Titanite(CaTi2SiO5) is an important accessory mineral not only in magmatic and metamorphic rocks,but also commonly in hydrothermal mineral deposits. It incorporates appreciable amounts(usually 10×10-6 s to 100×10-6 s) of U and Th into its structure and has a closure temperature of Pb diffusion higher than 660～700 ℃(Scott and St-Onge,1995) ,making it an ideal U-Th-Pb geochronometer. The Daye metallogenic district is located in the west of the giant Middle-Lower Yangtze River(MLYR) Cu-Fe-Au province,eastern China and is well endowed with Fe,Cu,and polymetallic skarn deposits.
LI Jian-wei 1,2,DENG Xiao-dong1,2,ZHOU Mei-fu3,LIU Yong-sheng1(1. State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China
The geometry and timing of amalgamation of the North China craton(NCC) have been controversial,with three main models with significantly different interpretations of regional structure,geochronology,and geological relationships.The model of Zhao G C et al.suggests that the eastern and western blocks of the NCC formed separately in the Archean,and an active margin was developed on the eastern block between 2.5 and 1.85 Ga,when the two blocks collided above an east dipping subduc-tion zone.The model of Kusky et al.presumes that the eastern block rifted from an unknown larger continent at circa 2.7 Ga,and experienced a collision with an arc(perhaps attached to the western block) above a west-dipping subduction zone at 2.5 Ga,and the 1.85 Ga metamorphism is related to a collision along the northern margin of the craton when the NCC joined the Columbia supercontinent.The model of Faure et al.suggests two collisions in the central orogenic belt,at 2.1 and 1.88 Ga.Recent seismic results support both the models of Kusky et al.and Faure et al.,showing that subduction be-neath the central orogenic belt(COB) was west-directed,and that there is a second,west-dipping pa-leosubduction zone located to the east of the COB dipping beneath the western block(Ordos craton).The boundaries identified through geophysics do not correlate with the boundaries of the Trans-North China orogen suggested in the Zhao et al.model,and the subduction polarity is opposite that predicted by that model.The seismic profiles are consistent with an Archean collision above a west-dipping sub-duction zone beneath the COB predicted by the models of Kusky et al.,and the second west-dipping subduction zone is consistent with the two events suggested in the Faure et al.model.
A detailed knowledge of the thickness of the lithosphere in the North China craton(NCC) is important for understanding the significant tectonic reactivation of the craton in Mesozoic and Ce-nozoic.We achieve this goal by applying the newly proposed continuous wavelet transform theory to the Gravity Field Model(EGM 2008) data in the region.Distinct structural variations are identified in the scalogram image of profile Alxa-Datong(大同)-Qingdao(青岛)-Yellow Sea(profile ABC),trans-versing the main units of NCC,which we interpret as mainly representing the Moho and lithosphere-asthenosphere boundary(LAB) undulations.The imaged LAB is as shallow as 60-70 km in the south-east basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior.A rapid change of about 30 km in the LAB depth was detected at around the boundary between the Bohai(渤海) Bay basin(BBB) and the Taihang(太行) Mountains(TM),roughly coincident with the distinct gravity decrease of more than 100 mGal that marks the North-South Grav-ity Lineament(NSGL) in the region.At last we present the gravity modeling work based on the spectral analysis results,incorporating with the observations on high-resolution seismic images and surface to-pography.The observed structural differences between the eastern and western NCC are likely associ-ated with different lithospheric tectonics across the NSGL.Combined with seismic tomography results and geochemical and petrological data,this sug-gests that complex modification of the litho-sphere probably accompanied significant litho-spheric thinning during the tectonic reactivation of the old craton.
In this paper the authors briefly introduce and review the new progress of the newly dis- covered Proterozoic Miaowan (庙湾) ophiolite and Neoproterozoic Huangling (黄陵) granitoids in the southern Huangling anticline in the Yangtze craton, and the tectonic evolution significance of assemblage and breakup of Rodinia and Columbia supercontinent in South China.
