Jian-Bo Zhou

Pan-African metamorphic and magmatic rocks of the Khanka Massif, NE China: Further evidence regarding their affinity

The Khanka Massif is a crustal block located along the eastern margin of the Central Asian Orogenic Belt (CAOB) and bordered to the east by Late Jurassic–Early Cretaceous circum-Pacific accretionary complexes of the Eastern Asian continental margin. It consists of graphite-, sillimanite- and cordierite-bearing gneisses, carbonates and felsic paragneisses, in association with various orthogneisses. Metamorphic zircons from a sillimanite gneiss from the Hutou complex yield a weighted mean 206 Pb/ 238 U age of 490 ± 4 Ma, whereas detrital zircons from the same sample give ages from 934–610 Ma. Magmatic zircon cores in two garnet-bearing granite gneiss samples, also collected from the Hutou complex, yield weighted mean 206 Pb/ 238 U ages of 522 ± 5 Ma and 515 ± 8 Ma, whereas their metamorphic rims record 206 Pb/ 238 U ages of 510–500 Ma. These data indicate that the Hutou complex in the Khanka Massif records early Palaeozoic magmatic and metamorphic events, identical in age to those in the Mashan Complex of the Jiamusi Massif to the west. The older zircon populations in the sillimanite gneiss indicate derivation from Neoproterozoic sources, as do similar rocks in the Jiamusi Massif. These data confirm that the Khanka Massif has a close affinity with other major components of the CAOB to the west of the Dun-Mi Fault. Based on these results and previously published data, the Khanka Massif is therefore confirmed as having formed a single crustal entity with the Jiamusi (and possibly the Bureya) massif since Neoproterozoic time.

New SHRIMP U-Pb zircon ages from the Heilongjiang High-Pressure Belt: Constraints on the Mesozoic evolution of NE China

The Heilongjiang Complex is a high-pressure accretionary belt composed mainly of serpentinite, glaucophane-bearing metabasalt, epidote-glaucophane-albite 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 206Pb/238U 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.

Paleo‐Pacific subduction‐accretion: Evidence from Geochemical and U‐Pb zircon dating of the Nadanhada accretionary complex, NE China

The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert, and mafic-ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have normal mid-ocean ridge basalt (N-MORB) affinity, whereas the Raohe basaltic pillow lavas have an affinity to ocean island basalts (OIB). Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses of gabbro in the Raohe Complex yield a weighted mean 206Pb/238U zircon age of 216 ± 5 Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128 ± 2 and 129 ± 2 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167 ± 1 Ma. Two sandstone samples in the Raohe Complex record younger concordant zircon weighted mean ages of 167 ± 17 and 137 ± 3 Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137 Ma, and that final emplacement of the Raohe Complex took place at 137–130 Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 and 180 Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.

Detrital zircon U–Pb dating of low-grade metamorphic rocks in the Sulu UHP belt: evidence for overthrusting of the North China Craton onto the South China Craton during continental subduction

The Shiqiao–Pingshang low-grade metasedimentary rocks sporadically crop out in the Sulu ultrahigh-pressure (UHP) belt in east–central China. Major and trace element data indicate that they were deposited in a passive margin basin, probably located at the southern margin of the North China Craton. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb dating of detrital zircons from a quartzite (SD53) from Pingshang and a quartz schist (SD54) from Shiqiao records ages ranging from 2800 to 1900 Ma. Three age populations are defined: at 2660–2500, 2350–2150 and 2100–1900 Ma, with peak ages at 2522, 2212 and 2020 Ma, respectively. Muscovite Ar–Ar dating of the quartz schist (SD54) yields a plateau age of 265.9 ± 1.2 Ma. These data support the view that the Shiqiao–Pingshang low-grade metasedimentary rocks originated from the North China Craton, and underwent deformation in the Permian during subduction of the South China Craton (Yangtze block) beneath the North China Craton. This requires that the low-grade metasedimentary rocks were overthrust for several tens of kilometres onto the South China Craton, explaining why source materials from the North China Craton are common in Triassic to Jurassic sedimentary basins adjacent to the Dabie–Sulu orogenic belt.