高压下华北北缘二辉麻粒岩电导率的研究

借助于YJ-3000t紧装式六面顶固体高压设备,在1.0~2.0 GPa、523~1173 K条件下,利用Agilent 34401A数字电表和Solartron IS-1260阻抗-增益/相位分析仪,同时使用三种方法:交流阻抗谱法(频率范围0.05~106 Hz)、单频交流法(0.1 Hz)和直流法测量了华北北缘二辉麻粒岩的电导率.结果表明:在实验的温度和压力范围内,二辉麻粒岩电导率的变化在2.66×10-5~0.056 S·m-1之间,电导率对压力没有很强的依赖性;随着温度的升高,电导率增大,遵循Arrenhius关系式,其指前因子为8.95~17.9 S·m-1,活化能为0.569~0.605 eV.对比三种方法获得的电导率数据,发现阻抗谱法测量结果大于单频法测量结果,直流法测得的结果最低,但是,三种方法获得的电导率差值除两个低温点外,绝大多数都很小(Δlgσ<0.20 lg(S/m)).结合现今华北克拉通地热学参数及地壳分层结构,依据实验获得的电导率温度关系建立了电导率-深度剖面.并将其与大地电磁测深获得的地壳电性结构进行了对比,结果表明二辉麻粒岩的电导率与华北北缘的中地壳底部和下地壳底部电导率值的区域相交,再结合高温高压下二辉麻粒岩的弹性波速度剖面与地震折射剖面的对比,认为二辉麻粒岩有可能是组成华北北缘下地壳的岩石之一.     

Quantitative elemental mapping of granulite-facies monazite: Textural insights and implications for petrochronology

Texturally complex monazite grains contained in two granulite-facies pelitic migmatites from southern Baffin Island, Arctic Canada, were mapped by laser ablation-inductively coupled plasma-mass spectrometry (using spot sizes ≤5 µm) to quantitatively determine the spatial variation in trace element chemistry (with up to 1,883 analyses per grain). The maps highlight the chemical complexity of monazite grains that have experienced multiple episodes of growth, resorption and chemical modification by dissolution–precipitation during high-grade metamorphism. Following detailed chemical characterization of monazite compositional zones, a related U–Pb data set is re-interpreted, allowing petrologically significant ages to be extracted from a continuum of concordant data. Synthesis of these data with pseudosection modelling of prograde and peak conditions allows for the temporal evolution of monazite trace element chemistry to be placed in the context of the evolving P–T conditions and major phase assemblage. This approach enables a critical evaluation of three commonly used petrochronological indicators: linking Y to garnet abundance, the Eu anomaly to feldspar content and Th/U to anatectic processes. Europium anomalies and Th/U behave in a relatively systematic fashion, suggesting that they are reliable petrochronological witnesses. However, Y systematics are variable, both within domains interpreted to have grown in a single event, between grains interpreted to be part of the same age population, and between samples that experienced similar metamorphic conditions and mineral assemblages. These observations caution against generalized petrological interpretations on the basis of Y content, as it suggests Y concentrations in monazite are controlled by domainal equilibria. The results reveal a c. 45 Myr interval between prograde metamorphism and retrograde melt crystallization in the study area, emphasizing the long-lived nature of heat flow in high-grade metamorphic terranes. Such long timescales of metamorphism would be assisted by the growth, retention and dominance of high-Th suprasolidus monazite, as observed in this study, contributing to the radiogenic heating budget of mid- to lower-crustal environments. Careful characterization of monazite grains suggests that continuum-style U–Pb data sets can be decoded to provide insights into the duration of metamorphic processes.     

Petrology, geochemistry and isotopic ages of eclogites from the Dulan UHPM Terrane, the North Qaidam, NW China

The Dulan eclogite–gneiss region is located in the eastern part of the North Qaidam eclogite belt, NW China. Widespread evidence demonstrates that this region is a typical ultrahigh-pressure (UHP) metamorphic terrane. Eclogites occur as lenses or layers in both granitic and pelitic gneisses. Two distinguished sub-belts can be recognized and differ in mineralogy, petrology and geochemistry. The North Dulan Belt (NDB) has tholeiitic protoliths with high TiO₂ and lower Al₂O₃ and MgO contents. REE patterns and trace element contents resemble those of N-type and E-type MORB. In contrast, eclogites in the South Dulan Belt (SDB) are of island arc protoliths with low TiO₂, high Al₂O₃ and show LREE-enriched and HFSE-depleted patterns. Sm–Nd isotope analyses give isochron ages of 458–497 Ma for eclogite-facies metamorphism for the two sub-belts. The ages are similar to those of Yuka and Altun eclogites in the western extension of the North Qaidam-Altun eclogite belt. The Dulan UHP metamorphic terrane, together with several other recently recognized eclogite-bearing terrenes within the North Qaidam-Altun HP-UHP belt, constitute the key to the understanding of the tectonic evolution of the northern Tibetan Plateau. The entire UHP belt extends for more than 1000 km from the Dulan UHP terrane in the southeast to the Altun eclogite–gneiss terrane in the west. This super-belt marks an early Paleozoic continental collision zone between the Qaidam Massif and the Qilian Massif.     

