to ovoidal, the latter predominatingin occurr- gradational group of rocks within the Thamaga Gra- ences of foliatedrocks. Rapakivi zoning of albitic nite. They may show dyke-like forms but often occur plagioclase around microcline microperthite is variable with gradational margins and enclosing islands of the andirregular; it is frequentlydiscontinuous, some- host granite which are elongated parallel to the dyke times absent altogether and occasionally reversed. trend (Fig 6). Alternatively,there occurextensive The groundmass in the porphyritic rocks is granular outcrops of an intricate and gradational intermixture andranges from medium-grainedto microgranitic. of granite and microgranite in which the relative prop- Theproportions of groundmassto phenocrysts vary ortions of each may vary rapidly from place to place, but are about equal in the more typical granites. The makingit difficult or virtuallyimpossible to ascribe darkminerals, biotite and hornblende, are largely forms to either type. Associated aplitic and pegmatitic confined to the groundmass but in small amounts may schlieren are common, often with mafic selvedges, or occur enclosed by phenocrystal feldspar, where they the mafic minerals may independently occuraggre- mayshow a zonal distribution. They generallyform gatedinto distinctschlieren or massivestructureless discrete masses, granular in the massive granites, spin- aggregates. The microgranites frequently enclose a few

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FIG. 6. Microgranite dyke within Thamaga Granite. Note ‘islands’ of rapakivi granite and feldspar phenocrysts in dyke (1/15th natural size).

feldspar phenocrysts similar in appearance and zonal feldspar and occasionally pyroxene. In the felsites the characteristics to those in the host granite but gener- feldspar phenocrysts are commonly fragmentary and allysmaller and often fragmentary, cracked and corroded, and often showa marked elongation. The corroded-lookingand witha tendency to occur in felsitic groundmass is intersected by bands and streaks clusters. Thedark minerals,predominantly biotite, of various mineral aggregates including combinations occur finely diffused but are occasionally in swarms of of quartz,feldspar, biotite, calcite, ore,leucoxene, coarser aggregates similar to those in the host granite sphene, apatite and sometimes zircon. The same min- andare then generallyassociated with clusters of erals may also occurin round orelliptical massessome- feldsparphenocrysts. When within foliated granites, timeszonally arranged. Bands may locally penetrate thedirection of elongationas well asany internal feldsparphenocrysts or swirl aroundtheir margins. structures are generally concordant. Minor dykes may These various features suggest movement in a highly follow other trends, usually that of subordinate joint viscous magma with the concentrations of mafic and directions. The characters of themicrogranite dykes accessoryminerals having crystallized from original are more suggestive of a metamorphic than an igneous volatile-rich streaks. The feldspar phenocrysts in the formation,and are considered to have formed by granophyres show euhedral to rounded outlines and replacementalong lines of localized shear.Petrog- occur withina groundmass composedlargely of a raphicalstudy has provided much confirmatory evi- granophyric intergrowth of quartz and feldspar (Fig. dence and has indicated that the shearing stresseswere 7). The intergrowths frequently form spherical aggre- probablyinitiated at a late stage but prior tothe gates, usually around a phenocrystal core, and there is complete consolidation of the magma. commonlyclosea optical relation betweenthe phenocryst and the aureole. The relationshiphas made Mineralogy, petrography itpossible todate certain fractures intersectinga chemistry phenocryst as having formed prior to the crystalliza- and tion of the groundmass, since the optical orientationin the unfractured groundmass may vary in accordance Kanye Volcanics and Ntlhantlhe with displacements in the fractured phenocryst. Microgranites The feldsDar.. Dhenocrvsts are comDosed of crmto-<* The felsites andgranophyres are holocrystalline perthiticto microperthitic alkali feldspar with anal- rockswith no evidence of devitrification andare of ysedcompositions in therange Ab,,0r8An, to comparable mineralogy and chemistry (Tables 1 & 2 in AbS50r3*An7; (Table3 in Supplementary Publication). SupplementaryPublication). Phenocrysts include Perthitic structures are most marked in the feldspars

