Geochemistry, zircon U–Pb geochronology, and Hf isotopes of S-type granite in the Baoshan block, constraints on the age and evolution of the Proto-Tethys

Geochemistry, zircon U–Pb geochronology, and Hf isotope data for the Early Paleozoic granites in the Baoshan Block reveal the Early Paleozoic tectonic evolution of the Proto-Tethys. The samples are high-K, calcalkaline, strongly peraluminous rocks with A/CNK values of 1.37–1.46, are enriched in SiO2, K2O, and Rb, and are depleted in Nb, P, Ti, Eu, and heavy rare earth elements,which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed in ca. 480 Ma. The Nansa granites have εHf(t) values ranging from-16.04 to 4.36 with corresponding TC DMages of 2.10–0.81 Ga, which suggests the magmas derived from the partial melting of ancient metasedimentary with minor involvement of mantle-derived components. A synthesis of data for the Early Paleozoic igneous rocks in the Baoshan block and adjacent(Tengchong,Qiangtang, Sibumasu, Himalaya, etc.) blocks indicates that these blocks were all aligned along the proto-Tethyan margin of East Gondwana in the Early Paleozoic. The Early Paleozoic S-type granites from Nansa were generated in a high-temperature and low-pressure(HTLP) extensional tectonic setting, which resulted from Andean-type orogeny instead of the final assembly of Gondwana or crustal extension in a non-arc environment. In certain places, an expanding environment may exist in opposition to the tectonic backdrop of the lithosphere’s thickening and shortening, leading the crust to melt and decompress,mantle-derived materials to mix, and a small quantity of peraluminous granite to emerge.

Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt:New Constraints from U-Pb Geochronology and Sr-Nd-Hf Isotopes

The Nianzi granite unit,which includes the Nianzi,Xiaolianghou and Xiawopu granitic intrusions,is a significant component of the northern part of the North China Craton(NCC)and is situated in the Yanshan fold and thrust belt(YFTB).However,there is still debate regarding the tectonic evolutionary history of the YFTB during the late Permian to Triassic period,specifically regarding the timing of subduction and collision between the NCC and the Paleo-Asian Ocean.The Nianzi granite unit exhibits unique petrological,geochronological and geochemical signatures that shed light on the tectonic evolutionary history of the YFTB.This study presents detailed petrology,whole-rock geochemistry,together with Sr-Nd isotopic,zircon U-Pb dating and Lu-Hf isotopic data of the granites within the Nianzi granite unit.Our findings demonstrate that the granites primarily consist of subhedral K-feldspar,plagioclase,quartz,minor biotite and hornblende,with accessory titanite,apatite,magnetite and zircon.Zircon U-Pb dating indicates that the Xiaolianghou granite was emplaced at 247.5±0.62 Ma.Additionally,the adakitic characteristics of the Nianzi,Xiawopu and Xiaolianghou granitic intrusions,such as high Sr and Ba contents and high ratios of Sr/Y and(La/Yb)N,combined with negative Sr-Nd and Lu-Hf isotopes(87Sr/86Sr)i=0.705681–0.7057433,εNd(t)=−21.98 to−20.97,zirconεHf(t)=−20.26 to−9.92,as well as the I-type granite features of high SiO_(2),Na_(2)O and K_(2)O/Na_(2)O ratios,enriched Rb,K,Sr and Ba,along with depleted Th,U,Nb,Ta,P and Ti,suggest that the Nianzi granitic unit was mainly derived from the partial melting of a thickened lower crust containing hydrous,calc-alkaline to high-K calc-alkaline,mafic to intermediate metamorphic rocks.In light of these parameters,we further integrate our data with previous studies and conclude that the Nianzi granitic unit was generated in a post-collisional extensional environment during the Early Triassic.

