U-Pb zircon geochronology and Hf isotope study of metamorphosed basic-ultrabasic rocks from metamorphic basement in southwestern Zhejiang: The response of the Cathaysia Block to Indosinian orogenic event

A combined study using LA-ICP-MS U-Pb dating, Hf isotopes, trace elements and the Ti-in-zircon geo-thermometer was carried out on zircons from the metamorphosed basic-ultrabasic rocks in the meta-morphic basement of the Cathaysia Block, southwestern Zhejiang Province. The formation and meta-morphic ages of the rocks from the metamorphic basement of the Cathaysia Block were determined based on zircon U-Pb geochronology. The age for the magmatic crystalline zircons from the protolith is about 1.85 Ga. The εHf(t) values of the older zircons were from ?7 to ?3, with two-stage model Hf ages (TDM2LC) of about 2.9 to 3.4 Ga, indicating that the source material was derived from anatexis and recy-cling of the Archean crust. The newly formed metamorphic zircons yielded U-Pb ages of 260―230 Ma. The metamorphic temperature calculated using the Ti-in-zircon geothermometer ranged from 610 to 720℃, consistent with the results from petrographic observations, indicating that the Cathaysia Block experienced an amphibolite facies metamorphism during the Indosinian. Results from this study pro-vided an important timeframe for the tectonic evolution in South China and the Southeast Asia during the Late Permian and Early Triassic times.     

Late Cretaceous K-rich rhyolitic crystal tuffs from the Chuduoqu area in Eastern Qiangtang subterrane:evidence for crustal thickening of the central Tibetan Plateau prior to India–Asia collision

In order to constrain whether the Lhasa–Qiangtang collision contributed to an early crustal thickening of the central Tibetan Plateau prior to the India–Asia collision,we present zircon LA–ICP–MS U–Pb ages,wholerock geochemistry,and zircon Hf isotopic compositions of the newly discovered rhyolitic crystal tuffs from the Chuduoqu area in the eastern Qiangtang subterrane,central Tibet.Zircon U–Pb dating suggests that the Chuduoqu rhyolitic crystal tuffs were emplaced at ca.68 Ma.The Chuoduoqu rhyolitic crystal tuffs display high SiO₂ and K2 O,and low MgO,Cr,and Ni.Combined with their zircon Hf isotopic data,we suggest that they were derived from partial melting of the juvenile lower crust,and the magma underwent fractional crystallization and limited upper continental crustal assimilation during its evolution prior to eruption.They should be formed in a post-collisional environment related to lithospheric mantle delamination.The Chuduoqu rhyolitic crystal tuffs could provide important constraints on the Late Cretaceous crustal thickening of the central Tibetan Plateau caused by the Lhasa–Qiangtang collision.     

U-Pb zircon age and Hf isotope compositions of Mesoproterozoic sedimentary strata on the western margin of the Yangtze massif

The Mesoproterozoic sedimentary strata on the western margin of the Yangtze massif are a clastic-carbonate rock association intercalated with a small amount of tuff and basalt and deposited in a relatively stable environment. They are termed as the Kunyang Group, the Huili Group, and the Dongchuan Group respectively in different regions. We performed zircon U-Pb dating of the tuff from the groups. The results, coupled with the detrital zircon U-Pb ages of clastic rocks from the Kunyang Group and the Dongchuan Group, indicate that the sedimentation ages of the Kunyang Group and the Huili Group range from 1050 to 1000 Ma and that the Kunyang Group and the Huili Group belong to a sedimentary association with contemporaneous heterotopic facies. The detrital zircon ages and Hf isotope compositions reveal that the clastic materials in the Kunyang Group and the Huili Group are derived primarily from the Cathaysia massif. Zircons of the tuff in the Dongchuan Group yields an age of ca.1.5 Ga and all the zircon ages of clastics in the Dongchuan Group are older than 1.5 Ga, indicating that the sedimentation of the Dongchuan Group occurred during the late Mesoproterozoic Changcheng Period. Age spectra of the detrital zircons indicate that the clastic materials of the Dongchuan Group are derived primarily from the ancient basement of the Yangtze massif. A systematic Hf isotope determination of various types of zircons in the above three stratigraphic units shows that there is a rapid elevation in the initial Hf value of zircon at ~1.5 and 1.0 Ga. Previous studies on the sedimentary characteristics of the Kunyang Group and the Huili Group show that both were deposited in a foreland basin. Combining our data with previous studies, we suggest that the Kunyang Group and the Huili Group are foreland basin sedimentary successions formed along the southern side of the Yangtze massif after an amalgamation between the Yangtze massif and the Cathaysia massif during the Grenvillian. The assembly of the Yangtze massif and the Cathaysia massif developed gradually from the west to the east and was finally completed in the eastern segment of the Yangtze massif at 0.9 Ga, representing the last stage of the Rodinia supercontinent assembly. Hf isotope compositions in zircon indicate that the supercontinent cycle has an intimate relation with crustal growth.     