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015 ± 4.4 Ma, 966.5 ± 3.5 and 808 ± 3.1 respectively. The plagiogranite shows high SiO₂, Na₂O and extremely low K₂O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO₂ and moderate Na₂O, together with high K₂O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO₂ moderately higher Na₂O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.     

滇西保山地块东缘晚三叠世何珠次英安斑岩的发现 及其地质意义

在野外地质调查和岩相学观察的基础上,对保山地块东缘何珠次英安斑岩进行了LA-ICP-MS锆石原位U-Th-Pb同位素测定,得到16个测定点的206Pb/238U年龄加权平均值为216.8Ma±2.2Ma。该年龄值反映了岩体的侵位时间,表明其属于晚三叠世,而非前人认为的新生代。结合岩相学和前人地球化学研究资料分析认为,岩体具有I型和S型花岗岩的特征,反映了其在矿物和地球化学组成上的不平衡,应为幔源高温基性岩浆与壳源岩浆混合形成的,且前者占主导地位。这是昌宁-孟连结合带西侧首次报道幔源物质对晚三叠世岩浆活动的参与,表明保山地块在碰撞后地壳熔融过程中伴随地幔物质的上涌。研究表明,以何珠岩体为代表的次英安斑岩并非前人认为的新生代,保山地块南端前人划分的新生代岩体群的分布范围和规模可能需要重估。     

麻粒岩p—T—t轨迹的研究现状

综述了近年来麻粒岩中p-T-t轨迹的研究现状。内容涉及到麻粒岩相变质作用的温压条件及时间跨度,同时指出:结合岩石中的矿物组合、后成合晶及反应结构,利用矿物温压计是目前确定p-T-t轨迹的主要手段。     

贺兰山群变质杂岩主期变质作用的温压环境研究

分布于贺兰山脉北部的贺兰山群变质杂岩的主期变质作用达角闪麻粒岩相,其矿物组合特征与低压相系麻粒岩相似、利用共生矿物组合和矿物对的温度计、压力计估算主期变质温度740—764(?),压力460—501MPa。温度压力条件与低压相系的地热梯度线较接近,变质作用强度具面型分布的特征,与东部密云、冀东等地的麻粒岩相区相似,但变质压力明显低,压力类型也不同,应属不同构造背景下的变质产物。     

东亚大陆边缘的构造格架及其中-新生代演化

燕山运动在亚洲大陆雏形东缘形成2条北东向的剪切带:郯庐断裂带和长乐-南澳-中央构造线断裂带,晚侏罗世—古近纪早期沿之发生地体/地块的拼贴。系统叙述了各移置地体/地块的主要岩石记录和拼贴时代,据起源分为3类:异地的(包括源自冈瓦纳的和源自盘古大洋的)、半异地的和准原地的;据拼贴位置分为2组:拼贴后基本位于原地的(日本海张开以前) 和发生过向北东错移的。新生代内东亚大陆边缘发生解体,可以台湾岛以北的菲律宾海盆断裂为界将东亚大陆边缘弧分为2段,北段仍处于剪切-拉张中,南段已进入剪切挤压-造山阶段。强调该地区中—新生代演化经历了2个里丁旋回, 形成早白垩世的北东向和新近纪的北东东向2期新生构造。     

Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC

Phase diagrams involving solid solutions are calculated by solving sets of non-linear equations. In calculating P–T  projections and compatibility diagrams, the equations used for each equilibrium are the equilibrium relationships for an independent set of reactions between the end-members of the phases in the equilibrium. Invariant points and univariant lines in P–T  projections can be calculated directly, as can coordinates in compatibility diagrams. In calculating P–T  and T–x / P–x pseudosections – diagrams drawn for particular bulk compositions – the equilibrium relationship equations are augmented by mass balance equations. Lines in pseudosections, where the mode of one phase in the lower variance equilibrium is zero, and points, where the modes of two phases are zero, can then be calculated directly. The software, THERMOCALC, allows the calculation of these and a range of other types of phase diagram. Examples of phase diagrams and phase diagram movies, with instructions for their production, along with the THERMOCALC input and output files, and the Mathematica^TM functions for assembling them, are presented in this paper, partly in hard copy and partly on the JMG web sites (http://www.gly.bris.ac.uk/www/jmg/jmg.html, or equivalent Australian or USA sites).
     

Geochronological Study of Caledonian Granulite and High-Pressure Gneiss in the Dabie Mountains

Using the single-zircon evaporation technique and U-Pb method, the authors have conducted an isotope geochonological study of the Huilanshan granulite and Shima garnet-bearing plagioclase gneiss (" country rocks" of the Shima eclogite) in the Dabie Mountains. The study shows that these rocks have peak metamorphic ages of 443-455 Ma, which are essentially consistent with that of the Caledonian high-ultrahigh pressure eclogites. This indicates the existence of the Caledonian collisional orogeny in the Dabie Mountains.