Modelización de la fusión parcial en equilibrio en una fuente mantélica metasomatizada. El volcanismo Cuaternario de La Garrotxa (Cataluña)

Trace element modelling together with the use of radiogenic and stable isotopic ratios, is a widely used tool to constrain the characteristics of the mantle sources of basaltic series. However, as the composition of the volcanic rocks may be the result of several combined magmatic processes, it is necessary to identify them before any attempt to their quantitative modelling. On the other hand, the information provided by partial melting modelling is limited to the final composition of the mantle source whereas some characteristics, like the presence of minor phases, can be cloaked if those minerals have not remained in the residue. A good example to illustrate the problems of this method is found in the Quaternary primitive basaltic rocks of La Garrotxa (ME Volcanic Region of Spain) which, according to a combined study o f trace element modelling and Sr-Nd-Pb isotopic ratios, can be interpreted as the result of the melting of a previously enriched lithospheric source which has been subsequently pervaded by basaltic melts derived from a common European sublithospheric reservoir

Petrogenesis of the alkaline and calcalkaline monogenetic volcanism in the northern sector of the Michoacán-Guanajuato Volcanic Field (Central Mexico)

In the Michoacán-Guanajuato Volcanic Field, Quaternary monogenetic alkaline volcanism is mostly present in its northern sector in association with coeval calcalkaline volcanism. Their respective geochemical signatures reveal the existence of two suites of differentiated alkaline lavas, one with a typical OIB-like signature and another that displays intermediate characteristics with the coeval calcalkaline lavas. Petrogenetic modelling supports that each suite followed independent fractionation paths from distinct initial magmas with no interaction between them. In all cases similar AFC processes prevailed with ~ 30% crystallization and contamination (r ~ 0.3) with heterogeneous granitic and rarely granulitic components, involving different paragenetic assemblages mainly controlled by olivine, plagioclase, pyroxenes as well as some minor phases such as apatite and oxides. The geochemical characteristics of the initial magmas suggest that they derived from distinct mantle sources. The approximate N-S distribution of alkaline volcanism in the area, which coincides with a major geodynamic boundary at about the 101° W longitude, both at crustal levels and also concerning the configuration of the Cocos plate in the area, supports the existence of some disruption of the subducting plate that would favour the ascent of enriched sublithospheric melts. These would either ascend to the surface to produce OIB-like magmas or act as metasomatic agents in the mantle wedge, also influenced by components derived from the subducting slab. Such a hybrid mantle would represent the source of both the calcalkaline and the intermediate mildly alkaline melts, resulting from selective sampling (by different degrees of melting) from compositionally distinct domains within the mantle wedge.This work was supported by the Government of Spain “Ministerio de Economía y Competitividad” grants [Research Project CGL2011-23422 and FPI Research grant BES-2012-052390].Peer reviewe