The oldest basement rock in the Yangtze Craton revealed by zircon U-Pb age and Hf isotope composition

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 der...     

Zircon Hf isotope composition of metamorphic eclogite from Xindian,Dabie Terrain

Hf isotope measurement has been carried out for UHP metamorphic eclogite from Xindian by using LA-MC-ICP-MS technique. The result indicates that metamorphic growth zircon has high 176Hf/177Hf (0.282544―0.282612) and low 176Lu/177Hf (0.000004―0.000211) ratio,inherited and recrystallized proto-lith zircon has low 176Hf/177Hf (0.282266―0.282466) and high 176Lu/177Hf (0.000090―0.002144) composi-tions. The low 176Lu/177Hf of growth zircon comes from its decreasing of Lu and increasing of Hf during UHP process. The high 176Hf/177Hf deduced from high radiogenic 176Hf,which was produced from long-term evolution of high Lu/Hf ratio minerals. Partial recrystallization of protolith zircon would not cause reworking of Lu/Hf isotope in zircon. Compared to U-Pb,zircon Lu-Hf system has better stability. The initial Hf isotope composition of metamorphic growth zircon may represent the Hf isotope compo-sition of whole rock system at the same time. The initial εHf of 3.0 for metamorphic precursor formation of Xindian eclogite indicates that the source material mainly derived from weak depleted mantle or mixing of depleted mantle with old crust.     

U–Pb dating and geochemistry of granite porphyry dykes in the Xicha gold–(silver) deposit,southern Jilin Province,China,and their metallogenic significance

We report U–Pb dating of zircon,as well as geochemical and Hf isotope data,in order to constrain the formation time,magma source,and tectonic setting of granite porphyry dykes in the Xicha gold–(silver) district in southern Jilin Province,Northeast China.The zircon grains are euhedral–subhedral,display oscillatory growth zoning and have Th/U ratios varying between 0.11 and 0.78,which together imply a magmatic origin.The dating results indicate the porphyry formed in the Early Cretaceous (122±1 Ma)and it contains SiO_2=70.64–72.31 wt%,Al_2O_(3-)=13.99–14.64 wt%,K_2O+Na_2O=6.96–7.81 wt%K_2O/Na_2O=1.24–2.10,and A/CNK=1.11–1.41.Chemically,the porphyry belongs to a high-K calc-alkaline S-type granite.Chondrite-normalized rare earth elements (REE)patterns show LREE enrichment,light rare earth elements(LREE)/heavy rare earth elements (HREE)=9.93–11.97(La/Yb)_N=11.08–15.16,and d Eu=0.69–0.95.On the trace element spider diagram,large ion lithophile elements such as Rb,Ba,K,Th,and U are enriched,whereas the high field strength elements Ti and P are depleted.The e Hf(t) values of zircon from the granite porphyry vary between-17.1 and-13.2,and their Hf two-stage model ages vary from 2.01 to 2.26 Ga,implying that the magma was derived from partial melting of old lower crust.The granite porphyry dykes and many A-type granites in the region formed at the same time,suggesting an extensiona environment.The combination of the occurrence of strong magmatism,large-scale mineralization,and extensiona tectonics throughout much of Eastern China indicate that the Early Cretaceous was a period of significant lithospheric thinning.The southern Jilin Province,therefore,experienced lithospheric thinning during the Early Cretaceous.     

Rare earth element and yttrium compositions of the Paleoproterozoic Yuanjiacun BIF in the Lüliang area and their implications for the Great Oxidation Event(GOE)

