黔东北地区钒矿床地球化学特征及成因研究

近年来在黔东北地区先后发现了多个中_大型沉积型钒矿床,它们均赋存在寒武系下统九门冲组(∈1jm)下部的黑色碳质泥岩中。为了从地球化学方面深入探讨该类钒矿床中含矿岩系的沉积环境以及矿床成因类别,选用电感耦合等离子体质谱(ICP_MS)法对钒矿体及其上、下地层的样品进行了主量、微量和稀土元素测试,采用层序地层学方法对典型钒矿床的含矿岩系地层进行剖析,充分利用地球化学元素对环境的敏感特性探索了含矿岩系形成的氧化还原条件和矿床成因。结果表明,含矿岩系主要为海相深(浅)水陆棚相沉积物,因沉积过程中沉积相的多次演变,形成了现在具有一定差异的岩性组合。同时,因地球化学场的不断变化,沉积物中主量、微量和稀土元素的含量呈现了特有的分布规律。文章首次从地球化学方面揭示了该区含矿岩系及钒矿层均是还原环境下的产物,钒矿床以正常海相沉积成因为主,仅与生物作用有一定关系,受热水影响并不明显。这对钒矿床的成矿预测以及实现更大找矿突破,具有一定意义。     

新疆西天山智博铁矿岩浆-热液成矿作用—来自安山岩矿物学的证据

本文以智博铁矿区内的安山岩为研究对象,通过详细的野外地质调查,并利用电子显微镜和电子探针,对安山岩中的主要矿物进行了系统的岩相学观察和矿物学研究。研究表明,智博安山岩中斜长石主要为Na-高钠长石,具低TiO2,高Na2O和Al2O3的特点;辉石主要为普通辉石,具高TiO2,高Al2O3的特点;角闪石主要为镁角闪石和阳起石,具低TiO2和Al2O3,高MgO的特点;副矿物磁铁矿具高TiO2,低MgO和Al2O3的特点。辉石、角闪石矿物化学特征表明,智博铁矿安山岩的母岩浆属于壳幔混源的玄武质岩浆,构造环境为火山岛弧环境。智博安山岩中单斜辉石结晶温度为1225℃左右,结晶压力约0.795GPa,结晶深度约26km。智博铁矿后期的热液作用也参与了磁铁矿成矿,对智博铁矿的成矿有一定的贡献。     

江南造山带东段桃岭岩体的地球化学特征及其成因

位于江南造山带东段赣东北地区的桃岭岩体地球化学特征总体表现为高钾钙碱性系列,弱过铝质-强过铝质,高Mg#值。桃岭岩体富集轻稀土和大离子亲石元素,亏损高场强元素,具弱的Eu负异常(δEu=0.62~0.81)(样品TL-1除外δEu=0.47)。锆石LA—ICPMS U—Pb定年获得桃岭岩体的成岩年龄为早白垩世(140±1)Ma。岩体初始~(87)Sr/~(86)Sr为0.7131~0.7141,ε_(Nd)(t)值为-7.43~-6.71。结合前人研究,笔者认为早白垩世(约140 Ma)交代的地幔发生部分熔融,幔源岩浆底侵,使地壳深处(至少40 km)的变质沉积岩发生部分熔融,并且与少量的幔源岩浆发生了岩浆混合作用,形成了桃岭岩体。     

Geochemistry of mafic rocks and melt inclusions and their implications for the heat source of the 14.0°S hydrothermal field,South Mid-Atlantic Ridge

Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.     

嵩山地区熊耳群火山岩岩石学、地球化学及成因探讨

近年在河南省嵩山地区发现有熊耳群火山岩地层。通过岩石学、地球化学特征及成因研究,认为嵩山地区熊耳群火山岩主要为弱过铝质流纹岩,属于高钾钙碱性系列-钾玄岩系列,以高硅、高钾、过铝、贫钠、贫钙镁为特征;轻稀土富集,重稀土亏损,轻重稀土分馏较为明显,Eu强烈负异常,Ce微弱负异常;富Ba而贫Sr,Rb,U,富Zr而贫Nb,Th,Hf,Ta;岩石属A型岩浆作用产物,形成于板内拉张环境。     

义敦岛弧带夏塞早白垩世A型花岗岩成因:锆石U-Pb年代学、地球化学及Hf同位素制约

义敦岛弧带晚中生代侵入岩体目前仍缺乏高精度的年代学数据制约,其成因也存在争论。作者首次在岛弧带中段夏塞银铅锌多金属矿区发现与成矿关系密切的黑云母二长花岗岩。本文对其开展了年代学、地球化学和Hf同位素分析,探讨成因及构造背景。LA-ICP-MS锆石U-Pb定年结果为103±1 Ma(MSWD=0.5),为早白垩世晚期岩浆活动产物。花岗岩属高钾钙碱性岩系,具有高硅、富碱和铁、贫钙和镁特征,Si O2含量为72.94%~74.98%,K2O+Na2O=7.56%~8.08%,铝饱和指数A/CNK=1.06~1.10,属弱过铝质岩石。岩石富集Zr、Hf等高场强元素和U、Th等大离子亲石元素,明显亏损Ba和Sr。REE具有明显的Eu负异常(δEu=0.13~0.25),总体呈较陡右倾的LREE富集和HREE相对亏损特征。岩相学和地球化学显示其为铝质A型花岗岩。Hf同位素组成εHf(t)=–2.7~0.6,二阶段模式年龄TDM2=925~1095 Ma。地球化学及Hf同位素揭示夏塞岩体为软流圈地幔与壳源长英质岩浆混合成因,并经历了斜长石、正长石和褐帘石等矿物的分离结晶。夏塞花岗岩体具有后碰撞花岗岩特征,形成于早白垩世晚期弧-陆碰撞造山后伸展构造背景。     

Structural and metamorphic controls on the distribution of zircon in an evolving quartzofeldspathic migmatite: an example from the Reynolds Range, central Australia

Three types of zircon occur in a complexly deformed and variably migmatized quartzofeldspathic gneiss from the Reynolds Range, central Australia. The oldest type is inherited from the granitic precursor of the gneiss, and is overgrown by a second group of zircon grains that formed during prograde, granulite facies metamorphism. Partial melting of the gneiss resulted in solution of both the inherited and metamorphic zircon. No new zircon growth accompanied crystallization of the partial melt, suggesting loss of zirconium–rich residual fluids. Hydrous, amphibolite facies retrogression of the gneiss and its migmatized variants during late shearing produced new, idiomorphic zircon in both the shear zone and its wall rocks.
Important implications of this study are that (i) zircon has a tendency to dissolve if it comes into direct contact with a melt produced from anhydrous biotite breakdown in a quartzofeldspathic granulite, (ii) melt crystallization is not necessarily accompanied by zircon growth, and (iii) euhedral zircon can grow from a hydrous fluid phase under subsolidus, amphibolite facies conditions, e.g. within shear zones.     

Contact Partial Melting of Granitic Country Rock, Melt Segregation, and Re-injection as Dikes into Ferrar Dolerite Sills, McMurdo Dry Valleys, Antarctica

Numerous, interconnected, granitic dikes (<30 cm in widthand hundeds of meters in length) cut Ferrar dolerite sills ofthe McMurdo Dry Valleys, Antarctica. The source of the graniticdikes is partial melting of granitic country rock, which tookplace in the crust at a depth of about 2–3 km adjacentto contacts with dolerite sills. Sustained flow of doleriticmagma through the sill generated a partial melting front thatpropagated into the granitic country rock. Granitic partialmelts segregated and collected at the contact in a melt-rich,nearly crystal-free reservoir adjacent to the initial doleritechilled margin. This dolerite chilled margin was subsequentlyfractured open in the fashion of a trapdoor by the graniticmelt, evacuating the reservoir to form an extensive complexof granitic dikes within the dolerite sills. At the time ofdike injection the dolerite was nearly solidified. Unusuallycomplete exposures allow the full physical and chemical processesof partial melting, segregation, and dike formation to be examinedin great detail. The compositions of the granitic dikes andthe textures of partially melted granitic wall rock suggestthat partial melting was characterized by disequilibrium mineraldissolution of dominantly quartz and alkali feldspar ratherthan by equilibrium melting. It is also unlikely that meltingoccurred under water-saturated conditions. The protolith granitecontains only 7 vol.% biotite and estimated contact temperaturesof 900–950°C suggest that melting was possible ina dry system. Granite partial melting, under closed conditions,extended tens of meters away from the dolerite sill, yet meltsegregation occurred only over less than one-half a meter fromthe dolerite chilled margin where the degree of partial meltingwas of the order of 50 vol.%. This segregation distance is consistentwith calculated length scales expected in a compaction-drivenprocess. We suggest that the driving force for compaction wasdifferential stress generated by a combination of volume expansionas a result of granite partial melting, contraction during doleritesolidification, and relaxation of the overpressure driving doleriteemplacement. On a purely chemical basis, the extent of meltsegregation necessary under fractional and batch melting tomatch the Rb concentrations between melt and parent rock isa maximum of 48 and 83 vol.% melt, respectively. KEY WORDS: Antarctica; dike injection; disequilibrium; granite partial melting; silicic melt segregation     

Trace element and Nd isotope analyses of apatite in granitoids and metamorphosed granitoids from the eastern Central Asian Orogenic Belt: Implications for petrogenesis and post-magmatic alteration

We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. ε_Nd(t) values of the these apatites are decoupled from zircon ε_Hf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO₂ contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.     

江西德兴斑岩铜矿石英闪长玢岩成因及成矿学意义

为探究石英闪长玢岩成因及幔源基性岩浆对斑岩铜矿的贡献,本文选取德兴矿床石英闪长玢岩开展了锆石U-Pb定年、Hf同位素和全岩地球化学研究。获得石英闪长玢岩LA-ICP-MS锆石U-Pb年龄为169 Ma,与成矿花岗闪长斑岩侵位时间一致,岩体为中侏罗世岩浆活动的产物。石英闪长玢岩具有低的SiO₂(58.41%~63.12%)和K₂O(1.68%~2.94%)含量及A/CNK值(0.85~1.04),富集大离子亲石元素和轻稀土元素,亏损高场强元素Nb、Ta、Ti和重稀土元素,属于钙碱性到高钾钙碱性系列岩石。具有相对亏损的锆石Hf同位素组成,εHf(t)=2.20~7.93(最大值7.93),指示其源区为岩石圈地幔。锆石稀土元素配分模式图显示出明显的正Ce异常,岩浆氧逸度(lg f_O2)为-20.05~-6.66,达到磁铁矿-赤铁矿氧逸度等级,指示石英闪长玢岩结晶自高氧逸度岩浆。全岩地球化学特征显示,德兴石英闪长玢岩与成矿花岗闪长斑岩及其暗色包体符合岩浆混合的演化趋势,说明成矿花岗闪长斑岩可能是中侏罗世幔源基性岩浆和地壳酸性岩浆大规模混合作用的产物,并且石英闪长玢岩代表了岩浆混合过程中的幔源基性端员。结合前人研究成果,认为在中侏罗世伸展构造背景下,软流圈物质上涌导致新元古代受交代的岩石圈地幔部分熔融形成幔源基性岩浆,基性岩浆的底侵作用诱发下地壳物质熔融并与之发生一定程度的岩浆混合作用,形成了花岗闪长斑岩的母岩浆。高氧逸度幔源岩浆的加入可抑制斑岩体系硫化物的过早饱和,同时为德兴矿床注入了成矿所需的部分挥发分和金属元素。