阿尔金山米兰红柳沟的席状岩墙群：海底扩张的重要证据

米兰红柳沟一带的蛇绿岩位于青藏高原的北部边缘,属于北阿尔金蛇绿岩带的西段,由变质橄榄岩、辉长岩、辉绿岩和基性熔岩组成。野外调查发现约50m厚的均由岩墙组成的岩墙群,其产状近直立,单个岩墙厚约30～40cm。岩墙和与其相伴的熔岩成分相同,属拉斑玄武岩系列,可与典型的洋底玄武岩对比。成分特点:SiO2含量为45wt%～49wt%,TiO2为～1wt%,K2O小于0.3wt%,P2O5为0.1wt%);LREE轻微富集到平坦型,(La/Sm)N0.86～1.74;Cr、Ni、Co和V等过渡金属元素含量高。岩墙群的发现为古海底扩张形成洋壳提供了关键性证据,并可进一步研究洋壳的形成过程和构造背景。     

A Mechanism for the Building of the Qinghai-Tibet Plateau

l.IntroductionSinceArgand's(l924)recognitionthattheCenozoictectonicsoftheHighAsiaareprincipallytheresultoftheconvergenceofGondwanancontinentalfragmentswithLaurisic,therehasbeenageneralacceptanceoftheviewthattheHimalayasandQinghai-TibetPlateauhavebeenformedbyacollisionbetweentheIndianandTibetcontinentsalongtheIndus-Zangbosuture(MolnarandTapponnief,l975,Sengor,l979;ChenBingwei,l982;DengWanming,l978,HuangJiqing,l954;LoZhongshu,l982;PanGuitangetal.,l984,WangQianshenetal.,l982,Besseetal.…     

峨眉山大火成岩省底部枕状玄武岩的地质地球化学特征及成因探讨

云南上沧二叠纪枕状玄武岩位于峨眉山大火成岩省内带底部。该玄武岩全岩SiO₂含量为49.36%～51.36%,TiO₂含量为1.28%～1.91%,MgO含量为6.08%～10.79%,Mg^#值为53～69,Al₂O₃含量为13.91%～16.31%,Na₂O含量为1.59%～3.72%,K₂O含量较低,为0.18%～0.76%,并且Na₂O/K₂O比值较高,为4.27～10.48,属拉斑系列玄武岩。具有右倾斜型稀土元素配分模式,重稀土和部分高场强元素与N-MORB接近,轻稀土和部分大离子亲石元素介于N-MORB和上地壳之间。(La/Sm)_N比值大多数低于2.0(1.4～2.2),(Ce/Yb)_N比值介于2.2～3.4之间,明显低于OIB,不具有OIB的地球化学特征,在Th/Hf-Ta/Hf构造环境判别图解中位于陆缘裂谷玄武岩区域,推测上沧枕状熔岩是古特提斯洋俯冲到扬子板块边缘形成的初始裂谷扩张的产物。     

Relationship between Granite and Eclogite on the Southern Margin of the Qilian Mountains: Evidence from Zircon SHRIMP Agesof the Aolaoshan Granite

Zircon SHRIMP ages of the Aolaoshan granite on the south margin of the Qilian Mts. range from 445±15.3 to 496±7.6 Ma (averaging 473 Ma), belonging to the Early Ordovician. Geochemically, the granite is similar to I-type granite and, tectonically, was formed in an island-arc environment based on relevant diagrams for structural discriminations. Considering also the regional geology, the authors suggest that the granite is part of an ultrahigh-pressure belt on the south margin of the Qilian Mts. and that its formation bears a close relationship to this belt.     

Two Contrasting Eclogite Types on the Northern Margin of the Qaidam Basin, Western China

Eclogites have been recently discovered in the Xitieshan area in the middle segment of the northern margin of the Qaidam basin. These eclogites, together with those recognized earlier in the Yuka area of the western segment of the northern margin of the Qaidam basin and in the Dulan area of the eastern segment of the northern margin of the Qaidam basin, form an eclogite belt with a length of 350 km. A comparison of the eclogites from the Yuka and Xitieshan areas suggests that they show different country rocks, microtextures, mineral assemblages, and especially, different peak temperatures, PT paths during decompression and isotopic features. Eclogites from the Yuka area bear evidence of prograde metamorphism, such as prograde mineral relics in garnet and growth zoning of garnet, and hairpin-shaped PT paths with coincidence of the baric and thermal peaks of metamorphism, which reflect rapid burial and uplift. Sm-Nd isotopic determination shows obvious Sm-Nd disequilibrium, and no isochron ages of Early P     

蛇绿岩中铬铁矿研究的问题与思考

蛇绿岩中的铬铁矿广泛分布在全球不同时代和地区的造山带中，是工业铬矿的重要来源。金属铬的物理化学性质决定了其耐高温、耐腐蚀和抗氧化等许多特性，是生产不锈钢等特种钢、用于军工、航天等领域的重要战略资源。尽管研究历史悠久，但许多关键的科学问题并没有解决，是地球科学研究的前沿和热点。本文简述了铬铁矿的性质，综述了全球铬铁矿资源的分布与需求现状；通过大数据分析，回顾了铬铁矿的研究历史和进展；概述了蛇绿岩铬铁矿的主要产出特征和地质背景；探讨了研究中存在的关键科学问题，包括蛇绿岩豆荚状铬铁矿成矿母岩浆性质，成矿物质来源和聚集成矿作用以及成矿动力学背景；讨论了铬铁矿成因的不同模式以及相关的高温高压实验岩石学的研究进展；综述了蛇绿岩铬铁矿中新发现的一系列新矿物及其地质意义，提出蛇绿岩铬铁矿是研究俯冲物质深地幔循环和地球深部动力学的重要窗口等新认识。     