Zircon SHRIMP U–Pb geochronology,geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province,China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed.Herein,the middle-acid volcanic rock,dacite,exposed to the Zhesang gold district,southeast Yunnan Province,has been analyzed.The results show that the dacite belongs to a calc-alkaline series,SiO_(2)contents range from 62.79 to 76.66 wt%.Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic(247.8±1.7 Ma,MSWD=1.2).All samples exhibit enrichment in LILE(e.g.Rb,K,Th,and U),and depletion in HFSE(e.g.Nb,Ta,and Ti),which has the geochemical affinity of I-type granite.La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting,mainly of the mafic lower crust,of rocks,plays a major role in the formation process.The dacite has low initial ^(87)Sr/^(86)Sr ratios(0.706954 to 0.708589)and negative ε_(Nd)(t)values(-11.77 to-10.88).Zircons in dacite have ε_(Hf)(t)values of-16.2 to-8.3,and the two-stage Hf model ages are 1799–2301 Ma,mostly concentrated between 1800 and 1900 Ma,indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials,and crystal fractionation process underwent in the late stage of magma migration.This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam.

Zircon U-Pb geochronology,geochemistry and tectonic implication of volcanic rocks from Manketouebo Formation in Keyihe area of northern Great Xing'an Range

Zircon U-Pb isotope dating and whole-rock geochemical analyses were undertaken for the rhyolite,rhyolitic lithic crystal tuff and dacitic tuff from the Manketouebo Formation in the Keyihe area,in order to constrain their genesis and tectonic significance.Zircon LA-ICP-MS U-Pb data indicate that the rhyolite and rhyolitic lithic crystal tuff were formed during 137±5 Ma and 143±1 Ma,respectively.These volcanic rocks have high SiO2(70.03%–76.46%)and K2O+Na2O(8.10%–9.52%)contents,but low CaO(0.03%–0.95%)and MgO(0.07%–0.67%)contents,which belong to the peraluminous and high-K calc-alkaline rocks.They are enriched in light rare earth elements(REEs),and exhibit fractionation of light over heavy REEs,withδEu values of 0.37–0.83.The volcanic rocks are enriched in LILEs(e.g.,Rb,U and K)and depleted in HFSEs(e.g.,Nb,Ti,P and Ta).The chemical composition suggests that these volcanic rocks formed by partial melting of crust material.Combined with previous regional research results,the authors consider that the volcanic rocks of the Manketouebo Formation in the Keyihe area were formed under an extensional environment related to the closure of the Mongolia–Okhotsk Ocean.

Paleo-to Mesoproterozoic Tectono-Magmatic Events Recorded in the Huwan Complex from the Dabie Orogen,Central China:Evidence from Petrology and U-Pb Geochronology

To better understand the Paleo-to Mesoproterozoic tectonic evolution of the Dabie Orogen in the northern margin of Yangtze Block,we present geochronological data for metasedimentary and metavolcanic rocks in the Huwan complex.A total of 385 detrital zircon LA-ICP-MS analyses for metasedimentary rocks yielded three^(207)Pb/^(206)Pb age populations:1.50-1.80 Ga,1.81-1.87 Ga and 1.93-2.0 Ga,providing a maximum depositional timing of ca.1.50 Ga;while metafelsic volcanic gneisses yielded protolith U-Pb ages of 1893±54 Ma.The peak ages are remarkably consistent with the tectonothermal events that occurred in the northern Yangtze Block,indicating the presence of Paleo-to Mesoproterozoic magmatism in the Dabie Orogen.The age range of 1.93-2.0 Ga correlates with the Paleoproterozoic collision;the age range of 1.81-1.87 Ga coincides with the period of the post-orogenic extension;and the age range of 1.50-1.80 Ga is interpreted to associate with an extensional regime.Zircon cores with age of 1732-1965 Ma haveε_(Hf)(t)values ranging from-11.70 to-2.47,indicating that juvenile crust involved in their magma sources.Owing to the similar age spectra,we proposed that the nucleus of the Dabie Orogen was close to the Yangtze Block since the Paleoproterozoic.The Huwan complex has an intimate affiliation to the Yangtze Block,and implies multiple orogenic cycles.It was not only experienced the Paleo-Tethys ocean subduction and collision,but also recorded Paleo-to Mesoproterozoic tectono-magmatic events in the Dabie Orogen.

