A STUDY OF ISOTOPIC DATING AND GEOCHEMISTRY OF GRANULITES IN THE EASTERN TIANSHAN BELT

Basic rock type granulitic slabs exist in Weiya and Yushugou areas in the Tianshan Paleozoic belt, NW China. Some obvious lithological differences and geological attitudes exist between the Weiya and the Yushugou granulites. The Weiya granulite was derived from protolites of basic and intermediate acid rocks contains three phases of metamorphic mineral assemblages. M 1=plagioclase+quartz+muscovite + biotite, M 2=clinopyroxene+garnet+quartz+antiperthite±orthopyroxene, M 3=garnet+cordierite+epidote+amphibole. These granulitic rocks are outcropped as the fault bounded slabs and as the roof pendant blocks of the foliated Paleozoic granite. The Yushugou granulitic rocks are basic type and occurred as thrust slabs in the allochthonous ophiolitic melange. Two phases of metamorphic mineral assemblages were observed, namely, M 1= clinopyroxene+ garnet+ plagioclase±orthopyroxene, M 2=pyroxene+amphibole+plagioclase+quartz±biotite. We obtained a Sm Nd isochron age of (540.8±11.3) Ma for the Weiya granulite (pligioclase +pyroxene+ whole rock), corresponding to a peak metamorphic period during Early Cambrian. This paper reported also the 40 Ar/ 39 Ar ages of (432.1±0.7) Ma (flat age) and of (434.7±2.5) Ma (isochron age) for the amphibole mineral of Weiya granulite, corresponding to a retrograde metamorphic age, suggesting a strong Caledonian tectonothermal event.