Estimation of Temperature, Pressure and Oxygen Fugacity of the Cal-Alkaline Basin-Type Granitoids in the Winneba Area, Ghana

Transcription

1 Research Journal of Environmental and Earth Sciences 4(1): 41-47, 2012 ISSN: Maxwell Scientific Organization, 2012 Submitted: May 09, 2011 Accepted: June 28, 2011 Published: January 01, 2012 Estimation of Temperature, Pressure and Oxygen Fugacity of the al-alkaline Basin-Type Granitoids in the Winneba Area, Ghana 1 Nyarko Saah Esther, 2 D.K. Asiedu, 1 Dampare Samuel, 1 Osae Shiloh, 2 Sakyi Patrick and 1 Hanson John 1 Department of hemistry, Ghana Atomic Energy ommission, P.O. Box LG 80, Legon, Ghana 2 Department of Earth Science, University of Ghana, P.O. Box LG 80 Legon, Ghana. Abstract: In Ghana, the granitoids rocks are in two different groups, thus the basin type granitoid and the belt type granitoids. These granitoids have been studied petrographycally mineralogically and geochemically, especially the belt-type granitoids since it is the major host of gold occurrences in Ghana, however there are little studies on the petrogenesis and emplacement conditions of this important rock. In this paper we present the lacking knowledge on the emplacement conditions (temperature, pressure and the oxygen fugacity) of the basin-type granitoids in the winneba area of Ghana.The calc-alkaline basin-type granitoids in the Winneba area is composed of quartz+ plagioclase +potash feldspar+ alkali feldspar+ Biotite+ Hornblende +amphibole+ titanite. The plagioclase composition ranges from Ab 58 An 1.7 Or 0.45 to Ab 98 An 41 Or 41 and mainly in the field of albite and oligoclase. The amphiboles however have a compositional range of Mg/(Mg +Fe) ranging from 0.52 to 0.62 and a Si content of 7.1 to 7.4 atom per formula unit (afu). Their end-member compositions in the classification diagram are controlled by Magnesio-Hornblend, tschermakitic-hornblend, and tschemakite substitution types. Scanning Electron Microscope (SEM) analyses of coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the assemblage titanite-magnetite-quartz were used to constrain the P, T and fo2 during the crystallization of the parent magmas. The estimated temperature indicated an average temperature of crystallization of 677º reflecting late crystallization from highly oxidized magma (log fo2-20 bars).the rocks were emplaced at an average pressure of 2.2 kbars corresponding to approximately 8 km depth of below the crust. Key words: Amphibole, barometry, crystallization, hornblende, magma, plagioclase, thermometry INTRODUTION Winneba is found in the entral Region of Ghana and is the principal town in the district linked by first class motor road to Accra in the east and Saltpond in the west. (Layton, 1958). The area falls within the Birimian terrene. The terrene is believed to have been accreted onto the Archean continental crust around 2.1 Ga (Taylor et al., 1992) during the Eburnean orogeny. The terrene comprises of metasedimentary and metavolcanic rocks with the former forming a sedimentary basin and the later, a volcanic belt. The volcanic belts are typically km wide and between 60 and 90 km apart. The sedimentary basin show extensive isoclinals folding. (Leube et al., 1990; Leube and Hirdes, 1986). The birimian sedimentary basin is characterized by extensive intrusive activities consisting of granites and pegmaties, contacts between granitiods and metasediments are irregular, with rafts and relict structures of metasediments persisting into the granitoids. Several geochemical studies have been carried out on these granitoids, mainly, mineralogical since they intruded the gold hosting rocks (Birimian rock). However this work seeks to add to the existing knowledge on the birimian granitoids. The aluminum in hornblende barometer, hornblendeplagioclase