India–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

Melt extraction is a process with a length scale that spans many orders of magnitude. Studies of residual migmatites and granulites suggest that melt has migrated from grain boundaries to networks of leucosome-filled structures to steeply inclined cylindrical or tabular granites inferred to have infilled ascent conduits. For example, in anatectic rocks from southern Brittany, France, during decompression-induced biotite-breakdown melting, melt is inferred to have been expressed from foliation-parallel structures analogous to compaction bands to dilation and shear bands, based on location of residual leucosome, and from this network of structures to ascent conduits, preserved as dykes of granite. The leucosome-filled deformation band network is elongated parallel to a sub-horizontal lineation, suggesting that mesoscale melt flow was focused primarily in the plane of the foliation along the lineation to developing dilatant transverse structures. The leucosome network connects with petrographic continuity to granite in dykes; however, the orientation of dykes discordant to fabric anisotropy suggests that their formation was controlled by stress, which indicates that the process is a fracture phenomenon. Blunt fracture tips and zigzag propagation paths indicate that the dykes represent ductile opening-mode fractures; these are postulated to have formed by coalescence of melt pockets. The structures record a transition from accumulation to draining; quantitative volume fluxes are calculated and presented for the generalised extraction process. The anatectic system may have converged to a critical state at some combination of melt fraction and melt distribution that enabled formation of ductile opening-mode fractures, but fractal distribution of inferred mesoscale melt-filled structures has not been demonstrated; this may reflect the inherent anisotropy and/or residual nature of the drained source. Melt extraction has been modelled as a self-organised critical phenomenon, but the mechanism of extraction is not described and the relationship between these models and the spatial and temporal granularity of lower continental crust is not addressed. Self-organised critical phenomena are driven systems involving ‘avalanches’ with a fractal frequency-size distribution; thus, the distribution of melt batch sizes might be expected to be fractal, but this has not yet been demonstrated in nature.

The Miocene Baltoro granite forms a massive plutonic unit within the Karakoram batholith, and is composed of comagmatic monzogranites and leucogranites with a mineralogy consisting of quartz-K-feldspar-plagioclase-biotite ± muscovite ± garnet, with accessory sphene, zircon, monazite and opaques. Geochemically the Baltoro granites are mildly peraluminous, and show a calc-alkaline trend on trace-element normalised diagrams with high LIL/HFS element ratios and negative Nb, P and Ti anomalies. REE are strongly fractionated with little or no Eu anomaly. Leucogranites are depleted in most elements compared to monzogranites with notable exceptions being Rb, K and the HREEs. Initial 87Sr/86Sr ratios are 0·7072-0·7128, considerably lower than High Himalayan leucogranites (0·74-0·79), and are indicative of a lower continental crust source. The probable petrogenesis of the Baltoro granite involves dehydration melting of a biotite-rich pelite to produce a voluminous, hot, water-undersaturated magma which could then separate from its source and intrude through an already thickened and still hot crust. Fractional crystallisation of the monzogranites produced the leucogranites and a pegmatite dyke swarm. A suite of lamprophyre dykes including amphibolerich vogesites and biotite-rich minettes intrude the country rock, dominantly to the north, around the Baltoro granite. These calc-alkaline shoshonitic lamprophyres are volatile-rich mantle-derived melts intruded around the same time as the granite, indicating simultaneous melting of the mantle and lower crust beneath the Karakoram during the Miocene, approximately 30 Ma after the India-Asia collision which initially caused the crustal thickening. Intrusion of mantle melts provided heat to promote crustal melting and may have selectively contaminated the granite magma.

The Baltoro granite intrudes sillimanite gneisses with melt pods along the southern margin indicating temperatures above 700°C at the time of intrusion. Locally, internal fabrics and numerous aligned xenoliths along the southern margin in the Biafo glacier region indicate steep, southward-directed thrusting during emplacement. Along the northern contact, the Baltoro granite intrudes anchimetamorphic to greenschist facies metasedimentary rocks with an andalusite-bearing contact aureole. Northward-directed culmination collapse normal faulting during Miocene emplacement is inferred, in order to explain the P-T differences either side of the pluton. This also provided an extensional stress regime in the upper crust to accommodate the rising magma.

Felsic I-type granites and associated volcanic rocks of Carboniferous age are extensively developed over an area of 15,000 km2 in northern Queensland. These granites have been subdivided into four supersuites: Almaden, Claret Creek, Ootann and O'Briens Creek.

Granites of the Almaden Supersuite are intermediate to felsic (56-72% SiO2) and are characterised by high K2O, K/K(K + Na), Rb, Rb/Sr, Th, U and relatively low Ba and Sr. The Claret Creek Supersuite granites are a little more felsic (65-77% SiO2), and are chemically distinctive, having higher A12O3, CaO, Na2O and Sr, and lower K2O, Rb, Th and U than granites of the Almaden Supersuite.

