Identification of clayey altered ophiolite in the Nujiang tectonic belt and new understanding of its impacts on engineering stability

1. Objectives Ophiolites from the oceanic crust are important indicators for identifying tectonic suture zones. Recently, a continuous ophiolite belt was found near the Guola Mountain in the Nujiang tectonic belt. Due to intensive hydrothermal alteration during tectonic evolution, clayey altered ophiolite with special engineering geological characteristics was formed, which has an extremely adverse impact on engineering stability. However, the adverse properties of clayey altered ophiolite are still not well understood in engineering practices(Zhang YS, et al., 2011).

Analysis and assessment of nickel and chromium pollution in soils around Baghejar Chromite Mine of Sabzevar Ophiolite Belt,Northeastern Iran

The key objective of this research was to estimate the Ni and Cr contents of soil around the Baghjar Chromite Mine（BCM）of Sabzevar Ophiolite Belt,Northeastern Iran,and assess the degree of soil pollution using the pollution indices.Soil samples（0-20 cm depth） were collected at various distances from the BCM.In the present research,heavy metals（Cr and Ni） in soil samples were analyzed by atomic absorption spectrometry to detect their concentrations and contour maps were produced to explain the metal spatial distribution.Also,the degree of metal pollution was quantified.The results indicate that the soils in the studied area are contaminated by Cr and Ni.The corresponding concentrations for Cr and Ni are（156.19±24.45） and（321.7±133.27） mg/kg,respectively,which exceed the corresponding maximum allowable concentrations in soils.The different indices demonstrate that soils around chromite mine are significantly contaminated with Cr and Ni,suggesting several times higher levels of toxic metals than normal ranges.The above results revealed that the heavy metal concentrations increase with increasing the distance from the mine and mining pollutants can be transported to long distances from their sources.

Arc-trench System of the Paleo-Tethys Ocean: Inferred from Ophiolite in the Southern Lancangjiang Belt, SW China

The Paleo-Tethys Ocean is usually interpreted as a Paleozoic ocean basin located between the Gondwana and Laurasia supercontinents. The Paleo-Tethyan orogenic record is well preserved in the Sanjiang area of SW China. However, ophiolites are commonly dismembered in orogenic belt, and complete ophiolite sequences are rare in the Sanjiang area. The southern Lancangjiang belt is the most complicated tectonic complex of the Sanjiang Paleo-Tethyan orogen, SW China, and is key to understanding the evolution of the orogen. In this study, we focused on mafic–ultramafic rocks in the Yakou and Banpo areas of the southern Lancangjiang belt, of which newly discovered Yakou rocks show a complete ophiolite sequence. These rocks are composed of serpentinized peridotite, isotropic and cumulate gabbros, massive and pillow basalts, and plagiogranite. Whole-rock geochemical data indicate that these rocks were formed in an oceanic ridge setting, and they show depletions in Nb, Ta and Ti, and enrichment in Pb, suggesting a supra-subduction zone affinity of a back-arc setting. Furthermore, positive εNd(t)(+4.5 to +6.7) and zircon εHf(t) values(+12.4 to +14.3), as well as mantle-like δ18O values(~5.5‰), suggest that these rocks were derived from a long-term depleted mantle source. All of these features suggest that the Yakou mafic-ultramafic complex represents an ophiolite suite, making it the first complete ophiolite sequence to be discovered in the southern Lancangjiang orogenic belt. The Banpo complex gabbroic rocks have similar whole-rock geochemical and Sr-Nd isotopic, and zircon O-Hf isotopic compositions to those of the Yakou complex, suggesting an N-MORB affinity. Thus, maficultramafic rocks from the Banpo and Jinghong areas are most likely dismembered ophiolite suites. Considering these various characteristics, we consider that the Yakou, Banpo, and Jinghong mafic-ultramafic complexes represent an ophiolite belt but not a magmatic arc belt. SHRIMP zircon U-Pb dating yield weighted mean ages of 305±3 Ma, 310±2 Ma, and 313±6 Ma. Therefore, we suggest that the Banpo-Jinghong mafic-ultramafic complex represents a Late Carboniferous(313–305 Ma) ophiolite belt in the Sanjiang Paleo-Tethyan orogen of SW China. Finally, we propose that an arc-trench system could have developed in the Sanjiang Paleo-Tethyan orogenic belt of SW China during the Late Carboniferous.

