Xiao-Long Huang

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.

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.

Origin of two differentiation trends in the Emeishan flood basalts

Both the Bowen and Fenner differentiation trends have been recognized in the Emeishan flood basalts. While the Longzhoushan lavas in Panxi paleorift evolved on a trend of silica enrichment (Bowen trend), the lavas from Guizhou evolved along the Fenner trend leading to the magmas with high Fe2O3 (23%) and low SiO2 (44%) contents. This provides evidence for the existence of Fe-rich and Si-poor magmas in nature. Such contrasting differentiation trends, marked by different timing of crystallization of Fe-Ti oxides, are correlated with the extent of crustal contamination of the magmas. Limited crustal contamination in the Guizhou lavas is coupled with the low oxygen fugacity which delayed the onset of magnetite crystallization. In contrast, significant involvement of crustal components in the Longzhoushan lavas increased ƒO2 which in turn triggered early crystallization of magnetite. The close spatial association between the Longzhoushan lavas and the synchronous V-Fe-Ti deposit-bearing layered intrusions suggests a potential relationship between them.

Paleoproterozoic lower crust beneath Nushan in Anhui Province: Evidence from zircon SHRIMP U-Pb dating on granulite xenoliths in Cenozoic alkali basalt

Zircon SHRIMP U-Pb dating was carried out for an intermediate granulite xenolith in Cenozoic alkali basalt from Nushan. The results suggest that the lower crust beneath Nushan may have formed at about 2400–2200 Ma, and have been subjected to granulite-facies metamorphism at 1915 ± 27 Ma. The old age of the Nushan lower crust is consistent with the geochemical similarities between Nushan granulite xenoliths and Archean-Paleoproterozoic granulite terrains in the North China craton, but it is not distinguishable from high-grade metamorphic rocks in the Yangtze craton where such old ages were also reported. Significant Pb-loss occurs in the Nushan zircons, implying important influence of widespread Mesozoic to Cenozoic underplating in East China on the lower crust beneath Nushan.

Late Permian Emeishan Flood Basalts in Southwestern China

Abstract Current debates over mantle plumes are centered on three assumptions namely: (1) cylindrical thermally anomalous materials which arise from the core mantle boundary; (2) high temperature of mantle plumes; (3) mantle plumes are stationary. A full test of the validity of these assumptions requires a comprehensive evaluation using techniques including high resolution seismic tomography, petrology and paleomagnetism. It is argued in this paper that the plume hypothesis can fairly well explain first order geologic observations. Geoscientists against the plume hypothesis often tend to over emphasized some details that may be also influenced by other factors, in addition to mantle plumes. Five criteria can be used to identify ancient mantle plumes: (1) crustal uplift prior to volcanism; (2) dyke swarms; (3) volcanic chain; (4) physical characteristics of volcanism and (5) chemical characteristics of plume derived basalts. Our investigation shows that at least three, and perhaps, four of these criteria are met for the Emeishan large igneous province, suggesting that a mantle plume played a central role in the generation of the Emeishan basalts.

Highly magnesian olivines and green-core clinopyroxenes in ultrapotassic lavas from western Yunnan, China evidence for a complex hybrid origin

Oligocene ultrapotassic rocks from western Yunnan have abundant olivines of varying size and green-core clinopyroxenes. Olivine macrocrysts (1–5 mm) have very high forsterite contents (Fo 92–94 ), and high CaO (0.15–0.30 wt%) and Cr 2 O 3 (up to 0.23 wt%) contents. They are believed to be magmatic rather than disintegrated crystals from the upper mantle. These magnesian olivines are not in equilibrium with melts corresponding to the whole-rock compositions of host lavas and, as such, may represent xenocrysts entrained by ultrapotassic magmas during their ascent to the surface. While komaiites, boninites and olivine lamproites are potential sources of these xenocrysts, a genetic link to the late Permian Emeishan large igneous province (LIP) is favoured, because the Emeishan flood basalt is the only known geologic event in the studied area that could have produced highly magnesian olivines. The olivine xenocrysts may have been entrained from the high velocity lower crust (V p = 7.1–7.8 km/s), which is believed to have formed from high Mg basalts (picrites?) that underplated and intruded the crust during the Emeishan flood basalt episode. Given the presence of olivine xenocrysts and reverse-zoned clinopyroxenes, it is apparent that some magnesian ultrapotassic lavas from Yunnan have a complex hybrid origin. While the former point to ultramafic/primary melts the latter indicate interaction between evolved melts and relatively primary melts thus indicating complex magma mixing processes.

Plume‐orogenic lithosphere interaction recorded in the Haladala layered intrusion in the Southwest Tianshan Orogen, NW China

The plume-orogenic lithosphere interaction may be common and important for the generation of large igneous provinces. The information regarding such a process is recorded by the Haladala gabbroic intrusion (~300 Ma), the largest layered ultramafic-mafic intrusion hosting V-Ti magnetite deposits in the Southwest Tianshan Orogen, NW China. The Haladala gabbros exhibit unfractionated chondrite-normalized rare earth element patterns with negative Nb and Ta anomalies and positive Pb anomaly on the primitive mantle-normalized multielement variation diagram. They are characterized by low initial Sr isotopes, slightly decoupled but high positive bulk rock eNd(t) and eHf(t), and high 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, delineating a DUPAL signature in the sources. The Haladala gabbros cannot be arc or postcollisional magmatism, given the lack of hydrous minerals and low K contents, respectively. This is further supported by the relatively low oxygen fugacity required for the gradual enrichment of V-Ti magnetite during the magma fractionation and by an overall anhydrous mantle source suggested by troctolite mineral assemblage (olivine + plagioclase). The emplacement age of the Haladala gabbros is identical to that of the Wajilitag kimberlites in the Tarims interior, which have been interpreted as the first magmatic expression of the Tarim mantle plume. We thus propose that the Haladala gabbroic intrusion was generated in a hybrid geodynamic setting in which the Southwest Tianshan Orogen was impacted by an upwelling mantle plume. In this sense, the Haladala layered gabbroic intrusion records the early phase of magmatism of the Tarim plume, which was preferentially emplaced in a lithospheric weak zone.