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FIG.7. Photomicrograph of porphyritic granophyre (Ntlhantlhe Microgranite) showing euhedral phenocrystsof alkali feldspar with aureoles of radiating aggregate of quartz and feldspar in felsitic matrix (crossed polarizers, Xll). from the interior granophyres and tend to bevague or The textural relations of the feldspar phenocrysts in indefinitein those fromthe transitional granophyres the granophyres areindicative of a primary origin with and felsites. The more prominent structures show as the feldspars continuing to crystallize until the quartz- thin,irregular, wavy anddiscontinuous streaks and feldspar cotectic boundary was reached, with the re- patches whose most obvious orientation is parallel to sultant simultaneous crystallization of both these min- the (100) plane. An X-ray oscillation photograph aroundthe b axis of a (010) cleavageflake of a - 91 e* feldspar from a peripheral granophyre showed that the MAX. feldspar was unmixed and composed of two triclinic, MICRO. albite-twinned potash and soda phases, whose orienta- ~ 90 tions accord with unmixing from an original homogeneous feldspar. The values of the lattice angles were calculated in accordance with the method of MacKen- zie & Smith (1955) and it was found (Fig. 8) that the -89 potash phase corresponded with maximum microcline and the soda phase to a value intermediate between low albite and microcline. The values suggest that the .88 soda phase includes a fair proportion of potash which wasnot exsolved andas such isin anintermediate I thermal state. The normative feldspar composition of the analysed rocks and the compositionsof several feldspar phenocrysts have been plotted on the feldspar ternary diagram (Fig. 9). It can be seen that the phenocrystsin the felsites andperipheral granophyres are, ingeneral, moresodium-rich than those from the interior granophyres, which approach the normative feldspar composition of the total rock. The proportion of the FIG. 8. Feldspar X-ray data. A, represent Ntlhan- anorthite moleculein these phenocrystsmay seem tlheMicrogranite feldspars; m, representKgale high but it must be remembered that no independent Granite perthites; 0, represents Thamaga Granite plagioclase occurs and presumably the greater part of perthites; 0, represents Thamaga Granite plagio- the calcium available has entered the alkali feldspar. clase rim.

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FIG. Feldspar9. plot on a ternaryalbite- FIG. 10. Plot on ternarydiagram albite- orthoclase-anorthitediagram. The limits of solid orthoclase-quartz of theaverage composition 'A' solution are after Tuttle & Bowen (1958) and are of analysedgranophyres and the estimated com- fordry rhyolites and phonolites (BB') and wet positions of groundmassthe of selected rhyolitesand phonolites (AA', DD'). 0. felsite granophyres byremoval of theobserved modal phenocrysts; U, peripheral Ntlhantlhe Microgranite amounts of phenocryst 1 feldspar ofknown com- phenocrysts; A, interiorNtlhantlhe Microgranite position. The quartz-feldspar boundary andthe ter- phenocrysts; 0, felsite normative composition; M, nary minimum 'T' for a water vapour pressure of granophyre normative composition. Corresponding 500kg/cm2are after Tuttle & Bowen(1958). 4 rocks and their phenocrysts are numbered approp- and 3 are compositions of phenocrystsfrom two riately. peripheral granophyres. B: average composition of phenocrysts from two interior granophyres. 3', 4', and B': calculated groundmass compositions. erals. The same deductions can be drawn if the compositions andtotal amounts of thephenocrystal feldspars are considered in relation to a hypothetical groundmass of one of theperipheral granophyres magma derived from the average composition of the analysed (W4) is atypical andanomalous, since the analysedgranophyres and recalculated in terms of residualliquid has apparently crossed thethermal Ab-Or-Qtz. The diagram (Fig. 10) is afterTuttle & divide. The explanationcould lie in experimental or Bowen(1958) and shows theposition of the observational error in the feldspar or modal analyses experimentally-determinedquartz-feldspar boundary or in some other factor, such as the anorthite content, and the ternary minimum for a pHzO of 500 kg/cmz. affecting the influence of the thermal divide. Several The composition of the felsite magma (A) lies some- repeatobservations did, however, give confirmatory what to the orthoclase side of the subsidiary thermal results and no satisfactory explanation is as yet availa- trough which extends from the Or-Ab minimum to the ble. The presence of a single feldspar provides a re- cotectic boundary (not shownin diagram) and theoret- semblance to some of the Icelandic pitchstones (Car- ically a feldspar richer in orthoclase would have been michael 1963) and to certain rhyoliteswith low cal- expected to crystallize. The compositions of the cium content from the San Juan region (Larsen 1938). hypotheticalmagma after removal of theobserved The occurrence in rocks of this general composition is, modalamounts of feldspar of knowncompositions however, unusual and could be significant. Carmichael have also been plotted and it is seen that they lie close (1963) suggested that such rocks aregenerally the pro- toand along the cotectic curve for the pH20 of duct of fractionation from a tholeiitic magma. 500 kg/cmz. The phenocrystalfeldspars in the felsites and Kgale Granite granophyres thus exhibita single series with increasing potashcontent converging towards the normative Thisgranite consists typically of a granularstruc- composition of thewhole rock and the binary turelessaggregate of quartzand feldspar which to- minimum on the alkali feldspar join. The groundmass getherconstitute 96-98 vol. YO of thetotal compositions in the granophyres closely approach the rock. Other minerals are mainly of accessory pro- ternaryminimum in theAb-Or-SiOz triangular plot portions and apart from pyroxene which is limited to indicatingequilibrium reactions. The potash-rich someperipheral rocks, all arelate magmatic or

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TABLE 2. Modalanalyses of the threeinternal phases of the Gaborone Granite (fromWright 1961)

