Large-scale Lithospheric Thinning Research Articles

Upper mantle shear velocity structure of the Cathaysia Block and surrounding areas: New insight into deep geodynamics

The Cathaysia Block (CAB) and its surrounding areas experienced intensive magmatism and mineralization in the Yanshanian period (ca. 200–90 Ma), but their mechanism and deep geodynamics are still debated. In addition, the origin and structures of the Hainan mantle plume are still unclear. To resolve these issues and investigate the large-scale lithospheric thinning and extension in the eastern South China Block, we determine a detailed 3-D S-wave velocity (Vs) model down to 700 km depth by collecting 24,190 S-wave teleseismic data recorded at 164 permanent stations and 125 portable stations deployed in the CAB and surrounding areas. Our results show that high-Vs anomalies exist separately in the study volume. Two high-Vs anomalies appear in the shallow upper mantle and the mantle transition zone, which may reflect the present thin lithosphere and the stagnant Paleo-Pacific slab, respectively. Two other high-Vs anomalies exist in the upper mantle, which may reflect the detached lithosphere and subducting slabs. In contrast, low-Vs anomalies appear widely beneath the CAB, which reflect a tilting magmatic conduit beneath the Wuyishan metallogenic belt (WYMB) and magmatic chambers beneath the Nanling metallogenic belt (NLMB). In addition, our results show that the Hainan plume has a double-layered appearance. Combining our tomographic results with previous multidisciplinary findings, we consider that (1) the subduction and rollback of the Paleo-Pacific Plate may have played different roles in the Yanshanian mineralization of the WYMB and the NLMB; (2) the double-layered appearance of the Hainan plume may be formed due to the influence of plume self-evolution dynamics and pre-existing deep structures; and (3) lithospheric delamination triggered by gravity instability may have occurred beneath the Xuefengshan Mountain in the late Mesozoic.

Heterogeneity of the late Mesozoic lithospheric mantle beneath the Jiaodong Peninsula, eastern North China Craton: a synthesis from geochemistry and Sr-Nd-Pb isotopes of mafic dykes

ABSTRACT The Late Mesozoic lithospheric mantle evolution of the Jiaodong Peninsula is crucial for understanding the destruction of the eastern North China Craton (NCC) and large-scale gold mineralization, but it still remains highly controversial. Here, we comprehensively compiled published Late Mesozoic mafic dyke magmatism data from the Jiaodong Peninsula to provide key constraints on the nature and evolution of the lithospheric mantle. Mafic dykes are widely emplaced in the Jiaodong Peninsula and have been divided into two groups (Jiaobei terrane and Sulu orogenic belt); they mainly comprise lamprophyre, dolerite, and gabbro. The ages of the mafic dykes in the Jiaobei terrane are predominantly between 120 and 125 Ma, and those of the Sulu orogenic belt are between 110 and 120 Ma. The Jiaobei and Sulu mafic dykes both have high contents of Mg and compatible elements (Cr, Ni), as well as low SiO2 content; they are characterized by arc-like mafic magma, including enrichment in Large Ion Lithophile Elements and Light Rare Earth Elements, depletion in High Field Strength Elements, high (87Sr/86Sr)i, and low εNd(t), implying a metasomatized lithospheric mantle source. However, the Jiaobei mafic dykes have relatively higher (87Sr/86Sr)i, lower εNd(t), and a larger range of (206Pb/204Pb)i, (208Pb/204Pb)i, and (207Pb/204Pb)i compared with the Sulu mafic dykes. The Jiaobei and Sulu mafic dykes exhibit Ba/Rb ratios of 1.29‒64.39 (average 29.3) and 6.61‒108.28 (average 33.1), respectively, indicating the presence of hornblende in the melts. The high Fe/Mn (>60) and Zn/Fe (>12) ratios of Jiaobei and Sulu mafic dykes indicate the mixed mantle source of peridotite and pyroxenite. But the content of pyroxenite in the mantle source of Jiaobei terrane is higher with wider range of Fe/Mn and Zn/Fe ratios. These features indicate the heterogeneity of the mantle sources of the mafic dykes in the Jiaobei and Sulu regions. It is suggested that both regions originated from the partial melting of hornblende-bearing peridotite, with the addition of more pyroxenite to the mantle source of the Jiaobei terrane compared to Sulu orogenic belt. The mafic dykes have the low Nb/Zr (Jiaobei: 0.024–0.229; Sulu: 0.030–0.166) and Th/Zr ratios (Jiaobei: 0.017–0.081; Sulu: 0.010–0.068), and the high Rb/Y (Jiaobei: 0.347–12.162; Sulu: 0.224–9.600) and low Nb/Y ratios (Jiaobei: 0.202–1.486; Sulu: 0.140–1.322). It implies that the mantle source area in both regions was metasomatized by subducted slab-derived fluids. The increase in (La/Yb)N and decrease in Hf/Sm implies carbonate metasomatism in both regions. In addition, some Jiaobei and Sulu mafic dykes have high Ti/Eu ratios, implying that the silicate metasomatism occurred in the mantle source of both regions. Based on the regional evolution, the lithospheric mantle source in the two regions beneath Jiaodong Peninsula experienced carbonated fluid metasomatism resulting from the subduction of the Palaeo-Asian Ocean plate beneath the northern NCC (ca. ~250 Ma), silicate melt-related metasomatism from subduction of the Yangtze Craton plate beneath the southern NCC (ca. 240‒220 Ma), and oceanic slab-derived fluid metasomatism from the subducted Palaeo-Pacific plate (at 180‒110 Ma). It is believed that the mantle source of Jiaobei terrane had undergone stronger carbonated fluid metasomatism and silicate melt-related metasomatism, compared to Sulu orogenic belt. Moreover, the continental fluid-related metasomatism only occurred in the Sulu orogenic belt. During the Late Mesozoic, the eastern NCC experienced large-scale lithospheric thinning, leading to upwelling of the asthenospheric mantle. The mafic dykes in Jiaobei and Sulu, show different geochemical features, were derived from partial melting of the metasomatized lithospheric mantle through upwelling of the asthenospheric mantle.

