Petrology and Geochemistry of Hama Koussou Dolerite Dyke Swarms (North Cameroon, Central Africa)

The Pan African granitoid basement of Hama Koussou Cretaceous half basin in North Cameroun (Central Africa) is transected by near N-S, NE-SW and ENE-WSW giant doleritic dykes trending along the same Pan African directions. Hama Koussou dolerites are compliant with the regional distension that occurred after the Pan African basement consolidation prior to the development of West and Central African Rift System at Late Jurassic-Early Cretaceous times. Studied lavas are composed of large clinopyroxene oïkocrysts, plagioclase and alkali feldspar laths and oxides phenocrysts exhibiting ophitic, sub-ophitic and intercertal textures. Microprobe chemical analyses carry out on the main mineral phases show that clinopyroxenes are diopside and augite, plagioclases are labradorite, andesine, oligoclase and albite and alkali feldspars are mainly sanidine with a few percent of orthoclase. ICP-MS and ICP-AES geochemical analyses of Hama Koussou lavas exhibit basalt, basaltic trachyandesite and trachyandesite compositions of continental tholeiite features. Tholeiite basalts of Hama Koussou are the results of high partial melting of E-MORB mantle source of spinel lherzolite composition, located at 65-55 km depth. More evolved tholeiite lavas of Hama Koussou basin are the products of tholeiite basalt differentiation trough assimilation and fractional crystallization coupled with fluids circulation.

Geochronological and geochemical studies of the OIB-type Baiyanghe dolerites: implications for the existence of a mantle plume in northern West Junggar (NW China)

In this paper, zircon U–Pb geochronology, major and trace elements, and Sr–Nd isotope geochemistry of the Baiyanghe dolerites in northern West Junggar of NW China are presented. The U–Pb dating of zircons from the dolerites yielded ages of 272.2±4 Ma and 276.7±6.2 Ma, which indicate the emplacement times. The dolerites are characterized by minor variations in SiO2(46.89 to 49.07 wt%), high contents of Al2O3(13.60 to 13.92 wt%) and total Fe2O3(11.14 to 11.70 wt%), and low contents of MgO (2.67 to 3.64 wt%) and total alkalis (Na2O+K2O, 5.1 to 5.97 wt%, K2O/Na2O = 0.37–0.94), which indicate affinities to metaluminous tholeiite basalt. The REE pattern ((La/Sm)N= 2.25–2.34, (La/Yb)N= 7.42–8.36), V–Ti/1000 and 50*Zr–Ti/50–Sm discrimination diagrams show that these rocks are OIB-type. The high contents of Zr and Ti indicate a within-plate tectonic setting, and samples plot in the ‘plume source’ field shown on the Dy/Yb(N)versus Ce/Yb(N)diagram. The positive εNd(t) values (+7.09 to +7.48), high initial87Sr/86Sr ratios (0.70442 to 0.70682) and depletions of Nb and Ta elements in the samples can be explained by the involvement of subducted sediments. In summary, it is possible that the Baiyanghe dolerites were derived from an OIB-like mantle source and associated with a mantle plume tectonic setting. Therefore, our samples provide the youngest evidence for the existence of a mantle plume, which may provide new insights into the Late Palaeozoic tectonic setting of West Junggar.

The chemistry and origin of some basement amphibolites between Ivigtut and Frederikshåb, South-West Greenland

Chemical and modal analyses of 54 amphibolites, 5 biotite gneisses and 5 hornblende-biotite gneisses, into which the amphibolites grade, confirm the field and petrographic evidence that the amphibolites were probably basic tuffs and lavas which, when mixed with varying amounts of quartzo-feldspathic sediment, gave rise to the hornblende-biotite gneisses on metamorphism. Plotted in an alkali-silica diagram 34 samples fall in the field of tholeiite basalt and 20 in the field of alkali basalt while traces of normative nepheline, never exceeding 4 %, occur in only 10 samples. It is shown that the sporadic occurrence of garnet and diopside and the occurrence of up to 15 % quartz are primarily controlled by the original composition of the samples. Garnet is entirely found in amphibolites with low Niggli mg values (0.36-0.45), diopside in samples with relatively high CaO, while modal quartz increases as mg falls. It is therefore possible that the more quartzose and garnetiferous amphibolites formed from late differentiates of the original basaltic magma, but the possibility cannot be excluded that contamination with sedimentary material has played a role, especially in increasing the modal quartz.