The origin of melt pockets in mantle xenoliths from southern Patagonia, Argentina

ALIANI, P; NTAFLOS, TH; BJERG, E A

La Plata

Congreso; XVI Congreso Geológico Argentino; 2005

The hills of La Huerta and Valle Fertil, wich belong to the Western Pampean Ranges in the Province of San Juan, NW Argentina, mainly consist of metagabboic to metadioritic rocks that intruded the Proterozoic basement in the Early Middle Ordovician (Famat <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:21.0cm 842.0pt; margin:70.9pt 3.0cm 70.9pt 3.0cm; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Mantle xenoliths carried by alkali basaltic rocks frequently show evidences of mantle metasomatism processes that took place prior Lo their inclusion in the host basalt. It is now well documented that infiltration and percolation of fluids and melts are responsible for the formation of hydrous phases (amphibole and/or phlogopite) in peridotitic xenoliths. Silicate glasses occur as inclusions in minerals and/or as intergranular blebs and/or as irregular patches. Silicate glass inclusions and patches in particular provide useful information about the nature of the melts and fluid phases that can influence and modify parts of the upper mantle. In recent years there have been many debates about the origin of melt pockets in mantle xenoliths . A number of authors (e.g. Neumann and Pedersen, 1997; Coltorti et al, 1999) support the hypothesis that ephemeral metasomatic agents react with upper mantle minerals prior to their incorporation as xenoliths in host magmas. According to Yaxley and Kamenetsky (1999) and references therein, in situ melting of amphibole or clinopyroxene (plus or minus phlogopite), either in closed system or during reaction with a metasomatic fluid allow fluid/rock ratios is responsible for the generation of glassy patches prior to entrainment of the xenoliths in the host lava. Glasses in xenoliths could also be produced by adiabatic decompression melting of clinopyroxene, amphibole and phlogopite during their transport to the surface (Frey and Green, 1974).