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.

Diamonds and Other Exotic Minerals Recovered from Peridotites of the Dangqiong Ophiolite, Western Yarlung-Zangbo Suture Zone, Tibet

Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

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.

Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China

The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(>20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.

Magnetic Signature of Serpentinization at Zedang in the South Tibetan Ophiolite Belt

Magnetic signature of serpentinized mantle peridotite has crucial importance in understanding the serpentinization process and interpreting the origin of strong magnetization anomalies at ultramafic-hosted hydrothermal settings. However, different groups of serpentinized peridotites from both ocean drillings and ophiolite complexes have shown considerable variations in the abundance of magnetite(Oufi et al., 2002;Bonnemains et al., 2016;Li et al., 2017). We examined the magnetic properties, petrography and mineral chemistry of variably serpentinized peridotites from Zedang ophiolite in the eastern Yarlung-Zangbo suture in south Tibet to evaluate the conditions of serpentinization and magnetite formation as well as magnetic sources in suture zones. The studied samples were 0–90% serpentinized with densities from 3.316 to 2.593 g cm–3 and show typical mesh textures of olivine replaced by serpentine on thin sections of core specimen. Serpentines were divided into type-1 Fe-poor serpentine mesh(1.84–2.88 wt% FeO) associated with magnetite in the early stage and type-2 Fe-rich serpentine cores(3.92–5.12 wt% FeO) with no formation of magnetite in the late serpentinization. Brucite vein appeared in central serpentine veins and show Mg/(Mg+Fe) values of 0.74–0.87 at ~50–70% of serpentinization. Pure magnetite was identified as the main magnetic carrier by thermomagnetic analyses, but minor Cr-magnetite(~0.8 mole fractions of Fe3O4) was also detected due to oxidation of early spinel. All the peridotite samples show a rapid increase of magnetic susceptibility from ~0.001 to ~0.03 SI before 40–50% of serpentinization and a following flat trend in values 0.02–0.03 SI at > 50% of serpentinization. This density-susceptibility relationship differs from the rapid production of magnetite above 60-70% of serpentinization for many abyssal peridotites(Oufi et al., 2002;Bach et al., 2006) and suggests that magnetite formation was coupled with hydration of olivine in the early serpentinization but the two decoupled at ~ 40–50% of serpentinization. This transition is consistent with the petrographic observation that magnetite-free serpentinization was developed in higher degrees(> 50%) of serpentinization. Prior studies suggested that serpentinization of < 200℃ would generate Fe-rich brucite, serpentine and little magnetite, whereas magnetite-rich serpentinization was associated with Fe-poor brucite and occurred at higher temperatures of 200–300℃(Klein et al., 2014). The petromagnetic features of serpentinized peridotites from the Zedang ophiolite indicate that the serpentinization process took place initially above 250℃(estimate from brucite composition) and continued to lower temperatures of < 200℃, probably during the mantle lithosphere cooling down in forearc settings(Xiong et al., 2017). These serpentinized peridotites have higher magnetization intensities(average 2.26 Am-1) than mafic dolerite dykes and basaltic volcanic rocks(mostly < 1 Am-1) and should be significant sources of aeromagnetic highs in the Yarlung-Zangbo suture.

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.

Geochronology and Geochemistry of the Magmatic Rocks from Zedong Ophiolite, Eastern Yarlung-Zangbo Suture Zone, Tibet

The Yarlung Zangbo suture zone extends more than2000 km along southern Tibet and marks the boundary between the Indian subcontinent and Eurasia.The Zedong terrane has been not suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean,as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic.In this study,we present detailed geochemical and geochronological data of various types of magmatic rocks widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane.We found that the Zedong volcanic rocks belong to high K2O calc-alkaline series,whereas the diabase and gabbro plotted in the low-K calcalkline.The basalt rocks are highly enriched in LREE and LILE,but strongly depleted in HFSE,indicating they were derived from a metasomatized mantle.Both gabbros and diabase have similar N-MORB geochemistry indicates that the cumulates were produced from MOR setting.Zircons from four samples,including the basalt rocks(158-161Ma)are older than the gabbro(131 Ma),certificate the gabbro are as the vein intrude into the basalt rocks.This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedonghave similar positiveεHf(t)values(+2.0 to+15.6)and(+8.6 to+18.4),indicating they were stemmed from similarly depleted mantle sources,same with the gabbro and granitic rocks from the Gangdese arc.Therefore,we proposed that the basalt rocks in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts.A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane,which gave rise to the cumulate and granitic rocks.This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic,although a possible intra-oceanic arc setting cannot be excluded.

