Petrology and geochemistry of Abyssal Peridotites from the Manipur Ophiolite Complex,Indo-Myanmar Orogenic Belt,Northeast India: Implication for melt generation in mid-oceanic ridge environment

The Manipur Ophiolite Complex (MOC) located in the Indo-Myanmar Orogenic Belt (IMOB) of Northeast India forms a section of the Tethyan Ophiolite Belt of the Alpine–Himalayan orogenic system. Whole rock compositions and mineral chemistry of mantle peridotites from the MOC show an affinity to the abyssal peridotites, characterized by high contents of Al₂O₃ (1.28–3.30 anhydrous wt.%); low Cr# of Cr-spinel (0.11–0.27); low Mg# of olivine (∼Fo₉₀) and high Al₂O₃ in pyroxenes (3.71–6.35 wt.%). They have very low REE concentrations (∑REE = 0.48–2.14 ppb). Lherzolites display LREE-depleted patterns (La_N/Sm_N = 0.14–0.45) with a flat to slightly fractionated HREE segments (Sm_N/Yb_N = 0.30–0.65) whereas Cpx-harburgites have flat to upward-inflected LREE patterns (La_N/Sm_N = 0.13–1.23) with more fractionated HREE patterns (Sm_N/Yb_N = 0.13–0.65) than the lherzolite samples. Their platinum group elements (PGE) contents (<50 ppb) and distinct mantle-normalised PGE patterns with the Pd/Ir values (1.8–11.9) and Pt/Pt^* values (0.2–1.1) show an affinity to the characteristic of the residual mantle material. Evaluation of mineralogical and petrological characteristics of these peridotites suggests that they represent the residues remaining after low degree of partial melting (∼2–12%) in the spinel stability field of a mid-oceanic ridge environment. The well-preserved mid-oceanic ridge characteristics of these peridotites further suggest that the mantle section was subsequently trapped in the forearc region of the subduction zone without undergoing significant modification in their chemistry by later subduction-related tectonic and petrological processes before its emplacement to the present crustal level.     

Petrological and geochemical study of serpentinised peridotites from the southern part of Manipur Ophiolitic Complex, Northeast India

The southern Manipur Ophiolitic Complex in the Indo-Myanmar Orogenic Belt, Northeast India comprises a set of serpentinised peridotites, pelagic sediments, podiform chromitites with minor mafic and felsic intrusives. Peridotites in the study area suffered various degrees of serpentinisation and also have been highly deformed and metamorphosed. Striations and slickenside surfaces are common features seen in these rocks indicating the effect of faulting or shearing due to tectonic movement. On the basis of mineralogy and modal composition these peridotites have been identified as serpentinised harzburgite, lherzolite, serpentinite with minor wehrlite. The serpentine minerals are mainly lizardite and chrysotile with sub-ordinate antigorite, whereas, pyroxenes are enstatite and diopside. The primary composition of Cr-spinels in these serpentinised peridotites are characterized by low SiO₂ (<0.11 wt.%), Fe₂O₃ (0.02 to 2.32 wt.%), TiO₂ (<0.15 wt.%), Cr# (0.13 to 0.23) and high Al# (0.74 to 0.86), Mg# (0.68 to 0.73). Olivine is relatively uniform in composition, ranges from Fo_89.64 to Fo_90.1. The mineralogical compositions and primary Cr-spinel chemistry of these peridotites are similar to Abyssal peridotites that formed in a mid-ocean ridge setting after low degree of partial melting. Thus, it supports that the mantle peridotites of the Manipur Ophiolitic Complex initially formed in a spreading regime.     

Petrogenesis of chromites from the Manipur ophiolite belt,NE India: evidence for a supra-subduction zone setting prior to Indo-Myanmar collision

