陇县904地区混合岩型铀矿成矿特征的讨论

含铀混合岩大部分位于古陆隆起带边部、ＮＷ向花岗岩外接触带。深大断裂系统在成矿成岩中占有相当重要的地位。燕山期构造和岩浆活化，对原有的构造和矿化进行了强烈改造。在混合岩中发现了两种主要成矿类型，即铀一赤铁矿型和铀一碳酸盐型。     

衡山西缘界牌混合岩带若干问题探讨

位于衡山西缘的界牌混合岩带是一个由早期的眼球状混合岩和晚期的条痕混合岩组成的复式岩带,其原岩为高变质相的韧性变形变质岩。根据其产出的地质环境及其中丰富的同混合和晚混合变形组构,界牌混合岩属断裂混合岩,不应划归白石峰岩体或南岳岩体。它们之间的紧密共生构成了“韧性变形变质岩—断裂混合岩—重熔花岗岩”三位一体组合,代表了断裂从韧性变形变质→混合岩化→重熔的递进变形变质序列。     

东喜马拉雅构造结多雄拉混合岩和花岗片麻岩锆石U-Pb年龄及其地质意义

位于东喜马拉雅构造结的南迦巴瓦岩群是研究喜马拉雅构造带基底演化的重要对象之一.在构造样式上,南迦巴瓦岩群为一个背形构造,该背形构造的核部由多雄拉混合岩和花岗片麻岩组成.本文开展了南迦巴瓦岩群多雄拉混合岩的锆石LA-ICP-MS U-Pb定年研究.结果表明.多雄拉混合岩深色体的原岩形成年龄为1759±10Ma,浅色体的形成年龄为1594±13Ma,代表发生混合岩化的年龄.另外,一个多雄拉花岗片麻岩的原岩形成年龄为1583±6Ma,该年龄在误差范围内与区域发生混合岩化作用的时代相近,表明在混合岩化过程中存在着一定程度的地壳深熔作用.区域对比表明,低喜马拉雅和高喜马拉雅构造单元内存在着明显不同的构造一岩浆事件,其中低喜马拉雅构造单元广泛存在1.6～1.8Ga的构造-岩浆事件.与之相对比,多雄拉混合岩和花岗片麻岩的锆石U-Pb年代学说明南迦巴瓦岩群核部应属于低喜马拉雅结晶岩系,而明显不同于高喜马拉雅结晶岩系,这与西喜马拉雅构造结相似,表明东喜马拉雅构造结与西喜马拉雅构造结有着相似的地质演化.     

江西武功山热穹隆变质--岩浆作用及演化

处于扬子与华夏古板块加里东期拼贴带的武功山热穹隆,以造山期变形花岗岩为中心,南华一寒武系遭受绿片岩相一低角闪岩相变质并形成变质矿物分带,环绕岩体分布,与中生代以来伸展形成的脆性断裂系统构成外环,共同组合形成屹立于晚古生代地层分布区的穹隆状构造。在区域递增变质带基础上,造山早期,深部初始重熔岩浆与深埋的变质岩发生交代形成原地型英云闪长质“混染岩浆”,岩浆上浸过程中,活动组分不断作用于围岩,致使围岩的成分、结构构造产生重新调整,形成岩体边缘混合岩。同时,由于硅铝质围岩组分加入,酸度增加,而渐变过渡为花岗闪长质岩浆。造山晚期,进一步演化为岩浆型正常花岗岩,并呈套叠状侵入于早期岩体之中。从早到晚构成同源岩浆演化系列,并发育明显构造应力变形。热穹隆区加里东期形成递进渐增变质一岩体边缘混合一变形花岗岩“三位一体”分布格局。中生代伸展一岩浆作用的复合叠加,进一步强化了热穹隆的构造背景。大致在旱白垩世末基本定型。     

Zircon U–Pb ages and Hf isotope compositions of migmatite from the North Dabie terrane in China: constraints on partial melting

Migmatite gneisses are widespread in the Dabie orogen, but their formation ages are poorly constrained. Eight samples of migmatite, including leucosome, melanosome, and banded gneiss, were selected for U–Pb dating and Hf isotope analysis. Most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains. Morphological and internal structure features suggest that their growth is associated with partial melting. According to the Hf isotope ratio relationships between metamorphic zircon and inherited cores, three formation mechanisms for metamorphic zircon can be determined, which are dissolution–reprecipitation of pre‐existing zircon, breakdown of Zr‐bearing phase other than zircon in a closed system and crystallization from externally derived Zr‐bearing melt. Four samples contain magmatic zircon cores, yielding upper intercept U–Pb ages of 807 ± 35–768 ± 12 Ma suggesting that the protoliths of the migmatites are Neoproterozoic in age. The migmatite zircon yields weighted mean two‐stage Hf model ages of 2513 ± 97–894 ± 54 Ma, indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation. The metamorphic zircons give U–Pb ages of 145 ± 2–120 ± 2 Ma. The oldest age indicates that partial melting commenced prior to 145 Ma, which also constrains the onset of extensional tectonism in this region to pre‐145 Ma. The youngest age of 120 Ma was obtained from an undeformed granitic vein, indicating that deformation in this area was complete at this time. Two major episodes of partial melting were dated at 139 ± 1 and 123 ± 1Ma. The first episode of partial melting is obviously older than the timing of post‐collision magmatism, corresponding to regional extension. The second episode of partial melting is coeval with the widespread post‐collision magmatism, indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the Dabie orogen, which were possibly triggered by the uprising of the Cretaceous mid‐Pacific superplume.     

