东南极格罗夫山变基性岩初步研究

东南极格罗夫山存在一套经历了峰期麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。对含石榴石的镁铁质麻粒岩的详细研究则显示了近等温降压 (ITD)的顺时针PT演化轨迹 ,与拉斯曼丘陵有相似的演化历史。岩石的主量元素组成和玄武岩一致 ,并且具有拉斑玄武岩演化趋势。进一步的地球化学研究表明 ,这套玄武岩为洋岛型玄武岩 (OIB)和洋中脊型玄武岩(MORB)的组合。OIB型具有大体类似的地球化学性质 ,它们均富集Ti(TiO2 =2 .68% )、REE( =2 0 2 μg/g)、LREE[(La/Yb) N=4.8]、Ti/Y( =343)、Zr/Y( =3.1 ) ,具洋岛玄武岩的特征 ,推测岩浆来源于富集地幔源区 (EM)。而MORB型以低Ti (TiO2 =1 .1 %— 1 .31 % ) ,明显低于OIB的P的含量 (P2 O5=0 .1 %— 0 .2 % ) ,低REE ( 4 7— 93μg/g)、LREE/HREE( 2 .2 7— 2 .5 4 )、(La/Yb) N( =1 .30— 1 .62 )为特征 ,具洋中脊玄武岩的特征。MORB和OIB组合的出现说明在泛非期该区可能存在过洋盆。     

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).
     

Assessing changes in the rainfall regime in Nigeria between 1961 and 2004

The study assessed changes in the rainfall regime in Nigeria between 1961 and 2004 in terms of (a) absolute seasonality—the length of dry and wet season; (b) relative seasonality—rainfall contrast during the year; (c) number of rainfall maxima and minima; and (d) timing of rainfall maxima and minima. Trends in the mean monthly surface locations of the Inter-tropical Discontinuity (ITD) were also examined as a prominent factor of the rainfall regime. Changes in the regime were examined over four time slices: 1961–1971, 1972–1982, 1983–1993 and 1994–2004. The results show that, in the area of single rainfall maximum regime, the length of the wet season has increased from 4 months (in 1961–1971) to 5 months (since 1972–1982). The rainfall relative seasonality has consistently been ‘most rain in 3 months or less’. The rainfall maxima still indicate single rainfall maximum but shift in the peak from August to July. Rainfall during the months of June to September appears to have witnessed declining trends over the first three time slices. The last time slice however indicates trends towards a wetter condition. For the area of double maxima rainfall regime, the length of the wet season has consistently been 8 months. The rainfall relative seasonality has consistently been ‘rather seasonal with a short drier season’. The rainfall maxima and minima still indicate double rainfall maxima with August as the month of the minimum but shift in the primary peak from July to September. The northward latitudinal distance of the surface location of the ITD from the equator indicates significant upward trends during the months of May to September only and for a period of 1983–2000. The trend results of the ITD appears to account for most of the observed changes in the rainfall regime in Nigeria.     

两种识别结构模态参数时域法的对比研究

模态参数是有效评估结构安全状况的关键参数,在结构抗震加固和健康诊断领域得到广泛应用。与频域法相比较,时域法直接利用实测的振动信号识别模态参数,不需要进行频域变换,减少数据处理带来的误差,并且可以实现大型结构的在线识别,真实地反应结构的现状。以同济大学12层钢筋混凝土标准框架振动台模型试验完整数据为对象,在详细介绍ITD法和复指数法2种时域法理论的基础上,通过编程选取结构不同测点的振动加速度时程数据,识别了小震和强震工况下12层钢筋混凝土框架模型振动台试验模型的模态频率和阻尼比,并结合移动谱识别结构模态参数的时变特性。结果表明:ITD法和复指数法可有效地识别结构的模态参数,自振频率的识别精度较高,而阻尼比的离散度较大;小震工况频率变化值不大,而强震工况频率值较初始时刻有明显的下降,这与试验现象是吻合的,进一步说明移动谱与这2种时域法相结合可以反应结构在塑性阶段的参数时变特性。     