thermometer and estimation of fo2, are parameters used to calculate pressure, temperature and oxygen fugacity, respectively. The recent application of Al (IV) and Al (tot) in hornblende, as both a geothermometer and geobarometer, respectively, provides new information on the likely temperatures and pressures that exited during the emplacement of the granitic magma within the crust. Hornblende and plagioclase are commonly coexisting minerals in calc-alkaline igneous rocks, so they are usually used for thermometry Based on hornblende solidsolution models and well constrained natural and experimental systems, two hornblende-plagioclase geothermometers (thermometer A and B) were calculated by Holland and Blundy (1994) and Masoudi and Jamshidi (2008). orresponding Author: Nyarko Saah Esther, Department of hemistry, Ghana Atomic Energy ommission, P.O. Box LG 80, Legon, Ghana, Tel.:

2 Fig. 1: Geological map of study area after Layton (1958) Thermometer A is based on the edenite-tremolite reaction (edenite+4 quartz tremolite + albite), which is applicable to quartz-bearing igneous rocks: and thermometer B is based on the edenite-richterite reaction (edenite + albite richterite + anorthite), which is applicable not only to quartz-bearing but also quartz-free igneous rocks. According to Anderson (1996). thermometer B (edenite-richterite) is preferable based on comparison to other igneous thermometers. In this study we analyze for the major element composition of feldspar, plagioclase, biotite and amphilbole minerals in the granitoids, and using thermometer B, estimated for the temperature and pressure of the granitoids during emplacement and further calculated for the oxygen fugacity. Equilibration temperatures for hornblende - plagioclase assemblages were calculated based on iteration using Anderson and Smith (1995) pressure at various thermometrs. The study area: The geological formation of the area falls under the precambian birimian terrain in Ghana. The geology of the area is as shown in Fig. 1. The rocks of the area may be classified as follows from older to younger in ascending succession. Tertiary to Recent (Lagoonal deposite, alluvium, gravels, salt beds, beach and raised beach sands, laterites, Boulder beds, beded and laminated sands, clay of uncertain age), the Togo Series (Quartzites, purple + white facies, breccias, sericite phyllite near base) which is in unconformity with Tarkwaian and the Acids intrusive ( aplites and pegmatites, quartz reefs, series of granites veins quartz, migmatites and the Western gneiss),then, the Post Birimian (Basic intrusive- Dykes now amphibolites, Epidiorites dykes Takwanian), underlain by the Birimian supergroup (Metamorphosed porphyritic and vesicular lavas, quartz Biotite and hornblende schist; hornblend schist, coarse amphibolites, actinolites schites, quartz schist. Gondites, manganiferous phyllite, epidosites and granulites, Mica schist finely laminated. (Ahmed et al., 1977; Layton, 1958). The granites in the area forms part of the acid intrusion in the Birimian. The junction between the granites and the surrounding rocks is a sharp one, though a slight veining of the coarse amphibolites with which it is in contact on the western part of the area exist. The granites as describe by Layton (1958) are porphyroblastic microcline-biotite adamellite. The Western and northern margins of the granite are relatively fine grained and non-porphyroblastic, while the margin at the east show similar characteristics but microcline granite may occur. (Layton, 1958). Apart from linearly arranged Biotite-rich ghosts, there are larger inclusions of hornblende schists pften contorted, finely schistose and cut by irregular feldspar veining. Typically, the porphyroblastic granite of the Winneba area is grey in colour and medium to coarse grained in hand specimen. They contain minerals such as Biotite, microcline, amphibole, feldspar (plagioclase is more abundant than other feldspars). Apatite, sphene, and rare magnetite are also occasionally seen as accessory minerals. (Ahmed et al., 1977; Layton, 1958). 42