Diamonds, Super-Reduced and Crustal Minerals in Chromitites of the Hegenshan and Sartohay Ophiolites, Central Asian Orogenic Belt, China

The Central Asian Orogenic Belt(CAOB)is a huge tectonic mélange that lies between the North China Craton and the Siberian Block.It is composed of multiple orogenic belts,continental fragments,magmatic and metamorphic rocks,suture zones and discontinuous ophiolite belts.Although the Hegenshan and Sartohay ophiolites are separated by nearly 3000 km and lie in completely different parts of the CAOB,they are remarkably similar in many respects.Both are composed mainly of serpentinized peridotite and dunite,with minor gabbro and sparse basalt.They both host significant podiform chromitites that consist of high-Al,refractory magnesiochromite with Cr#s[100Cr/(Cr+Al)]averaging<60.The Sartohay ophiolite has a zircon U-Pb age of ca.300 Ma and has been intruded by granitic plutons of similar age,resulting in intense hydrothermal activity and the formation of gold-bearing listwanites.The age of the Hegenshan is not firmly established but is thought to have formed in the Carboniferous.Like many other ophiolites that we have investigated in other orogenic belts,the chromitites in these two bodieshave abundant diamonds,as well as numerous super-reduced and crustal minerals.The diamonds are mostly,colorless to pale yellow,200-300μm across and have euhedral to anhedral shapes.They all have low carbon isotopes(δ14C=-18 to-29)and some have visible inclusions.These are accompanied by numerous super-reduced minerals such as moissanite,native elements(Fe,Cr,Si,Al,Mn),and alloys(e.g.,Ni-Mn-Fe,Ni-Fe-Al,Ni-Mn-Co,Cr-Ni-Fe,Cr-Fe,Cr-Fe-Mn),as well as a wide range of oxides,sulfides and silicates.Grains of zircon are abundant in the chromitites of both ophiolites and range in age from Precambrian to Cretaceous,reflecting both incorporation of old zircons and modification of grains by hydrothermal alteration.Our investigation confirms that high-Al,refractory chromitites in these two ophiolites have the same range of exotic minerals as high-Cr metallurgical chromitites such as those in the Luobusa ophiolite of Tibet.These collections of exotic minerals in ophiolitic chromitites indicate complex,multi-stage recycling of oceanic and continental crustal material at least to the mantle transition zone,followed by uprise and emplacement of the peridotites into relatively shallow ophiolites.

Comparison of Different-sized Chromite Mineralizations in the Yarlung-Zangbo Ophiolite Belt, Southern Tibet