In China, most Precambrian banded iron formations(BIFs) are situated in the North China Craton. The Yuanjiacun iron deposit, located in the Lüliang area, is arguably the most representative Superior-type BIF. This iron deposit is coherent with the sedimentary rock succession of the Yuanjiacun Formation in the lower Lüliang Group, and was interpreted to be deposited at 2.3–2.1 Ga, based on ages of overlying and underlying volcanic strata. This age overlaps with the time range of the Great Oxidation Event(GOE, 2.4–2.2 Ga). The Yuanjiacun BIF consists mainly of subhedral-xenomorphic magnetite and quartz and rarely other minerals with a lower degree of metamorphism, from greenschist to lower amphibolite facies. The geochemical characteristics of this BIF are similar to those of Superior-type BIFs. Prominent positive La, Y, and Eu anomalies normalized by the Post Archean Australian Shale(PAAS) indicate that the primary chemical precipitate is a result of solutions that represent mixtures of seawater and high-T hydrothermal fluids. The contamination from crustal detritus found is negligible based on low abundances of Al2O3 and TiO2(0.5%) and of trace elements such as Th, Hf, Zr, and Sc(1.5 ppm), as well as the lack of co-variations between Al2O3 and TiO2. In particular, the Yuanjiacun BIF samples do not display significant negative Ce anomalies like those of the Archean iron formations, but rather, the Yuanjiacun BIF samples exhibit prominent positive Ce anomalies, low Y/Ho ratios, and high light to heavy REE((Pr/Yb)SN) ratios, which are essentially consistent with the late Paleoproterozoic(2.0 Ga) BIFs around the world. These characteristics of the Yuanjiacun BIF samples imply that the ancient ocean(2.3–2.1 Ga) was redox-stratified from oxic shallow water to deeper anoxic water. The specific redox conditions of the ancient ocean may be related to the GOE, which gave rise to the oxidation of Ce and Mn in the upper water, and to the presence of a Mn oxide shuttle in the ocean, resulting in varying REE patterns due to the precipitation and dissolution of this Mn oxide shuttle under different redox states. Therefore, the Yuanjiacun BIF appears to have formed near the redoxcline and lower-level reduced marine water.     

Recycling of Paleo-Asian Ocean carbonates and its influence on the lithospheric composition of the North China Craton

The deep carbon cycle, which plays a critical role in mantle evolution and Earth habitability, is closely linked to the recycling of carbonate-bearing rocks through subduction. Marine carbonates are subducted to different depths during the closure of oceanic basins, thus carry important signatures of the disappeared oceanic basins. Petrological and geochemical features of the Hannuoba carbonatites in the northern North China Craton indicate that they were formed by melting of limestone subducted to mantle depths. Here, we show that detrital zircons carried by these carbonatites have a broad spectrum of U-Pb ages from Precambrian to Phanerozoic. Precambrian age peaks are at ~2.5 Ga, 2.1–2.3 Ga, 1.8–2.0 Ga, ~1.65 Ga, 1.3–1.4 Ga, ~1.1 Ga,0.91–0.94 Ga, 0.74–0.81 Ga, and 0.62–0.63 Ga, respectively. The recorded age peaks are different from those in the northern North China Craton and thus preclude an origin of crustal contamination. Nevertheless, the Precambrian age spectra are compatible with those of the Xingmeng Orogen in the southeastern Central Asian Orogenic Belt. Furthermore, the significantly positive εHf(t) values of 7.7–13.5 for the 300–373 Ma zircons are similar to those in the Xingmeng Orogen but different from those in the northern North China Craton. All these features suggest that the limestone precursor for the Hannuoba carbonatites was originated from the Paleo-Asian Ocean, and its deposition time was not earlier than 300 Ma. This indicates that the PaleoAsian Ocean still existed in the late Carboniferous to early Permian. The widespread distribution of metamorphic carbonates in the Central Asian Orogenic Belt indicates that there may have been widespread sedimentary carbonates in the Paleo-Asian Ocean. A large amount of sedimentary carbonates was probably carried into mantle during subduction of the Paleo-Asian oceanic slab, which significantly modified the chemical and physical properties of the lithospheric mantle.     

Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane,eastern China

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz + Phe + Ap + impurities and Qtz + Phe + impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe + Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe + Ap and Coe + Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP metamorphic stage and overpr     

The genesis of Archean supracrustal rocks in the western Shandong Province of North China Craton: Constraints on regional crustal evolution

