ALBERT

Description: The North China Craton (NCC) is the only place currently recognized where an Archaean craton developed a continental root in the Archaean, and subsequently lost half of that root in younger tectonism. In this volume, various authors have advanced different models of root loss, and provided geological, geophysical and geochemical data that help constrain the geometry and timing of root loss. Understanding why and how roots are lost may help us understand how often this process may have occurred in the geological past, and how much lithospheric material has been recycled to the convecting mantle through this mechanism, potentially drastically changing our current understanding of crustal growth rates and processes. With current data, there are several equally plausible possibilities that require further data collection for testing. There are several possible tectonic triggers that may have caused half the root to be lost, acting either separately or together. These include collisional, extensional, plume-related, fluid-weakening, spontaneous, and more complex hybrid mechanisms. We also do not know why only the eastern half of the root was lost, and not the root from beneath the whole craton. One tantalizing idea is that the root grew independently, by tectonic underplating of subducted buoyant oceanic lithosphere, beneath the previously separate eastern and western halves of the craton by 2.5 Ga, with modification at 1.8 Ga. If so, perhaps only the eastern half of the root was lost in younger tectonism because there was some physical or geometric difference between the two halves. Alternatively, collisional or subduction-related tectonic processes acting only on the Eastern Block may have caused the disruption of the tectosphere there in the Mesozoic. The timing of and mechanism for loss of the root is not uniquely resolvable with current data, but a solution to the problem is in reach. Possible triggering mechanisms include, but are not limited to, collision of the South China (Yangtze) and North China Cratons in the Triassic, the IndiaAsia collision, closure of the Solonker and MongolOkhotsk oceans, Mesozoic subduction of the Pacific plate beneath Eastern China, impingement of mantle plumes, mantle hydration from long-term subduction, and several rifting events. In this concluding review, we link studies of crustal tectonics with investigations aimed at determining the nature and timing of the formation and loss of the root, to better understand mechanisms of continental root formation, evolution and recyclingremoval.

Description: The North China Craton contains one of the longest, most complex records of magmatism, sedimentation, and deformation on Earth, with deformation spanning the interval from the Early Archaean (3.8 Ga) to the present. The Early to Middle Archaean record preserves remnants of generally gneissic meta-igneous and metasedimentary rock terranes bounded by anastomosing shear zones. The Late Archaean record is marked by a collision between a passive margin sequence developed on an amalgamated Eastern Block, and an oceanic arcophiolitic assemblage preserved in the 1600 km long Central Orogenic Belt, an ArchaeanPalaeoproterozoic orogen that preserves remnants of oceanic basin(s) that closed between the Eastern and Western Blocks. Foreland basin sediments related to this collision are overlain by 2.4 Ga flood basalts and shallow marinecontinental sediments, all strongly deformed and metamorphosed in a 1.85 Ga Himalayan-style collision along the northern margin of the craton. The North China Craton saw relative quiescence until 700 Ma when subduction under the present southern margin formed the QinglingDabie ShanSulu orogen (700250 Ma), the northern margin experienced orogenesis during closure of the Solonker Ocean (500250 Ma), and subduction beneath the palaeo-Pacific margin affected easternmost China (200100 Ma). Vast amounts of subduction beneath the North China Craton may have hydrated and weakened the subcontinental lithospheric mantle, which detached in the Mesozoic, probably triggered by collisions in the Dabie Shan and along the Solonker suture. This loss of the lithospheric mantle brought young asthenosphere close to the surface beneath the eastern half of the craton, which has been experiencing deformation and magmatism since, and is no longer a craton in the original sense of the word. Six of the 10 deadliest earthquakes in recorded history have occurred in the Eastern Block of the North China Craton, highlighting the importance of understanding decratonization and the orogencratonorogen cycle in Earth history.

Description: The Archaean and Proterozoic geology, structure and metamorphism of the North China Craton (NCC) reveal that the amalgamated Eastern and Western blocks of the craton collided as a single entity with the Columbia supercontinent at 1.93 Ga, and that the Northern Hebei orogen correlates with the Transamazonian-Eburnian belts of Africa and South America, and the Svecofennian of Baltica. This metamorphic belt preserves evidence for extreme crustal metamorphism with diagnostic ultrahigh-temperature (UHT) assemblages such as sapphirine+quartz, spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature perthites which record temperatures exceeding 1000 {degrees}C and pressure above 12 kbar. The metamorphic P-T trajectory is characterized by initial isobaric cooling followed by steep isothermal decompression, defining an overall anticlockwise exhumation history. Electron probe monazite geochronology and precise SHRIMP zircon dating of sapphirine-bearing granulites constrain the timing of the UHT event at c. 1.92 Ga, suggesting that the UHT metamorphism coincided with collisional orogenesis as the North China Craton joined the Columbia supercontinent amalgam along the Northern Hebei orogen during the Palaeoproterozoic. Fluid inclusion studies in the UHT rocks provide evidence for the involvement of synmetamorphic pure CO2, linking the thermal anomaly and fluid flux to underplated mafic magmas during asthenospheric upwelling. Evidence for probable-plume related mafic magmatism is also provided by the extensive mafic dyke swarms cutting the region and elsewhere within the NCC, with geochemical characters testifying to emplacement within rifts that opened up during the extensional collapse of the orogen subsequent to the collisional event. Recognition of the Palaeoproterozoic Columbia suture in the Northern Hebei orogen represents a major paradigm shift, as one popular group of models for the NCC suggests that the Palaeoproterozoic suture resides in an older orogen, the Central Orogenic belt. However, either model for the location of the suture is able to explain metamorphic P-T-t data for crustal thickening at 1.85 Ga, whereas only the model for a Late Archaean collision in the Central Orogenic belt and a Palaeoproterozoic collision in the Northern Hebei orogen can explain the structural, sedimentological, geochronological, and petrological data. The Central Orogenic belt contains several hundred fragments of a c. 2.505 Ga ophiolite suite, a contemporaneous 2.5-2.4 Ga foreland basin deposited on 2.7-2.5 Ga passive margin sediments on the Eastern Block, and contains rare evidence for c. 2.5 Ga granulite facies metamorphism that was largely overprinted by 1.92-1.85 Ga high-grade assemblages. East-directed 2.5 Ga fold-thrust structures are overprinted by 1.92-1.85 Ga south-directed thrusts associated with large-scale thickening of the craton, succeeded by strike-slip shear zones that slice the orogen into numerous fault-bounded terranes that preserve different levels of exhumation.

