ALBERT

Description: Sensitive high-resolution ion microprobe and inductively coupled plasma mass spectrometry U-Pb dating of zircons from granitoids and paragneiss in the Chinese segment of the Khanka Block reveals that granite magmatism occurred at 518{+/-}7 Ma and was followed shortly after by high-grade metamorphism at c. 500 Ma (timing ranging from 491{+/-}4 Ma in medium-grained granitoid, through 499{+/-}10 Ma in porphyritic granite, to 501{+/-}8 Ma in paragneiss). Such a scenario has previously been established on similar lithologies in the Jiamusi Block to the west, with identical ages. This suggests that the Khanka and Jiamusi blocks form part of a single terrane and that the Dunhua-Mishan Fault, which was previously considered to separate two unique terranes, cannot be a terrane boundary fault. Previous suggestions of a link between the Khanka Block and the Hida Block in Japan are not supported following a comparison of the new zircon data with published ages for the Japanese terranes. A granitoid with an age of 112{+/-}1 Ma in the Khanka Block probably records the effect of Pacific plate subduction, as such ages are common further south in the extreme eastern part of the North China Craton, where they have been related to post-collisional extension and lithospheric thinning in the Jiaodong Peninsula. The presence of such young granitoids, and the previous dating of blueschist-facies metamorphism as late Early Jurassic in the Heilongjiang Complex of the Jiamusi Block, supports the view that the current location of the Jiamusi-Khanka terrane is a product of circum-Pacific accretion rather than it being a microcontinental block that was trapped by the northward collision of the North China Craton with Siberia as part of the assembly of the main Central Asian Orogenic Belt.

Description: Arc system' is used here as a collective term for a variety of arcs that occur along continental margins or in oceanic plates; it includes associated units from adjacent plates. Four major arc systems (Mariana-, Japan-, Cordillera- and Alaska-type) can be distinguished along the Circum-Pacific region. Some Japan-type arc systems in ancient orogens (e.g. the Altaids) may have been largely regarded as microcontinents because they have so-called Precambrian basement. Often the Cordillera-type arc systems can be very complicated, and if they are rifted away from the host continent they become more difficult to recognize. Commonly these arc systems interact mutually and with continental marginal sequences, leading to complicated accretionary and collisional orogens. The alternation between Western Pacific archipelagos and the Eastern Pacific active margin is the stereotype of accretionary and collisional orogenesis. More importantly, these four main types of arc systems can be juxtaposed into a final orogenic collage, which is another main expression of accretionary orogenesis. Only some parts of accretionary and collisional orogens can be terminated by attachment of a continent-size craton such as Tarim or even India, and even so the accretionary and collisional processes may continue elsewhere along strike. The significance of the interactions among these arc systems and their final juxtaposition has not been fully appreciated in ancient orogens. The Altaids together with the Circum-Pacific orogens offers a good opportunity to study such accretionary-collisional orogenesis.

Description: SHRIMP U-Pb zircon analyses from eight samples of metamorphosed intermediate to felsic volcanic rocks from the lower, middle and upper subgroups' of the Wutai sequence in the North China Craton define a weighted mean 207Pb/206Pb age of 2523 {+/-} 3 Ma. Although individual rock ages range from 2533 {+/-} 8 Ma to 2513 {+/-} 8 Ma, all overlap within the error of the mean and do not support a stratigraphic interpretation for the sequence, since variations within individual previously assigned formations' in the sequence match the total age range. Contrary to previous interpretations, there is no correlation in age with metamorphic grade. These features highlight the need to reformulate stratigraphic schemes when defining the Precambrian geology of the North China Craton. The similarity in age between volcanic rocks of the Wutai Complex and higher-grade gneisses of the adjacent Fuping and Hengshan complexes supports the view that all three complexes represent portions of a Late Archaean arc complex that was tectonically dismembered and then re-assembled. There is no Fuping or Wutai orogeny in this, its type area: all three complexes were deformed and metamorphosed during collision of the eastern and western blocks of the North China Craton in the Luliang orogeny c. 1.8 Ga ago.

Description: The Hengshan-Wutai-Fuping mountain belt constitutes the middle segment of the Trans-North China Orogen, which separates the North China Craton into the Eastern and Western Blocks. The belt consists of the high-grade Hengshan and Fuping complexes, and the intervening low- to medium-grade Wutai Complex. Previous tectonic models assumed that the high-grade complexes were an older basement (Archaean to Palaeoproterozoic) to the low-grade Wutai Complex. However, new geochronological data show that the emplacement of granitoid rocks and eruption of volcanic rocks in the Wutai Complex occurred essentially coeval with or slightly earlier than intrusion of the tonalitic-trondhjemitic-granodioritic (TTG) suites in the Hengshan and Fuping complexes. New isotopic data also reveal the widespread presence of Palaeoproterozoic granitoid rocks in these complexes. Structural and metamorphic data demonstrate similar tectonothermal histories for the three complexes, which are characterized by peak medium- to high-pressure metamorphism accompanied by the development of thrusting, isoclinal folding (F2) and penetrative foliations, followed by near-isothermal decompression and cooling and retrogression associated with the formation of large-scale ductile shear zones and asymmetrical folds (F3) with nearly vertical axial planes. These geochronological, structural and metamorphic data suggest that the tectonic evolution of the Hengshan-Wutai-Fuping mountain belt may not be related to local interaction of the three complexes, as suggested in earlier models, either through closure of a Wutai rift or collision between a Wutai arc and the Hengshan and Fuping micro-continental blocks. Instead, they may represent elements of a single Late Archaean to Early Palaeoproterozoic magmatic arc that was subsequently incorporated into the Trans-North China Orogen along which the Eastern and Western blocks amalgamated to form the North China Craton at around 1.85 Ga.

