REE and Sr-Nd isotope characteristics of hydrothermal chimney at Jade area in the Okinawa Trough

Hydrothermal chimney is a product of hydrothermal activity on the seabed. Chimney samples dredged from Jade hydrothermal area in Izena depression of the Okinawa Trough, are characterized by relatively enriched light rare earth elements (LREE) and strongly positive Eu anomalies. 87Sr/86Sr and 143Nd/144Nd of these samples are exactly between those of seawater and of acidic pumice, averaged at 0.708928 and 0.512292, respectively. These characteristics imply that the main source of hydrothermal sulfide at Jade area is possibly the undersurface acidic rocks. The mineralizing mechanism can be summarized as follows: Large amount of mineralized material would be leached out and LREE-enriched hydrothermal solution would be subsequently produced as a result of thermo-chemical exchange reaction between acidic volcanic rocks and heated seawater that penetrated in advance from upper water mass. The spurting out from the seabed and quickly crystallizing in the seawater of hydrothermal solution are responsible for the formation of Cu-Zn sulfide and barite-amorphous SiO2 minerals that are characterized by enriched LREE and positively strong Eu anomalies.

Magma Origin and Evolution of Tengchong Cenozoic Volcanic Rocks from West Yunnan,China:Evidence from Whole Rock Geochemistry and Nd-Sr-Pb Isotopes

Tengchong Cenozoic volcanics that have record key information on the tectonic evolution and mantle features of the southeast margin of the Tibetan Plateau are of great importance because of its unique eruption history spanning the entire Quaternary period. Magma origin and evolution of Tengchong Cenozoic volcanic rocks were studied on the basis of Nd-Sr-Pb isotope and major and trace element data from different eruptions in the Ma＇anshan area. Different samples within one eruption show relative identical lithologies, chemical and isotopic compositions. However, the geochemical features for the five eruptions are distinct from each other. These volcanic rocks show low Mg# values （〈45）, moderate to high fractionation of LREEs and HREEs, and enrichment of Pb and Ba and depletion of Nb. Tengchong Cenozoic volcanic rocks were derived from an enriched mantle based on Nd-Sr-Pb isotopic studies. And lines of evidence show that crustal contamination should be involved before the eruption of different periods of Tengchong Cenozoic volcanic rocks. Older subducted components may be responsible for adakite recycling at various stages of evolution, which results in the origin of the enriched mantle source magma accounting for the isotopic features of Tengchong Cenozoic volcanic rocks. Segregated primitive magma pulsating injected into magma chamber, fractional crystallized and contaminated with crust component. Finally, magmas with distinct chemical and isotopic compositions for each eruption formed. The extension of the northeast segment of the Yingjiang tectonic belt triggered the pulsating eruption of the Cenozoic volcanics in the Tengchong area.

Geochemistry of trace elements and Sr-Nd isotopes of foraminifera shell from the Okinawa Trough

Trace elemental associations and Sr-Nd isotopic compositions are of important to recognition of biogenic material from mixed marine sediments. The foraminifera shell from the Okinawa Trough strongly enriches Sr,P,Mn and Ba, enriches Li,U,Th,Sc,Co,Cu,Pb,Zn,Cr,Rb,Y,Sb and light rare earth elements,slightly enriches V,Ga,Zr,Nb,Cd and middle rare earth elements,is short of Mo,In,Sn,Cs,Hf,Ta,W,Ti,Bi and heavy rare earth elements. The mechanism of elemental enrichment in forminifera is the concentrations of trace elements in sea water and selective absorption of trace elements during foraminifera living, as well as the geochemical affinity between major elements and trace elements.The REE (rare earth elements)partition pattern of foraminifera shell of the Okinawa Trough shows enrichment of middle rare earth elements with slightly negative Ce anomaly,which are different from those of foraminifera of the Pacific Ocean.The Sr,Nd isotopic ratios of the Okinawa Trough foraminifera are 0 709 769 and 0 512 162,respectively, which are different not only from those of oceanic water, but also from those of river water of China's Mainland, the former is slightly higher than those of oceanic water,but much lower than those of river water;the latter is slightly lower than those of oceanic water,but higher than those of river water,demonstrating that the Okinawa Trough sea water has been influenced by river water of China's Mainland.

