Establishing metamorphic history from clay minerals

Figure 1. Thornton Force, Ingleton, Yorkshire. Carboniferous sediments overlie likely Ordovician rocks of the Ingleton Group.

Analysis of clay minerals using X-ray diffration (XRD) techniques has revealed a complex history of metamorphism in the Precambrian to Silurian mudstone, greywacke and conglomerate sequences of north-west Yorkshire, England. Use of illite and chlorite crystallinity measurements has established a metamorphic inversion with higher grade metamorphic rocks of the Late Ordovican to Silurian Windermere Supergroup overlying low-grade earlier Ingleton Group strata. The differences suggest differing geotectonic settings, with Ingleton rocks most likely evolving in high-heatflow conditions consistent with an extensional basin, and Windermere strata evolving in a low-heatflow convergent plate setting. The data provide further information to support an Ordovician age for the Ingleton Group.

XRD as a tool to indicate clay mineral evolution

Figure 2. b cell dimensions against d001 spacing for K-white mica from the Windermere and Ingleton strata

Changes in clay-mineral structure during diagenesis and metamorphism are well-characterised and predictable patterns of change in mineral assemblages can reveal much information about changing conditions of formation. Variations in clay mineralogy in rocks can be studied routinely using XRD techniques and particularly useful are the illite crystallinity of dioctohedral clay minerals and chlorite crystallinity (14Å or 7Å) of trioctohedral clays. We have also used the b cell dimension of K-white mica to establish the pressure conditions at the time of mica formation or recrystallisation of the Ingleton and Windermere rocks.

The data record the separate tectonic settings of the two groups and their likely origin from two separate metamorphic events.

Further information

Kemp, S J and Merriman, R J. 2008. Polyphase low-grade metamorphism of the Ingleton Group, northern England, UK: a case study of metamorphic inversion in a mudrock succession. Geological Magazine, 146, 237 251.

Contact

Contact Simon Kemp for further information