Western Block Of North China Craton Research Articles

Edge-driven asthenospheric convection beneath the North China Craton: A numerical study

The North China Craton (NCC) is one of the most tectonically active cratons in the world today. Its scattered late Cenozoic volcanism, as well as small-scale low-velocity anomalies in the upper mantle, suggests a major role of asthenospheric upwellings in the NCC tectonism. Such asthenospheric upwellings, however, cannot be readily explained by the Indo-Asian continental collision or subduction of the Pacific plate, two of the plate boundary processes previously suggested for causing the Cenozoic tectomagmatism in the NCC. Here we propose that edge-driven convection, resulting from the large changes of lithospheric thickness under the NCC, is likely a major cause of the observed upper mantle structure and late Cenozoic volcanism in the NCC. Using a two-dimensional thermo-mechanical finite element model, we found that the contrast of lithospheric thickness between the Eastern and Western blocks of the NCC can cause significant edge-driven small-scale convection, especially if the viscosity of the asthenosphere is reduced by melts and volatiles from dehydration and decarbonization of the stagnant slabs of the subducted Pacific plate under the NCC.

High-resolution crustal velocity structure in the Shanxi rift zone and its tectonic implications

The Shanxi rift zone, located in the Trans-North China Orogen (TNCO) of the North China Craton (NCC), is wellknown for hosting large intraplate earthquakes in continental China. The TNCO is a suture zone formed by the amalgamation of the eastern and the western blocks of the NCC. After its formation, it was reactived and deformed by later tectonic activities, which result in complex lithospheric heterogeneities. Thus, the detailed crustal structure of the Shanxi rift zone is critical for understanding the tectonics and seismogenic mechanism in this area, which will shed new lights on the formation and dynamic evolution of the NCC. In this study, we applied ambient noise tomography based on 18 months continuous records from 108 seismic stations located in Shanxi and its surroundings, in order to constrain its detailed crustal structure. We measured 4437 Rayleigh wave phase velocity dispersion curves in the period of 5–45 s from the cross-correlation functions. Next, a surface wave direct inversion algorithm based on surface-wave ray tracing was used to resolve a 3-D S-wave velocity model in the upper 60 km with lateral resolution of ∼50–80 km. The tomographic images show that the sedimentary thickness of the Taiyuan Basin is less than 5 km. At depth of 0–10 km, we observe a good correlation between the imaged structural variations with geological and topographic features at the surface. For example, the center of rift shows low-velocity anomalies and the uplifting areas on both sides are characterized by high velocity anomalies. The western and eastern boundaries of the slow materials coincide with the faults that control the basin. The slow material extends from the shallow surface to depth of about 15 km but it getting smaller in shape at deeper depth. For the Taiyuan Basin, Linfen Basin, and Yuncheng Basin in the central and southern parts, the structure is dominant by slow materials in the upper crust but changes to strong high-velocity anomalies in the lower crust and the uppermost mantle at depth deeper than 25 km. We interprete these high-velocity anomalies to be associated with the cold remnant of the underplated basalt in the lower crust that were formed in early Tertiary before the basin was stretched. We also observe the low-velocity anomaly beneath the Datong volcanic area, which extends from the uppermost mantle to a depth of 20 km vertically and migrates from west to east laterally. It may reflect the upwelling channel of the magmatic material in Datong. Moreover, the strong low-velocity anomalies presented north of 38°N could be related to the heated crustal materials with paritial melting as a result of the intensive magmatic activities of the Datong Volcano since the Cenozoic. In our study region, seismicity mainly concentrates in the depth range of 5–20 km and we find that most earthquakes appear to occur in places where velocity changes from high to low rapidly, with slight higher concentration in the faster material areas. In summary, our high-resolution 3-D crustal velocity model provides important seismological constraints to understand the tectonic evolution and seismicity across the Shanxi rift zone.

