4 U -Pb ~ 32 Ma Fig. 1 Major tectonic units of Qinghai -Tibet Plateau km 2 U -Pb 1 2 Fig. 2 Geological map of Zhalaxi

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1 GLOBAL GEOLOGY Vol. 35 No. 4 Dec U -Pb U -Pb U -Pb ~ Ma ± Ma - La /Yb N = ~ δeu = ~ LILE Rb Ba K HFSE Nb Ta P Ti Zr Th C U -Pb P595 P597 A doi /j. issn Zircon U -Pb chronology and geochemistry of Zhalaxiageyong trachyte in Tuotuohe area Qinghai NING Chuan-qi 1 LI Bi-le 1 WANG Bin 2 WANG Guo-zhi 1 YANG Wen-long 1 1. College of Earth Sciences Jilin University Changchun China 2. No. 5 Exploration Team of Hebei Geological and Prospecting Bureau Tangshan Hebei China Abstract The authors studied the characteristics of U -Pb chronology and geochemistry of trachyte in Zhalaxiageyong of Tuotuohe area Qinghai. The results show that the zircon U -Pb age is ~ Ma with weighted average age of ± Ma which indicate the trachyte formed in Oligocene. The rock belongs to high - SiO 2 high - K peraluminous-metaluminous shoshonite series. The REE pattern appears rightward sloping with significant fractionation of LREE and HREE La /Yb N = ~ mean = with slight positive Eu anomalies δeu = ~ mean = The samples are characterized by enrichment in large ion lithophile elements LILEs e. g. Rb Ba K depletion in high field strength elements HFSEs e. g. Nb Ta P Ti but HFSEs Th and Zr are richer. The rock has the characteristics of C-type adakite as a new product of the partial melting of mafic lower crust. On the basis of the previous research results in combination with regional comparison it is considered that Zhalaxiageyong trachyte formed in extensional stage after orogenic thickening in Early Cenozoic. Key words Qinghai Tuotuohe area Zhalaxiageyong trachyte zircon U -Pb age chronology geochemistry libl@ jlu. edu. cn

4 U -Pb 921 0 2 1 2 3 4 40 ~ 32 Ma 1 1 - Fig. 1 Major tectonic units of Qinghai -Tibet Plateau 5 0.

2 4 U -Pb ~ 32 Ma Fig. 1 Major tectonic units of Qinghai -Tibet Plateau km 2 U -Pb 1 2 Fig. 2 Geological map of Zhalaxiageyong mining in Tuotuo River area of Qinghai R

世 922 界区的粗面岩采样坐标 92 01' 44 E 34 24' 14 N 西安 80 坐标系岩石类型为粗面岩图 3 岩石呈浅灰色斑状结构块状构造斑晶矿物成分为碱性长石 35% 黑云母 10% 碱性长石呈自形--半自形晶 3 5 mm 可见正长石的卡式地质第 35 卷双晶部分长石高岭石化绢云母化黑云母为自形结构 2 3 mm 大部分出现暗化现象

3 世 922 界区的粗面岩采样坐标 92 01' 44 E 34 24' 14 N 西安 80 坐标系岩石类型为粗面岩图 3 岩石呈浅灰色斑状结构块状构造斑晶矿物成分为碱性长石 35% 黑云母 10% 碱性长石呈自形--半自形晶 3 5 mm 可见正长石的卡式地质第 35 卷双晶部分长石高岭石化绢云母化黑云母为自形结构 2 3 mm 大部分出现暗化现象仅个别晶体可见多色性其他多呈黄褐色无多色性基质由微粒碱性长石 50% 组成岩石中含有少量其他暗色矿物 5% 主要为角闪石 a d 正交偏光 b c 单偏光 kp 钾长石 Bi 黑云母图3 Fig 扎拉夏格涌粗面岩显微照片 Microphotographs of trachytes in Zhalaxiageyong area 分析方法锆石单矿物挑选制靶反射光阴极发光在河北廊坊区域地质调查研究所完成利用标准重矿物分离技术在双目镜下仔细挑选表面整洁无裂纹形状规则的锆石颗粒再把挑选出来的锆石粘贴在双面胶上并用无色透明环氧树脂将锆石颗粒固定待环氧树脂固化之后对其表面进行抛光至锆石中心测试分析之前通过反射光和阴极发光 CL 图像仔细研究锆石的晶体形态并揭露其内部结构特征以选择测试分析的最佳点U--Pb 年龄测定在吉林大学测试实验中心利用 LA--ICP--MS 分析完成激光剥蚀系统为 GeoLas2005 ICPMS 为 Agilent 7500 a本次激光剥蚀过程分别采用氦气和氩气作为载气和补偿气每个时间分析的数据包括 s 的空白信号和 50 s 的样品信号对分析数据的离线处理包括对样品和空白信号的选择仪器灵敏度校正元素含量及 U--Th--Pb 同位素比值和年龄计算采用实验室软件 ICPMS --DataCal 等完