Biotife and Formation Quartz PerthitePlagioclase chloriteHornblende Accessories

31.7 64.6 - Ntlhantlhe32.5 64.1 - M icrogranite 32.3 63.9Microgranite 32.3 - 33.7 64.3 33.7 -

40.5 58.2 40.5 - Kgale 59.0 40.2 - Granite 40.6 58.0 - 41.0 44.8 10.9 44.8 41.0

28.0 57.0 7.1 57.0 28.0 6.51.3 trace T hamaga 26.6 44.3 18.1 44.3 26.6Thamaga 2.5 7.5 1.1 Granite 53.5 27.5 0.2 10.2trace 8.6 40.1 51.92.8 trace3.6 1.6

deuteric in origin and include biotite, muscovite, seri- andgranophyres. The peripheralThamaga Granite cite,chlorite, zircon, apatite, fluorite and iron ore. accords more closelywith the Kgale Granite,the Quartzoccurs as granular individuals with round, groundmass of the porphyritic granophyres andmicro- ovoidal, subhedral to euhedral forms and may consti- graniteswith the interior Thamaga Granite. In tute more than 40% of the total rock (Table 2). The general,the chemicalproportions in theThamaga bulk of the feldspar is microcline microperthite with Granite(Table 5, Supplementary Publication)show minor amounts of small albite crystals. Perthitic struc- greater variability than occurs in the outer membersof tures exhibit awide range of textural variations. In the the Gaborone complex. The feature is attributed to peripheral,finer-grained granites, they resemble the regular and even intergrowths of the feldspar in the porphyritic granophyres. In the coarser-grained, inner granites, the perthitic plagioclase occurs irregularly in patches, braided networks, veins and streaks. Much of this perthite is clearly derived from redistribution of exsolvedplagioclase andoften showsstructurally- controlledpatterns. The larger patches, whilstalso A demonstrably of ionic replacement, commonly enclose abundant, fine sericitic mica and are closely associated withlate-stage accessory minerals. Replacement appears tohave been closely contemporaneous with unmixing of the original alkali feldspar but part of the sodiumseems likely to havebeen derived fromthe residualmagmatic fluids. The chemical compositions of Kgalegranites are shown in Table 4 of theSup- plementary Publication. f -L Ab W. % Or Thamaga Granite FIG. 11. Plot of the normativecompositions of Granites and microgranites of this unit have a simi- analysedrocks of the GaboroneGranite onthe lar mineralogy to the Kgale Granite in respect of both ternarydiagram quartz-albite-orthoclase. The main and accessory minerals but with significant differ- stippledarea is the field of LobatseVolcanic ences of proportions, notably of quartz and dark min- Groupwhole-rock analyses. 0, felsites and granophyres; 0, groundmass of ananalysed erals (Table 2). Normative values of various analysed granophyre; 0, Kgale Granite: V, peripheral rockshave been plotted on athree-phase diagram Thamaga Granite; A, interiorThamaga Granite; (Fig. 11). It can beseen that the interior Thamaga m, microgranite in Thamaga Granite; 6,average Granitecorresponds in compositionwith the felsites rapakivi granite (after Sahama 1945).

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more significant redistributions of material which oc- percentages of alkalis in the rapakivi granites are fairly curred during the late magmatic and deuteric stagesin constant. The close association of the plagioclase with crystallization. the crystalloblastic form of quartz and with late-stage Thepetrography is complex,with two ormore hydrousand deuteric accessoryminerals, including generations of the main minerals occurring. Feldspar biotite, favours the conceptof replacement by residual occurs either as phenocrysts orin the groundmass, and magmatic fluids. There is no evidence for adifferential includesmicrocline microperthite and albiticplagio- build-up in sodiumin the residualfluids and potash clase, both mineralscommonly intergrown andwith the seems to have been fixed by iron in the formation of plagioclase of the same composition and orientation as biotite.Confirmation is provided byX-ray analysis associated perthitic plagioclase. The same textural re- (Fig. 8). Whereasperthitic feldspar in theThamaga lationships exist as inthe Kgale Granite except for the Granite shows bothphases, the rapakivi plagioclase generalabsence of the veryregular lamellar inter- exhibits only one low-temperature sodic-phase. growths andthe common presence of zonalplagioclase. The zonal arrangements are variable and grada- Petrogenesis tional(Figs 12-14). Thenarrow rapakivi rims are commonly bounded bycurving fractures andas- The suggested petrogenetic history of the Gaborone sociatedwith an apparent depletion of perthitic Granite complex is shown in aseries of schematic plagioclase in the interior core. Such rims are consi- sections(Fig. 5). The fundamental thesis is that the deredto have developed bymigration of exsolved entire sequence has been derived from a magma body plagioclase and marginal coalescence within the same emplacedinto the crust at a highlevel. Although crystals in response to differential pressures set up by differencesexist in consequence of fractionation or internalmovements in themagma during the late varying conditions of crystallization, there are funda- stages of consolidation.Other phenocrysts include mental similarities in overall chemical and mineralogi- much higher proportions of plagioclase. Although the cal compositions which favour this interpretation. The latter’s identical characteristics leave little doubt of a wide lateral extent and considerable thickness of the comparable replacement origin, the bulk compositions Kanye Volcanics, their uniform character, acid com- of the phenocrysts are clearly too variable to indicate position andfreedom from inclusions arefeatures original feldspar compositions. At the same time, an more suggestive of the high-level emplacement of a internal source of sodium is required, since the overall viscousliquid mass than of anextrusive formation.

