Textural immaturity of cumulates as an indicator of magma chamber processes; infiltration and crystal accumulation in the Rum Eastern Layered Intrusion

• Published in: Journal of the Geological Society Vol. 164; no. 3; pp. 529 - 539
• Main Author: Holness, M. B
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01.05.2007; The Geological Society of London; Geological Society; Geological Society Publishing House
• Subjects: allivalite; Chemicals; compaction; crystal mush; Crystalline rocks; crystals; cumulates; Earth sciences; Earth, ocean, space; Eastern Layered Intrusion; Europe; Exact sciences and technology; Field study; gabbros; Geology; grain boundaries; Grain growth; Grain size; Great Britain; Hebrides; Highland region Scotland; History; igneous and metamorphic rocks; Igneous and metamorphic rocks petrology, volcanic processes, magmas; igneous rocks; Inner Hebrides; Interfaces; intrusions; Inverness-shire Scotland; layered intrusions; magma chambers; magmas; Mineralogy; nesosilicates; olivine; olivine group; orthosilicates; peridotites; petrography; Petrology; plutonic rocks; Rhum; Scotland; settling; silicates; Solidification; Solids; textural maturity; textures; thermal history; troctolite; ultramafics; United Kingdom; Western Europe; thermal history; Allivalites; crystals; layered intrusions; olivine gabbro; textures; accumulation; maturity; topology; gabbros; igneous rocks; indicators; magma chambers; infiltration; grains; solidus; plutonic rocks; cooling; compaction; equilibrium; cumulates; populations
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/0016-76492006-021

Textural maturity describes the extent to which rocks have approached textural equilibrium. For igneous rocks, this is a function of the time-integrated thermal history in the sub-solidus which, for cumulate rocks, generally translates to the cooling history via the rate of solidification of the accumulating crystal mush. The most straightforward way to quantify the textural maturity of orthocumulates (sensu lato) is to measure the angle subtended at the corners of interstitial grains. Texturally mature cumulates have angle populations with median values of c. 120°, whereas texturally immature cumulates have lower medians, inherited by the interstitial phase, which infills either an impingement texture in the accumulating mush or a texturally equilibrated pore topology. Textural observations of allivalites from the Rum Layered Suite demonstrate that stratigraphically localized excursions towards textural immaturity are the result of either transient, relatively rapid, solidification rates (perhaps a result of the arrival of cold crystal-laden bodies of fluid on the chamber floor) or late-stage infiltration of fully solidified cumulates by evolved liquids expelled upwards by compaction of underlying horizons. These two processes have distinct textural and compositional signatures.