雅鲁藏布江蛇绿岩的形成与日喀则弧前盆地沉积演化

雅鲁藏布江蛇绿岩被时代连续的日喀则群沉积覆盖及其形成时代(120-110Ma)与冈底斯弧开始发育的时代(115-100Ma)十分相近的事实使人们有理由提出:雅鲁藏布江蛇绿岩是否代表着印度板块与拉萨地块间的特提斯-喜玛拉雅洋残迹的疑问。根据近期的研究,笔者认为雅鲁藏布江蛇绿岩不是形成于三叠纪的特提斯-喜玛拉雅洋的残迹,而是特提斯-喜玛拉雅洋向拉萨地块俯冲的初期(阿普第-阿尔必期),由俯冲作用在冈底斯弧前地区引发的海底扩张作用形成的一种俯冲带上叠型蛇绿岩(supra-subduction zone ophiolites).至森诺曼期,弧前海底扩张作用停止,雅鲁藏布江蛇绿岩开始向南仰冲,在其南侧形成增生杂岩楔。仰起的蛇绿岩开始向日喀则弧前盆地提供蛇绿质碎屑,如冲堆组。森诺曼期-土仑期,盆地接受了一套深水复理石沉积,沉积物源部分来自南部边缘脊的蛇绿质碎屑,而大部分则来自北侧的弧火山岩和岩浆岩碎屑。森诺期-路坦丁期,盆地逐渐变浅,接受了浅海-滨海沉积,物源均来自北部的岩浆弧。至始新世末期,发育在盆地南侧的增生杂岩楔与印度板块发生碰撞,日喀则弧前盆地闭合。     

Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)

Cr-rich magnesiochloritoid in the eclogitized ophiolites of the Monviso massif occurs in the least differentiated rocks of the gabbroic sequence (troctolites to melatroctolites). Chloritoid ( X _Mg=0.63–0.85; Cr≤0.55, atoms) co-exists with omphacite, talc and garnet. Minor, syn-eclogitic minerals are chromite, rutile and sometimes magnesite and Cr–Ti oxides.
Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs.
Syn-eclogitic volatile components are indicated by the development of OH-bearing minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), and supported by the presence of coarse-grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization, most likely during ocean-floor metamorphism. It is suggested that syn-eclogitic fluids formed by breakdown reactions of pre-eclogitic volatile-bearing minerals.
Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4  GPa and temperatures of 620±50  °C. The attainment of high-pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58  mol%). P–T  estimates point to a very low thermal gradient (about 9  °C km⁻¹), comparable to that deduced in the adjacent Dora-Maira ultra-high pressure unit.     

Deformation history of the ophiolite sequence from the Balagne Nappe,northern Corsica: insights in the tectonic evolution of Alpine Corsica

In Alpine Corsica, the Jurassic ophiolites represent remnants of oceanic lithosphere belonging to the Ligure‐Piemontese Basin located between the Europe/Corsica and Adria continental margins. In the Balagne area, a Jurassic ophiolitic sequence topped by a Late Jurassic–Late Cretaceous sedimentary cover crops out at the top of the nappe pile. The whole ophiolitic succession is affected by polyphase deformation developed under very low‐grade orogenic metamorphic conditions. The original palaeogeographic location and the emplacement mechanisms for the Balagne ophiolites are still a matter of debate and different interpretations for its history have been proposed. The deformation features of the Balagne ophiolites are outlined in order to provide constraints on their history in the framework of the geodynamic evolution of Alpine Corsica. The deformation history reconstructed for the Balagne Nappe includes five different deformation phases, from D1 to D5. The D1 phase was connected with the latest Cretaceous/Palaeocene accretion into the accretionary wedge related to an east‐dipping subduction zone followed by a Late Eocene D2 phase related to emplacement onto the Europe/Corsica continental margin. The subsequent D3 phase was characterized by sinistral strike‐slip faults and related deformations of Late Eocene–Early Oligocene age. The D4 and D5 phases were developed during the Early Oligocene–Late Miocene extensional processes connected with the collapse of the Alpine belt. Copyright © 2003 John Wiley & Sons, Ltd.     

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO₂ (0.18–0.57 ​wt.%), P₂O₅ (<0.05 ​wt.%), K₂O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu1 ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)_N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)_N ​= ​0.10–0.27 and (La/Yb)_N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.     

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin,Central Iran)

This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#^olivine < ~ 90, Cr#^{clinopyroxene} < ~ 0.2, Cr#^spinel < ~ 0.5, Al₂O₃^{orthopyroxene} > ~ 2.5 wt%, Al₂O₃^{clinopyroxene} > ~ 4.5 wt%, Al₂O₃^spinel > ~ 41.5 wt%, Na₂O^{clinopyroxene} > ~ 0.11 wt%, and TiO₂^{clinopyroxene} > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO₂ and Na₂O of clinopyroxene, and TiO₂ and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.     

新疆东昆仑阿其克库勒湖西南缘蛇绿岩--铬铁矿证据及其构造环境探讨

新疆东昆仑阿其克库勒湖西南缘蛇绿岩沿近东西走向的昆中断裂出露,以构造岩片的形式夹持于泥盆纪和志留纪地层中,由变质橄榄岩、堆晶岩和火山岩三部分组成.对变质橄榄岩中的铬铁矿成分分析证实了它们属蛇绿岩的成员,并且铬铁矿的Cr#=Cr/(Cr Al)为0.59～0.65,指示蛇绿岩经历了从洋中脊到俯冲带环境的演化,形成机制为岩石-熔体反应机制.     

