Age and origin of sillimanite schist from the Chinese Altai metamorphic belt: implications for late Palaeozoic tectonic evolution of the Central Asian Orogenic Belt

Reconstructing late Palaeozoic metamorphism of the Central Asian Orogenic Belt (CAOB) can provide a better understanding of how the CAOB formed. The petrology of sillimanite-bearing metapelitic schists from high-grade portions of the Permian Chinese Altai metamorphic belt (andalusite-type) reflects the effects of poorly understood high-T, low-P metamorphism. Phase equilibria modelling in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–TiO₂–O (NCKFMASHTO) system restricts P–T conditions of the sillimanite schists to approximately 635–670°C at approximately 5.8–6.8 kbar. SHRIMP U–Pb analyses of zircon from the rocks yield a concordant age of 299.2 ± 3.4 Ma. Combined with the slightly younger (292.8 ± 2.3 Ma) areally restricted pelitic granulite with peak P?T conditions of approximately 780–800°C at approximately 5–6 kbar and high-T granulite with P?T conditions of approximately 860°C at approximately 6 kbar, these metamorphic rocks reflect prograde heating at relatively low pressure in early Permian time. Together with contemporary and widespread magmatic activities, they are best explained in the context of a post-orogenic extensional environment related to a mantle plume.     

Age and origin of charoitite,Malyy Murun massif,Siberia, Russia

Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by ⁴⁰Ar/³⁹Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation ⁴⁰Ar/³⁹Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of published K–Ar ages (112 ± 5 Ma) for one K-feldspar sample of the Malyy Murun syenite. Tinaksite is characterized by a similar spread of ages (from 133.0 ± 3 Ma to 115.7 ± 4.3 Ma) within a single grain. We suggest that charoitites originated due to the interaction of metasomatic agents derived from the Malyy Murun magma and country rocks. Timing of magma emplacement and charoitite crystallization is reflected by the older cluster of ages, whereas the younger ages are due to a secondary process.     

夕线石成因讨论

本文通过夕线石的结构、成分及产出状态,系统讨论和总结了其变质形成过程.含夕线石的变质岩原岩未必对应泥质岩,富夕线石岩石成分上更不能与任何的泥质岩成分对应.原岩本身富铝如多数泥质岩是形成夕线石非常有利的成分条件,但是,即使有合适的温压条件,也未必能够形成夕线石,组分的差异性迁移才是夕线石形成的必要条件.夕线石的形成与变形...     

Geochemistry and origin of the Mirny field kimberlites,Siberia

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

     

Precambrian granulites of the Aldan shield, eastern Siberia, USSR

Abstract Precambrian granulites of the Aldan shield in southern Yakutia, USSR, form a massif of 200,000 km² bounded by younger fold-belts to the south, west and east. The massif consists of several blocks that reflect a primary heterogeneity of composition and differences in structural and thermodynamic evolution of different parts of the area. According to structural and petrological data the massif can be divided into two megablocks: eastern Aldan and western Aldan. They are separated by a narrow meridional fold-belt. Structural evolution of this central zone was determined by the geodynamics of the mega-blocks and was completed in the late Archaean. Towards the south, this central zone is ‘transformed’into the relatively small Sutam block adjoining the Stanovoy fold-belt that bounds the Aldan shield on the south. The Sutam block is separated from the other structural units of the Aldan shield by a system of north trending grabens filled by post-Archaean sediments. The Aldan shield is composed of Archaean high-grade granulites, while the Stanovoy fold-belt, to the south, consists of highly foliated Proterozoic rocks metamorphosed under relatively lower-grade conditions. However, relics of the granulites are mapped within the fold-belt. They contain high-grade assemblages (e.g. Opx + Sil + Qz, Sap + Qz, Opx + Gr + Sil, etc.). One of the relics, the Tokskii block, which is only slightly touched by diaphthoresis, is located in the southeastern part of the Stanovoy fold-belt. Metamorphic conditions of the Tokskii block are compared with those of the Sutam block and a similar evolution of the units is revealed. Mineral assemblages and mineral compositions do not vary within each unit, but they change in a north-south direction. The Opx + Sil + Qz assemblage has been found only in Sutam and Tok, but not in eastern Aldan and western Aldan. The Sap + Qz assemblage has been found in the Tokskii block but has not yet been found in the Sutam block. The pyrope content in garnets, from metapelites of both blocks, is significantly higher than that from the Aldan (eastern and western blocks) rocks to the north. The most important assemblages from different units of the Aldan shield have been studied using the electron microprobe in order to unravel the metamorphic evolution of the granulites and thus to deduce the thermodynamic regime of this evolution. A geodynamic model for the Aldan shield is discussed in terms of Archaean island arc development.     

