新疆西天山备战铁矿成矿作用初探

该文在野外地质调查基础上,对备战铁矿区内的火山岩和矿石进行了详细的矿物学和岩石地球化学研究,并深入讨论了矿床成因。研究表明,该区的主要赋矿围岩为火山岛弧玄武岩,其与磁铁矿有着类似的稀土元素配分模式,说明二者有成因上的联系。成矿物质可能来源于北天山洋块向伊犁板块之下俯冲过程中软流圈上涌形成的富铁玄武质岩浆。受洋块俯冲作用影响,富铁玄武质岩浆沿北天山压扭性深大断裂底侵,并经历了一定的结晶分离或同化混染作用,最后在中地壳形成一套演化的玄武质（安山质）岩浆,并在岩浆晚期阶段发生富铁岩浆和硅酸盐岩浆的分离,此时,火山岩中的普通辉石受岩浆晚期的热液作用影响蚀变为透辉石,发生了Si,Ca,Mg的富集和Fe,Al,Ti的缺失,一定程度上促进了含铁矿浆的富集。     

Petrogenesis of Middle-Eocene granitoids and their Mafic microgranular enclaves in central Urmia-Dokhtar Magmatic Arc (Iran): Evidence for interaction between felsic and mafic magmas

Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0 ± 0.7 Ma for the granodiorite phase. Mafic microgranular enclaves(MMEs) are common in these plutons, and have relatively low SiO_2 contents(53.04-57.08 wt.%) and high Mg#(42.6-60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous(A/CNK = 0.69-1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO_2 contents(59.7-66.77 wt.%) and lower Mg#(38.6-52.2); they are considered a product of partial melting of the mafic lower crust. Chondritenormalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies(Eu/Eu* = 0.60-0.93). The host granodiorite samples yield(⁸⁷Sr/⁸⁶Sr);ratios ranging from 0.70498 to 0.70591,positive eNd(t) values varying from +0.21 to +2.3, and TDM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves(⁸⁷Sr/⁸⁶Sr)i = 0.705111-0.705113, ε_Nd(t)= +2.14 to +2.16, T_DM2 = 697-785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantlederived mafic magmas in an active continental margin arc environment.     

班公湖-怒江缝合带西段阿翁错复式岩体的岩浆混合成因:地球化学、年代学和暗色微粒包体证据

阿翁错复式岩体位于班公湖-怒江缝合带西段,是班公湖-怒江特提斯洋俯冲消减,造山过程中岩浆响应的重要组成部分,以广泛发育暗色微粒包体和岩浆混合、不协调现象为特征。本文以阿翁错复式岩体为研究对象,对寄主岩和暗色微粒包体开展了系统的地质学、地球化学和锆石U-Pb年代学研究,探讨了阿翁错复式岩体的岩浆混合成因。暗色微粒包体塑性变形特征明显,与寄主岩呈截然或渐变接触,偶见反向脉发育,包体具细-中粗粒结构,含斜长石、钾长石、角闪石、暗色镶边石英等斑晶,偶见角闪石斑晶横跨包体和寄主岩,在包体及包体周围寄主岩中见长柱状斜长石、角闪石和针状磷灰石等结构特征,表明暗色微粒包体为岩浆混合作用的产物。寄主岩与包体均为准铝质、钙碱性-高钾钙碱性系列岩石,主要氧化物含量在Harker图解上具有良好的线性关系,稀土元素配分曲线图和微量元素蛛网图具有高度一致性,表明二者具有强烈的地球化学亲源关系,且经历了相似的岩浆演化过程。寄主岩和暗色微粒包体的成岩年龄分别为109. 1±1. 0Ma和107. 4±0. 7Ma,岩浆混合作用发生在早白垩世晚期,处于班公湖-怒江特提斯洋由弧-陆碰撞向陆陆碰撞的转换阶段即软碰撞阶段。研究表明,在班公湖-怒江特提斯洋向北向羌塘地块之下俯冲的背景下,洋壳脱水,引起上覆地幔楔发生部分熔融,形成镁铁质岩浆,镁铁质岩浆向上运移,并底侵于壳-幔边界,引发下地壳物质发生部分熔融,形成长英质岩浆,当镁铁质岩浆从底部注入长英质岩浆房时,镁铁质岩浆快速冷凝,形成部分色率高、粒度细,具冷凝边的包体,与寄主岩呈截然型接触,随着端元岩浆之间的温差逐渐降低,包体色率降低,粒度变大,与寄主岩呈渐变过渡。     

