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.

SHRIMP zircon U-Pb age and O isotopic analysis of the dunite from Kudi ophiolite in the West Kunlun,China

1.Objective The West Kunlun in Xinjiang is located on the northwestern margin of the Qinghai-Tibet Plateau(Fig.1a)and at the junction of the Paleo-Asian tectonic domain and the Tethys tectonic domain.It serves as an important area for the study on the geologic evolution of the Karakorum-West Kunlun due to its special tectonic position.

Report of 2.7 Ga zircon U-Pb age of orthogneiss in the Wenquan metamorphic complex,West Tianshan,China

1.Objective The Wenquan Group is one of the important medium-to low-grade metamorphic units within the Wenquan metamorphic complexes which distributes in Chinese segment of the Tianshan Belt,the southern part of the Central Asian Orogenic Belt.It mainly consists of pre-Neoproterozoic metamorphosed volcanic and sedimentary rocks(Wang B et al.,2014).

Zircon U-Pb geochronology,Hf isotopes,and geochemistry constraints on the age and tectonic affinity of the basement granitoids from the Qiongdongnan Basin,northern South China Sea

Studies in the northern South China Sea(SCS)basement remain important for understanding the evolution of the Southeast Asian continental margin.Due to a thick cover of sediments and scarce borehole penetration,little is known about the age and tectonic affinity of this basement.In this study,an integrated study of zircon U-Pb geochronology,Hf isotopes,and whole-rock major and trace elements on seven basement granitoids from seven boreholes of Qiongdongnan Basin has been carried out.New zircon U-Pb results for these granitoids present middle-late Permian((270.0±1.2)Ma;(253±3.4)Ma),middle to late Triassic((246.2±3.4)Ma;(239.3±0.96)Ma;(237.9±0.99)Ma;(228.9±1.0)Ma)and Late Cretaceous ages((120.6±0.6)Ma).New data from this study,in combination with the previous dataset,indicates that granitoid ages in northern SCS basement vary from 270 Ma to 70.5 Ma,with three age groups of 270–196 Ma,162–142 Ma,and 137–71 Ma,respectively.Except for the late Paleozoic-Mesozoic rocks in the basement of the northern SCS,a few old zircon grains with the age of(2708.1±17)Ma to(2166.6±19)Ma provide clues to the existence of the pre-Proterozoic components.The geochemical signatures indicate that the middle Permian-early Cretaceous granitoids from the Qiongdongnan Basin are I-type granites formed in a volcanic arc environment,which were probably related to the subduction of the Paleo-Pacific Plate.

Discovery of~2.5 Ga zircon U-Pb age from the diabase,Ar Horqin Banner Area,Inner Mongolia:Implications for Neoarchean crystalline basement of Bainaimiao Arc Belt

1.Objective The Bainaimiao Arc Belt(BAB),one of the most important units of the southeastern Central Asian Orogenic Belt(CAOB),locates along the northern margin of the North China Craton(NCC).There is a lack of understanding on the existence,scale,and affinity of the crystalline basement of the BAB.Based on the recent geochronologic studies.

Latest Zircon U-Pb Geochronology of the Qingshan Group Volcanic rocks along the Tan-Lu Fault Zone of Shandong Province, Eastern China

Objective Shandong Province is divided into two parts by the Tan -Lu fault zone： the western part （Luxi） and the eastern part （Jiaodong）. Large-scale volcanic activity occurred during the Late Mesozoic in Shandong Province, eastern China （Fig. lb）, and was controlled by the Tan-Lu fault zone and its secondary faults. Mesozoic volcanic rocks in Shandong Province mainly occur within the Cretaceous Qingshan group, overlying the Laiyang group and underlying the Wangsi group. The Qingshan group has been divided into four volcanic cycles, i.e., the Houkuang, Bamudi, Shiqianzhuang and Fanggezhuang formations from the oldest to the youngest. Although geochronology data indicate the volcanic activity occurred during the Early Cretaceous, the starting time and duration of volcanic activity are still equivocal. Two zircon U-Pb ages of volcanic rocks from strata at the lower base of the volcanic sequence along the Tan-Lu fault zone were reported in this paper, which provide new evidence for the discussion of the geological age.

