6 PRF_SENT-DAYMAGMATISMANDCIOBALTECTONICFROCESSES

islands develop forming an island arc, -whacks if it is a continental plate the volcanism creates an active coniinental margin (Ch. 71. Magma generation in these iwo environments is broadly Nimilar, but in thc Latter the geochemicai characteristics of the ma n2.5 may be modified by continental crustal coniamination. Behind some volcanic arcs secon-dary seafloor spre2.ding occurs, resulting in ihe development of back-arc or marginal basins (Ch_8). Processes here are similar to those operating at mj4,1-ocearde rictRes, although the resultant basaltic magmas may be nriorc complex gerachemiLally, having characieristics of both destructive and consEructive margin tectonic seitings_

The process of subduction recycles material which has been in equilibrium with the continenial crust (sea water or pelagic sediments formed in equilibrium with %La water) or has been derived from it (oceanic terrigenous sediments) back into the mantle_ This has fundamental implicalions for the long--term chemical evoluticpn of the mantle. Additionally, some authors (Ch. 9) have suggillet1 that recycled ocear.iv lithosphere may provide a source component for some oceanic-island b:asalts. Subduction-related rnagrnatism appearS I° be the dominant mcchanivn kr crusial grow-t11 at ihe present, and probably throughoui the Phanerozoic.

In terms of Table 1.1 we can clearly sec rbaE much of present-day ...oicanic activity it concen-trated within, or adjacent to zones of plate diverg-ence or convergence.. NeveriheIns, within. bothlocum Lc and continental plates active volcanoesoccur, often at considerable distances &iim 131Ale boundaries, forming inira.(within)-plate volcanicprovinces (Part 4). Idiagmatirn within continentalplates. is volumetrically insignificant at the presenttime beilV primarily associated with intra-plate riftsystems such as the E.s_si African rift (Ch. 11). However, this may have been a much moreimporiarii magma ge.nemion environrcuem in thepast, for riple during the formation of Large-cale ourilinental flood basah provinces (Ch. 10)

which appear to predate continental fragrnenlalion.SimilarLy, kimberlite maginatism (Ch.. 12, is not a

raid-oceanic ridge tectonic settings is low, as these form pert of the oceanic lithcisphcre which is recycled back into the mantle on a umescale of the order of 101) Ma.. Only rarely are these rocks pmerved as olxlucted Slices Ln ophiolhe cotnplexes (Ch. 5). As a consequence intracontinefttal plate and active continental margin i8110113-5 rocks should., in ihcory, be niore common within the geologic[ record_

It is generally accepted that partial melting of mantle material produces primary magmas of basic or ultra is composition in most tectonic settings (Ch. 3\ and thai subsiecnient dilleremiation proces-ses, including fractional crystallization, magma rnixing and crustal comarnination fit._ 4), are resvonsible for -the generation of the wide composi-tional spectrum of terrotrial igneous rocks. The geiNhernical characteristics of these primary mag-mas depend upon parameters such as the source composition and mineralogy and the depth and degree of partial melting; factors which may varw (torn Gine tec[onic &mining to another. Primary. magmas appear to I 8entraied within a very resuicued depth range within the upper 100-2(10 krn of the mantle, although in detail their precise depths of origin are rmorly constrained- Diamond-bearing kimber]ites (Ch. 12) are probably the deepest terrestrial magmas, originating from depths wcatcr than 200-250 km_

A fundamental aim of petrogenetic studies of igneous rocks i5 to distinguish 5ci1111irce characiLeri.c. tics that arc inherited by the primary partial melts at their depth of segregation from those arising from Rthscquenii processes. Variations in the iso-topic compositio.ns of Sr, Nd and Pb iii oceanic basalts (Ch. 5 & a have provided important constraints. on the structure arid compcsitional heterogeneity of the upper mimic_ In particular, they have provided support for a rwo-layer convec-tion model (Ch. 3) wick the boundary between the two layers probably located at the #70 Inn SCi.Smic discontinuity. The upper layer is depleted in incompatible elements acid is the source of MORE-The formatitM Of the continental crust throughout

boson tray. phenomenon_ geologkal time is generally held responsible for theIn general, the pre5ereation potential of igneous depletion of this layer. In contrast, the lower toyer

rocks generated in oceanic-island, island-arc i5 considered to he less depleted and must have