Petrogenesis of Tholeiitic Lavas from the Submarine Hana Ridge, Haleakala Volcano, Hawaii

Hana Ridge, the longest submarine rift zone in the Hawaiianisland chain, extending from Maui 140 km to the ESE, has a complexmorphology compared with other Hawaiian rift zones. A totalof 108 rock specimens have been collected from the submarineHana Ridge by six submersible dives. All of the rocks (76 bulkrocks analyzed) are tholeiitic basalts or picrites. Their majorelement compositions, together with distinctively low Zr/Nb,Sr/Nb, and Ba/Nb, overlap those of Kilauea lavas. In contrast,the lavas forming the subaerial Honomanu shield are intermediatein composition between those of Kilauea and Mauna Loa. The compositionalcharacteristics of the lavas imply that clinopyroxene and garnetwere important residual phases during partial melting. The compositionsof olivine and glass (formerly melt) inclusions imply that regardlessof textural type (euhedral, subhedral–undeformed, deformed)olivine crystallized from host magmas. Using the most forsteriticolivine (Fo_90·6) and partition coefficients     

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.     

赤道东经90°海岭3.5Ma以来远洋记录反映的重大环境事件

通过对东经90°海岭ODP758钻孔岩心3.5Ma以来的浮游有孔虫远洋记录的研究,结合岩性地层、稳定同位素地层、碳酸盐地层等资料,探讨了喜马拉雅山加速隆升、"中更新世气候转型"、"中布容溶解"事件等重大构造、环境事件在研究区的表现及相关机制。通过对各项指标的分析和对比,根据ODP758岩心的有孔虫组合面貌、古温度、古盐度和古温跃层的变化情况,推断研究区在1.4～0.7Ma期间的气候变化可能是在"中更新世气候转型"条件下发生的。碳酸钙、碳酸钙沉积通量和非碳酸钙沉积通量曲线在1.7Ma时快速增长,与现有资料显示的青藏高原的隆升历史相符,分别与青藏高原隆升过程中的青藏运动A幕、B幕、C幕(1.7Ma)和昆仑-黄河运动(1.2～0.6Ma)相当。碳酸钙含量、大于150μm的粗组分含量、浮游有孔虫沉积通量和浮游有孔虫生物地层揭示出在0.5～0.4Ma期间研究区深海碳酸盐遭受了强烈溶解。此事件与大西洋、太平洋、印度洋及南沙海区沉积记录中的"中布容溶解"事件一致。     

Progressive metamorphism of the Taitao ophiolite; evidence for axial and off-axis hydrothermal alterations

We estimated metamorphic conditions for the  6 Ma Taitao ophiolite, associated with the Chile triple junction. The metamorphic grade of the ophiolite, estimated from secondary matrix minerals, changes stratigraphically downwards from the zeolite facies, through the prehnite–actinolite facies, greenschist facies and the greenschist–amphibolite transition, to the amphibolite facies. The metamorphic facies series corresponds to the low-pressure type. The metamorphic zone boundaries are subparallel to the internal lithological boundaries of the ophiolite, indicating that the metamorphism was due to axial hydrothermal alteration at a mid-ocean ridge.

Mineral assemblages and the compositions of veins systematically change from quartz-dominated, through epidote-dominated, to prehnite-dominated with increasing depth. Temperatures estimated from the vein assemblages range from  230 °C in the volcanic unit to  380 °C at the bottom of the gabbro unit, systematically  200 °C lower than estimates from the adjoining matrix minerals. The late development of veins and the systematically lower temperatures suggest that the vein-forming alteration was due to off-axis hydrothermal alteration.

Comparison between the Taitao ophiolite with its mid-ocean ridge (MOR) affinity, and other ophiolites and MOR crusts, suggests that the Taitao ophiolite has many hydrothermal alteration features similar to those of MOR crusts. This is consistent with the tectonic history that the Taitao ophiolite was formed at the South Chile ridge system near the South American continent (Anma, R., Armstrong, R., Danhara, T., Orihashi, Y. and Iwano, H., 2006. Zircon sensitive high mass-resolution ion microprobe U–Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications. The Island Arc, 15(1): 130–142).     

