Barrovian vs. Buchan Facies Series — Pelitic Mineralogy: 18-Geol-A1 Mineralogy and Petrology, December 2016, Q8

Question 8 (Part 2, Q2): Barrovian vs. Buchan Facies Series — Pelitic Mineralogy (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.

Both series describe the sequence of pelitic mineral assemblages developed with increasing metamorphic grade, but along different P/T gradients — and because the $\text{Al}_2\text{SiO}_5$ polymorphs (andalusite, kyanite, sillimanite) are pressure-sensitive, the two series produce recognizably different index-mineral suites for an identical grade increase.

Barrovian vs. Buchan index-mineral progression (pelitic protolith)
	Barrovian (medium P/T)	Buchan (low P/T)
Index sequence	Chlorite → biotite → garnet → staurolite → kyanite → sillimanite	Chlorite → biotite → andalusite → cordierite → sillimanite
Diagnostic $\text{Al}_2\text{SiO}_5$ polymorph	Kyanite (high-P polymorph) present at intermediate–high grade	Andalusite (low-P polymorph) present at intermediate grade; kyanite absent
Staurolite	Present, a key intermediate-grade index mineral	Typically absent or very restricted (staurolite's stability field is P-sensitive and largely by-passed along the low-P path)
Cordierite	Absent to rare (unstable at Barrovian pressures over most of the sequence)	Common at intermediate–high grade — cordierite's low-P, Mg-Fe-rich stability field is characteristic of the Buchan path

Topic. Facies-series-dependent index mineralogy in pelites.

Key relations. The $\text{Al}_2\text{SiO}_5$ triple point separates the kyanite (higher-P), andalusite (lower-P) and sillimanite (higher-T) stability fields; staurolite and cordierite stability fields are similarly P-dependent.

Why this works. Because a facies series is really just a specific P/T path through the same underlying phase diagram, walking that path at a lower P/T ratio (Buchan, high heat-flow arc/contact-dominated settings) intersects a different sequence of mineral stability fields than walking it at a medium P/T ratio (Barrovian, continent-continent collision) — which is exactly why the presence of kyanite+staurolite vs. andalusite+cordierite in an otherwise similar pelitic schist is used as a direct petrologic read-out of the metamorphic P/T gradient, and hence the tectonic setting, without needing quantitative geobarometry.

Common pitfall. Assuming any $\text{Al}_2\text{SiO}_5$ polymorph is grade-diagnostic on its own; kyanite vs. andalusite is a pressure (facies-series) indicator, not a temperature/grade indicator by itself — sillimanite alone can terminate either series.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §26–27 (facies series, $\text{Al}_2\text{SiO}_5$ phase relations, AFM zone mineralogy).

Question 8 of 13: Barrovian vs. Buchan Facies Series — Pelitic Mineralogy

Question 8 (Part 2, Q2): Barrovian vs. Buchan Facies Series — Pelitic Mineralogy (Part 2 – 10 marks)