Granites of the Ootann and O'Briens Creek supersuites all contain more than 70% SiO2 and these comprise more than 90% of the total area of granites. These two supersuites are characterised by low Sr, Sr/Y and large negative Eu/Eu*, with the more evolved rocks becoming strongly depleted in TiO2, FeO* MgO, CaO, Ba, Sr, Sc, V, Cr, Ni, Eu, CeN/YN and K/Rb, and enriched in Rb, Pb, Th, U and Rb/Sr. Granites belonging to the O'Briens Creek Supersuite contain significantly higher abundances of HFSE, HREE and F (0·2-0·5 wt%) than those of the Ootann Supersuite, and as such have developed some characteristics of A-type granites.

Geochemical and isotopic properties suggest that all granites are of crustal derivation. The granites of all supersuites have very similar initial 87Sr/86Sr and εNd of 0·710 and −7·0–−8·0, respectively, except where they outcrop within Proterozoic country rocks, when they have more evolved εNd (−8·0–−11·0). Depleted-mantle model ages cluster around 1·5 Ga. The isotope systematics and geochemistry indicate that these granites were not derived from the equivalents of any exposed country rocks.

Models for the petrogenesis of these granites all appear to require the involvement of a long-lived and isotopically homogeneous crustal protolith, that most probably underplated the crust in the Proterozoic. Granites of the two more felsic supersuites were either derived by varying degrees of partial melting from this protolith of andesitic to dacitic composition, and/or were produced by a two-stage process by remelting of intermediate rocks similar in composition to the mafic end-members of the Almaden Supersuite. The resulting primary partial melts for the Ootann and O'Briens Creek supersuites underwent extensive, high-level, feldspar-dominated, crystal fractionation.

I-type granites are produced by partial melting of older igneous rocks that are metaluminous and hence have not undergone any significant amount of chemical weathering. In the Lachlan Fold Belt of southeastern Australia and the Caledonian Fold Belt of Britain and Ireland there was a major magmatic event close to 400 Ma ago involving a massive introduction of heat into the crust. In both areas, that Caledonian-age event produced large volumes of I-type granite and related volcanic rocks. Granites of these two areas are not identical in character but they do show many similarities and are markedly different from many of the granites found in Mesozoic and younger fold belts. These younger, dominantly tonalitic, granites have compositions similar to those of the more felsic volcanic rocks forming at the present time above subduction zones. The Palaeozoic granites show little evidence of such a direct relationship to subduction. Within both the Caledonian and Lachlan belts there are some granites with a composition close to the younger tonalites. A particularly interesting case is that of the Tuross Head Tonalite of the Lachlan Fold Belt, which can be shown to have formed from slightly older source rocks by a process that we refer to as remagmatisation which has caused no significant change in composition. Since remagmatisation has reproduced the former source composition in the younger rocks, the wrong inference would result from the use of that composition to deduce the tectonic conditions at the time of formation of the tonalite. Granites, particularly the more mafic ones, will generally have compositions reflecting the compositions of their source rocks, and attempts to use granite compositions to reconstruct the tectonic environment at the time of formation of the granite may be looking instead at an older event. This is probably also the case for some andesites formed at continental margins.

Several arguments can be presented in favour of a general model for the production of I-type granite sources by underplating the crust, so that the source rocks are infracrustal. Such sources may contain a component of subducted sediments with the consequence that some of the compositional characteristics of sedimentary rocks may be present in I-type source rocks and in the granites derived from them. The small bodies of mafic granite and gabbro associated with island arc volcanism have an origin that can be related to the partial melting of subducted oceanic crust or of mantle material overlying such slabs and can be referred to as M-type. These rocks have compositions indistinguishable from those of the related volcanic rocks, except for a small component of cumulative material. The tonalitic I-type granites characteristic of the Cordillera are probably derived from such M-type rocks of basaltic to andesitic composition, which had been underplated beneath the crust. Some of the more mafic tonalites of the Caledonian-age fold belts may also have had a similar origin. More commonly, however, the plutonic rocks of the older belts are granodioritic and these probably represent the products of partial melting of older tonalitic I-type source rocks in the deep crust, these having compositions and origins analogous to the tonalites of the Cordillera. In this way, multiple episodes of partial melting, accompanied by fractionation of the magmas, can produce quite felsic rocks from original source rocks in the mantle or mantle wedge. These are essential processes in the evolution of the crust, since the first stages in this process produce new crust and the later magmatic events redistribute this material vertically without the addition of significant amounts of new crust.