Paleoproterozoic (1.96–1.86 Ga) granites in Xinyang record zoned deep crustal structure and multi-stage reworking beneath the southern North China Craton

As one of the oldest cratons on Earth, the North China Craton (NCC) is a frequent area for geological research due to its complex crustal evolution. Regarding the Paleoproterozoic crustal evolution of the craton, most studies have focused on continental amalgamation (i.e. horizontal crustal evolution), but few on the vertical crustal evolution. Here we report whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotope geochemistry for the Xinyang granites to constraint the vertical crustal structure and evolution of the southern NCC. The studied granites comprise two-mica K-feldspar granite and garnet-bearing K-feldspar granite. All the rocks have high SiO2 and total alkali, but low CaO, MgO and Fe2O3T contents. Their A/CNK values (1.05–1.28) suggest that they are peraluminous rocks. These granites are enriched in LILE (Rb, Th and K), depleted in Sr, P and HFSE (Nb and Ti). The two-mica K-feldspar granites show obviously fractionated REE patterns ((La/Yb)N = 18.3–387) with negative or positive Eu anomalies (δEu = 0.47–1.03, 4.85–6.72), while the garnet-bearing K-feldspar granites have flat REE patterns ((La/Yb)N = 1.69–4.89) with extraordinary negative Eu anomalies (δEu = 0.22–0.42). The magmatic zircons of the two-mica K-feldspar granites yield crystallization ages ranging from ~ 1.86 Ga to ~ 1.90 Ga, and those of the garnet-bearing K-feldspar granites give crystallization ages of ~ 1.90 Ga. Previous work also reported a biotite K-feldspar granite pluton formed at ~ 1.96 Ga in the Xinyang area. Taken together, three major episodes of Paleoproterozoic granitoid events (~1.86 Ga, ~1.90 Ga and ~ 1.96 Ga) are recognized in the area. Combined with regional geochronological investigations, the peraluminous granitic magmatic activities occurring during 1.96–1.86 Ga in the Xinyang area could be linked to the collision between the Eastern and Western Blocks of the NCC. Alternatively, zircon Hf isotopic compositions suggest that these rocks derived from partial melting of pre-existing crust with heterogeneous compositions. Zircons from the ~ 1.86 Ga two-mica K-feldspar granites yield εHf(t) values of −5.2–1.7 with Tcrust of 2.40–2.86 Ga, compared to zircons from the ~ 1.90 Ga two-mica K-feldspar granites which yield wide range of εHf(t) values of −13.5–1.0 with Tcrust of 2.50–3.42 Ga. In contrast, the ~ 1.90 Ga garnet-bearing K-feldspar granites have homogeneous negative zircon εHf(t) values ranging from −11.8 to −10.4, with Tcrust ranging from 3.23 to 3.30 Ga. Moreover, zircons from the biotite K-feldspar granites have εHf(t) values of −0.6–2.6, corresponding to Tcrust of 2.44–2.77 Ga. Based on abovementioned geochemical signals, we propose that the Paleoproterozoic crust in the southern NCC likely consisted of two parts in vertical, the upper Middle-Late Archean crust and the lower early Paleoproterozoic crust with Eo-Paleoarchean crustal remnants. During the Paleoproterozoic, the southern NCC experienced multi-stage crustal reworking, indicating that a stable continental crust likely had been formed by the end of Archean.

Geochemistry, geochronology and its geological implication of metamorphic rocks of the Jiehekou Group in the Lüliang complex

