LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207^ pb/206^Pb age in the range of （2 49±54 ） -- （2 500±180） Ma. The magmatic genesis-derived detrltal zircon domain gives a 207^pb/ 206^Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206^ pb/ 238^U age of （2 790 ± 150） Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a diesordia with an upper intercept age of （2 044. 7 ± 29.3 ） Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. -2.8 Ga magmatlsm and -2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at （2.04±0.03） Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT （ultrahigh temperature） metamorphosed slab.

Adakitic Rocks Resulting from Partial Melting of Metabasite at High-Pressure Granulite-Facies Condition during Continental Collision

Objective Previous studies on adakitic rocks with high Sr/Y and La/Yb ratios have established that such rocks may form in a variety of tectonic settings through different petrogenetic processes including： （1） partial melting of subducted young （〈25 Ma）, hot and hydrated oceanic slab; （2） partial melting of thickened lower crust; （3） assimilation and fractional crystallization processes involving basaltic magma; （4） partial melting of delaminated lower crust; and （5） partial melting of hydrous garnet peridotite. The various origins for adakites provide important constraints on crustal growth and evolution throughout the Earth＇s history.

Early Paleozoic Granulite-Facies Metamorphism and Magmatism in the Northern Wulan Terrane of the Quanji Massif:Implications for the Evolution of the Proto-Tethys Ocean in Northwestern China

The nature and evolution of the Proto-Tethys Ocean originated from the breakup of the supercontinent Rodinia remain controversial. Early Paleozoic magmatism and metamorphism can pro- vide important constraints on the closure of the Proto-Tethys Ocean. This paper reports on a set of geological, petrographical, geochronological, mineralogical and geochemical data for Early Paleozoic granite, gabbro, granulite and granitic leucosome in the northern Wulan terrane of the Quanji Massif. Zircon LA-ICP-MS U-Pb dating reveals two episodes of magmatism, with the emplacement of a gran- itic pluton at 476.7±2.8 Ma and a gabbroic dike at 423±2 Ma. Whole-rock geochemistry suggests an arc affinity for the magma of the granitic pluton but a post-collisional extension setting for the gabbroic dike. Zircon LA-ICP-MS U-Pb dating also shows that the peak granulite-facies metamorphism and anatexis occurred at --475 Ma, coeval with the formation of the granitic pluton in the Quanji Massif as well as the early lawsonite-bearing eclogites in the North Qaidam high-pressure and ultrahigh-pressure （HP-UHP） metamorphic belt to the south. The granulite-facies metamorphism with peak P-T condi- tions at 718-729 ℃ and 0.46-0.53 GPa is characterized by an anticlockwise P-T path. Our data provide compelling evidence for Early Paleozoic paired metamorphic belts with HP-UHP metamorphism in the North Qaidam to the south and low PIT metamorphism in the Quanji Massif as a continental arc to the north, hence suggesting a northward subduction polarity for the Proto-Tethys oceanic plate. The intrusion of the post-collisional gabbroic dike supports for the closure of the Proto-Tethys Ocean in north- western China before 423 Ma.

