Volcanic and igneous plumbing systems

[1] Channelled ascent mechanisms include the formation of dykes[3] and ductile fractures that transport the melt in conduits.

When the mantle materials rise, the pressure greatly decreases which significantly lowers the melting point of the rock.

Depending on the efficiency of the segregation and extraction, there will be different structures of the volcanic and igneous plumbing systems.

[1] For instance, if the magma channels are not well connected, the source may not be drained successfully, and dykes may freeze before propagating far enough to feed plutons.

A diapir forms when a blob of buoyant, hot, and ductile magma ascends to a higher lithospheric layer.

[12][5] As the melt is less dense than the surrounding rock, Rayleigh-Taylor instabilities will grow and amplify, and eventually become diapirs.

[12][5] Stoke diapirism is a viable mechanism preferably for the ascent of massive magma bodies in a weak and ductile crust.

[13] Recent studies demonstrated that a dyke-diapir hybrid model may be a more realistic mechanism of diapir formation.

[14] It also demonstrates that episodic injection of magma is crucial in maintaining the temperature of the diapir system and preventing it from freezing.

Therefore, some dykes may rise to the surface, but the majority of them terminates at depth because of solidification of a blockage of rigid layer.

[17] The geometry of the dyke is related to the stress field and the distribution of pre-existing faults and joints in the country rock.

Fault and shear zones act as lines of weakness for magma to flow in and transport to upper levels.

[19] Particularly, a transpressional fault that cuts through layers is related to the transportation and ascent of magma by creating space for emplacement.

[2] From field evidence, the formation of plutons involves multiple stages of magma injection instead of a single pulse.

[21] According to the depth of formation and geometry, magma emplacement can be classified into plutons, sills, laccoliths and lopoliths.

[4] From field evidence, when plutons are formed in a ductile environment, it will displace the surrounding rocks both laterally and vertically.

They may have roots that tapers downwards which eventually become cylindrical-shaped feeder structures which cause the floors to dip inward at different angles.

[1] Sills are generally defined as sheet intrusions which are tabular in shape and dominantly concordant to the surrounding rock layers.

[25] However, in some cases, sills may deform sedimentary layers and exhibit other geometries such as inclined or sub-vertical shapes.

Depending to its shape and concordance to the country rock, sills can be classified into five different types based on field evidence.

[27] Transgressive sills cut through and propagate to higher layers with an oblique angle to the host rock, displaying discordant properties.

[28] They typically display dome-shaped structures with slightly elevated roofs and flat floors that are concordant to rock layers.

[28] These lines of weakness provide pathways for the formation of initial sill-like structures that are horizontal in shape.

[29] After some time, when the cooling rate decreases, and when the sills continue to stack onto one another, sheet intrusion is no longer a favourable mechanism because the zones of weakness diminish.

The cantilever model describes the formation of the lopoliths as a result of the tilting of floor about a point at the pluton margin.

Schematic sketch of the volcanic and igneous plumbing systems (after Burchardt, 2018). [ 1 ] [ 2 ]
Microscopic view of melt segregation and extraction. [ 6 ] [ 7 ] [ 8 ] When the source rock experiences compaction, minerals start to melt at grain boundaries. Melt droplets then build up and connect into melt pools until they are being extracted.
End members of magma segregation, ascent, and displacement: Diapirism and Channeled ascent (after Cruden, 2018). [ 4 ] Diapirs transport melt in a large batch of magma and emplace as plutons. Transport channels transports melt in a fracture network and emplace as dykes and sills. [ 4 ]
Pegmatite dyke intruding quartzite in Marquenas Formation, New Mexico, United States
Shape of different magma emplacement structures: (a)sill, (b) pluton, (c) laccolith and (d) lopolith. [ 4 ] Sills are tabular sheet intrusions. Plutons are large, thick tabular bodies. Laccoliths are dome-shaped structures with elevate roofs and flat floors. Lopoliths are lenticular structures with flat roofs and depressed floors. [ 4 ]
Classification of plutons depending on the geometry of pluton floor. Wedge-shaped plutons are circular to elliptical in shape, whereas tablet-shaped plutons are in disc-shape. [ 22 ]
Sill intrusion in Yellowstone National Park.
Different geometry of sills (After Galland et al., 2018). [ 25 ] [ 27 ] [ 26 ] They can be concordant (parallel to layers), discordant (cutting across layers), or a mix of two.
Formation of a laccolith (After Morgan, 2018). [ 28 ] The joints in the country rock allow sills to intrude, stack on top of each other, and eventually lead to vertical inflation and roof lifting, forming laccoliths. [ 28 ]
Laccolith in Limestone Butte, Montana
Two models of lopolith formation: the cantilevel model and the pistol model (After Cruden & Weinberg, 2018). [ 4 ] In the Cantilever model, lopoliths form by tilting of the floor. In the Pistol model, lopoliths form.by vertical subsidence of the floor. [ 4 ]