Strategies towards statistically robust interpretations of in situ U-Pb zircon geochronology

Zircon U-Pb geochronology has become a keystone tool across Earth science, arguably providing the gold standard in resolving deep geological time. The development of rapid in situ analysis of zircon （via laser ablation and secondary ionization mass spectrometry） has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable （e.g. detrital zircons as a sedi- ment fingerprinting method）. The ease of acquisition, while bringing benefit to the Earth science com- munity, has also led to diverse interpretations of geochronological data. In this work we seek to refocus U -Pb zircon geochronology toward best practice by providing a robust statistically coherent workflow. We discuss a range of data filtering approaches and their inherent limitations （e.g. discordance and the reduced chi-squared; MSWD）. We evaluate appropriate mechanisms to calculate the most geologically appropriate age from both 238U/206pb and 207pb/206pb ratios and demonstrate the cross over position when chronometric power swaps between these ratios. As our in situ analytical techniques become progressively more precise, appropriate statistical handing of U-Pb datasets will become increasingly pertinent.

Neoproterozoic Volcanic Rocks in the Southern Quruqtagh of Northwest China:Geochemistry,Zircon Geochronology and Tectonic Implications

The Early Neoproterozoic Beiyixi Formation volcanic rocks of the southern Quruqtagh comprise mainly of a suite of tholeiitic basalts, alkaline andesites, and calc-alkaline rhyolites. The rhyolites are characterized by variably fractionated enrichment in light rare earth elements （LREE） and fiat in heavy rare earth elements （HREE）, and strongly negative Eu anomalies. Compared to the rhyolites, the andesites also exhibit enrichment in LREE and flat HREE （chondrite-normalized values of La/Yb,and La/Sm are 13.30-41.09, 3.18-6.89 respectively）. Their rare earth element patterns display minor negative Eu anomalies. Both of them exhibit coherent patterns with strongly to moderately negative anomalies of Nb, Zr, Ti, and Hf on spider diagrams. Two rhyolite and one andesite magmatic zircons with defined oscillatory zoning yielded weighted mean 206pb/23Su ages of 743 ± 7 Ma, 741±2 Ma, and 7274 Ma. These ages are interpreted to represent the timing of volcanic eruptions. According to geochemistry and rock type, these volcanic rocks formed within a continental island-arc environment following subduction of the oceanic crust during the Early Neoproterozoic period.

Detrital-zircon geochronology of the Jurassic coal-bearing strata in the western Ordos Basin, North China: Evidences for multi-cycle sedimentation

The western Ordos Basin(WOB), situated in a tectonic transition zone in the North China Craton, acts as an excellent example for studying the Mesozoic intraplate sedimentation and deformation in Asia. In this study, U-Pb ages for 1203 detrital zircons of 14 sandstone samples collected from 11 sections are presented to unravel the sediment source locations and paleogeographic environments of the Early-Middle Jurassic coal-bearing Yan'an Formation in the WOB. Data show that there are five prominent age groups in the detrital zircons of the Yan'an Formation, peaking at ca. 282 Ma, 426 Ma, 924 Ma, 1847 Ma, and2468 Ma. Samples from the northern, middle, and southern parts of the WOB contain these five age categories in various proportions. In the northern region, the Yan'an Formation exclusively contains Early Permian detrital zircons with a single age group peaking at 282 Ma, matching well with the crystallizing ages of the widespread Early Permian granites in the Yinshan Belt to the north and the Alxa Block to the northwest. While in the southern region, the Yan'an Formation mainly contains three groups of detrital zircons, with age peaks at 213 Ma, 426 Ma, and 924 Ma. These zircon ages resemble those of the igneous rocks in the Qilian-Qinling Orogenic Belt to the south-southwest. Samples in the middle region, characterized by a mixture age spectrum with peaks at 282 Ma, 426 Ma, 924 Ma, 1847 Ma and 2468 Ma, are previously thought to have mixed derivations from surrounding ranges. However, by referring to the detrital-zircon age compositions of the pre-Jurassic sedimentary successions and combining with paleontological and petrographic analysis, we firstly propose that the sediments of the Yan'an Formation in the middle region were partly recycled from the Triassic and Paleozoic sedimentary strata in the WOB.The occurrence of recycled sedimentation suggests that the Late Triassic-Early Jurassic intraplate compressional deformation was very intense in the WOB, especially for regions in front of the Qilian Orogenic Belt.