冀东杏山BIF铁矿形成时代及成因探讨

杏山BIF型铁矿位于冀东曹庄一带,矿体赋存于太古界三屯营组变质岩系中。矿石多为条纹-条带状构造,其围岩和夹层以斜长角闪岩为主,兼具少量黑云石英片岩和黑云钾长片麻岩。对BIF铁矿进行的地球化学分析表明,条带状铁矿石富集重稀土((La/Yb)*PAAS平均0.46),Eu正异常(Eu/Eu*PAAS平均2.19)、La正异常(La/LaPAAS平均2.14)和Y的正异常(Y/Y*PAAS平均1.85)明显,加之高的Y/Ho值(平均46.81),说明成矿物质来源于海底高温热液与周围海水的混合溶液;无Ce的负异常说明BIF形成于低氧逸度环境;BIF显示较低的Al2O3和TiO 2含量,且二者缺乏相关性,说明在BIF沉淀过程中几乎没有陆源碎屑物的加入。对矿体围岩斜长角闪岩、黑云石英片岩及黑云钾长片麻岩的原岩恢复,表明斜长角闪岩的原岩为玄武岩-安山质玄武岩,黑云石英片岩及黑云钾长片麻岩的原岩可能为泥质碎屑岩。元素地球化学分析表明,黑云钾长片麻岩具有较低的总稀土含量(59.46×10-6~66.99×10-6)、不明显的Eu负异常(Eu/Eu*=0.77~0.91)、略微的Th和U富集、无Nb和Ta的亏损;而黑云石英片岩具有较高的稀土总量(168.7×10-6~171.0×10-6)、强烈的轻稀土富集((La/Yb)N=6.81~7.07)和较弱的Eu负异常(Eu/Eu*=0.64~0.68),两者不一致的地球化学特征说明他们具有不同的物质来源。斜长角闪岩与N-MORB具有相似的特征,如富集大离子亲石元素(LILEs),亏损Th和U、Nb、Ta、Ti、Zr和Hf等无明显亏损。综合分析表明斜长角闪岩的母岩浆来源于尖晶石二辉橄榄岩30%的部分熔融,且其母岩浆在分异结晶过程中可能受到陆壳物质的混染。对矿体夹层斜长角闪岩LA-ICP-MS锆石U-Pb定年,结果表明其原岩形成于2859±22Ma至2491±13Ma之间,最有可能形成于新太古代晚期,这间接约束了杏山BIF型铁矿的形成时代,暗示着迁西岩群主体可能仍形成于新太古代晚期。而2859±22Ma的捕掳锆石年龄反映杏山-曹庄地区可能存在有中太古代表壳岩。     

冀东高板河锌、硫铁矿矿床中的微生物蔗与矿床成因

冀东高板河锌、硫铁矿矿床容矿岩石具有热水沉积岩的地球化学特点，而沿特定层位、一定的岩性分布的层状矿体，也证明该矿床具有热水沉积成因。作者在近年的工作中，于高板河矿床的硫铁矿层，发现呈席状产出的黄矿细球菌。丰富多彩的生物结构从一个侧面证实了该矿床中存在生物成矿作用。因此，高板河锌、硫铁矿床是海底热水沉积和微生物成矿双重作用的结果。     

辽宁高家堡子银矿区石榴云母片岩地球化学特征及地质意义

石榴云母片岩是高家堡子银矿区重要的容矿围岩。岩石地球化学分析显示,石榴云母片岩具有副变质岩特征,富集大离子亲石元素(Th,K),亏损高场强元素(Nb,Ta,Ti),稀土元素总量较高(ΣREE均值169.28×10~(-6)),轻、重稀土元素分馏明显,轻稀土元素富集,具有δCe弱正异常,δEu中等负异常。野外观察结合地球化学特征研究,高家堡子银矿区石榴云母片岩原岩为泥岩、砂岩、杂砂岩,具有上地壳长英质物源特征,形成环境为被动大陆边缘环境。     

胶东大庄子构造角砾岩型金矿床地质地球化学特征及其成矿流体来源探讨

大庄子金矿受胶莱盆地边缘发育于荆山群内的顺层状低角度断裂带控制,以大理岩质碎裂岩及角砾岩等张性构造岩胶结物发生矿化为特点.容矿岩石与成矿流体具有强烈的物质交换,矿化与SiO2、Fe2O3、Na2O及As、Cd、Hg、Cu、Pb等元素关系密切.矿石具有富集轻稀土元素、铕正异常的特征,其稀土元素特征和基性脉岩具有一定的相似性,又具有原岩的一些特点,表明容矿岩石受到了深部成矿流体的影响.硫、铅、碳-氧及氢-氧等同位素地球化学及流体包裹体研究显示,成矿物质较复杂,成矿流体来源于地壳深部或上地幔的岩浆热液,在成矿作用中,萃取了部分围岩,并与围岩发生了强烈反应.     