Petrology and geochemistry of South Mid-Atlantic Ridge (19°S) lava flows: Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge (SMAR) 19°S segment, approximately located along the line of Saint Helena volcanic chain (created by Saint Helena mantle plume), is an ideal place to investigate the issue whether the ridge-hotpot interaction process affected the whole MAR. In this study, we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment. Major oxides compositions show that all samples are tholeiite. Low contents of compatible trace elements (e.g., Ni ​= ​239–594 ​ppm and Cr ​= ​456–1010 ​ppm) and low Fe/Mn (54–67) and Ce/Yb (0.65–1.5) ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source. Using software PRIMELT3, this study obtained mantle potential temperatures (T_p) beneath the segment of 1321–1348 ​°C, which is lower relative to those ridges influenced by mantle plumes. The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of ~63 ​km and ceases melting at ~43 ​km with a final melting temperature of ~1265 ​°C. The extent of partial melting is up to 16%–17.6% with an average adiabatic decompression value of 2.6%/kbar. The correlations of major oxides (CaO/Al₂O₃) and trace elements (Cr, Co, V) with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.⁸⁷Sr/⁸⁶Sr (0.702398–0.702996), ¹⁴³Nd/¹⁴⁴Nd (0.513017–0.513177) and ²⁰⁶Pb/²⁰⁴Pb (18.444–19.477) ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle, PREMA mantle, and HIMU mantle materials. The simple, binary mixing results among components from plume-free SMAR MORB, Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials. The abovementioned mantle potential temperatures, together with the low Saint Helena (<10%) and Tristan (<5%) components remaining in the asthenospheric mantle at present, show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased. However, the trace element and Sr-Nd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.     

Textural and chemical evolution of a fractionated granitic system: the Podlesí stock, Czech Republic

The Podlesí granite stock (Czech Republic) is a fractionated, peraluminous, F-, Li- and P-rich, and Sn, W, Nb, Ta-bearing rare-metal granite system. Its magmatic evolution involved processes typical of intrusions related to porphyry type deposits (explosive breccia, comb layers), rare-metal granites (stockscheider), and rare metal pegmatites (extreme F–P–Li enrichment, Nb–Ta–Sn minerals, layering). Geological, textural and mineralogical data suggest that the Podlesí granites evolved from fractionated granitic melt progressively enriched in H₂O, F, P, Li, etc. Quartz, K-feldspar, Fe–Li mica and topaz bear evidence of multistage crystallization that alternated with episodes of resorption. Changes in chemical composition between individual crystal zones and/or populations provide evidence of chemical evolution of the melt. Variations in rock textures mirror changes in the pressure and temperature conditions of crystallization. Equilibrium crystallization was interrupted several times by opening of the system and the consequent adiabatic decrease of pressure and temperature resulted in episodes of nonequilibrium crystallization. The Podlesí granites demonstrate that adiabatic fluctuation of pressure (“swinging eutectic”) and boundary-layer crystallization of undercooled melt can explain magmatic layering and unidirectional solidification textures (USTs) in highly fractionated granites.     

