Abstract

The Kennack Gneiss comprises a suite of interlayered mafic and felsic igneous rocks that intruded the 397 Ma Lizard Ophiolite Complex, Cornwall, at 376.4±1.7 Ma (U–Pb single-zircon date) and were shortly thereafter (c. 370 Ma) metamorphosed to the amphibolite facies. Weakly deformed examples at the type-locality of Kennack Sands reveal net-veining of the mafic by the felsic component, dispersed enclaves of the former in the latter and flame-like interfingering of the two, features indicative of magmatic commingling and mixing.

The silicic components of the Kennack Gneiss range from granodiorite to syenogranite and from metaluminous to peraluminous, the least silicic rocks being the most peraluminous. They were generated through anatexis of a predominantly metasedimentary crustal source having trace element and isotopic compositions comparable to those of the Devonian Gramscatho Group of South Cornwall. The mafic component of the gneiss includes members (group 1) which are weakly enriched in light REE, have minor negative Ta, Nb, P and Ti anomalies in extended trace element plots, and display primitive, time-corrected 143Nd/144Nd and 87Sr/86Sr values, and others (group 2) which are more strongly enriched in all incompatible trace elements, exhibit more prominent negative Ta, Nb, P and Ti anomalies, and have isotopic ratios intermediate between group 1 mafic components and the associated felsic gneiss. Group 2 represents mixtures of group 1 and granitic melts, whereas group 1 rocks are comparable to EMORB and were probably generated through partial melting of a weakly enriched mantle source. Emplacement of two commingled magmas into the base of the Lizard ophiolite, one representing the products of crustal anatexis, implies that the ophiolite was removed from its oceanic setting and was in the process of obduction in the Givetian.

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