LA-ICP-MS Zircon U-Pb Geochronology of the Fine-grained Granite and Molybdenite Re-Os Dating in the Wurinitu Molybdenum Deposit,Inner Mongolia,China

The Wurinitu molybdenum deposit,located in Honggor,Sonid Left Banner of Inner Mongolia,China,is recently discovered and is considered to be associated with a concealed fine-grained granite impregnated with molybdenite.The wall rocks are composed of Variscan porphyritic-like biotite granite and the Lower Ordovician Wubin'aobao Formation.LA-ICP-MS zircon U-Pb dating of the fine-grained granite reveals two stages of zircons,one were formed at 181.7±7.4 Ma and the other at 133.6±3.3 Ma.The latter age is believed to be the formation age of the fine-grained granite,while the former may reflect the age of inherited zircons,based on the morphological study of the zircon and regional geological setting.The Re-Os model age of molybdenite is 142.2±2.5 Ma,which is older than the diagenetic age of the fine-grained granite.Therefore the authors believe that the metallogenic age of the Wurinitu molybdenum deposit should be nearly 133.6±3.3 Ma or slightly later,i.e.,Early Cretaceous.Combined with regional geological background research,it is speculated that the molybdenum deposits were formed at the late Yanshanian orogenic cycle in the Hingganling-Mongolian orogenic belt,belonging to the relaxation epoch posterior to the compression and was associated with the closure of the Mongolia-Okhotsk Sea.

U-Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling

LA-ICPMS U-Pb zircon dating of the Sanpinggou, Gangou and Fengzishan granitoids in the Douling Group of the Eastern Qinling yields ages of 760-685 Ma, which represents a strong tectono-magmatic event in the southern Qinling during the late Neoproterozoic. Geochemical data show that these intrusions have wide compositions ranging from minor gabbros through diorites to granodiorites. They are relatively enriched in LILE, poor in HFSE and strongly depleted in Nb and Ta, displaying affinities of Ⅰ-type granites formed in an active continental margin with oceanic subduction. In contrast to granitoids, gabbros and enclaves in the granitoids have higher REE abundances, relatively flat REE patterns, lower LILE, slightly higher HFSE and more depletion in Nb and Ta. All these suggest that the gabbros were formed by partial melting of the upper mantle above the subduction zone and the granitoids by the partial melting of the lower crust. Combined with regional geological data, the subduction-related granitoids in the Douling Group, together with the Tuwushan A-type granite with an age of 725 Ma and contemporaneous basic dikes in the Wudang Block, provide evidence for local subduction of oceanic basins between different blocks during the rifting in the Southern Qinling in the Neoproterozoic. Thus, the coexistences of various magmatic rocks formed in different tectonic environments indicate a complicated tectonic evolution and variety of tectonic frameworks in the Qinling area in the Neoproterozoic.

