Systematic variation in chemistry and Nd-Sr isotopes across a Caledonian calc-alkaline volcanic arc: implications for source materials

Abstract

Sr and Nd isotope compositions, and major and trace element chemistry, are reported for 42 primitive calc-alkaline lavas from the Siluro-Devonian Scottish Old Red Sandstone. Northwestward increases in concentrations of Sr, Ba, K, P and LREE associated with northwest subduction of Iapetus ocean crust [1] are not accompanied by change in Rb, Th or Ti-group elements. These relationships show that the spatial variation cannot be simply the result of change in the degree of partial melting with depth. Initial 143Nd/ 144Nd expressed as ε Nd at 410 Ma shows a wide range from +6.4 to −3.7, and in contrast to ε Sr decreases markedly to the northwest. Samples from the Midland Valley show a strong anticorrelation between ε Nd and ε Sr , with the correlation band offset to more radiogenic compositions than that defined by modern oceanic volcanic rocks (the “mantle array”). Such an offset is common in modern volcanic arc magmas, but cannot be explained by addition to the overlying mantle of a single component enriched in radiogenic Sr derived from subducted lithosphere. Southwest Highland lavas lie in a completely separate field on an ε Sr-ε Nd diagram: most fall within the mantle array, but samples with highest concentrations of Sr, etc., are offset to less radiogenic compositions. Consideration of the chemical and isotopic relationships with respect to several models of crustal contamination shows that the additional LREE in the northwest cannot be derived from a crustal source. The data imply the existence beneath Scotland of a large region of mantle with time-integrated LREE enrichment, parts of which may have anomalous Nd-Sr isotopic compositions less radiogenic than the mantle array. Midland Valley lavas are probably derived from a long-term LREE-depleted mantle source modified by interaction with subducted oceanic lithosphere. The coexistence of two mantle sources with differing histories is best explained by vertical stratification, with deeper, more LREE-enriched layers accessible only to magmas formed at great depth in a subduction zone. The spatial variation probably in part reflects this mantle layering and in part the operation of some mantle zone-refining process that permits efficient concentration of incompatible elements from large regions of mantle.