Characterisation and origin of New Zealand nephrite jade using its strontium isotopic signature

摘要

Nephrite jade occurs in three terranes (Dun Mountain-Maitai,Caples and Torlesse) in New Zealand,where it is associated with ultramafic and ophiolitic rocks in narrow metasomatic reaction zones at the margins of serpentinite (having harzburgite/gabbro/ dolerite precursors) with silicic metasediments and metavolcanics.True nephrite fabrics are developed only locally where marginal shearing is intense,and late in the metamorphic history.~(87)Sr/~(86)Sr values of these nephrites do not display the primitive values of their gabbro/dolerite precursor component i.e.0.7030-0.7035,as expected if formed during serpentinisation.Rather,the nephrites have more evolved ~(87)Sr/~(86)Sr values inherited from the metasediment component at a later stage,and which fall within particular terrane groups:Dun Mountain-Maitai 0.7045-0.7060,Caples 0.7058-0.7075 and Torlesse 0.7085-0.7110.Rb-Sr ages and initial ~(87)Sr/~(86)Sr ratios of the metasediment component from in situ nephrite localities,when compared with their counterparts throughout the host terrane,show that nephrite Sr isotopic compositions are characteristic of the host terrane.