We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo (赵家阳坡) area from the Kongling (崆岭) Group in the northern Huangling (黄陵) anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim tex tures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660-808 ℃ (714 ℃ in average), all indicating that the BIFs underwent granulite facies metamorphism. The age of zir-cons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indicating that there was a significant granulite facies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the assembly of the Columbia supercontinent with South China. Moreover, the REE pattern is characterized by depletion in LREE while relatively flat in HREE, La N /Yb N =0.26, with a positive Eu anomaly (Eu/Eu*=1.59), which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.
Much has been learned in the past 40 years about the great diversity of the internal structure and geochemical compositions of Phanerozoic ophiolites, indicating that these on-land fragments of ancient oceanic lithosphere formed in distinctly different tectonic settings during their igneous evolution. Recent studies in Archean and Proterozoic greenstone belts have shown that the Precambrian rock record may also include exposures of a diverse suite of ophiolite complexes as part of craton development in the early history of the Earth. We review the salient features of the Precambrian ophiolite record to highlight what has been learned about Precambrian oceanic spreading systems since the original Penrose definition of ophiolites in 1972. Some of the diagnostic, characteristic, typical, and rare aspects of ophiolites of all ages are presented in a table in order to help determine if tectonically deformed and metamophosed sequences in Precambrian shield areas may be considered as ophiolites. The results of this comparative study are important in that they enable researchers to more realistically characterize allochthonous mafic/ultramafic rock sequences as ophiolitic or non-ophiolitic. This approach is more deterministic in contrast to some other arbitrary classification schemes requiring three or four of the Penrose-style ophiolitic units to be present in the Precambrian record for a specific rock sequence to be considered ophiolitic. Once these tectonic fragments are recognized as remnants of ancient oceanic lithosphere, great progress shall be made in understanding early Earth history. We discuss the significance and implications of the Precambrian ophiolite record to constrain the mode and nature of the plate tectonics that operated in deep time.
Timothy M. KUSKYWANG LuYildirim DILEKPaul ROBINSONPENG SongBaiHUANG XuYa
The South China fold belt has experienced a complex series of tectonic events that span 1.0 billion years of earth history. Longhushan (龙虎山) World Geopark is located on the Proterozoic suture between the Yangtze craton and Cathyasia block and highlights the long history of this belt. Collision of the Cathyasia and Yangtze cratons 1.0 billion years ago was associated with the formation of the Rodinian supercontinent where most of the planet's landmasses were amalgamated into one block. Jurassic through Early Cretaceous magmatism was associated with the inland migration of the continental margin arc associated with the penetration of a flat slab after subduction of the Kula-Farallon ridge. Slab roll-back in the Early to Middle Cretaceous opened many extensional basins across the South China fold belt, including the Xinjiang (信江) basin in which Longhushan is located, and these were filled largely with continental red beds deposited by fluvial systems in the hot torrid climate. The beds are richly fossiliferous, including remains of many dinosaurs and dinosaur eggs. Subduction of the Kula-Pacific plate in the Middle Cretaceous caused a short magmatic pulse, and then, the basins subsided slowly through the remainder of the Cretaceous. Cenozoic uplift of the red bed basins was initiated by the India-Asia collision. The uplift was associated with the formation of many faults, joints, and brittle structures that dissected the red bed deposits. Fluvial erosion of the red beds was enhanced along the brittle structures, and different locations have developed very distinctive and structurally controlled geomorphological features including mesas, kopjies, and isolated stone peaks that are known in China as Danxia (丹霞) land-forms. Together, these features form Danxia landscapes, and Longhushan World Geopark exhibits a complete range of the Danxia landscapes from juvenile, to mature and to old stages of development. The United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Geopark progra
Here we report an integrated study of zircon U-Pb age and Hf isotope composition for a gneiss sample from the Kongling terrain in the Yangtze Craton. CL imaging reveals that most zircons are magmatic, and a few of them have thin metamorphic rims. The magmatic zircons gave a weighted mean U-Pb age of 3218±13 Ma, indicating the gneiss is the oldest basement rock in the Yangtze Craton found to date. They have εHf(t) value of -2.33±0.51,and two-stage Hf model age of 3679±49 Ma,indicating that the gneiss was derived from partial melting of >3.6 Ga crustal rock. The metamorphic rims yielded an age of 2732±16 Ma, implying that the metamorphic event occurred in the Neoarchean era, which may be also a major tectono-thermal event in the Yangtze Craton.
