The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Raman study of MgCO3–FeCO3 carbonate solid solution at high pressures up to 55 GPa

Magnesite, siderite and ferromagnesites Mg_1?x Fe _x CO₃ (x = 0.05, 0.09, 0.2, 0.4) were characterized using in situ Raman spectroscopy at high pressures up to 55 GPa. For the Mg–Fe-carbonates, the Raman peak positions of six modes (T, L, ν₄, ν₁, ν₃ and 2ν₂) in the dependence of iron content in the carbonates at ambient conditions are presented. High-pressure Raman spectroscopy shows that siderite undergoes a spin transition at ~40 GPa. The examination of the solid solutions with compositions Mg_0.6Fe_0.4CO₃, Mg_0.8Fe_0.2CO₃, Mg_0.91Fe_0.09CO₃ and Mg_0.95Fe_0.05CO₃ indicates that with increase in the amount of the Fe spin transition pressure increases up to ~45 GPa.     

http://www.sciencedirect.com/science/article/pii/S1674987110000307

The Polar Urals region of northern Russia is well known for large chromium (Cr)-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum (Pt)-group elements (PGE), geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium (Ir) values. These values indicate the predominance of ruthenium–osmium–iridium (Ru–Os–Ir)-bearing phases among the platinum-group mineral (PGM) assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite (up to 15 μm). Less common are cuproiridsite (up to 5 μm), irarsite (up to 4–5 μm), and laurite (up to 4 μm). The predominant sulfide is heazlewoodite, in intergrowth with Ni–Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and estimated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE.     

Movement of lithospheric plates relative to subduction zones: Formation of marginal seas and active continental margins

Subduction zones with deep seismicity are believed to be associated with the descending branches of convective flows in the mantle and are subordinated to them. Therefore, the position of subduction zones can be considered as relatively fixed with respect to the steady-state system of convective flows. The lithospheric plate overhanging a subduction zone (as a rule of continental type) may:

1. (1) either move away from the subduction zone; or

2. (2) move onto it. In the first case extensional conditions originate behind the subduction zone and the new oceanic crust of back-arc basins forms. In the second case active Andean-type continental margins with thickening of the crust and lithosphere are observed.

Behind the majority of volcanic island-arcs, along the boundary with marginal-sea basins, independent shallow seismicity belts can be traced. They are parallel to the main seismicity belts coinciding with the Benioff zones. The seismicity belts frame island-arc microplates. Island-arc microplates are assumed to be a frame of reference to calculate relative movements of the consuming and overhanging plates. Using slip vector azimuths for shallow seismicity belts in the frontal parts of the Kurile, Japan, Izu-Bonin, Mariana and Tonga—Kermadec arcs, the position of the pole of rotation of the Pacific plate with respect to the western Pacific island-arc microplates was computed. Its coordinates are 66.1°N, 119.2°W. From the global closure of plate movements it has been determined that for the past 10 m.y. the Eurasian and Indian plates have been moving away from the Western Pacific island-arc system, both rotating clockwise, around poles at 31.1°N, 164.2°W and 1.3°S, 157.5°W, respectively. This provides for the opening of the back-arc basins. At the same time South America is moving onto the subduction zone at the rate of 4 cm/yr. Some “hot spots”, such as Hawaiian, Tibesti, and those of the South Atlantic, are moving relative to the island-arc system at a very low rate, viz. 0.5–0.7 cm/yr. Presumably, the western Pacific subduction zone and “hot spots” form a single frame of reference which can generally be used for the analysis of absolute motions.     

Glacier extent in a Novaya Zemlya fjord during the "Little Ice Age" inferred from glaciomarine sediment records

Glacier activity at Russkaya Gavan', north-west Novaya Zemlya (Arctic Russia), is reconstructed by particle size analysis of three fjord sediment cores in combination with ¹⁴C and ²¹⁰Pb dating. Down-core logging of particle size variation reveals at least two intervals with sediment coarsening during the past eight centuries. By comparing them with reconstructions of summer temperature and atmospheric circulation, these intervals are interpreted to represent two cycles of glacier advance and retreat sometime during ca. AD 1400–1700 and AD 1700–present. Sediment accumulation thus appears to be sensitive to century-scale fluctuations of the Barents Sea climate. The identification of two glacier cycles in the glaciomarine record from Russkaya Gavan' demonstrates that during the "Little Ice Age" major glacier fluctuations on Novaya Zemlya occurred in broad synchrony with those in other areas around the Barents Sea.     