Petrogenesis of Late Mesozoic trachydacites in the Jiaolai Basin, Eastern China: Implications for the destruction of the North China Craton

The North China Craton has witnessed a large-scale lithospheric thinning event in the Mesozoic. The volcanic rocks of the Qingshan Group formed during the destruction of the North China Craton, therefore retaining important information to the deep processes in this region. Zircon U-Pb dating and geochemical analyses have been carried out on volcanic rocks of the Qingshan Group collected from southeastern margin of the Jiaolai Basin. In combination with previously published data on the volcanic rocks of the Qingshan Group, the newly acquired data show that: (1) the volcanic rocks of Qingshan Group in Mashan have SiO2 content of 62.82 ∼ 66.3 wt% and total alkali content (K2O + Na2O) of 7.64 ∼ 9.34 wt%. According to TAS diagram, they belong to trachydacite which experienced strong fractionation between LREEs and HREEs with depletion of HFSEs and enrichment of LILEs. (2) The crystallization age of trachydacite in Mashan is 116.4 ± 2.3 Ma (MSWD = 1.9, n = 9). The magma originated from the partial melting of the ancient lower crust and the enriched lithospheric mantle metasomatized by the fluids/melts released from the subducted slab of the Yangtze Craton. (3) The formation age of magmatic rocks in western Shandong is generally earlier than that in eastern Shandong, while the lower crust delamination in western Shandong preceded that in eastern Shandong. The occurrence of magmatism in eastern Shandong is probably attributed to the delamination process in western Shandong as well as the ongoing tectonic activities along the Tan-Lu fault zone. The lithospheric thinning of the North China Craton was mainly controlled by the change of the subduction angle of the Paleo-Pacific plate, and is probably influenced by the continuous activity of the Tan-Lu fault zone in eastern Shandong.

