Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

The equilibrium phase relations of a mafic durbachite (53 wt.% SiO₂) from the Třebíč pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338–335 Ma), have been determined as a function of temperature (900–1,100°C), pressure (100–200 MPa), and H₂O activity (1.1–6.1 wt.% H₂O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni–NiO (NNO) buffer and 2.6 log unit above NNO buffer (∆NNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3–4 wt.% H₂O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite–clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.     

Evidence of multiple evaporite recycling processes in a salt‐tectonic context,Sivas Basin,Turkey

The isotopic composition of evaporites can shed light on their environment of precipitation and their subsequent recycling processes. In this study, we performed Sr, O and S isotopic analyses on evaporitic sulphates in the halokinetic Sivas Basin. The main objectives were to decipher the age and origin of the evaporites responsible for the salt tectonics, and to test whether diapir dissolution acts as the source of younger evaporitic layers in continental mini‐basins. The Sr isotopes demonstrate that the first evaporites precipitated from seawater during the Middle–Late Eocene. The similar isotopic values measured in the halokinetic domain confirm that the Eocene evaporites triggered the salt tectonics and were continuously recycled in Oligo‐Miocene mini‐basins as lacustrine to sabkha evaporites. Modern halite precipitates suggest that the dissolution and recycling of diapiric halite is ongoing. This study demonstrates the efficiency of isotopic analyses in constraining evaporite recycling processes in continental halokinetic domains.     

新疆萨勒布尔山火山碎屑浊积岩的地球化学和构造环境分析

新疆萨勒布尔山上志留统克克雄参数表明其物源为布鲁克碎屑浊积岩。该组的ＲＥＥ,主元素地化学参数表明其物源为布鲁克其组的岛弧型火山主沙尔布尔组的岩浆弧建造,其形成的构造环境为大洋岛弧至活动大陆边缘之间,代表着由大洋向活动大陆转变的趋势。     

土屋-延东斑岩铜矿田区域多元成矿信息分析

新疆土屋-延东斑岩铜矿田是近年来在我国发现的超大型铜矿之一。作为超大型铜矿,进一步认识其成矿地质背景、成矿地质特征及成矿规律为该区寻找和研究该类型矿床具有一定的指导意义。在阐述该矿田区域地质特征,并探讨和分析成矿地质信息、物化探信息和遥感信息等多元成矿信息的基础上,结合分析结果和野外地质调查依据,提出该研究区区域成矿特征的几点认识:(1)土屋-延东斑岩铜矿田经历过拉张、汇聚、碰撞-固结、陆内堆积和弛张作用等形成过程,其成矿主要与汇聚阶段的俯冲-碰撞作用有关;(2)大型基底构造(区域性深大断裂)及其衍生的线型次级构造或火山机构有重要的控制作用;(3)与矿田在时间和空间上有成因联系的区域型地质建造(早石炭世的陆源碎屑建造和晚石炭世的复理石及基-中-酸性火山岩建造)以及大规模的花岗岩链派生的浅成侵入岩对成矿有利;(4)具有表示区域性地质特征(区域性深大断裂、板块边缘岛弧等)的重力梯度带,重、磁场等值线形态的扭曲或线性的重、磁异常带反映与成矿有关的岩浆活动,是找矿远景区;(5)大规模高背景区域地球化学异常区的存在(以Cu为主、伴有Ag、Au、Mo、Cr、Ni、Co、Zn、W、Pb元素组合的高背景和异常区)提供成矿物质背景信息;(6)遥感信息能够一定程度上反映古火山机构、浅隐伏侵入岩体及区域构造格局等控矿因素特征。基于多元信息分析的成矿特征认识对该区进一步寻找和研究该类型矿床有参考价值和理论意义。     

西昆仑北缘钾镁煌斑岩及超镁铁岩地质特征

产于塔里木地台铁克力克隆起内的钾镁煌夺岩石化学成分与亚洲钾煌斑岩的平均岩这成分接近,与澳大利亚西后利地区钾镁煌斑岩一致。稀土元素、微量元素组成与世界已 知钾镁煌斑岩具有相同特征。铁克力克隆起南缘柯岗－塔伦深断裂南阔绰人串珠状分布的超镁铁岩体,与钾镁煌斑岩具有相同的地质背景。该深断裂与西昆仑北缘深断裂交汇处是寻找含矿钾镁煌斑岩的有利部位。     

Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition

Crystallization experiments at 400 MPa, oxidized condition (logfO₂= NNO + 1, where NNO is nickel–nickel oxide buffer) andover a range of temperatures (850–950°C) and fluidcomposition (XH₂O_in = 0·3–1) have been carriedout to constrain the storage conditions of the sulphur-richmagma of the Huerto Andesite (an anhydrite, pyrrhotite, andS-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). Theresults are used to evaluate the role of fluids released fromthe crystallization of magmas such as the Huerto Andesite onthe remobilization of the largely crystallized dacitic FishCanyon magma body. Experiments were performed using the naturalandesitic bulk composition with and without added sulphur. Thepresence of sulphur slightly affects the phase equilibria bychanging the phase proportions, stability fields of plagioclase,pyroxenes and ilmenite, and also affects the plagioclase composition.Phase equilibria and mineral composition data indicate thatthe magma may have contained 4·5 wt % water in the meltand that the pre-eruptive temperature was 875 ± 25°C.Assuming that the magma was in equilibrium with a fluid phase,the CO₂ concentration of the melt is estimated to be in therange 2000–4000 ppm (at 400 MPa). Before eruption, theandesite had an oxidation state very close to, or slightly within,the co-stability field of anhydrite–pyrrhotite at NNO+ 1·1. At these conditions, the sulphur content in themelt is 500 ppm. Assuming open-system degassing resulting fromcontinuing crystallization at depth, most of the CO₂ dissolvedin the andesitic melt should be released after the crystallizationof <10 vol. % of the magma, corresponding to a cooling from875 to 825–850°C. Thus, the fluids released owingto crystallization processes should be mainly composed of waterat temperatures below 825°C. KEY WORDS: experimental study; andesite; volatile; Fish Canyon Tuff; Huerto Andesite     

北秦岭丹凤地区早古生代花岗岩的Pb、Sr、Nd同位素地球化学特征

通过对北秦岭丹凤地区早古生代花岗岩的Ph、Sr、Nd同位素地球化学特征的系统研究,结合元素地球化学特征,证明早古生代具成分极性的枣园、黄柏岔、石门花岗岩是由于秦岭群斜长角闪岩和黑云斜长片麻岩部分熔融,近俯冲带以熔体与熔体的混合、远离俯冲带是前者产生的熔体与后者熔融后的残留相以不同比例混合形成的。     

新疆某地红宝石和蓝宝石矿物学研究

本文系统地研究了新疆某地红宝石和蓝宝石矿床,查明该矿床中红宝石和蓝宝石的粒度、形态、双晶、裂纹,包裹体形态和成分,硬度、密度、多色性、吸收性、Ne和No值,晶胞参数及化学成分等特征,从而为该类矿床的开发利用提供了重要信息。同时,根据这些宝石的物理性质和化学成分上的区别,进一步探讨了呈色机理。     

Experimental study into the petrogenesis of crystal-rich basaltic to andesitic magmas at Arenal volcano

Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H₂O/(H₂O + CO₂) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5?wt% SiO₂) from a nearby scoria cone containing olivine (Fo₉₂), plagioclase (An₈₆), clinopyroxene (Mg# = 82) and magnetite (X_ulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO₂) with 5 wt% H₂O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H₂O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H₂O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al₂O₃ samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.