The Manipur ophiolite belt within the Western Ophiolite Belt of the Indo-Myanmar Ranges (IMR), consists of tectonised to massive serpentinised peridotite, dunite pods, chromitite pods/lenses, cumulates, dykes, volcanic rocks and pelagic sediments. Chromitite pods and lenses hosted in peridotitic mantle rocks show magmatic textures, post magmatic brecciation and ferritchromitisation. Electron microprobe analyses show two types of massive chromitite, with one group having high-Cr (Cr# 75–76), medium-Al (Al₂O₃ 12.2–12.4 wt%) chromites (Sirohi-type) and the other group (Gamnom-type) having a wide range of compositions with generally lower Cr and higher Al (Cr# 65–71, Al₂O₃ 15.7–19 wt%). Accessory chromites in peridotitic mantle rocks have consistently low Cr (Cr# 38–39) and high Al (Al₂O₃ 34–35 wt%), whereas chromites in dunite pods have intermediate compositions (Cr# ~60; Al₂O₃ 20.7–21.2 wt%). The chromite chemistry suggests moderate (20 %) partial melting of the tectonised mantle harzburgite. The estimated Al₂O_3melt, (FeO/MgO)_melt and TiO_2melt for the Sirohi-type chromites indicate boninitic parentage, whereas chromite compositions from the Gamnom area suggest mixed boninitic—island arc tholeiitic magmas. The compositions of magmatic chromites suggest that the Manipur ophiolite was formed in a supra-subduction zone (SSZ) setting.     

Petrology, Geochemistry and Tectonic Significance of the Metamorphic Peridotites from Longmuco–Shuanghu Ophiolitic Melange belt, Tibet

Taoxinghu metamorphic peridotite is a firstly reported mantle sequence of ophiolite since Longmuco–Shuanghu–Lancangjiang suture zone (LSLSZ) was proposed, and it is also an important discovered for ophiolite studying in central Qiangtang. Based on detailed analyses of whole–rock geochemistry of Taoxinghu metamorphic peridotites and contrast to metamorphic peridotites in typical ophiolites worldwide, the paper investigates their petrogenesis and geological implication. The petrologic results show that the protolith of Taoxinghu metamorphic perdotites have the mineral assemblage and texture characteristic of mantle peridotite. Most metamorphic peridotites hav near global abyssal peridotites major elements contents, while the few is similar to SSZ–type peridotites. They exhibit typically U–shaped REE patterns, characterized by slight enrichment of LREE and HREE relative to MREE and a low fractionated LREE to HREE segment. Trace elements contents are low and all samples are strong enrichment in Cs, U, Pb, weak enrichment in Ba and depletion in Th, but negative Nb anomalies are only observed in few samples. That suggests Taoxinghu metamorphic peridotites have depletion mantle and suprasubduction affinities. A two–stage evolution history is considered: Taoxinghu metamorphic peridotites originated as the residue from melting at a ridge with 7%–20% degree of fraction melting and were subsequently modified by interaction with mafic melt and aqueous fluid within mantle wedge on subducted zone. Combined with previous studies, we preliminarily propose Taoxinghu metamorphic peridotites may be the Products of initial rifting of palo–Tethys, forming at middle Ordivician–upper Cambrian, and they may be the direct evidences for spreading of palo–Tethys.     

中国东北地区蛇绿岩

我国东北地区位于中亚造山带的东段,经历了复杂的增生造山过程,其所属微陆块的基底属性及拼贴位置、洋-陆转换一直是地学界研究的热点。根据近年来的研究进展,我们将东北地区微陆块划分为额尔古纳地块、兴安增生地体、松嫩-锡林浩特地块和佳木斯地块。同时综述了东北地区蛇绿岩/蛇绿混杂岩带的时空分布、年代学及地球化学的新资料,讨论了其构造背景及俯冲-增生过程。东北地区增生造山不仅涉及古亚洲洋和古太平洋,还可能与泛大洋有关,包括早奥陶世-晚三叠世古亚洲洋主洋盆及古亚洲洋分支——新元古代-晚寒武世新林-喜桂图洋、早寒武世-晚石炭世嫩江洋、新元古代-晚志留世黑龙江洋和晚二叠世-中侏罗世牡丹江洋的演化。早石炭世末-晚石炭世初,东北地区古亚洲洋分支洋盆全部闭合,所有微陆块完成聚合形成统一的东北陆块群。晚二叠世-早三叠世时期,古亚洲洋主洋盆沿索伦-西拉木伦-长春-延吉缝合带自西向东从早到晚以剪刀式最终闭合,完成东北陆块群与华北板块的拼接。晚三叠世-早侏罗世时期古太平洋板块俯冲启动,东北地区进入古太平洋俯冲增生构造体系。     

Geochemistry of peridotites and mafic igneous rocks from the Central Dinaric Ophiolite Belt,Yugoslavia

