Net-veined and mixed-magma ring intrusions are a characteristic simultaneous emplacement of mafic, intermediate and silicic magmas. Net-veined ring-intrusions consist of areas of granophyric net-veined mafic to intermediate rock which elsewhere forms rounded to elongate pillows in a silicic host rock. Mixed-magma ring intrusions contain small inclusions of intermediate to mafic rock dispersed within a silicic host rock.

The net-veined intrusions of St. Kilda and Ardnamurchan consist of a wide range of magmas intruded during several closely related intrusive phases into the same ring fracture. Volatile supersaturated silicic magma was intruded into elongate pillows on St. Kilda, rounded pillows on Ardnamurchan and intricate net-veining where silicic magma brecciated cooler more solidified mafic magma.

The ’mafic' component covers a wide compositional range and is dominated by basaltic-andesite and andesite. Detailed petrographical and geochemical (major, trace element and strontium isotope) analyses show these intrusions represent commingling of highly evolved tholeiitic magmas ranging from ferrobasalt through basaltic-andesite and andesite, to rhyodacite and rhyolite. The magmas represent both fractionates of basaltic magma and hybrids of silicic, intermediate and mafic magmas. Strontium isotope data also indicate that most of the magmas have been affected by minor crustal contamination. Processes of crystal fractionation and magma mixing have been modelled using a least-squares computer program and the Rayleigh fractionation equation.

Analogous compositional diversity and mixed-magma features in extrusive rocks from Icelandic central volcanoes of Hekla and Askja indicate that these ring intrusions originate from compositionally zoned magma chambers. Density relationships suggest that zonation in chambers of tholeiitic systems can result for magmas more evolved than ferrobasalt. Zonation can result from sidewall crystallisation, wall melting and magma chamber replenishment.

In St. Kilda and Ardnamurchan an early compositionally zoned (ferrobasalt to high-silica andesite) magma chamber is intruded into the ring fracture by central block subsidence. This is rapidly succeeded, within 10$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$-10$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ years, by intrusion of silicic magma, derived by further differentiation in the chamber and partial melting of crustal material, into hot intermediate and mafic magma/crystal mush, producing pillows and net-veining in the ring fracture. Mixing is incomplete due to contrasting viscosities and solidus temperatures of the different magmas. Volatile exsolution enhances mixing and net-veining and leads to the violent, explosive emplacement of magma at subvolcanic levels, sometimes accompanied by the eruption of mixed magma at the surface.