兰坪盆地大龙现代热泉锌多金属喷流-沉积成矿

大龙现代热泉是沿沘江断裂带分布的众多温泉之一,其喷流-沉积成矿作用较为直观地展示了热泉堆积泉华成矿过程,佐证了新生代在兰坪盆地热泉成矿的可能性和普遍性。热泉"热循环"作用形成三种成矿模式:喷出贯入陷沟和湖泊形成致密块状、角砾状矿体——兰坪"金顶式"Pb-Zn矿床;于构造带中沿一系列构造界面形成脉状矿体——兰坪"白秧坪式"Ag-Pb-Zn多金属矿床;喷流的地表沟谷形成热泉型——云龙"大龙式"Ag-Pb-Zn多金属矿床(低品位)。     

辽东大石桥组蛇纹石化大理岩中铀矿化特征及形成时代

在辽东大石桥组蛇纹石化大理岩中新发现晶质铀矿矿化现象。晶质铀矿呈角砾状发育在蛇纹石化白云石大理岩中,并叠加有辉钼矿、黄铁矿等矿化。U-Pb同位素年龄测定显示,晶质铀矿形成于1763~1794Ma。EPMA U-Th-Pb化学年龄显示,晶质铀矿形成后经历了1512±20Ma的热事件改造,对应一次岩浆侵入事件。辽东地区经历了古元古代的裂谷拉张、碰撞造山、造山后伸展等重大地质事件。大石桥组中蛇纹石化大理岩中的铀矿化,以及连山关铀矿床、翁泉沟地区铁-硼-铀矿床的热液铀成矿作用均形成于古元古代晚期造山后/非造山区域伸展环境,可能与区域伸展体制下的地幔柱活动有关。     

试论大火成岩省与成矿作用

根据组成大火成岩省的岩浆类型不同,大火成岩省可以分为两类,一是以基性火成岩为主的镁铁质大火成岩省（MLIPs）,二是以酸性火成岩为主的长英质大火成岩省（SLIPs）。它们都是由于在异常高的地幔热流参与下导致地幔或地壳大规模熔融形成的。大火成岩省独特的巨量岩浆活动是引起多层次物质和能量交换的重要场 所。成矿物质的聚集导致成矿作用和矿床的形成是必然的,因此大火成岩省本身就是一个大成矿系统。在这个成矿系统中,由于物源、成分、温度、压力、流体和氧逸度等条件的差异性,形成不同种类的矿化和矿床,并构成一定的成矿系列。镁铁质大火成岩省中形成的矿床类型有岩浆硫化物型Cr-Cu-Ni-PGE矿床和Ti-Fe 氧化物型V-Ti-Fe 矿床,热液型的Cu-Pb-Zn-Au-Ag矿床,以及远程低温热液矿床等。长英质大火成岩省形成的矿床类型为岩浆和交代型、热液型Cu-Pb-Zn-Au-Ag,W-Sn,U-Th-REE矿床,以及Sb-As矿床等。加强对大火成岩省及其成矿机理的研究,有望形成新的成矿理论和加速超大型矿床的发现。     

云南丽江苦橄岩Re-Os同位素地球化学初步研究

报导了云南丽江地区大具和仕满剖面12个苦橄岩和6个玄武岩的Re，Os含量和Os同位素组成。苦橄岩和玄武岩具有明显不同的Re-Os体系的特征。苦橄岩具有高的Os元素丰度［(1.5~3)×10-9］和低的Re元素丰度（<0.05×10-9）；共生的玄武岩具有低的Os元素丰度（<0.5×10-9）和相对高的Re元素丰度（<0.8×10-9）；苦橄岩具有低放射成因的 187Os/188Os 比值（0.123 3~0.126 6），而玄武岩具有高放射成因的187Os/188Os比值（0.133 8~0.157 7）。苦橄岩的Re-Os同位素特征与越南西北部二叠—三叠纪科马提岩具有低放射成因Os同位素特征相似，而玄武岩的Re-Os同位素特征与峨眉山大火成岩省（LIP）其他地区玄武岩的高放射成因的Os同位素特征相似。苦橄岩的Re-Os同位素特征表明，形成峨眉山LIP的地幔柱可能来自对流上地幔而不是深部的核-幔界面。换言之，峨眉山LIP的形成受控于岩石圈地幔过程而不是地幔柱过程。     

峨眉山玄武岩浆喷发对贵州西部区域成矿贡献研究

早、晚二叠世之间大规模峨眉山玄武岩喷发是峨眉地幔热柱基性岩浆活动的高峰期.峨眉山玄武岩在贵州西部地区广泛分布,它不仅以矿源层形式参与钼、铜、铅、锌、金、锑、汞、铊等以地下水热液成矿作用为主的层控矿床的形成,而且它自身形成火山气液型矿床--玄武岩铜矿及伴生热液型铂、钯等矿化点;再者它是外生矿床--高砷煤、高氟煤、高硫煤、高汞煤形成的主要原因.     

