Major-element geochemistry of Proterozoic Prince ' s Town granitoid from the southern Asbanti volcanic belt ,

The Paleoproterozoic metavolcanic rocks ofthe southern Ashanti greenstone belt ofGhana are intruded by three major suites ofgranitoids, locally called Prince's Town. Dixcove and Ketan plutons. The Prince's Town pluton is the largest intrusive body in the Axim area, and tends to separate the Axim volcanic branch from the Cape Three Points branch. The plutonconsists ofgranitic to dioritic rocks, which are generallymassive but occasionally display alignment offerromagnesian minerals. The rocks containmainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration ofamphibole and biotite to epidote and chlorite is common. Accessory minerals include apatite, sphene and zircon. The geochemical data indicate that the rocks are tonalitic to granodioritic in composition, metaluminous (ASI < 1) and have I-type characteristics. The granitoids have the Si02 contentof63-70%; total iron. as FepJ of3.lo-S.80%; (Nap+~O) content of5.01-6.%% andNap~Oratios from 1.34 to 2.70; andarecharacterlzed byMg#ranging from 53 to 48. The Fe· (=FeOIQ/{FeO,,,,,+MgO) and modified alkali-lime index (MALI) of the rocks indicate that the Prince's Town pluton is dominantly magnesian and calcic in nature. Higher values in molar CaO/(MgO+Fe0 lol ) coupled with low molar AlPl (MgO+FeO IOl ) may suggest their derivation from partialmelting ofmetabasaltic to metatonaliticsource, with apossible contributionfrom metagreywacke, but preclude any contribution from metapelitic sources. The Birimianmetavolcanic rocks are the likely source material candidate for the rocks. CIPW nOlll1 calculations yieldeda CI)'stallization temperature of650-685°C and a pressure of 4-7kb for the rocks, suggesting a lower crustal source. The Prince's Town plutonic rocks also show characteristics of plutons emplaced in a volcanic arc tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from otherparts ofthe southernAshanti greenstone belt c andthe belt-type granitoids ofGhana as awhole.

[1]  Calvin G. Barnes,et al.  A Geochemical Classification for Granitic Rocks , 2001 .

[2]  C. Pin,et al.  Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Quérigut Complex, French Pyrenees , 2000 .

[3]  R. Klemd,et al.  The metamorphic evolution of the Paleoproterozoic (Birimian) volcanic Ashanti belt (Ghana, West Africa) , 1999 .

[4]  M. Norman,et al.  Generation of Tonalite and Trondhjemite by Subvolcanic Fractionation and Partial Melting in the Zarza Intrusive Complex, Western Peninsular Ranges Batholith, Northwestern Mexico , 1999 .

[5]  W. R. Schmus,et al.  Geochemistry, Nd and Sr isotopes, and U/Pb zircon ages of granitoid and metasedimentary xenoliths from the Navajo Volcanic Field, four corners area, southwestern United States , 1999 .

[6]  J. Whalen,et al.  Geochemical and Nd isotopic evolution of Paleoproterozoic arc-type granitoid magmatism in the Flin Flon Belt, Trans-Hudson orogen, Canada , 1999 .

[7]  D. Davis,et al.  Two generations of Birimian (Paleoproterozoic) volcanic belts in northeastern Côte d'Ivoire (West Africa): consequences for the ‘Birimian controversy’ , 1996 .

[8]  V. Gardien,et al.  Experimental melting of biotite + plagioclase + quartz ± muscovite assemblages and implications for crustal melting , 1995 .

[9]  P. Wyllie,et al.  Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time , 1994 .

[10]  P. Sylvester,et al.  Lithostratigraphy and Composition of 2.1 Ga Greenstone Belts of the West African Craton and Their Bearing on Crustal Evolution and the Archean-Proterozoic Boundary , 1992, The Journal of Geology.

[11]  S. Moorbath,et al.  Early Proterozoic crustal evolution in the birimian of Ghana: constraints from geochronology and isotope geochemistry , 1992 .

[12]  W. Hirdes,et al.  The structural development of the early Proterozoic Birimian and tarkwaian rocks of southwest Ghana, West Africa , 1992 .

[13]  D. Davis,et al.  Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating , 1992 .

[14]  F. Albarède,et al.  Crustal growth in West Africa at 2.1 Ga , 1992 .

[15]  D. Lindsley,et al.  Occurrence of iron-titanium oxides in igneous rocks , 1991 .

[16]  F. Albarède,et al.  A major 2.1 Ga event of mafic magmatism in west Africa: An Early stage of crustal accretion , 1990 .

[17]  R. Batchelor,et al.  Petrogenetic interpretation of granitoid rock series using multicationic parameters , 1985 .

[18]  A. Tindle,et al.  Trace Element Discrimination Diagrams for the Tectonic Interpretation of Granitic Rocks , 1984 .

[19]  D. Wood,et al.  A RE-APPRAISAL OF THE USE OF TRACE ELEMENTS TO CLASSIFY AND DISCRIMINATE BETWEEN MAGMA SERIES ERUPTED IN DIFFERENT TECTONIC SETTINGS , 1979 .

[20]  Julian A. Pearce,et al.  Tectonic setting of basic volcanic rocks determined using trace element analyses , 1973 .

[21]  H. Furnes,et al.  Geochemistry and Petrogenesis of the Neoproterozoic Granitoids in the Central Eastern Desert, Egypt , 2002 .

[22]  E. Hegner,et al.  High-potassium, calc-alkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany) , 2000 .

[23]  Columbus,et al.  STRUCTURAL GEOLOGY OF THE OHIO SHALE , 1996 .

[24]  M. J. Bas.  Using Geochemical Data: Evaluation, Presentation, Interpretation , 1994 .

[25]  J. Whalen.  Geology, petrography, and geochemistry of Appalachian granites in New Brunswick and Gaspésie, Quebec , 1993 .

[26]  D. Pohl.  A plate tectonic re-interpretation of the 2.2-2.0 Ga Birimian supergroup, Tarkwaian system and metallogenesis in West Africa , 1992 .

[27]  G. Kesse,et al.  The mineral and rock resources of Ghana , 1985 .

[28]  F. Barker.  Trondhjemite: Definition, Environment and Hypotheses of Origin , 1979 .

[29]  R. Burton.  Gold on the Gold Coast , 1882 .