@phdthesis{DjoukaFonkwe2005,
  author    = {Djouka-Fonkw{\´e}, Merline Laure},
  title     = {Association of S-type and I-type granitoids in the Neoproterozoic Cameroon orogenic belt, Bafoussam area, West Cameroon : geology, geochemistry and petrogenesis},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-14526},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2005},
  abstract  = {The Bafoussam area in west Cameroon is located within the Cameroon Neoproterozoic orogenic belt (north of the Congo craton) which is part of the Central African Fold Belt (CAFB).The evolution of the CAFB is related to the collision between the convergent West African craton, the S{\~a}o Francisco - Congo cratons and the Sahara Metacraton. The outcrop area stretches over a surface of ~1000 km2 and dominantly consists of granitoids which intruded wall-rocks of gneiss and migmatite during the Pan-African orogeny. The Bafoussam granitoid emplacement was influenced by the N 30 °E strike-slip shear zone in the prolongation of the Cameroon Volcanic Line, but also by the N 70 °E Central Cameroon Shear Zone. In the field, these two shear directions are expressed in the schistosity and foliation trajectories, fault orientation and the alignment of the volcanic cones as well. In the Bafoussam area, four types of granitoids can be distinguished, including: (i) the biotite granitoid, (ii) the deformed biotite granitoid, (iii) the mega feldspar granitoid, and (iv) the two-mica granitoid. These granitoids occur as elongated plutons hosting irregular mafic enclaves (amphibole-bearing, biotite-rich, and metagabbroic types) and are frequently cut by late pegmatites, aplite dykes and quartz veins. Petrographically, they range in composition from syenogranite (major), alkali-feldspar granite, granodiorite, monzogranite, quartz-syenite, quartzmonzonite to quartz-monzodiorite. Potassium feldspar, quartz, plagioclase and biotite are the principal phases, in cases accompanied by amphibole and accessory minerals such as apatite,zircon, monazite, titanite, allanite, ilmenite and magnetite. Sericite, epidote and chlorite are secondary minerals. In addition, the two-mica granitoid contains primary muscovite and sometimes igneous garnet. In the granitoids, potassium feldspar is orthoclase (microcline and orthoclase: Or81-97Ab19-3), and plagioclase is mainly oligoclase with some albite and andesine (An3-35Ab96-64).Biotite is Fe-rich (meroxene and lepidomelane, with some siderophyllite), having high Fe2+/(Fe2+ + Mg) ratios of 0.40-0.80. It is a re-equilibrated primary biotite and suggests calc-alkaline and peraluminous nature of the host granitoids. Amphibole is edenitic and magnesian hastingsitic hornblende, with high Mg/(Mg + Fe2+) ratios of 0.50-0.62. The evolution of the hornblende was dominated by the edenitic, tschermakitic, pargasitic and hastingsitic substitution types. Primary muscovite is iron-rich [Fe2+/(Fe2+ + Mg) = 0.52-0.82] and has experienced celadonite and paragonite substitutions. Igneous garnet is almandine-spessartine (XFe = 0.99 and XMn = 0.46-0.56). The euhedral grain shapes of garnet crystals and the absence of inclusions coupled with the high Mn and Fe2+contents (2.609-3.317 a.p.f.u and 2.646-3.277 a.p.f.u,respectively) and low Mg contents (0.012-0.038 a.p.f.u) clearly point to its plutonic origin. The Mn-depletion crystallization model is suggested for the origin of the analyzed garnet, i.e. initial crystallization of garnet inducing early decrease of Mn in the original melt. Aluminum-in-hornblende and phengite barometric estimates show that the granitoids crystallized at 4.2 ± 1.1 to 6.6 ± 1.0 kbar, corresponding to emplacement depths of 15-24 km.Zircon and apatite saturation temperature calibrations and hornblende-plagioclase thermometry yielded emplacement temperatures between 772 ± 41 and 808 ± 34 °C. Except the two-mica granitoid, the titanite-magnetite-quartz assemblage gives oxygen fugacities ranging from 10-17 to 10-13, suggesting that