Podiform chromitites are characteristically occurred in ophiolites(e.g.,Thayer,1964;Dickey,1975).However,the metallogenic processes for podiform chromitites are still unclear.Early models involved fractional crystallization and crystal settling from picritic or basaltic melts in magma chambers(Dickey,1975;Boudier and Coleman,1981),but it was also proposed that podiform chromitites formed from partial melting and melt extraction in host mantle peridotites(Dick,1977;Dick and Bullen,1984).Recent studies by the majority of authors have suggested that melt-rock interaction at the Moho transition zone may have played a key role in the formation of podiform chromitites(Zhou and Robinson,1994;Zhou et al.,1996,2005,2014;Robinson,2008;Page and Barnes,2009;Uysal et al.,2009,2012;González-Jiménez et al.,2011,2015).Based on the occurrence of some ultrahigh pressure minerals(e.g.diamond and coesite)in chromitites,it has been proposed recently that the formation of podiform chromitite is likely related to multiple processes inclusing mantle recycling(Yang et al.,2007;Yamamoto et al.,2013).Although geat progresses have been made towards understanding the genesis of podiform chromitites,some fundamental issues in remain unanswered.For examples,what are the major controls on the size of chromitites?And why some ophiolites contain large podiform chromitite bodies,whereas most ophiolitic massifs are essentially chromitite-barren? The Yarlung-Zangbo Ophiolite belt is one of the most famous ophiolite zone in the world.It contains fresh peridotites as well as different-sided podiform chromitites.The Luobusha ophiolite in the eastern segment of the belt hosts the largest chromite deposit in China.In the central and western segments of belt the Dazhuqu and Dongbo ophiolitic massifs contain some small-scale chromitite bodies.Such characteristics make the Yarlung-Zangbo Ophiolites an ideal subject to investigate the major controls on the metallogenesis of podiform chromitites. The Luobusha chromitites are large lens and enclosed in dunite.In contrast,the Dazhuqu and Dongbo chromitites display generally as narrow dykes or irregular seams with dunite envelopes.The closely spatial association of the chromitites and dunite envelopes,together with their textural features,support a petrogenetic model that the chromitites from the Luobusha,Dazhuqu and Dongbo massifs form from reaction of melt with host peridotite.In terms of chemical composition of chromite,there are distinctive differences between those from the Luobusha and the Dazhuqu or the Dongbo.Chromite from the Luobusha chromitites has high Cr#(71-82),whereas Chromite in the Dazhuqu chromitites show relatively low Cr#(16-63),and chromite in the Dongbo chromitites includes low Cr#(11-47)and high Cr#(70-81)types.For the Dongbo and Dazhuqu massifs,linear trends of Cr#with Mg O,Fe Ot,Ni,Ga,V and Sc in chromite from the chromitites and dunites of are similar to those of the host peridotites,suggesting that the melt-rock reaction may provide major budget of Cr for the chromitites.The similar compositions at a given Cr#in chromite from these rocks also demonstrate that the chromitites may have been formed by in-situ crystallization of chromite under low melt/rock ratio.In contrast,the Luobusha chromitites have different trends of compositions in chromite from that of the host peridotites,implying that the formation of the chromitite bodies requires a continual replenishment of Cr-rich melts from deeper mantle.Fractionation and accumulation of chromite from a large volume of Cr-rich melt may play an important role on the formation of the Luobusha chromitites.MORB-normalized trace element patterns of chromite from the Luobusha chromitites suggest that it has been formed from Cr-rich boninitic melt at surpra-subduction zone(SSZ)setting.However,the Dongbo and Dazhuqu chromitites have formed originally from a MORB-affinity melt at a mid-ocean ridge(MOR)environment. In summary,the Luobusha chromitites crystallized from a Cr-rich melt in a dynamic conduit,where fractional crystallization and crystal settling play a key role in formation of the large chromitites.In contrast,the small-scale mineralizations of the Dongbo and Dazhuqu chromitite pods are formed from in situ produced melts.Podiform chromitites can be formed in MOR environment,whereas the higher Cr content in boninitic melt and assimilation of subducted slab materials at SSZ setting may benefit the formation of large chromite deposit.

Geochemistry and Geochronology of the Jinghong Ophiolites:Implications for the Tectonic Evolution of the Eastern Paleo-Tethys

The Jinghong mafic-ultramafic complex,exposed in the eastern margin of the Lancangjiang tectonic belt,is related to the subduction of the Paleo-Tethys Ocean.Its petrogenesis plays a key role in constraining the tectonic evolution of the eastern Paleo-Tethys Ocean in southwestern China.In this study,we present petrological,geochemical and geochronological results of the Jinghong complex rocks,in order to decipher their origin and tectonic significance.The Jinghong mafic-ultramafic complex was composed of peridotite,gabbro,basalt and minor plagiogranite.Whole-rock geochemical data of the mafic rocks indicate that they have both MORB and IAB affinities and plot in the back-arc basin basalt(BABB)field in the FeO^(*)/MgO vs.TiO_(2) diagram.Combined with their trace element characteristics,it can be concluded that the Jinghong mafic-ultramafic complex represents an ophiolite suite that was formed in a back-arc ocean basin.Precise LA-ICP-MS zircon U-Pb dating yielded weighted mean ^(206)Pb/^(238)U ages of 298.4±1.7 Ma,294.3±1.6 Ma,and 292.8±2.0 Ma for gabbroic rocks from this complex,which indicates that the Jinghong ophiolites were formed during the early Permian(298-293 Ma).We propose that during subduction of the main Paleo-Tethys Ocean,a back-arc ocean basin was formed at the east of the Lancangjiang tectonic belt.

