SLAB-MANTLE INTERACTIONS .3. PETROGENESIS OF INTRAPLATE MAGMAS AND STRUCTURE OF THE UPPER MANTLE

Intraplate magmas worldwide display a set of geochemical and isotopic characteristics which seem to have been produced by a common petrogenetic process connected with the subduction of oceanic lithosphere. Their source regions are to be found in quite diverse locations in the sub-continental and sub-oceanic lithospheres, and also much deeper in the mantle. However, minor-element partition relationships demonstrate that the petrogenetic processes which generated these source regions did not take place under perovskititic-facies conditions in the lower mantle and were therefore almost certainly restricted to the upper mantle. It is proposed that the source regions of intraplate magmas are formed initially in two principal locations within the upper mantle. One of these is situated near the interface between subducted lithosphere and overlying mantle, and is produced as a consequence of slab-mantle interactions mainly at depths between ∼150 and ∼300 km. Partial melting of the eclogitic crust of the slab in this region is caused by water released by bodies of subducted former serpentinite. The melts migrate into adjacent entrained refractory peridotite, hybridizing with it and thereby rendering it fertile. This is essentially an extension of the same processes which are believed to be responsible for the formation of calc-alkaline source regions, and may account for certain similarities in the geochemical and isotopic characteristics of the two associations. The relative depletions of Ti, Nb and Ta displayed by the calc-alkaline association are attributed to the retention of these elements by residual rutile during partial melting of the upper layers of the subducted eclogitic crust at depths of 80-100 km. At greater depths between 150- and 300 km, when the deeper layers of the subducted oceanic crust experience partial melting, rutile is no longer a residual phase. Therefore, Ti, Nb and Ta become highly incompatible and are effectively transferred from the slab into future source regions of intraplate magmas. Subduction may occur at shallow angles and extend for considerable horizontal distances beneath continents, producing source regions by slab-mantle interaction in or near the base of the subcontinental lithosphere. These may initiate the generation of intraplate magmas within continents, not only during subduction, but also for a considerable period following its cessation. A second type of intraplate magma source region is formed when subducted former oceanic crust is buoyantly trapped to form a gravitationally stable layer of garnetite situated on top of the 650-km seismic discontinuity. This layer is believed to be ∼70-100 km thick and to be of global extent. Partial melting of trapped oceanic crust occurs near the top of this layer, and the resultant liquids react and hybridise with an overlying boundary layer of pyrolite, causing the development of characteristic chemical and isotopic signatures in this lithology. Large diapirs eventually ascend from the fertilised boundary layer, undergoing partial melting at shallower depths and producing extensive "hot-spot" volcanism, both within both oceanic and continental regions. Small ascending diapirs from the same source region may be trapped in or near the base of the continental and oceanic lithosphere, causing local chemical heterogeneity in these regions. © 1990.