Geodynamic Background of Intracontinental Cenozoic Alkaline Volcanic Rocks in Laojiezi, Western Yangtze Craton: Constraints from Sr-Nd-Hf-O Isotopes

The Laojiezi alkaline volcanic rocks, which are located in the intraplate region of the Yangtze craton, coincide with the formation of the Jinshajiang-Ailaoshan-Red River alkaline rock belt. Although this belt has been widely studied by geologists because of its porphyry-related Pb-Ag-Au polymetallic deposit and geotectonic location, the material sources of this belt are still debate. Whole-rock analyses show that these rocks have high total alkali contents(3.73–11.08 wt%), and their aluminum saturation index(ASI) values widely vary from 0.82 to 3.07, which comprise a metaluminous-peraluminous magma series. These rocks are characterized by high K(K2 O/Na2 O>1) and low Ti and Mg contents; enrichment in large-ion lithophile elements, such as Rb, Ba, K and light rare earth elements; and depletion in high field strength elements, such as Ta, Nb, P, and Ti. These rocks exhibit moderate Eu(Eu/Eu*=0.86–1.04) and Ce(Ce/Ce*=0.63–0.96) anomalies. Their(87 Sr/86 Sr)i, εNd(t), zircon εHf(t) and δ18 O values range from 0.70839 to 0.71013, from-10.16 to-12.45, from-19.6 to-5.8, and from 5.69‰ to 8.54‰, respectively, and their Nd and Hf two-stage model ages(TDM2) are 1.67–1.86 Ga and 1.27–2.02 Ga, respectively. These data reflect the primary partial melting of Paleoproterozoic to Mesoproterozoic lower crust with minor residual continental lithospheric mantle and supracrustal metasediments. The lithosphere was likely thickened along the southeastern margin of the Tibetan Plateau following the Indian-Asian continentcontinent collision(65–41 Ma). During the post-collision phase(36–16 Ma), the transition from a compressional to extensional setting triggered the convective removal of the over-thickened CLM beneath the Yangtze craton, which led to the upwelling of asthenospheric materials. This process created alkali-rich and high-K magma through the partial melting of the thickened lower crust. Magma that carried Cu-Au-Pb-Ag minerals was emplaced by strike-slip motion along the E-to W-or ENE-to WSWtrending tectonically weak zone, finally forming an alkaline porphyry Cu-Au-Pb-Ag polymetallic deposit.

Geochemistry of Late Mesozoic mafic dykes in western Fujian Province of China: Sr-Nd isotope and trace element constraints

The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%–17.54% and K2O/Na2O=0.65–0.77; as for the Bali dyke, Al2O3=16.89%–17.81% and K2O/Na2O=0.93–0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low εNd (t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their εNd (t) values vary between -6.8 and -6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their εNd (t) values are -4.7– -4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.

Geochemistry and Sr-Nd-Pb Isotopes of the Granites from the Hashitu Mo Deposit of Inner Mongolia, China： Constraints on Their Origin and Tectonic Setting