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.

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.

蛇绿岩内部结构

蛇绿岩记录了地球特别是大洋演化的历史,是地球科学关注的热点。同时,蛇绿岩也是多种关键金属矿床的赋矿岩体,在铬铁矿、铂族矿产、贵金属找矿中地位突出。围绕蛇绿岩构造环境、地球化学、地质年代学、岩石学研究已取得重大进展,在地质固碳和地质氢新能源研究方面也有突破。但蛇绿岩的内部结构,特别是小尺度的内部结构,至今仍是蛇绿岩研究的薄弱环节。本文介绍了蛇绿岩内部结构研究方法与进展,依托矿床电磁学观测,结合地质、钻探结果,讨论了蛇绿岩的内部结构特征,特别是蛇绿岩内部超基性岩体的内部结构特征。矿床电磁学分析结果表明:从岩石物理分析到电磁法探测都发现西藏罗布莎、东巧蛇绿岩内部发育显著的高低阻电阻率异常分带。CSAMT探测结果表明新疆萨尔托海蛇绿岩内部也有类似的电性结构分带特征。依据罗布莎、东巧、萨尔托海、Semail蛇绿岩的地球物理探测和钻探结果以及乌拉尔一些矿区的剥露与深钻结果,我们提出了蛇绿岩电阻率-蚀变带结构模型。基于这一模型,蛇绿岩中的超基性岩主要发育高阻新鲜-弱蚀变方辉橄榄岩和低阻蚀变-强蚀变纯橄岩-蛇纹岩两个岩相结构带,两个岩相之间的过渡带是铬铁矿富集成矿的有利空间。电阻率-蚀变带模型可用于铬铁矿成矿预测以及蛇绿岩内部地质作用研究。

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.

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。

北山造山带增生造山过程

北山造山带位于中亚造山带南缘中段,处于东西向构造衔接的关键大地构造位置。近年来,北山造山带构造演化成为研究热点,在基底时代与构造属性、岩浆岩成因、蛇绿岩时代与构造背景、沉积物源与大地构造背景、构造变形样式与时限等方面取得了重要进展。本文以这些新进展为主线,总结北山造山带各构造单元的基本特征,梳理岩浆作用时空格架和蛇绿混杂岩时代,在此基础上探讨北山增生造山过程。北山造山带前寒武纪岩浆记录为零星分布的中元古代(约1.4 Ga)及新元古代(0.9~0.8 Ga)花岗片麻岩,不存在大规模的前寒武纪结晶基底;早古生代—早中生代发育连续的岩浆作用。蛇绿岩时代具有从中部寒武纪—奥陶纪向南北两侧石炭纪—二叠纪变年轻的特点,最终的缝合带位于柳园—后红泉一带,基性岩时代和最年轻的沉积岩基质时代共同限定了最终的增生拼贴时代为中—晚三叠世。北山造山带从中元古代开始位于超大陆外围增生边缘,此后经历了新元古代和古生代的长期增生,在三叠纪完成了最终的增生造山过程并进入陆内演化阶段。增生造山过程中幔源岩浆形成岛弧新生地壳、增生楔和岛弧侧向拼贴增生,对大陆生长具有重要意义。

阿拉善南部地区石炭纪—二叠纪期间的构造体制转换

阿拉善地区位于中亚造山带南缘中段,该地区出露大量古生代的蛇绿混杂岩、增生杂岩和岩浆岩,记录了古亚洲洋闭合过程对大陆地壳的改造。本文系统总结了阿拉善南部地区蛇绿混杂岩及石炭纪—早三叠世岩浆岩的研究进展,收集整理了该地区岩浆岩的年代学、全岩地球化学和同位素数据。结果发现,阿拉善南部地区分布有早二叠世俯冲相关蛇绿混杂岩,中、晚二叠世高镁安山岩和埃达克岩,表明该地区的古大洋在晚二叠世尚未闭合。阿拉善南部地区岩浆岩的时空分布反映出该地区岩浆岩带向北西方向迁移的特征,揭示了阿拉善地块晚古生代向北增生的过程。阿拉善南部地区从早石炭世的前进型俯冲带转换为晚石炭世—二叠纪的后撤型俯冲带。

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。