Upper Mantle Dynamic and Evolution beneath Lake Nyos Volcano (Cameroon Volcanic Line), Documented by Peridotite Xenoliths Enclosed in Alkali Basalts

Volcanic deposits from the lake Nyos contain ultramafic xenoliths: lherzolites, harzburgites and wehrlites, sometimes containing amphiboles and phlogopites. The lithospheric mantle beneath Nyos, as inferred from chemical diagrams, has experienced partial melting and variably cryptic and modal metasomatism of the two groups of samples that have been distinguished: Group 1 samples are characterized by spoon-shaped REE patterns, and Group 2 samples show light (L) REE-enriched patterns. Metasomatic events were associated with pervasive infiltration of volatile (Ti, CO2, H2O) or alkali-rich small melts fractions and fluids. Later on, hydrous phases, Ti-rich Cpx, CaO-rich Ol, Ti-rich Ol, Cr poor and low values of NiO and FO (%) in wehrlite compared to other xenoliths, precipitated from alkali enrichments due to the percolation of the mantle by basaltic magmas. The metasomatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that 1) cryptic metasomatism affected Group 1 samples, 2) the spinel-free wehrlite is a Group 2 sample corresponding to a cumulate of a similar melt and 3) amphibole may be a potassium-bearing mineral in addition to phlogopite at shallower levels of Nyos upper mantle. P-T estimated indicates that xenoliths were initially equilibrated in the garnet stability field, at depth of 85 Km, and then they were re-equilibrated in the spinel field owing to isobaric heating up to 1000℃. Adiabatic decompressions occur from 85 to 50 Km materialized by sample NK14 showing transitional porphyroclastic to equigranular texture and displaying pyroxene-Cr spinel symplectites, and from 50 to 30 Km corresponding to 8 - 18 Kbar, pressures in which most xenoliths were incorporated in the host lavas. Therefore, the presence of rising mantle plumes from 85 to 50 Km (sample NK14) and from 50 to 30 Km (all studied samples) is probably related to the evolution of heterogeneous translithospheric mantle diapirs beneath this section of the continental Cameroon Volcanic Line. Local diapirs here may be interpreted as rift-zone initiators.

Deep Tectonophysical Process of the Uplift of the Northern Qinghai--Tibet Plateau--Evidence from the Integrated Geological--Geophysical Profile from Golmud to the Tanggula Mountains, Qinghai Province, China

The tectonic activities occurring since the Cenozoic in the northern part of theQinghai-Tibet Plateau (the region from the East Kunlun Mountains to the Tanggula Mountains)were probably caused by the intense intraplate deformation propagation after the collision be-tween the Indian plate and the Eurasian plate. Their main expressions include the substantial up-lifting of the plateau, alternation of horizontal extension and compression under the verticalgreatest principal stress σ_1, occurrence of rift-type volcanic activity, formation of thebasin-range system, and successive eastward extrusion of blocks resulting from large-scalestrike-slip faulting. Geophysical exploration and experiments have revealed that there exist close-ly alternating horizontal high-velocity and low-velocity layers as well as lithospheric faults of aleft-lateral strike-slip sense in the lower part of the lithosphere (the lower crust and lithosphericmantle, 60-120 km deep). Based on an integrated study of the geological-geophysical data avail-able, the authors have proposed a model of deep-seated mantle diapir and the associatedtectonophysical process as the dynamic source for the uplift of the northern part of theQinghai-Tibet Plateau.