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.

Chronology and Genesis of S-type Granites in Hetai District, Guangdong Province: Constraints from LA-ICP-MS Zircon U-Pb Dating and Tourmaline Boron Isotope In-situ Analyses

1 Introduction Hetai district,which is a mountainous area,situated on Guangning and Zhaoqing city,west Guangdong Province.Hetai district is generally located on southwest of South China Caledonian fold belt,east margin of Yunkai post-Caledonian uplift.Multiple type granites are widely distributed in Hetai district,including Caledonian,Indosinian and Yanshanian granites.Based on different

Zircon U-Pb dating,Geochemical Features of the Halunwusu Composite Granites at the Northern Edge of Southern Qilian,Qinghai

1 Introduction Halunwusu Composite Granites,locates in the northern margin of Belt,between the North Qaidam block and the Central Qilian block,with two phases of magmatic activities.The granites mainly consisits of the Early

Petrogenesis of the Xihuashan Granite in Southern Jiangxi Province,South China:Constraints from Zircon U-Pb Geochronology,Geochemistry and Nd Isotopes

Mesozoic granitic intrusions are widely distributed in the Nanling region, South China. Yanshanian granites are closely connected with the formation of tungsten deposits. The Xihuashan granite is a typical representative of tungsten-bearing granite. The Xihuashan granite consists mainly of medium-grained porphyritic biotite granite, medium-grained biotite granite and fine-grained twomica granite, which correspond to LA-ICP-MS zircon U-Pb ages of 555.5±0.4 Ma, 553.0±0.6 Ma and 552.8±0.9 Ma, respectively. Rocks from the Xihuashan mining area displays high SlOe （73.85% to 76.49%） and NaeO＋K20 contents （8.09% to 9.43%）, belonging to high-K calc-alkaline series. They are metaluminous to weakly peraluminous with A/CNK values ranging from 0.96 to 5.06. All granites in this study area are rich in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti, Nb and Eu, especially depleted in medium-grained biotite granite and fine-grained two-mica granite. The medium-grained porphyritic biotite granites usually have high LREE concentrations, whereas medium-grained biotite granite and fine-grained two-mica granite displays high HREE contents. Our geochemical data reveal that the studied rocks are highly fractionated I-type granite. The magma underwent strong magma differentiation with decreasing temperature and increasing oxygen fugacity, which may explain the formation of three types of distinct granites. Variations of Rb, Sr and Ba concentrations in different type granites were controlled by fractional crystallization of biotite and feldspar. Fractional crystallization of monazite, allanite and apatite resulted in LREE changes in granite, and formation of garnet mainly caused HREE changes. Granites from the Xihuashan mining area have relatively high εd（t） values （-9.77 to -55.46）, indicating that they were probably generated by partial melting of underlying Proterozoic metasedimentary rocks with minor addition of juvenile crust or mantlederived magmas.

Single Zircon LA-ICP-MS U-Pb Dating of the Guandimiao and Wawutang Granitic Plutons in Hunan,South China and Its Petrogenetic Significance

The Guandimao and Wawutang plutons are located at the center of Hunan, South China. The former is mainly composed of biotite monzonitic granites/granodiorites and two-mica monzonltic granites, but the latter only consists of biotite monzonitic granites. The zircon ages of 203.0±1.6 Ma （biotite monzonitic granites） and 208.0-23.2 Ma （two-mica monzonltic granites） for the Guandimao pluton and 204±3 Ma for the Wawutang pluton obtained with the LA-ICP-MS U-Pb dating indicate that they were formed during the late Indosinian. In consideration of other geochronological data from Indosinian rocks of South China and adjacent regions, it is inferred that the two plutons were derived from crustal materials by decompressional melting in a post-collisional tectonic setting during spontaneous thinning of the thickened curst. Moreover, the inherited zircon age of 1273±57 Ma from the Wawutang pluton indicates that the source of the two plutons is related to the early Proterozoic crustal basement.