Mesozoic Bimodal Volcanic Suite in Zhalantun of the Da Hinggan Range and Its Geological Significance: Zircon U-Pb Age and Hf Isotopic Constraints

Mesozoic bimodal volcanic rocks of basaltic andesite and rhyolite are widely distributed in the Da Hinggan Range, but their petrogenetic relationships and geodynamic implications are rarely constrained. Detailed studies on doleritic and porphyry dikes in the Zhalantun area indicate that they display features of magma mixing, suggesting their coeval formation. In situ zircon U-Pb dating shows that the porphyry was emplaced in the Early Cretaceous with a ^206Pb/^238U age of 130±1 Ma. Zircons from the dolerite also yield an Early Cretaceous emplacement age of 124±2 Ma although some inherited zircons have been identified. These age results indicate that the Early Cretaceous was an important period of magmatism in the Da Hinggan Range. Zircons from porphyry are characterized by positive value of εHf（t） as high as 10.3±0.5 with Hf depleted mantle model age of 349-568 Ma, whereas magmatic zircons from the dolerite have εHf（t） value of 11.0±1.4 with Hf depleted mantel model age of 342-657 Ma, consistent with those from the porphyry. Considering other data on the geological evolution of this area, it is concluded that the mafic magma originated from the partial melting of Paleozoic enriched lithospheric mantle, whereas the felsic magma came from recycling of juvenile crust formed during the Paleozoic. Both of the protoliths are closely related to the subduction of the Paleo-Asian Ocean during the Paleozoic, indicating that the Paleozoic is an important period of large-scale crustal growth in the area.

An Important Spreading Event of the Neo-Tethys Ocean during the Late Jurassic and Early Cretaceous: Evidence from Zircon U-Pb SHRIMP Dating on Diabase in Nagarze, Southern Tibet

Widely distributed in Gyangzê-Chigu area, southern Tibet, NW- and nearly E-W-trending diabase（gabbro）-gabbro diorite dykes are regarded as the product of the large-scale spreading of the late Neo-Tethys Ocean. In order to constrain the emplacement age of these dykes, zircons of two samples from diabases in Nagarzê were dated by using the U-Pb SHRIMP method. Two nearly the same weighted mean ^206pb/^23SU ages were obtained in this paper, which are 134.9±1.8 Ma （MSWD=0.65） and 135.5 ± 2.1 Ma （MSWD=1.40）, respectively. They not only represent the crystallization age of the diabase, but also documented an important spreading event of the Neo-Tethys Ocean during the late Jurassic and early Cretaceous. This dating result is of great significance to reconstruct the temporal framework of the late Neo-Tethys Ocean in the Qinghai-Tibet Plateau.

SHRIMP Zircon U-Pb Chronology and Geochemistry of the Henglingguan and Beiyu Granitoids in the Zhongtiao Mountains,Shanxi Province

Henglingguan and Beiyu metamorphic granitoids, distributed in the northwest of the Zhongtiaoshan Precambrian complex, comprise trondhjemites and calc-alkaline monzogranites, displaying intrusive contacts with the Archean Zhaizi TTG gneisses. And the Beiyu metamorphic granitoids consist mainly of trondhjemites, distributed at the core of the Hujiayu anticline fold. New SHRIMP zircon U-Pb dating data show that the weighted mean ^207pb/^206pb ages are 2435.9 Ma and 2477 Ma for the Henglingguan metamorphic calc-alkaline monzogranites and Beiyu metamorphic trondhjemites, respectively, and reveal -2600 Ma inherited core in magmatic zircons. Whole-rock geochemical data indicate that all the Henglingguan and Beiyu metamorphic trondhjemites and calc- alkaline monzogranites belong to the metaluminous medium- and high-potassium calc-alkaline series. These rocks are characterized by relatively high total alkali contents (Na2O+K2O, up to 9.08%), depleted Nb, Ta, P and Ti, and right-declined REE patterns with moderate to high LREEs/HREEs fractionation (the mean ratio of (La/Yb)n = 25). The Henglingguan and Beiyu metamorphic trondhjemites display negative Rb, Th and K anomalies in the multi-dement spider diagrams normalized by primitive mantle. Sm-Nd isotopic data reveal that these granitoids have initial εNd(t) =-1.2 to +2.4 and Nd depleted mantle model ages of TMD = 2622 Ma-2939 Ma. All these geochemical features indicate that these granitoids were formed in an continent-marginal arc, and the trondhjemites mainly originated from partial melting of juvenile basaltic materials and, howbeit, the Henglingguan metamorphic calc-alkaline monzogranites derived from recycling of materials in the ancient crust under a continent-marginal arc. The granitic magma underwent contamination and fractional crystallization during their formation.

