I LL IT IZAT ION AND ORGANIC MATURITY IN S ILURIAN SEDIMENTS FROM THE SOUTHERN UPLANDS OF SCOTLAND

ABSTRACT: Discrepancies between clay mineral and organic indicators of very low-grade metamorphism have been observed from the Southern Uplands of Scotland. In the Southern Belt, I-S expandability ranges from 18-5 % and chitinozoan reflectance from 0.8--2.1 % R0 mean; a clear correlation exists between I-S expandability and organic maturity. At Dob's Linn in the Central Belt, I-S expandabilities range from 18-10%, and chitinozoan reflectance from 3.6-4-8 % R 0 mean. The results from Dob's Linn show anomalously high I-S expandabilities with respect to maturity, both in comparison with the Southern Belt and with previous work from other egions. K-feldspar ispresent in variable amounts in the Southern Belt, but is not detectable by XRD at Dob's Linn. Low K + activity is believed to have resulted in a slower eaction rate at Dob's Linn, and a consequent "lag " in I-S expandability with respect to the Southern Belt. The degree of expandabil ity of ill ite-smectite (I-S) is often used as a general indicator of metamorphic grade. However, recent work indicates that illitization is kinetically rather than thermodynamically controlled. Indeed, many workers (e.g. Lippmann, 1981; Jiang et al., 1990) consider that smectite, I-S and illite are all metastable under normal burial conditions