HR: 09:45h
AN: V21A-06 INVITED     [Abstracts]
TI: Hydraulic Inflation and Buoyancy Pumping: A Model for Large, Fracture-Mediated Felsic Intrusions
AU: * Clemens, J D
EM: j.clemens@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AU: Ablay, G J
EM: Girayablay@aol.com
AF: seeking affiliation, at home,
AU: Grocott, J
EM: j.grocott@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AU: Petford, N
EM: n.pet@kingston.ac.uk
AF: School of ESG, CEESR, Kingston University, Penrhyn Rd, Kingston-upon-Thames, KT1 2EE United Kingdom
AB: Large, intraplate, felsic intrusions with crustal anatectic sources, pipe-like conduits and tabular plutons are analysed, emphasising the magma-intrinsic factors that control their development. Conductively heated magma sources are broad and domical. Volume changes during melting generate buoyancy and hydraulic magmatic loads. The deformation state of the lithosphere determines its response. Rock strength depends on loading rate and limits differential stress. Tensile failure requires magma pore pressure, to reduce confining stress, while magma wedging in cracks modifies the stress field for vertical cracking. Non-magmatic loads include gravity and, critically, horizontal tensile stress from uplift, which favours vertical cracks. Tectonic forces are secondary. Intrusion begins with instability, due to the presence of magma and feedback between magmatic and ambient source loading. Hydrostatic magma pressure P_M is augmented by buoyancy and melting dilativity overpressures (ΔP_B and ΔP_V). ΔP_B^max increases with source height h while non-relaxed dilation creates ΔP_V (proportional to h³), which arises, instantaneously, to the wall-rock strength. ΔP_V confers high mechanical efficiency and increases faster than ΔP_B to a max. that is orders of magnitude greater. Inelastic uplift results but, since full inelastic relaxation of ΔP_V is impossible, elastic source compression results, providing hydraulic impetus for brittle source rupture and magma flow. For extensive melting, a molten cavity may develop. At lower melt fractions, melt veins form, connect and propagate as dykes. Magma pore overpressure and wedging fulfil the stress criteria for tensile rupture, whose geometry depends on the initial stress field (σ_H- or σ_V-dominant). In domed crust, radial ruptures focus to form a wide, central, pipe-like conduit. Requirements for a viable conduit are; (i) σ_V1; (ii) a positive gradient in P_M, and; (iii) an aperture adequate to prevent magma freezing. If P_M in dykes increases σ_H to σ_H1, then dykes re-orientate to sills, terminating ascent and initiating emplacement. Once stagnated, static ΔP_B is typically sufficient to force sill injection at depths less than a critical value D, where ΔP_B = σ_V. Sills growth is dominated by floor depression. Underburden subsidence suppresses roof uplift, influences the sill's plan geometry, expels source magma, processes crust through the melting zone, decreases σ_H in down-warped crust to favour conduit widening and magma ascent, and drains the source. Hydraulic inflation may end by melting cessation or exhaustion of excess magma volume EMV. Inelastic source swelling (φ) or crack growth (η) relax EMV. Exhaustion of non-relaxed EMV (EMV*) divides intrusion into two regimes; hydraulic inflation and buoyancy pumping. Loss of hydraulic drive (EMV* = 0) occurs either during dyke or sill growth, depending on the initial EMV (source volume and melting dilativity). Once rupture occurs, disequilibrium cracking initiates, and η increases abruptly as stored EMV* converts to crack volume. Equilibrium cracking begins once reduced EMV* balances new crack growth. If a sill is available to decouple shallow and deep crust, buoyancy pumping initiates, where underburden subsidence empties the magma source.
DE: 8035 Pluton emplacement
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly