Eocene potassic and ultrapotassic volcanism in south Tibet: New constraints on mantle source characteristics and geodynamic processes

In the Yangbajing area, southern Tibet, several monogenic volcanoes were conformably superimposed on the Linzizong calc-alkaline volcanic successions. According to their petrologic and geochemical characteristics, these monogenic volcanoes are composed of three rock varieties: tephritic phonolitic plugs and shoshonitic and trachytic lavas. Their geochemical systematics reveals that low-pressure evolutionary processes in the large voluminous Linzizong calc-alkaline magmas were not responsible for the generation of these potassic–ultrapotassic rocks, but the significant change in petrologic and geochemical characteristics from the Linzizong calc-alkaline to potassic–ultrapotassic magma is likely accounted for the change of metasomatic agents in the southern Tibetan lithospheric mantle source during the Paleocene to Eocene. The tephritic phonolites containing both leucite and plagioclase show primary ultrapotassic character similar to that of Mediterranean plagioleucititic magmas. Radiogenic Sr increases with SiO₂ in the xenolith-bearing trachytes strongly suggesting significant crustal assimilation in the shoshonitic magmas. The Yangbajing ultrapotassic rocks have high K₂O and Al₂O₃, and show depletion of high field strength elements (HFSEs) with respect to large ion lithophile elements. In primitive mantle-normalized element diagrams, all samples are characterized by positive spikes at Th (U) and Pb with negative anomalies at Ba, Nb–Ta and Ti, reflecting the orogenic nature of the ultrapotassic rocks. They are characterized by highly radiogenic ⁸⁷Sr/⁸⁶Sr_(i) ratios (0.7061–0.7063) and unradiogenic ¹⁴³Nd/¹⁴⁴Nd_(i) (0.5125), and Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 18.688–18.733, ²⁰⁷Pb/²⁰⁴Pb = 15.613–15.637, and ²⁰⁸Pb/²⁰⁴Pb = 38.861–38.930) similar to the global subducting sediment. Strong enrichment of incompatible trace elements and high Th fractionation from the other HFSEs (such as Nb and U) clearly indicate that the Th-enriched sedimentary component in a network veined mantle source was mainly introduced by sediment-derived melts. In addition, the ultrapotassic rocks have significant Ce (Ce/Ce* = 0.77–0.84) and Eu (Eu/Eu* = 0.72–0.75) anomalies, suggesting a subduction sediment input into the southern Tibetan lithospheric mantle source. In contrast, high U/Th (> 0.20) and Ba/Th (> 32) and low Th/La (< 0.3) in the shoshonites indicate that the Eocene potassic magma originated from partial melting of the surrounding peridotite mantle pervasively affected by slab-related fluid addition from the dehydration of either the subducting oceanic crust or the sediment. Thus, at least two different subduction-related metasomatic agents re-fertilized the upper mantle. According to the radiometric ages and spatial distribution, the Gangdese magmatic association shows a temporal succession from the Linzizong calc-alkaline to ultrapotassic magmas. This indicates a late arrival of recycled sediments within the Tibetan lithospheric mantle wedge. The most diagnostic signatures for the involvement of continent-derived materials are the super-chondritic Zr/Hf (45.5–49.2) and elevated Hf/Sm values (0.81–0.91) in the ultrapotassic rocks. Therefore, the occurrence of orogenic magmatism in the Gangdese belt likely represents the volcanic expression of the onset of the India–Asia collision, preceding the 10 Ma Neo-Tethyan slab break-off process at 42–40 Ma. The absence of residual garnet in the mantle source for the ultrapotassic volcanism seems to imply that the southern Tibetan lithosphere was not been remarkably thickened until the Eocene (～ 50 Ma).     

安徽铜陵矿集区与块状硫化物矿床有关的热水沉积岩

通过对铜陵地区系统的填图观察和室内研究 ,初步确定了铜陵地区海西期的热水沉积岩。它们是 :石炭系黄龙组 ,由下而上、由内到外的层序包括含硫化物滑石蛇纹石岩 (由原岩为含水富镁碳酸盐岩 +二氧化硅的热水沉积岩变质而成 )→块状、层状 (含铜金 )黄铁矿±铁碧玉±硬石膏±菱铁矿±菱锰矿±硅质岩→含炭质粘土→白云岩 ;二叠系为栖霞组 (含铅锌金 )硅质岩、孤峰组 (含锰 )硅质岩和大隆组 (含钼 )硅质岩。这些热水沉积岩总体构成铜陵地区晚古生代多期热水活动的格局。常量元素、微量元素和稀土元素的分析结果表明 ,本区的硅质岩具有热水沉积岩的特征。氧同位素估算的热水硅质岩古温度在 4 9℃以上。铜陵地区的热水沉积岩对层控矿床的勘查和隐伏矿床的预测评价具有重要的理论和实际意义 ,尤其是黄龙组底部的热水沉积岩是铜陵地区找矿的重要标志 ,对于指导长江中下游地区 ,如宁芜等地 ,寻找该层位的矿床有着不可替代的指示意义。     