The Hashitu molybdenum deposit is located in the southern part of the Great Hinggan Range, NE China. Molybdenum mineralization is hosted by and genetically associated with monzogranite and porphyritic syenogranite. Sr-Nd-Pb isotopes of the intrusions show that the porphyritic syenogranite has initial ^87Sr/^86Sr ratios of 0.70418-0.70952, ENd（t） values of 1.3 to 2.1 （t=143 Ma）, ^206Pb/^204Pb ratios of 19.191-19.573, ^207Tpb/^204pb ratios of 15.551-15.572, and ^208Pb/^204Pb ratios of 38.826-39.143. The monzogranite has initial 87Sr/86Sr ratios of 0.70293-0.71305, εNd（t） values of 1.1 to 2.0 （t=-147 Ma）, ^206Pb/^204pb ratios of 19.507-20.075, ^207Pb/^204Pb ratios of 15.564-15.596, and ^208Pb/^204Pb ratios of 39.012-39.599. The calculated Nd model ages （TDM） for monzogranite and porphyritic syenogranite range from 866 to 1121 Ma and 795 to 1020 Ma, respectively. The granitic rocks in the Hashitu area have the same isotope range as granites in the southern parts of the Great Hinggan Range. The isotope composition indicates that these granites are derived from the partial melting of a juvenile lower crust originating from a depleted mantle with minor contamination by ancient continental crust. The integrating our results with published data and the Late Mesozoic regional tectonic setting of the region suggest that the granites in the Hashitu area formed in an intra-continent extensional setting, and they are related to the thinning of the thickened lithosphere and upwelling of the asthenosphere.

Geochronology, Geochemistry and Sr-Nd-Pb-Hf Isotopes of No. I Complex from the Shitoukengde Ni–Cu Sulfide Deposit in the Eastern Kunlun Orogen, Western China: Implications for the Magmatic Source, Geodynamic Setting and Genesis

The Shitoukengde Ni-Cu deposit, located in the Eastern Kunlun Orogen, comprises three mafic-ultramafic complexes, with the No. I complex hosting six Ni-Cu orebodies found recently. The deposit is hosted in the small ultramafic bodies intruding Proterozoic metamorphic rocks. Complexes at Shitoukengde contain all kinds of mafic-ultramafic rocks, and olivine websterite and pyroxene peridotite are the most important Ni-Cu-hosted rocks. Zircon U-Pb dating suggests that the Shitoukengde Ni-Cu deposit formed in late Silurian （426-422 Ma）, and their zircons have ~Hf（t） values of-9.4 to 5.9 with the older TDMm ages （0.80-1.42 Ga）. Mafic-ultramafic rocks from the No. I complex show the similar rare earth and trace element patterns, which are enriched in light rare earth elements and large ion iithophile elements （e.g., K, Rb, Th） and depleted in heavy rare earth elements and high field strength elements （e.g., Ta, Nb, Zr, Ti）. Sulfides from the deposit have the slightly higher ~34S values of 1.9-4.3%o than the mantle （0 ~ 2%o）. The major and trace element characteristics, and Sr-Nd-Pb and Hf, S isotopes indicate that their parental magmas originated from a metasomatised, asthenospheric mantle source which had previously been modified by subduction-related fluids, and experienced significant crustal contamination both in the magma chamber and during ascent triggering S oversaturation by addition of S and Si, that resulted in the deposition and enrichment of sulfides. Combined with the tectonic evolution, we suggest that the Shitoukengde Ni-Cu deposit formed in the post-collisional, extensional regime related to the subducted oceanic slab break-off after the Wanbaogou oceanic basalt plateau collaged northward to the Qaidam Block in late Silurian.

Zircon LA-ICP-MS U-Pb dating and Sr-Nd isotope study of the Guposhan granite complex, Guangxi, China

Zircon U-Pb dating by the LA-ICP-MS method was applied to determining the ages of different units of the Guposhan granite complex, among which the East Guposhan unit is 160.8±1.6 Ma, the West Guposhan unit is 165.0±1.9 Ma, and the Lisong unit is 163.0±1.3 Ma in age. Much similarity in ages of the three units has thus proved that the whole Guposhan granite complex was formed in the same period of time. They were the products of large-scale granitic magmatism through crust-remelting in the first stage of the Middle Yanshanian in South China. However, the three units have differences both in petrology and in geochemistry. Besides the differences in major, trace and rare-earth elements, they are distinct in their Rb-Sr and Sm-Nd isotopic compositions. The East Guposhan unit and Lisong unit and its enclaves have a similar (87Sr/86Sr)i value of 0.7064 with an average of εNd(t)=-3.03, indicating that more mantle material was evolved in the magma derivation; whereas the West Guposhan unit has a higher (87Sr/86Sr)i value of 0.7173 but a lower εNd(t) value of -5.00, and is characterized by strong negative Eu anomalies and higher Rb/Sr ratios, suggesting that its source materials were composed of relatively old crust components and new mantle-derived components. In addition, an inherited zircon grain in the East Guposhan unit (GP-1) yielded a 206Pb/238U age of 806.4 Ma, which is similar to the ages of the Jiulin cordierite granite in northern Jiangxi and of the Yinqiao migmatic granite in Guangxi in the HZH granite zone. All this may provide new evidence for Late Proterozoic magmatism in the HZH granite zone.