Zircon U-Pb-Hf isotopic and geochemical characteristics of the Xierzi biotite monzogranite pluton,Linxi,Inner Mongolia and its tectonic implications

The opening, subduction and final closure of the Paleo-Asian Ocean led to the formation of the Central Asian Orogenic Belt. Controversy has long surrounded the timing of final closure of the Paleo-Asian Ocean. Here we present zircon U-Pb ages and petrological, geochemical and in situ Hf isotope data for the Xierzi biotite monzogranite pluton, Linxi, SE Inner Mongolia. U-Pb dating of zircon by LA-ICP-MS yields a middle Permian emplacement age(268.7 ± 2.3 Ma) for the Xierzi pluton that is dominated by biotite monzogranites with high SiO_2(71.2-72.8 wt.%),alkali(Na_2 O + K_2 O =8.05-8.44 wt.%), Al_2 O_3(14.4-15.2 wt.%) and Fe_2 O_3~T relative to low MgO contents, yielding Fe_2 O_3~T/MgO ratios of 2.87-3.44, and plotting within the high-K calc-alkaline field on a SiO_2 vs. K_2 O diagram. The aluminum saturation indexes(A/CNK) of the biotite monzogranites range from 1.06 to 1.19, corresponding to weakly to strongly peraluminous. They are enriched in rare earth elements(REE), high field strength elements(HFSEs; Zr,Hf). and large ion lithophile elements(LILEs; Rb, U, Th). The LREEs are enriched relative to the HREEs,with a distinct negative Eu anomaly in a chondrite-normalized REE diagram. Geochemically, the Xierzi biotite monzogranite is classified as an aluminous A-type granite, with all samples plotting within the A2-type granite field on a Y/Nb vs. Rb/Nb diagram. Zircon ε_(Hf)(t) values and two-stage modal ages of the zircons within the pluton range from +4.80 to +13.65 and from 983 to 418 Ma, respectively, indicating that the primary magma was generated through partial melting of felsic rocks from juvenile crust.Consequently, these results demonstrate that the Xierzi pluton formed under the post-orogenic extensional setting after arc-continent collision in the middle Permian.

Petrogenesis and Mineralization of Two-Stage A-Type Granites in Jiuyishan,South China:Constraints from Whole-rock Geochemistry, Mineral Composition and Zircon U-Pb-Hf Isotopes

The Jiuyishan complex massif,located in the northern section of the Nanling region,is a combination of five plutons,namely,the Xuehuading,Jinjiling,Pangxiemu,Shaziling and Xishan plutons.Whole-rock geochemistry,Imineral electron microprobe analysis,zircon U-Pb dating and Hf isotope analysis were carried out for the Jinjiling and Pangxiemu plutons.The zircon U-Pb dating yields weighted mean ages of 152.9±0.9 Ma for the Jinjiling pluton and 151.7±1.5 Ma for the Pangxiemu pluton,with a narrow gap between them.The Jinjiling and Pangxiemu plutons both have geochemical characteristics of high SiO2,Al2 O3,Na2 O,K2 O and low TiO2,MgO,CaO,P2 O5 contents,with intense depletions in Sr,Ba,Ti,Eu and enrichments in Ga,FeoOT and HFSE,and these characteristics reflect an A-type affinity.From the Jinjiling to the Pangxiemu plutons,the mineral composition of mica changes from lepidomelane to zinnwaldite,with increases in F,Li2 O and Rb2 O contents.The mineral composition of zircon changes from low Zr/Hf to high Zr/Hf,with increasing HfO2,P2 O5 and UO2+ThO2+Y2 O3 contents.The mineral compositions of feldspar indicate that the Pangxiemu pluton contains more alkali feldspar than the Jinjiling pluton.The whole-rock geochemistry and mineral compositions reveal a higher degree of differentiation for the Pangxiemu pluton.The nearly uniformεHf(t)indicates the same source region for the two plutons:both were derived from partial melting of the lower crust,with small contributions of mantle materials.In addition,higher F,lower Nb/Ta and Zr/Hf ratios in the Pangxiemu Pluton suggest a closer relationship with the rare metal mineralization than for the Jinjiling pluton.

Petrogenesis of Kejie Granite in the Northern Changning-Menglian Zone, Western Yunnan: Constraints from Zircon U-Pb Geochronology, Geochemistry and Hf Isotope

The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.

Formation Age of the Sujiagou Komatiites in Western Shandong: Further Constraints from SHRIMP U-Pb Zircon Dating on Granitic Dykes

Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U-Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.