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FIG. 13. Photomicrograph of zoned feldspar in Thamaga Granite. Note depletion of perthitic plagioclase in core and the dark islands of residual perthite in plagioclase mantle (crossed polarizers, X 16). The occurrence of such a monolithic sheet can reason- a period of tranquil conditions allowing slow cooling ably be related to crystallization during a slow, con- andequilibrium reactions (Fig. 5-2). No change in tinuousupdoming of themagma massfollowing its external conditions is implied during the groundmass emplacement. (Fig. 5-1). Theunderlying porphyritic formation, for theincreased viscosity of the liquid near granophyres may be supposed to have formed below the eutectic compositionwould result in arapid in- the felsite cap on cessation of such movements during crease in the numbers of centres of crystallization.

FIG. 14. Mantled feldspar phenocryst in Thamaga Granite. Note the irregularity of the mantle and the abundance of perthitic plagioclase in the core. The mantles contain diffuse inclusions of fine biotite and have formed probably by deuteric replacement (crossed polarizerf, X 18).

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The close spatial relations of the Kgale Granite with water in themagma chamber (Kennedy 1954) (Fig. the overlying granophyres and felsites and the combi- 5-4). The composition of the Thamaga Granite is not nation of transitional and abrupt contactssuggests that far removedfrom that of the felsites, and varieties the granite represents a lowerdifferentiated horizon occur which are enriched in those constituents which which had locallyintruded the earlier consolidated require removal to transform a liquid of felsite to one phases of the same mass (Fig. 5-5). The nature of the of theKgale Granitecomposition. The theory sup- abrupt contacts suggests the emplacement of a viscous poses that anearly separation of the primary fer- liquid magma intruded with little accompanying defor- romagnesian andsoda-rich alkali phases resulted in mation and probably occupying spaces formed by ten- theformation of aresidual liquid of lowerdensity sional relief. It is significant that the abrupt contacts which began to rise. The continuation of this convec- mainly occur on the edges of the flat roof region of the tional process would eventually produce a liquid rich complex, where tensionalrelief in adoming body in silica and with K20 in excess of Na,O in the upper would be expected. The rarity of dykes, apophyses and portion of the magma chamber but below the viscous xenolithicinclusions is probablya reflection of the and more rapidlyconsolidating granophyre cap. The high viscosity of the magma, and the minor scale of upwarddiffusing water wouldalso carry silica, the upward movements.The slightness of the whereby an excess of this component over and above metamorphicand metasomatic effects was probably eutectic proportions would developin the final liquid. theresult of the shallow temperatureand composi- The Kgale Granite locally extendsthrough the tional gradients across the contacts. microgranite-granophyre rocks andforms abrupt in- The differentiation of the hypothetical parent trusivecontacts with theouter felsites and, in one magmato produce the granite-granophyre-felsite as- location, with Black Reef Quartzite(Transvaal sociation must explain theconsiderable diversity of Supergroup). The impetus for intrusion is deemed to composition of thegranite from that of the have been provided by the upward movement of the felsite/granophyres.Since the massive felsite can be crystallizinginterior (Thamaga)granite mush which regarded as a quenched liquid, any homogeneity de- also initiated groundmass crystallization, primary flow veloped by a differential exchange must have occurred foliation,and the development of rapakivirimming. in the liquid phaseprior to crystallization of any At thistime, itseems likely that the peripheral primaryphases, and would require the improbable rapakivigranites on thedipping margins andthe diffusion of such elements as magnesium, calcium, etc. deeper-seated,more basicgranites had almost com- The massive felsites may, therefore, be regardedas the pletely consolidated. Following initial movement, it is quenched originalmagma andthe granophyre as believed thatthe core rocks continuedthe upward having formed from this same magma with no change movement with thedevelopment of extremeproto- in composition. A magma of felsite composition could clastic and cataclasticfoliation, replacement micro- betransformed into one comparable with the Kgale granites, boudins and various mineral schlieren. The Granite by an upward migration of SiO, and ‘ortho- protracted upward movement of the core rocks could clase’ and a downward migration of iron oxide. The possibly relate also to the formation of massive quartz upward migration is a reasonable supposition, since it reefs, sometimes with galena and fluorite, which inter- hasbeen shown(Morey & Hesselgesser 1951)that sect the peripheral granites and felsites and with the water vapour in equilibrium with hydrous granite mag- associatedfaults extendinginto the surrounding mas is rich in these molecular components. The down- Lobatse Volcanics. Fhorite is a major late-stage ac- ward migration of iron, magnesium etc. appears to be cessoryin both Kgale andThamaga granites. The possible only in a crystal phase. trend of the dykes is commonly ENE, which conforms A more probable hypothesis is based on fractional with themajor axis of theNorthern Sill and with crystallization with removal of the early-formed crystal pronounced foliation trends in the core rocks. It is also phases. The composition of the Kgale Granite is fairly aprimary trend in Archaeanrocks (Key 1977) and, close tothat of thegroundmass of theaverage indeed, is a recurrent trend in younger rocks than the granophyre. Nonetheless, a simple gravitative separa- Gaborone Granite. The feature is thought to indicate tion of the primary ferromagnesian and soda-rich alk- acurvature of thesupposed feeder zone extending ali feldspar phases is not likely to have occurred in an fromthe Molapo Wa Bojang pluton through the acid magma of presumably high viscosity, and there is Sephatlhaphatla Hills. The cause of thecontinuing no directevidence to support the viewpoint.Nor movement in thecore rocks abovethe feeder zone would the process accountfor the granites whose maypossibly be related toa build-up of the partial abundance of quartz phenocrysts is indicative of com- pressure of water, eventually attaining sufficient prop- positions in excess of quartz-feldspar eutectic propor- ortions to lift the roof and, by reducing the pressure tions. The explanation which satisfiesboth these on the supply source, allow a fresh influx of magma difficultiesinvokes an early crystallization in the frombelow whose impetusprolonged the upward deeperparts of themagma mass, dueeither to the doming. proximity of the floor or perhaps to the distribution of Theoccurrence of zonalplagioclase feldspar is