华北中部造山带是华北克拉通内部一条重要的前寒武纪碰撞造山带，该造山带的形成经历了一个漫长而复杂的演化过程。吕梁变质杂岩位于造山带中段，阜平杂岩西南，是透视华北克拉通早前寒武纪构造-热演化的一个重要窗口。本文针对该地区的界河口（岩）群黑云斜长片麻岩和含榴斜长角闪岩开展了系统的野外地质调查、显微岩石学、地球化学和变质年代学研究。结果表明，界河口（岩）群黑云斜长片麻岩的原岩多为富铝富钾的变泥质岩石，其物源主要来自长英质火成岩，形成于有演化岛弧发育的活动大陆边缘构造背景；含榴斜长角闪岩的原岩多数是拉斑玄武岩，源区来自于富集地幔，形成于岛弧构造环境。显微岩相学研究显示，含榴夕线黑云斜长片麻岩和含榴斜长角闪岩中均保留了早期进变质、峰期变质和晚期退变质三个阶段的变质矿物组合，且石榴子石边部多发育明显的白眼圈反应结构。其中，含榴夕线黑云斜长片麻岩的峰期矿物组合包括石榴子石、黑云母、夕线石、斜长石、石英和少量钾长石等；含榴斜长角闪岩的峰期矿物组合主要由石榴子石、角闪石、斜长石、磁铁矿和石英等组成。前期研究表明，二者的变质演化均记录了类似的顺时针近等温降压型的变质作用P-T轨迹，其峰期变质条件分别为>750℃/>7.0kbar和>750℃/>6.3kbar。本文LA-ICP-MS锆石U-Pb年代学研究表明，含榴黑云斜长片麻岩中记录了1928~1920Ma和1882~1855Ma两组变质年龄。结合中部带前人研究成果，本文推断吕梁变质杂岩的区域变质事件与华北克拉通东、西部陆块间的碰撞造山作用有关，该地区与碰撞阶段同期的峰期变质作用发生在1928~1920Ma，而后于1882~1855Ma期间进入与快速构造隆升有关的退变质作用阶段。;As an important Precambrian orogenic belt in the North China Craton (NCC), the Trans-North China Orogen (TNCO) was formed by the collision between the Eastern Block and Western Block through an extremely protracted evolution process. To the southwest of the Fuping complex in the central TNCO, the Lüliang metamorphic complex provides an ideal target for studying the Precambrian tectonothermal evolution of the NCC since many upper amphibolite- to granulite-facies metamorphic rocks of the Jiehekou Group were exposed in this area. Petrographic, geochemical and metamorphic geochronological studies were carried out on the biotite plagiogneisses and garnet-bearing amphibolites in this contribution. It was indicated that the protoliths of biotite plagiogneisses were mostly metapelites with high aluminium and potassium contents whose sedimentary provenances were demonstrated to have mainly come from felsic igneous rocks formed in an active continental margin environment. The protoliths of garnet-bearing amphibolites were tholeiite basalts from the enriched mantle, and formed in the island arc tectonic setting. Microscopic petrographic studies showed that there were three stages of mineral assemblages preserved in the garnet-bearing sillimanite-biotite plagiogneisses and amphibolites, including the prograde, peak and retrograde stages. The white-eye socket reaction microstructures commonly grew surrounding the garnet rims of these rocks. The peak assemblages of garnet-bearing sillimanite-biotite plagiogneisses mainly consist of garnet, biotite, sillimanite, plagioclase, quartz and some K-feldspar. Those of garnet-bearing amphibolites include garnet, amphibole, plagioclase, magnetite and quartz. Our previous studies showed that both two types of rocks revealed clockwise P-T paths ended with a near-isothermal decompression, and their peak conditions were > 750℃/>7.0kbar and >750℃/>6.3kbar, respectively. LA-ICP-MS zircon U-Pb dating in this study indicated that two groups of metamorphic ages, including 1928~1920Ma and 1882~1855Ma, were recorded in the garnet-bearing biotite plagiogneisses. Combined with previous metamorphic and geochronological studies of other complexes in the TNCO, the regional metamorphism occurred in the Lüliang complex was thought to be related to the orogenic collision between the Eastern Block and Western Block of the NCC. During this orogenic process, the metamorphic peak occurred at 1928~1920Ma in the Lüliang area, and then followed with 1882~1855Ma retrograde stages resulted by the rapid tectonic uplift after collision.

Metamorphic Age Comparison and Its Implications between the Zuoquan and Zanhuang Complexes in the Central North China Craton, Based on LA-ICP-MS Zircon U–Pb Dating