4 4 U -Pb ICP -MS BH- 206 Pb / 238 U 6 VO -2 BCR -2 GBW07103 GBW07104 NISTSRM Si > < 5% < μm 7 < 10% 3 X - XRF GB /T % ~ 5% 15 Agilent 7500A U-Pb 1 Table 1 1 LA -ICP MS U -Pb LA -ICPMS U -Pb analysis result of zircons from trachyte in Zhalaxiageyong area Pb Th U Th /U 207 Pb / 206 Pb 1σ 207 Pb / 235 U 1σ 206 Pb / 238 U 1σ 207 Pb / 206 Pb 1σ /Ma 206 Pb / 238 U 1σ 207 Pb / 235 U 1σ CL % ~ % % w Al 2 O 3 = 80 ~ 100 μm Th /U % ~ % % w CaO ~ U = 0. 81% ~ 3. 07% 1. 33% w MgO -Pb 15 = 0. 32% ~ 0. 37% 0. 34% w Na 2 O 206 Pb / 238 U ~ Ma = 0. 54% ~ 1. 01% 0. 76% w K 2 O ± Ma 5 = 9. 30% ~ % % 2 TAS 6 δ A / CNK w SiO 2 = - 7 a K 2 O - SiO 2

924 35 Fig. 4 4 CL CL images of zircons from trachyte in Zhalaxiageyong area Fig. 5 5 U -Pb a b Diagrams of U - Pb concordia a and weighted average age b for trachyte in Zhalaxiageyong area 7b 8a 3.

5 Fig. 4 4 CL CL images of zircons from trachyte in Zhalaxiageyong area Fig. 5 5 U -Pb a b Diagrams of U - Pb concordia a and weighted average age b for trachyte in Zhalaxiageyong area 7b 8a δeu = ~ ΣREE = ~ b 10-6 LREE /HREE = ~ LILE Rb Ba K La /Yb N = HFSE Nb Ta P Ti ~ Zr Th Ti Rb / Sr ~ 0. 96

6 4 U -Pb 925 Fig. 6 6 TAS TAS diagram of trachyte in Zhalaxiageyong area Th /La ~ La /Nb ~ Th /Nb ~ Y Yb Y = ~ Yb = ~ O C LREE HREE Table 2 2 % Analysis results and eigenvalues for major elements of trachyte in Zhalaxiageyong area ZL -H -1 ZL -H -2 ZL -H -3 ZL -H -4 ZL -H -5 ZL -H -6 ZL -H -7 ZL -H -8 ZL -H -9 SiO TiO Al 2 O FeO Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total δ A /CNK R R δ = w Na 2 O + K 2 O 2 /w SiO 2-43 A /CNK = Al 2 O 3 / CaO + Na 2 O + K 2 O R1 = 4Si - 11 Na + K - 2 Fe + Ti R2 = 6Ca + 2Mg + Al.

7 Table 3 Analysis results of REE and trace elements for trachyte in Zhalaxiageyong area /10-6 ZL -H -1 ZL -H -2 ZL -H -3 ZL -H -4 ZL -H -5 ZL -H -6 ZL -H -7 ZL -H -8 ZL -H -9 Cs Rb Sr Ba Ga Nb Ta Zr Hf Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y ΣREE LREE HREE LREE /HREE La N /Yb N δeu δce

8 4 U -Pb A /NK - A /ACNK a K 2 O - SiO 2 b Fig. 7 Diagrams of A /NK - A /ACNK a and K 2 O - SiO 2 b for trachyte in Zhalaxiageyong area Fig. 8 8 a b Chondrite-normalized REE patterns a and spider diagram of primitive mantel-normalized trace elements b 8 K 2 O 1. 72% K 2 O 3. 9% 9 9 K 2 O LILE Rb Ba K w K 2 O = 9. 30% ~ % HFSE Nb Ta P Ti % LREE Zr Th HREE Nb Ta Zr Th 8a δeu = ~ Y Yb Rb /Sr ~ C Th /La ~ 0. 53

9 La /Nb ~ Th /Nb ~ C C Yb N - La /Yb N 9a 11 C Nb /Ta ± 2 Nb /Ta 11 ~ Nb /Ta ~ > 50 km Nb /Ta < 43 km > 50 km C 18 R1 - R2 9b C Ma ± 65 ~ 41 Ma P Ti Eu NWW Y Yb 19 9 Yb N - La /Yb N a R1 - R2 b Fig. 9 Yb N - La /Yb N a and R1 - R2 b diagrams of trachyte in Zhalaxiageyong area