Archean greenstone belts are supracrustal sequences, the lower part of which is usually composed of voluminous ultramafic-mafic volcanics. Intermediate and acid volcanic rocks increase in abundance towards the upper domains. Greenstone belts constitute ~30% of the total volume of Archean cratons, and preserve significant information on the surface environment and magmatism in the early earth, which are useful in unraveling the nature of crustal formation and evolution. The western Shandong Province(WSP) is located at the eastern part of the North China Craton(NCC), where greenstone sequences formed at ~2.7 and ~2.5 Ga were well preserved. The early Neoarchean supracrustal rocks include komatiite-basalt sequence, some meta-sediments of the lower part of the Taishan Group and the Mengjiatun Formation. The volcanism had been correlated to mantle plume, which resulted in vertical crustal accretion. The late Neoarchean supracrustal rocks were composed of metamorphosed felsic volcano-sedimentary sequences and BIFs of the upper part of the Taishan Group and the Jining Group. The geochemical features of the meta-volcanics show calc-alkaline affinities, similar to modern arc-related magmatism, suggesting that the continental crust in the western Shandong Province witnessed horizontal plate movements at ~2.5 Ga. The metasediments and leucosomes in the Qixingtai area display regional upper amphibolite facies metamorphism and anatexis at 2.53–2.50 Ga, coeval with formation of large volumes of crustally-derived granites. These tectono-thermal events suggest that a unified continental crust was formed in the western Shandong Province at the end of Neoarchean.     

Detrital zircon U-Pb dating in the southern Hefei Basin: Evidence for exhumation of HP-UHP rocks of the Dabie Orogen

The Hefei Basin of eastern China developed in response to uplift of the Dabie Orogen, and zircon dating can be used to assess the exhumation history of the orogen. Zircons were collected from samples of the Lower Jurassic Fanghushan Formation and Middle Jurassic Sanjianpu Formation in the southern Hefei Basin, and mica-quartz schist and biotite granite gneiss from the Susong Complex of the Dabie Orogen. The zircon U-Pb dating was undertaken using laser ablation-inductively coupled plasma-mass spectrometry. The detrital zircons from conglomerates of the Fanghushan Formation and from clasts within the conglomerates have age-frequency distributions with the main clusters between 2.0 and 1.8 Ga, similar to age data of the Susong Complex. On the other hand, the zircons of the Fanghushan Formation do not show the age cluster at 1000–900 Ma that characterizes zircons in the underlying metasediments of the lower Paleozoic Foziling Group. A cluster of Triassic zircon ages also appears in the arkosic sandstones of the Fanghushan Formation. These data indicate that the provenance of the Fanghushan Formation was a mixture of high-pressure(HP) and ultrahigh-pressure(UHP) Triassic metamorphic rocks, Paleozoic magmatic rocks, and the Susong Complex, but not the lower Paleozoic Foziling Group even though it directly underlies the sediments of the Hefei Basin. Two samples from the Sanjianpu Formation show zircon age clusters at 797 and 791 Ma(middle Neoproterozoic)and 226 Ma(Triassic), and again, these are markedly different from the age clusters that characterize the Foziling Group. It seems, therefore, that despite the Foziling Group being at the surface in the underwater depositional area of the Hefei Basin, it was not exposed in the source area of the Hefei basinal sediments during the Jurassic, and there are two possible reasons for this.First, the exhumation of the Dabie Orogen was directed partly towards the north, in the process of which some of the Foziling Group was covered. Second, the Susong Complex rocks became involved in the development of an accretionary wedge, thus covering some of the Foziling Group during the process of subduction.     

SHRIMP zircon U-Pb dating in Jingshan "migmatitic granite", Bengbu and its geological significance

The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433-722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by t     

Zircon U-Pb geochronology,Hf isotopic composition,and geological implications of the Neoproterozoic meta-sedimentary rocks in Suizhou-Zaoyang area,the northern Yangtze Block

Suizhou-Zaoyang area is located in the southern Qinling-Tongbai-Dabie Orogen, China. A combined research on U-Pb ages and Lu-Hf isotopes for detrital zircons from three meta-sedimentary rocks in the Wudang Group is reported. The upper Wudang Group has a major age population of ca. 750 Ma and a sub-major of ca. 860 Ma. But the lower part only yields one age peak at ca. 2.03 Ga. In situ Lu-Hf analyses of the young age group of ca. 750 Ma for zircons from the upper Wudang Group yield an average εHf(t) value of ?8.6 and two-stage Hf model ages(TDM2) from 1837 to 2230 Ma, respectively. However, zircons from the lower Wudang Group give an average εHf(t) value of 4.5 and TDM1 of 2220±22 Ma, close to the timing of zircon crystallization. Thus, it is suggested that, in the study area, the continental crust grew during the middle Paleoproterozoic and reworked during the middle Neoproterozoic, which shows the affinity to the Dabie Orogen. In addition, in the lower Wudang Group, two metamorphic zircon ages of 1992±91 and 1999±61 Ma are consistent with that of the middle Paleoproterozoic metamorphism event in the northern Yangtze Block, which may represent the assemblage of the Columbia Supercontinent. On the basis of the U-Pb ages and Hf isotopes, it is proposed that Suizhou-Zaoyang area was involved in the subduction-collision event in the middle Paleoproterozoic and the Yangtze Block was one of the components of the Paleo-Mesoproterozoic supercontinent.     