Early Palaeozoic North Qaidam UHP metamorphic belt on the north-eastern Tibetan plateau and a paired subduction model

ABSTRACT An Early Palaeozoic UHP metamorphic belt was recently discovered in the North Qaidam Mountains in the north-eastern part of the Tibet Plateau. The belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss. The paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz. The calculated peak metamorphic conditions for the eclogite are T  = 730 °C and P  = 2.8 GPa. The garnet peridotites are magnesium-rich with Mg#s [100 × Mg/(Mg + Fe)] up to 92. Olivine in the peridotites is highly magnesian (Fo > 91.5) and TiO₂-poor and is typical of low- T garnet peridotites. Calculated peak metamorphic conditions for the peridotites are T  = 837 °C and P  = 2.5 GPa. U–Pb and Sm–Nd isotope dating indicate that continental subduction started during the early Ordovician (∼ 495 Ma). The North Qaidam UHP, together with the North Qilian HP belt about 300 km to the north, constitute a pair of genetically related early Palaeozoic subduction zones along the northern margin of the Tibet Plateau.     

Dur'ngoi Ophiolite in East Kunlun, Northeast Tibetan Plateau: Evidence for Paleo-Tethyan Suture in Northwest China

The A'nyemaqen (阿尼玛卿) ophiolite belt along the southern margin of the East Kunlun (昆仑) Mountains marks the suture formed by the closure of paleo-Tethys. The Dur'ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, Iherzolite, feldspar-bearing lherzolite and garnet-bearing Iherzolite and contain residual spinel with Cr# [100×Cr/(Cr+AI)] ranging from 30 to 57 and Mg# [100×Mg/(Mg+Fe2+)] ranging from 50 to 75, indicating an AI- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2-92.6, Al2O3 contents of (1-4) wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the Iherzolite are andradite, enriched in Ca and Fe and depleted in Mg and Al (And=95-97, Pyr=0.3-5, Gro=0-3), indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, py-roxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in pla-gioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular (Gro=69-90, And=9-19, Br=1-12), also metamorphic origin. The diabase dikes are moderately depleted in LREE [(La/Sm)N=0.5-0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies (δEu=1.1-1.3). The basaltic lavas have REE patterns similar to those of MORB with (La/Sm)N ratios of 0.5-1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basaits yields 206pb/238U ages of 276-319 Ma (average 308.0±4.9 Ma). The Dur'ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area: island arc volcanic (IAV) rocks, possible back arc basin (BAB) basalts and possible post-collisional vol-canic (PCV) and plutonic rocks. The distribution of these rocks suggests north-directed subduc-tion. Opening of the A'nyemaqen oceanic basin started at least as early as Late Carboniferous(308 Ma) and the basin probably closed during the Early Triassic. The IAV formed in Late Permian (260 Ms), the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistrai strike-slip fault zones, extending hun-dreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun (200-220 Ma), the Aityn Tagh fault (220-230 Ma), and the North Qaidam fault zone (240-250 Ma). These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.     

Petrogenesis and Dating of Two Types of Granite from North Qilian, China

1 IntroductionThree E-W-trending lithologic belts of the Paleozoic ageoccur in the eastern part of north Qilian. From north tosouth they are (a) the Huangyanghe-Gulang-Dajing-Beidengzi belt (the northern lithologic belt), (b) theLeigongshan-Maomaoshan-Laohushan belt (the middlelithologic belt) and (c) the Mayaxieshan-Shihuigou-Beiyinbelt (the southern lithologic belt) (Fig. 1). The rocks inthese belts are mainly Paleozoic volcanics and sandysiltstone. There are some Mesozoic and Cenozo…     

铬同位素体系及其在铬铁矿成矿过程中的应用

近年来铬(Cr)同位素体系在示踪地幔部分熔融、岩浆结晶分异及俯冲带流体相关的地质过程中均取得了重要进展。本文通过实例研究综述了Cr同位素在铬铁矿成矿作用方向的主要进展,包括:(1) Stillwater层状岩体橄榄岩带和K?z?ldaˇg蛇绿岩壳幔过渡带内铬铁矿及共生硅酸盐矿物的Cr同位素研究,揭示了层状/似层状铬铁矿成矿过程中可能发生明显的Cr同位素分馏,且主要受结晶分异和岩浆补给过程控制;(2) K?z?lda?蛇绿岩豆荚状铬铁矿床的Cr同位素研究,证明了俯冲带地幔橄榄岩中尖晶石的部分熔融,可能是豆荚状铬铁矿床主要的成矿物质来源之一,同时俯冲带流体也可能直接参与成矿;(3)层状岩体及蛇绿岩中普遍存在矿物间的Cr同位素不平衡分馏现象,不仅可用以记录岩浆作用的冷却时间,同时也证明了铬铁矿成矿过程中释放的流体对矿物间的元素交换具有明显促进作用。