A geochemical comparison of the Dalradian Leven Schists and the Grampian Division Monadhliath Schists of Scotland

Abstract

Suites of samples from the type area of the Leven Schist and from localities within the Monadhliath Schists of the Central Highlands have been analysed for major and trace elements and for their Sr isotopes. Petrologically the two schist groups are consistently different: the Leven Schists are muscovite-rich, plagioclase-poor semipelites to pelites, whereas the Monadhliath Schists are a typical ‘Moinian’ semipelite, consisting almost entirely of biotite, sodic andesine and quartz. Equally, consistent geochemical differences between the two include more Nb, Zr, Y and Rb, and less Ca, Sr and Ni in the Leven than the Monadhliath Schists. Ratios of Rb: Sr and of Y to P 2 ,O 5 , Sr and CaO discriminate the Leven from the Monadhliath schists. Factor analyses of the chemical data also serve to distinguish the two suites Rb-Sr isochron studies give 655±f2.5Ma, initial 87 Sr/ 86 Sr = 0.7232±f10, for the Leven Schists, and 486±9Ma, initial "Sr 87 /"Sr 86 = 0.7190±1, for the Monadhliath Schists and also separate the two formations from one another. The 655 Ma age for the Leven Schists is thought to record sedimentation and diagenesis, whereas the 486±9 age for the Monadhliath Schists is probably related to the cessation of Rb and Sr migration at or after the peak of the Grampian orogeny. Their Sr isotopic compositions were also completely different hack to 1300 Ma ago and serve to confirm the lack of any continuity between the Leven and Monadhliath schists. Sets of schists located between the main outcrop areas were also analysed and can be assigned without ambiguity either to the Leven or the Monadhliath schists on the basis of their chemistry. The results of this study imply that the Lower Dalradian Appin Group rests on the Grampian Division unconformably and that the latter is closely related, through its geochemistry, to the Moinian NW of the Great Glen.