Description: The basement of the North China Craton can be divided into the Archaean Eastern and Western Blocks, separated by major Palaeoproterozoic terrane boundaries that roughly correspond with the limits of a 100-300 km wide zone, named the Trans-North China Orogen. Some mafic granulites from the orogen and adjoining areas in the Eastern and Western Blocks preserve textural evidence for two granulite facies events involving contrasting P-T paths. The first event is characterized by three distinct mineral assemblages, M1a to M1c. M1a is represented by fine-grained orthopyroxene + clinopyroxene + plagioclase {+/-} quartz, which is surrounded by the M1b garnet + quartz symplectite, which itself is mantled by the M1c plagioclase + biotite symplectite. These assemblages and their P-T estimates define an anticlockwise P-T path, with peak metamorphism of 7.0-8.0 kbar and 800-850{degrees}C (M1a) followed by isobaric cooling to 700-750{degrees}C (M1b) and pressure-decreasing cooling to 630-700{degrees}C (M1c). The second event also includes three mineral assemblages, M2a to M2c. M2a represents growths of garnet porphyroblasts and matrix orthopyroxene + plagioclase + clinopyroxene + quartz; M2b consists of orthopyroxene + plagioclase {+/-} clinopyroxene symplectites or coronas; and M2c is represented by plagioclase + hornblende symplectites. These assemblages and their P-T estimates define a clockwise P-T path, with peak metamorphism of 9.2-9.8 kbar and 820-850{degrees}C (M2a), followed by near-isothermal decompression (M2b) of 7.0-7.6 kbar and 760-810{degrees}C and cooling (M2c) to 690-760{degrees}C. The isobaric cooling, anticlockwise, P-T path of the first granulite facies event is similar to the P-T paths inferred for the c.2.5 Ga metamorphosed mafic granulites from the Eastern and Western Blocks, whereas the near-isothermal decompression, clockwise, P-T path of the second granulite facies event is similar to the P-T paths inferred for the c. 1.8 Ga metamorphosed khondalite series in the Western Block and some mafic granulites in the Trans-North China Orogen. These relations suggest that the polymetamorphic granulites were derived from the reworking of the 2.5 Ga metamorphosed granulites during the 1.8 Ga collision between the Eastern and Western Blocks that resulted in the final amalgamation of the North China Craton.

Description: The evolution of the northern margin of Gondwana, especially to the north of India and Australia, remains enigmatic. Much controversy concerns when and where the Tarim craton was amalgamated with northern Gondwana due to final closure of the North and South Altyn Oceans (two branches of the Proto−Tethys Ocean between southeastern Tarim and northern Gondwana). This study addressed these issues through systematic field-based zircon U-Pb dating and Hf-isotope analyses of early Paleozoic sedimentary rocks in the Altyn Tagh orogen. New dating results reveal depositional ages from ca. 494 to 426 Ma. Provenance tracing indicates the ca. 494−477 Ma samples were dominantly sourced from local Altyn Tagh areas to the south of the North Altyn Ocean, whereas the ca. 465−449 Ma samples are characterized by a significant increase in ca. 2.7−2.4 Ga, 2.0−1.7 Ga, and 840−780 Ma detrital zircons, indicating an augmented supply of detritus from the Tarim craton to the north of the North Altyn Ocean. This change indicates a major provenance shift from a single to multiple source regions between ca. 477 and 465 Ma, marking the timing of the final closure of the North Altyn Ocean. Zircon U-Pb and Hf-isotopic data from the ca. 444−426 Ma samples resemble those from the ca. 465−449 Ma samples, suggesting local sediment recycling related to a postcollisional regime. Considering the South Altyn Ocean and other branches of the Proto−Tethys Ocean, we infer that the entire Proto−Tethys Ocean might have been progressively closed at ca. 500−420 Ma, leading to the amalgamation of most East Asian blocks with northern Gondwana. Detrital zircon U-Pb and Hf-isotope comparisons indicate that Tarim shared a North Indian affinity with many East Asian terranes (such as North Qilian, North Qinling, South China, Indochina, South Qiangtang, etc.), rather than with Arabia-Iran or other terranes (e.g., Lhasa and Sibumasu) that were adjacent to western Australia along the northern margin of Gondwana.

Description: Although subduction–accretion is proposed as a major regime in making new continental crust, how the lithospheric mantle forms remains unclear. Formed after the closure of the Palaeo-Asian Ocean, the Ashele basalt shows normal mid-ocean ridge basalt (N-MORB)-like characteristics with light REE-depleted patterns and extremely low contents of high field strength elements. The low Zr/Y and Nb/Y ratios of the basalt are significantly different from those of asthenosphere-derived melts, and the excess Eu and Sr suggest that the basalt was probably derived from accreted oceanic lithospheric mantle. The presence of the N-MORB-like terrestrial basalt implies that subduction–accretion is an effective mechanism in building the refractory lithospheric mantle of Phanerozoic continents.Supplementary material: A photograph of the outcrop, and age and geochemical data are available at http://www.geolsoc.org.uk/SUP18464.