HOW TO DATE A CLAY GOUGE? A CASE STUDY FROM TATRA MOUNTAINS, POLAND

In this study seven clay gouges from Tatra Mountains, Poland were investigated for using X-ray diffraction and Ar-Ar dating. Constraining timing of fault activity using radiometric dating is often based on the assumption that dioctahedral mica (“illite”) polytypes in fault zones form following the sequence: 2M₁ polytype the oldest, followed by 1M and 1M_d. A key aim of this study was to test this assumption.

The bulk mineral composition of gouge materials was quartz, plagioclase, K-feldspar, dioctahedral mica (2M₁, 1M, and 1M_d), smectite, and chlorite. Investigation of clay size fractions indicated the presence of mixed-layer R1 illite-smectite and kaolinite along with phases present in bulk. Large variability of gouge compositions among the investigated zones was noted.

Vacuum encapsulated Ar-Ar dates were obtained for four size fractions of one feldspar-free gouge. Three different approaches were used for extracting end-member ages (Tab. 1), including a three-component model proposed in this study.

Tab. 1. Preliminary age values of illite polytypes present in the clay gouge sample (in Ma) obtained with different approaches.

Polytype	Illite Age Analysis*	MODELAGE**	3 component system
2M1	204.6 ± 31	183.8 ± 29	33.1 ± 23
1M	10.4 ± 11	5.4 ± 13	189.8 ± 21
1Md			2.8 ± 5

* Pevear (1992)

**Szczerba and Środoń (2009)

Timing of fault activity was highly dependent on the method used for age calculation. Methods handling two-component system returned age values in rough agreement, pointing towards a young age of the authigenic component and at least a partial age reset of the “inherited” component during Alpine orogeny. The results of three-component age modeling indicated, however, that the assumption underlying conventional age analysis i.e. that 1M and 1M_d illite polytypes form at the same time, may not be always valid.

Acknowledgments:

AK thanks the Clay Minerals Society for founding the research with a Student Research Grant.

References:

Pevear, D.R. (1992) Illite age analysis, a new tool for basin thermal history analysis Pp. 1251-1254 in: Water-Rock Interaction (Y.K. Kharaka and A.S. Maest, editors). A.A. Balkema, Rotterdam, The Netherlands.

Szczerba, M. and Środoń, J. (2009) Extraction of diagenetic and detrital ages and of the ⁴⁰K_detrital/⁴⁰K_diagenetic ratio from K-Ar dates of clay fractions. Clay Clay Miner, 57, 93-103.