NEW SHRIMP U-Pb ZIRCON AGES FROM THE HEILONGJIANG HIGH-PRESSURE BELT: CONSTRAINTS ON THE MESOZOIC EVOLUTION OF NE CHINA

摘要

The Heilongjiang Complex is a high-pressure accretionary belt composed mainly of serpentinite, glaucophane-bearing metabasalt, epidote-glaucophanealbite schist, greenschist, marble, two-mica schist, muscovite-albite schist, quartz schist, and quartzite (chert), located along the suture zone separating the Jiamusi and Songliao blocks in NE China. SHRIMP U-Pb zircon analyses of two samples of granitic gneiss from Yilan that are interleaved with the Heilongjiang Complex record diverse protolith ages that are consistent with derivation from regional Permian granitoids (263 ± 7 Ma) and the adjacent Mashan Complex (492 ± 6 Ma), indicating tectonic incorporation of older granitic components into the accretionary complex. The age of the basaltic protolith of two samples of glaucophane-albite schist from Yilan is ～245 Ma and is slightly younger than the protolith age of two samples of amphibole schist from Mudanjiang, located about 200 km south of Yilan, that define weighted mean ~(206)Pb/~(238)U ages of 257 ± 4 and 257 ± 5 Ma. All four samples contain a few older inherited zircon grains and also show the effects of an isotopic disturbance at ～200 to 190 Ma, most likely related to the subsequent high-pressure metamorphism, the timing of which has been determined in previous studies. The basaltic rocks record rifting and ocean development from ～260 to 210 Ma, based on previous studies in which both E-MORB and OIB protolith signatures were identified. Detrital zircons in two samples of mica schist from Mudanjiang show distinct populations at ～700 Ma, 500 to 450 Ma, 350 to 213 Ma (peak at 255 Ma), 290 to 200 Ma and ～200 Ma. All except the youngest ages can be matched with rocks from the older components of the Jiamusi block and with other areas in the Central Asia Orogenic Belt (CAOB), supporting the view that the Jiamusi block was most likely the eastern segment of the CAOB prior to rifting. The youngest concordant magmatic detrital grain with an age of 207 ± 3 Ma defines the maximum age of sedimentation. This also limits the timing of high-pressure metamorphism in the Heilongjiang Complex to post-Late Triassic, consistent with argon data obtained in previous studies. Final closure between the Jiamusi and Songliao blocks took place in the latest-Triassic-Early Jurassic, with the two blocks accreted as a result of Pacific Ocean subduction. This suggests that the Heilongjiang Complex records the time when north-south closure of the CAOB in China gave way to westward movement related to the Pacific Ocean subduction, which has dominated the tectonics of NE China and Far East Russia since that time.