Geological Characteristics and Zircon U-Pb Dating of Volcanic Rocks from the Beizhan Iron Deposit in Western Tianshan Mountains, Xinjiang, NW China

The Beizhan large iron deposit located in the east part of the Awulale metallogenic belt in the western Tianshan Mountains is hosted in the Unit 2 of the Dahalajunshan Formation as lens, veinlets and stratoid, and both of the hanging wall and footwall are quartz-monzonite; the dip is to the north with thick and high-grade ore bodies downwards. Ore minerals are mainly magnetite with minor sulfides, such as pyrite, pyrrhotite, chalcopyrite and sphalerite. Skarnization is widespread around the ore bodies, and garnet, diopside, wollastonite, actinolite, epidote, uralite, tourmaline sericite and calcite are ubiquitous as gangues. Radiating outwards from the center of the ore body the deposit can be classified into skarn, calcite, serpentinite and marble zones. LA-ICP-MS zircon U-Pb dating of the rhyolite and dacite from the Dahalajunshan Formation indicates that they were formed at 301.3±0.8 Ma and 303.7±0.9 Ma, respectively, which might have been related to the continental arc magmatism during the late stage of subduction in the western Tianshan Mountains. Iron formation is genetically related with volcanic eruption during this interval. The Dahalajunshan Formation and the quartz-monzonite intrusion jointly control the distribution of ore bodies. Both ore textures and wall rock alteration indicate that the Beizhan iron deposit is probably skarn type.

Paleozoic and Mesozoic Basement Magmatisms of Eastern Qaidam Basin,Northern Qinghai-Tibet Plateau:LA-ICP-MS Zircon U-Pb Geochronology and its Geological Significance

The eastern margin of the Qaidam Basin lies in the key tectonic location connecting the Qinling, Qilian and East Kunlun orogens. The paper presents an investigation and analysis of the geologic structures of the area and LA-ICP MS zircon U-Pb dating of Paleozoic and Mesozoic magmatisms of granitoids in the basement of the eastern Qaidam Basin on the basis of 16 granitoid samples collected from the South Qilian Mountains, the Qaidam Basin basement and the East Kunlun Mountains. According to the results in this paper, the basement of the basin, from the northern margin of the Qaidam Basin to the East Kunlun Mountains, has experienced at least three periods of intrusive activities of granitoids since the Early Paleozoic, i.e. the magmatisms occurring in the Late Cambrian （493.1±4.9 Ma）, the Silurian （422.9±8.0 Ma-420.4±4.6 Ma） and the Late Permian-Middle Triassic （257.8±4.0 Ma-228.8＋1.5 Ma）, respectively. Among them, the Late Permian - Middle Triassic granitoids form the main components of the basement of the basin. The statistics of dated zircons in this paper shows the intrusive magmatic activities in the basement of the basin have three peak ages of 244 Ma （main）, 418 Ma, and 493 Ma respectively. The dating results reveal that the Early Paleozoic magmatism of granitoids mainly occurred on the northern margin of the Qaidam Basin and the southern margin of the Qilian Mountains, with only weak indications in the East Kunlun Mountains. However, the distribution of Permo-Triassic （P-T） granitoids occupied across the whole basement of the eastern Qaidam Basin from the southern margin of the Qilian Mountains to the East Kunlun Mountains. An integrated analysis of the age distribution of P-T granitoids in the Qaidam Basin and its surrounding mountains shows that the earliest P-T magmatism （293.6-270 Ma） occurred in the northwestern part of the basin and expanded eastwards and southwards, resulting in the P-T intrusive magmatism that ran through the whole basin basement. As the Cenozoic basement thrust system developed in the eastern Qaidam Basin, the nearly N-S-trending shortening and deformation in the basement of the basin tended to intensify from west to east, which went contrary to the distribution trend of N-S-trending shortening and deformation in the Cenozoic cover of the basin, reflecting that there was a transformation of shortening and thickening of Cenozoic crust between the eastern and western parts of the Qaidam Basin, i.e., the crustal shortening of eastern Qaidam was dominated by the basement deformation （triggered at the middle and lower crust）, whereas that of western Qaidam was mainly by folding and thrusting of the sedimentary cover （the upper crust）.

