Eastern China continental lithosphere thinning is a consequence of paleo-Pacific subduction: A review and new perspectives

Understanding the processes that lead to the lithosphere thinning is a key aspect of continental geology research. In this paper, we present essential observations and summarize our understandings on the lithosphere thinning and accompanying magmatism in eastern continental China since the Mesozoic as a straightforward consequence of plate tectonics. We show that the lithosphere thinning in the Mesozoic resulted from basal hydration weakening with the water coming from dehydration of the paleo-Pacific plate in the mantle transition zone. The weakening effect is to convert the basal lithosphere into asthenosphere by reducing its viscosity, having thus thinned the lithosphere while triggering mantle melting and crustal magmatism marked by the widespread Mesozoic basalts and granitoids in space and time. These observations and logical reasoning require the existence and effect of subducted paleo-Pacific plate in the mantle transition zone, whose active subduction ended at ~ 90 Ma with the suture located off the continental China marked by the arc-shaped southeast coastline. As a result, the thinned lithosphere began a 40-Myr period (i.e., ~ 90 to ~ 50 Ma) of basal accretion manifested by compositional systematics of basalts erupted in this period. The initiation of the present-day western Pacific subduction at ~ 50 Ma and its eastward retreat caused eastward drift of continental China, leaving the older portions of the present-day Pacific slab stagnant in the mantle transition zone with resumed water supply in the form of hydrous melt to maintain the thinned lithosphere, which is the same as creating and maintaining the oceanic-type seismic low velocity zone (LVZ) beneath eastern China, responsible for the Cenozoic alkali basalt volcanism in the region. That is, the present-day lithosphere-asthenosphere boundary (LAB) beneath eastern China is a petrological boundary, either as an amphibole dehydration solidus or water-saturated solidus. As predicted, the Cenozoic alkali basalts in eastern China demonstrate that lithosphere thickness (i.e., the LAB depth) controls the compositions of mantle melts, i.e., the lid effect. The latter further confirms the LAB beneath eastern China as a solidus, below which decompression melting happens, and above which melt solidifies or ascends rapidly to the surface. Our studies thus lead us to the unavoidable conclusion that the lithosphere thinning in the Mesozoic, the present-day LAB, the seismic LVZ and the widespread Mesozoic-Cenozoic magmatism in eastern China are all consequences of plate tectonics in response to paleo-Pacific plate subduction, which is of global significance for understanding intra-continental magmatism at present and in Earth’s histories

Sublithosphere mantle crystallization and immiscible sulphide melt segregation in continental basal magmatism: evidence from clinopyroxene megacrysts in the Cenozoic basalts of eastern China

This study explores the effects of high-pressure crystallization and immiscible sulphide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to > 10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulphide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulphide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulphide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulphide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g., Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of highpressure crystallization of clinopyroxene and garnet. The calculated P-T conditions of the clinopyroxene megacrysts are close to those of the lithosphere-asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~ 4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts

Petrogenesis of the early Cretaceous intra-plate basalts from the western North China Craton: Implications for the origin of the metasomatized cratonic lithospheric mantle

We present new bulk-rock 40Ar/39Ar age, major and trace elements and Sr-Nd-Hf isotopic data on the early Cretaceous intra-plate alkali basalts from the Western North China Craton (WNCC) to study the origin of the metasomatized cratonic lithosphere mantle. The age of these basalts is ~116 Ma. These basalts have elevated incompatible element abundance with high [La/Sm]N (2.80–4.56) and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.7062–0.7075, εNd(t) = −6.0 to −13.0 and εHf(t) = −8.3 to −17.4), being similar to the contemporary analogues from the Western North China Craton and Paleozoic kimberlites and mantle xenoliths. The WNCC basalts also show good correlations between ɛNd(t) and ɛHf(t), and high [La/Sm]N. All these geochemical observations are consistent with the interpretation that these basalts originated from partial melting of the lithospheric mantle that experienced melt metasomatism. Two types metasomatism melts are required to explain the geochemical characteristics of these rocks. The obvious negative Nbsingle bondTa (compared with K)-Ti and positive Basingle bondPb anomalies observed in these basalts further constrain that one of the metasomatic melts was derived from the subducted terrigenous sediment. Furthermore, the overall higher P/Nd, Nb/La and Nb/Th and lower Lu/Hf of basalts in the WNCC suggest that there is also contribution of low-F melts from asthenosphere mantle. Collectively, we suggest that the formation of the metasomatized lithosphere mantle beneath the WNCC is the process of metasomatic reaction between mantle peridotite and the melts of different origin to generate metasomatic veins containing amphibole/phlogopite. Partial melting of the metasomatic lithospheric mantle at 106–120 Ma in the WNCC was considered to be induced by thermal perturbation that was ultimately related to the breakoff of the subducted oceanic slab following the closure of the Mongolia-Okhotsk ocean

Iron isotope compositions of coexisting sulfide and silicate minerals in Sudbury-type ores from the Jinchuan Ni-Cu- sulfide deposit: A perspective on possible core-mantle iron isotope fractionation

Many studies have shown that the average iron (Fe) isotope compositions of mantle-derived rocks, mantle peridotite and model mantle are close to those of chondrites. Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope composition. However, this is a brave assumption because nearly 90% Fe of the earth is in the core, whose Fe isotope composition is unknown, but is required to construct bulk earth Fe isotope composition. We approach the problem by assuming that the earth’s core separation can be approximated in terms of the Sudbury-type Ni-Cu sulfide mineralization, where sulfide-saturated mafic magmas segregate into immiscible sulfide liquid and silicate liquid. Their density/buoyancy controlled stratification and solidification produced net-textured ores above massive ores and below disseminated ores. The coexisting sulfide minerals (pyrrhotite (Po) > pentlandite (Pn) > chalcopyrite (Cp)) and silicate minerals (olivine (Ol) > orthopyroxene (Opx) > clinopyroxene (Cpx)) are expected to hold messages on Fe isotope fractionation between the two liquids before their solidification. We studied the net-textured ores of the Sudbury-type Jinchuan Ni-Cu sulfide deposit. The sulfide minerals show varying δ56Fe values (-1.37 ~ -0.74‰ (Po) < 0.09 ~ 0.56‰ (Cp) < 0.53 ~ 1.05‰ (Pn), but silicate minerals (Ol, Opx, Cpx) have δ56Fe values close to chondrites (δ56Fe = -0.01±0.01‰). The heavy δ56Fe value (0.52 ~ 0.60‰) of serpentines may reflect Fe isotopes exchange with the coexisting pyrrhotite with light δ56Fe. We ob- tained an equilibrium fractionation factor of Δ56Fesilicate-sulfide = ~ 0.51‰ between reconstructed silicate liquid (δ56Fe = ~ 0.21‰) and sulfide liquid (δ56Fe = ~ -0.30‰), or Δ56Fesilicate-sulfide = ~ 0.36‰ between the weighted mean bulk-silicate minerals (δ56Fe[0.70ol,0.25opx,0.05cpx] = 0.06‰) with weighted mean bulk- sulfide minerals (δ56Fe = ~ -0.30‰). Our study indicates that significant Fe isotope fractionation does take place between silicate and sulfide liquids during the Sudbury-type sulfide mineralization. We hypothesize that significant iron isotope fractionation must have taken place during core-mantle segregation, and the bulk earth may have lighter Fe isotope composition than chondrites although Fe isotope analysis on experimental sulfide-silicate liquids produced under the varying mantle depth conditions is needed to test our results. We advocate the importance of further research on the subject. Given the close Fe-Ni association in the magmatic mineralization and the majority of Earth’s Ni is also in the core, we infer that Ni isotope fractionation must also have taken place during the core separation that needs attention

Aluminum impairs rat neural cell mitochondria in vitro.

Exposure to aluminum has been reported to lead to neurotoxicity. Mitochondria are important organelles involved in maintaining cell function. This study investigates the effect of aluminum on mitochondria in rat neural cells. The ultrastructure of mitochondria was observed, and the cell death rate (CDR), reactive oxygen species (ROS), mitochondrial membrane potential (MMP) and 3-[4,5demethyl-2-thiazalyl]-2,-5diphenyl-2H-tetrazolium bromide (MTT) were measured to investigate the effect of aluminum on the mitochondrial structure and its function in neural cells. Results observed from the mitochondrial ultrastructure show that aluminum may impair the mitochondrial membrane and cristae. Increased CDR, enhanced ROS, decreased MMP, and decreased enzyme activity in mitochondria were observed in the Al-exposed neurons (100 – 500 μM). The present study demonstrates that alteration in the mitochondrial structure and function plays an important role in neurotoxic mechanisms induced by aluminum

The lithospheric thickness control on the compositional variation of continental intraplate basalts: A demonstration using the Cenozoic basalts and clinopyroxene megacrysts from eastern China

Studies on intra‐plate ocean island basalts have demonstrated the control of lithosphere thickness on the extent of melting and pressure of melt extraction (i.e., the lid‐effect). However, whether lithosphere thickness also controls the composition of within‐continent basalts remains unclear. Here we test this hypothesis by studying the Cenozoic basalts containing clinopyroxene megacrysts from 10 localities throughout eastern continental China with a north‐south spatial coverage in excess of 2500 km. Indeed, the geochemical parameters (e.g., abundances and ratios of major and trace elements) correlate well with the depth of the lithosphere‐asthenosphere boundary (LAB) calculated using the clinopyroxene barometry, showing significant lithospheric thickness control on basalt compositions. These observations offer further evidence for melt pooling (a melt rich layer) close beneath the LAB as a “stable magma reservoir” for crystallizing compositionally uniform clinopyroxene megacrysts to be carried by subsequent pulses of melt transport and eruption

Elemental and Sr–Nd–Pb isotope geochemistry of the Cenozoic basalts in Southeast China : insights into their mantle sources and melting processes.

We analyzed whole-rock major and trace elements and Sr–Nd–Pb isotopes of the Cenozoic basalts in Southeast China to investigate their mantle source characteristics and melting process. These basalts are spatially associated with three extensional fault systems parallel to the coast line. After correction for the effect of olivine microlites on bulk-rock compositions and the effect of crystal fractionation, we obtained primitive melt compositions for these samples. These primitive melts show increasing SiO2, Al2O3 but decreasing FeO, MgO, TiO2, P2O5, CaO and CaO/Al2O3 from the interior to the coast. Such spatial variations of major element abundances and ratios are consistent with a combined effect of fertile source compositional variation and increasing extent and decreasing pressure of decompression melting from beneath the thick lithosphere in the interior to beneath the thin lithosphere in the coast. These basalts are characterized by incompatible element enrichment but varying extent of isotopic depletion. This element-isotope decoupling is most consistent with recent mantle source enrichment by means of low-degree melt metasomatism that elevated incompatible element abundances without yet having adequate time for isotopic ingrowth in the mantle source regions. Furthermore, Sr and Nd isotope ratios show significant correlations with Nb/Th, Nb/La, Sr/Sr⁎ and Eu/Eu⁎, which substantiates the presence of recycled upper continental crustal material in the mantle sources of these basalts. Pb isotope ratios also exhibit spatial variation, increasing from the interior to the coastal area. The significant correlations of major element abundances with Pb isotope ratios indicate that the Pb isotope variations also result from varied extent and pressure of decompression melting. We conclude that the elevated Pb isotope ratios from the interior to coast are consistent with increasing extent of decompression melting of the incompatible element depleted mantle matrix, which hosts enriched Pb isotope compositions

Lithosphere thickness controls the continental basalt compositions: An illustration using the Cenozoic basalts from eastern China

Recent studies demonstrate that lithosphere thickness variation exerts the primary control on global seafloor basalt compositions. If the mechanism of such control, i.e., the lid effect, is indeed at work, lithosphere thickness variation must also influence basaltic compositions in continental settings. To test this hypothesis, we chose to study Cenozoic basalts in eastern continental China over a distance of ∼260 km along a southeast-to-northwest traverse with a steep topographic gradient (∼500 to ∼1500 m above sea level) mirrored with a steep lithospheric thickness gradient (∼80 to ∼120 km). The basalts erupted on the thinned lithosphere to the east are characterized by lower pressure (e.g., higher Si72, lower Mg72, Fe72, and [Sm/Yb]N; subscript “72” refers to corresponding oxides corrected for fractionation effect to Mg# = 72; N—primitive mantle normalized) and higher extent (e.g., low Ti72, P72, K72, Rb, Ba, Th, and ratios of more- to less-incompatible elements such as [La/Sm]N, Ba/Zr, and Zr/Yb) of melting than basalts erupted on the thickened lithosphere to the west. Importantly, these geochemical parameters all show significant correlations with both lithosphere thickness and topographic elevation. These first-order observations are a straightforward manifestation of the lid effect. Lithospheric contamination and mantle-source compositional variation can indeed contribute to the compositional variability of these continental basalts, but these latter effects are averaged out and are overshadowed by the lid effect. This finding emphasizes the importance of evaluating the lid effect before interpreting the petrogenesis of continental basalts and mantle dynamics. Our results also indicate that the continental surface elevation is isostatically balanced above a mantle depth that is deeper than the lithosphere-asthenosphere boundary