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Abstract: The petrology, mineral compositions, whole rock major/trace element concentrations, including highly siderophile elements, and Re–Os isotopes of 99 peridotite xenoliths from the central North China Craton were determined in order to constrain the structure and evolution of the deep lithosphere. Samples from seven Early Cretaceous–Tertiary volcanic centers display distinct geochemical characteristics from north to south. Peridotites from the northern section are generally more fertile (e.g., Al2O3 =0.9–4.0%) than those from the south (e.g., Al2O3 =0.2–2.2%), and have maximum whole-rock Re-depletion Os model ages (T RD) of ∼1.8Ga suggesting their coeval formation in the latest Paleoproterozoic. By contrast, peridotites from the south have maximum T RD model ages that span the Archean–Proterozoic boundary (2.1–2.5Ga). Peridotites with model ages from both groups are found at Fansi, the southernmost locality in the northern group, which likely marks a lithospheric boundary. The Neoarchean age of the lithospheric mantle in the southern section matches that of the overlying crust and likely reflects the time of amalgamation of the North China Craton via collision between the Eastern and Western blocks. The Late Paleoproterozoic (∼1.8Ga) lithospheric mantle beneath the northern section is significantly younger than the overlying Archean crust, indicating that the original lithospheric mantle was replaced in this region, either during a major north–south continent–continent collision that occurred during assembly of the Columbia supercontinent at ∼1.8–1.9Ga, or from extrusion of ∼1.9Ga lithosphere from the Khondalite Belt beneath the northern Trans-North China Orogen, during the ∼1.85Ga continental collision between Eastern and Western blocks. Post-Cretaceous heating of the southern section is indicated by high temperatures (>1000°C) recorded in peridotites from the 4Ma Hebi suite, which are significantly higher than the temperatures recorded in peridotites from the nearby Early Cretaceous Fushan suite (<720°C), and likely reflects significant lithospheric thinning after the Early Cretaceous. Combining previous Os isotope results on mantle xenoliths from the eastern North China Craton with our new data, it appears that lithospheric thinning and replacement may have evolved from east to west with time, commencing before the Triassic on the eastern edge of the craton, occurring during the Jurassic–Cretaceous within the interior, and post-dating 125Ma on the westernmost boundary. [Copyright &y& Elsevier]

Abstract: Xenolithic peridotites having a similar range of major element compositions from two nearby localities in the Trans-North China Orogen, North China Craton, provide a rare opportunity to explore effects resulting from both primary partial melting and secondary processes on Os isotopes and highly siderophile element (HSE) abundances. HSE patterns of peridotites from Hannuoba are similar to those of orogenic peridotite massifs worldwide, but are rare for xenolithic peridotites. These patterns can be explained by relatively low degrees of melt depletion, coupled with long-term preservation of sulfides. By contrast, peridotites from Yangyuan have major element compositions similar to or slightly more depleted than Hannuoba xenoliths, but are characterized by distinct, highly fractionated HSE patterns with lower total HSE abundances and Os, Pd and Re depletions relative to Ir. Some of the latter HSE characteristics must reflect secondary processes. The low S and Se contents of Yangyuan peridotites, coupled with scarcity of observable sulfides, suggest that they experienced sulfide breakdown, possibly as a result of interaction with a S-undersaturated melt/fluid. This may have occurred under oxidizing conditions, as suggested by the somewhat higher ƒO2 recorded in the Yangyuan peridotites compared to the Hannuoba peridotites, as well as the metal-deficient composition of rare, mono-sulfide-solid solution (mss) sulfides within the Yangyuan peridotites. We speculate that under such conditions, Os, Pd, and possibly Re, more strongly partition into a sulfide liquid, or the oxidizing medium (melt or fluid), than Ir and Pt and, thus, become depleted. These effects would have been imposed on original patterns that were similar to those in the Hannuoba suite. The good correlation between 187Os/188Os and major element indices of melt depletion in the Yangyuan rocks, coupled with the poor correlation between 187Os/188Os and 187Re/188Os, suggests that the S, Os, Pd and Re removal was recent. Hence, the long-term Re–Os isotopic systematics of these rocks would not have been affected, and Re depletion model ages, based on Os isotopes, remain viable to constrain the timing of melt deletion in these peridotites. The similarity of model age distributions between Yangyuan and Hannuoba peridotites (TRD =0 to 1.7 and 0 to 1.5Ga, respectively) is consistent with this, and further indicates that these peridotites formed in the Paleoproterozoic. [Copyright &y& Elsevier]

