Orogeny-modified lithosphere

Abstract

One of the major mantle reservoirs on Earth is the continental lithospheric mantle. Our understanding of the early continental lithosphere sometimes is limited by its subsequent complex history. Continental tracts can be involved in collisional processes that coalesce them into larger masses and obscure their original composition and structure. Depending on the intensity of collisional processes, the continental interiors can be modified by far-field stresses that lead to hot ductile belts and crustal magmatism. The lithospheric mantle according to orogenic peridotites exposed at the surface has primary (peridotites) and cumulated assemblages (pyroxenites). The relationship among the mantle assemblages is detected in subsequent mantle-derived lavas that have interacted with (e.g., plume-induced lavas) or were generated in the lithosphere (e.g., extension-induced lavas) by carrying signatures of mineralogic and isotopic changes from the time of lithosphere modification. This study examines extruded lavas surrounded by orogenic terrains and the entrained mantle assemblages (xenoliths). The first part shows that asthenospheric derived melt is feasible if the lithosphere has an irregular configuration. The second part of this study examines the temporal changes of the SCLM (sub-continental lithospheric mantle) using mantle xenoliths. New xenolith analyses from the Sahara Craton yield one of the most precise bulk-rock Re-Os isochrons yet reported in peridotitic assemblages. Yielding an exact age in SCLM primary lithology reveals a thermal adjustment event in the SCLM that was synchronized with the cessation of Pan African collisions. The last part of this study presents evidence from five different continents showing that mantle plumes or extensional stresses in the Cenozoic time can trigger or cause the melting of secondary lithologies of SCLM (Cumulate assemblages, such as pyroxenites). The young lavas convey Re-Os orogenic ages that show that SCLM modifications are syn-orogenic.