Geochemistry and SHRIMP Zircon U-Pb Age of Post-Collisional Granites in the Southwest Tianshan Orogenic Belt of China:Examples from the Heiyingshan and Laohutai Plutons

The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A-type granites geochemically. Analysis shows that the both are ferron calc-alkalic peraluminous or ferron alkali-calcic peraluminous with a relatively high concentration of SiO2 （〉70%）, high alkali contents （Na20 ＋ K20 = 7.14%-8.56%; K20〉N20; A/CNK = 0.99-1.20）, and pronounced negative anomales in Eu, Ba, St, P and Ti. A SHRIMP zircon U-Pb age of 285±4 Ma was obtained for the Heiyingshan hornblende biotite granite intrusion. The geochemical and age dating data reported in this paper indicate that these granites were formed during the post-collisional crustal extension of the Southwest Tianshan orogenic belt, in agreement with the published data for the granites in the South Tianshan.

Ages of the Laocheng Granitoids and Crustal Growth in the South Qinling Tectonic Domain,Central China:Zircon U-Pb and Lu-Hf Isotopic Constraints

The Laocheng granitoid pluton is located in the South Qinling tectonic domain of the Qinling orogenic belt,southern Shaanxi Province,and consists chiefly of quartz diorite,granodiorite and monzogranite.A LA-ICP-MS zircon U-Pb isotopic dating,in conjunction with cathodoluminescence images,reveals that the quartz diorite and granodiorite were emplaced from 220 Ma to 216 Ma,while the monzogranite was emplaced at～210 Ma.In-situ zircon Hf isotopic analyses show that theε_（Hf）（t） values of the quartz diorite and granodiorite range from-8.1 to ＋1.3,and single-stage Hf model ages from 809 Ma to 1171 Ma,while theε_（Hf）（t）values of the monzogranite are-14.5 to ＋16.7 and single-stage Hf model ages from 189 Ma to 1424 Ma.These Hf isotopic features reveal that the quartz diorite, granodiorite and monzogranite were formed from the mixing of the magmas derived from partial melting of the depleted mantle and the lower continent crustal materials,and there were two stages of continental crust growth during the Neoproterozoic（～800 Ma）and Indosinian（～210 Ma）eras, respectively,in the south Qinling tectonic domain of the Qinling orogrnic belt,Central China.

Geochemical Characteristics and LA-ICP-MS Zircon U-Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