High-resolution crustal velocity structure in the Shanxi rift zone and its tectonic implications

The Shanxi rift zone, located in the Trans-North China Orogen(TNCO) of the North China Craton(NCC), is wellknown for hosting large intraplate earthquakes in continental China. The TNCO is a suture zone formed by the amalgamation of the eastern and the western blocks of the NCC. After its formation, it was reactived and deformed by later tectonic activities,which result in complex lithospheric heterogeneities. Thus, the detailed crustal structure of the Shanxi rift zone is critical for understanding the tectonics and seismogenic mechanism in this area, which will shed new lights on the formation and dynamic evolution of the NCC. In this study, we applied ambient noise tomography based on 18 months continuous records from 108 seismic stations located in Shanxi and its surroundings, in order to constrain its detailed crustal structure. We measured 4437 Rayleigh wave phase velocity dispersion curves in the period of 5–45 s from the cross-correlation functions. Next, a surface wave direct inversion algorithm based on surface-wave ray tracing was used to resolve a 3-D S-wave velocity model in the upper 60 km with lateral resolution of ～50–80 km. The tomographic images show that the sedimentary thickness of the Taiyuan Basin is less than 5 km. At depth of 0–10 km, we observe a good correlation between the imaged structural variations with geological and topographic features at the surface. For example, the center of rift shows low-velocity anomalies and the uplifting areas on both sides are characterized by high velocity anomalies. The western and eastern boundaries of the slow materials coincide with the faults that control the basin. The slow material extends from the shallow surface to depth of about 15 km but it getting smaller in shape at deeper depth. For the Taiyuan Basin, Linfen Basin, and Yuncheng Basin in the central and southern parts, the structure is dominant by slow materials in the upper crust but changes to strong high-velocity anomalies in the lower crust and the uppermost mantle at depth deeper than 25 km. We interprete these high-velocity anomalies to be associated with the cold remnant of the underplated basalt in the lower crust that were formed in early Tertiary before the basin was stretched. We also observe the low-velocity anomaly beneath the Datong volcanic area, which extends from the uppermost mantle to a depth of20 km vertically and migrates from west to east laterally. It may reflect the upwelling channel of the magmatic material in Datong. Moreover, the strong low-velocity anomalies presented north of 38°N could be related to the heated crustal materials with paritial melting as a result of the intensive magmatic activities of the Datong Volcano since the Cenozoic. In our study region, seismicity mainly concentrates in the depth range of 5–20 km and we find that most earthquakes appear to occur in places where velocity changes from high to low rapidly, with slight higher concentration in the faster material areas. In summary, our high-resolution 3-D crustal velocity model provides important seismological constraints to understand the tectonic evolution and seismicity across the Shanxi rift zone.     

SHRIMP zircon U-Pb dating of late Paleoproterozoic kondalites in the Daqing Mountains area on the North China Craton

The Daqing Mountains area comprises a typical occurrence of the Khondalite Belt in the Western Block of the North China Craton (NCC). In this area, both early and late Paleoproterozoic metasedimentary rocks have been identified in what was originally called the Upper Wula Mountains "Subgroup". Six metasedimentary rock samples yielded SHRIMP U-Pb zircon ages of 2.56-2.04 Ga for detrital and 1.96-1.83 Ga for metamorphic zircons. Based on these data and previously published results, the following conclusions can be drawn: 1) The source region for the late Paleoproterozoic detrital sedimentary rocks is mainly 2.55 2.4 and 2.2 2.04 Ga in age, consistent with the early Precambrian geological history identified widely in the basement of the NCC. 2) The majority of sedimentary rocks of the khondalite series were deposited between 2.04 and 1.95 Ga, and then in a protracted period (1.96 and 1.83 Ga) underwent a complex history of amphibolite to granulite-facies metamorphism.     

Geochronology and geological significance of the strata of the Neoproterozoic Nanhua System,SW North China Craton

A set of low-grade clastic metamorphic and carbonate rocks, and greenschists outcropping in the southwestern(SW) margin of the North China Craton(NCC), was originally classified as the Paleoproterozoic Xiong’er Group according to stratigraphic correlation. To verify the age, this paper carried out detrital zircon U–Pb LA-ICP-MS dating of low-grade clastic metamorphic rocks exposed in the Changqing area at the SW margin of the Ordos Block in the SW part of the NCC. Results from detrital zircon da...     

