庐山地区星子群碎屑锆石SIMS U-Pb年龄及其地质意义

江西庐山地区出露的星子群为一套遭受角闪岩相变质的中深变质岩系,前人将其划分为古元古代地层,并将其与华南早期地壳的形成与演化和中生代变质核杂岩构造相联系。本文通过对星子群浅粒岩中碎屑锆石U-Pb SIMS年代学研究,获得了(834±4)Ma碎屑锆石年龄,代表了该套地层沉积时代的下限;再结合其上覆筲箕洼组(830±5)Ma的火山岩年龄,将星子群的地层时代限定于新元古代。这一研究结果表明庐山地区并未出露古元古代的变质基底;而且野外也并未找到典型大规模拆离断层的存在,因此前人厘定的中生代变质核杂岩构造也值得商榷。根据碎屑锆石U-Pb不一致线下交点年龄为(133±20)Ma,推断星子群的变质作用发生在中生代。另外,由星子群单一的碎屑锆石年龄、锆石形态特征及矿物组成特征推断,星子群是源区花岗质岩石快速抬升、风化剥蚀和搬运沉积的产物,暗示着强烈的地壳运动过程,可能与Rodinia超大陆的裂解有关。     

Extreme crustal metamorphism during Columbia supercontinent assembly: Evidence from North China Craton

A vast supracrustal belt of khondalites (granulite facies metapelites) occur along the northern margin of the North China Craton. We report here for the first time spinel + quartz equilibrium assemblage from these rocks in two textural settings: (1) high ZnO (up to 14.47 wt.%) spinel with quartz as inclusions within garnet porphyroblasts defining pressure above 12 kbar and temperature of 900 °C; and (2) low ZnO (down to 1.2 wt.%) spinel in association with quartz in the matrix assemblage formed during peak ultrahigh-temperature conditions (ca. 975 °C and 9 kbar). We present a unique case of decompression where the metamorphic conditions of the rocks traversed mostly through the spinel + quartz (extended) stability field. Monazite grains in textural association with both types of spinel + quartz textures were analysed for age determination, and the data define two age peaks at 1927 ± 11 Ma and 1819 ± 11 Ma. Since the peak thermal regime of the khondalites was close to or exceeded the theoretical closure temperature of Pb in monazite, we infer the 1819 ± 11 Ma age as the timing of ultrahigh-temperature event in this craton. Our data lend support to the idea of ca. 1.9–1.8 Ga E–W collisional orogen at the northern margin of the North China Craton. We correlate the extreme crustal metamorphism with tectonics associated with the assembly of the North China Craton within the Columbia supercontinent.     

元古代基性岩墙群的成因机制,构造背景及其古板块再造意义

大陆克拉通广泛发育元古代镁铁质岩墙群，其形成时限短，侵位机制复杂，可以侵位于不同的地壳层次，岩墙群的岩浆可能有多种来源，但主要来自陆下软流圈或地幔岩石圈，很少经历结晶分异与地壳混染，岩墙群的总体化学成分量富集型的，从古元古代到新元古代，其碱性组分逐渐增多，反映地幔演化特征，板块构造理论可应用于晚太古代－古元古代的构造研究，元古代以超级大陆的聚合及裂解为特征，岩墙群的形成普遍与伸展体制相联系，是超级大陆开始裂解或地幔柱活动的重要标志。华北克拉通区广泛发育中元古代镁铁质岩墙群，对其进行多学科的研究，将为确定华北克拉通在中元古代超级大陆中的构造位置及其古板块再造提供重要依据。     