辽东后仙峪硼矿床含硼岩系中电英岩的地球化学特征及其成因

辽东后仙峪硼矿床含硼岩系中产出有电英岩,与硼矿床的形成有密切关系。岩石学和地球化学等研究发现矿区内电英岩呈层状分布于硼矿体的顶、底板,常夹于变粒岩层中,具完好的条纹条带状构造;在判别硅质岩成因的TiO2-Al2O3和（K2O＋Na2O）-A l2O3二元图解上,投影点均落在海底热水沉积硅质岩区;δEu和δCe均为中等程度的负异常,经北美页岩标准化后的稀土元素配分曲线左倾明显,这些特征表明后仙峪硼矿床含硼岩系中的电英岩为热水沉积岩,与其关系密切的后仙峪硼矿床为热水喷流沉积硼矿床。     

胶东东部文登金矿床地质特征

文登金矿是受脆性剪切带控制的构造角砾岩 碎裂岩型金矿床 ,是脆性剪切作用形成的 ,为典型的动力成岩成矿。矿石类型主要是含金碎裂岩 ,含金构造角砾岩 ,网状含金构造角砾岩 ,碎裂化、矿化粗中粒二长花岗岩 4种。成矿作用分为 2期 5个阶段 ,第一期碎裂构造作用———热液期 (内生期 )是主成矿期 ,分 4个成矿阶段。成矿物质来源于原岩 ,主要是昆嵛山超单元中晚期粗粒花岗岩。成矿时代可能为燕山晚期 [成矿期绢云母K Ar年龄为 (10 1 97± 2 4 9)Ma]     

Boron,sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt,eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73 Ga volcanic and sedimentary sequences, as well as 2.75–2.72 Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70 Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5 kbar pressures and at 500–600 °C temperatures affected the belt at around 2.70 Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe^3 +–Al^3 +-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ¹¹B data for this kind of tourmaline is from − 17.2‰ to − 12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe^3 +–Al^3 + substitution) and δ¹¹B values between − 19.0‰ and − 9.6‰. The uvite–foitite compositional trend and δ¹¹B ‰ values between − 24.1% and − 13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ¹¹B values between − 14.5‰ and − 10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ³⁴S_VCDT values measured in pyrite, chalcopyrite and pyrrhotite is from − 9.1 to + 8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ³⁴S_VCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ³⁴S_VCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ³⁴S_VCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ⁶⁵Cu_NBS978 values from chalcopyrite is from − 1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ⁶⁵Cu_NBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ³⁴S_VCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.     

新疆芒拉克艾肯红柱石矿床地质特征及成因

芒拉克艾肯红柱石矿床位于南天山博洛霍坦北坡,南天山古生代裂陷槽之石炭纪野云沟拗拉槽东部,北与霍拉山地块相邻,二者之间为霍拉沟大断裂.区域出露地层为下元古界和下石炭统野云沟组以及中上石炭统塔拉克组.以霍拉沟大断裂为界,北部为角闪岩相,由下元古界中深变质岩系组成,岩性有黑云母角闪石英片岩、云母石英片岩、角闪斜长片麻岩等;南部为低绿片岩相,由石炭系组成,岩性有变砂岩、千枚岩、粉砂岩、含红柱石板岩等.本区岩浆活动比较强烈,岩浆岩分布严格受区域大断裂的控制.其构造线呈NWW向展布(图1). 图1 区域地质矿产…     

内蒙古毛登锡铜矿床地质及成因

内蒙古毛登锡铜矿床是大兴安岭南段的锡多金属矿床之一。该矿床是与阿鲁包格山花岗斑岩（Ｒｂ－Ｓｒ等时线年龄１４９×１０＾－６ａ），锶同位素初始比值为０．７０５）有关的具中等成矿深度和成矿温度的热液脉型锡铜矿床。矿床中主要产出锡矿体，它们分别以微脉、细脉和大脉的形式充填于自接触带向外的ＮＷ向断裂裂隙系统中。钾化（磁铁矿矿化）、云英岩化（锡石或辉钼矿矿化）和硅化（硫化物矿物）三个蚀变－矿化阶段流体的温度和     

