岡山大学 地球科学研究報告
Published by Okayama University Earth Science Reports Editorial Committee

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

Samuel Dampare Graduate School of Natural Science and Technology, Okayama University
柴田 次夫 Department of Earth Sciences, Okayama University
Daniel Asiedu Department of Geology, University
Shiloh Osae National Nuclear Research Institute, Ghana Atomic Energy Commission
抄録
The Pateoproterozoic metavolcanic rocks of the southern Ashanti greenstone belt of Ghana are intruded by three major suites of granitoids, 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 Pluton consists of granitic to dioritic rocks, which are generally massive but occasionally display alignment of ferromagnesian minerals. The rocks contain mainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration of amphibole 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 composoition, metaluminous (ASI<1) and have I-type characteristics. The granitoids have the SiO2 content of 63-70% ; total iron, as Fe2 O3 of 3.10-5.80% ; (Na2O+K2O) content of 5.01-6.96% and Na2O/K2O ratios from 1.34 to 2.70 ; and are characterized by Mg# ranging from 53 to 48. The Fe*(=FeOtot/FeOtot+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+FeOtot) coupled with low molar AI2O3/(MgO+FeOtot) may suggest their derivation from partial melting of metabasaltic to metatonalitic sourcc, with a possible contribution from metagreywacke, but preclude any contribution from metapelitic sources. The Birimaian metavolcanic rocks are the likely source material candidate for the rocks. CIPW norm calculations yielded a crystallization temperature of~650-685℃ and a pressure of 4-7kb for the rocks, suggesting a lower crustal souree. The Prince's Town plutonic rocks also show characteristice of plutons emplaced in a volcanic are tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from other parts of the southern Ashanti greenstone belt c and the belt-type granitoids of Ghana as a whole.
キーワード
Geochemistry
tectonic setting
granitoids
Birimian
Ghana
ISSN
1340-7414
NCID
AN10506284
NAID