Crustal accretion beneath the Koyna coastal region (India) and Late Cretaceous geodynamics

In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Geochronology,geochemistry, Hf isotopic compositions and formation mechanism of radial mafic dikes in northern Tibet

A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.     

Petrogenesis of the Piqiang mafic-ultramafic layered intrusion and associated Fe-Ti-V oxide deposit in Tarim Large Igneous Province,NW China

ABSTRACT

The Piqiang intrusion is one of the two important ma?c-ultrama?c layered intrusions that host giant Fe-Ti-V oxide deposits in the Permian Tarim Large Igneous Province, NW China. The intrusion mainly consists of gabbro, anorthosite and minor plagioclase-bearing clinopyroxenite in the marginal zone. Disseminated to massive Fe-Ti oxide ores occur as layers and lenses within the gabbro. SHRIMP zircon U-Pb results from both a gabbro from the Piqiang intrusion and a granite from the surrounding granitic dyke yield ages of ~270 Ma. Geochemically, the Piqiang silicate rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high ?eld strength elements (HFSE), and have a limited range of Sr-Nd-Hf isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the layered units suggest that all the rocks are co-magmatic. The parental magma is Fe-Ti-rich and is akin to the most primitive diabasic dyke which is associated with the Piqiang intrusion. Partial melting of the Tarim mantle plume with involvement of a subduction-metasomatized lithospheric mantle source best explains the geochemistry and petrogenesis of the parental magmas of the Piqiang intrusion. We propose that the lithospheric mantle source may have been metasomatized by subduction-related materials and the metasomatic enrichment of this source region which may be correlated with oceanic sediment recycling during southward subduction of the South Tianshan oceanic slab during the Early-Middle Paleozoic. Crystal settling and mechanical sorting is the predominant process responsible for the formation of the massive Fe-Ti oxide ores in the Piqiang intrusion. Central to ore formation is a combination of the protracted differentiation history of a Fe-Ti-enriched parental magma and the later addition of external H₂O from the country rocks to the slowly cooling magma chamber.     

Fates of dissolved and particulate materials from the Mississippi river immediately after discharge into the northern Gulf of Mexico,USA, during a period of low wind stress

In June 2003, we conducted a two-part field exercise to examine biogeochemical characteristics of water in the lower Mississippi river during the 4 days prior to discharge and in the Mississippi river plume over 2 days after discharge. Here we describe the fates of materials immediately after their discharge through Southwest Pass of the Mississippi delta into the northern Gulf of Mexico. Changes in surface water properties immediately after discharge were much larger and more rapid than changes prior to discharge. Total suspended matter (TSM) declined, probably due to sinking, dissolved macronutrients were rapidly diminished by mixing and biological uptake, and phytoplankton populations increased dramatically, and then declined. This decline appeared to begin at salinities of approximately 10 and was nearly complete by 15. A large increase in dissolved organic carbon (DOC) occurred over approximately the same salinity range. Weak winds (<2 m s⁻¹) during and preceding this cruise apparently led to the formation of an extensive but thin freshwater lens from the river. This lens spread widely without much mixing, and the bloom of phytoplankton that occurred between discharge and a salinity of 10 was probably a freshwater community seeded from the lower river. Phytoplankton bloomed for a period of about 1–2 days, then declined dramatically, apparently releasing large amounts of DOC. Macronutrients from the river were utilized by the river phytoplankton community in the extensive freshwater lens. This contrasted with the more typical situation in which river nutrients stimulate a marine phytoplankton bloom at salinities in the mid-20s. We concluded that the direct effects of dissolved and particulate bio-reactive materials discharged by the Mississippi river were spatially restricted at this time to low-salinity water, at least as surface phenomena. After being transported through the lower river essentially unaltered, these materials were biogeochemically processed within days and tens of km. More generally, the mixing rate of plume water with receiving oceanic water has profound effects on the food web structure and biogeochemical cycling in the plume.