江西宜丰甘坊岩体中黑云母花岗岩与花岗伟晶岩的锆石、独居石U-Pb年龄及成因联系

江南造山带东段宜丰地区是我国重要的Nb、Ta、Be、Li等稀有金属成矿区,成矿作用与中生代甘坊岩体有着密切关系。近些年,随着富Be花岗伟晶岩的发现,更是增添了成岩、成矿作用的复杂性。笔者等对甘坊岩体早期黑云母花岗岩及晚期花岗伟晶岩开展了锆石、独居石同位素年代学及地球化学分析。结果显示,黑云母花岗岩锆石年龄为145.4±1.2 Ma,独居石年龄为145.6±1.1 Ma,两者的元古宙206 Pb/238U捕获年龄图谱与双桥山群一致。花岗伟晶岩独居石206 Pb/238U年龄比较集中,其年龄为139.5±0.9 Ma。伟晶岩与黑云母花岗岩中的独居石具有相似的EPMA数据,在不同成因独居石判别图中,两者呈明显的线性关系,分布在岩浆成因范围内。黑云母花岗岩锆石△FMQ=-5.36~-0.77(平均值为-2.84)、Ti饱和温度为668~757℃(平均为710℃),具有低氧逸度、低温特点,具备形成亲氧元素矿床能力。锆石的球粒陨石标准化REE配分曲线呈左倾配分型式,与壳源花岗岩锆石在稀土元素含量及配分模式一致。锆石微量元素图解显示,黑云母花岗岩具有大陆地壳、S型、演化程度高的花岗岩特征。综上,甘坊岩体花岗伟晶岩应该来自黑云母花岗岩母岩浆的演化分异末期,源区为双桥山群变质砂岩、泥岩。

Determination of the Neoproterozoic Shicaogou Syn-collisional Granite in the Eastern Qinling Mountains and Its Geological Implications

The Shicaogou granite has been identified as a magnesian (Fe-number=0.71-0.76), calcic to calc-alkalic (MALI=3.84-5.76) and peraluminous (ASI=1.06-1.13) granite of the syn-collisional S-type, with high SiO2(>71%), A12O3 (>13%) and Na2O+K2O (6.28%-7.33%, equal for NaO2 and K2O). Trace element and REE analyses show that the granite is rich in LILE such as of Rb, Sr, Ba and Th, and poor in HFSE like Yb, Y, Zr and Hf. Its Rb/Sr ratio is greater than 1; the contents of Nb and Ta, and the ratio of Nb/Ta as well as the REE geochemical features (e.g. REE abundance, visible fractionation of LREE and HREE and medium to pronounced negative Eu anomalies) are all similar to those of crust-origin, continent-continent syn-collisional granite. Moreover, the granite exhibits almost the same pattern as that of the typical continent-continent syn-collisional granite on the spider diagram and all samples fall within the syn-collisional granite field.The cathodoluminescence (CL) investigations have revealed that the zircon from the Shicaogou granite represents a typical magmatic product characterized by its colorless, transparent and euhedral crystals, and distinct zoning of oscillatory bands. Residual cores of irregular zircon can be found in a few enhedral grains. Trace element studies of the zircon grains, with high contents of P, Y, Hf, Th, U and REE and high ratios of Th/U, obviously positive Ce anomalies and HREE enrichment compared to LREE, also result in the same conclusion.The LA-ICP-MS U-Pb isotopic data from 24 spots of 21 zircon grains demonstrate that 20 spots in the oscillatory zone yield an average weighted 206Pb/238U age of 925±11 Ma, indicating that the Shicaogou granite was formed in the Neoproterozoic. Combined with other Neoproterozoic syn-collisional granites found in the study area, the present geochronological determination can further reveal that collision-amalgamation events could have occurred among some continental blocks in the Qinling orogenic belt during the Neoproterozoic. This in turn provides an accurate chronological constraint on the Neoproterozoic break-up and convergence in the belt.

Protracted zircon growth in migmatites and In situ melt of Higher Himalayan Crystallines:U-Pb ages from Bhagirathi valley, NW Himalaya,India

The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.