LA-ICPMS Zircon U-Pb Dating for Three Indosinian Granitic Plutons from Central Hunan and Western Guangdong Provinces and Its Petrogenetic Implications

The LA-ICPMS zircon U-Pb geochronology of three typically Indosinian granitic plutons yielded weighted mean ^206pb/^238U ages of 214.1±5.9 Ma and 210.3±4.7 Ma for the biotite monzonitic granites from the Xiema and Xiangzikou plutons in Hunan Province, and 205.3±1.6 Ma for biotite granite from the Napeng pluton, western Guandong Province, respectively, showing a similar late Indosinian age of crystallization. In combination with other geochronological data from Indosinian granites within the South China Block （SCB）, it is proposed that those late Indosinian granites with an age of -210 Ma and the early Indosinian granites （230-245 Ma） have the similar petrogenesis in identical tectonic setting. The Indosinian granites within the SCB might be the products of anatexis of the thickening crust in a compressive regime. These data provide a further understanding for the temporal and spatial distribution of the Indosinian granites and the dynamic evolution of the SCB.

Laser Ablation ICP-MS U-Pb Zircon Geochronology of Granitoids and Quartz Veins in the Shihu Gold Mine, Taihang Orogen, North China: Timing of Gold-mineralization and Tectonic Implications

The Shihu gold deposit, situated in the Taihang Mesozoic orogen of the North China Craton （NCC）, is hosted by ductile-brittle faults within Archean metamorphic core complex. The deposit is characterized by gold-bearing quartz-polymetallic sulfides veins. The Mapeng granitoids stock and intermediate-basic dikes intruded the metamorphic basement rocks, and are spatially related to gold mineralization. Detailed laser ablation inductively coupled plasma mass spectrometry （LA ICP-MS） U-Pb zircon ages of the granitic rocks, dykes and mineralized quartz veins in the studied area reveal its magmatic and mineralized history. The mineralized quartz veins contain inherited zircons with ages of about 2.55 Ga and 1.84 Ga, probably coming from the basement. These two Precambrian events are coeval with those in other parts of the NCC. The Mapeng granitoid stock, the largest intrusion in the area, was emplaced at ca. 130 Ma, and is coeval with magmatic zircon populations from diorites and quartz diorite pophyrites in the same region. The ca. 130 Ma magmatism and gold mineralization were most likely related to an underplating event that took place in the Taihang orogen at Late Mesozoic. The timing of gold mineralization with respect to felsic magmatism in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern margin of NCC, indicative of a widespread late Yanshanian metallogenic event that was a response to the Early Cretaceous lithosphere in the eastern NCC, in which the mesothermal gold deposits were formed from similar tectono-magmatic environments.

Detrital zircon geochronology of the Lutzow-Holm Complex,East Antarctica:Implications for Antarctica-Sri Lanka correlation

The Lützow-Holm Complex(LHC) of East Antarctica has been regarded as a collage of Neoarchean(ca.2.5 Ga), Paleoproterozoic(ca. 1.8 Ga), and Neoproterozoic(ca. 1.0 Ga) magmatic arcs which were amalgamated through the latest Neoproterozoic collisional events during the assembly of Gondwana supercontinent. Here, we report new geochronological data on detrital zircons in metasediments associated with the magmatic rocks from the LHC, and compare the age spectra with those in the adjacent terranes for evaluating the tectonic correlation of East Antarctica and Sri Lanka. Cores of detrital zircon grains with high Th/U ratio in eight metasediment samples can be subdivided into two dominant groups:(1) late Meso-to Neoproterozoic(1.1-0.63 Ga) zircons from the northeastern part of the LHC in Prince Olav Coast and northern Soya Coast areas, and(2) dominantly Neoarchean to Paleoproterozoic(2.8-2.4 Ga) zircons from the southwestern part of the LHC in southern Lutzow-Holm Bay area. The ca.1.0 Ga and ca. 2.5 Ga magmatic suites in the LHC could be proximal provenances of the detrital zircons in the northeastern and southwestern LHC, respectively. Subordinate middle to late Mesoproterozoic(1.3-1.2 Ga) detrital zircons obtained from Akarui Point and Langhovde could have been derived from adjacent Gondwana fragments(e.g., Rayner Complex, Eastern Ghats Belt). Meso-to Neoproterozoic domains such as Vijayan and Wanni Complexes of Sri Lanka, the southern Madurai Block of southern India, and the central-western Madagascar could be alternative distal sources of the late Meso-to Neoproterozoic zircons. Paleo-to Mesoarchean domains in India, Africa, and Antarctica might also be distal sources for the minor ~2.8 Ga detrital zircons from Skallevikshalsen. The detrital zircons from the Highland Complex of Sri Lanka show similar Neoarchean to Paleoproterozoic(ca. 2.5 Ga) and Neoproterozoic(ca. 1.0 Ga) ages, which are comparable with those of the LHC, suggesting that the two complexes might have formed under similar tectonic regimes. We consider that the Highland Complex and metasedimentary unit of the LHC formed a unified latest Neoproterozoic suture zone with a large block of northern LH-Vijayan Complex caught up as remnant of the ca. 1.0 Ga magmatic arc.