Archean (about 2500 Ma) anatexis in eastern North China Block

Two Neoarchean alkaline feldspar-rich granites sourced from partially melted granulite-facies granodioritic orthogneiss have been here recognised in the eastern part of the North China Block(NCB).These poorly foliated granites have previously been assumed to be Mesozoic in age and never dated,and so their significance has not been recognised until now.The first granite(AG1)is a porphyritic syenogranite with megacrystic K-feldspar,and the second(AG2)is a quartz syenite with perthitic megacryst.Zircons from the granites yield LA-ICP-MS U-Pb ages of 2499±10 Ma(AG1),and 2492±28 Ma(AG2),which are slightly younger than the granodioritic orthogneiss that they intrude with a crystallisation U-Pb age of 2537±34 Ma.The younger granites have higher assays for SiO_(2)(71.91%for AG1 and 73.22%for AG2)and K_(2)O(7.52%for AG1 and 8.37%for AG2),and much lower assays for their other major element than the granodioritic orthogneiss.All of the granodioritic orthogneiss and granite samples have similar trace element patterns,with depletion in Th,U,Nb,and Ti and enrichment in Rb,Ba,K,La,Ce,and P.This indicates that the granites are derived from the orthogneiss as partial melts.Although they exhibit a similar REE pattern,the granites have much lower total REE contents(30.97×10^(−6) for AG1,and 25.93×10^(−6) for AG2),but pronounced positive Eu anomalies(Eu/Eu^(*)=8.57 for AG1 and 27.04 for AG2).The granodioritic orthogneiss has an initial ^(87)Sr/^(86)Sr ratio of 0.70144,εNd(t)value of 3.5,and εHf(t)values ranging from−3.2 to+2.9.The orthogneiss is a product of fractional crystallisation from a dioritic magma,which was derived from a mantle source contaminated by melts derived from a felsic slab.By contrast,the AG1 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.6926 that is considered too low in value,εNd(t)value of 0.3,andεHf(t)values between+0.57 and+3.82;whereas the AG2 sample has an initial ^(87)Sr/^(86)Sr ratio of 0.70152,εNd(t)value of 1.3,andεHf(t)values between+0.5 and+14.08.These assays indicate that a Sr-Nd-Hf isotopic disequilibrium exists between the granite and granodioritic orthogneiss.The elevatedεHf(t)values of the granites can be explained by the involvement of Hf-bearing minerals,such as orthopyroxene,amphibole,and biotite,in anatectic reactions in the granodioritic orthogneiss.Based on the transitional relationship between the granites and granodioritic orthogneiss and the geochemical characteristics mentioned above,it is concluded that the granites are the product of rapid partial-melting of the granodioritic orthogneiss after granulite-facies metamorphism,and their crystallisation age of about 2500 Ma provides the minimum age of the metamorphism.This about 2500 Ma tectonic-metamorphic event in NCB is similar to the other cratons in India,Antarctica,northern and southern Australia,indicating a possible connection between these cratons during the Neoarchean.

A Review of Ultrahigh Temperature Metamorphism

Ultrahigh-temperature （UHT） metamorphism represents extreme crustal metamorphism with peak metamorphic temperatures exceeding 900 ℃ and pressures ranging from 7 to 13 kbar with or without partial melting of crusts, which is usually identified in the granulite-facies rocks. UHT rocks are recognized in all major continents related to both extensional and compressive tectonic environments. UHT metamorphism spans different geological ages from Archean to Phanerozoic, providing information of the nature, petrofabric and thermal evolution of crusts. UHT metamorphism is traditionally identified by the presence of a diagnostic mineral assemblage with an appropriate bulk composition and oxidation state in Mg-Al-rich metapelite rocks. Unconventional geothermobarometers including Ti-in-zircon （TIZ） and Zr-in-rutile （ZIR） thermometers and phase equilibria modeling are increasingly being used to estimate UHT metamorphism. Concentrated on the issues about UHT metamorphism, this review presents the research history about UHT metamorphism, the global distribution of UHT rocks, the current methods for constraints on the UHT metamorphism, and the heat sources and tectonic settings of UHT meta- morphism. Some key issues and prospects about the study of UHT metamorphism are discussed, e.g., identification of UHT metamorphism for non-supracrustal rocks, robustness of the unconventional geothermometers, tectonic affinity of UHT metamorphic rocks, and methods for the constraints of age and duration of UHT metamorphism. It is concluded that UHT metamorphism is of great importance to the understanding of thermal evolution of the lithosphere.

Early Paleozoic (Pan African) thermal event of the Larsemann Hills and its neighbours, Prydz Bay, East Antarctica

The early Paleozoic (Pan African) thermal event of the Larsemann Hills and its adjacent areas, East Antarctica is discussed based upon the isotope ages we obtained. An Sm-Nd internal isochron for a representative mafic granulite yields an age of 540 Ma±75 Ma. Another Sm-Nd internal isochron, which is made up of the assemblage of the peak metamorphism and its whole rock as well, gives an age of 497 Ma ± 7 Ma The isotopic chronological data of single zircon stepwise evaporation dating and 40Ar-39Ar analysis provide further evidence for the early Paleozoic event of high-grade metamorphism in the region. The data from the field geological investigation in the Larsemann Hills also show that there is not only strong regional partial melting but also low-pressure granulite-facies metamorphism accompanied by it in the region. The early Paleozoic (Pan African) thermal event of the region may be related to the final formation of the East Antarctica craton, even of Gondwanaland.