Hong-Yan Li

Subduction initiation and ophiolite crust: new insights from IODP drilling

ABSTRACT International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Evolution of the mantle beneath the eastern North China Craton during the Cenozoic: Linking geochemical and geophysical observations

Recent discoveries related to the geochemistry of Cenozoic basalts and the geophysics of the deep mantle beneath eastern Eurasia make it possible to place constraints on the relationship between the seismic tomography of sub-continental mantle domains and their geochemical heterogeneities. Basalts with ocean island basalt (OIB)-like trace elements erupted during (56-23u2009Ma) and after (≤23u2009Ma) rifting of the eastern North China Craton (NCC) show evidence for the mixing of an isotopically depleted source and an EMI pyroxenitic mantle. NCC rifting stage basalts exhibit anomalously low MgO and Fe2O3T and high SiO2 and Al2O3, as well as low Dy/Yb and Y/Yb and high eHf at a given eNd, as compared to the post-rifting basalts. Temporal compositional variations and their association with basin subsidence indicate that heterogeneity in the eastern NCC asthenospheric mantle is the primary driver for intraplate magmatism in this region. The specific magmatic sources shifted in terms of depth, related to lithospheric thinning and thickening in the eastern NCC. The NCC EMI mantle domain most likely developed due to ancient events, is persistent through time, and is not related to dehydration of the stagnant Pacific slab in the MTZ. Based on the chemical signatures of post-rifting basalts, contributions from the Pacific slab are likely to be carbonatite-rich. Mantle metasomatism by carbonatite melts from the Pacific slab, and the interaction of these melts at shallower depths with EMI pyroxenitic mantle domains to trigger melting, are contributors to the observed low P-wave velocity zone beneath eastern Eurasia.

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 (~300u2009Ma), 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 (olivineu2009+u2009plagioclase). 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.