CHANGE FROM CALC-ALKALINE TO ADAKITIC MAGMATISM RECORDED IN THE EARLYCRETACEOUS DARRAN COMPLEX, FJORDLAND, NEW-ZEALAND

The Early Cretaceous Darran Complex of the Median Tectonic Zone near M ilford, South Island, New Zealand, consists dominantly of biotite - tw o-pyroxene diorites cut by biotite-hornblende microdiorite and quartz monzodiorite dikes. The host diorite gives a 138 +/- 2.9 Ma SHRIMP age on zircons, which is interpreted to be the age of igneous crystallisa tion. The calc-alkaline geochemistry of the host diorites and the micr odiorites is attributed to melting of a mantle wedge source fluxed by slab-derived fluids. Ductile deformation (D-1) of the host diorite and the microdiorite dikes took place at mid-upper amphibolite facies con ditions, with extension lineations indicating a top to the NNE sense o f shear. A SHRIMP age of 136 +/- 1.9 Ma on zircons from a quartz monzo diorite dike injected along D-1 shears is statistically indistinguisha ble from that of the host diorite. This suggests that D-1 was synmagma tic and that subduction, during or shortly after magma emplacement, wa s oblique to the Gondwana margin at c. 138 Ma. The quartz monzodiorite dikes are enriched in Na2O, Al2O3, and Sr, depleted in Y, and have a distinctly adakitic geochemistry. The change in chemistry from calc-al kaline magmas to alkali-calcic adakitic magmas reflects the melting of a mafic, garnet-bearing, essentially plagioclase-free source in the r oot of a volcanic are system.