Description: The Jiao-Liao massif is located in the hanging wall of the north-dipping DabieSulu suture zone and is an important part of the Eastern Block of the North China Craton. Several important tectonic models for the tectonic evolution of Eastern Asia rely on critical information from the Jiao-Liao massif. This paper combines new sensitive high-resolution ion microprobe (SHRIMP) UPb zircon ages of the Dandong Granite in the southern Liaoning Province, China, with extensive field data for the eastern North China Craton, including the Bohai Bay Basin. Combined with other recent SHRIMP dating, we use this information to summarize the Mesozoic tectonic reactivation and evolutionary processes of the Jiao-Liao massif of the Eastern Block of the North China Craton. In this study we identify a c. 160 Ma episode of partial melting of Palaeoproterozoic plutons in the Jiao-Liao massif. Cathode luminescence and backscatter electron imagery reveal c. 167157 Ma magmatic euhedral single zircons and magmatic zircon rims surrounding c. 2100 Ma cores in the Dandong Granites near the Liaonan Neoarchaean terrane. This partial melting is probably related to in situ remelting of ancient lower continental material, mostly the North China Craton. The Dandong plutons are aligned in a NESW direction and are extensively deformed by subhorizontal ductile thrust-related shearing and subsequent NNESSW trending folds. Here, we show that for the Dandong area the first deformation occurred between 195 and 193 Ma, based on KAr and 40Ar/39Ar ages of muscovites from eastwest-trending shear zones on the Liaodong Peninsula. Based on the field relationships between the plutons and structural fabrics, a range from 153 to 145 Ma is defined as the duration of the second deformation in the Dandong Granites. The third deformation is marked by the formation of NNESSW strike-slip faults between 135 and 95 Ma. This deduced age range is similar to an 40Ar/39Ar age range of 128132 Ma of initial sinistral strike-slip faulting of the Tan-Lu fault in Anhui Province and to a biotite cooling age of 100{+/-}2.3 Ma of the YilanYitong segment of the Tan-Lu fault in the Jilin Province. These faults are transtensive and controlled the formation of pull-apart basins. However, during the third deformation, some metamorphic core complexes in Eastern China formed in the overlapping area between the large-scale sinistral faults. Our SHRIMP data also indicate that the Liaodong basement and its Early Mesozoic magmatism are similar to the Jiaodong basement and its Mesozoic magmatism. Therefore, the Early Mesozoic evolution of the Liaodong area, similar to that of the Jiaodong area, was also closely related to the Sulu orogen in the Early Mesozoic and to the Pacific subduction throughout the Mesozoic.

Description: Cretaceous granites are widespread in the North Dabie orogen, Central China, but their emplacement sequence and mechanism are poorly known. The Tiantangzhai Complex in the North Dabie Complex is the largest Cretaceous granitic suite consisting of six individual intrusions. In this study, zircon U–Pb ages are used to constrain the crystallization and protolith ages of these intrusions. The Shigujian granite is a syn-tectonic intrusion with an age of 141 Ma. This granite was emplaced under a compressional regime. Oscillatory rims of zircons have yielded two peaks at 137±1 Ma and 125±1 Ma. The 137±1 Ma peak represents the beginning of orogenic extension and tectonic collapse, whereas the 125±1 Ma peak represents widespread granitic magmatism. Zircon cores have yielded concordant ages between 812 and 804 Ma, which indicate a crystallization age for the protolith. The Tiantangzhai granites show relatively high Sr contents and high La/Yb and Sr/Y ratios. The Shigujian granite has positive Eu anomalies resulting from partial melting of a plagioclase-rich source in an over-thickened crust. Correspondingly, in situ Lu–Hf analyses from zircons yield high negative Hf ( t ) values from –24.8 to –26.6, with two-stage Hf model ages from 2748±34 to 2864±40 Ma, suggesting that the magmas were dominantly derived from partial melting of middle to lower crustal rocks. The Dabie orogen underwent pervasive NW–SE extension at the beginning of the early Cretaceous associated with subduction of the Palaeo-Pacific plate beneath eastern China.