3 Or Annite 1.0 Siderophyllite Peraluminous Fe2 /mg+fe Biotite Lepidomelane Meroxene 0.2 alc-alkaline Phlogopite Ab Abl Olig And Lab Fig. 2: Plagioclase chemistry in the Or- Ab- An diagram of Deer et al. (1966) Or An Phlogopite Altot (a.p.f.u) Eastonite Fig. 5: Biotite chemistry plotted in the Al vs. Fe 2+ / (Fe 2+ + Mg) diagram of TrÅger.,(1982) MgO Sanidine A P Ab Mg/(Mg+Fe) 1.0 Anorthoclase Oligoclase Andesine Labradorite Bytownite Anorthite Fig. 3: Alkali- feldspar chemistry in the Or- Ab- An diagram of Deer et al. (1966) Tremolite Tr Hb Actionlite Act Hbl Magnesio- Hbl Tsch Hbl Tschermakite TSI Fig. 4: Diagram showing the classification of amphibole according to the nomenclature of Leake et al. (1997) MATERIALS AND METHODS Sampling: This study was conducted in the Winneba Township about 52 km from Accra the capital town of An FeOtot A12O3 Fig. 6: Biotite composition diagram showing the classification of magmas after Abdel-Rahman (1994) Ghana. Sampling was carried out in September- October These samples were obtained from the outcropped granitiods along the beach and near the John Boscoss School in the Winneba Township. Sampling was problematic though, since there were few fresh unweathered outcrops of granitiods due to the extended settlement to the areas where there were outcrops. However there were relatively fresh once to sample. The weathered surface were broken off to reveal fresh surface before sampling, in addition extra care was taken in order not to sample pegmatites which were in abundant and so similar to the granitiods due to their unusual smaller grain size. Analytical method: Scanning electron microscope: Prior to analysis by Scanning Electron Microscope (SEM), the thin section were washed with deionized water in an ultrasonic bath and coated with carbon film to eliminate electrostatic charge-up on the sample surface. Mineral analyses were performed at the Institute for the Earth s Interior, Okayama University, Misasa, Japan. 43

4 Table 1: Representative SEM plagioclase mineral analyses from the studied basin type granitoids Oxide W-01 W-02 W-03 W-05 W-06 W-07 W-08 W-09 W-10 SiO Al 2 O ao Na 2 O K 2 O Total Number of Ions calculated on 8 oxygen basis Si Al a Na K Molecular Ratio Ab An Or Table 2: Representative SEM amphibole mineral analyses from the studied basin type granitoids Wt % W-01 W-02 W-03 W-04 W-06 W-07 W-08 W-09 WG-10 SiO TiO Al 2 O FeO MnO MgO ao Na 2 O K 2 O Number of Ions calculated on 23 oxygen basis Si Ti Al Fe Mn Mg a Na K Mg/(Mg+Fe 2 ) Table 3: Representative SEM biotite mineral analyses from the studied basin type granitoids Wt % W-1 W-2 W-3 W-4 W-5 W-6 W-7 W-8 W-9 W-10 SiO TiO Al 2 O r 2 O FeO MnO MgO ao Na 2 O K 2 O P 2 O Total Number of Ions on the basis of 11 oxygens Si Ti Al r Fe Mn Mg a Na K P Total Fe/Mg+Fe ASI

5 Table. 4: Result of geothermobarometry and oxygen fugacity for the basin type granitoids studied compared to other geothermobarometry Sample ID Plag Ab Amph Al(T) T-HB(ed-tr) (º) T-HB(ed-ri) (º) T-BH (º) Pschmidt (kb) P-A&S (Kbar) LogfO2 WG WG WG WG WG WG WG WG-09A WG-09B Back-Scattered Electron (BSE) images and major element compositions of all mounted grains were obtained using a HITAHI S-3100H Scanning Electron Microscope coupled with an EMAX-7000 Energy Dispersive X-ray Spectrometer, which operated at an accelerating voltage of 20 kv and 0.3 na beam current. orrection for the major element compositions was done by the standardless ZAF correction method of the EMAX-7000 program (HORIBA Ltd., Ver. 1.32). Analytical points