Geochemical studies on the arnphibolites in the Songshugou ophiolite from Shangnan County, Shaanxi Province demonstrate that the protolith of the amphibolites is tholeiitic. The arnphibolites can be classified into two groups according to their REE patterns and trace element features. Rocks of the first group are depleted in LREE while rocks of the second group are slightly depleted in LREE or flat from LREE to HREE without significant Eu anomaly. The first group of rocks have (La/Yb)N=0.33-0.55, (La/Sm)N= 0.45-0.65, and their La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are averaged at 1.20, 0.12, 31.02, 2.92 and 198, respectively, close to those of typical N-MORB. The second group of rocks have (La/Yb)N=0.63-0.95, (La/ Sm)N = 0.69--0.90, and their average La/Nb, Ce/Zr, Zr/Nb, Zr/Y and Ti/Y ratios are 0.82, 0.83, 1.15, 0.16, 19.00, 2.58 and 225, respectively, which lie between those of typical N-MORB and E-MORB but closer to the former. The two groups of rocks both exhibit flat patterns from Th to Yb in the highly incompatible elements spider diagram, but the first group of rocks have lower element abundances than the modern N-MORB, indicating a derivation of their mantle source from more depleted mantle source than the present N-MORB. The abundances of Th, Ta, Nb, La and Ce in the second group of rocks are slightly higher than those of the present N-MORB, and other elements, such as Hf, Zr, Sm, Ti, Y and Yb, are close to those of the N-MORB, indicating that the original magma was derived from depleted mantle but mixed with the enriched mantle. These characteristics, combined with the regional geology and previous studies, provide further evidence that the mafic-ultramafic rocks have the features of a typical ophiolite.Zircon grains from the amphibolite are generally rounded, and in most of them a distinguishable core-mantle texture is preserved as shown in the cathodoluminescence (CL) images. The core or core-mantle parts of the zircon grains are also rounded, same as those in basalts from other regions of the world. The LA-ICP-MS trace element and U-Pb isotopic analyses show that the zircon grains from the amphibolites are similar to the typical magmatic zircon in terms of their very low U and Th contents (62.36-0.10 μg/g and 78.47-0.003 μg/g, respectively). Seven pits from the core and core-mantle parts of the zircon grains yielded an average weighted 206Pb/ 238U age of 973±35 (2σ) Ma with the Th/U ratios range from 0.01 to 8.38 and mostly greater than 0.23. This age is consistent within the error range with the whole-rock Sm-Nd isochron age of 1030±46 Ma for the same kind of rocks reported by Dong et al. (1997a). In a combined analysis with the zircon positions on the CL images and the corresponding Th/U ratios, the age of 973±35 Ma is probably the formation age of tholeiite, the protolith of the Songshugou amphibolite. The geochronological determination gives further evidence that the Songshugou ophiolite was formed during the Neoproterozoic. In addition, there is one pit from the rim of a zircon grain giving a 206Pb/ 238U age of 5721199 (1σ) Ma with a Th/U ratio of 0.08. It may represent the age of the accretionary zircon in the amphibolite-facies metamorphism.

Petrogenesis of Late Cretaceous Jiangla＇angzong I-Type Granite in Central Lhasa Terrane, Tibet, China： Constraints from Whole-Rock Geochemistry, Zircon U-Pb Geochronology, and Sr-Nd-Pb-Hf Isotopes

The Jiangla＇angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA-ICP-MS zircon U-Pb analyses suggest that syenogranite has a weighted mean 2±6pb/23SU age of 86±1 Ma （mean square weighted deviation=0.37）, which is in accordance with the muscovite Ar-Ar age （85±1 Ma） of Cu-Au ore-bearing skarns and the zircon U-Pb age （84±1 Ma） of adamellite. This suggests that the Jiangla＇angzong magmatism and Cu-Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al-bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO2 （65.10 -70.91wt%）, K20 （3.44-5.17wt%）, and total K20＋Na20 （7.13-8.15wt%）, and moderate contents of A12Oa （14.14-16.45wt%） and CaO （2.33-4.11wt%）, with a Reitman index （δ43） of 2.18 to 2.33, and A/ CNK values of 0.88 to 1.02. The P205 contents show a negative correlation with SiO2, whereas Pb contents show a positive correlation with SiO2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla＇angzong granite is a high K calc-alkaline metaluminous I-type granite. It is enriched in light rare earth elements （LREE） and large ion lithofile elements （LILE）, and depleted in heavy rare earth elements （HREE） and high field strength elements （HFSE）, with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies （δCe=l.00-1.04）. The relatively low initial a7Sr/a6Sr ratios of 0.7106 to 0.7179, positive ε±nt（t） values of 1.0 to 4.1, and two-stage Hf model ages （TDM2） ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The （143Nd/144Nd）t values from the Jiangla＇angzong granite range from 0.5121 to 0.5123, its eNd（t） values range from -10.17 to -6.10, its （^206pb /^204pb）t values range from 18.683 to 18.746, its （^207pb /^204pb）t values range from 15.695 to 15.700, and its （^208pb /^204pb）t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla＇angzong granite was formed during the post- collision extension of the Qiangtang and Lhasa terranes.

