Residuum

Abstract: The concept of 'urKREEP' (primeval KREEP), a magma residuum hypothetically produced early in lunar history by fractional distillation of the global magma ocean which hypothetically created the lunar crust, is used to explain the origin of KREEPy lunar rocks. The incompatible-rich last dregs of the magma ocean left their trace in the form of incompatible patterns that show no relative fractionation from site to site on the moon and that, with the exception of minor fractionals in two pristine clasts, are the same in pristine samples as in breccias. The high concentration on the lunar surface of these urKREEP remnants demands a high efficiency in upward transport of the incompatibles. This transport may have been enhanced by urKREEP's presumably low density and by high temperatures produced by radioactive decay in the K-, U-, and Th-rich residuum.

Abstract: Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH 2 O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH 2 O<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH 2 O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; eNd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; eNd(0)=+3.3 to +6.9). The fH 2 O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.

Abstract: (Taylor & McLennan, 1985), the physical processes inA continuous section through reworked Archaean crust records the volved, particularly how granite magma is formed, remain generation of granitic magma and its subsequent development in the controversial (Miller et al., 1988; White & Chappell, Opatica subprovince in the Canadian shield. There, the transition 1990). A partially melted source can form a granitic from palaeosome to granite was a closed-system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite. The average most of their residuum. Understanding this process is degree of partial melting was less than 30%, but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs. For example, melt–residuum separation deformation. Regions that lost melt became residual diatexites could be a nearly perfect, single-stage process at the enriched in TiO2, FeOT, MgO, CaO, Sc, Cr, Co, Sr, rare earth site of partial melting, as described for leucosomes in elements (REE) and high field strength elements (HSFE). Melt metatexite migmatites (Wickham, 1987a; Sawyer, 1991, accumulated to create diatexite magmas enriched in large ion 1994; Brown et al., 1995). Alternatively, the separation lithophile elements (LILE), but contaminated with residuum macould be an imperfect, multi-stage process that yields a terial. Such diatexite magmas are parental to granites found at magma with a large residual component, as is observed higher crustal levels in the terrane. Flow of the diatexite magma in in diatexite migmatites (Bea, 1991; Greenfield et al., response to deformation separated some of its residuum into schlieren. 1996; Sawyer, 1996). The metatexite model necessarily Parautochthonous plutons were created where ascending granitic produces leucocratic, residuum-free granites, such as magma locally ponded below impermeable layers and structures. those described by Le Fort (1981) and Montel et al. Magma left the anatectic region in dykes and lost its remaining residuum as it crystallized. Consequently, the allochthonous granite (1991), and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation. In contrast, the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum-rich granites, and so reduce the residuum. geochemical effect of crustal differentiation, as Chappell (1996) noted. How anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known, because few crustal sections