西南印度洋中脊地质构造特征及其地球动力学意义

西南印度洋中脊（SWIR）增生的洋壳面积仅占印度洋的15%左右,但其具有比东南印度洋中脊和西北印度洋中脊更悠久而复杂的演化历史.基于已有的地质、地球物理和地球化学等资料,系统总结了SWIR的地质构造特征,并讨论了SWIR的演化过程、洋脊地幔的不均一性、洋脊周边海底高原成因等核心问题.SWIR地形中段高、东西两段低,空间重力异常基本与地形变化一致.按转换断层一级边界可将SWIR划分为20个一级段.SWIR的磁异常条带呈现两端渐进式分布和中段带状分布特征,对应洋脊的三期演化历史.SWIR的地幔源区极不均一,尤其是中新元古代造山带根部集中拆离的中段.源区地幔的不均一性与大陆裂解和洋脊演化过程密切相关.SWIR的东端与西北印度洋中脊和东南印度洋中脊的邻近洋脊段具有地球化学亲缘性,西端与大西洋中脊和南美洲—南极洲洋中脊的邻近洋脊段具有地球化学亲缘性,这与SWIR的渐近式扩张有关.SWIR周边海底高原普遍具有较大的地壳厚度,其成因除了陆壳基底之外,可能与热点火山作用、热点-洋脊相互作用或热点-三联点相互作用有关,目前尚未形成统一的认识.SWIR的形成演化及其作用域内的熔融异常（如海底高原）是冈瓦纳大陆裂解、残留岩石圈地幔、软流圈地幔和深部地幔热柱物质共同作用的结果.了解SWIR的演化过程对揭示冈瓦纳大陆的裂解过程和印度洋的演化具有重要意义.     

滇东Pt-Pd-Cu含矿建造地球化学特征及其含矿性分析

滇东Pt-Pd-Cu含矿建造包括黑色页岩建造和玄武岩建造,黑色页岩建造由不同层位的黑色岩系组成.作者重点对元古宙昆阳群、下寒武统黑色岩系和二叠纪峨眉山玄武岩的地球化学特征进行了系统研究.研究表明昆阳群黑色岩系以As、B(K≥5)显著富集,Sb、Pd、Mo、Ag、U、Pt、V、Zn、W、Cu(K≥1.2)富集为特征;滇东下寒武统黑色岩系以Mo、Ag、U、As、V、B、Pd、Sb(K≥5)显著富集,Pt、Pb、Zn、W、Cu、Ni(K≥1.2)富集为特征;峨眉山玄武岩则以Pt、Pd(K≥5)显著富集,Au、Ag、Cu、Zn、Sb、B、U、V(K≥1.2)富集为特征.总之,分布于研究区不同层位的黑色岩系和二叠纪峨眉山玄武岩建造构成了该区贵金属(Pt、Pd、Ag、Au等)和有色金属(Cu、Mo、V等)元素成矿的初始矿源层,上述区域成矿要素最佳匹配地段是可能产出非传统贵金属(pt、Pd、Ag、Au等)和有色金属(Cu、Mo、V等)矿床的潜在地段.     

Thermal equation of state of majorite with MORB composition

In situ synchrotron X-ray diffraction experiments were conducted using the SPEED-1500 multi-anvil press at SPring-8 on majoritic garnet synthesized from natural mid-ocean ridge basalt (MORB), whose chemical composition is close to the average of oceanic crust, at 19 GPa and 2200 K. Pressure-volume-temperature data were collected using a newly developed high-pressure cell assembly to 21 GPa and 1273 K. Data were fit to the high-temperature Birch-Murnaghan equation of state, with fixed values for the ambient cell volume (V₀ = 1574.14(4) Å³) and the pressure derivative of the isothermal bulk modulus (K^′T = 4). This yielded an isothermal bulk modulus of K_T0 = 173(1) GPa, a temperature derivative of the bulk modulus (∂K_T/∂T)_P = −0.022(5) GPa K⁻¹, and a volumetric coefficient of thermal expansivity α = a + bT with values of a = 2.0(3) × 10⁻⁵ K⁻¹ and b = 1.0(5) × 10⁻⁸ K⁻². The derived thermoelastic parameters are very similar to those of pyrope. The density of subducted oceanic crust compared to pyrolitic mantle at the conditions in Earth's transition zone (410-660 km depth) was calculated using these results and previously reported thermoelastic parameters for MORB and pyrolite mineral assembledges. These calculations show that oceanic crust is denser than pyrolitic mantle throughout the mantle transition zone along a normal geotherm, and the density difference is insensitive to temperature at the pressures in lower part of the transition zone.     

泥堡金矿与水银洞金矿地质特征初步对比

泥堡金矿及水银洞金矿是大型金矿床，两矿床赋矿地层、容矿岩石、矿体产出特征、金的赋存状态等诸多特征相似；并处于富金的峨眉山玄武岩东侧，其矿床成因与该玄武岩有密切关系。应加强从泥堡-戈塘-紫木凼-水银洞一线地层中P3l 的研究，指导金矿找矿及勘查工作。     

Volcaniclastic deposits from the North Arch volcanic field, Hawaii: explosive fragmentation of alkalic lava at abyssal depths

Submarine explosive eruptions are generally considered to become less likely with increasing depth due to the increasing hydrostatic pressure of the overlying water column. Volcaniclastic deposits from the North Arch volcanic field, north of Oahu, have textural characteristics of explosive fragmentation yet were erupted in water depths greater than 4,200 m. The most abundant volcaniclastic samples from North Arch are clast-supported with highly vesicular, angular pyroclasts. They are most likely near-vent pyroclastic fall deposits formed in eruption columns of limited height. Interbedded with highly vesicular pillow lava, they form low (50 to 200 m), steep-sided cones around the vents. Less common are stratified samples with graded bedding; one such sample includes a layer of roughly aligned, platy, bubble-wall glass fragments (resembling littoral limu o Pele) that may have been deposited by density currents. In addition to bubble-wall glass shards, numerous glass fragments with spherical, delicate spindle and ribbon shapes, and Pele's hair-like glass strands occur in the finer size fraction (<0.5 mm) of some samples. They are probably more distal fallout. Another sample, consisting of glass fragments dispersed in a marine clay matrix, was apparently reworked and deposited farther from the vents by bottom currents. Glass compositions include low-(∼0.4-0.6 wt%) and medium-K₂O (>0.6 wt%) alkalic basalt, basanite, and nephelinite. Sulfur and chlorine abundances are high, reaching a maximum of 1,800 and 1,300 ppm, respectively. The ubiquitous presence of limu o Pele fragments, regardless of glass composition, suggests that bursts of Strombolian-like activity accompanied most eruptions. Coalescing vesicles observed in larger pyroclasts and some pillow lava suggests accumulation of volatiles. Since the great hydrostatic pressure makes steam expansion impossible, a volatile-rich, supercritical magmatic fluid probably drove the eruptions. If these volatile-rich magmas had erupted in shallow water or subaerially, tall fountains would most likely have resulted. The great hydrostatic pressure (>40 MPa) limited fountain and eruption column heights.     

The emplacement of pahoehoe lavas on Kilauea and in the Deccan Traps

There is a growing interest in deciphering the emplacement and environmental impact of flood basalt provinces such as the Deccan, India. Observations of active volcanism lead to meaningful interpretations of now-extinct volcanic systems. Here, I illustrate and discuss the morphology and emplacement of the modern and active lava flows of Kilauea volcano in Hawaii, and based on them, interpret the compound pahoehoe lavas of the Deccan Traps. The latter are vastly larger (areally extensive and voluminous) than Kilauea flows, and yet, their internal architecture is the same as that of Kilauea flows, and even the sizes of individual flow units often identical. Many or most compound flows of the Deccan Traps were emplaced in a gentle, effusive, Kilauea-like fashion. Bulk eruption rates for the Deccan province are unknown, and were probably high, but the local eruption rates of the compound flows were no larger than Kilauea’s. Large (≥ 1000 km³) individual compound pahoehoe flows in the Deccan could have been emplaced at Kilauea-like local eruption rates (1 m³/sec per metre length of fissure) in a decade or less, given fissures of sufficient length (tens of kilometres), now exposed as dyke swarms in the province.     