云开隆起区热中心式区域混合岩的成因研究

云开隆起区不同混合岩化程度的混合岩在空间多以黑云母花岗岩为中心，呈环带状分布，混合岩的矿物空间分布研究表明，亲成体矿物具有随机分布特点，说明为熔浆结晶成因，结晶温度在５６０－５８０℃，条带状混凳夺的质量平衡研究表明，新成体和古成体的成分具有很好的相关性，递进熔融作用合理解释了这种混合岩的成因及其空间分布规律。     

Low-P–high-T metamorphism and the role of heat transport by melt migration in the Higo Metamorphic Complex, Kyushu, Japan

This paper characterizes the metamorphic thermal structure of the Higo Metamorphic Complex (HMC) and presents the results of a numerical simulation of a geotherm with melt migration and solidification. Reconstruction of the geological and metamorphic structure shows that the HMC initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels. Subsequently, this initial thermal structure has been collapsed by E–W and NNE–SSW trending high‐angle faults. Pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field P–T array that may be divided into two segments: the array at apparent higher structural levels has a low‐dP/dT slope, whereas that at apparent lower structural levels has a high‐dP/dT slope. This composite array cannot be explained by heat conduction in subsolidus rocks alone. Migmatite is exposed pervasively at apparent lower structural levels, but large syn‐metamorphic plutons are absent at the levels exposed in the HMC. Transport and solidification of melt within migmatite is a potential mechanism to generate the composite array. Thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth. The model also predicts that strata beneath the crossing point will consist of high‐grade solid metamorphic rocks and solidified melt products, such as migmatite. This prediction is consistent with the observation that migmatite was associated with the very high‐dP/dT slope. The melt migration model is able to generate the very high‐dP/dT segment due to the high rate of heat transfer by advection.     

Large-scale melt-depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

This study uses field, petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane, and to determine what the grain‐ and outcrop‐scale melt‐flow paths were during the melt segregation process. The Ashuanipi subprovince, located in the north‐eastern Superior Province of Quebec, is a large (90 000 km²) metasedimentary terrane, in which > 85% of the metasediments are of metagreywacke composition, that was metamorphosed at mid‐crustal conditions (820–900 °C and 6–7 kbar) in a late Archean dextral, transpressive orogen. Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase = melt + orthopyroxene + oxides. Using melt/orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting. The metagreywackes are enriched in MgO, CaO and FeO_t and depleted in SiO₂, K₂O, Rb, Cs, and U, have lower Rb/Sr, higher Rb/Cs and Th/U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt %, granitic melt from typical Archean low‐grade metagreywackes. A simple mass balance indicates that about 640 000 km³ of granitic melt was extracted from the depleted granulites. The distribution of relict melt at thin section‐ and outcrop‐scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks pervasive grain boundary flow of melt occurred along the layers for a few, to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits, visible as the net‐like array of leucosomes in some outcrops. The leucosomes contain very little residual material (< 5% biotite + orthopyroxene) indicating that the melt fraction was well separated from the residuum left in situ as melt‐depleted granulite. Only 1–3 vol percentage melt remained in the melt‐depleted granulites, hence, the extraction of melt generated by biotite dehydration melting in these granulites, was virtually complete under conditions of natural melting and strain rates in a contractional orogen.     

混合岩中浅色体的有限迁移及其对变形分解的影响

熔体在地壳和地幔中怎样迁移及其动力学是地质学中的一个重要问题。作为地壳深融作用的产物 ,混合岩中的浅色体提供了一个极好的机会来探讨影响地壳熔体在中下地壳迁移的因素。为此 ,我们对美国加州南 Sierra Nevada岩基中典型的混合岩、变泥质岩及邻近的花岗闪长岩进行了详细的主要元素地球化学和野外构造变形分析 ,同时应用流体动力学理论估算了在中下地壳条件下 ,典型浅色体的迁移距离。南 Sierra Nevada岩基中的混合岩中的浅色体厚度为 1mm至 1cm。在部分熔融程度较高的区域 (>10 % ) ,浅色体相互连接而成网结状构造 ,应变的承载方式主要以 IWL(Interconnected Weak L ayers)形式进行 ,即熔融体表现为弱相而承载大部分的应变。相反地 ,在部分熔融程度较低的区域 (<5 % ) ,浅色体孤立地出现 ,应变的承载方式主要以 L BF(L oad- Bearing Frame-work)形式进行 ,即应变主要由非熔融体的基质来承担。这表明在混合岩形成过程中 ,熔体的出现强烈地制约着应变分解作用。应用 Shaw的岩石粘度模型 ,我们根据浅色体的主要元素地球化学成分计算了浅色体在熔融状态下的粘度。根据流体动力学原理 ,估算了浅色体在不同条件下的迁移距离。计算结果表明 :1和典型花岗岩相比 ,浅色体具有较高的粘度 ,为 10 9～ 10 1 2