LA-ICP-MS Zircon U-Pb Geochronology of Quartz Porphyry from the Niutougou Gold Deposit in Songxian County,Henan Province

The Niutougou gold deposit, located in the center of the Xiong＇ershan gold district, western Henan Province, is a large gold deposit with many quartz porphyries found in the area. Based on the field geological investigation of quartz porphyry of Niutougou gold deposit and by using the cathodoluminescence （CL） images analysis and in situ LA-ICP-MS U-Pb isotope dating method of zircons, the inner structure, trace element compositions and U-Pb age of the zircons separated from quartz porphyry were analyzed and determined. Cathodoluminescence （CL） images of zircons show clear magmatic zonations. Trace element analyses of zircons reveal that all zircons show high concentrations of Th, U, and HREE, and the REE patterns of depletion in LREE, with a positive Ce anomaly. Zircon LA-ICP-MS U-Pb dating results show that the quartz porphyry in the Niutougou gold deposit was formed at 159.714-0.99 Ma （about 160 Ma）, belonging to the product of magmatic activity in late Middle Jurassic. Combined with the geological characteristics of the Niutougou gold deposit, the formation age of the quartz porphyry and the analysis of the formation age of the granite body exposed in the Niutougou gold deposit, the study suggests that the metallogenic epoch of the Niutougou gold deposit may be Yanshanian in age.

Geochemistry and Zircon U-Pb Dating of Changshagou Adakite from the South Altyn UHPM Terrane: Evidence of the Partial Melting of the Lower Crust

Changshagou adakite, an outcrop in the middle segment of the South Altyn Tagh ultra-high pressure metamorphism （UHPM） terrane, contains medium-K cal-alkaline and weakly peraluminous compositions （SiO2 = 66.79% to 68.65%, Al2O3 = 17.48% to 18.31%, K20 ＋ Na20 = 6.32% to 6.88%, K2O/Na2O = 0.25 to 0.33, A/CNK = 1.01 to 1.06）. This outcrop is also enriched with large ion lithophile elements but with depleted high-field strength elements （HFSE） showing clearly negative Nb, Ta, and Ti anomalies. REE distribution patterns show a positive anomaly of Eu （6Eu = 1.15 to 1.31） and weakly enriched with LREE compared with HREE （LREE/HREE = 1.02 to 4.20）. Experimental results and several characteristics, including relatively low Nb/Ta ratios （6.03 to 8.45） and high Sr, Sr/Y, （La/Yb）N and low Y and Yb, which indicate the presence of residual garnet and the absence of plagioclase in the source region, show that adakite may form at a pressure ranging from 1.2 GPa to 1.5 GPa and at a temperature of approximately 900~C. Low Cr, Ni, and Mg# values, trace element patterns, and SiO2- Mg# and SiO2-MgO diagrams indicate that rocks are formed by the partial melting of a thickened lower continental crust. LA-ICP-MS in situ U-Pb dating yields two group ages： 503.1±1.7 Ma （core） and 453.1±3.0 Ma （rim）. The Th/U ratios of the core and the rim are 0.11 to 0.40 and 0.03 to 0.07, respectively. Considering the zircon CL image characteristics, Th/U ratios, and previous studies on regional UHPM rocks, adakite formed at 503.1 ± 1.7 Ma and underwent a tectothermal event as a result of the break-off of the Altyn deep subducted continental crust at 453.1 ± 3.0 Ma.

Zircon U-Pb Dating on Granitoids from the Northern South China Sea and its Geotectonic Relevance

Zircon U-Pb ages of 163.8-100.4 Ma and 146.6-134.5 Ma are obtained for the granitoids from the Pearl River mouth basin, and from southern Guangdong Province, respectively. These new dating data accord well with the crystallization ages of Yanshanian granitoids broadly in the Nanling. The active continental margin of South China, as revealed by a combination of zircon U-Pb data, underwent a key granitoid-dominated magmatism in 165-100 Ma. Its evolution varied temporally, and spatially, registering under control of the paleo-Pacific slab subduction. The granitoids that occurred in 165-150 Ma broadly from the South China Sea to the Nanling are preferably related to two settings from volcanic-arc to back-arc extension, respectively. The activities of Cretaceous granitoids migrated from the southeastern Guangdong （148-130 Ma） to the Pearl River Mouth basin （127-112 Ma）, corresponding to the model of a retreating subduction. The subduction-related granitoid magmatism in South China continued until 108-97 Ma. A tectonic transformation from slab-subduction to extension should occur at -100 Ma.