The Tectonic Implications of the Hongliuhe-Xichangjing Ophiolitic Mélanges Belt in the Central Region of the Beishan Orogen, NW China——Constrained by the U-Pb Ages of Detrital Zircons of the Metasandstones

The tectonic attributes of different blocks within orogenic belts are of great significance for the study of accretionary processes and the evolution of Earth. The Hongliuhe-Niujianzi-Baiyunshan-Xichangjing ophiolitic mélange belt(HXOMB) is distributed in the heart of the Beishan Orogen, the Shuangyingshan and Minshui-Hanshan blocks being distributed in the south and north of the HXOMB respectively, and a large number of Early Paleozoic geological units are exposed on the blocks. According to the zircon age populations of the metasandstones in the Baiyunshan area recovered in this paper, when compared with the zircon age populations of the Paleozoic metasandstones reported in the Niujuanzi and Hanshan areas, we found that the metasandstones of the Shuangyingshan Block have age peaks at c. 598 Ma, 742 Ma, 828 Ma, 941 Ma, 990 Ma, 1168 Ma, 1636 Ma, 2497 Ma with non-significant age populations of 1500–1300 Ma, showing a possible affinity with the Tarim Craton;the metasandstones of the Minshui-Hanshan Block have age peaks at c. 606 Ma, 758 Ma, 914 Ma, 1102 Ma, 1194 Ma, 1304 Ma, 1672 Ma with significant age populations of 1500-1300 Ma, showing a possible affinity with the Chinese Central Tianshan Block. Therefore, the HXOMB of the Beishan Orogen is of great significance in plate segmentation, which separates the Tarim Craton in the south and the Chinese Central Tianshan Block in the north. Based on the evolutionary process of the Hongliuhe-Xichangjing ocean in the Beishan Orogen, we believe that break-up and convergence can be recognized as having occurred twice between the Chinese Central Tianshan Block and the Tarim Craton since the Mesoproterozoic in the Beishan area. This was related firstly to the break-up of the Columbia Supercontinent and the convergence of the Rodinia Supercontinent, mainly during the Middle Mesoproterozoic to Early Neoproterozoic, and secondly to the opening and closing of the Hongliuhe-Xichangjing ocean, mainly during the Early Paleozoic.

Multi-stage Process of the Bulqiza Chromitites, Eastern Ophiolitic Belt, Albania

The ultramafic massif of Bulqiza,which belongs to the eastern ophiolitic belt of Albania,is the most important area for metallurgical chromitite ores.The massif consists of a thick(>4 km)rock sequence,with a generalized

Petrology, Geochronology and Geochemistry of the Xar Moron River Ophiolite: Implications for the Tectonic Evolution of the Paleo-Asian Ocean

As the largest accretionary orogen, the crustal tectonic framework and evolution of the Central Asian Orogenic Belt(CAOB) have always been one of the hot topics among geologists(Seng?r et al., 1993, 1996;Jahn et al., 2000 a;Badarch et al., 2002;Windley et al., 2007;Li et al., 2009). The formation of the main part of the crust in the CAOB involved continuous lateral accretion of island arcs and accretionary complexes along the margins of the Siberian, Sino-Korean and Tarim paleocontinents and the final collision between these continental margins because of the subduction of the Paleo-Asian Ocean plate since Mesoproterozoic. The ophiolites, which represent the fragments of ancient oceanic lithosphere, are the direct evidence for the study of the evolution of orogenic belts. Based on field geological survey, the mantle peridotite(serpentinite), gabbro, basalt and radiolarian bedded chert, which were deemed as the "ophiolite trinity", were identified as isolated blocks in the matrix of pelitic siltstone and silty mudstone in the Kedanshan, Xingshuwa and Jiujingzi areas along the Xar Monron River in southeast Inner Mongolia of China. Besides, there were plenty of other exotic blocks, such as limestone and sandstone, in the matrix. Both of the matrix and blocks underwent strong foliated deformation. All of these rocks above constitute a tectonic mélange. Zircon U-Pb dating for the gabbro blocks in the Xingshuwa and Jiujingzi ophiolites reveals that they were formed in early Permian(275-280 Ma). The ages of the gabbros, together with the middle Permian radiolaria fossils in the chert reported by Wang and Fan(1997), indicate that the oceanic basin was not closed in early-middle Permian. The geochemical compositions of the basaltic blocks distributed in different locations in the Xingshuwa tectonic mélange display different genetic types of normal mid-ocean ridge basalt(N-MORB), enriched mid-ocean ridge basalts(E-MORB), oceanic island basalt(OIB), island arc basalt and continental marginal arc basalt, which indicates what they represented is a complex oceanic basin. Combining with the studies on regional magmatism, strata and structure data, it is suggested that the Xar Moron River Ophiolite belt represented the final suture zone of the Paleo-Asian Ocean in the southeast Inner Mongolia, and the ocean did not close before late Permian.