四川天宝山铅锌矿床硫化物微量元素组成:LA-ICPMS研究

"川滇黔接壤铅锌矿集区"是我国西南大面积低温成矿域的重要组成部分,对于其中铅锌矿床是否属于MVT型矿床存在较大争议。本文以该矿集区中代表性矿床——四川天宝山矿床为例,通过LA-ICPMS原位元素分析,结合元素Mapping,以认识该矿床中闪锌矿和方铅矿微量元素组成特征及其赋存状态。研究表明,矿床中闪锌矿以富集Cd、Ge贫Fe、Mn、In、Sn、Co为特征,这些元素均以类质同象形式赋存于闪锌矿中,但含量变化范围较大,这可能与其成矿流体属于低温混合流体有关,这类盆地卤水流体在长期和长距离运移过程中,流经不同基底地层,活化出其中不同微量元素,因此成分变化较大,但以低温元素为主。此外,矿床中闪锌矿Ge和Cu呈现较好正相关关系,暗示其与Zn置换方式为:nC u~(2+)+Ge~(2+)(n+1)Zn~(2+),这可能是该矿床富集Ge的重要原因之一;矿床中方铅矿以富集Ag、Sb贫Bi为特征,含微量Cd和Tl,类质同象是这些元素主要赋存形式,其置换方式为(Ag)~(1+)+(Sb)~(3+)2Pb~(2+);矿床中Ge主要赋存于闪锌矿中,而方铅矿中不含Ge。总体上,本矿床硫化物微量元素组成与MVT型矿床基本一致,明显有别于喷流沉积型矿床、岩浆热液型矿床和远源夕卡岩型矿床,其成矿温度属于低温范围,成矿流体运移方向可能为深部→浅部。结合其矿床地质地球化学特征,本文认为天宝山铅锌矿床属于MVT型矿床,但其中闪锌矿中富集Cu,而方铅矿中富集Ag,可能暗示其形成具有一定特殊性。     

云南富乐铅锌矿床黄铁矿微量(稀散)元素组成及成因信息:LA-ICPMS研究

川滇黔铅锌矿集区是华南大面积低温成矿域的重要组成部分,区内铅锌矿床是否属于MVT型矿床长期存在争议。该区铅锌矿床以富集Ge等稀散元素为特征,闪锌矿是其主要载体矿物,但稀散元素在黄铁矿中是否富集、赋存状态及微量元素组成特征等研究基本属于空白。本文通过LA-ICPMS研究富乐黄铁矿中微量元素(尤其是稀散元素)的富集特征,发现黄铁矿中也相对富集Ge。本研究样品选自富乐矿床的富乐和富盛两个矿段,包括1350、1410和1536三个中段(由深到浅),LAICPMS分析结果表明,该矿床黄铁矿以富集Cu、As、Co、Ni为特征,局部富集Pb(Sb)和Zn(以方铅矿和闪锌矿显微包裹体形式赋存于黄铁矿中),该类黄铁矿富集的稀散元素主要为Se、Ge及少量Tl、Te,而Cd和In以类质同象形式赋存于含Zn的显微包裹体(闪锌矿)中,类质同象是其余稀散元素主要赋存形式,且黄铁矿中Ge与Cu存在较好相关关系,可能存在Cu~(2+)+Ge~(2+)?2Fe~(2+)耦合置换方式。此外,黄铁矿中稀散元素的富集与成矿元素(特别是Cu)的富集密切相关,随着成矿作用的进行,从矿体深部到浅部,成矿温度逐渐降低,Se/Te比值逐渐升高,且稀散元素与成矿元素呈逐渐增加趋势。研究表明,该矿床黄铁矿的Co/Ni比值基本都小于1. 00,暗示其属于沉积改造型黄铁矿,在Co-Ni和稀散元素Se-Tl含量投影图上,富乐矿床黄铁矿的投影点与MVT型矿床投影区基本一致,而明显有别于SEDEX、VMS和矽卡岩型矿床中黄铁矿的投影区,结合富乐矿床类似于MVT型的地质特征,我们认为富乐矿床属于MVT型铅锌矿床。     

Geochemistry of zircons from basic rocks of the Korosten anorthosite-mangerite-charnockite-granite complex,north-western region of the Ukrainian Shield

Mineralogy and Petrology - The concentrations of 26 trace elements have been determined by laser ablation ICP-MS in zircons from four samples of basic rocks of the Korosten...     

Role of radiogenic heat generation in surface heat flow formation

Heat generation due to decay of long-lived radioactive isotopes is considered in the Earth’s crust of the Archean–Proterozoic and Paleozoic provinces of Eurasia and North America. The heat flow that forms in the mantle is calculated as the difference between the heat flow observed at the boundary of the solid Earth and radiogenic heat flow produced in the crust. The heat regime in regions with anomalously high radiogenic heat generation is discussed. The relationship between various heat flow components in the Precambrian and Phanerozoic provinces has been comparatively analyzed, and the role of erosion of the surfaceheat- generating layer has been estimated.