Petrogenesis and source characteristics of metatholeiites from the Archean Ramagiri schist belt, eastern part of Dharwar craton, India

The N–S trending, 2–4 km wide Ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks, separated and surrounded by granitic rocks of distinct characteristics. The metavolcanic rocks are tholeiitic in composition and are very similar in their major element composition as well as in their abundances of some trace elements. However, the rare earth elements (REE) require distinct sources. The rocks of the amphibolite facies eastern block have LREE depleted REE patterns ([Ce/Yb] = 0.7–0.9), requiring derivation from depleted mantle-like sources. The greenschist facies metatholeiitic rocks of the central block have LREE enriched REE patterns ([Ce/Yb] = 3–6), reflecting the nature of their source(s). The Nd isotopic data require that the LREE enriched nature could not have been attained significantly prior to its melting. The fine-grained, upper greenschist facies metatholeiites of the western block have flat to slightly LREE depleted patterns ([Ce/Yb] = 0.8–0.95). Minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of the three blocks. Different extents of partial melting of distinct mantle sources have played a dominant role in the generation of the parent magmas to the central versus eastern and western block metatholeiites. The geochemical data suggest that the mantle sources were non-lherzolitic, and that these sources may have seen previous episodes of melt addition and extraction prior to melting that gave rise to the parent melts to the rocks ∼2750 Ma ago. The REE data indicate that while the sources of the eastern and western block rocks were similar to depleted mantle (ɛ_{Nd( i )} about +2), the source of the central block rocks (ɛ_{Nd( i )} about +3.5) were enriched in large ion lithophile element (LILE)-rich fluids/melts probably derived from subducting oceanic crust. This and other trace element signatures point to magma extraction in tectonic settings similar to modern island arcs. Subsequent to magma emplacement and crystallization, all the three suites of rocks were affected by interaction with low-temperature, crustal derived fluids (ɛ_{Nd 2750Ma} of about −8 to −12), probably during the accretion of the three blocks of the belt in the present form. The inferred source characteristics, tectonic setting of magma generation and the crustal fluid processes seem to suggest that Phanerozoic-style tectonic processes may have been important in the generation of Archean crust in the Dharwar craton. Received: 31 July 1995 / Accepted: 12 May 1997     

Sulfur isotope studies of the felbertal scheelite deposit,eastern alps

The Felbertal scheelite deposit is the largest known strata-bound tungsten concentration. It lies in an up to 400 m thick rock pile in the lowermost part of the volcanic rock sequence, probably of the Early Paleozoic Habach Formation. Both ore fields (eastern and western) have been affected by Variscan and Alpine metamorphism and tectonism, resulting in a remobilization of the ore mineralization. This ore deposit and the neighboring rocks show a strikingly low sulfur content. The eastern field with one major orebody has very little sulfide mineralization. The western field, with 8 orebodies (K1–K8) and two remobilized vein zones (S1 and S2), reveals somewhat more minor sulfide enrichments that are mainly within and around the K1 and K2 orebodies and in some parts of the interlayered schist sequence. Sulfur isotope compositions of 90 sulfide minerals (37 pyrrhotite, 20 chalcopyrite, 19 pyrite and 11 molybdenite and/or WS₂-MoS₂ solid solutions and 3 Pb-Bi sulfosalts, including 7 sulfides within scheelite grains) from 60 ore and host rock samples have been determined with a standard error of less than ±0.2 per mil. All data range from –3.6 to +4.3 ³⁴S. There are small differences in the sulfur isotope values from place to place and in time from the first and second to the third generation. In the western field, the K1 orebody differs from other orebodies (K2, K4, K7) due to isotopically heavier ³⁴S values. The three scheelite generations show differences in the ³⁴S values of the sulfide microphases within scheelite grains, from +1.0 to +4.3 per mil for the first and the second, and from –1.8 to –3.3 per mil for the third generation. Sulfide phases within molybdoscheelites may have crystallized under the same conditions as the other coeval sulfide minerals in the same orebody. They commonly formed later than scheelite. These changes may be explained using data from Ohmoto and Rye (1979): Small changes in temperature, pH, and/or may result in large changes in the ³⁴S values with the precipitation of isotopically heavier sulfides under more reducing conditions. Only four samples with sulfide mineral pairs show isotopic equilibrium. All others display some disequilibrium. We suggest that the sulfides in the ores and surrounding volcanogenic host rocks formed contemporaneously from the same hydrothermal ore fluids, and that the sulfur species in these fluids may have been dominantly H₂S.