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Substantial quantities of sediments are known to enter the deep lithosphere at subduction zones, but the extent to which sediments melt and the process involved in sediment contribution to the deep lithosphere are inadequately understood. Vigorous debate continues about whether the subducted sediment component is terrigenous or pelagic and transported as a hydrous melt, an aqueous fluid, or bulk sediment. In this contribution, we conduct an integrated study on a variety of deep-seated xenoliths in the Neogene Hannuoba basalts from the northern margin of the North China Craton. Among these xenoliths, clinopyroxenite xenoliths are compositionally and isotopically distinct. Mineral chemistry shows that the clinopyroxenite xenoliths come from a depth near the MOHO, rather than from the mantle as suggested previously. The clinopyroxenite xenoliths have extremely evolved Sr-Nd-Hf isotopic compositions and are interpreted to have a late Archean protolith age. The extremely low contents of Cr and Ni for the clinopyroxenite xenoliths preclude a magmatic origin. Instead, a metasomatic origin is suggested, which is strongly supported for the clinopyroxenites by the occurrence of hydrous minerals and high contents of large-ion-lithophile elements (K, Rb, Ba, Th and Sr) and light rare earth elements, as well as elevated δ18O (9.9–11.3‰) and light δ26Mg (−1.04‰ to −1.42‰) isotopic compositions. Furthermore, their high high-field-strength element (Nb, Ta, Zr and Hf) contents indicate that the metasomatic agent is a hydrous melt, rather than an aqueous fluid. The metasomatic melts are considered to be derived from a mixed source of sedimentary carbonates and ancient, felsic continental materials. A combination of zircon ages and oxygen isotope data for the clinopyroxenite xenoliths further restricts the timing of metasomatism to the late Paleozoic. Considering the regional tectonic setting, the sediments most likely came from the subducted Paleo-Asian oceanic slab. Thus the Hannuoba clinopyroxenite xenoliths provide direct evidence for melting of the subducted Paleo-Asian oceanic slab sediment and its interaction with the deep lithosphere. The data show that melting of subducted sediments can take place at a much shallower depth than commonly thought and place an independent constraint on future models of slab geotherms. [ABSTRACT FROM AUTHOR]

We conduct a study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton. Most zircons from both types of granulite have core-rim structure in their CL images. All the magmatic zircon cores yield essentially identical ∼2.5 Ga ages, whereas metamorphic zircons yield ages of ∼1.85 Ga and 120–140 Ma, respectively. Most the ∼1.85 Ga metamorphic zircons have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120–140 Ma) metamorphic zircons (0.12–3.48) overlaps that of the magmatic zircons (0.12–2.44). The Mesozoic and ∼1.85 Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the ∼1.85 Ga ones show less enrichments and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. The ∼1.85 Ga metamorphic zircons, however, have lower formation temperatures, oxygen fugacity and 176 Lu/ 177 Hf ratios than the Mesozoic ones. We speculate that the ∼1.85 Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites. [ABSTRACT FROM AUTHOR]

U–Pb zircon dating for 10 rock samples collected from the Archean Jiaobei terrain of the eastern block of the North China craton (NCC) yield three groups of ages: ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga. Thirteen rock samples studied can be classified into a low Mg# (molecular Mg/(Mg + Fe)) (Mg# = 34–52) and a high Mg# group (Mg# = 52–61), respectively. The low Mg# rocks are typical Archean TTGs with emplacement ages of ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga whereas the high Mg# rocks are Archean sanukitoids with age of ∼2.5 Ga. Despite having different ages, all the Jiaobei TTGs have zircon δ 18 O values between 5.6‰ and 6.3‰, essentially identical to those of the ∼2.7 Ga TTGs from adjourning Taishan area and Archean TTGs from other parts of the world. Magmatic zircons from the Jiaobei sanukitoids have δ 18 O values between 6.4‰ and 7.5‰, significantly higher than the Archean TTGs. The whole-rock Nd and the zircon Hf isotopic compositions indicate that the ∼2.9 Ga and ∼2.5 Ga TTGs and ∼2.5 Ga sanukitoids represent crustal growth at ∼2.9 Ga and ∼2.5 Ga in the Jiaobei terrain. Unlike the ∼2.7 Ga TTGs in the adjourning Taishan area that are considered to reflect juvenile crust, the Jiaobei ∼2.7 Ga TTGs, however, are most likely resulted from the reworking of the pre-existing ∼3.0 Ga crust. This is in contrast to the previous suggestion that ∼2.7 Ga is the most significant crust-forming episode and ∼2.5 Ga a period of crustal reworking in the Jiaobei terrain. The high Mg# and high contents of Cr, Ni, LREE and LILE of the ∼2.5 Ga Jiaobei sanukitoids, require a metasomatized peridotitic mantle source for their origin. The significant negative Zr and Hf anomalies and the elevated δ 18 O values of the ∼2.5 Ga sanukitoids, are inconsistent with a mantle source metasomatized solely by the TTG melts. Instead, we suggest that the Jiaobei sanukitoids could have originated from a mantle wedge metasomatized by a high δ 18 O component, most likely the slab-derived fluids. The similarity of multi-element patterns and the zircon δ 18 O values between the Jiaobei and Superior Province sanukitoids may imply similar processes involved to form sanukitoids in both Jiaobei and Superior Province terrains. [ABSTRACT FROM AUTHOR]