the granitoids were produced by an oxidized magma. Since the twomica granitoid lacks magnetite, it was originated from a magma under reducing conditions, below the quartz-fayalite-magnetite buffer. Fluid inclusions in quartz from hydrothermal veins are secondary in nature and are found in trails along healed microcracks or in clusters. Two types of fluid inclusion have been recognized, mixed aqueous-non-aqueous volatile fluid inclusions subdivided into aqueous-rich mixed and non-aqueous volatile-rich mixed fluid inclusions, and pure aqueous fluid inclusions.The non-aqueous volatile-rich mixed fluid inclusions are one-, two-, or three-phase inclusions, whereas the aqueous-rich mixed fluid inclusions are exclusively three-phase inclusions. Both have similar low to moderate salinities (1 to 10 equiv. wt. \%). The total homogenization temperatures of the aqueous-rich mixed fluid inclusions are slightly lower than those of the nonaqueous volatile-rich mixed fluid inclusions, ranging from 150 to 250 °C and 170 to 300 °C,respectively. They contain nearly pure CO2, or CO2 with addition of 4.1-13.5 mole \% CH4 as volatile constituents. Pure aqueous fluid inclusions are two-phase with lower total homogenization temperatures (130-150 °C) and salinities ranging from 3 to 8 equiv. wt. \%. They display mixing salt system characteristics, having NaCl as the dominant salt and considerable amounts of other divalent cations. Aqueous-rich mixed fluid inclusions and pure aqueous fluid inclusions exhibit a low geothermal gradient value of 18 °C/km, whereas the non-aqueous volatiles-rich mixed fluid inclusions have a high density which correspond to high geothermal gradient of 68 °C/km. The studied granitoids are intermediate to felsic in compositions (56.9-74.6 wt. \% SiO2)and have high contents of alkalis K2O (1.73-7.32 wt. \%) and Na2O (1.25-5.13 wt. \%) but low abundances in MnO (0.01-0.20 wt. \%), MgO (0.10-3.97 wt. \%), CaO (0.37-4.85 wt. \%), P2O5(up to 0.90 wt. \%). They display variable contents in TiO2 (0.07-0.91 wt. \%), Fe2O3* (total Fe = 0.96-7.79 wt. \%) and Al2O3 (12.0-17.6 wt. \%) contents. The granitoids show a wide range of high-field-strength elements (HFSE) and large ion lithophile elements (LILE) contents, with felsic granitoids being enriched in HFSE and the intermediate granitoids displaying in contrast high LILE concentrations. They exhibit chemical characteristics of non-alkaline to mid-alkaline, alkali-calcic, calc-alkaline, K-rich to shoshonitic, ferriferous affinities. Chondrite-normalized rare earth element (REE) patterns are characterized by a strong enrichment in light compared to heavy REEs [(La/Sm)N = 3.23-9.65 and (Ga/Lu)N = 1.45-5.54, respectively], with small to significant negative Eu anomalies (Eu/Eu* = 0.28-1.08). Ocean ridge granites (ORG)normalized multi-elements spidergrams display typical collision-related granites pattern, with characteristic negative anomalies of Ba, Nb and Y, and positive anomalies in Rb, Th and Sm. The granitoids under study are genetically I-type granitoids (biotite granitoid, deformed biotite granitoid and mega feldspar granitoid) and one S-type granitoid (two-mica granitoid). The I-type granitoids are metaluminous (ASI: 0.70-1.00) or moderately peraluminous if highly fractionated (ASI: 1.01-1.06). The geochemistry and petrological features of these I-type granitoids argue for close genetic relationships and it is suggest that they originated from a single parent magma. The observed variability in mineralogy and major and trace element compositions in these granitoids are then the reflection of the fractional crystallization that evolved separation of plagioclase, biotite, K-feldspar and accessory minerals at the level of emplacement. The two mica S-type granitoid is exclusively peraluminous (ASI: 1.07-1.25) and classified as a peraluminous leucocratic granitoid or leucogranite. It is marked in its CIPW normative composition by the permanent presence of corundum, ranging between 