Petrochemical and thermodynamic evidence on the origin of kimberlites

The petrochemistry of kimberlites from Yakutia and Lesotho has been studied using a silicate melt model with the SiO₂, CO₂ and H₂O derivatives as the main anions.A model has been developed, according to which the dissolution of H₂O in an ultramafic melt results in orthosilicates (H₂SiC ₄ ^-2 , H₃SiO ₄ ^– , H₄SiO₄ etc.) rather than metasilicates, while the dissolution of CO₂ produces additional hydrocarbonate complexes. It suggests that at high PCO ₂ ¹ , and where the orthosilicic calcium salt clusters are likely to be present in the magma, the kimberlite melt can break down into carbonate and silicate liquids. Therefore, the composition of kimberlite magma will be determined by the H₂O/CO₂ ratio under the relatively constant fluid pressure. This can be seen from the distinct fluidrs trend in the H₂O-CO₂-SiO₂ diagram for the Yakutia and Lesotho diamond-bearing kimberlites. The H₂O/CO₂ ratio changes with the liquidus temperature along this trend (Perchuk and Vaganov 1977) which suggests that liquid immiscibility predominates over the simple CO₂ solubility in the melts of kimberlite composition. The well-known Boyd's diagrams for the equilibrium PT-conditions in peridotites have been applied along with new experimental data to natural Cpx and Opx, and the PT-parameters were correlated for peridotite inclusions in kimberlite pipes in Yakutia and Lesotho. The liquidus temperatures for the extrapolated area of these correlations gave depths (pressures) at which kimberlite magmas are formed (200–250 km).The hypothesis on SiO₂ partitioning between the melt and the fluid was used to calculate the composition of dry initial kimberlite which characterised the average mantle composition: SiO₂ — 45.12; TiO₂ — 2.49; Al₂O₃ — 3.58; Cr₂O₃ — 0.12; FeO — 9.32; MnO — 0.16; CoO — 0.11; MgO — 23.47; CaO — 13.44; Na₂O — 0.20; K₂O — 1.12; P₂O₅ — 0.69; S — 0.18; sum — 100 wt.%. This kimberlite is close to wehrlite in composition.     

Textural and chemical evidence bearing on sulfide-silicate reactions in metasediments

Two textural relationships between pyrrhotite and ferromagnesian silicates were found to occur in a metamorphosed sandy pelite at the staurolite grade of metamorphism. (1) Increased grain size of silicates at the pyrrhotite-silicate boundary; (2) Oriented intergrowths of pyrrhotite in large grains of chlorite and biotite. Both textures were interpreted as resulting from sulfide-silicate reactions. Ferromagnesian silicates in these sulfidic rocks show higher Mg/Fe ratios than those in non-sulfidic rocks in the same area.

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Zusammenfassung In einem bis zur Staurolith-Fazies metamorphosierten ehemaligen sandigen Pelit werden zwei Gefügebeziehungen zwischen Magnetkies und Eisen-Magnesium-Silikaten festgestellt: (1) Eine Kornvergrößerung der Silikate an der Grenze Magnetkies—Silikate; (2) Eine orientierte Verwachsung von Magnetkies mit großen Kristallen von Chlorit und Biotit. Beide Gefüge werden als Reaktionsgefüge zwischen Sulfiden und Silikaten gedeutet. Die Eisen-Magnesium-Silikate zeigen in diesen Gesteinen reich an Sulfiden höhere Mg/Fe-Verhältnisse als in sulfidfreien Gesteinen desselben Gebietes.

     

Recent dynamics and probable origin of the Tulva Upland in the Perm Foreurals

The Tulva Upland is a meridional neotectonic swell that complicates the eastern Russian Plate in its recent manifestation. The intense recent uplift is expressed in the rise and splitting of terraces of the Kama River and anomalously increasing lateral ruggedness of topography. Having a steep western and a gentle eastern limb, the swell is sharply asymmetric in cross section and additionally is complicated by a chain of local NE-trending uplifts. Several morphostructural indications testify to the substantial role of NW-trending strike-slip faulting in the structure of the swell, which was formed under conditions of latitudinal compression and conjugated meridional extension. Such a stress-strain field is confirmed by the study of mesotectonic structural elements in the western steep limb of the swell regarded as a flexure above a suggested reverse fault. Like many other zones of within-plate dislocations in the Russian Plate, the recent Tulva Swell was formed as a result of folding of sedimentary fill and inversion of long-lived platform trough. In our case, this trough inherited the Riphean Kaltasy Aulacogen. Together with the unilateral, probably, reverse-fault-line (?) Ufa Horst, the Tulva Swell is situated opposite to the area of maximum near-latitudinal compression of the recent Urals (the socalled Ufa amphitheater, or Central Ural pinch) and along with other within-plate arches similar in structure—Bugul’ma-Belebei and Obschii Syrt—marks a zone of neotectonic reactivation of the Russian Plate near the Urals.     

滇西梁河—潞西地区新生代英安岩的源区及成因——来自地球化学的证据

报道了滇西梁河—潞西地区新生代英安岩的主量、微量、稀土元素和Nd同位素地球化学研究成果。英安岩富集轻稀土元素、大离子亲石元素(Rb、Th、U、K),相对亏损重稀土元素和Nb、Ta、Sr、P、Ti,具有Eu负异常和Pb正异常。火山岩的εNd(0)为-10.3~-8.72,Nd同位素模式年龄(tDM)为1.2~1.4Ga。这些地球化学特征表明,研究区英安岩源于区域基底岩系的部分熔融,并与该地区同期幔源玄武质岩浆发生了不同程度的混合。     

滇西梁河—潞西地区新生代英安岩的源区及成因——来自地球化学的证据

报道了滇西梁河—潞西地区新生代英安岩的主量、微量、稀土元素和Nd同位素地球化学研究成果。英安岩富集轻稀土元素、大离子亲石元素(Rb、Th、U、K),相对亏损重稀土元素和Nb、Ta、Sr、P、Ti,具有Eu负异常和Pb正异常。火山岩的εNd(0)为-10.3~-8.72,Nd同位素模式年龄(tDM)为1.2~1.4Ga。这些地球化学特征表明,研究区英安岩源于区域基底岩系的部分熔融,并与该地区同期幔源玄武质岩浆发生了不同程度的混合。     

Oxygen isotope evidence for the origin of discrete granitoid plutons from the Archean western Wabigoon Belt,northwestern Ontario

The whole-rock δ ¹⁸O values of samples from twelve discrete Archean plutons intruding the western Wabigoon granite-greenstone belt, northwestern Ontario, range from 6.8 to 9.5‰. Most samples with δ ${}^{18}\text{O}\text{> 8.7‰}$, however, come from portions of the Burditt Lake stock, or the Esox Lake area, that have been affected by deuteric or metasomatic activity. The distribution of δ ¹⁸O values for the remaining samples is very similar to that known for the large batholithic complexes and gneissic terrains that dominate this portion of northwestern Ontario. The generally low δ ¹⁸O values of the discrete granitoid plutons suggest that ¹⁸O-rich supracrustal rocks were unimportant in their genesis. Like the granitoids of the batholithic complexes and gneissic terrains, the discrete granitoid plutons represent new additions to the Archean sialic crust.     

川西凹陷地区更新统砾石层沉积成因探讨

笔者通过野外地质调查和相关资料,从宏观形态和沉积特征多方面对川西凹陷地区的更新统砾石层的成因进行综合分析.发现各时期更新统砾石层平面上均呈扇状,具多级阶地.各砾石层的岩性结构柱、基本层序、砾石的组构特征均表现为河流相冲洪积成因.并通过ESR和孢粉、充填序列和地貌标志探讨了区内各更新统砾石层的沉积时代、古气候特征.综合研究表明区内存在4套不同特征的更新统砾石层,它们不仅在空间位置上存在密切联系,而且在沉积特征和形成时代上也具继承性.沉积环境均为温暖潮湿热环境下,出龙门山的古青衣江在盆地中不同时期、不同位置的沉积响应.并反演古青衣江水系在前陆盆地中不同时期的迁移及变化规律.     

On the origin of the western Mediterranean Sea basins

This is a report of a symposium organized by the Netherlands Commission for the Upper Mantle Project. The data relative to the generation of the western Mediterranean Sea basins, presented during this symposium, are summarized in the Appendix.

Several modes of origin have been discussed:

1. (1) the basins are remnants of a former larger ocean;

2. (2) they are formed in the wake of drifting continental blocks;

3. (3) by an erosion and denudation of a continental crust;

4. (4) by an upheaval and later subsidence of an ocean floor; or

5. (5) by sub crustal erosion of a continental crust.

It is concluded that, although many data are in agreement with the drift model, this process cannot have been the sole agent in the generation of the basins.     

Geochemical evidence for the origin of moldavites

Forty-eight moldavites and samples of rocks from the impact crater of Ries were analyzed using non-destructive neutron activation analysis. The following elements have been determined: La, Ce, Sm, Eu, Lu, Sc, Co, Cs, Hf and Th; and Rb and Cr in two moldavites. The darker moldavites, more common in Moravia, show higher contents of trace elements than those lighter in colour, from Southern Bohemia.Terrestrial igneous rocks cannot be regarded as a suitable source material, but terrestrial sandy to silty claystones show strong similarities in major and trace element abundances, and exhibit analogous inter-element variations to moldavites. The Tertiary claystones and sandstones which probably covered the Ries area before the impact, are a possible source rocks for moldavites.     

柴达木盆地西部卤水特征及成因探讨

柴达木盆地自中生代末-新生代初以来,四周山体不断抬升,形成"高山深盆"的沉积环境,为盆地带来了大量盐类物质,再加上新近纪干旱封闭的气候环境,使得其西部沉积了广阔而厚层的岩盐.在对柴达木盆地西部新近系、第四系地层出露盐矿点实地考察的过程中,采集、分析了18件卤水样品的水化学组成.结果表明,卤水矿化度高,油田卤水富B3 、Li 、Sr2 和Br-等有益组分,其前缘第四纪形成的盐湖也有大量K 资源分布.通过离子含量及水化学特征系数,查明了卤水的水化学类型主要为氯化物型和硫酸镁亚型,地表浅层卤水富Mg2 、SO24-贫Ca2 ,而油田卤水富Ca2 ,贫Mg2 、SO24-.反映了油田卤水具有深部CaCl2型水体的特征,这种富Ca2 的卤水可能与白云岩化作用使卤水中的Ca2 增加而Mg2 减少,以及硫酸盐的还原作用使SO24-减少有关;地表浅层卤水接近青海湖湖水蒸发线,说明盆地卤水主要是由大气降水汇聚蒸发形成.通过卤水氢氧同位素的分析,发现卤水明显偏离于大气降水线,发生了明显的"氧同位素正漂移"现象,且卤水演化趋势线和当地大气降水线的交点与周围山体前缘的盐泉水的氢氧同位素值相近,地表浅层卤水主要分布在大气降水线附近,而油田卤水集中分布在变质水的范围内.说明地表浅层卤水主要是由大气降水汇聚而形成的;而深部油田卤水推测基本上来源于冰雪融水或雨水补给,这些冰雪融水或雨水沿着断裂带下渗,在迁移的途中发生了变质作用和深部地热水体掺杂作用,形成了深部油田卤水.     

Correlation between the Wonominta and the Broken Hill Blocks in western New South Wales: Evidence from Nd isotope studies of metasediments

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (T_DM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.     

Substitution mechanisms and solubility of titanium in phlogopites from rocks of probable mantle origin

Previously proposed substitution mechanisms for Ti in phlogopites, based on experimental studies and crystal chemistry, have been examined using data for 81 phlogopites from mantle-derived rocks (primarily as nodules in kimberlites and also from alkali basalts, lamprophyres and carbonatites), 49 phlogopites from high-K rocks with basaltic affinities, and from 32 phlogopites crystallized in high pressure experiments mainly on high-K rock compositions. For the majority of phlogopites from the kimberlite group and for all those crystallized in the experimental studies, the substitution of Ti can be represented by a combination of the mechanisms represented by 2Mg^[VI]⇌ Ti^[VI]□^[VI] and Mg^[VI]2Si^[IV]⇌Ti^[VI]2Al^[IV]. Some phlogopites in ultrapotassic rocks have only the former substitution mechanism. The Ti contents of phlogopites generally increase with decreasing octahedral site occupancy and decreasing Si+Al^[VI]. For the phlogopites crystallized in the experiments on high-K rocks, the solubility of Ti increases with increasing fO₂ and temperature, and possibly with decreasing pressure at constant fO₂. The effect of the composition of the liquids used in the experimental studies from which these phlogopites have crystallized has only minor effect on either the substitution mechanism or the solubility of Ti in phlogopites. This suggests that phlogopite in high-K rocks may be a potential geothermometer and possibly a geobarometer.     

Isotopic evidence on the origin of the Shamrocke copper mine,Rhodesia

The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide ³⁴S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar ³⁴S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734     

西藏拉萨地块MORB型榴辉岩的岩石地球化学特征

松多榴辉岩的SiO2、Al2O3含量分别为45.3%～50.6%、12.93%～14.27%,K2O Na2O为1.3%～2.8%,TFeO/MgO为1.11～2.02,CaO为10.4%～14.47%,K2O<0.3%,Na2O为0.97%～2.47%。总体上具有玄武质或辉长质岩石的成分特征,属于钙碱性系列,具有MORB的特征,表明其原岩来自大洋环境。松多榴辉岩大多数样品的稀土元素配分模式显示轻稀土元素亏损,无明显的Eu异常,重稀土元素平坦。微量元素蛛网图均显示从Zr到Yb属于平坦型,基本上未发生分馏。松多榴辉岩的LaN/YbN比值为0.19～0.56,LaN/CeN为0.75～0.81,LaN/SmN为0.37～0.81,三者均小于1;Hf/Th除了石英榴辉岩中的06Y-344样品为6.81外,其他样品为11.10～33.97,均大于8;Ce/Nb为2.72～19.02,均大于2;Th/Yb为0.01～0.10,均小于或等于0.1,也说明其原岩具有典型MORB的特征。锆石SHRIMPU-Pb定年获得松多榴辉岩的变质年龄为261Ma±5.3Ma,推测其原岩可能为古特提斯洋盆的残留。     

Fluid inclusion evidence for progressive folding during decompression in metasediments of the Voltri Group (Western Alps, Italy)

Quartz veins syntectonic to distinct folding events in metasediments from the Voltri Group (Ligurian Alps) were studied in order to compare fluid and structural evolution. Studied veins (VS1, VS2, VS3) pertain to three distinct generations of folds (F₁, F₂, F₃) that formed during the retrograde metamorphic evolution. Two types of fluids characterize the different generations of veins and are represented essentially by aqueo-carbonic mixtures of moderate salinity with decreasing densities (1.01–0.41 g/cm³). The chemical evolution is characterised by a progressive decrease of H₂O, from early fluids associated with opening of VS1 and VS2 (XCO₂≈0.08) to fluids related to VS3 formation (XCO₂≈0.3). The close match between the fluids in VS1 and VS2 suggests that the development of two superimposed systems of folds (F₁ and F₂ folds) occurs under very similar P–T conditions, during a progressive and continuous deformational event at glaucophanic and/or barroisitic metamorphic grade. A different evolution is outlined for the formation of VS3 during low greenschist grade. Successive isochores allow us to define a retrograde decompression path for the Voltri Group. Present results indicate that fluid inclusions are powerful markers to constrain the P–T–t conditions of different folding events.     

The origin of diamonds in western Minas Gerais,Brazil

Important diamond mineralized alluvium/colluvium occurs in western Minas Gerais (WMG) in the Brasília Orogenic Belt (700-450 my). Diamonds are geographically related to glacial and periglacial sediments (Upper Proterozoic and Permo-Carboniferous), wadi and debris flow conglomerates (Lower Cretaceous and Upper Cretaceous), and ultramafic alkaline rocks (intrusive and extrusive, including kimberlites, leucitites, kamafugites, carbonatites, etc.) of Cretaceous age. Petrological data indicate that the rocks originally petrographically classified as kimberlites present mineralogical, chemical, and isotopical differences with Group I and II kimberlites and more closely resemble kamafugites. The Brasília Orogenic Belt presents features of a Wilson Cycle implying that WMG does not fulfil the geotectonic and geothermometric requisites to host primary diamond sources. An analysis of field relations and sedimentology in WMG shows that the majority of the diamonds have been transported by glacial events from the São Francisco Craton further east.     

Eclogitic metasediments from central Qiangtang, Northern Tibet: Evidence for continental subduction during the eastern and western Qiangtang collision

Eclogitic metasediments from the central Qiangtang metamorphic terrane provide new insights into the continental subduction during the eastern and western Qiangtang collision. Petrologic observations show that the metasediments correspond to meta-sandstones of a continental margin. It is characterized by the garnet + omphacite + rutile + phengite + quartz assemblage, and the peak metamorphic temperature and pressure were estimated to be 535 ± 40 °C and ～27 kbar, respectively, by major element partitioning thermobarometry. Subsequent retrogression occurred at ～472 ± 30 °C and ～6–9 kbar. The occurrence of eclogitized central Qiangtang terrane indicates the subduction of the western Qiangtang passive continental margin beneath eastern Qiangtang when eastern and western Qiangtang collided along the Shuanghu suture.     

扬子西缘瓦斯沟花岗岩的元素-Nd同位素地球化学——岩石成因与构造意义

对扬子地块西缘康滇裂谷北段的瓦斯沟花岗质杂岩进行了系统的岩石学、元素-Nd同位素地球化学研究,结果表明该岩体为I型花岗岩,是由前存年轻(中元古代末-新元古代初)岛弧地壳物质部分熔融形成的.早期的花岗岩形成于扬子地块西缘由会聚挤压向陆内伸展的转折环境,晚期的花岗闪长岩形成于板内环境,很可能与新元古代地幔柱事件有关.它们显示的"岛弧地球化学特征"是继承了源岩地球化学特征的结果,不代表其形成时的构造环境.     

Geochemical constraints for the origin of thermal waters from western Turkey

The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO₃ type with high CO₂ content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na–SO₄ type that is also associated with metamorphic rocks of the Menderes Massif with H₂S addition; and (4) a Ca–Mg–HCO₃–SO₄ type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Çanakkale–Tuzla (Br/Cl<sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water–rock interactions; (2) intensification of reactions due to high dissolved CO₂ and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ¹¹B=2.3‰ to 18.7‰; n=6) can indicate either leaching of B from the rocks, or B(OH)₃ degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. ⁸⁷Sr/⁸⁶Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (⁸⁷Sr/⁸⁶Sr ratio as high as 0.719479) and carbonate rocks (low ⁸⁷Sr/⁸⁶Sr ratio of 0.707864).