IGNEOUS ROCKS/Granite 245 although the viscosity contrast may be less than originally thought (104–106 Pa s for hydrous crystalpoor granite versus 102 Pa s for a typical basalt) The thermal constraints are satisfied by models in which granitic magmas are generated by the wholesale assimilation of crustal materials by thick convecting basaltic sills in thermally perturbed lower crust, or by the combination of felsic liquids derived from the crystallization of such sills with partial melts of the overlying crust Compelling field evidence for such scenarios can be directly observed in deep-crustal exposures upthrust in continental-collisional zones, such as the Ivrea Zone of the Italian Alps and in the Norwegian Caledonides A promising new way of unravelling the various source components of granitic magmas involves decoding the isotopic information preserved in the fine-scale growth zoning of certain minerals (Figure 10) This is done by in situ microanalysis, using laser-ablation sampling or secondary ion mass spectrometry, both of which are capable of measuring precise isotope ratios and trace-element contents at sub-ppm levels in very small areas of the crystal (20 mm or less) Zircon is an ideal mineral for this approach since the growth zones can be directly dated by uranium–lead isotopes Zircon is also robust and may survive as a refractory residue in granitic magmas (see Minerals: Zircons), thereby preserving age and compositional information about the granite’s source rocks and the otherwise inaccessible deep crust Hafnium is a particularly abundant trace element in zircon, and fluctuations in hafnium isotope composition within zircon crystals record the progress of processes such as magma mixing and crustal assimilation In this way, the crystallization history of silicic magmas can be reconstructed as they ascend from their deep crustal source to their final emplacement site The Time-Scales of Granitic Magmatism The time-scales of geological processes provide important clues about the formation of granitic magmas and the various factors that influence their compositional evolution For silicic magmas, this area of study is still in its relative infancy However, there is growing evidence that the segregation, ascent, and emplacement of granitic magmas occur on time-scales of hundreds or thousands, rather than millions, of years (Figure 11) When coupled with the crystallization rates, this places constraints on the sites and nature of magmatic differentiation and on partial-melting processes in the source region Information on the time-scales of melt segregation and extraction in the granite’s source region comes from the accessory minerals zircon and monazite, Figure 10 Cross sections of zircon crystals imaged by back scattered electron microscopy (scale bar is 100 mm) to show internal growth structures Circles in (A) (C) represent ion probe pits, and the circles in (D) represent laser ablation pits (A) Zircon with a 390 Ma igneous crystallization age showing corroded, partially resorbed older core dated at 1050 Ma (B) Small, irregularly shaped zircon nucleus (too small to analyse) enclosed by strongly zoned magmatic zircon (C) Elongate 550 Ma core surrounded by 385 Ma igneous zircon Note the cracks radiating from the older zircon core (D) Magmatic zircon analysed by laser ablation, showing strong core to rim zoning in hafnium isotopic composition, expressed in the standard epsilon notation (see caption to Figure 9) This reflects the operation of an open system process, such as crustal assimilation, during crystallization