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 122 R. M. Key and E. P. Wright closelyassociated with this proposedsequence of Large disparities in their data for the Kanye Vol- events.hasIt formed by exsolution of original canics precluded any meaningful estimate of the age of homogeneous alkalifeldspar and as a late-stage or these rocks. In their summary they stated: 'the Inner deutericreplacement. Replacement has occurred by Marginal Assemblage (Kgale Granite) of the Complex differentialredistribution of exsolvingplagioclase is 2328*Ma oldand theCentral Assemblage under a structural control and/or under the influence (Thamaga Granite) is either the same age or 2683" Ma of late-stageresidual solutions rich in alkalis. The depending upon which interpretation of theRb-Sr condition favouring the former was the slow imposi- data is favoured'. On the basis of the present authors' tion of deforming forces prolonged through the temp- interpretation of asingle intrusion, all thedata of eraturerange of exsolution.Such forces in the Harding et al. (1974)were plotted on one isochron peripheral rapakivi granites were probably small and diagram(Fig. 15). Four of the felsitesamples were not imposed until consolidation was almost complete. omitted because of their dubious geographical location Redistributedexsolved plagioclase anddeuteric (remapping suggests thatthey may bepart of the plagioclase has tended to coalesce in irregular patches Lobatse Volcanic Group).This singleplot produces and discontinuous rims. Regular zoning is most pro- agood linear spread witha low MSWD factor of nounced in the inner porphyritic granites where defor- 3.2, and gives anage of 2394k26 Ma, Ri = mation was prolongedthrough final consolidation. 0.74991 zkO.00643. Under such conditions it is thought that residual fluids The Kanye Volcanics and Thamaga Graniteanal- would tendto be distributed more evenly. In the yses plot near the intercept on the 87Sr/86Sraxis. The extremely foliated porphyritic rocks, deformation ap- four Kanye Volcanics samples in fact plot just below pears to have beenprotoclastic and mainly effective but parallel to the isochron. This indicates that they above the main exsolution temperature range. Margi- are the sameage but have a lower initial ratio than the nal rimmingmay occurbut the replacement plagio- granitic phases. This could be due to contamination of clase is clearly post-deformationaland a structural the felsites (the outer phase of the Gaborone Granite) control of exsolved perthite distribution is not marked. by the country rock during emplacement (C. Rundle, pen. comm.). In fact, if these four samples are omitted from Fig. 15 and only the Kgale and Thamaga Granite Geochronology samples are regressed, the MSWD reduce to 1.21 and the age is 2428+55 Ma, Ri=0.75502+0.00660 (C. McEIhinny (1966) recorded an age of 2290~t44Ma* Rundle, pen. comrn.) On the basis of this new analysis from a three point plotof Rb-Sr whole-rock data from of existing data the authorsconclude that the Gaborone theThamaga Granite. Confirmation of thisage was Granite was emplaced 'in toto' at about 2400 Ma. given by Evans (1967), on the basis of palaeomagne- A minimum age of 2630* 100 Ma was established tic evidence which suggested an age of c. 2300 Ma for forthe Lobatse Volcanic Group by van Niekerk & the whole intrusion. Subseauentisotovic data obtained Burger (1968) with U-Pb studies of zircons. A marker by Harding et al. (1974) was considered to favour the qua& porphyry referred to as the Plantation Porphyry Poldervaart/Crockett thesis(see introduction) of was sampled for this study. A later Rb-Sr study on the polyphase igneous activity. same rock by Harding et al. (1974)failed to give a r t3 /O'O