喂子坪秦岭杂岩麻粒岩相变沉积岩的变质作用p-T演化及其构造意义

秦岭杂岩位于秦岭造山带北秦岭构造带,是研究秦岭造山带早古生代构造演化的重要岩石构造单元。喂子坪地区发育有典型的秦岭杂岩,其中的混合岩是由变沉积岩经历了强烈的深熔作用形成的。本研究对其进行了岩相学、变质相平衡模拟和LA-ICP-MS锆石U-Pb年代学研究,以深入揭示它们的变质温压演化特征,进而阐明它们指示的构造意义。混合岩的中色体由含石榴子石角闪黑云斜长片麻岩和含石榴子石黑云角闪斜长片麻岩组成。含石榴子石角闪黑云斜长片麻岩只记录了峰期变质矿物组合,为镁铁闪石+石榴子石+斜长石+石英+黑云母+钛铁矿+熔体,而含石榴子石黑云角闪斜长片麻岩记录了3个变质演化阶段,分别是早期进变质阶段（M1）：黑云母+斜长石+石英; 峰期变质阶段（M2）：镁铁闪石+石榴子石+斜长石+石英+黑云母+钛铁矿+熔体; 退变质阶段（M3）：普通角闪石+斜长石+黑云母+石英+熔体。全岩成分视剖面图模拟计算显示含石榴子石角闪黑云斜长片麻岩和含石榴子石黑云角闪斜长片麻岩压力峰期的变质温压条件分别为790~810℃/990~1 040 MPa和840~862℃/1 000~1 190 MPa。含石榴子石黑云角闪斜长片麻岩3组局部矿物组合域成分视剖面图模拟计算得到压力峰期后变质阶段的温压条件为735~814℃/400~810 MPa、721~794℃/430~700 MPa、740~810℃/470~780 MPa。因此,本研究揭示了喂子坪地区秦岭杂岩片麻岩记录了近等温降压的p-T轨迹。LA-ICP-MS锆石U-Pb定年得到含石榴子石角闪黑云斜长片麻岩和2个浅色体样品中的变质锆石²⁰⁶Pb/²³⁸U加权平均年龄分别为383.2±7.0 Ma、400±3.6 Ma和406.7±7.8 Ma。结合已发表的数据,喂子坪地区变沉积岩麻粒岩相峰期变质作用和强烈的混合岩化作用的时代约为410~390 Ma,而约380 Ma的年龄可能代表退变质冷却到固相线的时代。片麻岩近等温降压的变质演化轨迹指示喂子坪地区秦岭杂岩的变沉积岩在下地壳经历了麻粒岩相变质作用和随后的快速抬升,与碰撞造山引起的地壳增厚和随后的地壳伸展有关。     

东南极格罗夫山镁铁质麻粒岩的变质作用

东南极内陆－格罗夫山存在一套经历了麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。变质反应结构显示该区多为单一的区域性麻粒岩相变质作用。但是，对含石榴石的镁铁质麻粒岩的详细工作则显示了包括3个阶段的近等温降压（ITD）的顺时针PT演化轨迹，M1：0．93GPa＞800℃：M2:0.65GPa,733-850℃;M3:0.46-0.61GPa，并有着与拉斯曼丘陵相似的演化历史。根据矿物组合和成分、变质反应结构及温压计算结果，格罗夫山镁铁质麻粒岩可能为高压麻粒岩。     

The P-T path of metamorphism and age of migmatites from the northwestern Irkut block (Sharyzhalgai uplift of the Siberian Platform)

We study migmatized aluminous gneisses in the northwest of the Irkut granulite complex in the southeastern Sharyzhalgai uplift of the Siberian Platform basement. Migmatized gneisses with the mineral assemblage Grt + Sil + Bt + Kfs + Pl + Qz (+ Crd + Opx + Spl) contain a leucosome and widespread cordierite-bearing (+ orthopyroxene, quartz, and spinel) symplectites developed after garnet and sillimanite. Study of the microstructural relationships of minerals and modeling using the PERPLEX 672 software have shown a retrograde P-T path of metamorphism for the metasedimentary gneisses, close to the isothermal decompression (ITD). The parameters of the peak of metamorphism are T = 850-870 °C and P > 7 kbar. The weighted average age of zircon from the metasedimentary gneisses (1856 ± 13 Ma, SHRIMP) corresponds to the time of metamorphism. The decompression type of retrograde metamorphism of the rocks in the northwest of the Irkut block indicates their formation in the crust extension and thinning setting. The presence of domal structures in the section of the Irkut block on the shore of Lake Baikal suggests that the dome tectogenesis was involved in the exhumation processes. The Paleoproterozoic metamorphism and granite formation were associated with the same stage of collision processes, when the compression setting was changed by an extension one (1.88-1.85 Ga).     