The Trans-North China Orogen (TNCO) is well-known as an early Precambrian orogenic belt in the North China Craton (NCC). Three meaningful groups of metamorphic ages have been reported in the metamorphic complexes throughout the TNCO, including c. 1.85 Ga, c. 1.95 Ga, and c. 2.5 Ga. The spatial distributions and relationships of these ages provide notable insight into the formation timing and tectonic evolution of the NCC basement. The Zuoquan and Zanhuang complexes are exposed in the south–central TNCO and are adjacent to the Eastern Block. In order to place new constraints on the timing of two phases of metamorphism that occurred in the complexes, combined U–Pb and rare earth element analyses were performed on zircons from different types of metamorphic rocks. Uranium–Pb zircon dating in this study shows that two groups of metamorphic ages of 1.88–1.85 Ga and 2.48–2.46 Ga were commonly recorded by metamorphic rocks in the Zanhuang and Zuoquan complexes, respectively. Our previous geochronological studies showed that metamorphic ages of c. 2.51 Ga and c. 1.90 Ga were locally recorded in the Zanhuang and Zuoquan complexes, respectively. These data indicate that metamorphic rocks in the two complexes underwent at least two phases of metamorphism, i.e., 2.51–2.46 Ga (Phase I) and 1.90–1.85 Ga (Phase II). In combination with previous studies regarding reaction microstructures, metamorphic pressure–temperature paths, and geochronology, the Phase II metamorphic ages are interpreted to be linked to the collision between the Western and Eastern Blocks along the TNCO between 1.97 Ga and 1.80 Ga, whereas the Phase I metamorphic ages, as a result of an earlier and extensive tectono-thermal event that occurred in the Eastern and Western Blocks of the NCC, were related to underplating of mantle-derived magma. It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and then were tectonically involved in the TNCO and experienced c. 1.90–1.85 Ga metamorphism. Metamorphic peaks occurred at different crustal levels in the orogen, resulting in distinct metamorphic ages and peak conditions preserved by metamorphic rocks in the two complexes.

Metamorphic evolution and tectonic significance of ultrahigh‐temperature mafic granulites in the southern part of the Shiguai area, Daqingshan, China

Ultrahigh‐temperature (UHT) mafic granulites in the southern part of the Shiguai area in Daqingshan, China, crop out within the Shanheyuan Gneiss of the Khondalite Belt (KB), a Paleoproterozoic tectonic belt in the Western Block of the North China Craton (NCC). The UHT mafic granulites occur within hypersthene‐bearing dioritic–granitic gneiss as irregular lenses and deformed dike swarms. Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. M1 is defined by the enclosed orthopyroxene and clinopyroxene within the “red eye” corona of the garnet, representing the prograde metamorphic stage. M2 (UHT granulite‐facies) is marked by the appearance of “red eye” corona of the garnet, with mineral assemblages of grt–cpx–opx–pl–ilm–q. M3 is defined by spinel‐rich symplectites between garnet and orthopyroxene, including grt–opx–pl–spl–mt–ilm. M4 (amphibolite facies) is featured by the development of amphibole. Phase equilibrium modelling and some critical thermobarometers are combined to constrain the P–T evolution of the studied samples. The P–T conditions for the M1–M4 are respectively estimated at ~8 kbar/~771–852°C, ~9.4–10.0 kbar/~930–965°C, ~5.0–5.4 kbar/~865–910°C, ~5.0–5.2 kbar/~675–682°C, which define a clockwise P–T path, involving near‐isothermal decompression following the peak metamorphism. The P–T path indicates that the rocks underwent a sequence of tectonothermal processes that started with crustal thickening (M1–M2), followed by rapid crustal uplift (M3) and cooling (M4). This process is inferred that the metamorphism of the UHT mafic granulites was associated with rapid exhumation and uplift that followed the continent–continent collision and amalgamation of the Yinshan and Ordos blocks. This amalgamation formed the Paleoproterozoic KB in the Western Block of the NCC.

Three-dimensional conductivity model of crust and uppermost mantle at the northern Trans North China Orogen: Evidence for a mantle source of Datong volcanoes

While the Eastern Block of North China Craton (NCC) had experienced significant lithospheric destruction in the Mesozoic, the Western Block of NCC and the Trans North China Orogen (TNCO) have undergone localized lithospheric modification since the Cenozoic. The northern TNCO is highlighted by the Cenozoic magmatic activities including Hannuoba basalts and Datong volcanoes and is a seismically active region. In this study 3-D electrical conductivity model of the crust and uppermost mantle is derived by the 3-D inversion technique using data from 72 broadband magnetotelluric (MT) stations. The final model shows that a 15 km thick resistive layer of about 3000 Ω m dominates the upper crust, which may represent the intact Archean and Paleoproterozoic terrains. Whereas in the mid-crust there are marked high conductivity anomalies of about 10 Ω m beneath Shanxi rifting basin, which may result from the interconnected saline fluid of 0.2% to 6% volume fraction. The most important finding is that one significant conductor extended into the mantle is located between Hannuoba field and Datong volcanoes and it connects with the mid-crust conductor beneath the Datong volcanoes. We suggest that this could be the mantle source (partial melting region) for the Quaternary volcanic activities of Datong volcanoes and the melt fraction is estimated as 6.6%. Its location inside the Western Block suggests that the volcanic activities at Datong volcanoes are irrelevant to the tectonic process to the east of TNCO. It is likely to be related to the mantle flows from the Tibetan Plateau around the Ordos block which converges at the northeastern corner of the Ordos block and local upward flow along the slope of the thinning lithosphere resulted in decompression partial melting and the melt percolated upward through the crust to feed the lava eruptions at the Datong volcanoes to the east. Finally, large crustal earthquakes in this region are generally located in resistive zones with high conductivity anomalies beneath. The fluids brought up by hot mantle material may play an important role in fault strength weakening and rupture nucleation at mid-crust depths.