Geochemistry, U-Pb Geochronology and Sr-Nd-Hf Isotopes of the Early Cretaceous Volcanic Rocks in the Northern Da Hinggan Mountains

Whole-rock geochemical, zircon U-Pb geochronological and Sr-Nd-Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2 （73.19-77.68 wt%） and Na20＋K20 （6.53-8.98 wt%） contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206pb/23SU ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate （STSr/S6Sr）i values of 0.704912 to 0.705896, slightly negative eNd（t） values of -1.4 to -0.1, and positive Cur（t） values of 3.7 to 8. Their zircon Hf and whole-rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol-Okhotsk Ocean.

Devonian alkaline magmatism in the northern North China Craton:Geochemistry,SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton（NCC）.The intrusion is mainly composed of quartz-monzonite.Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca.381.5 Ma.The rock is metaluminous with high（Na2O ＋ K2O） values ranging from 8.46 to 9.66 wt.%.The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies.The Wulanhada rocks exhibit high initial values of（87Sr/86Sr）t = 0.70762-0.70809,low εNd（t） =-12.76 to-12.15 values and negative values of εHf（t） =-23.49 to-17.02 with small variations in（176Hf/177Hf）,（0.281873-0.282049）.These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC.The Wulanhada rocks,together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions,constitute a post-collisional magmatic belt along the northern NCC.

东昆仑大格勒地区碱性杂岩体中辉石岩的年代学、地球化学、Sr-Nd同位素特征及其地质意义

近年团队在东昆仑大格勒地区发现了Nb、P矿化碱性杂岩体,并对杂岩体的岩石组合、岩石学特征及含矿性开展了研究,初步圈定了Nb、P矿化体,显示东昆仑具有寻找与碱性岩-碳酸岩型稀有稀土矿潜力。对碱性岩的形成时代、地球化学组成、矿物成分以及形成环境等方面的研究,不仅对重建东昆仑古构造环境具有重要的意义,还可推动东昆仑稀有金属成矿规律研究与找矿突破。本文在野外地质调查的基础上,选取杂岩体中重要的含矿辉石岩为研究对象,开展了单矿物电子探针原位分析、磷灰石和榍石U-Pb年代学、岩石地球化学及Sr-Nd同位素研究。结果显示,东昆仑大格勒地区辉石岩的形成时代为418 Ma。辉石岩主要矿物中存在似长石(霞石)、碱性暗色矿物(富铁黑云母),单斜辉石为透辉石,角闪石为钙质角闪石(铁韭闪石),黑云母为铁质黑云母和镁质黑云母。岩石地球化学特征显示辉石岩具有富K(K_(2)O>Na_(2)O)、CaO含量高、轻稀土元素强烈富集、富集Rb、Ba、Sr等大离子亲石元素,不亏损Nb、Ta元素,亏损Zr、U、Ti等高场强元素的特征。全岩的(87Sr/86Sr)i为0.704058~0.704278,ε_(Nd)(t)为-0.4~-0.2。矿物组成、元素和同位素地球化学特征均指示大格勒辉石岩为钾质碱性岩,具有与OIB相似的特征,岩浆源区为EMⅠ型地幔端元。岩石的形成过程为母岩浆在相对较深的地幔源区经历了1%~3%较低程度的部分熔融作用,在上侵过程中经历了较强的分离结晶作用和微弱的同化混染作用,其形成时代为该地区岩浆活动最强烈的时期,可能与碰撞后软流圈地幔的上涌和岩石圈的强烈伸展相关。

