New Sr and Nd isotope and rare earth element data obtained from primary apatite microphenocysts indicate that the Dob’s Linn metabentonites originated from a mildly depleted mantle source, probably generated in a continental margin or back-arc basin environment. There is a small but notable reduction in crustal contamination of the source magmas through two graptolite zones (convolutus to sedwickii) indicated by a shift to higher ɛNd values. This shift is consistent with a magma chamber being periodically refilled with mantle-derived melts.

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