Petrogenesis of Late Mesozoic Calc-Alkaline Lamprophyres from Sulu UHP Terrane, Eastern China: Implications to Paleo-Pacific Plate Subduction and Destruction of the North China Craton

Destruction of the North China Craton has caused extensive concern on its multiple potential mechanisms including thermal erosion,chemical erosion and delamination.It is widely accepted that thinning of the

Temperature of Prograde Metamorphism, Decompressional Partial Melting and Subsequent Melt Fractional Crystallization in the Weihai Migmatitic Gneisses,Sulu UHP Terrane:Constraints from Ti-in-Zircon Thermometer

In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. Zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indi- cating that a process of fractional crystallization of the partial melt during exhumation.

Age, Trace Elements and Hf-Isotope Composition of Zircon in Eclogites from the Sulu UHP Belt, East-Central China: Neoproterozoic Intrusion and Subsequent Paleozoic and Early Mesozoic Metamorphism

The Sulu （苏鲁）-Dabie （大别） orogen in East-Central China formed during the subduction and collision of the Yangtze block with the North China block in Early Mesozoic （240-225 Ma）. Constraints on the nature and derivation of eclogites, which are a significant component of the orogen, can provide useful information about subduction-zone metamorphism and crust-mantle interaction. The U-Pb ages, Hf-isotope ratios and trace-element compositions of zircons in eclogites from the Jiangzhuang （蒋庄） （Jiangsu （江苏） Province） and Rongcheng （荣成） （Shandong （山东） Province） areas indicate that the protoliths of the eclogites derived from ultramafic-mafic complexes or mafic intrusion in the subducted continental lithosphere. The upper intercept age of 852±10 Ma and high tHf （up to 14.7） of the Neoproterozoic zircons in a Jiangzhuang sample indicate that the protoliths represent products of the Neoproterozoic addition of juvenile materials to the older （i.e., Paleo-Mesoproterozoic） continental crust. The zircon ages of eclogites from both localities mainly record the Triassic （230-220 Ma） metamorphism, consistent with the formation of the Sulu orogen in Early Mesozoic. The lower intercept age of 316±4 Ma in a Jiangzhuang sample suggests that thermal activity relating to the paleo- Tethyan in Late Carboniferous also affected the eclogitic protolith.

Serpentinized Peridotite as Source of Aeromagnetic Anomalies

The source of long-wavelength aeromagnetic anomalies appears to originate from the earth's deep crust. Constrained by previous studies on geochemical, petrologic analysis, the eclogite and serpentinized peridotite samples from drill hole ZK703 at Donghai in the western Sulu ultrahigh-pressure (UHP) terrane, East China, were unambiguously exhumed from the lower crust and the upper mantle, providing significant information about the magnetic properties of rocks at a deeper part of the crust. Results show that the serpentinization process favors the neoformation of nearly stoichiometric magnetite, resulting in the enhancement of its magnetization up to 8.6 A/m, which is sufficient enough to contribute to some magnetic anomalies. In contrast, eclogite samples have only weaker magnetization (generally less than 0.05 A/m) compared to serpentinized peridotite. Nevertheless, experiments under the lower crustal conditions are necessary to further support these conclusions.