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We report new U–Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr–Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43–48En19–35Fs18–38), sanidine (An0Ab3–11Or89–97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (δ18O = 5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean. [ABSTRACT FROM AUTHOR]

The Zhangguangcai Range in the Xing'an Mongolian Orogenic Belt, NE China, contains Early Jurassic (c. 188 Ma) Dabaizigou (DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have (87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive ɛNd(t) values of 2.3–3.2 and positive ɛHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet- to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia. [ABSTRACT FROM AUTHOR]

The Dongbo peridotite massif in the western part of the Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet represents a mantle thrust sheet of more than 400 km 2 , tectonically overlying a Cretaceous mélange. Spinel-harzburgite with a porphyroclastic texture is the main rock type, accompanied by subordinate dunite. Olivine (Fo 90.3–91.3 ), orthopyroxene (En 87–90 ), clinopyroxene (En 49–51 Wo 46–51 Fs 0–4 ) and spinel [Mg# = 100Mg/(Mg + Fe)] = 41.1–73.1; Cr# = (100Cr/(Cr + Al) = 23.6–87.3] are the major minerals. The peridotites are characterized by “spoon-shaped”, chondrite-normalized REE patterns with (La/Yb) CN = 0.09–0.40, and (Gd/Yb) CN = 0.23–0.95. The patterns show relative uniform depletion from Lu to Nd, followed by slight enrichment from Nd to La. These rocks represent residues of mantle rocks after variable degrees of basaltic magma extraction beneath a seafloor spreading environment, followed by modification by percolating, suprasubduction-related melts. The Dongbo peridotites have variable, unradiogenic Os isotopic compositions with 187 Os/ 188 Os = 0.1235–0.1282 and rhenium-depletion ages ( T RD ) = 0.22–0.96 Ga, implying heterogeneity of Os isotopes in the mantle beneath Neotethys. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. [ABSTRACT FROM AUTHOR]