Proto-Tethys ophiolitic mélange in SW Yunnan: Constraints from zircon U-Pb geochronology and geochemistry

An early Paleozoic Proto-Tethys ocean in western Yunnan has long been postulated although no robust geological evidence has been identified.Here we investigated the recently-identified Mayidui and Wanhe ophiolitic mélanges in SW Yunnan,which occurs in a N-S trending belt east of the late Paleozoic Changning-Menglian suture zone.The ophiolites consist mainly of meta-basalts(amphibole schists),meta-(cumulate)gabbros and gabbroic diorites,and meta-chert-shale,representing ancient oceanic crust and pelagic and hemipelagic sediments,respectively.Six samples of gabbros and gabbroic diorites from 3 profiles(Mayidui,Kongjiao and Yinchanghe)yielded zircon U-Pb ages between 462±6 Ma and 447±9 Ma,constraining the formation of the Mayidui and Wanhe ophiolites to Middle Ordovician.Gabbros from the Mayidui and Kongjiao profiles share similar geochemical characteristics with affinities to tholeiitic series,and are characterized by depleted to slightly enriched LREEs relative to HREEs with(La/Sm)N=0.69-1.87,(La/Yb)N=0.66-4.72.These,along with their predominantly positive wholerock eNd(t)and zircon eHf(t)values,indicate a MORB-like magma source.By contrast,the meta-mafic rocks from the Yinchanghe profile show significantly enriched LREEs((La/Sm)N=0.97-3.33,(La/Yb)N=1.19-14.93),as well as positive whole-rock eNd(t)and positive to negative zircon eHf(t)values,indicating an E-MORB-type mantle source.These geochemical features are consistent with an intra-oceanic setting for the formation of the Mayidui-Wanhe ophiolites.Our data,integrated with available geological evidence,provide robust constraints on the timing and nature of the Mayidui-Wanhe ophiolitic mélange,and suggest that the ophiolites represent remnants of the Proto-Tethys Ocean,which opened through separation of the Indochina and Simao blocks from the northern margin of Gondwana before the Early Cambrian,and evolved through to the Silurian.

Zircon Geochronology and Trace Element Geochemistry from the Xiaozhen Copper Deposit, North Daba Mountain: Constraints on Albitites Petrogenesis

Albitite often accompanies with various metal and gem mineral deposits and a large number of occurrences have been reported globally, including the South Qinling orogen, China. The Xiaozhen copper deposit is a typical deposit in the North Daba Mountain area of the South Qinling orogen whose distribution is controlled by albitite veins and fractures. As there are few studies on the petrogenesis of albitite in Xiaozhen copper deposit, this paper focuses on the petrogenesis of albitite and its mineralization age. Detailed fieldwork and mineral microscopic observations initially suggest that albitite from the Xiaozhen copper deposit is igneous in origin. Further zircon trace element geochemistry studies indicate that these zircons have high Th/U ratios（〉0.5）, low La content, high（Sm/La）N and Ce/Ce＊values, and a strong negative Eu anomaly, which are commonly seen in magmatic zircons. The chondrite–normalized rare earth element（REE） patterns are consistent with magmatic zircons from throughout the world, and they fall within or near the field of magmatic zircons on discriminant diagrams. The calculated average apparent Ti–in–zircon temperature for young zircons is 780°C, consistent with magmatic zircon crystallization temperatures. Therefore, zircon geochemistry indicates that the albitite origin is magmatic. SIMS U–Pb dating on nine magmatic zircons yielded a concordia age of 154.8±2.2 Ma, which represents the formation of albitite and the metallogenic age. More importantly, it is consistent with the ages of Yanshanian magmatism and metallogenesis in the South Qinling orogen, so formation of the Xiaozhen copper deposit may be a closely related geological event.