Inherited zircons in Early Cretaceous dyke: Implication of growth and reworking of continental crust in the Jiaodong Peninsula

Inherited zircons from Early Cretaceous dykes across the Jiaodong Peninsula provide a means of exploring magmatic episodes. Negative Eu anomalies, high Th and U contents, and high U/Yb ratios confirmed a zircon source of ancient continental crust. Differences in mineralogy, chronology, geochemistry, and Hf isotopes showed that these inherited zircons were not brought in by the Triassic Yangtze Craton collision. Zircons within 2938–2800 Ma confirmed that the ancient crust in the Jiaobei Terrane originated from the Mesoarchean crustal growth. This crustal growth continued until the Early Paleoproterozoic and was accompanied by crustal transformation activities in the later period. After the collision of the Jiao–Liao–Ji Belt, the Jiaodong Peninsula entered a typical craton period until it was reactivated at the end of the Neoproterozoic, which was characterised by the Archean crustal transformation. During the Palaeozoic, smooth crustal thickness change with limited addition of oceanic crust material indicated the Qinling–Dabie–Sulu Orogenic Belt as a distal island arc system that slowed the impact of Paleo-Tethys subduction. There was no record of positive-εHf(t) zircons in the Jiaodong Peninsula during the Mesozoic, but the crust thickened rapidly. This confirmed that the Paleo-Pacific Plate may have directly subducted beneath the lithospheric mantle and had little contact with the lower crust. This subduction pattern released little hydrous fluid but much heat, which prompted the remodelling of the lower crust in the Mesozoic. Furthermore, the large-scale lithospheric thinning event around 120 Ma was identified as a result of the delamination of the lithospheric mantle together with lower crust.

Geochemistry and Isotopic Characteristics of Apatite and Zircon From Late Jurassic Granites in the Jiaobei Terrane, East China: Implications for Petrogenesis and Geodynamic Setting

The North China Craton (NCC) was stable for more than 2.0 Gyr before a Jurassic–Cretaceous large-scale lithospheric thinning event, but the geodynamic setting during the early phases (Late Jurassic) of NCC reworking remains controversial. We present new petrological and whole-rock geochemical data, zircon and apatite geochemistry, U–Pb ages, O isotopic data, and Sr–Nd isotopic data for two phases of Late Jurassic granite (Linglong and Luanjiahe) from the Jiaobei terrane, southeastern NCC. LA-ICP-MS zircon U-Pb dating suggests that the Linglong granite formed about 6 Myr earlier than Luanjiahe granite (158 Ma vs 152 Ma), after the inception of the paleo-Pacific plate subduction. High zircon U/Yb ratios, high δ18O values [7.89 ± 0.10‰ to 7.67 ± 0.14‰ (2σ)], and inherited zircon age spectra, as well as high apatite F/Cl ratios and Sr–Nd isotopic compositions, suggest that the Linglong and Luanjiahe granites formed by partial melting of ancient thickened lower continental crust of the NCC and Yangtze Craton. Magma evolution modelling based on Rb and Rb/Nb data suggests a similar decoupled assimilation-fractional crystallization process for the generation of Linglong and Luanjiahe granite but with different assimilation degrees. The water contents of parental magma evaluated by using whole-rock Ba, Sr and apatite F, Cl data indicate that the Linglong granite was formed in a relatively water-rich environment than Luanjiahe. This is consistent with the presence of amphibole and minor negative Eu anomalies in the Linglong granite, as water input can promote amphibole fractionation and suppresses plagioclase crystallization. Considering the similar magma sources but distinct water contents of the granites, and the oblique Paleo-Pacific plate subduction setting in the Late Jurassic, the fluids were likely released from the ocean plate beneath a stacked thickened crust. Since the earliest mafic dikes (OIB-type) in the NCC are coeval with the Luanjiahe granite, we suggest that the lower water contents of the Luanjiahe granite were associated with roll-back that resulted in an increasing distance from slab to continental crust. Such a tectonic transition from subduction compression (158 Ma) to initial extension (152 Ma) in the Late Jurassic perhaps possibly marks the beginning of the reworking of the NCC.

Magnetotelluric evidence for crustal decoupling: Insights into tectonic controls on the magmatic mineral system in the Nanling–Xuancheng area, SE China