In the Central Dinaric Ophiolite Belt (CDOB) peridotites and associated metamorphic rocks of various grades tectonically overlie an olistostrome melange of middle to late Jurassic age. Peridotites and underlying slices of mafic granulites (partially transformed to gamet amphibolites) are intruded by doleritic dikes which do not occur in the melange. The melange contains blocks of subgreywackes and cherts as well as those of pillow lavas and massive diabase (spilites). CDOB peridotites are in the spinel peridotite facies, but locally spinel-plagioclase peridotites occur as well. All peridotites have lherzolitic compositions showing several significant element correlations: Al₂O₃, CaO, TiO₂, Na₂O and Cu are negatively correlated and Ni is positively correlated with MgO. Recent estimates of primitive mantle compositions lie near the low-MgO end point of each correlation trend. Al/Ti and Ca/Al ratios of CDOB lherzolites are for the most part higher than the range observed in chondrites. However, when a few samples with extreme compositions are excluded, Al/Ti and Ca/Al are positively correlated with MgO, and the samples at the low-MgO end have near-chondritic Ca/Al but slightly higher than chondritic Al/Ti ratios. Chondrite-normalized REE patterns of CDOB lherzolites show extreme depletions in LREE providing strong evidence for the absence of any metasomatic renrichment. The lack of correlation between highly incompatible elements (LREE) and moderately incompatible elements (HREE, Ti, Na, Al, Ca) together with the extremely low La/Sm ratios suggest that fractional or very small increment melt removal played a role in the genesis of these lherzolites. Four out of five lherzolites yield and apparent Sm-Nd isochron age of 136±15 Ma with an Nb of 6.0±1.1 (bulk rocks and clinopyroxene separates). One sample has an exceptionally high Nd of about 23. The mafic igneous rocks scatter around the lower end of the 136 Ma reference isochron allowing, but not proving, a genetic relationship with a mantle having a Nd isotopic composition which is similar to that of CDOB lherzolites. LIL element abundances of spilites and doleritic dike rocks suggest some hydrothermal alteration. In primitive mantle-normalized concentration diagrams none of these mafic igneous rocks shows a significant negative Nb-Ta anomaly. Chondrite-normalized REE patterns of both rock types are essentially flat. Whereas the inferred primary compositions of the spilites compare well with those of E-type MORBs, the doleritic dike rocks show elemental ratios similar to those normally found in back-arc basin tholeiites.     

Geochemistry of Gneisses from Dabie Complex and Tongbai Complex in Qinling-Tongbai-Dabie Orogenic Belt: Implications for Location of Yangtze-Sino-Korean Suture

ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅＱｉｎｌｉｎｇ Ｔｏｎｇｂａｉ ＤａｂｉｅｏｒｏｇｅｎｉｃｂｅｌｔｉｓａｃｏｌｌｉｓｉｏｎａｌｏｒｏｇｅｎｉｃｂｅｌｔｂｅｔｗｅｅｎｔｈｅＳｉｎｏ ＫｏｒｅａｎａｎｄＹａｎｇｔｚｅｃｒａｔｏｎｓ.Ｕｌｔｒａｈｉｇｈｐｒｅｓｓｕｒｅ (ＵＨＰ)ａｎｄｈｉｇｈｐｒｅｓｓｕｒｅ (ＨＰ)ｍｅｔａｍｏｒ ｐｈｉｃｒｏｃｋｓｓｕｃｈａｓｃｏｅｓｉｔｅ ｂｅａｒｉｎｇｅｃｌｏｇｉｔｅｓａｒｅｗｉｄｅｌｙｄｉｓｔｒｉｂｕｔ ｅｄｉｎｔｈｅｅａｓｔｅｒｎｅｎｄｏｆｔｈｅｏｒｏｇｅｎｉｃｂｅｌｔ …     

Geology, Geochemistry and Chromite Mineralization Potential of the Amnay Ophiolitic Complex, Mindoro, Philippines

The Amnay Ophiolitic Complex in Mindoro, the Philippines, is considered an emplaced Cenozoic South China Sea oceanic lithosphere as a result of the collision between the Palawan microcontinental block and the Philippine mobile belt. Middle Oligocene sedimentary rocks intercalated with dominantly MORB-like pillow lavas and volcanic flows suggest the generation of this ophiolite complex in an intermediate spreading ridge within a back-arc basin setting. The volcanic rock suite geochemistry also manifests a slab component suggesting that it is a supra-subduction zone ophiolite. Petrography of the gabbros shows a plagioclase-clinopyroxene crystallization order consistent with a back-arc basin setting. Spinel and pyroxene geochemistry shows that the lherzolites and aluminous-spinel harzburgites are products of low degrees of partial melting. The chromitites hosted by the harzburgites could have not been associated with the MORB-like volcanic suites, gabbros, lherzolites and aluminous-spinel harzburgites. The chromitites are products of mantle sources that have undergone higher degrees of partial melting that would have involved the presence of water. The study of this ophiolitic complex gives us a glimpse of the characteristics of the South China Sea.     

大别造山带银水寺铅锌矿区正长花岗斑岩脉锆石U-Pb年代学、地球化学特征和地质意义

银水寺铅锌矿床位于大别造山带北缘,是大别山地区最大的矽卡岩型矿床.目前对矿区岩浆作用时限、岩石地球化学、岩石形成环境等方面的研究较为薄弱,在一定程度上制约了对该区铅锌成矿规律的认识.在详细野外地质调查的基础上,对矿区出露的正长花岗斑岩脉的岩石学、成岩年代学、岩石化学和同位素地球化学进行了系统研究.结果表明,银水寺矿区正长花岗斑岩脉锆石LA-ICP-MS U-Pb年龄为125.4±0.4 Ma,表明其形成时代为早白垩世.全岩地球化学分析显示,正长花岗斑岩脉表现为高硅（SiO₂=71.43%~72.71%）、高钾（K₂O=4.62%~4.88%）、富碱（7.47%~7.81%）、弱过铝质（A/CNK=1.03~1.06）、富Fe低Ca,贫Sr、Ba、Ti、P,轻稀土富集、重稀土亏损,具Eu负异常,Ce异常不明显,稀土配分图呈典型的右倾型,显示出A型花岗岩的特征.岩石具有富集的Sr-Nd-Hf同位素组成:全岩（87Sr/86Sr）_i值为0.710 21~0.710 53,ε_Nd（t）值在-20.0~-19.2之间;锆石ε_Hf（t）值为-26.7~-23.8,位于地幔演化线之下;T_DM2值变化于2 663~2 845 Ma之间,指示其起源于大别杂岩与扬子板块北缘的古老下地壳物质的混合.结合区域地质背景,认为矿区正长花岗斑岩脉形成于扬子克拉通与华北克拉通碰撞造山后的大规模伸展环境,软流圈地幔上涌作用于减薄的岩石圈,与西向俯冲的古太平洋板片俯冲角度改变造成的强烈弧后拉张有关.      

秦岭造山带秦岭杂岩研究中有关问题讨论

分布于北秦岭造山带南部,夹持于商丹断裂带与朱夏断裂带之间的秦岭杂岩是秦岭造山带中最古老的前寒武纪基底块体之一.它是秦岭造山带中前寒武纪地壳形成与演化的地质记录,经历了晚元古代以来多旋回俯冲碰撞造山作用过程中多期构造热事件的改造,包含了大量反映秦岭造山带形成与演化的动力学过程地质信息.虽然前人对秦岭杂岩已经进行了广泛深入研究,发表了大量的研究论著,提出了许多新认识和新见解,如从原秦岭群中解体出松树沟蛇绿岩片、峡河山群、古深成花岗片麻岩体等;高压变质岩(榴辉岩,高压麻粒岩)的发现;多期变形与变质特征的认识等.但由于秦岭杂岩问题的复杂性,某些认识或问题仍有待进一步研究探讨.其中(1)松树沟蛇绿岩片是否真正蛇绿岩?松树沟超镁铁质岩体与其围岩变质镁铁质岩是否同时代?它们的构造变形与变质历史是否相同?(2)峡河山群组成、分布、时代、构造意义及其与秦岭群片麻岩的关系;(3)秦岭岩群的组成、主变质期和变形期的时代及性质;(4)晋宁期花岗岩带形成的构造背景及动力学意义;(5)已有同位素测年数据确切的地质意义;(6)高压变质岩的分布、产出状态、与围岩的关系、时代、成因及构造意义是秦岭杂岩研究中的基本问题.文中对上述有关问题进行了简要讨论,提出了今后秦岭杂岩研究的重点应放在构造事件、变质热事件、岩浆事件及其相互关系研究和这些地质事件的精确的同位素测年工作上.     

Boninitic geochemical characteristics of high-Mg mafic dykes from the Singhbhum Granitoid Complex,Eastern India

Acta Geochimica - The high-Mg mafic dykes from the Singhbhum Granitoid Complex in East India have geochemical characteristics [e.g., enrichment of the large ion lithophile elements and light rare...     

Petrology of Cr-rich Ophiolitic Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

<正>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 profile from the bottom to the top as follows.The tectonite     

南秦岭晚三叠世胭脂坝岩体的地球化学特征及地质意义

本文对出露于佛坪穹窿东部宁陕地区的胭脂坝岩体进行了详细的岩石学、锆石U-Pb年代学和地球化学特征的研究,讨论了胭脂坝岩体的岩石成因、成岩物质来源及其地质意义。岩体主要由黑云母花岗岩组成。锆石LA-ICP-MS U-Pb定年结果表明胭脂坝黑云母花岗岩的成岩年龄为200Ma±4Ma。该花岗岩的地球化学特征为富硅(70.09%～73.35%)、富碱(Na2O+K2O=7.49%～8.59%),A/CNK=1.01～1.08,里特曼指数(σ=2.07～2.62),钾大于钠(K2O/Na2O=1.12～1.39)和高CaO/Na2O值(0.3),微量元素主要富集Rb、Th、U、K,亏损Nb、Ta、Sr、Ba、P和Ti,稀土总量129.98×10-6～189.97×10-6,轻稀土富集[LREE/HREE=8.00～10.73,(La/Yb)N=8.62～15.68],Eu亏损明显(δEu=0.41～0.50)。这些特征表明,胭脂坝岩体属于高钾钙碱性系列,为准铝—弱过铝质壳源S型花岗岩。结合对区域地质背景的全面分析表明,可能是印支运动晚期的造山作用造成本区地壳加厚,之后在伸展—减薄的背景下,中部地壳深度的变质砂屑质岩石,通过黑云母脱水发生部分熔融而形成的产物。     

Geochemistry of Archean Metavolcanic Rocks from Kadiri Schist Belt, Andhra Pradesh, India

The basic volcanic group exposed in the Kadiri schist belt includes high Mg-basalt, basalt, basaltic andestite and dacite. The basalts are tholeiitic in composition and high Mg-basalts, basaltic andesites and dacites show calc-alkaline affinity. Major and trace element characteristics suggest that the volcanic suite has been derived from an initial tholeiitic magma which has given rise to an early basaltic type and a later calc-alkaline type of rocks. An island arc and active continental margin tectonic setting was inferred for these rocks.     

Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt in NE China

The eastern Central Asian Orogenic Belt (CAOB) in NE China is a key area for investigating continental growth. However, the complexity of its Paleozoic geological history has meant that the tectonic development of this belt is not fully understood. NE China is composed of the Erguna and Jiamusi blocks in the northern and eastern parts and the Xing’an and Songliao-Xilinhot accretionary terranes in the central and southern parts. The Erguna and Jiamusi blocks have Precambrian basements with Siberia and Gondwana affinities, respectively. In contrast, the Xing ’an and Songliao-Xilinhot accretionary terranes were formed via subduction and collision processes. These blocks and terranes were separated by the Xinlin-Xiguitu, Heilongjiang, Nenjiang, and Solonker oceans from north to south, and these oceans closed during the Cambrian (ca. 500 Ma), Late Silurian (ca. 420 Ma), early Late Carboniferous (ca. 320 Ma), and Late Permian to Middle Triassic (260 –240 Ma), respectively, forming the Xinlin-Xiguitu, Mudanjiang-Yilan, Hegenshan-Heihe, Solonker-Linxi, and Changchun-Yanji suture zones. Two oceanic tectonic cycles took place in the eastern Paleo-Asian Ocean (PAO), namely, the Early Paleozoic cycle involving the Xinlin-Xiguitu and Heilongjiang oceans and the late Paleozoic cycle involving the Nenjiang-Solonker oceans. The Paleozoic tectonic pattern of the eastern CAOB generally shows structural features that trend east-west. The timing of accretion and collision events of the eastern CAOB during the Paleozoic youngs progressively from north to south. The branch ocean basins of the eastern PAO closed from west to east in a scissor-like manner. A bi-directional subduction regime dominated during the narrowing and closure process of the eastern PAO, which led to “soft collision” of tectonic units on each side, forming huge accretionary orogenic belts in central Asia.©2022 China Geology Editorial Office.     