LA-ICPMS Zircon U-Pb Dating in the Jurassic Daohugou Beds and Correlative Strata in Ningcheng of Inner Mongolia

LA-ICPMS Zircon U-Pb dating is applied to volcanic rocks overlying and underlying the Salamander-bearing bed in the Daohugou beds of Ningcheng in Inner Mongola and Reshuichang of Lingyuan and Mazhangzi of Jianping in western Liaoning. The results indicate that the youngest age of the rocks in Daohugou of Ningcheng is 158 Ma, and the oldest one is 164 Ma. Synthesized researches indicate that the salamander-bearing beds in Daohugou of Ningcheng, Reshuichang of Lingyuan and Mazhangzi of Jianping were developed in the same period. The Daohugou beds were formed in the geological age of 164-158 Ma of the middle-late Jurassic. Whilst, the Daohugou beds and its corrdative strata should correspond to the Tiaojishan Formation （or Lanqi Formation） of the middle Jurassic in northern Hebei Province and western Liaoning Province, based on the disconformity between the Daohugou beds and its overlaying beds of the Tuchengzi Formation of Late Jurassic and the Jehol Beds of early Cretaceous, and the disconformity between the Daohugou Beds and its underlying Jiulongshan Formation, which is composed of conglomerate, sandstone, shale with coal and thin coal beds.

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block,NE Scotland:Implications for the Grampian Orogeny

The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny.

U-Pb Zircon and Re-Os Molybdenite Geochronology of the W-Mo Mineralized Region of South Qinling, China, and their Tectonic Implications

A W-Mo mineralized region is located along the northern margin of the South Qinling tectonic belt of China. WMo mineralization occurs mainly in Cambrian–Ordovician clastic and carbonate rocks, and the ore bodies are structurally controlled by NW–SE-and NNE–SSW-striking faults. Evidence for magmatism in the area is widespread and is dominated by intermediate–felsic intrusives or apophyses, such as the Dongjiangkou, Yanzhiba, Lanbandeng, and Sihaiping granitic bodies. Quartz-vein-type mineralization and fault-controlled skarn-type mineralization dominate the ore systems, with additional enrichment in residual deposits. At present, there are few or insufficient studies on(1) the age of mineralization,(2) the relationship between intermediate–felsic granite and W-Mo mineralization,(3) the source of ore-forming materials, and(4) the metallogenic and tectonic setting of the mineralized area. In this paper, we present geochronology results for numerous intrusive granitic bodies in the South Qinling tectonic belt. U-Pb zircon geochronology of the Lanbandeng monzogranite and Wangjiaping biotite monzogranite yields ages of 222.7 ± 2.3 and 201.9 ± 1.8 Ma, respectively. In contrast to the Late Triassic age of the Lanbandeng monzogranite, the age of the newly discovered Wangjiaping biotite monzogranite places it at the Triassic–Jurassic boundary. Re-Os molybdenite geochronology on the Qipangou W-Mo deposit yielded a model age of 199.7 ± 3.9 Ma, indicating the deposit formed in the early Yanshanian period of the Early Jurassic. Granitoid intrusions in the mineralized area are characterized by composite granite bodies that crystallized at ca. 240–190 Ma. While there were multiple stages of intrusion, most occurred at 210–220 Ma, with waning magmatic activity at 200–190 Ma. The Re-Os age of molybdenite in the region is ca. 200–190 Ma, which may represent a newly discovered period of W-Mo metallogenesis that occurred during the final stages of magmatism. The heat associated with this magmatism drove ore formation and might have provided additional ore-forming components for metallogenesis(represented by the Wangjiaping biotite monzogranite). Ore materials in the mineralized area were derived from mixed crustal and mantle sources. Enrichment of the region occurred during intracontinental orogenesis in the late Indosinian–Yanshanian, subsequent to the main Indosinian collision. At this time, the tectonic environment was dominated by extension and strike-slip motion.

