AN EVALUATION OF ELEMENTAL MOBILITY DURING METAMORPHISM OF A TURBIDITE SEQUENCE (GREENLAND GROUP, NEW-ZEALAND)

The extent of elemental mobility during lower greenschist to amphiboli te facies metamorphism of a uniform turbidite suite (Greenland Group, New Zealand) has been evaluated using data for major elements and 19 t race elements. Simple comparison of average compositions at 2 wt % Al2 O3 intervals in the data suites shows little contrast between lower gr eenschist protolith and upper greenschist and amphibolite facies equiv alents, except for enrichment of CaO and Sr, and loss of Ba and Rb, pa rticularly in the sandier end members. Division into psammitic and pel itic suites using TiO2/Al2O3 and Zr/Al2O3 ratios allows delineation of individual residual enrichment models on Ti-reference element plots, and both lithotypes can be used to assess potential elemental mobility . These plots show that a large number of elements (Ti, Al, Fe, Mg, Ce , Cr, Ga, La, Nh, Ni, Sc, Th, V, Zn and Zr) constitute immobile refere nce species, with abundances equal to their equivalent lithotype in th e protolith, and little mass loss or residual enrichment. K and Rb als o largely conform to the residual model, but in the amphibolite facies some exchange between pelite and psammite may occur. A number of elem ents show enrichment (Mn, Cu, Pb, U, Na, P) or depletion (Y) in a smal l number of samples, but the significance of these contrasts is questi onable due to relatively large variation in the protolith. In contrast , Ca and Sr show progressive and marked enrichment with increasing gra de, and Ba and As are clearly depleted in amphibolite facies psammites . In the amphibolite facies some Si may have been lost by psammites, a nd gained by the pelites, although there has been no mass change from the suite as a whole. A large part of the Ca and Sr enrichment in the amphibolite facies can be accounted for by metasomatic homogenization of calcareous concretions which occur in the lower grade protolith. Th e metamorphism of the Greenland Group is thus considered to be essenti ally isochemical.