东昆仑大格勒地区富铌橄榄岩矿物学、地球化学及Sr-Nd同位素特征

东昆仑大格勒地区碳酸岩-碱性杂岩富铌稀土矿化的发现,对青海省优质稀有稀土矿产的勘查及综合研究具有重要启示。本文以大格勒杂岩体中蛇纹石化富铌橄榄岩为研究对象,开展了矿物学、岩石地球化学和Sr-Nd同位素研究。矿物学分析显示,富铌橄榄岩中铌矿物主要为铌易解石(Nb_(2)O_(5)=50.33%~52.50%)、铌钙矿(Nb_(2)O_(5)=48.20%~50.27%)、褐铈铌矿(Nb_(2)O_(5)=42.80%~43.45%)、Stefanweissite矿(Nb_(2)O_(5)=33.14%~34.78%),次为钛铁矿(Nb_(2)O_(5)=3.45%~4.89%)。样品具低SiO_(2)(20.16%~29.20%)和K_(2)O+Na_(2)O(0.02%~2.47%)含量,高Mg O(22.88%~27.69%)含量和Mg^(#)值(74~82),稀土元素总量较高(∑REE=772~1807μg/g),稀土元素配分模式强烈富集LREE、亏损HREE,Eu异常不明显(δEu=0.87~1.17)。原始地幔标准化微量元素蛛网图显示,样品富集Nb、Ta、Th、U、Sr、Pb,Nb含量高,为981~1753μg/g,平均1378μg/g,可能与晚阶段碳酸岩侵入流体交代作用有关。样品的(87Sr/86Sr)i值为0.70378~0.70396,ε_(Nd)(t)值为-0.49~0.31,岩浆源区可能为EMⅠ型地幔端元。综合区域构造背景,大格勒岩体中富铌橄榄岩的形成可能与该地区拉张环境下岩石圈的强烈伸展和软流圈地幔的上涌相关。

松嫩沙地元素和Sr-Nd同位素组成特征及其对区域粉尘物源的指示

松嫩沙地位于欧亚黄土带最东端,其物质组成的研究有利于重建松嫩平原冰期—间冰期粉尘传输路径。为此,系统采集了松嫩沙地123个河流沙和风成沙样品,对其进行分粒级的常量、微量和稀土元素以及Sr-Nd同位素组成等地球化学分析,并利用Frequentist模型进行风尘物源定量重建,探讨松嫩沙地不同区域、不同粒级组分对哈尔滨黄土的贡献及搬运路径。结果表明,松嫩沙地经历了初级的化学风化过程(<63μm、<30μm、<10μm组分CIA平均值分别为55.20、57.46、57.51),有较低的再循环历史(<63μm、<30μm、<10μm组分CIA/WIP比值的平均值为0.98、1.08、1.04)。根据不同粒度组分的元素地球化学与Sr-Nd同位素组成特征,将松嫩沙地划分为西北部和西南部两个地球化学分区。不同粒度组分地球化学组成的定量重建结果表明,这两个分区不同粒级组分(<63μm、<30μm、<10μm)对哈尔滨黄土的贡献度分别为:75.7%~88.5%、73.4%~84.9%、61.0%~89.7%(西南部)和11.5%~24.3%、15.1%~26.6%、10.3%~39%(西北部)。本研究揭示了末次冰期以来松嫩平原粉尘传输路径以西南方向为主,与冰期以西北风为主导的环流模式存在差异。

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.