塔里木北缘库鲁克塔格地区南华纪花岗质火山-侵入杂岩岩石成因及地质意义

位于亚洲中部的塔里木克拉通是中国三大古老克拉通之一,经历了新元古时期罗迪尼亚(Rodinia)大陆聚合和裂解。然而,关于塔里木在该时期的构造演化仍然存在较多争议。本文对塔里木北缘库鲁克塔格地区南华系地层中流纹岩及伴生的正长花岗岩开展岩石学、地球化学、锆石U-Pb年代学和Hf同位素研究,结果表明:正长花岗岩与流纹岩是一套具有"时、空、源"一致的花岗质火山-侵入杂岩组合,锆石U-Pb年龄分别为735±10 Ma和738.9±5.4 Ma,同为新元古代南华纪岩浆作用的产物。它们具有高硅(69.85%~73.87%)、低铁(2.36%~2.96%)、贫镁(0.16%~0.75%)和富碱(Na₂O+K₂O=7.51%~9.05%)的特征,属碱性系列、过铝质岩石。富集K、Rb、Th等大离子亲石元素和轻稀土元素,亏损Nb、Ta、Ba、Sr、P和Ti等。具A₂型花岗岩的特点,是俯冲挤压向减压伸展构造转换背景下岩浆活动的产物。锆石Hf同位素组成较均一, ε_Hf(t)值大多为-17.4~-10.6之间,其二阶段模式年龄T_DM2为2732~2321 Ma,表明岩浆来源于太古宙-元古宙古老结晶基底。结合前人研究表明,大约在735 Ma塔里木北缘开始进入减压伸展的拉张环境,之后可能发生了Rodinia超大陆的裂解。     

阿尔金造山带青白口纪亚干布阳片麻岩年龄、地球化学特征及其地质意义

出露于阿中地块库木塔什萨依一带的亚干布阳片麻岩主要岩性为黑云斜长片麻岩、黑云二长片麻岩。利用LA-ICPMS方法进行锆石微区U-Pb同位素定年,得到206Pb/238U年龄加权平均值为900.2±2.9Ma,表明亚干布阳片麻岩原岩形成于新元古代早期青白口纪;地球化学结果显示,主量元素具有高SiO_2、Al_2O_3、K_2O+Na_2O含量,低Na_2O、MgO、CaO和TiO_2含量的特征,A/CNK值介于0.95～1.22之间,属于高钾钙碱性系列的过铝质花岗岩。岩石富集Rb、Th、K等大离子亲石元素,亏损Nb、Sr、P、Hf、Ti等高场强元素;岩石轻稀土元素分馏较强而重稀土元素分馏较弱,具有明显的负Eu异常,总体呈右倾的"V"字形稀土元素配分模式,显示典型的地壳重熔型花岗岩特征。亚干布阳片麻岩的源岩主要为地壳中沉积岩类的部分熔融,形成于俯冲-同碰撞构造环境。综上说明亚干布阳片麻岩是新元古代早期俯冲碰撞热事件的产物,反映阿中地块和柴达木地块青白口纪处于汇聚碰撞阶段,构造岩浆活动强烈,与Rodinia超大陆汇聚事件具有一致性。     

Paleoproterozoic Nb-enriched meta-gabbros in the Quanji Massif,NW China:Implications for assembly of the Columbia supercontinent

Diverse models have been proposed for the role of the Tarim Craton within the Paleoproterozoic Columbia supercontinent assembly. Here we report a suite of-1.71 Ga Nb-enriched meta-gabbro lenses in the eastern Quanji Massif, within the Tarim Craton in NW China. The meta-gabbroic rocks have Nb contents of 11.5-16.4 ppm with Nb/La ratios varying from 0.84 to 1.02((Nb/La)_N = 0.81-0.98) and Nb/U ratios from 38.0 to 47.2. They show low SiO_2(45.1-48.5 wt.%) and MgO(5.96-6.81 wt.%) and Mg#(Mg# = Mg/(Mg + Fe) = 43.5-47.7), high FeO~t(13.0-15.7 wt.%) and moderate Ti02(1.70-2.51 wt.%).with tholeiitic affinities. These rocks possess low fractionated REE patterns without obvious Eu anomalies(Eu/Eu~* = 0.87-1.02). Their primitive mantle-normalized elements patterns display significant Zr-Hf troughs, positive Nb anomalies, weak negative Ti and P anomalies, and high contents of Rb and Ba,resembling Nb-enriched basalts generated in arc-related tectonic settings. Their arc-like geochemical signatures together with whole rock εNd(t) values of 0.4-2.1 and corresponding old T_(DM)(2.22-2.37 Ga)as well as(~(143)Nd/~(144)Nd)_t and(~(87)Sr/~(86)Sr)t(t = 1712 Ma) values of 0.5104-0.5105 and 0.7030-0.7058,respectively, suggest that their precursor magma originated from mantle wedge peridotite metasomatised by subduction-derived melts. The results from our study reveal subduction along the eastern periphery of the Tarim Craton and marginal outgrowth continuing to ~1.7 Ga within the Columbia supercontinent.     