Geochronology and Geochemistry of the Early Permian A-Type Granite in the Hongol Area, Central Inner Mongolia:Petrogenesis and Tectonic Implications

We performed geochronological and geochemical analyses of the A-type granite in the Hongol area, central Inner Mongolia, to determine its age, petrogenesis and tectonic setting, which are significant for clarifying the Late Paleozoic tectonic evolution of the Xing’an Mongolian Orogenic Belt(XMOB). The rock type of the A-type granite in the Hongol area is alkali-feldspar granite, and it constitutes a western part of the Baiyinwula-Dongujimqin A-type granite belt. Zircon U-Pb geochronology yieldsPb/U ages ranging from 293 to 286 Ma for the alkali-feldspar granite, indicating this granitic pluton formed in the Early Permian. The alkali-feldspar granite is high in silica(SiO=75.13 wt%-80.17 wt%), aluminum(AlO=10.59 wt%-13.17 wt%) and alkali(NaO+KO=7.33 wt%-9.11 wt%), and low in MgO(0.08 wt%-0.39 wt%) and CaO(0.19 wt%-0.70 wt%). It is obviously enriched in LILEs such as Rb, Th and K,depleted in HFSEs such as Nb, Ti, La and Ce, with pronounced negative anomalies of Nb, Ti, P, Eu, Sr and Ba. Its Sr-Nd-Pb isotopic compositions show positive ε(t)(+0.72-+3.08), low T(805-997 Ma),and high radioactive Pb with(Pb/Pb)of 18.710-19.304,(Pb/Pb)of 15.557-15.604 and(Pb/Pb)of 37.887-38.330. Petrological characteristics and geochemical data suggest that the alkalifeldspar granite in the Hongol area belongs to aluminous A-type granite. This A-type granite formed in a post-collisional extensional setting and was generated by the partial melting of felsic rocks in the middlelower crust resulting from post-collisional slab breakoff. It is suggested that the Paleo-Asian Ocean was closed before the Permian in central Inner Mongolia.

Early Cretaceous I-type granites in the Tengchong terrane:New constraints on the late Mesozoic tectonic evolution of southwestern China

The Early Cretaceous granitoids that are widespread in the Tengchong terrane of Southwest China play a critical role in understanding the tectonic framework associated with the Tethyan oceans. In this study, we present a detailed description of zircon U-Pb ages, whole-rock geochemistry and Hf isotopes for the Laoxiangkeng pluton in the eastern Tengchong terrane and elucidate their petrogenesis and geodynamic implications. Zircon U-Pb dating of the Laoxiangkeng pluton yields ages of 114 ± 1 Ma and 115 ± 1 Ma,which imply an Early Cretaceous magmatic event. The Laoxiangkeng pluton enriched in Si and Na, is calcalkaline and metaluminous, and has the characteristics of highly fractionated I-type granites. Zircons from the pluton have calculated ε_(Hf)(t) values of-12.7 to-3.7 and two-stage model ages of 1327-1974 Ma,respectively, indicating a mixed source of partial melting of Paleo-Neoproterozoic crust-derived compositions with some inputs of mantle-derived magmas. By integrating all available data for the regional tectonic evolution of the eastern Tethys tectonic domain, we conclude that the Early Cretaceous magmatism in the Tengchong terrane was produced by the northeastward subduction of the Meso-Tethyan Bangong-Nujiang Ocean.

Petrogenesis and tectonic setting of granite porphyry in Keyouzhongqi area,Inner Mongolia

The authors studied the geochemistry, zircon U-Pb ages, Hf isotope of the granite porphyry and its petrogenesis and tectonic setting in the studied area. The zircon U-Pb dating indicates the formation of the granite porphyry is in Early Cretaceous （ 125.1 ± 1.5 Ma）. The granite porphyry has high-SiO2 and alkali-rich features, which belongs to high-K talc-alkaline series rocks （ A/CNK = 0. 95%-1. 25% ）. The analyses of trace elements show the characteristics of a swallow-shaped REE distribution pattern with enrichment in LREEs and most of LILEs and HFSEs, depletion in Ba, Sr, Nb, P and Ti, and especially strong depletion in Eu, which indicates the granite porphyry belongs to the aluminous A-type granite. Their εHf（t） range from 5.94 to 8.80 with Hf two-stage model ages （TDM2 ） of 620 Ma to 803 Ma. Combining with the regional tectonic background, we conclude that the source of the rocks is the new crust materials accreted from depleted mantle in Neoproterozoic and is the product of partial melting of middle and lower crustal rocks, which may be suffered from the dual impact of the subduction of the Palaeo-Pacific Ocean and the closure of the Mongol-Okhotsk Ocean.

