Mineralogy & Petrology

Metamorphic Petrology: Facies & Grade

Metamorphic facies, Barrovian index minerals and P–T conditions — reading the temperature and pressure history of a rock.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Metamorphism rearranges minerals in the solid state under changing pressure and temperature. Facies group assemblages by P–T: zeolite → greenschist → amphibolite → granulite with rising T; blueschist and eclogite record high-P/low-T subduction; hornfels records low-P contact heating. In Barrovian (regional) metamorphism, index minerals mark increasing grade: chlorite → biotite → garnet → staurolite → kyanite → sillimanite. Pressure converts to depth via the lithostatic relation.

The Core Formula Matrix

Lithostatic pressure:

P = ρ g z

(≈ 27 MPa/km for crust).

Field gradient:

Δ T /Δ z

between isograds.

Barrovian sequence: chlorite → biotite → garnet → staurolite → kyanite → sillimanite.

Protolith pairs: shale→slate→phyllite→schist→gneiss; limestone→marble; sandstone→quartzite.

The ‘IIT Traps’

⚠
Blueschist = high P, low T. Don't read 'blue' as cold-and-shallow; it is deep subduction.
⚠
Grade ≠ facies. Grade is the intensity (low→high); facies is the specific P–T field.
⚠
Contact vs regional. Hornfels (contact) is non-foliated; schist/gneiss (regional) are foliated.

📚 Standard references

Petrogenesis of Metamorphic Rocks — Bucher & Grapes
Metamorphic Petrology — Vernon & Clarke

PART 2

Progressive 3-Tier Question Suite

Q1MEDIUM1 Mark · MCQ

The metamorphic facies characterised by high pressure and low temperature is

Q2MEDIUM1 Mark · MCQ

Among these Barrovian index minerals, the one marking the highest grade is

Q3HARD2 Marks · NAT

A metamorphic assemblage equilibrated at

0.8

GPa in crust of density

2.8

g/cm³. The depth is _____ km.