The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Summary. In the summer of 1984 an electrical survey using magnetometric off-shore electrical sounding (MOSES) was conducted at two sites in Middle Valley, part of the northern Juan de Fuca Ridge complex. MOSES has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer. Site 1, at 48°32N, 128°42W, is in the central part of the turbidite-filled basin. Using a two-layer model of conductive sediments overlying a fractured basalt basement, the sediment resistivity and thickness were found to be 0.82 ± 0.06 Ωm and 1800 ± 300 m, respectively. The basement resistivity, although not well constrained by the data is consistent with the results obtained at site 2.
Site 2, located at 48°10N, 128°50W, has a thinner sediment layer, which appears to vary with position. The sediment conductivity—thickness product is the parameter determined by the data. If the sediment resistivity were the same as at site 1, the sediment thickness would be 140 ± 30 m to the SE of site 2, and 240 ± 55 m to the NW. The fractured basalt basement has a resistivity of 8.5 ± 3.4 Ωm and is at least 1000 m thick.
Using temperature-corrected pore fluid resistivity, the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement. These values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment-filled basin and determined for layer 2A/B basalts in DSDP hole 504B, respectively.     

西藏札佐晚白垩世中期埃达克岩年代学、地球化学及其构造意义

为解释南冈底斯晚白垩世埃达克质岩石成因及其地球动力学机制,本文对西藏扎囊县札佐地区二长花岗岩开展了锆石U-Pb年代学、Hf同位素和全岩地球化学分析。札佐二长花岗岩锆石U-Pb定年为80.43±0.62 Ma,其SiO₂为66.19%~66.84%,Al₂O₃为15.17%~15.48%,MgO为1.67%~1.91%,Mg#为47.4~51.5,K₂O为3.86%~4.09%,A/CNK=0.91~1.01,属准铝质高钾钙碱性岩石。岩石轻稀土富集明显,高Sr（492×10^-6~670.2×10^-6）,低Y（8.27×10^-6~14.99×10^-6）,Yb（1.07×10^-6~1.79×10^-6）,高Sr/Y（35.0~81.0）,高La/Yb（17.4~21.4）,弱负Eu异常,具埃达克岩地球化学特征。相对富集大离子亲石元素（LILE）,亏损高场强元素（HFSE）和重稀土。锆石ε_Hf（t）值为10.5~14.1,单阶段Hf模式年龄（t_DM1）为184.8~326.1 Ma,平均为203.4 Ma,二阶段Hf模式年龄（t_DM2）为247.2~476.0 Ma,平均为287.1 Ma,略大于侵位年龄,指示岩浆物质来源于俯冲洋壳,并可能卷入俯冲沉积物。岩石中地幔组分印记Mg#值和相容元素Ni、Cr含量较高,表明熔体在上升过程中与上覆地幔楔发生反应。研究分析表明,在新特提斯洋洋脊俯冲作用下,高温热流透过板片窗导致洋壳（及俯冲沉积物）部分熔融形成札佐埃达克质二长花岗岩。同时表明,在80 Ma左右,新特提斯洋仍处于洋脊俯冲阶段。     

一种改进的全极化SAR图像MCSM-Wishart非监督分类方法

针对H/Alpha/A-Wishart非监督分类算法存在的未充分提取SAR图像极化信息和分类精度低等问题,引入多分量散射模型(multiple-component scattering model,MCSM)分解,提出了一个适用于全极化SAR图像非监督分类的MCSM-Wishart算法。首先对全极化SAR图像进行MCSM分解,提取体散射、二次散射、螺旋体散射、表面散射和线散射极化信息,采用迭代自组织数据分析技术(iterative self-organizing data analysis technique,ISODATA)的非监督分类算法进行聚类;然后通过基于描述多视协方差矩阵的复Wishart分布的迭代分类得到分类结果。以南京溧水和盐城滨海湿地的ALOS PALSAR图像为研究数据,比较了H/Alpha-Wishart算法、H/Alpha/A-Wishart算法、MCSM-Wishart算法和监督-Wishart算法4种分类方法。研究结果表明,MCSM-Wishart分类算法在效率、总体准确率和Kappa系数等指标上均较原始分类器有一定的提高;将ISODATA聚类算法应用于复Wishart分布的迭代分类器中,可有效提高分类的精度。