20 40 BD 80 100 120 I I l l J FIG. 15. Isochron diagram. +, Thamaga Granite samples.; El, felsites; 0,Kgale Granite samples

*All ages are recalculated using hRb= 1.42~10-"a-'

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meaningfulresult. Confirmation of the age forthe ing ENE on Archaean gneisses with subsiding Trans- Lobatse Volcanic Group is indirectly given by an age vaal Supergroup basins to the N (Molepolole) and S of 2643 f 100 Ma (U-Pb on zircons) for a quartz por- (Kanyeand Bushveld). TheLobatse Volcanics and phyry from the Platburg Group of the Ventersdorp of Transvaal Supergroup probably thinned over the base- South Africa (van Niekerk & Burger 1978). The Lob- mentplatform and the Gaborone Granite magma atse Volcanic Group is a northern extension of these spread laterally along the flatinterface between the rocks. volcanics and the basement platform, thusforming the Therefore the isotopic data supports the fieldevi- Northern Sill. The Molapo Wa Bojang Pluton occurs dence presented in this paper that the Lobatse Vol- betweenthe Bushveld andKanye basins alongthe canic Group was emplaced prior to the intrusionof the same linear feature (Vryburg Arch) which has appar- Gaborone Granite. Quenching of the marginal felsitic ently controlled the occurrence of the Lobatse Vol- facies of the granite has given these outermost rocks a canics in this area. slightly lower Ri than the interior cooler phases. Although there is therefore a close spatial and structural correlation between the Gaborone Granite and Geophysics the Lobatse Volcanics, a marked dissimilarity in chemical compositionexists (Fig. ll), which,incidentally, A ground magnetic traverse across the short, N-S axis precludesthe concept that the KanyeVolcanics are of the Northern Sill was completed (along the main derivedby recrystallization fromthe Lobatse Vol- Gaborone to Francistown road). The outer felsites are canics. Theemplacement of theGaborone Granite strongly magnetic relative to the coarser-grained gra- produced a weak hornfelsing of the Lobatse Volcanic nites although both the Ntlhantlhe Microgranite and Group strata over a horizontal distance of lessthan Kgale Granite have an erratic magnetic character. By 3 km from theMolapo Wa Bojang Pluton. Spotted comparison the Thamaga Granite is magnetically very textureswere locally produced in the more fissile quiet and the western half of the Northern Sill defines rocks. Chiastolite grains toup 0.5 cmlongdefine this tex- a magnetic low in the aeromagnetic survey of Reeves ture (N of Lobatse), but usually the spots are due to (1978). The inliers of Archaean gneisses W of microcrystalline opaque aggregates. Secondary biotite Thamaga are strongly magnetic compared to the adja- growth in the volcanics is attributedto the granite centThamaga Granite and have been accurately emplacement. The emplacement of the Northern Sill defined by ground geophysics. Ground-truth traverses granitizedvarious clastic sedimentary rocks of the across the gneisses reveal a high proportion of mafic TransvaalSupergroup at their onlyplace of contact rock which probably accounts for their strong magne- (on Tsele Hill) over a vertical distance in the roof zone tic contrast. of several metres. Elsewhere no noticeable mineralog- On the National Gravity Map of Botswana (Reeves icalchanges arepresent in thecountry rock tothe & Hutchins 1976) the contours (at 100 g.u. intervals) Northern Sill. Theemplacement of theMolapo Wa overthe area of theGaborone Granite follow the Bojang Pluton also produced a weak cleavage in the surfaceshape of the Northern Sill (-1250 g.u. con- marginal Lobatse Volcanic Groupstrata, notablyin tour). The Gaborone Granite is generally represented the fissile tuffaceous beds. This fabric weakens north- by agravity low. There is no abrupt ‘edge-effect’ wards. To the E and SW, however, in the Kanye and around the Northern Sill, which confirms that it is a Bushveldbasins, theupper parts of theTransvaal thin sill-like feature rather than a vertical mass. Supergroup show intense faulting interpreted by Croc- kett (1969, 1972) as due to gravity sliding consequent Origin and structural relations on subsidence in both these basins adjacent to more stable platforms. These tectonic features are unique to The occurrence of the Gaborone Granite appears to the western margin of the Bushveld basin and in the be closelycontrolled by structural trends in the presentauthors opinion can more likely be ascribed Archaean basement which have also affected the dis- to the emplacement of the Gaborone Granite. Lahar tribution of the Lobatse Volcanics and the Transvaal depositsseen in theupper parts of theTransvaal and Waterberg Supergroup sediments. The northerly Supergroup also indicate an unstable crustal surface at strike of the Lobatse Volcanics in SE Botswana ap- this time (Key, in press). pears to be an extension of a linear feature in South The high initial ratios of 87Sr/s6Sr indicate that the Africa known as the Vryburg Arch (Hutton 1978, fig. magma contained a large proportion of crustal mater- 1). Farther N,two majorENE-trending lineaments ial. Harding et al. (1974) noted that this crustal compo- parallel the grainin the underlying Archaean rocks nent would have to have had high Rb/Sr ratios. Their (Crockett & Jones 1975; Hutton 1978). The distribu- analyses of theLobatse Volcanic Group, which tion andstrike of theLobatse Volcanics appearto formeda thickened crustalunit along theVryburg havebeen constrained tobend eastwards inaccor- Arch,do show high Rb/Sr values. TheGaborone dance with these pronounced structural features. The Granite is depleted in strontium, with a mean value of two lineaments define a more stable platform extend- only 16 ppm forall phases (40 analyses) compared to a