河南桐柏老湾花岗岩岩浆动力学与成矿

基于岩浆岩岩石学、流体动力学、热力学研究。本文计算了河南桐伯老湾花岗岩岩浆过程的上升速度、冷凝速度及岩浆熔体的密度、粘度、含水量等物理参数，探讨了熔体中晶体的成核密度和生长速度以及岩浆对流形式等动力学行为，并分析了它们与成矿作用的联系。研究表明，老湾花岗岩岩浆含水量为4．76％，在侵位的温度和压力下是饱和的，较高的水含量有利于矿化。老湾花岗岩熔体上升较快而冷却缓慢，晶体成核密度和生长速度较低，以挥发分为迁移形式的成分对流是熔体中成矿物质迁移、富集的主要方式。老湾花岗岩特殊的岩浆物理性质和动力学行为指示其岩浆作用与老湾金矿床的形成具有密切的成因联系。     

滇西北独龙江花岗岩中浆混包体特征

班公湖-东巧-怒江结合带(南段)西侧的独龙江花岗岩可划分为2个序列。早期序列(龙岗不若序列)富含混杂包体和混成包体。前者包括(石英)辉长苏长岩、辉长岩及辉长辉绿(玢)岩;后者由闪长岩、石英闪长玢岩、安山岩、安山玄武岩组成。从地球化学特征看,这些岩石由同源岩浆分异,并与生成寄主岩石的酸性岩浆混合而成。地球化学图解判别,浆混包体属钙碱性玄武岩系列,形成于岛弧环境,与寄主岩石———龙岗不若序列的生成环境一致。该类包体的存在,进一步证明了独龙江花岗岩早期序列(龙岗不若序列)由壳幔混源岩浆形成。     

中国东南部P波速度结构与构造分析

利用中国东南部地震台站的波形数据,通过远震P波层析成像方法反演了福建及台湾地区的上地幔P波速度结构,据此分析了华夏块体的构造属性并探讨了中国东南部上地幔的深部动力学机制.结果表明,研究区上地幔速度结构存在明显的横向非均匀性,它们与区域构造的深部动力学成因密切相关.宏观上,NE向展布的低速异常与东南沿海地区中新生代火成岩的条带状分布保持了较好的一致性,推测其上地幔可能存在幔源物质上涌;以政和—大埔断裂带为界,东、西华夏块体的速度结构存在差异,结合前人的研究结果分析,东、西华夏块体岩石圈减薄和中新生代岩浆活动的驱动机制有所不同,西华夏块体为"板块俯冲+岩浆底侵(岩石圈拆沉)"的动力学模式,东华夏块体是"玄武质岩浆迁移+岩浆底侵"的动力学模式.     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

新疆库鲁克塔格地区兴地Ⅱ号镁铁-超镁铁质岩体的地球化学特征与岩石成因

兴地Ⅱ号镁铁-超镁铁质岩体位于塔里木盆地东北缘的库鲁克塔格地区,形态为不规则的漏斗状岩盆,出露面积约10km²。岩石类型主要有方辉橄榄岩、二辉橄榄岩、(含长)单辉橄榄岩、橄榄辉石岩、方辉辉石岩、二辉辉石岩、单辉辉石岩、橄榄辉长苏长岩、辉长苏长岩、橄榄辉长岩、辉长岩等,岩石类型属拉斑玄武岩系列。岩体的稀土元素含量低(∑REE=10.73×10^-6~77.44×10^-6),富集轻稀土元素(La/Yb)_N=4.06~7.73和大离子亲石元素(Cs、Rb、Sr、Ba),亏损高场强元素尤其是Nb、Ta、Ti、P等,δEu为0.90~2.46多数岩石显示出正铕异常。岩相学及地球化学特征表明,岩浆运移过程中主要发生了橄榄石、斜方辉石、单斜辉石的分离结晶作用以及斜长石的聚集/堆晶。原生岩浆为富镁拉斑玄武岩,岩浆源区为富集型岩石圈地幔,岩浆上升侵位过程中局部遭受了陆壳的同化混染。兴地Ⅱ号岩体与金川岩体具有很多的相似之处,而且兴地Ⅱ号岩体已见有铜镍矿化。因此,兴地Ⅱ号岩体具有很好的找矿前景。