AR-40 AR-39 ILLITE DATING OF LATE CALEDONIAN (ACADIAN) METAMORPHISM AND COOLING OF K-BENTONITES AND SLATES FROM THE WELSH BASIN, UK/

The vacuum-encapsulation laser Ar-40-Ar-39 technique allows extremely small (10(-6) g) samples of fine-grained materials such as diagenetic clays to be dated. Here we show that the method can be extended to hig her-grade clay minerals. The integration of transmission electron micr oscopic (TEM) characterization with Ar-40-Ar-39 dating of vacuum encap sulated samples permits the resolution of the timing of metamorphic gr owth/cooling from the time of diagenesis. We have applied this techniq ue to well characterized Lower Paleozoic slates and K-bentonites from the Welsh Basin, which span the transition from anchizonal to epizonal grade, which had been previously studied using Rb-Sr and Sm-Nd dating . TEM observations of epizonal K-bentonites and slate showed that illi te in these samples is of 2M(1) polytype, of muscovite-like compositio n, and oriented parallel to cleavage, suggesting that they are of meta morphic origin. Total gas ages (equivalent to conventional K-Ar ages) for encapsulated epizonal K-bentonites and slate (340-40& Ma) are cons iderably variable. The Ar retention ages (calculated from Ar-39 and Ar -40 atoms retained in the sample after irradiation) are more consisten t (383-411 Ma). The Ar-39 recoil losses are minor for illites from who le rock samples of epizonal K-bentonites but very significant for clay separates of epizonal slate. Plateaus in age spectra were observed in epizonal K-bentonites and slate. The plateau ages (414-421 Ma) and re tention ages (383-411 Ma) can be correlated with the onset of Acadian metamorphism and culmination of uplift and inversion of the Welsh Basi n, respectively. These ages are significantly younger than the similar to 450 Ma ages previously reported for diagenetic clays using the sam e method, suggesting that diagenetic history has been lost in these ep izonal K-bentonites and slate. TEM observations of anchizonal slates s howed that there are two modes of illite. The first mode is similar to that observed in epizonal samples, suggesting a metamorphic origin. T he second mode consists of the 1M(d) polytype, has typical diagenetic illite composition, and is oriented parallel to bedding, suggesting a diagenetic origin. Total gas ages for encapsulated anchizonal slates v ary considerably (361-422 Ma). The retention ages are more consistent (413-432 Ma) than the total gas ages. The Ar-39 recoil losses are more significant than those for epizonal K-bentonites and slate. Plateaus in age spectra are generally not observed. However, the consistent ret ention ages for the anchizonal slates correspond to the plateau ages f or the epizonal samples, and are inferred to represent the onset of Ac adian metamorphism. These data, when combined with our previously publ ished results for diagenetic shales, suggest that thermal conditions n ear the boundary of anchizonal and epizonal grades are necessary to co mpletely reset Ar systems in shares and slates. (C) 1997 Elsevier Scie nce B.V.