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elementconcentrations), which it was suggested are inheritedfrom the ancient sialiccrust (see above). Fyfe(1978) emphasized that hot-spot phenomena dominated crustalprocesses in the Precambrian, and theevidence from Southern Africasupports this theory.(Horizontal tectonic processes are of subsidiary importance to verticalcrustal movement, see E also Kroner 1977.) a The Bushveld Igneous Complex, which is situated a immediately E of the Gaborone Granite, contains a a granitic phase identical lithologically and chemically to a the Gaborone Granite (see Hunter 1975). The Bush- veld Igneous Complex,however, has been datedat c. 2050Ma (Hamilton 1977) and is, therefore, younger thanthe Gaborone Granite. There is no evidencefor amajor mafic phaserelated tothe wt. K Gaborone Granite (the Northern Sill defines a gravity low andthe drillhole through thisbody penetrated FIG. 16. Plot of potassiumagainst rubidium. 0, Archaean gneisses). felsite; 0, granophyres; A rapakivi granites; U, Kgale Granite samples; V, late microgranites in the Thamaga Granite. Limits of scatter shown by Comparisons with Fennoscandian two diagonal lines are fromTaylor et al. (1956). rapakivi rocks

mean value of rubidum of 319 ppm. A plot of potas- The similarities betweenthe Gaborone Granite and sium against rubidiumin rocks of the Gaboronecomplex Fennoscandian rapakivi granites and associated rocks (Fig. 16) shows a distinct trend from the felsites to the aretoo numerous to list in detail.They include Kgale Graniteand interior micrograniteswith the similarities of structuralrelations, petrography, latter group occurring outside thelimits of scatter. The mineralogy,chemistry andtrends of differentiation. trend confirms the proposed differentiation sequence. TheThamaga Granite corresponds to the wiborgite Within the contextof southern Africa the Gaborone rapakivi type, the Kgale Granite to the pyterlite type. Graniteconforms patterntoa definedby the Fennoscandianrapakivi granites are now generally Archaeanand early Proterozoic granites and non- accepted ashaving crystallized from magma at high metamorphicsupracrustals of theKaapvaal Craton. crustal levels (Dawes 1966) and sheet-like morphology Severalauthors, notably Anhaeusser (1973), Hunter is common. Significantdifferences arerare and in- (1974) and Pretorious (1979), have pointed out that clude, in the main, the presence of directional textures there is a regular migration of granitic intrusions and and the abundance of aplites and pegmatites in parts their peripheral sedimentary basins in a northwesterly of the Thamaga Granite.In this connection, Wegmann directionacross the craton between c. 2800Ma and (1938) pointedout that the absence of pegmatitic c. 1800 Ma. The granites were emplaced in areas of materialin rapakivi granites is moreapparent than relative uplift (crustal arching) with the adjacent nega- real.According to him, thelate magmaticsolutions tiveareas forming the sedimentary/volcanic basins have been unable to separate into well-defined veins (e.g. Witwatersrand,Ventersdorp, Transvaal and but are to be found diffused throughout the granites, Waterbergsupracrustal units). All theintrusions are where they have reacted with earlier-formed minerals. non-orogenic and the supracrustal rocks are also not The occurrenceof foliated rocks have been reportedby affectedby any major orogenic imprint. It has been Bridgewater et al. (1974) in a post-orogenic Ketilidian suggested by Key et al. (in press) that the uppercrustal intrusion of rapakivigranite in S Greenland.The phenomenaare responses of thecoherent cratonic intrusionhas a mushroom shape, witha sheet-like plate moving over a hot spot in the underlying mantle. upper part with outcrop areas of 5000 km2 overlying a This initially produced underplating of the sialic crust vertical stem. Foliation is largely limited to the rocks whichled to crustalthickening and upliftlocally re- of thevertical stem, with pronounced deformation lated to extrusivevolcanism. The disturbance of the parallel tothe contacts. A close correspondence is, lowercrust led to the generation of graniticmagma therefore, apparentwith the morphology of the North- which forcedits way to high crustallevels. Hunter ern Sill and the vertical feeder zoneexposed in the (1974) noted that all the early Proterozoic granites of Sephatlhaphatlha Hills. the Kaapvaal Craton were emplaced at depths of less Various theories have been proposed to account for than10 km (5kbar pressure). He also notedthat the mantling on rapakivi feldspars, but none are en- thesegranites have unusual chemical features (trace tirely adequate. Gates (1953) proposed a replacement

