PETROLOGY, GEOCHEMISTRY AND ISOTOPE DATA ON A ULTRAHIGH-PRESSURE JADEITE QUARTZITE FROM SHUANGHE, DABIE MOUNTAINS, EAST-CENTRAL CHINA

In the Dabie ultrahigh-pressure terrane of east-central China, coesite -bearing jadeite quartzites occur locally as intercalated layers with marble and mafic eclogite. This rock assemblage is, in turn, enclosed within quartzofeldspathic gneisses, Metamorphic parageneses and kelyph itic textures reveal a multistage metamorphic evolution and complex ex humation history. The primary peak metamorphic assemblage consists of jadeite + garnet + coesite + rutile +/- apatite. Minor coesite and coe site pseudomorphs occur as inclusions in jadeite and garnet. Three sta ges of retrograde assemblages are observed in the jadeite quartzites. Stage A is represented by the polymorphic transformation of coesite to quartz aggregates. Stage B is characterized by formation of coronas a round jadeite porphyroblasts consisting of an inner layer of oligoclas e + amphibole and an outer layer of albite +/- aegirine-augite. The la st stage (stage C) involved total replacement of jadeite and most garn ets by taramitic amphibole + albite + aegirine-augite. peak metamorphi c P-T conditions were > 26 kbar at 660 degrees C and are consistent wi th the estimates from the adjacent coesite-bearing eclogites. The jade ite quartzites display clockwise P-T path that matches those of the ad jacent eclogites. Major and trace element data suggest that the protol ith of the jadeite quartzite could have been an albitized siltstone en riched in Na and depleted in K and Ca. The highly negative present-day epsilon Nd value (-24.7) indicates a very old age for the protolith. Its late Archean model age (TDM) of 2.58 Ga is among the oldest so far identified for rocks from the Dabie UHPM terrane. Concordant field re lations and petrogenetic considerations suggest that all mafic, peliti c, carbonate and gneissic rocks have experienced in-situ UHP metamorph ism during Triassic continental collision between the Sino-Korean and Yangtze cratons. (C) 1997 Elsevier Science B.V.