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Table 4 (continued) Tarkwaian Eburnean intrusions Eoeburnean intrusions Lithology Phyllites Sandstones Conglomerates Dolerite (sills) Granite Granodiorite Granodiorite Tonalite Mineralogy chl, ms, bt, gph, st, qz, mgt, bt qz, Birimian and cpx, hbl, pl, chl, bt, bt, ms, qz, pl, kfs pl, qz, bt, chl, ep gt Granitoid pebbles gt, cal Susceptibility range (10−3 SI) 0.08–0.16 0.08–0.32 0.13–13.10 No data 0.06–38.00 0.05–8.06 Sample GH397 GH512 No Sample GH395 GH237 GH507 U (ppm) 4.500 0.880 No data 0.740 0.650 0.110 Th (ppm) 9.290 3.580 No data 2.220 1.510 0.270 K20 (%) 2.640 1.050 No data 0.140 2.120 0.170 Map Airborne magnetic response Airborne radiometric response Moderate intensity layers High U High Th Medium K (white-yellow) Low intensity layers Medium U High Th Medium K (green-yellow) Moderate to high intensity layers Low U Low Th Medium K (dark red) Moderate to low intensity layers Medium U High Th Low K (light green) Moderate to high intensity bodies Low U Low to medium Th Medium K (dark red) Low intensity bodies Strong magnetic fabric (parallel to bt. alignment) Low U Low to medium Th Low K (dark red-green) ank, ankerite; bt, biotite; cal, calcite; chl, chlorite; cpx, clinopyroxène; dol, dolomite; ep, epidote; gph, graphite; gt, garnet; hbl, hornblende; mgt, magnetite; ms, muscovite; ol, olivine; opx, orthopyroxene; pl, plagioclase; qz, quartz; ser, sericite; st, staurolite. S. Perrouty et al. / Precambrian Research 204– 205 (2012) 12– 39 21
22 S. Perrouty et al. / Precambrian Research 204– 205 (2012) 12– 39 Fig. 4. Image of the gamma-ray data draped over a shaded digital elevation model. This image shows the relative proportion of potassium, thorium and uranium as an RGB image, and their correlation with the topography. Pixel size is 100 m. U and Th rich areas with high topography correspond to Sefwi Group metavolcanics as around Cape Three Points and proximal to the Ashanti Fault. K-rich granitoids are well defined in the eastern part of the area. gravity data can only be used to broadly differentiate regional rock packages and image the continuity of the major structures at depth. 3.2.4. ALOS PALSAR data Advanced Land Observing Satellite; Phased Array type L-band Synthetic Aperture Radar high-resolution data (ALOS PALSAR data) collected in 2009 were used to image structure within lithological packages. Two polarisation channels (HH and HV) (available from http://www.eorc.jaxa.jp/ALOS/en/) were gridded with a resolution of 50 m. A ternary RGB image was then produced (Fig. 6), where red, green, blue colours correspond respectively to the HH, HV and (HH + HV/2) polarisations. These radar data are able to image structural details within sedimentary units such as bedding, with better resolution than the combined radiometric and digital elevation model. 3.3. Petrophysics Petrophysical data were collected during regional reconnaissance, resulting in a database of over 300 susceptibility measurements of the major lithologies encountered in the Ashanti region. The data provided quantitative information on the magnetic susceptibility of the Birimian and Tarkwaian units and intrusives that were used to constrain the regional magnetic interpretation and subsequent geophysical modelling. These susceptibility data are consistent with common susceptibility range proposed by Clark (1997) and with values measured by Metelka et al. (2011) in Burkina Faso. The histograms in Fig. 7 show the range of measured magnetic susceptibilities for the lithologies seen in the study area using a Geofyzika KT-5 Kappameter hand held susceptibility meter. To ensure high quality measurements, some representative samples were re-measured using a Kappabridge KLY-3 magnetic susceptibility meter and no statistically significant differences were found. Natural remanent magnetisation (NRM) of the dolerite dykes was measured using AGICO JR5A spinner magnetometer, mounted within Helmholtz coil systems. Paleomagnetic laboratory experiments were carried out at the GET Magnetism Laboratory (Toulouse). Density data were acquired for 26 samples taken from all of the outcropping regional units. Calculated values derived from sample weighing in air and water (Table 5) agree with measurements made by Hasting (1978, unpublished report referenced in Barritt and Kuma (1998) and with equivalent samples from Burkina Faso (Baratoux et al., 2011).
Page 1 and 2: Precambrian Research 204- 205 (2012
Page 3 and 4: Table 1 Comparison between the tect
Page 5 and 6: 16 S. Perrouty et al. / Precambrian
Page 7 and 8: Table 3 Synthesis of structural and
Page 9: Table 4 Summary table of the lithol

Revised Eburnean geodynamic evolution of the ... - Tectonique.net

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