全球天然氢气勘探开发利用进展及中国的勘探前景

在全球能源脱碳背景下,天然氢气作为一种一次能源,因其零碳、可再生的优点而备受关注,但中国目前还未开展专门针对天然氢气的勘探工作。通过介绍全球已知高含量天然氢气(体积分数大于10%)气藏的主要形成地质环境及成因类型,系统总结了天然氢气富集的有利地质条件,并结合国外天然氢气的勘探开发现状,评价了中国天然氢气的勘探前景。研究结果表明:(1)全球高含量天然氢气主要发育于蛇绿岩带、裂谷和前寒武系富铁地层中,且以无机成因为主,富铁矿物的蛇纹石化过程是天然氢气最主要的成因来源,其次为地球深部脱气和水的辐解。(2)优质的氢源与良好的运移通道是氢气富集的前提,而盖层的封盖能力是天然氢气能否成藏的关键要素;天然氢气作为伴生气时,传统盖层对其具备封盖能力,但当其含量较高时,传统盖层可能难以形成有效封盖;裂谷环境、蛇绿岩发育区以及断裂发育的前寒武系富铁地层是富氢气藏的勘探有利区。(3)国外多个国家和地区已制定了天然氢气的勘探开发和利用计划,其中,马里已实现天然氢气的商业开采,美国、澳大利亚也已成功钻探天然氢气勘探井。(4)中国高含量氢气区与富氢地质条件高度匹配,天然氢气勘探前景良好,郯庐断裂带及周缘裂陷盆地区、阿尔金断裂带及两侧盆地区、三江构造带—龙门山断裂带及周缘盆地区的天然氢气勘探潜力较大;中国应尽快开展天然氢气普查工作,加强氢气成藏过程研究和潜力评价,并进行勘探技术、开采分离技术和储运技术的攻关,为天然氢气的大规模开发利用做好技术储备。

西南三江甘孜-理塘洋晚古生代构造演化:来自理塘蛇绿混杂岩堆晶辉长岩U-Pb年龄的约束

理塘县高城镇西北的擦岗隆洼岩组是一套以玄武岩为基质的蛇绿混杂岩,通过该蛇绿混杂岩中堆晶辉长岩的LA-ICP-MS锆石U-Pb定年,显示其年龄范围介于349~274 Ma之间,形成2个年龄组:第一组斜锆石^(206)Pb/^(238)U年龄加权平均值为346±17Ma,代表堆晶辉长岩早期岩浆活动的时代;第二组斜锆石^(206)Pb/^(238)U年龄加权平均值为286.2±5.1 Ma,代表了堆晶辉长岩晚期岩浆活动的结晶年龄。擦岗隆洼岩组的超基性—基性岩具有N-MORB(正常洋中脊玄武岩)和E-MORB(富集型洋中脊玄武岩)组合特征,指示这些岩石可能形成于洋中脊环境,岩浆物质源自N-MORB所代表的亏损上地幔源区受地幔柱交代混染后而形成的混合源区。研究认为,甘孜-理塘洋最早可能在中泥盆世,受区域地幔柱活动影响开始拉张,导致中咱地块从扬子陆块西缘裂离,并在早石炭世发育形成洋盆。在中晚三叠世甘孜-理塘洋壳开始向西俯冲,并于晚三叠世末闭合,区域进入弧陆碰撞造山阶段。

