Was the easternmost segment of the Central Asian Orogenic Belt derived from Gondwana or Siberia: An intriguing dilemma?

摘要

The Khanka Massif forms the easternmost segment of the Central Asian Orogenic Belt (CAOB) in NE China and its tectonic setting is highly controversial. Metasedimentary rocks from the Hulin Complex and the Mt. Huoshi sequence were selected to address this issue. For the Hulin Complex, SHRIMP detrital zircon U-Pb dating of a two-mica schist (sample HL-011) reveals a unimodal population with a weighted mean ~(206)Pb/~(238)U age of 257 ±3 Ma, suggesting local derivation from nearby Permian granitoids. In contrast, a quartz schist (sample 04H-138) contains five zircon populations with ages of 233-270 Ma, 350-400 Ma, 473-570 Ma, 720-760 Ma and 930-970 Ma, and peaks at 252, 350, 520, 758 and 970 Ma, indicating a much more diverse provenance. Two samples of fine-grained gneiss were selected from Mt Houshi, with sample 06H-15 containing four zircon populations, with groupings at 260-340 Ma, 470-560 Ma, 710-940 Ma and a single grain at 2090 Ma, whereas zircons in sample 06H-16 define two populations with ages of 727-854 Ma, with a peak at 776 Ma, and a single grain with an age of 404 ±13 Ma. The youngest detrital zircon was obtained from sample 04H-138 and has a ~(206)Pb/~(238)U age of 233±9Ma, limiting deposition of the Hulin Complex to after ～233 Ma. The late Pan-African and Paleoproterozoic to Neoproterozoic ages, together with the ～500 Ma age for granulite facies metamorphism previously determined for the Hutou Complex in the Khanka Massif, indicate that the Khanka and Jiamusi massifs (together with the Bureya Massif in Russia) formed a contiguous crustal unit. Detrital zircon data suggest that this combined block was not part of either the North China or the South China cratons. Instead, it was either derived from a peri-Gondwana position, a rifted portion of the Songliao Massif in the CAOB, an exotic block of unknown affinity, or from the Siberia Craton. We evaluate the possibility that the Khanka/Jiamusi/Bureya block may have originated in Siberia and once formed part of the Sayan-Baikal orogen at ～500 Ma. Whatever its origin, the block drifted westward to collide with the Songliao Massif of NE China in the latest Triassic-Early Jurassic as a result of the onset of Pacific-plate subduction. It thus forms part of the evolving supercontinent of Eurasia.