on the mounted grains were pre-recorded using a Visual Stage system that was developed to support the probe analyses using the SEM-EDX and SIMS with extremely high spatial resolution. hemical compositions and structural formulae of feldspars, amphibole, biotite, are listed in Table 1-3. The structural formula for the mineral chemistry was calculated on the basis of 8, 11, 23, and 4 oxygen for feldspar (K-feldspar and plagioclase), Biotite, amphibole, respectively. RESULTS AND DISUSSION Mineral chemistry: feldspars: Representative analyses of plagioclase and their calculated formulae are given in Table 1. The plagioclase composition ranges from Ab 58 An 1.7 Or 0.45 to Ab 98 An 41 Or 41. In the Deer et al. (1966) classification diagram for plagioclase and alkali feldspar (Fig. 2 and 3) the rocks plotted mainly in the albite and oligoclase field with only one in the Andesine filed and also mainly along the albite- oligoclase boundary with only one along the Andesine- orthoclase boundary respectively. The range of the plagioclase composition is necessary for the Al content in hornblende to be solely a function of pressure (Hollister et al., 1987). Amphibole: Amphibole compositions of the granitoids were determined and representative analyses together with their chemical formulae are given in Table 2. Amphiboles in the granitoids are of igneous origin, as their Si values do not exceed the 7.50 a.p.f.u. (atoms per formula unit) of the limit for igneous amphiboles (Leake, 1971; Leake et al., 1997). Amphiboles have a compositional range of Mg/ (Mg+Fe) ranging from 0.52 to 0.62 and a Si content of 7.1 to 7.4 atom per formula unit (afu). Their end-member compositions in the classification diagram (Fig. 4) are controlled by Magnesio-Hornblen, tschermakitic-hornblend, and tschemakite substitution types. Biotite: Biotite has relatively high Al and moderate Fe contents (Djouka-Fonkwe et al., 2008) and Microprobe analyses of the biotite from the basin type granitoids Table 3. indicate compositions of meroxenes and lepidomelanes (Fig. 5), following the classification of Troger (1982) with an intermediate Al content. The most important feature of biotites is that they are Mg-rich (Helmy et al., 2004). The range of molar Fe 2 / (Fe 2+ Mg) of the biotite composition is high (57-64) and the alumina saturation index (ASI = Al/a+Na+K,) is moderate ( ) and reflects some extend of alumina activity in the crystallizing magma (Zen, 1988). The progressive increase of the total Al contents and the Fe 2+ / (Fe 2+ + Mg) ratios in biotite (Fig. 5) are consistent with a compositional trend of Biotite in continental-collision related granites (Lalonde and Bernard, 1993). It indicates contribution of crustal material (metasediments) during the petrogenesis of the basin type granitoids magmas, either by assimilation or anatexis (Shabbani and Lalon-de, 2003 and references therein). The biotite compositions systematically range from the calc-alkaline to the peraluminous granitic of the discriminative trend defined by Abdel-Rahman (1994) among the different granitoid suites (Fig. 6). Temperature and pressure: Hornblende and plagioclase are both used to estimate temperature and pressure of granitic rocks respectively. The Al content provides information on the likely temperatures and pressures that exited during the emplacement of the granitic magma within the crust and apparently the depth of emplacement of plutonic rocks. There are several empirical Al-in-hornblende barometers that have been used to determine solidus pressures in calc-alkaline plutons (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989). The most recent barometer (Anderson, and Smith. 45