An Overview on the Composition and Age of Upper Crust of Proto-Tethyan Lajishan Intra-oceanic Arc,NE Tibet Plateau

Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin.This article summarizes the geological,geochemical,and geochronological characteristics of upper crust of Proto-Tethyan Lajishan intra-oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean.The intra-oceanic arc volcanic rocks,including intermediate-mafic lava,breccia,tuff,and minor felsic rocks,are distributed along southern part of the Lajishan ophiolite belt.Geochemical and isotopic compositions indicate that the intermediate-mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids,whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra-oceanic arc setting.Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma,suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra-oceanic arc.Combined with the Cambrian forearc ophiolite and accretionary complex,we suggest that the Cambrian intra-oceanic arc in the Lajishan ophiolite belt is belonging to the intra-oceanic arc system which was generated by south-directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.

中蒙边境地区主要蛇绿岩带特征及地质意义

【研究目的】中国与蒙古的交界地区构造位置上位于中亚造山带,沿边境两侧断续出露呈团块、带状分布的蛇绿(混杂)岩带。蛇绿岩作为古大洋岩石圈的残片,是研究岩石圈地幔演化、俯冲带物质循环和重建古地理格局的天然实验室。【研究方法】本文依托中蒙合作1/100万系列地质图件编制项目成果,结合前人针对该地区蛇绿(混杂)岩带的研究成果,进一步厘定中亚造山带的构造演化和恢复其构造格局。【研究结果】在研究区划分出十四条蛇绿(混杂)岩带,综合阐述了其时空分布特征,建立了蛇绿(混杂)岩带的时空分布格架。【结论】研究结果表明,中蒙边境蛇绿岩带自西向东呈年龄逐渐年轻的趋势(新元古代-早二叠世),揭示了中亚造山带西段和东段大洋岩石圈地幔演化的差异性。多数蛇绿(混杂)岩带属于与俯冲作用密切相关的SSZ(supra-subduction zone)型蛇绿岩,但弧前和弧后的构造背景的争议仍需进一步研究加以限定。

Neoproterozoic Trench-arc System in the Western Segment of Jiangnan Orogenic Belt, South China

The Jiangnan orogenic belt is a key to understanding of the Neoproterozoic tectonic evolution of the South China Block. We investigate the mafic-ultramafic suites of lherzolite, pyroxenite, gabbro, pillow basalt and gabbroic diorite as well as red jasper interbedded with marine marbles that are mainly exposed as fault-trapped blocks in the Yuanbaoshan and Longsheng domains of the western Jiangnan belt. The postcollisional granite plutons that intruded the ultramafic-mafic rocks are developed well. Zircons in the gabbro yield crystallization ages of 867±10 Ma, 863±8 Ma, 869±9 Ma and 855±5 Ma whereas those from the granites show ages of 823±5 Ma, 831±5 Ma, 824±5 Ma and 833±6 Ma. The Neoproterozoic serpentinited ultramafic samples display minor REE enriched pattern with depletion of Rb, Ba, Nb, Ta and Ti, similar to those of SSZ type ophiolite. The coeval gabbro shows tholeiitic features and is characterized by negative Ba, Nb, Ta, Zr, Hf and Sr anomalies and LREE enriched patterns, with a minor negative Eu anomaly. Some zircon grains from the Longshen gabbro yield Neoarchean-Paleoproterozoic ages(2859–2262 Ma), suggesting its continental arc setting. Geochemical signature of the maficultramafic rocks is consistent with subduction related setting. The pyroxene-bearing diorite exhibits a distinctive arc affinity. The zircons from the gabbro show positive εHf(t) values ranging from 3.9 to 13.8. The granitoids are typical S–type granites with high ACNK values(1.15–1.40) and negative εHf(t) values(–15.1 to –3.2), and are classified as collision–related granites. Combined with the occurrences of mafic-ultramafic rocks, siliceous marble and red jasper mixed with basalt, our new results suggest the presence of a Tonian(863–869 Ma) SSZ ophiolite system and continental arc-type magmatism in the western Jiangnan orogen.