A long-lived magma chamber in the Paleoproterozoic North China Craton: Evidence from the Damiao gabbro-anorthosite suite

The Damiao igneous complex is a composite gabbro-anorthosite suite that was emplaced during the post-collisional extensional stage following the amalgamation of the Eastern and Western Blocks within the North China Craton (NCC). This magmatic suite is composed of anorthosite, leuconorite, gabbroic anorthosite, norite, gabbronorite, noritic gabbro, ferrodiorite, Fe-Ti-(P) rich gabbro and Fe-Ti ore. We present zircon LA-ICP-MS U–Pb age data on noritic gabbro, norite, leuconorite, gabbronorite, and gabbroic anorthosite from the Damiao suite. The data yield weighted mean 207Pb/206Pb ages of 1725±13Ma, 1687±18Ma, 1751±15Ma and 1693±24Ma, 1721±17Ma, and 1729±14Ma respectively suggesting a relatively long-lived crystallization process within the magma chamber during late Paleoproterozoic. The different lithologies show similar rare earth element patterns indicating a co-magmatic nature and derivation from the same magma chamber through polybaric crystallization and differentiation. The high alumina dykes of gabbronorite and noritic gabbro which are chemically identical to the parental magmas of high alumina gabbros from elsewhere and exhibit an upper mantle origin with high degree melting of spinel-bearing mantle facies. Their zircon ɛHf(t) compositions plot along the evolution line of the 2.5–3.0Ga Neo- to Mesoarchean basement rocks in the NCC (−9.2 to −3.3) and the relatively low Cr, Ni contents (Cr: 9.33–338ppm; Ni: 8.62–182ppm), high Th/Ta (>2.90), Ba/Zr (>20.18), and low Nb anomaly (Nb/Nb*=NbPM/(ThPM×LaPM)<0.50) for chemically equivalent parental rocks deviate from typical mantle-derived melts, suggesting the input of Archean crustal components. We propose that the magma was sourced from spinel-bearing sub-continental lithospheric mantle at depths of up to 60km, within a post-collisional extensional setting, and assimilated Archean crustal components before ascent. The heat input for the extensive melting might have come from upwelling asthenosphere triggered by the break-off of the subducted oceanic slab following the collision between the Eastern and Western Blocks of the NCC. The deep seated magma was channeled to mid-crust depth along rift zone and underwent low pressure crystallization and differentiation to form the Damiao suite.

Crust structure of the North China Craton from a long-range seismic wide-angle-reflection/refraction data

We present an interpretation of a 1530-km-long wide-angle seismic reflection/refraction profile that extends across the North China Craton (NCC) approximately from east to west. This profile is the longest and the densest wide-angle seismic reflection/refraction profile in China up to now. Parameterizing the velocity model as layers with variable thickness, we perform seismic travel time tomography to resolve the velocity variation within layers and velocity contrast across Moho. Our model shows crustal thickness that varies from 35km under the Shandong peninsula, to 30km under the North China Plain (NCP) and 32–40km under central NCC, to 45km under the Ordos plateau. These results indicate obvious crustal thinning beneath the NCP in comparison with the ~35km crustal thicknesses typical of most cratonic regions. In particular, an abrupt Moho step of ∼10km is detected at the junction of the eastern block and western block of the NCC. These crustal characteristics are consistent with studies of lithosphere–asthenosphere boundary (LAB) with the method of receiver function migration, and they indicate a close correlation with lithospheric thickness and surface tectonics of the northeastern NCC. These results provide robust seismological evidence that the lithosphere from NCC to the western edge of the Taihang Mountain, might have been widely affected and thinned since its formation in the Archean era.

Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry, zircon U–Pb geochronology, and Hf isotope of the pyroxenite–gabbro–diorite suite from Xinghe, Inner Mongolia

The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The εHf(t) shows mostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO2 and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.

The formation and rejuvenation of continental crust in the central North China Craton: Evidence from zircon U–Pb geochronology and Hf isotope

The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U–Pb and Lu–Hf data on TTG (tonalite–trondhjemite–granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450–1900Ma suggesting a long-lived convergent margin. The εHf(t) values of these zircons range from −11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120–130Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the εHf(t) values of these zircons show wide a range from −30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.

Neoproterozoic S-type granites in the Alxa Block, westernmost North China and tectonic implications: In situ zircon U-Pb-Hf-O isotopic and geochemical constraints

The Alxa Block in northern China has been traditionally considered as the westernmost part of the Archean North China Craton (NCC). However, recent studies revealed that there are few Archean rocks exposed in the Alxa Block, and the Paleoproterozoic geology of this block is different from that of the western part of the NCC. Thus, the tectonic affinity of the Alxa Block to the NCC and/or other Precambrian blocks needs further investigations. In this study, we carry out integrated analyses of <i>in situ</i> zircon U–Pb age and Hf–O isotopes as well as whole-rock geochemistry and Nd isotopes for the Neoproterozoic Dabusushan and Naimumaodao granites from central Alxa Block. Secondary ion mass spectrometry (SIMS) U–Pb zircon dating results indicate that the Naimumaodao and Dabusushan granite plutons were formed at <i>ca.</i> 930 Ma and <i>ca.</i> 910 Ma, respectively. These granites are peraluminous (A/CNK value &gt;1.0), and contain peraluminous minerals such as muscovite and tourmaline, similar to those of S-type granites. They are characterized by high zircon δ¹⁸O values of <i>ca.</i> 8.2 to 12.1 permil, corresponding to a calculated magmatic δ¹⁸O value of <i>ca.</i> 10.5 to 14.3 permil, variable zircon ε_Hf(t) values of −6.2 to +3.8 (corresponding to Hf model ages of 2.2 to 1.6 Ga) and whole-rock ε_Nd(t) values of −10.1 to −4.5 (corresponding to Nd model ages of 2.4-1.9 Ga). The petrological and Nd–Hf–O isotopic study indicated that these granites were most probably generated by remelting of dominant (meta)sedimentary rocks in an orogenesis-related compressional environment. There is a clear contrast in the Precambrian geological evolution, including basement rock age data, Precambrian magmatism and detrital zircon age patterns, between the Alxa Block and the NCC. Furthermore, the new <i>in-situ</i> detrital zircon ages on Neoproterozoic (meta)sedimentary rock suggest that Alxa Block is likely related to the Cathaysia Block of South China during the Neoproterozoic, and amalgamated with the NCC since the Early Paleozoic. Thus, our new data suggest that the Alxa Block is most likely a separated Precambrian terrane from the Western Block of the NCC.

Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting

An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5Ga granite pluton and cross-cut by undeformed 2.5Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic–ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc–forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc–continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.

Zircon U–Pb age and Lu–Hf isotope constraints on Precambrian evolution of continental crust in the Songshan area, the south-central North China Craton

