PETROGENESIS AND SHRIMP DATING OF THREE GRANITE TYPES FROM THE NORTH QADAIM UHP BELT, NW CHINA

Three types of granite association occur in the early Paleozoic North Qadaim ultrahigh pressure metamorphic belt with the eclogite age of 466-495 Ma. The first type consists of quartz monzodiorite + granodiorite + adamellite as small stock. The second is adamellite + two mica granite + muscovite granite + syenogranite, and the third is granite + adamellite + biotite granite as batholith. These granitic rocks intruded into a Proterozoic metamorphic basement and its overlying Paleozoic deep-sea sediments, volcanics and continental clastic rocks and shallow-marine sequences. Petrochemically, both the first and third ones belong to I-type granite, with SiO2=61-69wt%, Na2O/K2O>1, ANK<1, _Eu=0.7-1.0; the first type granites have island-arc affinity whereas the third one are post-orogenic granites. The second association belongs to syn-collisional S-type granite, with SiO2=70-76wt%, Na2O/K2O<1, ANK>1, _Eu=0.1-0.3. The adamellite of the first association has SHRIMP zircon age of 473 Ma whereas the syenogranite of the second association and the granodiorite of the third association respectively yield SHRIMP age of 446 and 397 Ma. These geochemical and geochronological data constrain the formation of these granitic types and their associated HP-UHP belts through the subduction of the south Qilian ocean plate and the collision of the Central Qilian and Qaidam continents