Petrogenesis, geochemistry and geological significance of Paleocene Granite in South Gangdese, Tibet

The Gangdese magmatic belt,located along the southern margin of the Lhasa terrane,plays a critical role in understanding the tectonic framework associated with the Indian-Asian slab collision.In this paper,a chronology of zircon U-Pb and geochemical analysis of the rock of the Cuobulaguo granitic mass in the southern of the Gangdese,Tibet,revealed a series of results.The results show that the LA-ICP-MS monzonitic granite zircon U-Pb ages are 61-59 Ma,which corresponds to the same period as the magmatic arc of the southern limit of Gangdese.In terms of geochemical composition,the granite is rich inω(SiO2)70.09%to 72.64%,with a highω(Al2O3)14.40%to 15.99%,a lowω(TiO2)0.08%to 0.24%,ω(MgO)0.41%to 0.76%,ω(Fe2O)6.82%to 29.9%,ω(P2Os)0.07%to 0.12%,andω(CaO)1.06%to 1.75%.The granite mainly belongs to the high-K calc-alkaline series.The light rare earth element(LREE)content of skeletal granite is between 133.69×10^-6〜226.64×10^-6 and the heavy rare earth element(HREE)content is between 17.36×10^-6 and 32.11×10^-6.LREE/HREE is between 5.05 and 7.83.It is enriched in light rare earth elements(LREE)and large ion lithophile elements,such as Rb,K,U,etc.,depletes high field strength elements,such as P,Nb,and Ta,and has the geochemical composition of arc magmatic rocks.In addition,the aluminum saturation index(A/CNK=1.06 to 1.11)of Cuobulaguo granite,belongs toⅠ-type granite.The comprehensive analysis showed that with the beginning of the collision between the Indian-Asian continental,the subduction ocean plate was separated from the continental plate due to gravity,resulting in an increase in the asthenosphere,which made partial fusion of the lithospheric mantle.It invaded the bottom of the lower crust,which in turn induced a partial melting of the lower crust to form granite.

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 the UHPM. The UHPM could be the reason for the major Pb loss at ±220 Ma.

Zircon SHRIMP U-Pb dating of granites from Dulan and the chronological framework of the North Qaidam UHP belt, NW China

Zircon SHRIMP dating of granites from Dulan,east segment of North Qaidam UHP belt shows that they are 406.6±3.5 Ma for Yematan-E,407.3±4.3 and 397±6 Ma for Balijiehatan-W,404.5±4.0 and 397.0±3.7 Ma for Shuiwenzhan-N,380.5±5.0 Ma for Shuiwenzhan-S,382.5±3.6 and 372.5±2.8 Ma for Chachagongma.These granites from Dulan represent the products of the third and fourth periods of Paleozoic magmatism in North Qaidam.Geochemically,the granitoids with metalumious to weak peratuminous are quartz diorite,granodiorite,and granite in composition and mainly belong to calc-alkaline series,a few samples to calc or alkali-calc series.The third period of granites is a rock association of granodiorite+granite,with initial 87Sr/86Sr ratios from 0.7082 to 0.7110 and T2DM model ages from 1.41–1.90 Ga;and the fourth period of granites is a rock association of quartz diorite+granodiorite+granite,with initial 87Sr/86Sr ratios from 0.7072 to 0.7091 and T2DM model ages from 1.07–1.38 Ga.Therefore,the third period of granites has higher initial 87Sr/86Sr ratios and T2DM model ages.On the contrary,the fourth period of granites has Nd(t)values from 0.6 to-3.0,higher than that of the third granite with Nd(t)values-3.2 to-9.3.Thus,the data comparison indicates that the third granites may derive from Paleo-proterzoic continental crust with mantle material whereas the fourth granites may derive from the Meso-proterzoic basalt crust with continental material.Combined with regional geology,we thought that the third granites were formed relative to plate exhumation and the fourth granites to delamination of the lithospheric mantle.