Morphology,Chemistry and U-Pb Geochronology of Zircon Grains in Quartz Monzodiorite from the Sunzhuang Area,Fanshi County,Shanxi Province

The morphology, REE geochemistry and U-Pb geochronology of zircons from quartz monzodiorite in the Sunzhuang area, Fanshi County, Shanxi Province are presented in this study. The zircon crystals can be classified into four main types as： AB, L, S and P, and 24 subtypes such as AB4, ABs, Ls, and S3. The maximum crystallization temperature of zircon was estimated as 850℃, with the minimum of 550℃. The peak temperatures of the zircon crystallization range from 650℃ to 700℃. The abundances of Th and U in the zircon grains show large variation with the Th/U values 〉 0.4. The Th and U values also show a positive correlation in most zircons. The REE abundance of zircon in the quartz monzodiorite ranges from 280.4 ppm to 2143 ppm with an average of 856.4 ppm. The chondrite normalized zircon REE patterns show two types, one is characterized by HREE enrichment and LREE depletion with positive Ce-anomaly and negative Eu-anomaly whereas the other is HREE enriched and LREE depleted with negative Eu-anomaly but without positive Ce-anomaly, and relatively flat patterns. The LA-ICP-MS U-Pb geochronology on the zircons yields a mean age of 133-0.87 Ma. Our data on zircon morphology, composition and U-Pb geochronology reveal that the parent magma of the quartz monzodiorite which was emplaced during late Yanshanian had a mixed crust-mantle source, with crustal components dominating. The magma is inferred to have been water rich and alkaline with initial high oxygen fugacity. Post-magmatic hydrothermal activity occurred under relatively reducing conditions which was conductive for gold precipitation in the Yixingzhai gold deposit.

Integration of zircon and apatite U–Pb geochronology and geochemical mapping of the Wude basalts(Emeishan large igneous province):A tool for a better understanding of the tectonothermal and geodynamic evolution of the Emeishan LIP

Radiogenic isotopic dating and Lu–Hf isotopic composition using laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS)of the Wude basalt in Yunnan province from the Emeishan large igneous province(ELIP)yielded timing of formation and post-eruption tectonothermal event.Holistic lithogeochemistry and elements mapping of basaltic rocks were further reevaluated to provide insights into crustal contamination and formation of the ELIP.A zircon U–Pb age of 251.3±2.0 Ma of the Wude basalt recorded the youngest volcanic eruption event and was consistent with the age span of 251-263 Ma for the emplacement of the ELIP.Such zircons hadε_(Hf)(t)values ranging from7.3 to+2.2,identical to those of magmatic zircons from the intrusive rocks of the ELIP,suggesting that crust-mantle interaction occurred during magmatic emplacement,or crust-mantle mixing existed in the deep source region prior to deep melting.The apatite U–Pb age at 53.6±3.4 Ma recorded an early Eocene magmatic superimposition of a regional tectonothermal event,corresponding to the Indian–Eurasian plate collision.Negative Nb,Ta,Ti and P anomalies of the Emeishan basalt may reflect crustal contamination.The uneven Nb/La and Th/Ta values distribution throughout the ELIP supported a mantle plume model origin.Therefore,the ELIP was formed as a result of a mantle plume which was later superimposed by a regional tectonothermal event attributed to the Indian–Eurasian plate collision during early Eocene.