The Pliocene-Pleistocene magmatic activity of the ^amboanga arc is linked to the southward subduction of the Oligocene-Miocene Sulu Sea back-arc basin along the Sulu Trench. The magmatic products include small amounts ofadakites datedfrom 3-8 to 0-7 Ma, abundant Nb-enriched basalts and basaltic andesites (NEB) datedfrom 2 to 1 Ma and a lone calc-alkaline potassic basaltic andesite dated at 0 4 Ma. Three kinds of NEB are distinguished: nearly primitive Mg-rich (MG) basalts displaying positive or no Nb anomalies with respect to adjacent incompatible elements and more evolved low-K (LK) and calcalkaline (CA) lavas which, despite their Nb enrichment, display negative Nb anomalies. Although the role of OIB-type mantle components has been advocated to explain the HFSE enrichment of NEB, the spatial and temporal association of these rocks with adakites suggests a petrogenetic link between them. Trace element characteristics of the NEB imply that amphibole and ilmenite might be present in their source. We suggest that these minerals could be added metasomatically to the mantle through hybridization by percolating slab melts, during which Nb and Ti are preferentially extracted from the adakitic melts. In an early stage (4-3 Ma) of the subduction of the young and hot Sulu Sea basin crust beneath the ^jimboanga peninsula, adakitic liquids formed at depths of 75-85 km. A few of them were emplaced at the surface but most were consumed through slab melt-mantle metasomatic reactions. Adakite production and emplacement continued later (<2 Ma), while the Nb-enriched mantle was brought by convection to depths that allowed its melting and the subsequent emplacement of NEB behind the adakitic front of the ^pmboanga arc

It is generally accepted that the Cenozoic potassic volcanic rocks of northern Tibet were derived from a lithospheric mantle source. Here we report new chronological, geochemical, and isotopic data for the Miocene (ca. 18-15 Ma) K-rich adakitic volcanic rocks from the Hohxil area of the Songpan-Ganzi block in northern Tibet. We contend that these rocks were generated by partial melting of the mafic lower crust, in an intracontinental setting unrelated to subduction of oceanic crust. The Hohxil rocks exhibit high Sr/Y and La/Yb ratios, high Sr and La contents, but low Yb and Y concentrations, similar to adakites formed by slab melting associated with subduction. However, their relatively low Nd values (؊2.09 to ؊4.58); high 87 Sr/ 86 Sr (0.7072-0.7075), Th/U, Th/Ba, and Rb/Ba ratios; and distinctive potassium enrichments (K 2 O Ͼ Na 2 O) are very different from the composition of typical adakites. In addition, those K-rich adakitic rocks with the highest SiO 2 contents (Ͼ61 wt%) exhibit the lowest 87 Sr/ 86 Sr ratios and highest Nd values and are the oldest Cenozoic volcanic rocks exposed in the Songpan-Ganzi block, suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma. Taking into account the composition of lower-crustal mafic xenoliths in Tibet, as well as the tectonic and geophysical evidence, we conclude that the Hohxil adakitic magmas were produced by partial melting of amphibole-bearing eclogites with a K-rich mafic bulk composition, in the lower part (Նϳ55 km) of the thickened northern Tibetan crust. Partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward-subducted continental lithosphere.

No abstract

Oblique aseismic subduction below western Panama and southeastern Costa Rica has produced Recent arc-related volcanism. The aseismicity is probably related to the subduction of relatively hot oceanic lithosphere. The volcanism throughout this region over the past 2 Ma has been quite distinct, consisting of felsic magmas (andesites to rhyolites but mainly dacites) with geochemical signatures suggesting a metamorphosed basaltic source. It is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge. Rocks with slab-melt geochemistries and associated with young subducted crust have been termed adakites elsewhere. The young adakite melts are sometimes associated with a few rare young high-Nb basalts, but there is no obvious genetic link between them through differentiation. High-Nb basalts may also be derived from the partial melting of the subducted oceanic crust. High-Nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the Catalina Schist, California. Alternatively, the high-Nb basalts may be partial melts of phlogopite-rich mantle that has previously reacted with adakite magmas. Eruption of adakites and high-Nb basalts was preceded by a 2-3 Ma period of relative quiescence. Prior to this, there was a 7 Ma period of calc-alkaline volcanism typical of the present-day magmatism (associated with a distinct Benioff zone) found throughout the Central American arc. The abrupt transition in volcanism with time from an early calc-alkaline sequence to a later adakite-high-Nb basalt sequence may record a change in the tectonic setting of western Panama and southeastern Costa Rica over the past 12 Ma.

No abstract