6 1995) was chosen in this study because of the smaller margin of error and the correction of effect of temperature and pressure in the equation: P (±0.6) kbar = 4.76 Al {[T(/)-675]/85}* {0.530Al [T(/)-675]} where, P is in kbar, T in º and Al is the total Al content of hornblende in atoms per formula unit. For estimation of temperature in rocks, Blundy and Holland (1990) first proposed a very simple, empirical thermometer on the basis of the edenite-tremolite reaction; which could be applied only to quartz-bearing, intermediate to felsic igneous rocks with plagioclase An<0.92 and Si in hornblende<7. 8 atoms pfu. In this study, Holland and Blundy (1994) thermometer for the calculation of temperature is used and compared with other thermometers. Table 4 shows the types of thermometers and barometers applied for the estimation of the temperature and pressure. From the analyzed rocks, the calc-alkaline basin type granitoids, crystallized at a temperature range of º indicating a late crystallization. The emplacement pressure was estimated at 1.3± ±0.6 kbars. The pressure was converted to depths by using an average crustal density of 2.65 g/cm 3 and average pressure of 2.2±0.6 kbars. The average depth of emplacement of granitoids is approximately 8 km. T-HB(ed-tr) refers to Holland and Blundy Hbld-Plag thermometry calibration reaction edenite + 4 quartz = tremolite + albite T-HB(ed-ri) refers to Holland and Blundy Hbld-Plag thermometry calibration reaction edenite + albite = richterite + anorthite T-BH refers to Blundy and Holland Hbld-Plag thermometry calibration reaction edenite + 4 quartz = tremolite + albite P-Sch refers to Schmidt (1992) Al-in Hornblende barometer P-A and S refers to Anderson and Smith (1995) Al-in Hornblende barometer Oxygen fugacity: The oxygen fugacity of magma is related to its source material, which in turn depends on tectonic setting. Sedimentary-derived granitic magmas are usually reduced, while I-type granites are relatively oxidized. It is difficult to estimate the original oxygen fugacity of primary magmas from the study of granitoids, as magnetite usually becomes Ti free during slow cooling and ilmenite undergoes one or more stages or oxidation and exsolution (Haggerty, 1976). However, some inferences on the oxidation state of magma can be made using the rock mineral assemblage and mineral chemistry. The occurrence of Mg-rich, pargasitic, magnesiohornblende (Fig. 4) and Fe 2+ biotite in the rocks suggest relatively oxidized magma. According to Wones (1989), the assemblages of titanite + magnetite + quartz in granitic rocks permit an estimation of relative oxygen fugacity. The LogfO2 estimated bases on Wones (1989) equilibrium expression of LogfO 2 = /T (P-1)/T (where, T is temperature in Kelvin and P is pressure in bars) gave logfo 2 in the limit between to which shows that the calc-alkaline, peraluminous, and K-rich magma crystallized in low oxygen fugacity (fo 2 ). ONLUSION The application of Al in hornblend barometry of Anderson and Smith (1995) indicated a minimum pressure of ~2.12±0.6 kbars and a maximum of 2.9±0.6 kbars for the basin-type intrusion. The average pressure of the magma corresponded to approximately 8km depth of emplacement using an average crustal density of Temperature estimates based on the hornblendeplagioclase geothermometer of Blundy and Holland (1990) is º which probably reflect latestage crystallization from a highly oxidized magmas (log fo to 17.6). AKNOWLEDGMENT We also acknowledge the logistics, field assistance and laboratory assistance of Mr. Samuel Ganyaglo and Mr. Opata and the entire staff of the Department of Nuclear Engineering and mathematical science of the institute of Ghana Nuclear Research center. REFERENES Abdel-Rahman, A., Nature of biotites from alkaline, calcalkaline and peraluminous magmas. J. petrol., 35(2): Ahmed, S.M., P.K. Blay, S.B. astor and G.J. oakley, The Geology of ¼ fields Sheets No.33, Winneba N.E, 59, 61 and 62, Accra S.W. N.W and N.E. Ghana. Geol. Survey Bull., 32: pp. Anderson, J.L., Status of thermobarometry in granitic batholiths trans royal soc edinburgh. Earth Sci., 87: Anderson, J.L. and D.R. Smith, The effects of temperature and fo2 on the Al-in hornblende barometer. Am. Mineral., 80: Blundy, J.D. and T.J.B. Holland, alcic amphibole equilibria and a new amphibole-plagioclase geothermometer. ontrib. Min. Petr., 104: Deer, W.A., R.A. Howie and J. Zussman, An Introduction to Rock-Forming Minerals. 17th Edn., Longman Ltd., pp:

7 Djouka-Fonkwe, M.L., B. Schulz, U. Schüssler, J.P. Tchouankoué and. Nzolang, Geochemistry of the Bafoussam Pan-African I- and S-type granitoids in western ameroon. J. Afr. Earth Sci., 50: Haggerty, S.E., Opaque mineral oxides in igneous rocks. In: Rumble, D. (Ed.), Oxide Minerals, P. Hgl0t-300. Mineralogical Society of America Short ourse Notes, Hammarstrom, J.M. and E. Zen, Aluminum in hornblende: An empirical igneous geobarometer. Ameri. Mineral., 71: Helmy, H.M. A.F. Ahmed, M.M. Mahallawi and S.M. Ali-El, Pressure, temperature and oxygen fugacity conditions of calc-alkaline granitoids, Eastern Desert of Egypt, and tectonic implications. J. Afr. Earth Sci., 38: Holland, T. and J. Blundy, Non-ideal interactions in calcic amphiboles and their bearing on amphiboleplagioclase thermometry. ontrib. Mineral. Petrol., 116: Hollister, L.S., G.. Grissom, E.K. Peters, H.H. Stowell and V.B. Sisson, onfirmation of the empirical correlation of Al in hornblende with pressure of solidification of calc-alkaline plutons. Am. Mineral., 7(2): Johnson, M.. and M.J. Rutherford, Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley aldera (alifornia) volcanic rocks. Geol., 17: Lalonde, A.E. and P. Bernard, omposition and colour of biotite from granites: two useful properties in the characterization of plutonic suites from the Hepburn internal zone of the Wopmay orogen, Northwest Territories. an. Mineral., 31: Layton, W., The Geology of 1 º/4 field sheet No.32. Latitude 5º 15-5º 30 N. Longitude 0 º 30-0 º45 W. Ghana geol. survey bull., 24: pp. Leake, B.E., On aluminous and edenitic amphiboles. Mineral. Magazine, 38: Leake, B.E., J.. Schumacher, D.. Smith, N..N. Stephenson, L. Ungaretti, E.J.W. Whittaker and G. Youzhi, Nomenclature of amphiboles. Eur. J. Mineral., 9: Leube, A. and W. Hirdes, The Birimian Supergroup of Ghana: Depositional Environment, Structural Development and onceptual Model of an Eairly Proterozoic Suites Bundesanstalt fur Geowissenschaften and Rohstoffe, Hannover, pp: 260. Leube, A., W. Hirdes, R. Mauer and G.O. Kesse, The Early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization. Precambrian Res., 46: Masoudi, F. and M. Jamshidi Badr, Biotite and hornblende composition used to investigate the nature and thermobarometry of pichagchi pluton, northwest sanandaj-sirjan metamorphic belt, iran. J. Sci. Islamic Republic Iran, 19(4): Schmidt, M.W., Amphibole composition in tonalite as a function of pressure: An experimental calibration of the Al-in hornblende barometer. ontrib. Mineral. Petrol., 110: Shabbani, A.T. and A. Lalonde, omposition of Biotite from Granitic Rocks of the anadian Appalachian: A potential tectonomagmatic indicator? an. Mineral., 41: Taylor, P.N., S. Moorbath, A. Leube and W. Hirdes, Early Proterozoic crustal evolution in the Birimian of Ghana: onstraints from geochronology and isotope geology. Precambrian Res., 56: Troger, W.E., Optische Bestimmung Der Gesteinsbildenden Minerale, Teil 2. Scheweizerbartsche Verlagsbuchhandlung, Stuttgart, pp: 822. Wones, D.R., Significance of the assemblage titanite+magnetite +quartz in granitic rocks. Ameri. Mineral., 74: Zen, E., Phase relations of peraluminous granitic rocks and their petrogenetic implications. Annual Rev., Earth Planetary Sci., 16: