Yi-Gang Xu

Petrologic and geochemical constraints on the petrogenesis of Permian-Triassic Emeishan flood basalts in southwestern China

Abstract The Emeishan flood basalt is a large igneous province erupted during the Permian–Triassic period in southwestern China. Based on petrographic, major and trace element, and Sr–Nd isotope data, the Emeishan basalts can be classified into two major magma types. These are: (1) a low-Ti (LT) type that exhibits low Ti/Y ( 500). The HT lavas can be further divided into three subtypes. HT1 lavas exhibit significantly high TiO2 (3.65–4.7%), Fe2O3* (12.7–16.4%), Nb/La (0.75–1.1), coupled with higher eNd(t) (1.1–4.8) and lower SiO2 (45–51%); HT2 lavas are compositionally similar to the HT1 lavas but show conspicuous depletion in U and Th. The HT3 type has higher Mg# (0.51–0.61) than the HT1 and HT2 lavas. It differs from the LT type in having higher TiO2 (∼3%) at comparable Mg#. Elemental and isotopic data suggest that the chemical variations of the LT and HT lavas cannot be explained by crystallization from a common parental magma. Instead, they may originate from different mantle sources under various melting conditions and underwent distinct differentiation and contamination processes. REE inversion calculations indicate that the HT magmas were generated by low degrees of partial melting (1.5%) of a mantle source that has eNd(t) of ∼+5 and 87Sr/86Sr(t) of ∼0.704 within the garnet stability field. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variations. In contrast, parental magmas of the LT type were generated by higher degree of partial melting (16%) of a distinct mantle source (eNd(t)≈+2, 87Sr/86Sr(t)≈0.705) around the spinel–garnet transition zone. The chemical evolution of the LT lavas is controlled by an olivine (ol)+clinopyroxene (cpx) fractionation. The Emeishan flood basalts may result from a starting mantle plume. The petrogenesis of both the LT and HT magmas was further complicated by contamination of upper crust and lithospheric mantle. While the HT1 lavas have experienced an AFC style of contamination in the upper crust, the HT2 lavas that mark with U–Th depletions may result from additional interaction with melts derived from a gabbroic layer near the crust–mantle boundary. In contrast, a temperature-controlled style of contamination was associated with the LT lavas. Our data show that both temporal and spatial geochemical variations exist in the Emeishan flood basalt province. The occurrence of thick LT lavas in the western part of the province may record the main episode of the flood basalt emplacement. In contrast, the less abundant overlying HT basalts may imply a waning activity of the plume. In fact, the HT basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT lavas may be a result of relatively thicker lithosphere and lower geotherm.

Sedimentary evidence for a rapid, kilometer-scale crustal doming prior to the eruption of the Emeishan flood basalts

Biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the Middle Permian Maokou Formation that immediately underlies the Emeishan flood basalts in southwest China. The results suggest a domal crustal thinning before the emplacement of the Emeishan large igneous province. Variably thinned carbonates in the Maokou Formation are capped by a subaerial unconformity, which is generally manifested by karst paleotopography, paleoweathering zone, or locally by relict gravels and basal conglomerates. Provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost Maokou Formation. Therefore, the stratigraphic thinning likely resulted from differential erosion due to regional uplift. Iso-thickness contours of the Maokou Formation delineate a subcircular uplifted area, in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling. The duration of this uplift is estimated to be less than 3 Myr and the magnitude of uplift is greater than 1000 m. The sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the Emeishan flood basalts.

Geologic, geochemical, and geophysical consequences of plume involvement in the Emeishan flood-basalt province

Prevolcanic kilometer-scale lithospheric doming in the Emeishan large igneous province, southwest China, allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography, geology, geochemistry, and geophysics of the region. Systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments, the extent of erosion, thickness, and chemistry of volcanic rocks, and the crust-mantle structure. These features, which are best explained by a mantle plume, may be used to track older plume sites in the geologic record.

Destruction of the North China Craton

A National Science Foundation of China (NSFC) major research project, Destruction of the North China Craton (NCC), has been carried out in the past few years by Chinese scientists through an in-depth and systematic observations, experiments and theoretical analyses, with an emphasis on the spatio-temporal distribution of the NCC destruction, the structure of deep earth and shallow geological records of the craton evolution, the mechanism and dynamics of the craton destruction. From this work the following conclusions can be drawn: (1) Significant spatial heterogeneity exists in the NCC lithospheric thickness and crustal structure, which constrains the scope of the NCC destruction. (2) The nature of the Paleozoic, Mesozoic and Cenozoic sub-continental lithospheric mantle (CLM) underneath the NCC is characterized in detail. In terms of water content, the late Mesozoic CLM was rich in water, but Cenozoic CLM was highly water deficient. (3) The correlation between magmatism and surface geological response confirms that the geological and tectonic evolution is governed by cratonic destruction processes. (4) Pacific subduction is the main dynamic factor that triggered the destruction of the NCC, which highlights the role of cratonic destruction in plate tectonics.

Contrasting Cenozoic lithospheric evolution and architecture in the Western and Eastern Sino-Korean Craton : Constraints from geochemistry of basalts and mantle xenoliths

Cenozoic basalts from both sides of the Daxin’anling‐Taihang gravity lineament that separates the Sino‐Korean craton (SKC) into western and eastern parts have been studied. In the western SKC, magmas evolved from xenolith‐bearing alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and an increase in Yb content. Sr‐Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin, although lithospheric mantle may have also contributed to these basalts. This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern SKC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magmatism in the Late Cretaceous–Early Tertiary. Such contrasting lithospheric processes may reflect diachronous extension in the SKC, with initial extension in the eastern part owing to the Late Mesozoic paleo‐Pacific subduction and subsequent extension in the western SKC induced by the Early Tertiary Indian‐Eurasian collision. An implication is that the lithospheric mantle in the western SKC is relatively old compared with that beneath the eastern SKC, which may be a mixture of old lithospheric relicts and newly accreted mantle. This predicted lithospheric architecture is consistent with Sr‐Nd isotopic data and recent Re‐Os age determinations from mantle xenoliths included in Cenozoic basalts.

Evidence for crustal components in the mantle and constraints on crustal recycling mechanisms: pyroxenite xenoliths from Hannuoba, North China

Spinel and garnet pyroxenite xenoliths in Cenozoic basalts from Hannuoba, North China show extremely heterogeneous chemical and isotopic compositions (eNd=−27 to +34). Most of these pyroxenites are relatively young, probably late Mesozoic in age, although a few Al-pyroxenites could be very old (∼2 Ga). While their texture and major element compositions suggest an origin of high pressure cumulates, the trace element and isotopic compositions of the Hannuoba pyroxenites require multiple segregation processes from different parental magmas. Strong LREE enrichment, ubiquitous HFSE depletion and some Eu anomalies of the Al- and Cr-pyroxenites indicate the involvement of crust components in their source. Their Sr–Nd isotopic ratios are negatively correlated and plot below the MORB–OIB–IAB–sediment trend, suggesting that the parental melts of the Cr- and Al-pyroxenites may have been derived from a mixture of asthenospheric melts and a long-term evolved continental crust. The garnet pyroxenites significantly deviate from the isotopic array defined by the Al-pyroxenites, due to their relatively high 87Sr/86Sr at given eNd. They thus more likely represent segregates from melts derived from partial melting of hydrothermally altered oceanic crust (basalts+marine sediment). If the crustal component involved in the Al-pyroxenites is subducted terrigenous sediments or other continental materials from the Archean Sino-Korean Craton, the Al-pyroxenites and garnet pyroxenites may have formed contemporaneously at a palaeo-convergent plate margin. This may be related to the subduction of the Mongol–Okhotsk plate beneath North China during the late Jurassic. Alternatively, if the delaminated lower crust was involved, it implies that most of the Al-pyroxenites are younger than the garnet pyroxenites, and their formation may be temporally correlated with lithospheric thinning during the Cretaceous. This model is attractive because the inferred tectonic evolution from a convergent setting to an extensional environment is consistent with the geologic record in the area.

Geochronology, petrology and geochemistry of the granulite xenoliths from Nushan, east China: Implication for a heterogeneous lower crust beneath the Sino-Korean Craton

Abstract The occurrence of both Archean granulite terrains and granulite xenoliths in Cenozoic basalts from the Sino-Korean Craton (SKC) provides an ideal opportunity to define composition and evolution of continental lower crust of eastern China. The granulite xenoliths in Quaternary basanites from Nushan (southeastern SKC) show a basic-intermediate composition that is distinctly different from mafic granulites from Hannuoba (western SKC). They instead resemble the Archean granulite terrains in terms of mineral and whole rock compositions. Trace element modeling suggests that the “protoliths” of the Nushan granulites were likely subjected to fractional crystallization and assimilation of old crustal components. Zircon SHRIMP U-Pb dating shows at least two episodes in the formation of the lower crust at Nushan. The protoliths of the Nushan granulites were most likely formed at ca. 2.5 Ga and metamorphosed at 1.9 Ga. This late Archean crustal growth was followed by Mesozoic (∼140 Ma) basaltic underplating, which was probably coeval with the widespread thermo-tectonic lithospheric reactivation in eastern China. The Nushan granulites are therefore interpreted as dominantly derived from the late Archean crystalline basement and subordinately from the mafic layer that was accreted to the basement during late Mesozoic lithospheric thinning. The consistencies between the depth to seismic Moho and the depth to crust-mantle boundary, and between the calculated Vp (mostly Liu et al., 2001) . Such a compositional difference, in conjunction with contrasting age and seismic velocity structure of the lower crust at the two localities, highlights two fundamentally distinct tectonic domains in the SKC. The data presented in this study also yield implication for the origin of the compositional difference between granulite xenoliths and terrains.

Chemostratigraphic Correlation of Upper Permian Lavas from Yunnan Province, China: Extent of the Emeishan Large Igneous Province

Although the mantle plume model has been increasingly adopted for generation of the Emeishan basalts, doubts have been cast on the viability of this model because of the relatively small dimensions of this large igneous province (LIP) compared with typical LIPs. The Emeishan LIP was traditionally thought to be bounded on the southwest by the Ailaoshan-Red River (ASRR) fault zone. However, pre-Cretaceous strata in the Jinping area, southwest of the ASRR fault zone, are comparable to those at Binchuan in the western margin of the Emeishan LIP. This correlation is reinforced by similar chemostratigraphic variation of Upper Permian basalts from these spatially separated regions. Therefore, the Jinping basalts, which crop out ∼500 km southeast of the Binchuan counterpart, are interpreted as a dismembered part of the Emeishan basalts that were displaced to the present location by mid-Tertiary sinistral movement along the ASRR fault. We suggest that the ASRR fault does not bound the Emeishan LIP. Thus, this LIP has a much larger extent, and its western sector may have extended toward the Paleo-Tethyan ocean, and may have been destroyed by closure in the Triassic.

A Permian large igneous province in Tarim and Central Asian orogenic belt, NW China: Results of a ca. 275 Ma mantle plume?

New sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, geochemical data, and a synthesis of existing stratigraphic, geochronologic, and geochemical results from the Tarim block and the Central Asian orogenic belt in northwestern China suggest the presence of a Permian (ca. 275 Ma) large igneous province (the Bachu large igneous province). The large igneous province consists predominantly of coeval mafic rocks (basalts and mafic-ultramafic intrusions) having an aerial coverage of more than 600,000 km 2 , and its formation was accompanied by voluminous emplacement of A-type granites. This large igneous province, interpreted to be of mantle plume origin, is ∼15 m.y. older than the ca. 260 Ma Emeishan large igneous province in southwestern China and ∼25 m.y. older than the 251 Ma Siberian Trap in Russia. Such a sudden flair up of plume activity in the Permian may represent the early stage of the Pangean superplume event. The Permian plumes likely played a role in late Paleozoic rapid continental crustal growth in the Central Asian orogenic belt. In addition, there appear to be two types of mantle geochemical provinces (domains) in the region, a long-term enriched Tarim province and a subduction-metasomatized and depleted Central Asian orogenic belt province.

Reactive harzburgites from Huinan, NE China: Products of the lithosphere- asthenosphere interaction during lithospheric thinning?

Abstract Petrologic, trace element and Sr-Nd isotopic studies of mantle xenoliths in Quaternary basalts from Huinan, NE China provide constraints on the origin of coarse-grained harzburgites and the nature of lithosphere-asthenosphere interaction during lithospheric thinning. The Huinan harzburgites have a secondary recrystallized texture and their composition deviates from the partial melting trend of residual peridotites. The convex-upward REE pattern and a positive Cr-Yb correlation in clinopyroxene imply an interaction with basaltic melts at a high melt/rock ratio. The Huinan harzburgites are therefore not simple residues of partial melting, but likely resulted from melt-rock interaction during which the percolating melts preferentially dissolved pyroxenes by precipitation of olivine, transforming lherzolite to harzburgite. The melt percolation-reaction enhanced grain boundary diffusion kinetics, and gave rise to the characteristic texture of these mantle rocks. These “reactive” harzburgites were eventually metasomatized by compositionally distinct small volume volatile-rich melts, which may be derived from the main harzburgite-forming event as a result of melt-consuming reaction. Most likely the formation of the Huinan harzburgites was coeval with thermo-tectonic erosion of the continental lithosphere by upwelling asthenospheric melts. Thermometric considerations suggest a relatively long time interval between lithospheric thinning and eruption of the host basalts, consistent with the contention that lithospheric thinning in eastern China may have peaked in the late Cretaceous.