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The Genesis of the Gaborone Rapakivi Granite Complex in Southern Africa

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The Genesis of the Gaborone Rapakivi Granite Complex in Southern Africa 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. The Gaborone 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 Archaean gneisses, 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 110 R. M. Key and E. P. Wright 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 felsites may attain 1300m, whereas nature of the magmatic differentiation. the granophyres and leucocratic granite are appreci- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 Genesis of the Gaborone rapakivi granite complex 111 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 112 R. M. Key and E. P. Wright 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-called Kanye 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 Genesis of the Gaborone rapakivi granite complex 113 a MOLAPO WA BOJANG PLUTON 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). E 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). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/139/2/109/4887780/gsjgs.139.2.0109.pdf by guest on 26 September 2021 114 R. M. Key and E. P. Wright 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 bodies resembling 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
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