High-pressure mafic granulites of the South Muya Block (Central Asian Orogenic Belt)

Mineralogical, petrographic, and geochemical studies of mafic granulites of the South Muya Block (Central Asian Orogenic Belt) have been carried out. The granulite protoliths were olivine- and plagioclase- rich cumulates of ultramafic–mafic magmas with geochemical affinities of suprasubduction rocks. The isotope–geochemical characteristics of the granulites indicate the enriched nature of their source, associated with recycling into the mantle of either ancient crust or oceanic sediments, or intracrustal contamination of melts at the basement of the ensialic arc. Formation of garnet-bearing parageneses has occurred during high-pressure granulite metamorphism associated with accretion in the eastern part of the Baikal–Muya composite terrane.     

秦岭造山带中部两河口岩体花岗岩锆石U-Pb年代学、地球化学及地质意义

北秦岭两河口岩体位于太白地区,侵位于秦岭群杂岩中,主要岩性为眼球状花岗岩、片麻状花岗岩和二长花岗岩。 本文研究的眼球状花岗岩和片麻状花岗岩的结晶年龄分别为928±19 Ma 和940±12 Ma,岩石中还保留古元古代至中元古代的 继承锆石。眼球状花岗岩含有富铝矿物石榴子石和白云母。岩石的A/CNK 多大于1.1,具有高Si、富铝的特征,属于高钾钙 碱性系列。岩石轻、重稀土分馏明显,具有中等负Eu 异常。岩石富集大离子亲石元素（Rb, Ba, K 等）、亏损高场强元素 （Nb, Ta, Ti 等）,具有明显的Ba, P, Sr 负异常。矿物学和地球化学特征显示眼球状花岗岩和片麻状花岗岩为S 型花岗岩。两河 口岩体初始Sr 同位素组成变化大,87Sr/86Sr(t)=0.701067~0.739451,具有较低的εNd(t)=-5.7~-3.3, 两阶段Nd 模式年龄为TDM2= 1.9~2.1 Ga。样品具有高的放射成因Pb 同位素组成,指示两河口岩体是壳源成因岩石,其源岩可能为秦岭群斜长角闪岩和 片麻岩。结合区域地质背景,认为两河口岩体源于新元古代陆壳碰撞晚期的构造转换阶段古老中下地壳的熔融作用,是对 Rodinia 超大陆汇聚事件的响应。     

Geochemistry of metamorphic processes in mafic rocks of the Krasnaya Guba area,Belomorian Mobile Belt

New data are obtained on the major-component and minor- and trace-element composition of metamorphosed gabbronorites and Fe-Ti gabbro in the Krasnaya Guba area, Belomorian Mobile Belt. These two magmatic complexes, which are spatially combined but are of different age and were produced in different geodynamic environments, were simultaneously metamorphosed in the Paleoproterozoic to parameters transitional between the amphibolite and eclogite facies. Eclogite metamorphism was associated with perceivable changes in the concentrations of alkalis and alkali earth elements, and the metamorphic transformation of the plagioclase eclogite (which can be metamorphosed gabbronorite and/or gabbro) into amphibolite and biotite amphibolite was associated with changes in the concentrations of practically all trace elements. This implies that the metamorphic processes were not isochemical and were accompanied by significant removal and introduction of incompatible elements. However, some trace-element ratios (such as Ti/Y, Ti/Zr, Zr/Y, La/Sm, and Nb/Th) remained unmodified in the course of metamorphism and remained the same as in the pristine gabbronorite or gabbro and likely can be employed as indicators of the composition of the probable protolith and the conditions under which it was produced.     

阿尔泰晚古生代早期长英质火山岩的地球化学特征及构造背景

新疆北部哈巴河一带出露的一套晚古生代早期长英质火山岩主要由英安质—流纹质火山岩组成。其Rb-Sr等时线年龄为405±57Ma,为早泥盆世。地球化学特征显示它们具有Ti含量低,Mg#含量中等—富集(42.2~88.7),Th/Ta为8.6~16.4,低—中等Ti/Zr值(介于4.9~53.46);稀土配分型式为LREE富集,无Eu或中等的负Eu异常;且在蜘蛛网图上显示LILE富集,Nb和Ta亏损的特征。Y-Nb和Rb-(Y+Nb)构造判别图解显示这些长英质火山岩形成于岛弧环境。结合前人的研究成果,认为新疆北部哈巴河晚古生代早期长英质火山岩是在古亚洲洋北侧岛弧构造环境下形成的,是作为古亚洲洋从SW至NE方向朝西伯利亚古板块俯冲的产物。