古气候中的沙尘变化及其气候影响

大气中的沙尘通过对太阳光的吸收和散射对地表温度产生影响,进而对大气环流和降水等也产生影响,但是在地球的不同历史时期尤其是深时地球大气中的沙尘量如何变化以及如何影响气候还有很多不清楚的地方。本文通过模式模拟对地球上4个不同时期的沙尘变化及其气候影响进行了研究,这4个时期分别是陆地植被还没有出现的前寒武时期、有植被且全球气温较高的盘古超大陆时期、大陆分布接近现代但气候较寒冷的末次盛冰期和气候较温暖的中全新世。在前寒武时期,由于陆地起沙面积大,大气中的沙尘含量可以达到现代地球的近10倍,使得全球地表温度下降也有近10 ℃。在盘古超大陆时期,由于在副热带的陆地面积比现代稍大,因此大气沙尘量也比现代稍大,对全球平均温度的影响很小。在末次盛冰期,由于寒冷干燥,沙尘量是现代的约两倍,但是它对全球有强烈的增温作用,如果没有沙尘,全球温度可能降低约2.5 ℃。在中全新世,观测显示全球沙尘量比现代少很多,模拟显示,采用现代沙尘和把沙尘全部去掉对全球平均温度的影响可以忽略,但是大西洋经向翻转流有显著变化。     

Orosirian (ca. 1.96 Ga) mafic crust of the northwestern São Francisco Craton margin: Petrography,geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement,NE Brazil

The bulk of the Atlantic shield basement in the eastern South American platform is made of Rhyacian calc-alkaline magmatic rocks (∼2.2–2.15 Ga) affected by granulite-facies metamorphism at around 2.06 Ga, as part of a Paleoproterozoic orogenic system which joined Archean crustal fragments together. Orosirian (2050–1800 Ma) rocks, on the other hand, are scarce in all four major provinces of eastern South America (the Borborema, São Francisco, Tocantins and Mantiqueira provinces). Recently, however, sparse occurrences of Orosirian-aged rocks have been described, particularly in the northern São Francisco Craton margin and adjoining fold belts that compose the Borborema Province further north. In this paper, we present new geochemical, geochronological and isotopic data of amphibolites which compose the basement of the Rio Preto fold belt, in the northwestern São Francisco Craton margin. Geochemical data suggest tholeiitic gabbro protoliths intruded in a subduction-related setting, as suggested by tectonic discrimination diagrams and differentiation trends. U–Pb analyses of magmatic zircon crystals yielded a 1958.3 ± 16 Ma Concordia age. Slightly negative to positive εNd_(1.96) = (−0.3 to +1.0) and εHf_(1.96) from −0.76 to −4.55 suggest variable mixing of a depleted mantle source and older continental crust. We interpret the amphibolite body as a remnant of a subduction-related crustal fragment developed in the Orosirian, around 1.96 Ga ago, possibly in a continental back-arc setting. This crustal fragment was further reworked as part of the basement of the Rio Preto fold belt during the Brasiliano Orogeny (∼600–550 Ma). The Cristalândia do Piauí Complex gneisses further north might represent fragments of a coeval continental magmatic arc. These rocks could be the first well documented representatives of an Orosirian tectono-magmatic event which has been, up to now, broadly ignored within this portion of West Gondwana, and thus present major implications for the Proterozoic crustal evolution of South America and for Paleoproterozic supercontinent reconstructions in general.     

中国新元古代大陆拼合与Rodinia超大陆

在对前人研究成果分析的基础上,结合对中祁连块块前寒武纪基底的研究,认为新元古代（1000－9000Ma）中国各主要克拉通地块（包括华夏地块、扬子地块、华北地块、阿拉善－祁连－柴达木地块、塔里木地块）曾经通过晋宁期碰撞拼合带发生过一次全面的多块体复杂拼贴。这次拼贴过程是全球性新元古代格林威尔碰撞造山作用和Rodinia超大陆形成过程的一个组成部分。新元古代拼合的古中国地块在当时位于Rodinia超大陆中北部澳大利亚、劳伦提亚和西伯利亚克拉通地块之同。中国的各主要克拉通地块只不过是巨型格林威尔碰撞带之中夹裹的一些小型地块,且都处于新元古代晚期Rodinia超大陆破裂的中心部位,这些正是中国大陆上克拉通地块活动性大的早期地质背景。