扎河坝蛇绿混杂岩内富铌玄武(安山)岩的地球化学特征及其地质意义

在新疆北部东准噶尔的扎河坝蛇绿混杂岩中发现的富铌玄武（安山）岩,其SiO₂含量介于46.71%~57.65%,TiO₂含量为1.00%~1.76%,与太古代绿岩带内富铌玄武（安山）岩相似,Na₂O含量为3.86%~6.64%,P₂O₅为0.34%~0.82%,明显高于太古代绿岩带富铌玄武（安山）岩。扎河坝富铌玄武岩铌含量介于7.22×10^-6~21.91×10^-6之间,大于7×10^-6,与典型的富铌玄武岩相同。该岩石轻重稀土元素分馏较明显,其分布模式为无明显铕至弱负铕异常的右倾曲线。尽管铌的绝对含量较高,但由于钍和轻稀土元素更加富集,在微量元素蛛网图中扎河坝富铌玄武（安山）岩仍表现为铌的明显亏损,同时高场强元素Zr、Hf及Ti也表现出一定程度的亏损。微量元素地球化学特征显示,扎河坝蛇绿混杂岩内富铌玄武（安山）岩形成于古亚洲洋的洋内弧,它是被埃达克质岩浆交代的地幔楔橄榄岩部分熔融的产物,同时大洋沉积物及俯冲板块释放的流体对成岩作用也有一定的贡献。富铌玄武（安山）岩作为弧前增生楔定位在扎河坝蛇绿混杂岩体内,与早前报道的超高压变质岩共存表明,该蛇绿混杂岩体至少记录了两次性质不同的古亚洲洋洋壳俯冲,这更进一步证实新疆北部晚古生代新增陆壳是古亚洲洋多次俯冲作用的产物。     

Geology, Geochemistry and Minerogenesis of the Shijuligou Zinc–Copper Deposit in Gansu, China

Abstract: The Shijuligou deposit was separated by an arcuate ductile shear zone cross the center of the deposit region, resulting in the difference between the southern and northern ore bodies. The lead (Pb) isotopic data of ores of the Shijuligou copper deposit have averages of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in 17.634, 15.444, and 37.312, respectively. It has been shown that ore-forming metals originated from intrusive and extrusive rocks in the upper part of ophiolites. The sulfur isotopic data of pyrite and chalcopyrite in the northern part change from +7.61‰ to +8.09‰ and +4.95‰ to +8.88‰ in the southern part. Isotopes of δ18O in the Shijuligou copper deposit are between +11.1‰ and +18.6‰, with the calculated δ18OH2O at +0.65‰. It is suggested that the mineralized fluid is a mixture of magma fluid, meteorological water, and seawater through circulating and leaching metals from the volcanic rocks. The zircon uranium-lead (U–Pb) dating of gabbro is 457.9±1.2 Ma, and the lower crossing age of the discordant and concordia curves of pyroxene spilite of zircon is 454±15 Ma. It is indicated that the Shijuligou deposit formed in a new ocean crust (ophiolite) of the back-arc basin in the late Ordovician. Mineralization should occur in the intermittence period after strong volcanic activity, and the age should be the late Ordovician. Moreover, the mineralization of ophiolite-hosted massive sulfide deposits in the ancient orogenic belt of the late Ordovician in the northern Qilian Mountains was controlled by the primary fault/fracture, with the forming of a metallogenic hydrothermal system by a mixture of volcanic magma fluid and seawater, which circularly leached the metallogenic metals from the volcanic rocks, resulting in their accumulation. The ore bodies were transformed with morphology and metallogenic elements. Jasperoid is an important sign for prospecting such deposits. There were many island arcs in the continent of China. This study provides evidence for understanding and exploration of ophiolite-hosted massive sulfide deposits in western China, especially in the area of northern Qilian Mountains.     

Ultramafic Blocks in Sumdo Region, Lhasa Block, Eastern Tibet Plateau: An Ophiolite Unit

Recently, an over 100 kin long MORB-type eclogite belt of Permian was discovered in the Sumdo (松多) region of the Lhasa block, Tibet. A critical question thus is: what is the tectonic setting of the eclogite belt and is it related to an unrecognized suture in the region? Further investigations show that there are some mafic and ultramafic rocks spatially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling (罗马岭),Gongbupala (贡布爬拉) and Qiazhasumdo (卡扎松多) massifs. All the massifs are fault-contacted with greenschist (Chasagang (岔萨岗) Formation) or muscovite-quartz schist (Mabuku (马布库) Formation), and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents: SiO2(35.97-40.63) wt.%, MgO (37.02-38.60) wt.%, TiO2(0.01-0.08) wt.%, Al2O3 (0.80-1.64) wt.%, (Na2O+K2O) less than 0.1 wt.%, with high volatile contents (H2O+CO2) (11.24-14.91) wt.%. After recalculation without H2O+CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT(7.45-9.97) wt.% (8.55 wt.% in average), (MgO+FeOT) 52.09 wt.%, Mg# (100×Mg/(Mg+Fe*), where Fe* represents total Fe)=89.42-90.08, (m+f)/Si ((atomicity Mg+atomicity Fe) /atomicity Si)=1.53-1.75 (1.59 in average), respectively. The mean M/F (atomicity Mg/atomicity Fe) ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, Al and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low ∑ REE with (1.60-2.68)×10-6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with (Ce/Yb)N 1.03-2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature In REE from a relic mantle and common features In highly serpentinized ultramafic rocks in the Yarlung-Zangbo (雅鲁藏布) ophiolite and the Bangong (班公)-Nnjiang (怒江)ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element (LHSE) and depleted in Sin, Ti, Y, Yb element (HFSE), a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protofith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.     

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.