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origin, and Tuttle & Bowen (1958) a theory based on ing the early Proterozoic and has the following charac- experimental dataon the feldspar systems. The former teristics. based his theory on observationson the Waupaca 1. Differentiation of themagma mass produceda rapakivi granite, which shows mantled feldspars occur- suite of rocks of interrelatedbut essentially ring in zones quite sharply demarcated from areas of granitic compositions and varying textures. similarrocks lacking such feldspars. He suggested 2. Theshape of the intrusion was controlled by that deformation late in the crystallization history of major structural features in the country rocks. thegranite magma created low pressurezones, to- 3. Emplacementproduced a narrow metamorphic wardswhich migratedsodic material unmixed from aureole and caused crustal warping. This led to alkali feldspar in adjacent zones, replacing and mantl- gravitysliding in adjacentsupracrustal layered ing the crystals there. This has a certain similarity to strata. thetheory proposed for the Gaborone rapakivi 4. Mineralization in the Gaborone Granite is minor feldspars, except that, although late stage deformation and confined to fluorite and some galena in late is also regarded as a controlling factor, themobility of stage quartz reefs. exsolved plagioclase appears to have been essentially The field evidence outlined in this paper indicates limited to the parentcrystals. The hypothesis of Tuttle that the Gaborone Granite is older than the Lobatse & Bowen is based on thefact that at a certain pressure Volcanic Group (Ventersdorp) and most, if not all, of of water vapour the top of the solvusin the binary theTransvaal Supergroup. Re-interpretation of the systemalbite-orthoclase will beintersected and the isotopic data indicates an age of c. 2400 Ma for the feldspars will then crystallize together. Already exist- emplacement of the Gaborone Granite. Therefore, the ing feldspar may then zone either towards orthoclase authors favour the age of c. 2640 Ma for the Venters- or oligoclase. The explanation could not account for dorpproposed by van Niekerk & Burger (1978) as theformation of theGaborone mantled feldspars, opposed to theyounger age of c. 2300 Ma favouredby sincethere is strongevidence for the secondary re- previous authors. Similarly, the Transvaal Supergroup placement origin of the plagioclase. is regarded as c. 2400 Ma, which is older than previous Thinsections of typicalpyterlite and wiborgite estimates. rapakivigranite from Finland have been examined. The pyterlite is closely similar to the Kgale Granite. A ACKNOWLEDGMENTS.Thepaper ispublished with the per- dissimilarity of the Thamaga Granite with the wibor- mission of the Director of the Institute of Geological Sciences gite rapakivi concerned mainly the plagioclase feldspar.(NERC) and the Ministry of Mineral Resources and Water The plagioclase mantles in the latter rock tend to be Affairs of the Government of Botswana. Drs E. J. Cobbing regular and not necessarily in parallel orientation with and D. Mallick commented on earlier drafts of this paper and Mr C. Rundle undertook the re-examination of the isotope the perthitic plagioclase of the core. In some cases, the data. mantle was composed of several anhedral crystals of varying orientation. A primary origin for such plagioclase seems indicated, probably in the manner envisagedSUPPLEMENTARY PUBLICATION. Tables 1-5 (whole-rock anal- by Tuttle & Bowen. yses of the Kanye Volcanics felsites, Ntlhantlhe Microgranite, Kgale Granite andThamaga Granite andcomposition of feldsparphenocrysts in felsitesand granophyres) of Sup- Concluding remarks plementary Publication No. SUP 18037 (6 pages) are obtain- able upon payment from the British Library Lending Divi- TheGaborone Granite is theproduct of highlevel sion, Boston Spa, Wetherby, West Yorkshire LS23 7BQ or emplacement of a magma of granitic composition dur- from the Geological Society Library.

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Received 13 March 1981; revised typescript received 28 July 1981. R. M. KEY,Overseas Division, Institute of GeologicalSciences, Keyworth, Nottingham NG12 5GG. E. P.WRIGHT, Hydrogeology Unit, Institute of GeologicalSciences, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB.

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