Despite extensive efforts to understand the proliferation of metallogenic deposits in the Middle to Lower Reaches of the Yangtze River (MLYR) important knowledge gaps remain. Deep seismic reflection and geochemical studies suggest large-scale lithospheric thinning and basaltic magma underplating associated with Mesozoic magmatism and ore-formation, but the magmatic mineral system that caused the mineralization and its relationship to the regional tectonic evolution remains unclear. Three geodynamic models, i.e., collision deformation, subduction deformation, and tectonic regime conversion, have been used to explain the relationship between mineralization and underplating. However, none of these models can reconcile all the regional geological and geophysical data. For the first time, we present a 3D geo-electrical model, created by 3D inversion of magnetotelluric data, which reveals the electrical structure beneath the Nanling-Xuancheng ore district in the MLYR; an emerging new ore district in the MLYR metallogenic belt. Conductivity variations have distinctly different orientations in the upper and lower crust. The trend is northeast-southwest in the upper part of the crust and east–west in the deeper crust. This is interpreted as evidence for decoupling between the upper-to-mid and mid-to-lower crust. Two major conductive zones revealed in the crust are used to interpret the relationship between magmatism and deep underplating. According to the analysis of rheology parameters calculated from the electrical model, we propose a new mechanism, referred to as “crustal decoupling” to explain the tectonic-magmatic history of the Nanling – Xuancheng area. The upwelling fluids created a weak zone at ductile–brittle transition, and provided the conditions for crustal decoupling caused by the NW directed compression of Paleo-Pacific plate. Fluids migrated through NE-SW directed channels and interacted with the upper crustal rocks. The Cu-Au mineralization related to cryptoexplosive breccia developed in a subsequent process involving rapid release of the fluids due to a change in pressure. This process can be attributed to the extensional decoupling of the crust in changing the stress regime.

Meso‑Cenozoic lithospheric thermal structure and its significance in the evolution of the lithosphere in the Ordos Basin, WNCC, China

ABSTRACT The Ordos Basin located in the Western North China Craton (WNCC) has been confirmed to have undergone large-scale lithospheric thinning at the end of the Early Cretaceous (130 ~ 90 Ma), but investigations involving its lithospheric thermal structure in the Mesozoic and Cenozoic are limited. In this study, we employed a 1D, steady-state, heat conduction equation to calculate mantle heat flow and its partitioning, the Moho temperature and the thermal lithospheric thickness of different tectonic units in the Ordos Basin based on an analysis of thermal history reconstruction and crustal layering models. The results suggest that the peak of mantle heat flow at the end of the Early Cretaceous varied between 46.65 mW/m2 and 62.97 mW/m2, far exceeding 50% of surface heat flow, which shows a typical ‘hot mantle and cold crust’ type of lithospheric thermal structure, and the lithospheric thinning reached a minimum thickness of 50 ~ 75 km. This was followed by a thermal decay phase in the Late Cretaceous, and a significant vertical thickening of lithosphere occurred during this period, during which the lithospheric thickness increased to 78 ~ 133 km by the end of the Late Cretaceous. Since the Cenozoic, the ratio of mantle to surface heat flow gradually declined to less than or close to 50%. At present, the average mantle heat flow in the Ordos Basin ranges from 22.10 mW/m2 to 38.76 mW/m2, the lithospheric thickness is between 81 ~ 158 km, and the Moho temperature entirely surpasses 600°C. In addition, we discuss potential factors and uncertainties in the thermal structure calculation. Combined with previous studies, we suggest that the lithosphere in the WNCC has been thinned locally due to asthenosphere upwelling.

A genetic link between Late Cretaceous granitic magmatism and Sn mineralization in the southwestern South China Block: A case study of the Dulong Sn-dominant polymetallic deposit

The southwestern South China Block, composed of southwestern China and northeastern Vietnam, is one of the most important Sn-polymetallic ore districts in the world. The Dulong skarn Sn-polymetallic deposit, which is closely related to Late Cretaceous granitic magmatism, contains 5.0 mt Zn, 0.4 mt Sn, 0.2 mt Pb, and 7 kt In, and is one of the largest Sn-polymetallic deposits in the region. Sn mineralization occurs mainly as cassiterite at the contact zone between the Laojunshan granitic pluton and carbonates of the Cambrian Tianpeng Formation. The Laojunshan granitic pluton consists of three: medium- to coarse-grained two-mica monzonitic granite, medium- to fine-grained two-mica monzonitic granite, and granite porphyry, with zircon U–Pb ages of 87.5 ± 0.6 Ma, 85.3 ± 1.1 Ma, and 82.8 ± 1.6 Ma, respectively. Zircon grains from these three phases show εHf(t) values from −9.5 to −2.5, typical of a mantle origin. All samples exhibit strongly peraluminous and high-K calc-alkaline affinities, and show trace element patterns characterized by enrichment in Rb, Ta, La, Zr, and Hf, and depletion in Ba, Nb, and Ti, typical of S-type granites. Three main alteration/mineralization stages are recognized within the mineralized contact zones: (1) skarn stage; (2) sulfide mineralization stage; and (3) supergene stage. Cassiterite grains from the skarn alteration stage yield a 207Pb/206Pb–238U/206Pb concordia age of 89.4 ± 1.4 Ma, suggesting that mineralization was associated with the Laojunshan granitic pluton. Regionally, the Sn-polymetallic mineralization in the southwestern South China Block took place within a narrow time span from 97 to 79 Ma. The causative granitic magmas can be produced by partial melting of the Paleoproterozoic–Mesoproterozoic pelitic basement rocks during large-scale lithospheric thinning and asthenospheric upwelling.