An index for describing the core-stone shape in weathered columnar joints

The shape of core-stones remained in the weathering basalt column is an index to classify its weathered degree. In this paper, a roundness index (RI) like the box dimension using in fractal theory, was employed to quantitatively measure the shape of core-stone in various weathering grades. The rounded core-stone that is weathered seriously has a smaller RI value. According to the value of RI, the five classes shown in the Russell–Taylor–Pettijohn chart for visually classifying grain shape is available to approximately determine the weathered grades of core-stones remained in the basalt column in Taiwan. A slump test is preliminarily tested to show the difference in repose angle of a stability slope assembled by the grains in a similar shape. The collection of angular core-stones those are less weathered (with a higher RI value) indeed reveals a greater repose angle.     

Cotectic Proportions of Olivine and Spinel in Olivine-Tholeiitic Basalt and Evaluation of Pre-Eruptive Processes

The volume %, distribution, texture and composition of coexistingolivine, Cr-spinel and glass has been determined in quenchedlava samples from Hawaii, Iceland and mid-oceanic ridges. Thevolume ratio of olivine to spinel varies from 60 to 2800 andsamples with >0·02% spinel have a volume ratio ofolivine to spinel of approximately 100. A plot of wt % MgO vsppm Cr for natural and experimental basaltic glasses suggeststhat the general trend of the glasses can be explained by thecrystallization of a cotectic ratio of olivine to spinel ofabout 100. One group of samples has an olivine to spinel ratioof approximately 100, with skeletal olivine phenocrysts andsmall (<50 µm) spinel crystals that tend to be spatiallyassociated with the olivine phenocrysts. The large number ofspinel crystals included within olivine phenocrysts is thoughtto be due to skeletal olivine phenocrysts coming into physicalcontact with spinel by synneusis during the chaotic conditionsof ascent and extrusion. A second group of samples tend to havelarge olivine phenocrysts relatively free of included spinel,a few large (>100 µm) spinel crystals that show evidenceof two stages of growth, and a volume ratio of olivine to spinelof 100 to well over 1000. The olivine and spinel in this grouphave crystallized more slowly with little physical interaction,and show evidence that they have accumulated in a magma chamber. KEY WORDS: olivine; spinel; basalt glass; volume %; cotectic     

论玄武岩金矿的地质特征

滇黔桂金三角区内一些产于峨眉山玄武岩组(含底部大厂层)和龙潭组内的金矿,其含矿岩性主要是玄武熔岩、凝灰岩和含凝灰质、碳质的粉砂岩、页岩．一些学者将它们都归入“卡林型金矿”．但因为地幔柱成因的玄武岩,自有其独立的成矿系列,包含Cu、Fe、Mn、Au、Pt、Pd、Co、Pb、Zn、Sb、CaF2、FeS2等多种矿产,在不同地区,其矿化各有侧重．可以看到金的矿集区都在铜的矿集区之南,且以玄武岩喷溢早期(大厂层)更富金,应命名为玄武岩金矿．晚期(龙潭组)更富铜为特色(玄武岩铜矿)．在共同具备含金(P2β2-1)、含铜(P21)有利地层与有利构造区内,发现卡林型金矿后,应着重探索玄武岩金矿,反之亦然．     

Volcanoes of the southwestern extension of the active Mariana island arc: New swath-mapping and geochemical studies

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at ∼ 14°N immediately west of Guam. Sea floor mapping in 1995 shows a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequence of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate margin are becoming increasingly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of arc volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal arc volcanoes and tend to form along lineaments. Compositions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subducts nearly orthogonal to the strike of the trench along the southern part of the Mariana system and the distance to the arc line from the trench axis is only ∼ 150 km. Several deep fault-controlled canyons on the inner slope of the southern Mariana trench indicate an enhanced tectonic extension of this plate margin. The presence of these active arc volcanoes and the existence of the orthogonal normal faulting along the southern Mariana forearc supports a model of radial extension for formation of the Mariana Trough, a model previously dismissed because of the lack of evidence of these two major geological features.