2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 10
Presentation Time: 10:45 AM

WHAT'S IMPORTANT IN A ROCK NAME: INSIGHTS FROM DEVELOPING A COMMON MAP LEGEND


EASTON, Robert Michael, Precambrian Geoscience Section, Ontario Geological Survey, 933 Ramsey Lake Road, 7th Floor, Sudbury, ON P3E 6B5, Canada, mike.easton@ontario.ca

Naming a rock might at first seem to be straightforward.  After all, a term such as granodiorite is well-defined, worldwide.  However, when it comes to naming a rock in a geologic map legend, the task is more complex.  For example, there may be 2 granodiorite bodies in a map area, one with a gneissic fabric and an Archean age, another massive, coarse grained, and Proterozoic in age.  I can show the age difference simply by map colour, but what other key descriptive terms should be included in the description, especially if one is trying to create a common map legend for a large geographic area?  The problem becomes even more acute when naming metamorphic, altered, and/or deformed rocks (a.k.a. composite-origin rocks).  For example, is composition critical to the name (e.g., marble); or texture (e.g., schist); or protolith (e.g., metabasalt); or mineralogy (e.g., diopside marble); or a combination of factors (e.g., kyanite metapelite)?  Unfortunately, all recent rock nomenclature schemes for composite-origin rocks have focused on naming such rocks at the hand sample level, not at a macroscopic scale.  To address this problem, in the process of developing a common map legend for Ontario Geological Survey maps, a hierarchical scheme, is proposed, as outlined in brief in the table below.

Type

Subtype

Root Name (examples)

Optional Modifier (textural, mineralogical, descriptive)

Metamorphic

Structural

schist, gneiss, granolite

kyanite schist

Compositional

marble, amphibolite, pelite

calcitic marble

Textural

breccia, agmatite

Hybrid

migmatite

Alteration

Hydrothermal

silicification

Metasomatic

fenite, skarn

Structural

Brittle

cataclastite and related rocks

fault gouge

Ductile

mylonite and related rocks

Impact

melt, breccia