Zircon Geochronological Evidence for Participation of the North China Craton in the Protolith of Migmatite of the North Dabie Terrane

The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals par- ticipation of crustal rocks of the North China Craton （NCC） in the protolith in addition to the more common crustal rocks of the Yangtze Craton （YC）. The evidence of an NCC affinity for protolith of migmatite in the NDT is the -2.5 Ga （2 486±14 and 2 406±26 Ma） magamtic age and -1.8 Ga （1 717±79 Ma） metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7-0.8 Ga （e.g., 787±12 Ma） magamtic zircon age. Mid-Neoproterozoic magmatic age （0.7-0.8 Ga） is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite sam- ples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.

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.

Geochemistry,Zircon U-Pb Age and Hf Isotope of the Huilanshan Granitoids in the North Dabie Terrane:Implications for Syn-Collapse Magmatism of Orogen

Syn-collapse magmatism is a critical issue for evolution of the continental orogen.The Dabie Orogen is a typical orogen which was suffered from a complete collapse.Two kinds of granitoids,namely,the coarse-grained diorite and the fine-grained granite,are recognized at the center of the Luotian extensional dome,providing an opportunity to decipher the syn-collapse magmatism in the Dabie Orogen.The diorites(125±3 Ma)are high K calc-alkaline rocks,with low SiO2(51.9 wt.%–56.6 wt.%)and high MgO(3.5 wt.%–4.0 wt.%)contents.They are enriched in LREE and LILEs(e.g.,Ba,K,Rb)and depleted in HFSEs(e.g.,Ta,Nb,and Hf)with low ratio of Sr/Y(30.82–46.89).The granites(118±2 Ma)are shoshonite series rocks,with relatively high SiO2(68.9 wt.%–72.6 wt.%)and low MgO(0.32 wt.%–0.66 wt.%)contents.They are also enriched in LREE and LILEs with weakly negative Eu anomalies(δEu=0.81–0.85),and are depleted in HFSEs with low Sr contents(338 ppm–477 ppm)and Sr/Y ratios(23.80–33.13).Therefore,the two kinds of granitoids have no geochemical characteristics of adakitic rocks,suggesting that they were generated from a normal or thinned crust level.The diorites have quite negative zirconεHf(t)values(-18.4 to-21.1),suggesting they were from partial melting of the mafic lower continental crust.The granites have relatively higher zirconεHf(t)values(-14.4 to-18.1).The granites also contains a series of old inherited zircon cores,such as two upper intercept ages of 2 628±41 and 1 840±37 Ma,and a concordant age of 807±9 Ma.All these features suggest that the granites were generated from partial melting of the felsic middle-lower continental crust.Thus,the Huilanshan Early Cretaceous granitoids coupled with the Luotian extensional dome revealed the collapsed process of the Dabie Orogen.

Monazite recorded Paleoproterozoic granulite-facies metamorphism and Triassic fluid modification of the Weihai pelitic granulite,Sulu orogen

Monazite is an important accessory mineral in the pelitic granulites(Weihai area,Sulu orogen),and is also a powerful monitor for understanding the metamorphic evolution of the granulites.The pelitic granulites incongruously occur as lenses in granitic gneisses.The lithologies of the lenses gradually change from core to margin:The undeformed coarse-grained granulite,the foliated fine-grained granulite,the garnet-biotite-gneiss,and the migmatized granulite.The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet+plagioclase(antiperthite)+quartz+sillimanite+biotite with accessory minerals of rutile,zircon,and monazite.The lens margins display a fluid-induced retrogression.In order to decipher the metamorphic processes of the pelitic granulites,a combined study of BSE imaging,U-Pb dating,and trace element composition for the monazites from the metapelitic lens were conducted.Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age(1832±7 Ma,n=40).Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age.For example,monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma(n=56)with Triassic concordant age of 223.8±2.9 Ma.The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies,indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions.During Triassic fluid modification,the monazite bright rims assimilated Th and Si but released U,HREE,Y,and P.This process resulted in that the Triassic overgrowth monazites have higher HREE and Y contents,and lower Th and U contents with relatively low Th/U ratios.Thus,the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification.The Weihai pelitic granulites might have a tectonic affinity with the North China Craton.Therefore,the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision,and subsequently were remoulded by the fluids during its exhumation.