A combined whole-rock geochemical and zircon U–Pb and Lu–Hf isotope study on two Precambrian lithotectonic units in the Songshan area, south-central North China Craton (NCC), was performed to constrain the tectonic evolution of the Trans-North China Orogen (TNCO) during collision between the Eastern and Western Blocks of the NCC. The metamorphic unit at Shipaihe consists of dioritic gneiss with minor amphibolitic enclave, whereas the magmatic unit at Shicheng is composed of granite. Zircon U–Pb dating demonstrate that the dioritic gneiss and enclave amphibolite were originally emplaced contemporaneously at ca. 2.5Ga. The magmatic zircons exhibit ɛHf(t) values of 2.2–7.8 and Hf model ages of 2.52–2.73Ga, suggesting that the dioritic gneiss was produced by reworking of the juvenile crust. The magmatic zircons of amphibolite have ɛHf(t) values of 0.7–8.1 and Hf model ages of 2.51–2.79Ga. The granite primarily gave zircon U–Pb ages of ca. 1.78Ga, with inherited zircons in ca. 1.87Ga. Zircon Lu–Hf isotopic analyses yield negative ɛHf(t) values of −16.7 to −1.8 and Hf model ages of 2.55–3.47Ga, indicating that the granite was mainly derived from reworking of ancient Archean crust. The Neoarchean dioritic gneiss and amphibolite show enrichment of LREE and LILE but depletion of HREE and HFSE, suggesting their derivation from anatexis of juvenile arc-type crust and enriched lithospheric mantle, respectively. The dioritic gneiss also has highly fractionated REE patterns and negligible negative Eu anomalies, implying anatexis at high pressures, where garnet and possibly amphibole as residual phases. The Paleoproterozoic granite has high K2O+Na2O and Zr, high ratios of total FeO/MgO and Ga/Al, and low contents of CaO, Al2O3, Ba, Sr and Eu/Eu* values, consistent with the characteristics of A-type granites. The negative Eu, Ba and Sr anomalies suggest that plagioclase acted as a residual phase during partial melting at relatively low pressures. Taken together, the Songshan metamorphic complex would originally derive from the reworking of juvenile arc-type crust and coeval partial melting of juvenile, enriched lithospheric mantle. Thus, an active continental margin was present in the late Neoarchean in south-central NCC, generating arc-type magmatism between the Eastern and Western Blocks. During the collisional orogeny, the arc-type crust in southern NCC underwent intensive metamorphism that followed by post-collisional extension at ca. 1.78Ga, and the reworking of the old continent in the southern TNCO yield Shicheng A-type granite.

Precambrian geology of China

Precambrian geology of China

Crustal growth and intracrustal recycling in the middle segment of the Trans-North China Orogen, North China Craton: a case study of the Fuping Complex

AbstractThe most important crustal growth on Earth occurred at ~2.7 Ga, but the North China Craton (NCC) is characterized by prevalent development of ~2.5 Ga juvenile crust, with relatively rare records of ~2.7 Ga crustal growth. The Fuping Complex in the middle segment of the Trans-North China Orogen (TNCO) between the Eastern and Western blocks of the NCC is composed mainly of ~2.5 Ga Fuping tonalitic–trondhjemitic–granodioritic (TTG) gneisses and Longquanguan augen gneisses, ~2.1 Ga Nanying granitic gneisses and the Wanzi supracrustal rocks. Previous studies have suggested one major phase of crustal growth at ~2.5 Ga, possible intracrustal recycling at ~2.1 Ga and the presence of older rocks in the Fuping Complex, but there has been no record of ~2.7 Ga crustal growth. The Fuping TTG gneisses are dominated by stromatic migmatite, and new U–Pb dating of magmatic zircons from two stromatic migmatite samples yielded three different ages: (1) 2.75 Ga, which is the oldest age obtained from the Fuping TTG gneisses, (2) 2.54 Ga, which just falls in the published zircon U–Pb age range of 2.53 to 2.47 Ga for the Fuping TTG gneisses, and (3) 2.11 Ga, which is almost the same as the age of the Nanying granitic gneisses. Therefore, there are two generations of TTG gneisses in the Fuping Complex. Importantly, both of the 2.75 and 2.54 Ga zircons have the highest εHf(t) values, almost equal to the contemporaneous depleted mantle. This indicates high contributions of juvenile material to the two generations of TTG gneisses. In contrast, the 2.11 Ga zircons have apparently low εHf(t) values of −0.47 to +2.04, just falling in between 2.55 and 2.75 Ga continental crust values. This strongly suggests the reworking of the two generations of TTG gneisses at 2.1 Ga. Zircon U–Pb and Hf isotopes convincingly reveal two major phases of crustal growth in the Fuping Complex at ~2.7 and ~2.5 Ga, the same as in the northern and southern segments of the TNCO, and also confirm one major phase of intracrustal recycling at ~2.1 Ga, which may be responsible for the Nanying granitic gneisses.

Paleoproterozoic evolution of the eastern Alxa Block, westernmost North China: Evidence from in situ zircon U–Pb dating and Hf–O isotopes

The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34Ga and ca. 2.32–2.30Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394ppm) and Ti/V (>30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98Ga, and subjected to high grade metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.