Comparison of airborne laser scanning methods for estimating forest structure indicators based on Lorenz curves

The purpose of this study was to compare a number of state-of-the-art methods in airborne laser scanning (ALS) remote sensing with regards to their capacity to describe tree size inequality and other indicators related to forest structure. The indicators chosen were based on the analysis of the Lorenz curve: Gini coefficient (GC), Lorenz asymmetry (LA), the proportions of basal area (BALM) and stem density (NSLM) stocked above the mean quadratic diameter. Each method belonged to one of these estimation strategies: (A) estimating indicators directly; (B) estimating the whole Lorenz curve; or (C) estimating a complete tree list. Across these strategies, the most popular statistical methods for area-based approach (ABA) were used: regression, random forest (RF), and nearest neighbour imputation. The latter included distance metrics based on either RF (NN–RF) or most similar neighbour (MSN). In the case of tree list estimation, methods based on individual tree detection (ITD) and semi-ITD, both combined with MSN imputation, were also studied. The most accurate method was direct estimation by best subset regression, which obtained the lowest cross-validated coefficients of variation of their root mean squared error CV(RMSE) for most indicators: GC (16.80%), LA (8.76%), BALM (8.80%) and NSLM (14.60%). Similar figures [CV(RMSE) 16.09%, 10.49%, 10.93% and 14.07%, respectively] were obtained by MSN imputation of tree lists by ABA, a method that also showed a number of additional advantages, such as better distributing the residual variance along the predictive range. In light of our results, ITD approaches may be clearly inferior to ABA with regards to describing the structural properties related to tree size inequality in forested areas.     

提高阻尼识别精度的ITD两步法

由于无限介质中辐射阻尼的存在，地下结构抗震安全评价中正确地识别阻尼影响十分重要。阻尼从理论上指出了ITD时域识别技术在阻尼比识别方面存在误差的原因，并提出了一种改进的阻尼识别方法，算例表明，提出的方法，能显著地提高阻尼的识别精度。     

Ultrahigh temperature granulite metamorphism (1050 °C, 12 kbar) and decompression in garnet (Mg70)–orthopyroxene–sillimanite gneisses from the Rauer Group, East Antarctica

Highly magnesian and aluminous migmatitic gneisses from Mather Peninsula in the Rauer Group, Eastern Antarctica, preserve ultrahigh temperature (UHT) metamorphic assemblages that include orthopyroxene+sillimanite±quartz, garnet+sillimanite±quartz and garnet+orthopyroxene±sillimanite. Garnet that ranges up to X_Mg of 71.5 coexists with aluminous orthopyroxene that shows zoning from cores with 7.5–8.5 wt% Al₂O₃ to rims with up to 10.6 wt% Al₂O₃ adjacent to garnet. Peak P–T conditions of 1050 °C and 12 kbar are retrieved from Fe–Mg–Al thermobarometry involving garnet and orthopyroxene, in very good agreement with independent constraints from petrogenetic grids in FeO–MgO–Al₂O₃–SiO₂ and related chemical systems. Sapphirine, orthopyroxene and cordierite form extensive symplectites and coronas on the early phases. The specific reaction textures and assemblages involving these secondary phases correlate with initial garnet X_Mg , with apparent higher-pressure reaction products occurring on the more magnesian garnet, and are interpreted to result from an initial phase of ultrahigh temperature near-isothermal decompression (UHT-ITD) from 12 to 8 kbar at temperatures in excess of 950 °C. Later textures that involved biotite formation and then partial breakdown, along with garnet relics, to symplectites of orthopyroxene+cordierite or cordierite+spinel may reflect hydration through back-reaction with crystallizing melts on cooling below 900–850 °C, followed by ITD from 7 to 8 kbar to c. 5 kbar at temperatures of 750–850 °C. The tectonic significance of this P–T history is ambiguous as the Rauer Group records the effects of Archean tectonothermal events as well as high-grade events at 1000 and 530 Ma. Late-stage biotite formation and subsequent ITD can be correlated with the P–T history preserved in the Proterozoic components of the Rauer Group and hence with either 1000 or 530 Ma collisional orogenesis. However, whether the preceding UHT-ITD history reflects a temporally unrelated event (e.g. Archean) or is simply an early stage of either the late-Proterozoic or Pan-African tectonism, as recently deduced for similar UHT rocks from other areas of the East Antarctica, remains uncertain.