Mineral growth in melt conduits as a mechanism for igneous layering in shallow arc plutons: mineral chemistry of Fisher Lake orbicules and comb layers (Sierra Nevada, USA)

• Published in: Contributions to mineralogy and petrology Vol. 172; no. 7; p. 1
• Main Authors: McCarthy, Anders, Müntener, Othmar
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.07.2017; Springer Nature B.V
• Subjects: Apatite; Chemistry; Conduits; Control; Control systems; Cooling; Cooling rate; Crystallization; Crystals; Decompression; Dynamical systems; Dynamics; Earth and Environmental Science; Earth Sciences; Extraction; Forming; Fronts; Geochemistry; Geology; Gradients; Growth; Heat; Heat of crystallization; Igneous rocks; Lakes; Latent heat; Lava; Layering; Magma; Mineral Resources; Mineralogy; Original Paper; Petrology; Plagioclase; Plutons; Quartz; Reservoirs; Rims; Rock; Rocks; Sierra Nevada Mountain Range; Igneous layering; Comb layers; Orbicules; Sierra Nevada
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-017-1371-z

Different processes have been proposed to explain the variety of igneous layering in plutonic rocks. To constrain the mechanisms of emplacement and crystallization of ascending magma batches in shallow plutons, we have studied comb layers and orbicules from the Fisher Lake Pluton, Northern Sierra Nevada. Through a detailed study of the mineralogy and bulk chemistry of 70 individual layers, we show that comb layers and orbicule rims show no evidence of forming through a self-organizing, oscillatory crystallization process, but represent crystallization fronts resulting from in situ crystallization and extraction of evolved melt fractions during decompression-driven crystallization, forming a plagioclase-dominated cres-cumulate at the mm- to m-scale. We propose that the crystal content of the melt and the dynamics of the magmatic system control the mechanisms responsible for vertical igneous layering in shallow reservoirs. As comb layers crystallize on wall rocks, the higher thermal gradients will increase the diversity of comb layering, expressed by inefficient melt extraction, thereby forming amphibole comb layers and trapped apatite + quartz saturated evolved melt fractions. High-An plagioclase (An 90 –An 97.5 ) is a widespread phase in Fisher lake comb layers and orbicule rims. We show that a combination of cooling rate, latent heat of crystallization and pressure variations may account for high-An plagioclase in shallow melt extraction zones.