The onset of Pacific margin accretion in NE China: Evidence from the Heilongjiang high-pressure metamorphic belt

Abstract

The Heilongjiang Complex is a sequence of high-pressure metamorphic rocks, located along the suture zone that separates the Jiamusi–Khanka (–Bureya) and Songliao–Zhangguangcai blocks in NE China (and extending northward into Far East Russia). The complex consists of mafic–ultramafic rocks, various quartzo–feldspathic schists and radiolarian-bearing quartzite (formerly chert). The rocks were metamorphosed up to epidote–blueschist facies, with P– T conditions of approximately T = 320–450 °C and P = 0.9–1.1 GPa. The lithological association and major and trace element compositions indicate that the blueschists were metabasalts of OIB and E-MORB affinity, most likely generated in a rift setting at the western margin of the Jiamusi Block that later underwent subduction. Magmatic zircons extracted from two samples of epidote–blueschist facies metabasalts from Mudanjiang have SHRIMP U–Pb 206Pb/ 238U ages of 213 ± 2 Ma and 224 ±7 Ma, whereas similar rocks ∼ 200 km farther north at Yilan have ages of 258 ± 2 Ma and 259 ± 4 Ma. These data define the protolith ages of the metabasalts as Late Triassic and Late Permian, respectively. These ages limit the timing of high-pressure metamorphism in the Heilongjiang Complex to post-Late Triassic, consistent with argon data reported from previous studies. Inherited zircon components in all four epidote–blueschist facies samples show distinct populations at 290–330 Ma, 420–530 Ma, 670–910 Ma and > 1065 Ma. Such ages are also a feature of the Central Asia Orogenic Belt (CAOB) to the west, supporting the view that the Jiamusi Block was most likely the rifted easternmost segment of the CAOB and not an exotic block derived from Gondwana. Final closure between the Jiamusi–Khanka–Bureya and Songliao blocks took place in the latest Triassic to Early Jurassic, with the two blocks accreted as a result of Pacific Ocean subduction. This suggests that the Heilongjiang Complex records the time when northward movement of the combined Mongolia–North China Block toward Siberia was waning and becoming surpassed by the onset of Pacific accretion from the east, which has dominated the tectonics of NE China and Far East Russia since the Early Jurassic.