安徽铜陵朝山金矿床稳定同位素、稀土元素地球化学研究

朝山金矿床位于安徽铜陵狮子山矿田，属于矽卡岩型金矿床，侵入岩体为白芒山辉石二长闪长岩体。成矿过程包括矽卡岩阶段、石英-硫化物阶段和碳酸盐阶段3个主要成矿阶段。文章通过对朝山金矿床的氢、氧、碳、硫、硅同位素组成和稀土元素地球化学特征的研究，探讨成矿溶液中水、碳、硅和硫的来源以及成矿溶液的演化问题。研究表明，成矿热液早期以岩浆热液为主，随着成矿过程的进行，加入的大气降水比重越来越大，到晚期可能主要以大气降水为主。该矿床矿石中方解石的碳、氧同位素组成与矿区大理岩的碳、氧同位素组成明显不同，其δ^13 Cv-PDB、δ^18OV-SMOW值分别为-4．5～-5．3‰、13．9～14．0‰，与岩浆作用形成的CO2的碳、氧同位素组成一致，表明矿石中方解石的碳、氧来源于岩浆作用。硅和硫具深部岩浆或岩浆热液来源的特点。     

Underplating in the middle-lower Yangtze Valley and model of geodynamic evolution:Constraints from geophysical data

~~Underplating in the middle-lower Yangtze Valley and model of geodynamic evolution:Constraints from geophysical data1)East China petroleum Administration,1988(unpublished data). 1)Measurements of regional rock densities and magnetic parameters of Anhui Province(research report),Annul Geological and Mineral Bureau,1987(unpublished). 1)In this paper South China Block(SCB)includes the Yangtze block and Cathaysian block.In the Neoproterozoic the Yangtze block had been sutured to th…     

Porphyry Cu deposits linked to episodic growth of an underlying parental magma chamber

Saindak is one of the typical porphyry Cu deposits (PCDs) in the Chagai magmatic arc in Pakistan. Ore-forming porphyries at Saindak PCD are mainly composed of tonalite. Here, we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization. Although apatite inclusions have homogeneous intra-grain S and Cl compositions, there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase. Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An, FeO and Mg contents of the hosting plagioclase phenocryst. It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber, rather than by one major injection or by addition of mafic melt derived from different source region. Each primitive melt injection introduced essential ore-forming materials such as S and Cl, which were rapidly and effectively released to the coexisting fluids, causing mineralization. Once primitive melt injection stops, signaling the end of growth of underlying magma chamber, mineralization will cease quickly although the hydrothermal system can still survive for a long time. However, the later released fluids are relatively depleted in ore-forming materials, and thus have lower capability to generate mineralization. Accordingly, predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system.

     

安徽新桥块状硫化物矿床地球化学特征

新桥块状硫化物矿床发育双层结构,地球化学特征也呈现出明显的“二元性”和垂向变化。下部网脉状蚀变矿化岩石SiO₂、Al₂O₃、K₂O和Na₂O等含量较高,上部层状块状矿石和含矿岩石Fe₂O₃、FeO、CaO、MgO及SiO₂等明显富集。稀土含量相对较低,上部层状块状矿层平均值为10.73×10^-6,下部通道相蚀变矿化岩石平均值为126.1×10^-6。重晶石δ³⁴S值为+16.2‰,硬石膏δ³⁴S值为+11.2‰,黄铁矿δ³⁴S值为+1.5‰～+4.7‰。含矿硅质岩δ¹⁸O为+12.0‰～+13.9‰,下部通道相含黄铁矿石英脉δ¹⁸O值为+13.3‰～+18.6‰。自下部网脉状矿化到上部层状块状矿层,从粗晶细晶到变胶状胶状黄铁矿,δ³⁴S、δ¹⁸O和δ30Si值逐渐降低,²⁰⁶Pb/²⁰⁴Pb、²⁰⁷Pb/²⁰⁴Pb和²⁰⁸Pb/²⁰⁴Pb平均值逐渐增高。     

西藏念青唐古拉山帕戈勒中新世二云母花岗岩成因机制及对稀有金属铍成矿能力的指示

淡色花岗岩是研究浅部地壳物质成分和碰撞造山带地球动力学过程的理想对象,也是稀有金属成矿的重要载体。本文研究的帕戈勒二云母花岗岩发育在西藏冈底斯念青唐古拉复式花岗岩岩基的西南侧,以岩株和岩枝形式产出,局部偶见绿柱石。精细的锆石U-Pb定年显示,岩体结晶成岩年龄为18.4 Ma和17.6 Ma,形成于中新世。岩体具有高的SiO₂(72.78%～75.53%)和K₂O(4.83%～5.52%)含量,低的Al₂O₃(13.42%～14.60%)、MgO(0.10%～0.24%)、TFe₂O₃(0.86%～1.65%)含量,A/CNK为1.04～1.12,属于过铝质高钾钙碱性-钾玄质花岗岩。(⁸⁷Sr/⁸⁶Sr)_i=0.712139～0.713545,ε_Nd(t)=-9.0～-8.3,Nd二阶段模式年龄t_DM2=1211～1468 Ma,²⁰⁶     

云南省中西部红色风化壳中铂族元素分布特征

近年来,铂族元素以其广泛的地质活动环境受到越来越多的关注,人们对铂族元素的研究也从其在硫化物熔体、硅酸盐熔体和热液流体中的富集成矿行为逐渐延展到其在表生风化过程中的地球化学迁移行为.本文通过对我国云南省中西部潞西菲红、元谋朱布和弥渡金宝山3个具有铂族元素矿化的超基性岩体形成的红色风化壳研究发现,三地风化壳化学风化程度远...     