Petrogenesis and tectonic implications of Late-Triassic high εNd(t)-εHf(t) granites in the Ailaoshan tectonic zone(SW China)

High εNd(t)-εHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan(SW China). Zircon grains separated from the two samples(10HH-119 A and 10HH-120A) yield the weighted mean 206Pb/238 U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive εHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2(72.66 wt%–73.70 wt%), low Mg#(0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic(229 Ma). Genesis of the Huashiban high εNd(t)-εHf(t) granites involved into two processes:(1) underplating of the sub-arc mantle into the lower crust, and(2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.     

Geochemistry and Zircon U–Pb age of the Yao'an pseudoleucite porphyry,Yunnan Province,China

The Yao'an Pb–Ag deposit, located in the Chuxiong Basin, western Yangtze Block, is an important component of the Jinshajiang–Ailaoshan alkaline porphyry–related polymetallic intrusive belt. This complex suite of rock bodies includes a vein of pseudoleucite porphyry within deposits of syenite porphyry and trachyte.The pseudoleucite is characterized by a variable greyish,greyish-white, and greyish-green porphyritic texture. Phenocrysts are mainly pseudoleucite with small amounts of alkali feldspar and biotite. In an intense event, leucite phenocrysts altered to orthoclase, kaolinite, and quartz.Both the pseudoleucite porphyry and the syenite porphyry samples were typical alkali-rich, K-rich, al-rich rocks with high LaN/YbNratios; enriched in light rare earth elements and large-ion lithophile elements, and depleted in high field strength elements; and with strongly negative Ta, Nb, and Ti(TNT) anomalies and slightly negative Eu anomalies—all characteristics of subduction-zone mantle-derived rock.We obtained a LA-ICP-MS zircon U–Pb age of 34.1 ± 0.3 Ma(MSWD = 2.4), which is younger than the established age of the Indian and Eurasian Plate collision.The magma derived from a Type-II enriched mantle formed in a post-collisional plate tectonic setting. The geochemical characteristics of the Yao'an pseudoleucite porphyry are powerful evidence that the porphyry'sdevelopment was closely linked to the Jinshajiang–Ailaoshan fault and to the Indian-Eurasian collision.     

Crustal accretion and reworking processes of micro-continental massifs within orogenic belt: A case study of the Erguna Massif,NE China

This paper summarizes the geochronological, geochemical and zircon Hf isotopic data for Mesozoic granitoids within the Erguna Massif, NE China, and discusses the spatial-temporal variation of zircon Hf isotopic compositions, with the aim of constraining the accretion and reworking processes of continental crust within the Erguna Massif, and shedding light on the crustal evolution of the eastern segment of the Central Asian Orogenic Belt. Based on the zircon U-Pb dating results, the Mesozoic granitic magmatisms within the Erguna Massif can be subdivided into five stages: Early-Middle Triassic(249–237 Ma), Late Triassic(229–201 Ma), Early-Middle Jurassic(199–171 Ma), Late Jurassic(155–149 Ma), and Early Cretaceous(145–125 Ma).The Triassic to Early-Middle Jurassic granitoids are mainly I-type granites and minor adakitic rocks, whereas the Late Jurassic to Early Cretaceous granitoids are mainly A-type granites. This change in magmatism is consistent with the southward subduction of the Mongol-Okhotsk oceanic plate and subsequent collision and crustal thickening, followed by post-collision extension. Zircon Hf isotopic data indicate that crustal accretion of the Erguna Massif occurred in the Mesoproterozoic and Neoproterozoic. ZirconεHf(t) values increase gradually over time, whereas two-stage model(TDM2) ages decrease throughout the Mesozoic. The latter result indicates a change in the source of granitic magmas from the melting of ancient crust to more juvenile crust. Zircon εHf(t)values also exhibit spatial variations, with values decreasing northwards, whereas TDM2 ages increase. This pattern suggests that,moving from south to north, there is an increasing component of ancient crustal material within the lower continental crust of the Erguna Massif. Even if at the same latitude, the zircon Hf isotopic compositions are also inconsistent. These results reveal lateral and vertical heterogeneities in the lower continental crust of the Erguna Massif during the Mesozoic, which we use as the basis of a structural and tectonic model for this region.     

Petrogenetic simulation of the Archean trondhjemite from Eastern Hebei,China

It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures(0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2 O, and lower CaO and Mg~# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.