Three Types of Igneous Layering: 18-Geol-A1 Mineralogy and Petrology, May 2017, Q13

Question 13: Three Types of Igneous Layering (Part 2 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Three types of igneous layering (layered mafic/ultramafic intrusions)
Type	How it forms	Example
Modal layering	Repeated variation in the relative proportions of cumulus minerals, produced by episodic changes in the crystallizing assemblage or settling dynamics (density currents, periodic recharge of fresh magma, in-situ nucleation events).	Rhythmic chromitite seams within the Bushveld Complex, South Africa.
Phase layering	Marked by the abrupt appearance or disappearance of a cumulus phase, recording a discrete point on the liquid line of descent where the crystallizing mineral assemblage changed.	The base of the Middle Zone in the Skaergaard Intrusion, Greenland, marked by the entry of cumulus magnetite.
Cryptic layering	A smooth, progressive (not abrupt) change in the composition of a single cumulus mineral with stratigraphic height, recording continuous fractional crystallization of the parent magma without any change in modal proportions or phase assemblage.	The systematic upward decrease in olivine forsterite content through the Skaergaard Layered Series.

Topic. Igneous layering styles in layered mafic/ultramafic intrusions.

Key relations. Modal layering = variation in mineral proportions; phase layering = a discrete phase-assemblage change; cryptic layering = continuous compositional change within one mineral.

Why this works. The three types record three physically distinct aspects of the same underlying crystallization history — modal layering reflects episodic depositional dynamics on the chamber floor, phase layering marks discrete points where the crystallizing assemblage itself changed, and cryptic layering is the continuous chemical record of ongoing fractional crystallization — together they let a petrologist reconstruct both the depositional processes and the compositional evolution of a layered intrusion's parent magma.

Common pitfall. Confusing cryptic layering (invisible in hand specimen, detected only by mineral chemical analysis) with modal or phase layering, both of which are visible as physical banding in outcrop.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §10 (layered intrusions, modal/phase/cryptic layering).

Question 13 of 13: Three Types of Igneous Layering

Question 13: Three Types of Igneous Layering (Part 2 – 10 marks)