Igneous Layering in Basaltic Magma Chambers

Magmatic structures in the Aztec Wash pluton preserve a record of repeated injections of mafic magma into a felsic magma chamber and provide information about subsequent tilting. The pluton is divided into a southern (lower) heterogeneous zone (HZ), which contains the abundant magmatic structures, and a northern (upper) homogeneous granite (HG), which is virtually structureless. HG is entirely felsic (most-72 wt.% SiO2), whereas the HZ is extremely diverse in composition (44–76 wt% SiO2). Sequences of meter-scale mafic sheets with intervening felsic material mark repeated injection of mafic magma into the HZ. The sheets spread laterally and ponded, trapping more felsic magma or crystal mush beneath them. Most of the sheets are fine-grained with delicately crenulated, quenched margins against felsic rock, but thick sheets apparently cooled slowly enough to crystallize coarse cumulates. Gravitationally unstable felsic melt in the trapped felsic layers intruded the overlying mafic layers to form vertical sheets and pipes. The lateral spreading of the mafic sheets was interrupted near the margins of the HZ, where magma injected and disaggregated the wall rock to form abundant xenolithic blocks. After the HZ was semisolid, composite vertical sheets with mafic pillows in a felsic matrix intruded, apparently as a coarse slurry, in response to fresh injections of mafic magma into the base of the chamber. A gradational contact with the overlying HG suggests that HG was still at least partly molten when late HZ magmas intruded, but preservation of significant topography on the HZ-HG surface and of trains of mafic material above the contact indicate that it had appreciable strength. The structures in the HZ demonstrate the complex, protracted way in which magma chambers can fill. This provides a mechanism for maintaining a chamber in a partially molten state for a much longer interval than if it had been emplaced all at once, or had contained only mafic or felsic magma. Furthermore, because many of the structures are inferred to have been either horizontal or vertical when they formed, they provide a means of estimating crustal tilting and therefore of reconstructing the original geometry of the pluton. The tilt estimates also clarify important extensional structures in this area that were difficult to define because of the absence of stratified rocks.

Magma chamber

10.1201/9780429070877-10

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Citations (9)

Intercumulus igneous layering in the Kalka Layered Intrusion, central Australia

Geological Magazine (1977)

A. D. T. Goode

Summary Igneous layering resulting from variations in intercumulus mineralogy has been observed in the Kalka Intrusion in central Australia. The layering is parallel to cumulus layering, and is outlined by a mottled heteradcumulus ‘clump’ texture in leucogabbros, which occur in a generally adcumulate anorthositic sequence. Such intercumulus layering is thought to originate in slow cooling intrusions at times of slow crystal accumulation and/or relatively high rates of intercumulus crystallization when a zone of melt supersaturation (the cumulus nucleation zone) rises above the intrusion floor leaving the intercumulus melt undersaturated with respect to the settled cumulus phases. Adcumulates are produced when the supersaturated zone is coincident with the top of the crystal pile. The morphological elongation of clump plates parallel to the gross layering is probably controlled by the cumulus plagioclase fabric which tends to be planar laminate.

Layering

Layered intrusion

Texture (cosmology)

10.1017/s0016756800044794

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Citations (13)

Granites, dynamic magma chamber processes and pluton construction: the Aztec Wash pluton, Eldorado Mountains, Nevada, USA

Earth and Environmental Science Transactions of the Royal Society of Edinburgh (2004)

Brian E. Harper Calvin F. Miller G. Christopher Koteas N. L. Cates Robert A. Wiebe

ABSTRACT The mid-Miocene Aztec Wash pluton is divisible into a relatively homogeneous portion entirely comprising granites (the G zone, or GZ), and an extremely heterogeneous zone (HZ) that includes the products of the mingling, mixing and fractional crystallisation of mafic and felsic magmas. Though far less variable than the HZ, the GZ nonetheless records a dynamic history characterised by cyclic deposition of the solidifying products of the felsic portion of a recharging, open-system magma chamber. Tilting has exposed a 5-km section through the GZ and adjacent portions of the HZ. A porphyry is interpreted as a remnant of a chilled roof zone that marks the first stage of felsic GZ intrusion. Subsequent recharging by felsic and mafic magma, reflected by repeated cycles of crystal accumulation and melt segregation in the GZ and emplacement of mafic flows in the HZ, rejuvenated and maintained the chamber. Kilometre-scale lobes of mafic HZ material were deposited as prograding tongues into the GZ during periods of increased mafic input. Thus, they are lateral equivalents of the cumulate GZ granites with which they interfinger. Conglomerate-like units comprising rounded, matrix-supported intermediate clasts in cumulate granite are located immediately above the lobes. These ‘conglomerates’ appear to represent debris flows shed from sloping upper surfaces of the lobes. Thus, the GZ can be viewed as comprising distal facies, remote from the site of mafic recharging in the HZ, and the HZ as comprising proximal facies. Elemental chemistry suggests that the GZ cumulate granites represent a second-stage accumulation from an already evolved melt, and that coarse, more mafic, feldspar+biotite+accessory mineral ± hornblende rocks trapped between mafic sheets in the HZ are the initial cumulates. Fractionated melt accumulated roofward and laterally, and was the direct parent of the ‘evolved’ GZ cumulates. The most highly fractionated, fluid-rich melts accumulated at the roof.

Felsic

Magma chamber

Fractional crystallization (geology)

10.1017/s0263593300001073

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Citations (42)

Igneous Layering in Basaltic Magma Chambers

Springer geology (2015)

Olivier Namur Bénédicte Abily A. E. Boudreau François Blanchette John W. M. Bush

Layering

Magma chamber

Settling