0.12 and 3.03. The Bafoussam granitoids were emplaced in a syn- to post-collisional tectonic environment. The observed deformational features and the concentrations in Y, less than 40 ppm, confirm that they are related to an orogenesis. Whole-rock Rb-Sr isochrons defines an igneous crystallization ages of 540 ± 27 Ma for the biotite granitoid and 587 ± 41 Ma for the mega feldspar granitoid. These ages fit with the range of Pan-African granitoid ages (650-530 Ma) in West Cameroon and correspond to the Pan-African D2 deformation event in the Neoproterozoic Cameroon orogenic belt. The two-mica granitoid yields an older Rb-Sr isochron age of 663 ± 62 Ma which is considered to be probably a mixing age. The Nd-Sr isotopic compositions indicate that the I-type granitoids have been produced by partial melting of a tonalite-granodiorite source in the lower crust. This is supported by their initial 87Sr/86Sr(600 Ma) ratios (0.705-0.709) and by their WNd(600 Ma) values (0.2 to -6.3, mainly < 0). The two-mica granitoid was generated by partial melting of a greywacke-dominated source involving biotite-limited, biotite dehydration melting. Chemical data of the two-mica granitoid that support this hypothesis are low CaO/Na2O (0.11-0.38) and Sr/Ba (0.20-0.30), the high Rb/Sr (2.26-7.00), the high initial 87Sr/86Sr(600 Ma) ratios ranging from 0.708 to 0.720, the large range in Al2O3/TiO2 (47-204) and the negative WNd(600 Ma) values (-9.9 to -14.0). Moreover,the higher initial 87Sr/86Sr(600 Ma) ratios of the two-mica granitoid are consistent with an upper crust origin. The depleted mantle Nd model ages (TDM) of 1.3-2.3 Ga indicate that the studied granitoids originated by partial melting of Paleoproterozoic and Mesoproterozoic crust, with limited mantle-derived magma contribution. The high initial 87Sr/86Sr(600 Ma) ratios of these granitoids coupled with the wide negative WNd(600 Ma) values strongly suggest a very long residence time in the crust of their protoliths before the melting event. The petrologic signatures of the Bafoussam granitoids are similar to those described in other Pan-African belts of western Gondwanaland such as the neighbouring provinces of Nigeria and the Central African Republic, as well as in the Borborema Province of northeastern Brazil. This supports the previous hypothesis that the Central African fold Belt including Cameroon, Nigeria and the Central African Republic provinces has a continuation in Brazil.},
  subject      = {Kamerun <West>},
  language  = {en}
}
@phdthesis{Drueppel2003,
  author    = {Dr{\"u}ppel, Kirsten},
  title     = {Petrogenesis of the Mesoproterozoic anorthosite, syenite and carbonatite suites of NW Namibia and their contribution to the metasomatic formation of the Swartbooisdrif sodalite deposits},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-6987},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2003},
  abstract  = {During the Mesoproterozoic large volumes of magma were repeatedly emplaced within the basement of NW Namibia. Magmatic activity started with the intrusion of the anorthositic rocks of the Kunene Intrusive Complex (KIC) at 1,385-1,347 Ma. At its south-eastern margin the KIC was invaded by syenite dykes (1,380-1,340 Ma) and younger carbonatites (1,140-1,120 Ma) along ENE and SE trending faults. Older ferrocarbonatite intrusions, the 'carbonatitic breccia', frequently contain wallrock fragments, whereas subordinate ferrocarbonatite veins are almost xenolith-free. Metasomatic interaction between carbonatite-derived fluids and the neighbouring and incorporated anorthosites led to the formation of economically important sodalite deposits. Investigated anorthosite samples display the magmatic mineral assemblage of Pl (An37-75) ± Ol ± Opx ± Cpx + Ilm + Mag + Ap ± Zrn. Ilmenite and pyroxene are surrounded by narrow reaction rims of biotite and pargasite. During the subsolidus stage sporadic coronitic garnet-orthopyroxene-quartz assemblages were produced. Thermobarometry