Genesis and Uranium Sources of Leucogranite-hosted Uranium Deposits in the Gaudeanmus Area, Central Damara Belt, Namibia: Study of Element and Nd Isotope Geochemistry

This paper presents a systematic study of major and trace elements and Sm-Nd isotopes in leucogranites closely related to uranium mineralization in the Gaudeanmus area, Namibia. The results illustrate that the uraniferous leucogranites possess high SiO2 （68.8wt%-76.0wt%, average 73.1wt%） and K （4.05wt%-7.78wt%, average 5.94wt%） contents, and are sub-alkaline and metaluminous to weakly peraluminous, as reflected by A/CNK values of 0.96-1.07 with an average of 1.01. The leucogranites are rich in light rare earth elements （LREE/HREE = 2.53-7.71; （La/Yb）s = 2.14-10.40）, have moderate Eu depletion and high Rb/Sr ratios （2.03-5.50 with an average of 4.36）; meanwhile, they are enriched in Rb, K, Th, U and Pb, and depleted in Ba, Nb, Ta, and Sr. The tNd（t） values of uraninites range from -14.8 to -16.5, and the two-stage Nd model ages are 2.43-2.56 Ga. Detailed elemental and Sm-Nd isotopic geochemical characteristics suggest that the leucogranites were formed in a post- orogenic extensional environment. The U-rich pre-Damara basement was the main source of uranium during the primary mineralization event, which is disseminated in leucogranites, whereas the uranium mineralization in veins possibly resulted from remobilization of the primary uranium minerals.

Refnement of the supercontinent cycle with Hf, Nd and Sr isotopes

The combined use of Hf,Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons.Sr and Nd seawater isotopes,although not as precise as εNd and εHf distributions,also record input from ocean ridge systems.Unlike detrital zircons where sources cannot be precisely located because of crustal recycling,both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data.Furthermore,primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly.Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions,river system sampling,or geologists,and these may be responsible for most of the decorrelation observed between isotopic systems.Distributions between 3.5 and 2 Ga based on εHf median values of four detrital zircon databases as well as our compiled εNd database are noisy but uniformly distributed in time,whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations.Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens.Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases,which may partially reflect variations in both the number of sample locations and the number of samples per location.External and internal orogens show similar patterns in εNd and εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions.However,both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode.Many secular changes in εHf and εNd distributions correlate with the supercontinent cycle.Although supercontinent breakup is correlated with short-lived decreasing εHf and εNd （≤ 100 Myr） for most supercontinents,there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna.Assembly of supercontinents by extroversion is recorded by decreasing εNd in granitoids and metasediments and decreasing εHf in zircons,attesting to the role of crustal reworking in external orogens in advancing mode.As expected,seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion.Pangea is the only supercontinent that has a clear isotopic record of introversion assembly,during which median εNd and εHf rise rapidly for ≤ 100 Myr.Although expected to increase,radiogenic seawater Sr decreases （and seawater Nd increases） during assembly of Pangea,a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode.The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime,as supported by Nd and Hf model ages.

A Study of Sr, Nd and O Isotopes of the K-rich Melanocratic Dykes in the Late Mesozoic Gold Field in the Jiaodong Peninsula

Geologic, petrographic and petrochemical studies of the late Mesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr),=0.70895-0.71140) and low (143Nd/144Nd)1 ratios (varying from 0.51135 to 0.51231); and its δ18Osmow, whole rock values vary from +5.8%c to +10.6%c with a mean of +7.1%c. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced metasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enriched mantle in its mature stage in the back-arc spreading environment. The evolution of magmas is associated with two trends, i.e., fractional crystallization and mixing with or intensive contamination by palaeo-crust materials or metamorphic rocks. The former process is evident in the gold field system of quartz-vein type, whereas the latter is dominated in the gold field system of the altered-rock type. This conclusion is very important for more detailed study of petrogenesis and mineralization through the crust-mantle interaction (exchange) in the Mesozoic in this region.

