Late Archaean granitic rocks from the southern Yilgarn Craton of Western Australia have a close temporal relationship to the basaltic and komatiitic volcanism which occurs within spatially associated greenstone belts. Greenstone volcanism apparently began ∼2715 Ma ago, whereas voluminous felsic magmatism (both extrusive and intrusive) began about 2690 Ma ago. A brief but voluminous episode of crust-derived magmatism ∼2690–2685 Ma ago resulted in the emplacement of a diverse assemblage of plutons having granodioritic, monzogranitic and tonalitic compositions. This early felsic episode was followed immediately by the emplacement of mafic sills, and, after a further time delay, by a second episode of voluminous crust-derived magmatism dominated by monzogranite but containing plutons covering a wide compositional range, including diorite, granodiorite and tonalite. The products of this 2665– 2660 Ma magmatic episode now form a significant fraction of the exposed southern Yilgarn Craton. Later magmatism, which continued to at least 2600 Ma ago, appears largely restricted to rocks having unusually fractionated compositions. The magmatic sequence basalt–voluminous crust-derived magmatism–later diverse magmatism, is interpreted in terms of a dynamically-based model for the ascent of the head of a new mantle plume. In this model basalts and komatiites are derived by decompression melting of rising plume material, and the crust-derived magmas result after conductive transport of heat from the top of the plume head into overlying continental crust. This type of magmatic evolution, the fundamentally bimodal nature of the magmatism, the presence of high-Mg volcanics (komatiites), and the areal extent of the late Archaean magmatic event, are all suggested to be characteristic of crustal reworking above mantle plumes rather than resulting from other processes, such as those related to subduction.

Four chemically and mineralogically distinct rock suites formed in south-central Wisconsin during a major anorogenic intrusive-extrusive event 1.76 b.y. ago. This anorogenic event followed by at least 60 m.y. the terminal calc-alkalic plutonism and volcanism of the Penokean orogeny. The 1.76-b.y.-old rocks may represent the only surface expression in the Lake Superior area of a 1.69 to 1.78-b.y.-old felsic terrain that can be traced in the subsurface from Wisconsin into northern Illinois and possibly as far as western Arizona. The magma types formed during the anorogenic event are (1) a peraluminous suite of texturally variable ash-flow tuffs; (2) a meta-luminous suite containing quartz- and orthoclase-bearing rhyolites and closely associated granophyric granites; (3) a suite of low SiO 2 , high Sr, and REE depleted granites at Baxter Hollow and chemically similar rhyolite dikes on Observatory Hill and at Marquette (Baxter Hollow suite); and (4) a suite containing a diorite intrusion and numerous dikes of tholeiitic basalt and andesite (Denzer suite). Major and trace element modeling studies suggest the following scenario for the formation of these post-Penokean rocks. The peraluminous and metaluminous suiteswere probably derived by partial melting (16%) of different crustal sources of tonalitic composition having slightly different residue mineralogies. These source rocks may have formed earlier by igneous activity related to the Penokean orogeny. Compositional variation within the two suites may be related to feldspar-dominated fractional crystallization. The Baxter Hollow suite was probably generated by large-scale (66%) partial melting of a crustal source of dioritic composition. Heat for such a major melting episode may have been provided by magma of the Denzer suite rising from a mantle source. Modeling studies suggest that 10% melting of either garnet peridotite, or eclogite will provide liquid of the composition of the Denzer suite. Fractional crystallization is most probably responsible for the chemical variation observed within both the Denzer and Baxter Hollow suites.

Melting experiments with and without added H 2 O on a model metagreywacke and a natural metapelite demonstrate how pressure and H 2 O content control the compositions of melts and residual assemblages. Several effects are observed under isothermal conditions. Firstly, the stability field of biotite shrinks with decreasing pressure and with increasing H 2 O content, whereas that of plagioclase shrinks with increasing pressure and H 2 O content. Secondly, the ferromagnesian content of melts at the source (i.e. coexisting with their residual assemblages) decreases with decreasing H 2 O activity. Thirdly, with increasing pressure the Ca/Mg and Ca/Fe ratios of melts decrease relative to those of coexisting garnet. As a consequence, a wide spectrum of melts and crystalline residues can be generated from the same source material. For example, H 2 O-starved dehydration melting of metagreywacke at low pressure (⩽ 10 kbar) generates K-rich (granitic) melts that coexist with pyroxene- and plagioclase-rich residues, whereas melting of the same material at high pressure (≈ 15 kbar) and with minor H 2 O infiltration can generate leucocratic Na-rich and Ca-poor (trondhjemitic) melts that coexist with biotite- and garnet-rich residues. An increased H 2 O content stabilises orthopyroxene at the expense of garnet + biotite + plagioclase, causing melts to shift towards granodioritic or perhaps tonalitic compositions.