Late Mesozoic Ore-forming Events in the Ningwu Ore District, Middle-Lower Yangtze River Polymetallic Ore Belt, East China: Evidence from Zircon U-Pb Geochronology and Hf Isotopic Compositions of the Granodioritic Stocks

Late Mesozoic volcanic-subvolcanic rocks and related iron deposits, known as porphyry iron deposits in China, are widespread in the Ningwu ore district （Cretaceous basin） of the middle-lower Yangtze River polymetallic ore belt, East China. Two types of Late Mesozoic magmatic rocks are exposed： one is dioritic rocks closely related to iron mineralization as the hosted rock, and the other one is granodioritic （-granitic） rocks that cut the ore bodies. To understand the age of the iron mineralization and the ore-forming event, detailed zircon U-Pb dating and Hf isotope measurement were performed on granodioritic stocks in the Washan, Gaocun-Nanshan, Dongshan and Heshangqiao iron deposits in the basin. Four emplacement and crystallization （typically for zircons） ages of granodioritic rocks were measured as 126.1±0.5 Ma, 126.8±0.5 Ma, 127.3±0.5 Ma and 126.3±0.4 Ma, respectively in these four deposits, with the LA-MC-ICP-MS zircon U-Pb method. Based on the above results combined with previous dating, it is inferred that the iron deposits in the Ningwu Cretaceous basin occurred in a very short period of 131-127 Ma. In situ zircon Hf compositions of εHf（t） of the granodiorite are mainly from -3 to -8 and their corresponding 176Hf/177Hf ratio are from 0.28245 to 0.28265, indicating similar characteristics of dioritic rocks in the basin. We infer that granodioritic rocks occurring in the Ningwu ore district have an original relationship with dioritic rocks. These new results provide significant evidence for further study of this ore district so as to understand the ore-forming event in the study area.

LA-ICP-MS Zircon U-Pb Dating of Intermediate-Acidic Intrusive Rocks and Molybdenite Re-Os Dating from the Bangpu Mo (Cu) Deposit, Tibet and its Geological Implication

The multi-stage intrusions of intermediate-acid magma occur in the Bangpu mining district, the petrogenic ages of which have been identified. The times and sequences of their emplacement have been collated and stipulated in detail in this paper by using the laser ablation-inductively coupled plasma-mass spectrometry （LA-ICP-MS） zircon U-Pb dating method. The ages of biotite monzogranite that were formed before mineralization in the southwest of this mining district are 70±1 Ma （mean square of weighted deviates （MSWD） =9.5, n=8） and 60.60±0.31 Ma （MSWD=3.8, n=16）, which belong to the late Cretaceous-early Paleocene in age. That means, they are products of an early tectonicmagmatic event of the collision between the Indian and Asian continentals. The ages of ore-bearing monzogranite porphyry and ore-bearing diorite porphyrite are 16.23±0.19 Ma （MSWD=2.0, n=26） and 15.16±0.09 Ma （MSWD=3.9, n=5） separately, which belong to the middle Miocene in age; namely, they are products of the Gangdese post-collision extensional stage when crust-mantle materials melted and mixed as well as magmatic intrusion simultaneously occurred. Some zircons with ages of 203.6±2.2 Ma （MSWD=1.18, n=7） were captured in the ore-bearing diorite porphyrite, which shows that there had been tectono-magmatic events in the late Triassic-early Jurassic. Molybdenum （copper） ore-bodies produced in the monzogranite porphyry and copper （molybdenum） ore-bodies produced in the diorite porphyrite are the main ore types in this ore deposit. The model ages of Re-Os isotopic dating for the 11 molybdenite are 13.97-15.84 Ma, while isochron ages are 14.09±0.49 Ma （MSWD=26）. The isochron ages of seven molybdenite from molybdenum （copper） ore with monzogranite porphyry type are 14.11±0.31 Ma （MSWD=5.2）. There is great error in the isochron ages of four molybdenite from copper （molybdenum） ore with diorite porphyrite type, and their weighted average model ages of 14.6±1.2 Ma （MSWD=41）, which generally represent the mineralization age. The results about the Re-Os isotopic dating of molybdenite in the ore of different types have limited exactly that, the minerlazation age of this ore deposits is about 14.09 Ma, which belongs to the middle Miocene mineralization. The Bangpu deposit has a uniform metallogenic dynamics background with the porphyry type and skarn-type deposits such as Jiama, Qulong and others.