Zircon U-Pb dating and geochemical study of the Xianggou granite in the Ma'anqiao gold deposit and its relationship with gold mineralization

Single zircon LA-ICP-MS U-Pb dating and lithogeochemical studies have been performed on the Xianggou monzonitic granitic porphyry outcropped in the Ma’anqiao gold deposit.A weighted average U-Pb age of 242.0±0.8 Ma for Xianggou monzonitic porphyry has been obtained.This corresponds with the conclusions of previous studies indicating a syn-orogenic age (242±21 Ma) of the Qinling Orogenic Belt,suggesting that the formation of the Xianggou granite should be associated with the collisional event of the North China Plate and the Yangtze Plate in the Indosinian period.The Xianggou granite is characterized by the high silicon and alkali of high K calc-alkaline series granites.It is rich in Al (Al2O3=14.49%-15.61%) and Sr (457.10-630.82 ppm),poor in Y (【16 ppm) and HREE (Yb【0.45 ppm),and exhibits high ratios of Sr/Y (76.24-97.34) and (La/Yb)N (29.65-46.10),as well as strongly fractionated REE patterns.These geochemical characteristics suggest the Xianggou granite can be classified as C-type adakitic rock.The initial Sr isotope ratios for the Xianggou granite vary from 0.70642 to 0.70668,εNd(t) values from -4.54 to -3.98,and TDM values from 1152 Ma to 1220 Ma.The low εNd(t) and ISr and high TDM values,as well as Na2O/K2O ratios of the Xianggou granite are close to 1 (Na2O/K2O=0.95-1.10),indicating that it is not an I-type adakite formed by partial melting of the subducting oceanic crust,nor adakitic rock formed by melting of the underplated basaltic lower crust,but the product of partial melting of the nonunderplated basaltic thickened lower crust.Zircons from the Xianggou pluton have a homogeneous Hf isotopic composition with negative εHf(t) values (between -9.7 and -5.9,with an average of -6.9),indicating that the rock-forming materials were mostly extracted from the ancient crust,not from the depleted mantle.The Xianggou monzonitic granitic porphyry is rich in LILE and LREE and depleted in HSFE,HREE and Y;the composition of trace element and REE are similar to those of the syn-collisional granites.The geological and geochemical characteristics of the Xianggou granite reveal that it was a product of partial melting of the basaltic rocks from the thickened lower crust,triggered by continental collision,which occurred in the geodynamic background of continental-continental collision and shearing within the crust.The Xianggou granite was intruded in the compressive orogenic environment 242 Ma ago,but the gold mineralization occurred in the transitional environment of compression to extension around 170 Ma ago,lagging behind the intrusive age of the Xianggou granite by about 70 Ma.Meanwhile,the distribution of trace elements and REEs of the Xianggou monzonitic granitic porphyry is distinct from that of ores,suggesting the absence of direct genetic relationship between the Xianggou granite and gold mineralization.In contrast,the relatively high ore-forming elemental content of the Xianggou monzonitic granitic porphyry is due to the rock having experienced Au-bearing hydrothermal alteration.From the view of gold mineralization,considering the intrusive age,structural deformation,as well as alteration of the granite,we can conclude that the Xianggou pluton was a pre-ore-intrusion,whose intrusive age of 242 Ma constrains the lower time limit of gold metallogenesis.Following the intrusive event of the syn-collisional granitic porphyry and the intensively brittle-ductile shear deformation,large-scale fluid activity and gold mineralization took place.

新疆卡鲁安矿区伟晶岩锆石U-Pb定年、铪同位素组成及其与哈龙花岗岩成因关系研究

利用LA-ICPMS和LA-MC-ICPMS技术,对卡鲁安-阿祖拜伟晶岩田中4条伟晶岩脉以及哈龙岩体中锆石U-Pb定年、铪同位素组成进行了研究。研究显示,卡鲁安矿区805、806、807号脉的形成时代分别为(216.0±2.6)Ma、(223.7±1.8)Ma和(221±15)Ma,属三叠纪岩浆活动的产物。对于库卡拉盖650号脉,锆石U-Pb定年结果显示,它由形成时代为(227.9±2.6)Ma的早期钠长石伟晶岩与形成时代为(211.3±2.4)Ma的晚期锂辉石-钠长石-锂云母伟晶岩2期伟晶岩构成。哈龙岩体形成时代为400.9~403.3 Ma,由于伟晶岩与哈龙岩体之间存在形成时代上的差异(170 Ma以上),预示着它们之间不具成因上的联系。卡鲁安矿区伟晶岩脉中锆石显示较小的正εHf(t)值(+0.65^+2.50)和较大的模式年龄TDM2(1090~1213 Ma),相似于可可托海3号脉、柯鲁木特112号脉中锆石的铪同位素组成,指示伟晶岩由陆-陆碰撞体制伸展背景下的加厚地壳物质减压熔融所形成。