滇西北浪都夕卡岩矿床致矿岩体的岩石地球化学、年代学、Sr-Nd-Pb同位素组成及其地质意义

滇西北浪都侵入岩体位于义敦岛弧南段的中甸弧,在空间上属浪都夕卡岩型铜矿床的致矿岩体。本文以浪都致矿岩体中石英二长斑岩为对象,对其开展了锆石SIMS(二次离子质谱)U-Pb定年、岩石地球化学和Sr-Nd-Pb同位素研究。锆石U-Pb定年结果显示浪都石英二长斑岩的成岩年龄为219.2±1.8 Ma(2σ,MSWD=1.3,n=13),与中甸弧中晚三叠世侵入岩体成岩年龄基本一致。浪都石英二长斑岩具有中等SiO2含量(w(SiO_(2))为60.42%~66.44%),高Al_(2)O_(3)(w(Al2O3)为14.56%~16%)和高K2O(w(K_(2)O)为3.49%~4.49%)含量,以富集LREE和Rb、Ba、Sr等大离子亲石元素,相对亏损HREE和Ta、Nb、Ti等高场强元素为特征,在化学成分上属于高钾钙碱性系列。此外,浪都石英二长斑岩的87Sr/86Sr初始值为0.7054~0.7061,εNd(t)值为-3.5~-2.5,且所有样品在Pb同位素构造模式图上均落在造山带演化线附近。结合浪都石英二长斑岩的成岩时代、岩石化学和同位素特征,本文认为浪都致矿岩体形成于晚三叠世甘孜-理塘洋壳西向俯冲增生的构造背景,是交代地幔部分熔融形成的熔体在上升过程中受到下地壳物质混染,并经历分离结晶作用形成的产物。浪都石英二长斑岩的高Sr/Y比值和高V/Sc比值,指示岩浆具有高含量H2O和较高的氧逸度,是该区斑岩-夕卡岩铜矿床形成的关键。

Episodic crustal growth in the Tanzania Craton:evidence from Nd isotope compositions

The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crest-mantle separation ages,and events marking juvenile crustal addition and crustal recycling.Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton,although a rock record of such antiquity is yet to be found there.The most significant period of juvenile crustal addition as well as crustal recycling is 2.7-2.6 Ga.The Nd isotopes of marie samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton,with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle.The Nd isotope section reveals significant differences in Nd isotopes between the north eraton and central craton;compared to the north craton,the central craton yields a Nd model age that is approximately 100 Ma older, and its εNd(t)values are more negative,indicating that the two parts of the craton have different mantle source regions.Different types of granitoids are distributed in the Tanzania Craton,such as high-K and low-Al granite,ealc-alkaline granite,peraluminous granite and transitional types of tonalite-trondhjemite- granodiorites (TTGs).Most of the granitoids formed later than the marie rocks in syn-collision and postcollision events.

Characteristics of rare-earth elements (REE),strontium and neodymium isotopes in hydrothermal fluorites from the Bailashui tin deposit in the Furong ore field,southern Hunan Province,China

In this paper the authors present the REE concentrations and Sr and Nd isotopic compositions of fluorites from the Bailashui tin deposit of the Furong ore field,southern Hunan Province. The results showed that the total amount of REE in fluorites is usually low,ranging from 0.705 to 8.785 μg/g with the chondrite-normalized REE dis-tribution patterns similar to those of the Qitianling granites in the study area,characterized by LREE-enrichment patterns with pronounced negative Eu anomalies. The fluorites vary in Sr isotopic composition within the range of 0.7083-0.7091,the values are lower than those of the granites and higher than those of the host carbonate rocks in this area. The εNd(t) values of fluorites vary between -9.4 and +10.3,revealing that both the crust-and mantle-source materials were involved in the ore-forming hydrothermal fluids. Combined with previous studies on this ore deposit,the Bailashui tin deposit is temporally and spatially closely related with granitic magmatism in this area. The hydro-thermal fluorites are the product of fluid/rock interactions between granitic magmatic hydrothermal fluid and marine carbonate rocks. The REE and F in the ore-forming fluid were derived from the granites,whereas Sr in the ore-forming fluid came mainly from the granitic magmatic hydrothermal fluid and marine carbonate rocks,although variations in Sr isotopic composition cannot be explained by a simple mixture of these two end-members. Evidence demonstrated that the ore-forming fluids are of crustal-mantle mixing origin,but that the fluids were probably in-completely homogenized and this may be caused by inhomogeneous mixing of the fluids of different sources.