Northeastern South China Research Articles

Hf–Nd–O isotopic evidence for melting of recycled sediments beneath the Sulu Orogen, North China

The mantle source of late Mesozoic mafic magmas in the southeastern North China Craton is generally considered to have been enriched by deeply subducted continental lithosphere of the Yangtze Block. The mechanisms through which enrichment occurs are still unclear. Here we document an Early Cretaceous (zircon U–Pb age at 115±2Ma) dioritic intrusion with some geochemical features similar to sanukitoids (i.e., Al2O3>17.5%, Ba=1692–1819ppm, Sr=1042–1111ppm and δ18OZircon=6.2±0.2‰) from the Sulu Orogen in North China. Compared with the regional older (120–130Ma) mafic magmas (dykes), the diorite has less radiogenic Sr (87Sr/86Sr(i)~0.7075), higher εNd(t) (−9.0 to −8.6) with less radiogenic Hf than Nd (ΔεHf(t)=−6.1 to −5.2; ΔεHf(t)=εHf(t)−1.36×εNd(t)−2.89), signatures typically observed from continental zircon-bearing sediments. The primary magma of the dioritic intrusion was probably derived from a mantle enriched by the addition of a significant volume of a restitic component of recycled sediment that had been previously melted to leave residual zircon. Combined Hf–Nd–O isotopic data for the Rushan dioritic intrusion and the earlier mafic rocks requires the involvement of recycled sediments or their melts, incorporated into the lithospheric mantle as lenses or metasomatic veins, to produce a pudding-cake structure beneath the Sulu Orogen.

Seismic imaging of crustal reworking and lithospheric modification in eastern China

We processed teleseismic waveforms from a temporal 36-station seismic array to determine the crustal and lithospheric structure of the eastern South China Block. The NCISP-8 seismic profile spans a 480 km transect across the northeastern South China Block as well as the convergent boundary between the North China Craton and the South China Block. Common conversion point (CCP) stacking was used to image velocity discontinuities in the lithosphere along the profile. Synthetic test of CCP imaging along with forward modelling and inversion analysis of receiver function waveforms were used to identify attributes of the discontinuities obtained from CCP imaging and construct velocity models for each station. The crustal structures identified include a horizontally widespread low-velocity zone in the lower crust as well as local vertically diffuse crust–mantle boundary. These features indicate the effect of significant crustal reworking. We interpret the observed crustal fabrics as the effect of magmatism occurring since the Late Mesozoic. The thin lithosphere (57–86 km) and the heterogeneous lithospheric mantle structure provide compelling evidence of lithospheric modification in the eastern South China Block. Similarities between the lithospheric structures of the eastern South China Block and eastern North China Craton confirm that subducting ocean plate–continental lithosphere interaction controlled continental lithospheric modification in eastern China.

Distinct lateral variations of upper mantle anisotropy beneath eastern China revealed by shear‐wave splitting

AbstractA dense seismic array consisting of 28 temporary stations was deployed to study the crustal and upper‐mantle deformations beneath eastern China. We measured the splitting parameters in the crust and mantle by analyzing P‐to‐S phases converted at the Moho discontinuity (called PmS phases) and the core‐mantle boundary (i.e., core phases), respectively. The splitting parameters of core phases are retrieved at most stations while that of the PmS phases are retrieved at only a few stations. Distinct lateral variations of the fast polarization directions analyzed with the core phases are found in different tectonic blocks in eastern China. The delay times in the mantle and crust are moderately large (~1 s) and averagely smaller than 0.3 s, respectively. By the Fresnel‐zone analysis, the laterally variant lithospheric anisotropy is revealed between the two sub‐blocks (Southeast China Orogenic Belt and Yangtze Craton) of the eastern South China Block. In contrast, in the southeastern North China Craton, the anisotropy in a relatively deep layer contributes to the splitting observations.

Evolution of the lithospheric mantle beneath the southeastern North China Craton: Constraints from mafic dikes in the Jiaobei terrain

Two stages of mafic dikes with ages of ~117–116Ma and ~95–87Ma, respectively, are identified in the Jiaobei terrain based on zircon U–Pb ages. The Early Cretaceous mafic-intermediate dikes (ECMDs) and Late Cretaceous mafic dikes (LCMDs) show distinct geochemical and isotopic features. The ECMDs are enriched in large ion lithophile elements (LILE, Rb, Sr, Ba, U and Th) and light rare earth elements (LREE, La, Ce, Pr, Nd, Sm and Eu) with a strong depletion in high field strength elements (HFSE, Nb, Ta, Ti and P). They show uniformly high (87Sr/86Sr)i (0.70841–0.71387), low εNd(t) values (−18.1 to −13.2), positive (206Pb/204Pb)i (17.07–17.90) and negative εHf(t) (−21.0 to −6.1) values. These rocks were derived from partial melting of an enriched lithospheric mantle, and the enrichment of the mantle source resulted from source metasomatism imparted by the melts generated from remelting of the subducted lower crust of the Yangtze Craton (YC). In contrast, the LCMDs show lower SiO2 (41.5–50.1wt.%), lack of HFSE depletion and are depleted in whole rock Nd and zircon Hf isotope. They are interpreted to have sourced from a newly accreted depleted lithospheric mantle generated through continuous erosion of older enriched lithospheric mantle by asthenospheric upwelling, leaving out minor remnants of the older mantle in the newly accreted lithospheric mantle. The geochemical and isotopic characteristics reported in our study demonstrate that the lithospheric mantle beneath the southeastern North China Craton (NCC) was generally enriched during the Early Cretaceous (~120Ma) and the mechanism of the enrichment may be related to source metasomatism caused by the subducted lower crust of the YC. However, in Late Cretaceous, the lithospheric mantle beneath southeastern NCC was highly heterogeneous, which might be related to asthenospheric non-uniform erosion. The ~116–87Ma marks the transition from enriched lithospheric mantle to depleted asthenospheric mantle.

Slab–mantle interaction in continental subduction channel: Geochemical evidence from Mesozoic gabbroic intrusives in southeastern North China

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock major-trace elements and Sr–Nd isotopes as well as whole-rock and mineral O isotopes was carried out for Mesozoic gabbroic intrusives in the southeastern edge of the North China Block (NCB). The zircon U–Pb dating yields consistent ages of 131±2 to 136±2Ma for magma crystallization. The gabbroic rocks are characterized by arc-like trace element distribution patterns, enrichment of LILE (large ion lithophile elements) and LREE (light rare earth elements) but depletion of HFSE (high field strength elements). They exhibit variable enrichment of whole-rock Sr–Nd and zircon Hf–O isotopic compositions relative to the depleted MORB (mid-ocean ridge basalts) mantle. These geochemical features indicate their derivation from enriched mantle sources with incorporation of continental crustal-derived materials. The arc-like trace element distribution patterns and enriched Sr–Nd–Hf isotopic compositions for these Early Cretaceous mafic igneous rocks are comparable to those of Triassic ultrahigh-pressure metaigneous rocks in the Dabie–Sulu orogenic belt. Furthermore, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are characteristic time of crustal growth for the subducted continental crust of the South China Block (SCB). These results suggest contributions of the subducted SCB continental crust to the mantle sources for the gabbroic intrusives. It is thus inferred that the mantle sources were generated by melt-peridotite reaction in continental subduction channel, in which the overlying NCB subcontinental lithospheric mantle (SCLM) wedge peridotite was metasomatized by felsic melts derived from the subducted SCB continental crust during the Triassic continental collision. The gabbroic intrusives in the southeastern NCB edge exhibit geochemical gradients from southeast to northwest, with decreases of their 87Sr/86Sr, K2O/Na2O, Ce/Pb and La/Nb ratios but increase of their εNd(t) and εHf(t) values. This suggests the spatial effect of crustal metasomatism on the mantle sources with regard to distance from the Dabie–Sulu orogenic belt. Therefore, there are variable contributions from the subducted SCB continental crust to the mantle sources, with a possible range of about 200–300km from the continental collision zone for the slab–mantle interaction in the subduction channel.

U–Pb ages and Hf isotope data from detrital zircons in the Neoproterozoic sandstones of northern Jiangsu and southern Liaoning Provinces, China: Implications for the Late Precambrian evolution of the southeastern North China Craton

The tectonic evolution of the North China Craton (NCC) from the Late Mesoproterozoic to the Neoproterozoic has been unclear. This paper reports the results of LA–ICP–MS U–Pb dating and in situ Hf isotope analysis of detrital zircons from Neoproterozoic strata in the northern Jiangsu and southern Liaoning provinces of southeastern NCC, and we go onto discuss how these results help constrain the Late Precambrian evolution of the NCC. Most analyzed zircon grains exhibit oscillatory growth zoning, and they have relatively high Th/U ratios (>0.4) and positive Ce anomalies as well as negative Eu anomalies, all of which suggests a magmatic origin. The detrital zircons from the Xinxing and Jinshanzhai formations in northern Jiangsu Province give weighted mean ages of 1753, 1635, 1486, and 1121Ma, and 2515, 2305, 1856, 925, and 825Ma, respectively, and the detrital zircons from the Diaoyutai and Xingmincun formations in southern Liaoning Province give ages of 1745, 1619, 1484, 1175, and 1077Ma, and 1798, 1623, 1150, 1020, and 934Ma, respectively. These results, together with previously published ages, indicate that (1) the Neoproterozoic strata in the northern Jiangsu and southern Liaoning provinces were deposited during a period from the Late Mesoproterozoic to Neoproterozoic, and (2) the sediments in the Neoproterozoic strata were sourced mainly from igneous rocks of Late Paleoproterozoic, Mesoproterozoic, and Early Neoproterozoic age, as well as from the ancient Precambrian basement of the NCC. The concordia plots for 291 detrital zircon grains give three prominent age groups of 1.86–2.5, 1.5–1.8, and 0.9–1.2Ga. The detrital zircon ages of 1.86–2.5Ga in the Neoproterozoic strata tie in with most of the Precambrian tectonic–magmatic–thermal events that occurred in the NCC. Zircon ages of 1.6–1.8Ga can be linked to rift magmatism of Late Paleoproterozoic age in the NCC. The detrital zircon ages of 0.9–1.2Ga can be related to Grenville-aged magmatism in the southeastern NCC. The zircons show negative ɛHf (t) values (−8.8 to −0.1), suggesting that the source rocks were dominated by reworked ancient Precambrian crust. The positive ɛHf (t) values (+0.3 to +10.8) and Hf model ages indicate that growth of continental crustal occurred at ∼2.7Ga, ∼1.9Ga, and ∼1.5Ga in the southeastern NCC.

Modification of subcontinental lithospheric mantle above continental subduction zone: Constraints from geochemistry of Mesozoic gabbroic rocks in southeastern North China

The subcontinental lithospheric mantle (SCLM) in the southern margin of the North China Block was modified by the Triassic subduction of the South China Block, but less is known about subsequent events. This study presents a geochemical study of its anatectic products, Mesozoic gabbroic rocks from the southeastern edge of the North China Block. Zircon U–Pb dating gave ages of 119±4 to 131±1Ma for magma crystallization. The gabbroic rocks have high contents of MgO, Cr and Ni as well as high Mg# values, but low contents of SiO2. They exhibit arc-like trace element patterns, enriched Sr–Nd–Hf isotope compositions, and high δ18O values. These geochemical features indicate their derivation from partial melting of a fertile mantle source that is enriched not only in large ion lithophile elements and light rare earth elements but also in radiogenic isotopes. In particular, they have two-stage whole-rock Nd and zircon Hf model ages of Mesoproterozoic to Paleoproterozoic, which are similar to those of ultrahigh-pressure metaigneous rocks in the Dabie–Sulu orogenic belt. This suggests a genetic relationship in precursor origin between them. Therefore, the mantle source for the gabbroic rocks would be generated by crustal metasomatism through underplate reaction of the overlying SCLM-wedge peridotite with felsic melts derived from subducting continental crustal rocks of the South China Block during the Triassic continental collision. The fertile and enriched signatures in the mantle source were imparted by the crustally derived melts in the continental subduction channel, giving rise to the metasomatised SCLM in the Triassic. Because of its fertile and enriched properties, the metasomatised SCLM was susceptible to partial melting in an extensional tectonic setting, resulting in gabbroic magmatism in the Early Cretaceous. Therefore, these gabbroic rocks provide petrological and geochemical records of the crust–mantle interaction above a continental subduction zone. The spatial effect of continental deep subduction on the overlying SCLM wedge may extend to 200–300km distant from the collisional orogen. The melt metasomatism would have modified the chemical and physical properties of the SCLM in the southern North China Block, facilitating the SCLM thinning in the Mesozoic.

Zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotopes from the metamorphic basement in the Wuhe Complex: Implications for Neoarchean active continental margin along the southeastern North China Craton and constraints on the petrogenesis of Mesozoic granitoids

We report zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope data from mafic granulites and garnet amphibolites of the Wuhe Complex in the southeastern margin of the North China Craton (NCC). In combination with previous data, our results demonstrate that these rocks represent fragments of the ancient lower crust, and have features similar to those of the granulite basement in the northern margin of the NCC. A detailed evaluation of the Pb isotope data shows that Pb isotopes cannot effectively distinguish the role of the Yangtze Craton basement from that of the NCC basement with regard to the source and generation of magmas, at least for southeastern NCC. The age data suggest that the protoliths of the granulites or amphibolites in the Wuhe Complex were most likely generated in Neoarchean and that these rocks were subjected to Paleoproterozoic (1.8–1.9 Ga) high-pressure granulite facies metamorphism. This study also shows that the Precambrian metamorphic basement in the southeastern margin of the NCC might have formed in a tectonic setting characterized by a late Neoarchean active continental margin.

Spatial extent of the influence of the deeply subducted South China Block on the southeastern North China Block: Constraints from Sr–Nd–Pb isotopes in Mesozoic mafic igneous rocks

Spatial variations in whole-rock Sr–Nd–Pb isotopic compositions were investigated for Early Cretaceous mafic igneous rocks from western Shandong in the southeastern edge of the North China Block (NCB). The results are used to constrain the spatial extent of the influence of the deeply subducted South China Block (SCB) on the collided NCB. Five localities of mafic rocks were analyzed in this study, which are the Feixian basalt, the Shangyu gabbro–diorite, the Tietonggou high-Mg diorite, the Jinling high-Mg diorite, and the Jinan gabbro. The new data are combined with previous analyses on the other localities of Mesozoic mafic rocks (the Fangcheng basalt, the Jiaoyu diorite, the Yinan gabbro–diorite, and the Zouping gabbro). All these mafic rocks have SiO2 contents of 48.29 to 59.75%, Mg# values of 56 to 76, and Na2O/K2O ratios of 0.45 to 4.17. They are characterized by enrichment of LREE and LILE (e.g., Rb, Ba, Th, and U), depletion of HFSE (e.g., Nb, Ta, Zr, Hf, and Ti), and prominent positive Pb anomalies. Sr–Nd–Pb isotopic data indicate that the Jinan and Zouping gabbro and the Jinling high-Mg diorite, which are located far away from the Tan–Lu fault zone, have initial 87Sr/86Sr ratios of 0.7047 to 0.7054, εNd(t) values of −3.92 to −10.47, 206Pb/204Pb ratios of 16.671 to 17.111, 207Pb/204Pb of 15.320 to 15.378, and 208Pb/204Pb of 36.562 to 37.095. In contrast, the Shangyu gabbro–diorite, which are located close to the Tan–Lu fault zone, along with the contemporaneous Feixian and Fangcheng basalts, have initial 87Sr/86Sr ratios of 0.7096 to 0.7117, εNd(t) values of −13.04 to −16.60, 206Pb/204Pb ratios of 17.181 to 17.649, 207Pb/204Pb of 15.477 to 15.591, and 208Pb/204Pb of 37.536 to 38.472. The Tietonggou and Jiaoyu high-Mg diorites, which are located at intermediate distances from the fault zone, have initial 87Sr/86Sr ratios of 0.7067 to 0.7075, εNd(t) values of −7.82 to −13.30, 206Pb/204Pb of 17.466 to 17.667, 207Pb/204Pb of 15.442 to 15.472, and 208Pb/204Pb of 37.615 to 37.812. The Early Cretaceous mafic rocks in western Shandong show spatial variations in Sr–Nd–Pb isotopic compositions: from southeast to northwest, there is a decrease in initial 87Sr/86Sr, 206Pb/204Pb,207Pb/204Pb, and 208Pb/204Pb ratios, and an increase in εNd(t) values. Together with the Pb isotopic compositions of the contemporaneous Xuzhou–Huaibei adakitic rocks, the geochemical property of magma sources for the Mesozoic mafic rocks is inferred in view of the Pb isotopic difference in the NCB–SCB distinction. It is suggested that the magma sources for the Feixian, Fangcheng, Shangyu, Tietonggou, and Jiaoyu mafic rocks were mainly modified by the SCB basement. In contrast, the magma sources for the Jinan, Zouping, and Jinling intrusions, which are located far from the Tan–Lu fault zone, were mainly modified by melts derived from the delaminated NCB lower crust. These inferences are consistent with a tectonic model that outlines northwestward subduction of the SCB beneath the NCB during the Early Mesozoic. Along with data on the Sr–Nd–Pb isotopic compositions of coeval mafic–ultramafic rocks from southern Liaoning and southern Jilin provinces, it is concluded that the lateral extent of the influence of the deeply subducted SCB on the adjacent NCB lithospheric mantle is about 200km in its southeastern margin.

Paleobiogeography, paleoecology and paleoenvironmental significance of the Cambrian trilobites from the Zanskar region (Zanskar-Spiti-Kinnaur Basin), Northwest Himalaya

In present study the newly recorded latest Middle Cambrian trilobite fauna from the Cambrian succession of the Zanskar region of Zanskar-Spiti-Kinnaur Basin (Tethyan Himalaya) is analyzed critically to assess relationships with other Cambrian faunal elements of equatorial peri-Gondwanaland. The identification of genus Neoanomocarella, Parablackwelderia, Kunmingaspis, Fuchouia, Damesella and Dorypyge from the Cambrian of the Zanskar region and their comparison with those of South China and Australia is significant. It constitutes the basis for assessing the paleobiogeographic affinities during the Cambrian. The latest Middle Cambrian trilobite fauna from Zanskar shows proximity of Indian margin with that of southwest China “outboard” micro-continent. The recovery of analogous Middle Cambrian species i.e., Dorypyge perconvexlis, Fuchouia bulba, Fuchouia cf. oratolimba, Parablackwelderia sp. and Damesella sp. from the Zanskar region (Tethyan Himalaya) suggests a contiguous close proximity with south China and Australia during the latest Middle Cambrian, which supports the model of Meert and Van der Voo (1997) for assembly of Gondwanaland. The presence of Kunmingaspis in Zanskar and similar reports from northwestern Yunnan-Tibetan region, northern Henan, central and southeastern Hubei, north China, western Xinjiang and Yangtze platform reveal a close affinity between the Indian margin and the Yangtze platform during the Middle Cambrian. The trilobite fauna indicates the deeper shelf-shallow slope environment of deposition under fluctuating conditions of relative sea-level. The faunal elements of the Lejopyge acantha and Proagnostus bulbus zones indicate that the sea inundated the northern margin of Zanskar region during the latest Middle Cambrian time (Teta transgression) which is synchronous with globally recognized eustatic events during Lejopyge laevigata Zone.

Multiple crust–mantle interactions for the destruction of the North China Craton: Geochemical and Sr–Nd–Pb–Hf isotopic evidence from the Longbaoshan alkaline complex

In situ zircon U–Pb ages and Hf isotopic data, major and trace elements, and Sr–Nd–Pb isotopic compositions are reported for the Longbaoshan alkaline intrusive complex in the western Shandong Province (Luxi Block), southeastern North China Craton. The Longbaoshan complex, which consists of quartz syenite, aegirine–augite syenite, hornblende syenite, monzonite, and syenodiorite, was emplaced at 129.4–131.7Ma. The complex is characterized by high concentrations of SiO2, K2O+Na2O, Al2O3, LILEs (e.g., Sr and Ba), and LREEs, and low concentrations of CaO, Fe2O3, MgO, and HFSEs (e.g., Nb, Ta, P and Ti). The Sr, Nd, Pb, and Hf isotopic values of this complex are similar to the nearby mafic rocks, which were derived from EM2-type lithospheric mantle, indicating that the Longbaoshan complex was mainly derived from partial melting of the EM2 source. Combined with geochemical and isotopic features, the high Nb/Ta ratios (average 19.2) of the Longbaoshan complex suggest that the EM2 source was induced by modification of crust-derived melts, which were in equilibrium with rutile-bearing eclogite. This was attributed to the subduction of the Yangtze continental crust. The presence of inherited zircons (2.51–2.64Ga) with positive εHf(t) values (0.2–6.2) suggest that the ancient crust of the North China Craton was also involved in the formation of the Longbaoshan complex. Magma mixing modelling shows that those 10–35% crustal materials were assimilated in the complex. This complex may have experienced hornblende, apatite/monazite, and titanite crystal fractionation before emplacement in the shallow crust level. Two stages of crust–mantle interaction can be identified through the formation of the Longbaoshan complex, including (1) the lithospheric mantle was metasomatized by the melts/fluids derived from the subducted crust and then transformed into an EM2 counterpart, and (2) magma derived from the EM2 source underplated the ancient crust and assimilated the crust-derived materials. We thus propose that multiple crust–mantle interactions are the essential mechanism for the destruction of the NCC. The first intensive crust–mantle interaction was aroused by crustal subduction and was responsible for the lithospheric mantle destruction, and the second major crust–mantle interaction was induced by mantle-derived magma underplating and was responsible for the crustal activation.

Melt/rock interaction in remains of refertilized Archean lithospheric mantle in Jiaodong Peninsula, North China Craton: Li isotopic evidence

Li contents and its isotopes of minerals in mantle peridotite xenoliths from late Cretaceous mafic dikes, analyzed in situ by Cameca IMS-1280, reveal the existence of melt/rock interaction in remains of refertilized Archean lithospheric mantle in Qingdao, Jiaodong Peninsula, North China Craton. Two groups of peridotites exist, i.e., low-Mg# lherzolite and high-Mg# harzburgites. The low-Mg# lherzolite has a relatively homogeneous Li concentration (ol: 2.01–2.11 ppm; opx: 1.77–1.88 ppm; cpx: 1.75–1.93 ppm) and Li isotopic composition (δ7Li in ol: 4.2–7.6‰; in opx: 6.0–8.3‰; in cpx: 5.3–8.4‰). The similarity in δ7Li value to the fresh MORB provides further evidence for the argument that the low-Mg# lherzolite could be the fragment of the newly accreted lithospheric mantle. The high-Mg# harzburgites have heterogeneous Li abundances (ol: 0.83–2.09 ppm; opx: 0.92–1.94 ppm; cpx: 1.12–4.89 ppm) and Li isotopic compositions (δ7Li in ol: −0.5 to +11.5‰; in opx: −6.2 to +11.1‰; in cpx: −34.3 to +10.1‰), showing strong disequilibrium in Li partitioning and Li isotope fractionation between samples. The cores of most minerals in these high-Mg# harzburgites have relatively homogeneous δ7Li values, which are higher than those of fresh MORB, but similar to those previously reported for arc lavas. These harzburgites have enriched trace elemental and Sr–Nd isotopic compositions. These observations indicate that in the early Mesozoic the lithospheric mantle beneath the southeastern North China Craton was similar to that in arc settings, which is metasomatized by subducted crustal materials. Extremely low δ7Li preserved in cpxs requires diffusive fractionation of Li isotopes from later-stage melt into the minerals. Thus, the Li data provide further evidence that the Archean refractory lithospheric mantle represented by the high-Mg# harzburgites was refertilized through melt/rock interaction and transformed to the Mesozoic less refractory and incompatible element and Sr–Nd isotopes enriched lithospheric mantle.

Recycled crustal melt injection into lithospheric mantle: implication from cumulative composite and pyroxenite xenoliths

A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.

Glass melt inclusion in clinopyroxene from Linqu Cenozoic basalt, Shandong Province, China

Cenozoic basalts from the Linqu County, Shandong Province, China entrain some clinopyroxene crystals, of which many contain abundant glass melt inclusions. These melt inclusions are extremely irregular in shape with most grain sizes in a range of 10–50 μm and coexist with low-Mg# olivines, labradorites and Ca-rich potassium feldspars. In-situ major and trace element analyses show that the glass melt inclusions are high in alkalis (Na2O+K2O > 10 wt%), SiO2 (>54 wt%), CaO and FeO (>4 wt%), but low in MgO (Mg# 2), thus having phonolitic compositions. The compositional features of clinopyroxene crystals, glass melt inclusions and their coexistent minerals suggest that these melt inclusions were exotic melts in clinopyroxenes trapped prior to their entrainment in the host basalt. The discovery of these melt inclusions provides a new approach to further investigating the evolution of Mesozoic lithospheric mantle beneath the southeastern North China Craton.

Geochemistry of Mesozoic Basalts and Mafic Dikes, Southeastern North China Craton, and Tectonic Implications

Mesozoic basaltic magmatism in the North China craton (NCC) is a key for probing the nature of the Mesozoic lithospheric mantle, including the process of lithosphere thinning over Phanerozoic time. Early Cretaceous calc-alkaline basalts and mafic dikes are widespread in the Fangcheng, Mengyin, Jimo, and Jiaodong regions of the southeastern NCC. Both rock types have similar geochemical signatures; are high in SiO2 content; moderately high in MgO, CaO, and Al2O3; highly enriched in LREE ((Ce/Yb)N > 10) and LILE (Rb, Ba, U, Th); and depleted in HFSE (Ti and Nb). These rocks are highly enriched in Sr-Nd isotopes (87Sr/86Sri > 0.7072, εNd(t) = -11.5 ∼ -17.5) and depleted in Pb isotopes (206Pb/204Pb < 17.6, 207Pb/204Pb < 15.6, 208Pb/204Pb < 38.1). Their geochemistry demonstrates that these basalts and mafic dikes were derived from enriched lithospheric mantle, which differs considerably from the Paleozoic and Cenozoic lithosphere of the region. Calculations indicate that the Mesozoic lithosphere was extensively modified by silicic melt, derived from the partial fusion of subducted silicic crust. Tectonic underplating of the Yangtze continental crust beneath the base of the NCC lithosphere is well established. Extensive melting of the enriched lithosphere and tectonic collapse of the continental root provide a possible mechanism for lithosphere thinning linked to the Dabie collision.