Geochronology, geochemistry and Hf–Sr–Nd isotopes of the ore-bearing syenite from the Shapinggou porphyry Mo deposit, East Qinling-Dabie orogenic belt

The Shapinggou Mo deposit is located in the western Dabie mountains, the eastern part of the Qinling-Dabie molybdenum orogenic belt. Shapinggou Mo deposit is a concealed deposit with the ore body mainly hosted by explosive breccia of Gaijing and the granite porphyry as well as the syenite of Shapinggou. Geochemistry study show that the SiO2 contents of Shapinggou syenite range from 61.74 to 69.93%, and the A/CNK from 0.95 to 1.06, classified as metaluminous to weak peraluminous, belonging to alkalic to shoshonitic series. The Mo deposits in Qinling Mo belt formed in two main periods, i.e., the first period occurred in to the Early Cretaceous (145–130 Ma), the second period in the late Early Cretaceous (130–110 Ma). Most of the Mo deposits in Dabie region formed in the second period. The results of zircon U–Pb show that the age of the Shapinggou syenite is 111.3 ± 1.2 Ma, which belongs to the second period. Proterozoic-Archean inherited zircons suggest that it may include some more ancient crustal material like Kongling group. The ɛHf(t) values of Shapinggou syenite range from −15.6 to −8.0, TDM2(Hf) from 1.7 to 2.16 Ga, respectively. The ɛNd(t) values of the Shapinggou syenite range from −12.29 to −11.76, TDM2(Nd) from 1.85 to 1.89 Ga, the 87Sr/86Sr from 0.709 to 0.710, respectively. Results of zircon Hf isotope and whole rock Sr–Nd isotope of Shapinggou syenite indicate that the Mo ore-forming materials were mainly generated from old Yangtze craton, e.g., gneiss from Dabie orogeny, mixed with some juvenal mantle materials. The geodynamics of the Shapinggou Mo deposit corresponded to an extension period in Eastern China, which caused by large scale lithospheric thinning. The delamination caused asthenosphere upwelling and crust-mantle interaction, which provided the ore-forming material and heat.

Geochemistry, geochronology and Sr–Nd–Hf isotopes of two Mesozoic granitoids in the Xiaoqinling gold district: Implication for large-scale lithospheric thinning in the North China Craton