studies on amphiboles yield temperatures of 985-950°C whereas the chemical composition of coronitic garnet and orthopyroxene indicate a subsolidus re-equilibration of the KIC at conditions of 760 ± 100°C and 7.3 ± 1 kbar. In the syenites Kfs, Pl, Hbl and/or Cpx crystallized first, followed by a second generation of Kfs, Hbl, Fe-Ti oxides and Ttn. Crystallization of potassium feldspar occurred under temperatures of 890-790°C. For the crystallization of hastingsite pressures of 6.5 ± 0.6 kbar are obtained. In order to constrain the source rocks of the two suites, oxygen isotope analyses of feldspar as well as geochemical bulk rock analyses were carried out. In case of the anorthosites, the general geochemical characteristics are in excellent agreement with their derivation from fractionated basaltic liquids, with the d18O values (5.88 ± 0.19  per mille) proving their derivation from mantle-derived magmas. The results obtained for the felsic suite, provide evidence against consanguinity of the anorthosites and the syenites, i.e. (1) compositional gaps between the geochemical data of the two suites, (2) trace element data of the felsic suite points to a mixed crustal-mantle source, (3) syenites do not exhibit ubiquitous negative Eu-anomalies in their REE patterns, which would be expected from fractionation products of melts that previously formed plagioclase cumulates and (4) feldspar d18O values from the syenites fall in a range of 7.20-7.92  per mille, which, however, is about 1.6  per mille higher than the average d18O of the anorthosites. Conformably, the crustal-derived felsic and the mantle-derived anorthositic suite are suggested to be coeval but not consanguineous. Their spatial and temporal association can be accounted for, if the heat necessary for crustal melting is provided by the upwelling and emplacement of mantle-derived melts, parental to the anorthosites. In order to constrain the source of the 1,140-1,120 Ma carbonatites and to elucidate the fenitizing processes, which led to the formation of the sodalite, detailed mineralogical and geochemical investigations, stable isotope (C,O,S) analyses and fluid inclusion measurements (microthermometrical studies and synchrotron-micro-XRF analyses) have been combined. There is striking evidence that carbonatites of both generations are magmatic in origin. They occur as dykes with cross-cutting relationships and margins disturbed by fenitic aureoles, and contain abundant flow-oriented xenoliths. The mineral assemblage of both carbonatite generations of Ank + Cal + Ilm + Mag + Bt ± Ap ± pyrochlore ± sulphides in the main carbonatite body and Ank + Cal + Mag ± pyrochlore ± rutile in the ferrocarbonatite veins, their geochemical characteristics and the O and C isotope values of ankerite (8.91 to 9.73 and -6.73 to -6.98, respectively) again indicate igneous derivation, with the 18O values suggesting minor subsolidus alteration. NaCl-rich fluids, released from the carbonatite melt mainly caused the fenitization of both, the incorporated and the bordering anorthosite. This process is characterized by the progressive transformation of Ca-rich plagioclase into albite and sodalite. Applying conventional geothermobarometry combined with fluid-inclusion isochore data, it was possible to reconstruct the P-T conditions for the carbonatite emplacement and crystallization (1200-630°C, 4-5 kbar) and for several mineral-forming processes during metasomatism (e.g. formation of sodalite: 800-530°C). The composition and evolutionary trends of the fenitizing solution were estimated from both the sequence of metasomatic reactions within wallrock xenoliths in the carbonatitic breccia and fluid inclusion data. The fenitizing solutions responsible for the transformation of albite into sodalite can be characterised as of NaCl-rich aqueous brines (19-30 wt.\% NaCl eq.), that contained only minor amounts of Sr, Ba, Fe, Nb, and LREE.},
  subject      = {Namibia <Nordwest>},
  language  = {en}
}