THE ROCK ASSOCIATION IN JINSHAJIANG MELANGE BELT

Jinshajiang melange belt locates between Jianda\|Weixi island arc and Zhongzha massif. The melange belt and island arc makes up Jinshajiang plate junction. Although subsequent tectonic movements had complexed the structural form of Jinshajiang melange belt, there are still a lots of structural block remained which carried amount of information about the tectonic evolution of the belt. Recent researches have identified several kinds of rock association in the structural blocks.(1) Ophiolite:The ophiolite consists of serpentinization ultramafite, ultramafic cumulus crystal rock (pyroxenite, dunite), gabbro, diabase cluster, ocean\|ridge type basalt, plagiogranite and radiolarian silicalite. The isotopic age shows that the ultramafite and basalt formed during Upper Carboniferous and Lower Permian. The silicalite is high in radiolaria of Lower Permian.(2) Rock association of oceanic island\|arc:The liptocoenosis of oceanic island\|arc scatter in melange belt, it mainly consists of sandy slate, pyroclastic rock, silicalite, basalt and andesite. A part of volcanic rock belongs to calc\|alkaline volcanic suite and the other is tholeiite. The petrochemistry, REE and microelement of volcanic rock have the feature of the rock in ocean\|island arc. The isotopic age of basalt shows that the ocean\|island arc formed in Lower Permian.

青海扎巧合蛇绿混杂岩中辉长岩地球化学、锆石U-Pb年龄及对洋脊环境的限定

【研究目的】扎巧合蛇绿岩处于中祁连和南祁连构造带的结合部位,带内蛇绿岩组分由蛇纹岩、蛇纹石化辉橄岩、辉长岩,辉绿岩、块层状蚀变玄武岩,少量枕状玄武岩、透镜状硅质岩等组成,为拉脊山蛇绿混杂岩带在青海省内的西延部分。【研究方法】本文对扎巧合蛇绿岩中的辉长岩进行了年代学、岩石地球化学研究。【研究结果】辉长岩以主化学成分Si O_(2)、Na_(2)O、K_(2)O、(Fe_(2)O_(3)+Fe O)及MgO等的特征判断,为拉斑系列的辉长岩。从其REE分布模式看,铕无异常或弱负异常,说明岩石在熔融过程中经历了斜长石的分离结晶作用,稀土配分曲线为一条近似平直曲线;微量元素具明显的Ta和Sr正异常特征,表明本区蛇绿岩中辉长岩具有与E-MORB类似的特征。LA-ICP-MS锆石U-Pb测年结果显示辉长岩结晶年龄为(524.0±2.5)Ma。【结论】这些辉长岩起源于未发生大规模熔融的初始地幔,认为扎巧合蛇绿岩可能形成于古洋盆初始裂解阶段的洋脊环境。

新疆卡拉麦里蛇绿混杂岩带北侧英云闪长岩年代学、地球化学特征及地质意义

以出露于卡拉麦里蛇绿混杂岩带北侧元湖一带英云闪长岩为研究对象,在野外地质调查基础上,结合锆石LA-LCP-MS U-Pb年龄测试、岩石地球化学分析,对该岩体侵入时代及成因进行讨论。英云闪长岩锆石LA-ICPMS U-Pb年龄为(342.3±2.8)Ma(MSDW=0.49)。岩石SiO_(2)含量73.21%~77.15%,Al2O_(3)含量10.66%~13.00%,属偏铝质,A/CNK为0.89~0.98、富钠,Na_(2)O/K_(2)O值为6.99~22.31,低钾(拉斑)系列。稀土元素总量较低,∑REE=40.95~55.01,轻重稀土分馏较弱,(La/Yb)_(N)=0.47~0.72,Eu负异常明显,δEu=0.15~0.25。Sr/Y比值为3.07~4.36,Nb/Ta比值为3.61~9.86。岩石具较低的Sr初始值,(^(87)Sr/^(86)Sr)i=0.704485和正的Nd初始值,ε_(Nd)(t)=9.45~10.295。认为卡拉麦里蛇绿混杂岩带北侧英云闪长岩不属于蛇绿岩套组成部分,是由亏损地幔演化的新生低钾拉斑玄武岩部分熔融形成。