The Mesozoic granitoids at the southern margin of the North China Craton are of considerable geologic interest, because of their geodynamic context and likely relationship to Mesozoic lithospheric delamination, but also because they are associated with one of the most important gold belts in China. However, their detailed age relations and petrogeneses have not been described by systematic and comprehensive geochemical studies. Here we report a systematical zircon U–Pb dating, petrogeochemistry, and Sr–Nd–Hf isotopic data on two of these granitoids (namely the Wenyu and the Niangniangshan plutons) in the Xiaoqinling gold district, Henan Province, North China. Both plutons are metaluminous to weakly peraluminous, and can be classified as I-type granites with similar zircon U–Pb ages of 131±1Ma and 134±1Ma. They are enriched in Si, K, Ba, and Sr, while depleted in Nb, Ta, P, Ti, Mg, Ni, V and Y, and both have Archaean-type tonalite–trondhjemite–granodiorite (TTG) and adakitic affinity, e.g. enrichment in LREE with high Sr/Y and La/Yb ratios and without Eu anomalies. Both the plutons show relatively high initial 87Sr/86Sr ratios (0.7076–0.7089 for the Wenyu pluton and 0.7074–0.7086 for the Niangniangshan pluton), negative εNd(t) values (−15.0 to −10.1 for the Wenyu pluton and −18.5 to −12.2 for the Niangniangshan pluton), and negative εHf(t) values (−21.9 to −10.4 for zircons from the Wenyu pluton and −30.8 to −20.8 for zircons from the Niangniangshan pluton). All the isotopic data indicate old crustal sources for the magma. Furthermore, low concentrations of MgO, Ni, and V also indicate little or no mantle materials in the source. The local Neoarchaean to Palaeoproterozoic basement rocks are the most likely sources, but their isotopic compositions do not cover the total isotopic range of the plutons, hence there maybe other sources involved. The occurrence of 1.78Ga inherited zircon grains on the depleted mantle Hf isotopic evolution line indicates 1.78Ga underplating mafic rocks to be a possible source. Detailed petrologic and geochemical data indicate that the two plutons derived from partial melting of Neoarchaean to Palaeoproterozoic metamorphic rocks and possibly with minor amounts of 1.78Ga mafic lower-crustal rocks at a depth of at least 50km, with eclogite, garnet amphibolite or amphibolite residues. During the magma crystallization stage, amphibole, apatite and titanite fractionation caused the observed variations of the element compositions. Meanwhile, the existence of >50-km-depth crust indicates that large-scale lithospheric thinning in this part of the south margin of North China Craton did not take place prior to the emplacement of these granitoids 130Myr ago. However, a fuller understanding of the geological significance of the widespread occurrence of Mesozoic granitic magmatism in the south margin of North China Craton will require similarly detailed studies of other granitoids in the region. In this way it should be possible to constrain their source compositions and the nature of the heat source(s) that led to this lithospheric thinning event.

Seismic imaging of the crust and upper mantle under Beijing and surrounding regions

We determined a high-resolution 3D P-wave velocity model of the crust and upper mantle under Beijing and surrounding regions in North China by jointly inverting 48,750 arrival times from 2973 local events and 12,249 relative residuals from 234 teleseismic events recorded by the newly built Capital Seismic Network. Our results show considerable differences in the lithospheric structure down to about 200 km depth between the North China basin, the Taihangshan uplift and the Yanshan uplift, which are the three main tectonic units in the study area. At depths of 50–150 km, a pronounced low-velocity anomaly exists under North China basin and Bohai Bay, and right beneath it a high-velocity anomaly extends down to about 200–300 km depth, suggesting that this area has experienced large-scale lithospheric thinning and lithospheric detachment has occurred. The Taihangshan front fault and the Zhangjiakou-Penglai fault zone may have cut through the crust to lithosphere and they become flimsy tectonic boundaries, and the asthenospheric materials may upwell from the two channels. We consider that the deep and shallow structures beneath this region are closely related to dynamic processes that result from the deep subduction of the Pacific slab and its stagnancy in the mantle transition zone under East Asia.

Tectonic implications from Re–Os dating of Mesozoic molybdenum deposits in the East Qinling–Dabie orogenic belt

The East Qinling–Dabie molybdenum belt is part of a larger East–West trending metallogenic belt in eastern China. Most of the molybdenum deposits occur as porphyry or porphyry–skarn type, but there are also some vein type deposits. Following systematic Re–Os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits, we have recognized that the molybdenum mineralization in the East Qinling–Dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks. Three pulses of granitoid magmatism and Mo mineralization are recognized corresponding to significant tectonic events in the East Qinling–Dabie belt. Vein type deposits dated at 233–221 Ma were formed in detachment fractures, indicating localized extension within the collisional setting of the North China and Yangtze Cratons. I-type and transitional I- and S-type granites and related mineralization dated at 148–138 Ma may have formed part of a continental magmatic arc, with widespread magmatism and back-arc extension caused by subduction of the Izanagi or Paleopacific plate beneath the Eurasian continent in a WNW–ESE direction in the Late Jurassic–Early Cretaceous. Both S-type and transitional S- and I-type granite-associated porphyry molybdenum deposits dated at 131–112 Ma are part of an extensive mineralization event throughout East China that can be ascribed to regional large-scale lithospheric thinning, delamination and thermal erosion.