西藏列廷冈-勒青拉Fe-Cu-Pb-Zn矿区成矿岩体锆石U-Pb年代学与岩石地球化学特征

西藏冈底斯北缘发育一条矽卡岩型Fe-Cu-Pb-Zn多金属成矿带,列廷冈-勒青拉矿床是这条带上同时发育Fe-Cu-Pb-Zn四个矿种的最具代表性矿床。矿区内Fe-Cu矿体位于列廷冈矿段和勒青拉矿段东侧,Pb-Zn矿体位于勒青拉矿段西侧,与成矿作用相关的居布扎日岩体位于矿区东南部,呈复式岩体,以大面积岩基产状出露。岩体北侧列廷冈Fe-Cu-(Mo)矿段的花岗闪长斑岩锆石U-Pb年龄为62.85±0.58 Ma,为印度-欧亚大陆碰撞造山带主碰撞早期岩浆活动的产物。岩石地球化学分析表明该套岩石属于高钾钙碱性、准铝质向过铝质过渡花岗岩;其Rb、Ba、Th、U、Pb等大离子亲石元素(LILE)强烈富集,Nb、Ta、Ti、P等高场强元素(HFSE)和Sr强烈亏损;稀土总量变化于173.23×10-6~208.98×10-6之间,(La/Yb)N介于5.71~6.24之间,具有中等负Eu异常(0.45~0.54),富集轻稀土元素(LREE),相对亏损重稀土元素(HREE);总体具有弧火山岩的地球化学特征。锆石结晶温度平均为693℃,岩体的氧逸度较小,平均为-8.63,表明其岩浆经历了在水近饱和条件下发生的熔融过程。锆石176 Hf/177 Hf=0.282557~0.282927,εHf(t)=-6.25~6.79,平均地壳模式年龄TDMC=696~1522 Ma,表明成矿岩体岩浆源区具有幔源岩浆混染壳源岩浆特征,这也成为形成矽卡岩型Fe-Cu-Pb-Zn多金属矿化共存的主导因素。结合前人及本次研究结果,建立冈底斯北亚带Fe-Cu-Pb-Zn多金属矿化成矿过程如下:在印-亚大陆碰撞造山带主碰撞早期,俯冲的新特提斯洋板片发生回卷引起软流圈地幔上涌,诱发楔形地幔区部分熔融,经MASH过程产生的幔源岩浆上侵,并在部分地区遭受与壳源岩浆的混染甚至混合。当幔源岩浆与壳源岩浆分别上侵并与碳酸盐岩地层相互作用时,矿区形成矽卡岩型Fe-Cu和矽卡岩型Pb-Zn矿化,但当壳幔混源的岩浆上侵并与碳酸盐岩地层相互作用时,矿区则形成矽卡岩型Fe-Cu-Pb-Zn等多金属矿化。     

西藏甲岗雪山钨钼矿床成矿流体及成矿物质来源

甲岗雪山钨钼矿床位于西藏自治区申扎县境内,是西藏首例云英岩型钨矿床,关于该矿床的研究对探讨区域成矿机制和指导找矿都具有重要意义.成矿作用与矿区内的二长花岗岩紧密相关,矿体主要产于岩体内部和紧邻岩体的围岩中.矿体的类型包括云英岩型和石英脉型,矿石多呈细脉状或者浸染状产在云英岩或云英岩化二长花岗岩体内部,少量呈大脉状产于围岩地层中.为了研究该矿床成矿流体及成矿物质的来源,挑选云英岩型矿体和石英脉型矿体中的黑钨矿、石英进行H、O同位素测试,挑选金属硫化物进行S、Pb同位素测试.结果显示,黑钨矿δ18OV-SMOW(‰)值集中在3.7~4.7;石英的δ18O水值为2.0‰~4.3‰,δD值为-131‰^-84‰,表明成矿流体主要来源于脱气后的岩浆水,可能混入了极少量大气降水.矿石硫化物δ34S的值为+2.2‰^+5.3‰,表明硫来自岩浆;硫化物的206Pb/204Pb、207Pb/204Pb、208Pb/204Pb值分别为18.582 2~18.797 1、15.671 7~15.760 6、39.462 5~39.501 2,进一步表明成矿物质铅主要来源于中拉萨地体前寒武纪变质基底部分熔融产生的岩浆,可能有少量来自围岩地层.