Lithodemes in the new millenium: a bibliographical survey

The term “lithodeme” was proposed 30 years ago for the purpose of crystalline rock stratigraphy. However, there are inconsistencies in its understanding. For example, it is unclear whether it is suitable to apply this term to layered intrusions or not: different specialists/organizations offer different approaches. A bibliographical survey of the geological literature published after 2000 demonstrates that the term “lithodeme” is used too rarely, although its use for magmatic and metamorphic rocks is well balanced. In addition 65% of all papers considering lithodemes deal with North and South America. Moreover, some specialists employ genesis and age when they delineate lithodemes, whereas other do not. If modern geology really still needs the lithodemic approach, researchers should work towards reaching consensus on what the term “lithodeme” means, and they should also devote effort to promote this approach internationally.     

Helium and carbon isotope variations in Liaodong Peninsula,NE China

Chemical and C–He isotopic compositions have been measured for N₂-rich hydrothermal gases from the Liaodong (abbreviation of East Liaoning Province) Peninsula from which the oldest crustal rocks in China with ⩾3.8 Ga outcrop. With the exception of one sample containing tritogenic ³He and atmospheric ⁴He in Liaoyang, the observed ³He/⁴He ratios from 0.1 Ra to 0.7 Ra indicate 1–8% helium from mantle, 92–98% from crust and 0.1–0.8% from atmosphere. Despite the lack of Quaternary volcanism, such ³He/⁴He ratios suggest, together with geophysical evidences, the existence of intrusive magmas that contain mantle helium and heat within the Liaodong middle-lower crust. The ³He/⁴He ratios are high along the NE-trending Jinzhou faults and gradually decrease with the increase of distance from the faults. Such a spatial distribution suggests that the mantle helium exsolves from magmatic reservoir in the middle-lower crust, becomes focused into the root zones of Jinzhou faults, and subsequently traverses the crust via permeable fault zones. When transversely migrated by groundwater circulation in near surface, mantle helium with high ³He/⁴He ratio may have been further diluted to the observed values by addition of radiogenic helium produced in the crust. This pattern shows strong evidence that the major faults played an important role on mantle-derived components transport from mantle upwards.     

The Earth as a multiscale quantum-mechanical system

Major features of the Earth's structure and dynamics originate in the contrast between the rigidity of SiO bonds and the softness of SiOSi linkages. Because this contrast results from orbital hybridization, a real understanding of bonding relies on ab initio quantum-mechanical principles. As investigated with first-principles interatomic potentials, the α–β transitions of SiO₂ polymorphs illustrate how soft SiOSi linkages give rise to dynamical structures at rather low temperatures and yield the low melting temperatures of SiO₂-rich minerals that are at the roots of SiO₂ enrichment in magmatic differentiation. The increasing concentration of alkalis throughout this process is another aspect that must also be studied in terms of molecular orbitals in relation with the presence of aluminum in tetrahedral coordination. Finally, calculations of noble gas solubility show that some important features can be treated with “hybrid” calculations when, in addition to quantum-mechanical effects, the energy needed to create a cavity in the silicate melt is dealt with in a classical manner.     

阿尔泰东南缘泥盆纪花岗质岩石的锆石U-Pb年龄、成因演化及构造意义：钙碱性-高钾钙碱性-碱性岩浆演化新证据

中国阿尔泰广泛发育的花岗质岩石已获得大量研究,但是其东南缘研究薄弱,制约了对整个阿尔泰造山带构造岩浆演化的认识。本文新获得阿尔泰东南缘四个花岗质岩体(昆格依特、库吉尔特、布铁乌及卡拉特玉别)锆石U-Pb年龄,分别为382±4Ma、381±4Ma、385±5Ma和363±6Ma。岩石学、地球化学特征等显示这些花岗质岩石具有高钾钙碱性、准铝质—弱过铝质的I型特点,全岩εNd(t)值为-2.42~-0.53,Nd模式年龄tDM为1.6~1.3Ga;锆石εHf(t)值为-3.44~+13.26,绝大多数为正值,锆石Hf二阶段模式年龄tDM-2为2.5~0.6Ga,表明源区物质组成复杂,有较多的新生幔源物质参与花岗质岩石的形成,并含有古老地壳成分。综合已有年龄分析显示,中国阿尔泰花岗质岩石的形成时代可分为480~440Ma(峰期460Ma)、420~390Ma(峰期400Ma)、390~370Ma(峰期380Ma)、370~360Ma(峰期365Ma)、360~350Ma。处于岩浆发育峰期的早泥盆世(420~390Ma)多为准铝-过铝质的钙碱性系列;中晚泥盆世(390~360Ma)多为准铝—弱过铝质的高钾钙碱性系列;370~360Ma为高钾钙碱性系列。该地区363Ma的高钾钙碱性花岗质岩石的确定,为进一步厘定整个阿尔泰泥盆纪花岗质岩浆由钙碱性(480~390Ma),到高钾钙碱性(390~360Ma),再到354Ma的布尔根碱性花岗岩的演变特点提供了新的证据,进一步揭示阿尔泰造山带该时期由俯冲增生演变到碰撞及后碰撞的演化过程。     

Petrogenesis and metallogenic setting of porphyries of the Duobuza porphyry Cu–Au deposit,central Tibet,China

Given that the Duobuza deposit was the first porphyry Cu–Au deposit discovered in central Tibet, the mineralization and mineralized porphyry in this area have been the focus of intensive research, yet the overall porphyry sequence associated with the deposit remains poorly understood. New geological mapping, logging, and sampling of an early granodiorite porphyry, an inter-mineralization porphyry, and a late-mineralization diorite porphyry were complemented by LA–ICP–MS zircon dating, whole-rock geochemical and Sr–Nd isotopic analyses, and in situ Hf isotopic analyses for both inter- and late-mineralization porphyry intrusions. All of the porphyry intrusions are high-K and calc-alkaline, and were emplaced at ca. 120 Ma. The geochemistry of these intrusions is indicative of arc magmatism, as all three porphyry phases are enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. These similar characteristics of the intrusions, when combined with the relatively high (⁸⁷Sr/⁸⁶Sr)_i, negative ε_Nd(t), and positive ε_Hf(t) values, suggest that the magmas that formed the porphyries were derived from a common source region and shared a single magma chamber. The magmas were generated by the mixing of upwelling metasomatized mantle-wedge-derived mafic magmas and magmas generated by partial melting of amphibolite within the lower crust.The inter-mineralization porphyry has the lowest ε_Nd(t) and highest (⁸⁷Sr/⁸⁶Sr)_i values, suggesting that a large amount of lower-crust-derived material was incorporated into the melt and that metals such as Cu and Au from the enriched lower crust were scavenged by the parental magma. The relative mafic late-mineralization diorite porphyry phase was formed by the residual magma in the magma chamber mixing with upwelling mafic melt derived from metasomatized mantle. The magmatic–hydrothermal evolution of the magma in the chamber released ore-forming fluid that was transported mainly by the inter-mineralization porphyry phase during the mineralization stage, which ultimately formed the Duobuza porphyry Cu–Au deposit.These porphyritic intrusions of the Duobuza deposit have high Mg^# and low (La/Yb)_N values, and show some high LILE/HFSE ratios, indicating the magma source was enriched by interaction with slab-derived fluids. Combined with age constraints on the regional tectonic evolution, these dating and geochemical results suggest that the Duobuza porphyry Cu–Au deposit formed in a subduction setting during the final stages of the northward subduction of the Neo-Tethyan Ocean.     

The significance of magmatic erosion for bifurcation of UG1 chromitite layers in the Bushveld Complex

One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex.     

Assimilation of sulfate and carbonaceous rocks: Experimental study,thermodynamic modeling and application to the Noril’sk-Talnakh region (Russia)

Most of the intrusions in the Noril’sk-Talnakh region (Siberia) are hosted in thick sedimentary sequences including abundant evaporitic and terrigenous sedimentary rocks. Three mafic-ultramafic intrusions in this region contain unusually thick massive sulfide deposits, which represent one of the world’s largest economic concentrations of Ni, Cu and PGE. The interaction of Siberian magmas with sulfate and organic matter-rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated. Here we clarify, by both experimental petrology and thermodynamic modeling, how sulfate and organic matter assimilation occur in mafic-ultramafic magmas, affecting magma composition, crystallization and sulfide saturation.Interaction experiments were conducted at conditions relevant to the emplacement of Noril’sk type intrusions (1200 °C, ∼80 MPa) to simulate the assimilation of sulfate and/or organic compounds by ultramafic magmas. We used a picrite from Noril’sk1 intrusion, and coal and anhydrite from the area as starting materials. The experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt, which increases the magma’s sulfur content, but suppresses sulfide saturation and reduces olivine crystallization. Extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0.9 ± 0.1 wt% S. Conversely, coal assimilation promotes sulfide segregation and magma crystallization, while decreasing the dissolved H₂O content of the melt and increasing the amount of coexisting fluid phase.We also employed gas-melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the S content of the magma, and investigate the role of temperature, pressure, and initial gas content of the magma in the assimilation process. We quantify how sulfate assimilation strongly oxidizes the magma and increases its S content; both effects are intensified by increasing pressure (from 50 to 100 MPa in this study), decreasing temperature (from 1350 to 1200 °C in this study), and decreasing amounts of fluid phase initially coexisting with the magma (from 2 to 0 wt%). The interaction with organic matter (CH in this study) induces a strong reduction of the magma, even for extremely low degrees of assimilation (few tenths of wt%), and the dehydration of the melt.We therefore suggest that in the Noril’sk-Talnakh district (1) additional S was supplied to mantle derived magmas by the assimilation of evaporitic rocks, and was transported during magma ascent in the form of dissolved, oxidized S; (2) a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments. This mechanism can potentially produce massive sulfide deposits by important sulfate assimilation (locally higher than 3 wt% CaSO₄) and minor organic matter assimilation (few tenths of wt% CH); however, if one of the two steps does not occur, or the assimilation in (1) is not large enough, disseminated sub economic or no sulfide deposits are produced. We conclude that exceptional conditions favoring substantial assimilation of sediments are needed to generate exceptional ore deposits like those of the Noril’sk-Talnakh district.     

岩浆侵入对围岩储层的成岩及其变质特征的影响及意义——以松辽盆地南部辉绿岩侵入带露头研究为例

以松辽盆地南部团山子采石场辉绿岩侵入露头为例,采用连续密集取样、常规分析和碳氧稳定同位素测试等方法,对围岩储层的成岩及其变质特征进行分析,进而探讨岩浆侵入对围岩的影响及其储层意义。岩浆侵入对角岩围岩的影响包括：1）自生绢云母距离侵入体越远,其含量越少;2）粘土矿物主要为成岩晚期的伊利石和绿泥石;3）角岩中发育特殊“气孔-杏仁”构造。岩浆侵入对上覆砂岩的影响表现为：1）造成自生石英的特征分布;2）造成粘土矿物的不均匀分布;3）形成反映应力挤压特征的颗粒结构。角岩中浊沸石发育和高岭石不发育对应碱性环境,不利于热液微裂缝中碳酸盐矿物的溶蚀。因此,尽管角岩发育微孔、微缝等少量储集空间（平均孔隙度为1.70%）,但其渗透能力很低（平均渗透率小于0.1 mD）。侵入活动对研究露头砂岩储层物性造成的不利影响,主要是由于挤压作用和热液流动作用使自生矿物不均匀分布,从而导致或加剧了砂岩储层的非均质性。     

江苏栖霞山铅锌多金属矿床成因探讨:流体包裹体及氢-氧-硫-铅同位素证据

江苏栖霞山铅锌多金属矿床位于长江中下游成矿带宁镇地区,是华东地区最大的铅锌矿之一,近年来在其深部取得重大找矿突破,但该矿床的成因仍存在争议。本文在详细的野外调查基础上,重点对深部样品进行了系统的观察和采集。通过对其各成矿阶段流体包裹体及H、O、S、Pb同位素系统测定及分析,并结合地质事实,最终确定其矿床成因,为该矿床及区域下一步找矿提供方向。流体包裹体研究表明,栖霞山矿床流体包裹体类型以纯液相包裹体(L)和气液两相包裹体(V+L)为主。显微测温结果显示:主成矿期(热液成矿期)第一阶段磁铁矿-石英阶段(Ⅰ)的包裹体均一温度集中变化于280~380℃,盐度变化于4.24%~9.86%Na Cleqv;第二阶段石英-硫化物阶段(Ⅱ)的包裹体均一温度集中变化于180~320℃,盐度变化于1.74%~8.00%Na Cleqv;第三阶段石英-碳酸盐岩阶段(Ⅲ)的包裹体均一温度变化于80~160℃,盐度变化于0.53%~6.74%Na Cleqv。从第一阶段到第三阶段,均一温度和盐度均有降低的趋势,显示流体混合的特征,可能是其矿质沉淀的重要机制。H-O同位素分析(δ~(18)OH2O值为-1.9‰~5.5‰,δD值为-80.3‰~-69.9‰)显示成矿流体主要为岩浆流体,后期有大气降水的加入。硫化物S同位素研究显示,δ~(34)S值总体变化范围-4.6‰~3.8‰,呈塔式分布,位于零值附近,暗示着栖霞山矿床硫化物的S主要来源于岩浆,且可能有部分赋矿地层S的混入。矿石硫化物的(206)~Pb/(204)~Pb为17.616~17.817,207Pb/(204)~Pb为15.513~15.718,(208)~Pb/(204)~Pb为37.907~38.585,说明Pb主要来源于岩浆,可能有部分震旦系基底地层Pb的加入。综合矿床地质、流体包裹体及H、O、S、Pb同位素特征可知,栖霞山矿床属于主要受石炭系黄龙组灰岩与高丽山组砂岩之间"硅钙面"控制的岩浆热液矿床,与本区早白垩世晚期岩浆活动密切相关。