哈萨克斯坦乌拉尔肯皮赛铬铁矿资源基地地质背景、成矿特征及矿床成因

铬铁矿是中国极为短缺的战略性矿产,哈萨克斯坦乌拉尔肯皮赛铬铁矿资源基地是全球最大的豆荚状铬铁矿产地。笔者系统总结了肯皮赛铬铁矿资源基地的地质背景、蛇绿岩特征及成矿类型,并对基地的开发现状进行了梳理。肯皮赛铬铁矿资源基地位于乌拉尔造山带南部,伴随寒武纪—三叠纪古乌拉尔洋形成演化,大量蛇绿岩地体残存于乌拉尔造山带内,形成一系列豆荚状铬铁矿床。肯皮赛地块出露完整的蛇绿岩层序,地幔橄榄岩以方辉橄榄岩和纯橄岩为主,发育高铝型和高铬型两类富铬铁矿。其中,高铝型铬铁矿形成于早泥盆世(或更早)MOR环境下;而高铬型铬铁矿形成于晚泥盆世洋内俯冲环境下(SSZ),是俯冲带流体与残留地幔橄榄岩交代反应后形成的熔体与围岩(地幔橄榄岩)发生熔岩反应的产物。肯皮赛铬铁矿资源基地铬铁矿探明储量约3亿t,年产铬铁矿和铬铁合金约700万t和169万t,主要用于出口。中国应不断加强与哈萨克斯坦的铬铁矿资源潜力调查、勘查开发及产能合作。

甘孜-理塘蛇绿混杂岩带晚三叠世洋岛型岩石组合识别及其对甘孜-理塘洋盆构造演化的制约

甘孜-理塘蛇绿混杂岩带位于特提斯构造域东段,为西南“三江”多岛弧盆系的重要组成部分,具有完整的沟-弧-盆体系。以理塘地区拉扎嘎山一带下坝岩组洋岛型“玄武岩+碳酸盐岩”岩石组合为研究对象,开展岩石学、地球化学和锆石U-Pb测年分析,为甘孜-理塘洋晚三叠世洋陆格局的恢复重建提供新的证据。岩石地球化学分析结果表明,样品SiO_(2)含量为42.16%~48.32%,TiO_(2)为2.81%~3.75%,稀土元素总量为164.51×10^(-6)~414.40×10^(-6),轻稀土元素较重稀土元素富集,(La/Yb)_(N)值为9.35~34.31,明显富集Rb、Ba、Th、U、K等大离子亲石元素,以及Nb、Ta、Zr、Ti等高场强元素,玄武岩的稀土元素配分曲线和微量元素蛛网图与典型洋岛型玄武岩(OIB)相似。锆石U-Pb测年表明,洋岛型玄武岩形成于211 Ma。这些资料进一步表明,晚三叠世甘孜-理塘洋盆存在洋岛环境,同时,也为甘孜-理塘洋盆晚三叠世处于俯冲消减阶段的认识提供了新的证据。

Geochemistry of ophiolite cherts from the Qinling orogenic belt and implications for their tectonic settings

Paleozoic cherts from the Mianl and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14-683.52 in the Mianle cherts, 12.29-58.62 in the Erlangping cherts), Fe2O3 (0.01-0.35 and 0.02-1.24) and high Al2O3/(Al2O3+Fe2O3) ratios (0.82-0.99 and 0.83-0.99). The negative correlation between Si2O and Al2O3 in the cherts reflects the important role of terrigenous components. The Erlangping cherts have Lan/Cen=0.9-1.15 and Ce/Ce*=0.95-1.15 with low contents of V, Ni and Cu, consistent with those of cherts forming on the continental margin. In contrast, the Ce/Ce* ratios of the Mianle cherts range from 0.71 to 1.18 and Lan/Cen from 0.88 to 1.43 with slightly high V, Ni and Cu, which are similar to cherts found in the mid-ocean ridges and pelagic basins. Combined with the features of basic lavas associated with the cherts, it is suggested that during the Paleozoic, when the back-arc basin represented by the Erlangping ophiolite commenced shrinking in size in the mid-Ordovician, the southern Qinling was still in an extensional regime and finally grew into a new limited oceanic basin in the early Carboniferous.