Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China: geochronological, geochemical and Nd-Sr isotopic evidence

摘要

The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon Pb-206/U-238 age of 215 +/- 5 Ma was obtained for the quartz syenite and mineral Ar-40/Ar-39 dating gave emplacement ages of 214.4 +/- 0.3 and 214.6 +/- 0.6 Ma for the pyroxene syenite and 200.6 +/- 0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K2O approximate to 4.4-6.4 wt.% and K2O/Na2O approximate to 3.5, have high MgO (8.06-12.44 wt.%), Ni (119-319 ppm) and Cr (477-873 ppm) and moderately low CaO/Al2O3 (similar to0.76) and TiO2 (similar to1.12 wt.%). They also have high Sr (Sr-87/(16)Srsimilar to0.7073), low Nd (epsilon(Nd) = similar to-16.5) isotopic ratios, enriched LILE (Ba/La=66-74), LREE [(La/Yb)(N)=28-33] and depleted HFSE (La/Nb=4-6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle. (C) 2004 Elsevier B.V. All rights reserved.