An evaluation of elemental mobility during metamorphism of a turbidite sequence (Greenland Group, New Zealand)

• Published in: Geological Magazine
• Main Authors: ROSER, B. P., NATHAN, SIMON
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1997
• Subjects: Geology; Greenland; New Zealand
• Online Access: http://dx.doi.org/10.1017/s0016756897006638; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0016756897006638

The extent of elemental mobility during lower greenschist to amphibolite facies metamorphism of a uniform turbidite suite (Greenland Group, New Zealand) has been evaluated using data for major elements and 19 trace elements. Simple comparison of average compositions at 2 wt % Al 2 O 3 intervals in the data suites shows little contrast between lower greenschist protolith and upper greenschist and amphibolite facies equivalents, except for enrichment of CaO and Sr, and loss of Ba and Rb, particularly in the sandier end members. Division into psammitic and pelitic suites using TiO 2 /Al 2 O 3 and Zr/Al 2 O 3 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, Nb, Ni, Sc, Th, V, Zn and Zr) constitute immobile reference species, with abundances equal to their equivalent lithotype in the protolith, and little mass loss or residual enrichment. K and Rb also largely conform to the residual model, but in the amphibolite facies some exchange between pelite and psammite may occur. A number of elements show enrichment (Mn, Cu, Pb, U, Na, P) or depletion (Y) in a small number of samples, but the significance of these contrasts is questionable due to relatively large variation in the protolith. In contrast, Ca and Sr show progressive and marked enrichment with increasing grade, and Ba and As are clearly depleted in amphibolite facies psammites. In the amphibolite facies some Si may have been lost by psammites, and 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. The metamorphism of the Greenland Group is thus considered to be essentially isochemical.