U. Mass Lowell Prof. Nelson Eby Department of Environmental, Earth, & Atmospheric Sciences




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Cameroon Volcanic Line

The Cameroon volcanic line project is a collaborative study with Dr. Isaac Injilah, Yaounde University, Cameroon. The initial phase of the project was an investigation of the trace element chemistry of lavas from both the oceanic and continental sector of the province using a subset of the sample suite collected and analyzed by Godfrey Fitton, University of Edinburgh. Jaime Hussey and Kathy Milidakis, UML, participated in this portion of the study. Analysis of these same samples by instrumental neutron activation analysis (INAA) allowed us to investigate additional trace element systematics. The second phase of the project will involve geological mapping and sample collection from the various continental volcanic structures and subsequent geochemical and isotopic analysis. We will also be looking at the older period of magmatism that predates the Cameroon Volcanic Line. Preliminary results and pictures of the field area are found below.

Preliminary Results

The Cameroon volcanic line consists of a string of volcanoes that extend from the Atlantic ocean into Cameroon (Fig. 1). The current period of volcanic activity started approximately 38 Ma ago (Fig. 3) and extends to the present. Intrusive rocks associated with the province range in age from 66 to 30 Ma (Fig. 2). Since the volcanoes cross the oceanic-continental boundary they have been of considerable interest. We initiated the present study by determining additional trace elements, using INAA, for a subset of a suite of samples collected by Godfrey Fitton, University of Edinburgh.

The rocks of the Cameroon volcanic line range in composition from picro-basalt and basalt through intermediate compositions to phonolite and rhyolite (Fig. 4). Silica saturated rocks are largely confined to the continental portion of the province. A plot of Na2O versus SiO2 shows enrichment of Na in the evolved rocks from the oceanic part of the province and a decrease in Na in the evolved rocks from the continental part of the province. This bifurcation of the Na (and alkali) trend is commonly noted in provinces that contain both highly evolved silica-undersaturated and silica saturated rocks.

Fig. 4. Alkali-silica plot for volcanic rocks from the oceanic and continental parts of the CVL.
Fig. 5. Na2O versus SiO2 for volcanic rocks from the oceanic and continental parts of the CVL.

The Nb/Ta ratio for most of the samples falls within the normal mantle range of 15 - 20 (Fig. 6), but note the significant Ta depletion, and correspondingly high Nb/Ta ratios, for some samples from the oceanic sector. This may be a response to titanite fractionation. Some of the continental samples have Nb/Ta < 15 which can be interpreted as a sign of crustal contamination.

Fig. 6. Nb/Ta versus Ta abundance. Note that most samples fall between Nb/Ta = 15 - 20, ratios typical of the mantle.
Fig. 7. Projection of samples with mg# > 60 into the Ol-Di-Neph phase diagram. The samples plot in the olivine field indicating that olivine fractionation controlled the early stages of magmatic evolution.

Samples with mg# >60 plot in the olivine field on the Diopside-Olivine-Nepheline phase diagram (Fig. 7) indicating that the early stages of magmatic evolution are largely controlled by olivine fractionation.

Samples with mg#>60 fall in the OIB field (Fig. 8) and the HIMU field (Fig. 9) supporting an OIB-like source for these magmas.

Fig. 8. Samples with mg# >60 plot in the OIB field on the Yb/Ta versus Y/Nb diagram.
Fig. 9. Samples with mg# >60 largely plot within the HIMU field on the Zr/Nb versus La/Nb diagram.

Compared to the East African rift system, basalts from the CVL are relatively depleted in LIL elements (Fig. 10). This suggests that, relative to the mantle under the East African rift system, the mantle under the CVL was not as enriched in LIL elements. The REE systematics can be explained by small degree partial melts (1-3%) of a garnet lherzolite mantle (Fig. 11). Hence, as a preliminary conclusion we suggest that the CVL magmas were the result of small degrees of partial melting of an only slightly enriched garnet lherzolite mantle.

Fig. 10. OIB-normalized plots of basalts from the CVL and East African rift system. Note the significant relative enrichment in LIL elements for the East African basalts.
Fig. 11. Melting model for CVL basalts

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Photographs from the Field Area

Recent lava flows (1999) from Mt. Cameroon cutting the road. On the 1999 lava flow. Fanning columnar joints. Quarry in basalt flow. Columnar joints in basalt flow.
Another field vehicle in superb condition. Here comes the tire. Lake Nyos. Pyroclastic deposit at Lake Nyos.
Field party on pyroclastic dam at Lake Nyos. From left: Isaac Njilah, Wilson, Nancy, and Alex. Crater lake in a small volcanic cone. Cinder cones in Manengouba caldera. Crater lake in Manengouba caldera.

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