10 4 U -Pb SONG Zhong-bao DU Yu-liang LI Zhi-ming et al. km NE 2 Characteristics of mineral resources in Qinghai Province J. Journal of Earth Sciences and Environment Ma J CHEN Jian-ping TANG Ju-xing CHEN Yong et al. 23 Geological characteristics and metallogenic model of the Narigongma copper-molybdenum deposit in the north part of the Sanjiang region Southwest China J. Geoscience J SONG Zhong-bao JIA Qun-zi CHEN Xiang-yang et al. The petrogenic age of Narigongma granitic diorite-porphyry in the northern part of the Sanjiang region and its geo- - logical implications J. Acta Geoscientica Sinica J PAN Gui-tang XU Qiang WANG Li-quan. Theory and practice of mineral resource prognosis and evaluation from research on Sanjiang region west-eastern China 40 ~ 32 Ma J. Mineral Deposits Suppl J ZHANG Yu-zhong SONG Yu-cai HOU Zeng-qian et 5 al. Geological and geochemical studies of the Nariniya 1 lead-zinc deposit in Tuotuohe area Qinghai Province ± Ma J. Acta Petrologica et Mineralogica Wiedenbeck M Alle P Corfu F et al. Three natural - C zircon standards for U -Th -Pb Lu -Hf trace element and REE analyses J. Geostandards Newsletter Liu Y S Gao S Hu Z C et al. Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans- North China Orogen U -Pb dating Hf isotopes and trace elements in zircons from mantle xenoliths J. 1. Journal of Petrology / J J

11 ZHANG Qi QIAN Qing WANG Er-qi et al. Existence of East China Plateau in mid-late Yanshanian period implication from adakites J. Scientia Geologica Sinica J ZHANG Qi. Reappraisal of the origin of C-type adakitic rocks J. Acta Petrologica et Mineralogica ZHANG Qi WANG Yan QIAN Qing et al. The char acteristics and tectonic-metallogenic significances of the adakites in Yanshan Period from eastern China J. Acta Petrologica Sinica Defant M J Drummond M S. Derivation of some modern arc magmas by melting of young subducted lithosphere J. Nature M DENG Jin-fu LUO Zhao-hua SU Shang-guo et al. Petrogenesis tectonic setting and metallogenesis M. Beijing Geological Publishing House Taylor S R McLennan S M. The continental crust its composition and evolution M. Oxford Blackwell of Geophysics Weaver B L. The origin of ocean island basalt end-member compositions trace element and isotopic constraints J. Earth and Planetary Science Letters / Saunders A D Norry M J Tarney J. Origin of MORB and chemically depleted mantle reservoirs trace element constraints J. Journal of Petrology 1988 Spec Martin H. The adakitic magmas modern analogues of Archaean granitoids J. Lithos Geology in China J. 23. delamination HOU Zeng-qian GAO Yong-feng MENG Xiang-jin et al. Genesis of adakitic porphyry and tectonic controls on the Gangdese Miocene porphyry copper belt in the Tibetan orogen J. Acta Petrologica Sinica Hou Z Q Gao Y F Qu X M et al. Origin of adakitic intrusives generated during mid-miocene east-west extension in southern Tibet J. Earth & Planetary Science Letters / C J. 19. Pb -Zn -Ag -Cu J HOU Zeng-qian SONG Yu-cai LI Zheng et al. Thrust-controlled sediments-hosted Pb -Zn -Ag -Cu deposits in eastern and northern margins of Tibetan orogenic belt geological features and tectonic model J. Mineral Deposits Spurlin M S Yin A Horton B K et al. Structural evolution of the Yushu -Nangqian region and its relationship to syncollisional igneous activity east-central Tibet J. GSA Bulletin J LI Ya-lin WANG Cheng-shan YIN Hai-sheng et al. Cenozoic thrust system and uplifting of the Tanggula Mountain northern Tibet J. Acta Geologica Sinica J XU Zhi-qin YANG Jing-sui LI Hai-bing et al. The Qinghai-Tibet Plateau and continental dynamics a review on terrain tectonics collisional orogenesis and process and mechanisms for the rise of the plateau J. J GAO Shan JIN Zhen-min. Delamination and its geodynamical significance for the crust-mantle evolution J. Geological Science and Technology Information

NE NW NE NW 6 NE NW NE ~ 85 1 ~ 4 NW NE 702 NE NW 1 NE30 ~ 50 SE 50 ~ 60 1a I II I 73 m 3. 5 m g /t II 62 m 60 ~ 100

NE NW NE NW 6 NE NW NE ~ 85 1 ~ 4 NW NE 702 NE NW 1 NE30 ~ 50 SE 50 ~ 60 1a I II I 73 m 3. 5 m g /t II 62 m 60 ~ 100 34 2 2015 6 GLOBAL GEOLOGY Vol. 34 No. 2 Jun. 2015 1004 5589 2015 02 0385 11 1 1 2 2 1 1 1. 130061 2. 702 157021 σ < 3. 3 Rb Ba K Pb LILE Nb Ta P Ti HFSE LREE HREE Nb /Ta 14. 7 Zr /Hf 26. 23 ~ 38. 37 P618.