• In UHP eclogite, leucosomes form millimetric pockets that preserve in situ melt compositions. • Leucosome microstructures and compositions indicate melting dominated by omphacite breakdown. • Leucosomes are enriched in LILE, but depleted in Th and Ta, similar to rock-forming omphacite. • Thermobarometry yields P – T of 1.6–1.2 GPa/780–690 °C, consistent with calculated phase equilibria. • Omphacite breakdown melting during exhumation could be common, helping to drive exhumation of UHP crust. Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features and compositional data that indicate in situ partial melting dominated by breakdown of omphacite in UHP eclogite from the Sulu belt, China. Diagnostic microstructures include: (i) the presence of in situ leucosome pockets composed of plagioclase, euhedral amphibole, minor K-feldspar and epidote within host zoisite- and phengite-bearing eclogite; (ii) skeletal omphacite within the leucosome pockets that has a lower jadeite content (25–45 mol.%) than rock-forming omphacite (39–54 mol.%); and, (iii) seams of Na-rich plagioclase that extend along grain boundaries separating phengite, quartz and zoisite, and which commonly exhibit low dihedral angles where they terminate at triple grain-boundary junctions. Major oxide proportions of 57 leucosome pockets, calculated using mineral modes and compositions, yield leucodiorite bulk compositions characterized by intermediate SiO 2 , high Al 2 O 3 and Na 2 O, and low K 2 O contents. In primitive mantle-normalised trace element diagrams, the leucosome pockets show enrichment in large ion lithophile elements, U, Pb, Zr, Hf and Ti, but depletion in Th and Ta, patterns that are similar to those of rock-forming omphacite. Rather than forming predominantly by breakdown of phengite and/or zoisite, as widely proposed in the literature, the leucosome pockets have petrographic characteristics and major oxide and trace element compositions that are consistent with partial melting dominated by omphacite breakdown. Based on conventional thermobarometry, the eclogite was exhumed from pressure–temperature (P – T) conditions of 3.6–3.1 GPa and 900–840 °C. Partial melting led to the formation of the leucosome pockets, which equilibrated with the rims of surrounding rock-forming garnet and pyroxene during crystallisation. Conventional thermobarometry using rim compositions yields P – T conditions of 1.6–1.2 GPa and 780–690 °C, broadly consistent with calculated phase equilibria and Ti-in-zircon temperatures from zircon overgrowths. Weighted mean ages of ca 217–214 Ma from thin overgrowths on zircon are interpreted to record melt crystallisation. This study provides insight into an overlooked mechanism by which eclogites partially melt during exhumation from UHP conditions, and permits a better understanding of the processes that assist deeply-subducted continental crust to return to shallower depths. [ABSTRACT FROM AUTHOR]

Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UHP metamorphic conditions. Here, we investigate decimeter- to meter-thick leucosome sheets in retrogressed and migmatitic UHP eclogite in the central Sulu belt. The leucosomes have variable mineral modes and deformation fabrics; some are composite with marginal and interior facies, whereas others are not. They range from granite to trondhjemite, with high abundances of SiO 2 , K 2 O + Na 2 O and Al 2 O 3 , similar to hydrous melts, and have major oxide and trace element variations consistent with modal changes in phengite and albite + epidote sensu lato. The leucosomes are enriched in large ion lithophile elements relative to high field strength elements and have rare earth element (REE) patterns enriched in light REE relative to heavy REE, generally with small positive or negative Eu anomalies. Newly-crystallized zircon in the leucosomes records weighted mean ages of c. 223–218 Ma. Early-formed leucosome has Sr-Nd isotope compositions similar to those of nearby unretrogressed UHP eclogites, whereas younger leucosome has Sr-Nd isotope compositions intermediate between the eclogites and surrounding gneisses. Ti-in-zircon thermometry combined with Si-in-phengite barometry indicates crystallization of the leucosomes between ~850 and ~770 °C, over a wide range of pressure (P) from 3.5 to 2.2 GPa, which correlates with age from older (higher P) to younger (lower P). At the metamorphic peak, which may have exceeded 5.5 GPa, the source rocks were likely fluid deficient or fluid absent. During exhumation from UHP conditions, we posit that exsolution of water stored in nominally anhydrous minerals formed a silicate-rich supercritical fluid in eclogite that evolved to a denser, more viscous and more polymerized hydrous melt. Phengite barometry indicates that the early-formed leucosomes crystallized at pressures close to the critical line for the basalt plus water system, possibly by diffusive loss of water to the host eclogite. Infiltration of solute-rich supercritical fluid from the surrounding gneisses blended with melt in the eclogites generating variable Sr-Nd isotope composition intermediate between these end-members, as recorded by leucosomes that crystallized at lower pressures. The youngest leucosomes crystallized at pressures near the critical line for the Ca-granite plus water system creating a low volume of aqueous fluid that precipitated quartz veins associated with the leucosome sheets, particularly at their margins. Subsequently, limited phengite-breakdown melting in the leucosomes is recorded by aggregates of plagioclase + biotite around phengite and thin films and cuspate veinlets and patches of K-feldspar along grain boundaries. Phase equilibrium modelling indicates that this late stage melting occurred at P–T conditions near the transition from HP eclogite to amphibolite facies, with final subsolidus equilibration at 1.0–0.9 GPa and T < 640 °C. • Both simple and composite leucosome sheets, the latter with distinct marginal and interior facies, are recognized in retrogressed migmatitic UHP eclogites at General's Hill, in the central Sulu belt, China, recording crystallization from an evolving hydrous granite melt during exhumation; • The leucosomes crystallized at c. 223–218 Ma from the margins to the interior during exhumation from UHP to HP eclogite facies conditions at P = 3.5–2.2 GPa and T = 850–770 °C; • Based on Sr-Nd isotope compositions, we posit that early-formed leucosomes crystallized from hydrous granite melt that had evolved from solute-rich supercritical fluid initially derived from the eclogites; • The solute-rich supercritical fluid was generated by exsolution of structural water from nominally anhydrous minerals during exhumation; • Younger leucosomes crystallized from a blended hydrous melt after solute-rich supercritical fluid derived from the surrounding gneisses had infiltrated into the eclogite; • The crystallized leucosomes record evidence of lower pressure low-volume partial melting due to suprasolidus phengite-breakdown during ongoing exhumation from HP eclogite to amphibolite facies conditions. [ABSTRACT FROM AUTHOR]