Though typically exhibiting considerable scatter, geochemical variations in granitic plutons and silicic volcanic deposits are commonly modelled as products of differentiation of originally homogeneous magmas. However, many silicic igneous bodies, particularly those classified as S-types, are internally heterogeneous in their mineralogy, geochemistry and isotope ratios, on scales from hundreds of metres down to one metre or less. The preservation of these heterogeneities supports recent models for the construction of granitic magma bodies through incremental additions of numerous batches (pulses) of magma derived from contrasting sources. Such pulses result from the sequential nature of the melting reactions and the commonly layered structure of crustal magma sources. Internal differentiation of these batches occurs, but not generally on the scales of whole magma chambers. Rather than being created through differentiation or hybridisation processes, at or near emplacement levels, much of the variation within such bodies (e.g. trace-element or Mg# variation with SiO2 or isotope ratios) is a primary or near-source feature. At emplacement levels, the relatively high magma viscosities and slow diffusion rates of many chemical components in silicic melts probably inhibit processes that would lead to homogenisation. This permits at least partial preservation of the primary heterogeneities.

Abstract-Understanding the nature and composition of larger extraterrestrial bodies that may collide with the Earth is important. One source of information lies in the record of ancient impact craters, some of which have yielded chemical information as to the impacting body. Many deeply eroded craters have no remaining melt sheet or ejecta yet may contain impactor residue within basement fractures. The emplacement mechanism for fractionated siderophile residues is likely to be gaseous, although, melt droplets and some solid materials may survive. For breccia-and melt-filled fractures to contain extraterrestrial material, they must form very early in the impact process. Most current numerical models do not dwell on the formation and location of early major fractures, although, fractures in and around small craters on brittle glass exposed to hypervelocity impact in low Earth orbit have been successfully simulated. Modelling of fracture development associated with larger craters may help locate impact residues and test the models themselves.

The c. 280 Ma, post-orogenic, S-type Dartmoor pluton was assembled from numerous sheets of granitic magma, emplaced into the shallow crust. The main magma source lies in the middle crust, and is most probably Proterozoic metagreywackes, with minor metapelites and metavolcanic or plutonic rocks, possibly formed in a syn-collisional environment. Partial melting of this source may have occurred under fluid-deficient conditions, and the magmas most likely had relatively high initial H2O contents. The pluton contains substantial, wholerock-Sr and quartz-O isotope heterogeneities on scales down to a metre or less, and such small-scale heterogeneities are probably common in granitic intrusions derived from heterogeneous protoliths. Thus, variations in source terranes may not be fully captured with the sample numbers and scales commonly applied in studies of granitic plutons. The preservation of both large-and small-scale isotopic heterogeneities suggests that the 2 Dartmoor magmas were never efficiently homogenised by flow-driven mechanical mixing. This implies a source terrane with lithological variations on scales of tens of metres or less.The granitic rocks form five texturally, chemically and isotopically distinct groups, each of which had somewhat different sources or mixtures of sources. The main chemical variations cannot have been formed through fractionation of any combination of the major minerals in the rocks. Instead, entrainment of variable proportions of peritectic plagioclase, orthopyroxene and ilmenite was responsible, together with local crystal fractionation. Lowdensity, late-magmatic melts and aqueous fluids produced patchy enrichment in light elements, and extreme enrichment in some of the highly silicic, two-mica microgranites.However, although they are also enriched in light elements, the 'aplites' were not produced through fractionation, and seem to have had independent magmatic origins.

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