The Kennack Gneiss of the Lizard Peninsula, Cornwall, SW England: commingling and mixing of mafic and felsic magmas accompanying Givetian continentalincorporation of the Lizard ophiolite

The Kennack Gneiss comprises a suite of interlayered mafic and felsic igneo us rocks that intruded the 397 Ma Lizard Ophiolite Complex, Cornwall, at 37 6.4 +/- 1.7 Ma (U-Pb single-zircon date) and were shortly thereafter (c. 37 0 Ma) metamorphosed to the amphibolite facies. Weakly deformed examples at the type-locality of Kennack Sands reveal net-veining of the mafic by the f elsic component, dispersed enclaves of the former in the latter and flame-l ike interfingering of the two, features indicative of magmatic commingling and mixing. The silicic components of the Kennack Gneiss range from granodiorite to sye nogranite and from metaluminous to peraluminous, the least silicic rocks be ing the most peraluminous. They were generated through anatexis of a predom inantly metasedimentary crustal source having trace element and isotopic co mpositions comparable to those of the Devonian Gramscatho Group of South Co rnwall. 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 anom alies in extended trace element plots, and display primitive, time-correcte d Nd-143/Nd-144 and Sr-87/Sr-86 values, and others (group 2) which are more strongly enriched in all incompatible trace elements, exhibit more promine nt negative Ta, Nb, P and Ti anomalies, and have isotopic ratios intermedia te 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 meltin g of a weakly enriched mantle source. Emplacement of two commingled magmas into the base of the Lizard ophiolite, one representing the products of cru stal anatexis, implies that the ophiolite was removed from its oceanic sett ing and was in the process of obduction in the Givetian.