Fast switch from extensional exhumation to thrusting of the Ronda Peridotites (South Spain)

The Alboran Domain, situated at the western end of the Mediterranean subduction system, is characterized by the Ronda Peridotites, one of the world's largest exposures of sub‐continental mantle. Using U–Pb (LA‐ICP‐MS) and Ar–Ar dating, we precisely dated two tectonic events associated with the Tertiary exhumation of the Ronda Peridotites. First, shearing along the Crust–Mantle Extensional Shear Zone caused, at ca. 22.5 Ma, mantle exhumation, local partial melting in the deep crust and coeval cooling in the upper crust. Second, the Ronda Peridotites Thrust triggered the final emplacement of the peridotites onto the continental crust at c. 21 Ma, as testified by granitic intrusions in the thrust hangingwall. The tectonic evolution of the western Alboran Domain is therefore characterized by a fast switch from continental lithospheric extension in a backarc setting, with sub‐continental mantle exhumation, to a rift inversion by thrusting driven by shortening of the upper plate.     

Experimental study into the petrogenesis of crystal-rich basaltic to andesitic magmas at Arenal volcano

Arenal volcano is nearly unique among arc volcanoes with its 42 year long (1968–2010) continuous, small-scale activity erupting compositionally monotonous basaltic andesites that also dominate the entire, ~7000 year long, eruptive history. Only mineral zoning records reveal that basaltic andesites are the result of complex, open-system processes deriving minerals from a variety of crystallization environments and including the episodic injections of basalt. The condition of the mafic input as well as the generation of crystal-rich basaltic andesites of the recent, 1968–2010, and earlier eruptions were addressed by an experimental study at 200 MPa, 900–1,050 °C, oxidizing and fluid-saturated conditions with various fluid compositions [H₂O/(H₂O + CO₂) = 0.3–1]. Phase equilibria were determined using a phenocryst-poor (~3 vol%) Arenal-like basalt (50.5?wt% SiO₂) from a nearby scoria cone containing olivine (Fo₉₂), plagioclase (An₈₆), clinopyroxene (Mg# = 82) and magnetite (X_ulvö = 0.13). Experimental melts generally reproduce observed compositional trends among Arenal samples. Small differences between experimental melts and natural rocks can be explained by open-system processes. At low pressure (200 MPa), the mineral assemblage as well as the mineral compositions of the natural basalt were reproduced at 1,000 °C and high water activity. The residual melt at these conditions is basaltic andesitic (55 wt% SiO₂) with 5 wt% H₂O. The evolution to more evolved magmas observed at Arenal occurred under fluid-saturated conditions but variable fluid compositions. At 1,000 °C and 200 MPa, a decrease of water content by approximately 1 wt% induces significant changes of the mineral assemblage from olivine + clinopyroxene + plagioclase (5 wt% H₂O in the melt) to clinopyroxene + plagioclase + orthopyroxene (4 wt% H₂O in the melt). Both assemblages are observed in crystal-rich basalt (15 vol%) and basaltic andesites. Experimental data indicate that the lack of orthopyroxene and the presence of amphibole, also observed in basaltic andesitic tephra units, is due to crystallization at nearly water-saturated conditions and temperatures lower than 950 °C. The enigmatic two compositional groups previously known as low- and high-Al₂O₃ samples at Arenal volcano may be explained by low- and high-pressure crystallization, respectively. Using high-Al as signal of deeper crystallization, first magmas of the 1968–2010 eruption evolved deep in the crust and ascent was relatively fast leaving little time for significant compositional overprint by shallower level crystallization.     

东天山板块缝合区(带)的构造演化与多金属矿床成矿的关系

研究表明位于康古尔断裂和阿齐克库都克断裂之间的东天山板块缝合(区)带经历了4个阶段的演化过程, 即泥盆纪吐哈古大洋形成、早石炭世俯冲造山、晚石炭世碰撞造山以及二叠纪造山期后缝合带的进一步变形.自北而南划分为4个构造带, 共同组成了一个向下收敛的楔状缝合区(带).东天山的矿产严格受构造控制, 相应地从北向南分布有前缘推覆带斑岩型铜矿床→蛇绿混杂岩带铜(镍)硫化物矿床和韧性剪切带型金矿床→叠瓦岩片带矽卡岩型铜多金属矿床→热液型金矿床.这些类型的矿床集结成东天山地区4条大的成矿带: 康古尔断裂以北铜矿带, 金、铜镍硫化物矿床成矿带(北带), 阿齐山-雅满苏铁(铜)、银多金属成矿带(南带)和中天山地块铁、铅锌、银成矿带.      

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO₃ (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce₂O₃. Sulfur is incorporated in apatite via several independent exchange reactions involving (P⁵⁺, Ca²⁺) vs. (S⁶⁺, Si⁴⁺, Na⁺, and Ce³⁺). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO₃ (sulfate-saturated systems). The chlorine partition coefficients (D_Cl^{apatite/glass}) range from 0.4 to 1.3 (average D_Cl^{apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D_F^{apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D_F^{apatite/glass} determined experimentally in felsic rock. D_S^{apatite/glass} decreases from 9.1 to 2.9 with increasing SO₃ in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO₃). D_S^{apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO_{3 apatite} (wt%) = 0.157 * ln SO_{3 glass} (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.     

北准噶尔洪古勒楞蛇绿岩研究的新进展

通过对洪古勒楞东段蛇绿岩的研究，基本搞清了蛇绿者组合特征及其形成时代，首次指出该区在晚震旦世为广阔大洋环境，为研究准噶尔的大地构造演化提供了证据。依据蛇绿岩与围岩的接触关系，指出了其构造侵位时间为奥陶纪末，而与泥盆系则为断层接触。通过Ｓｍ－Ｎｄ等时线法测年结果确定蛇绿岩形成于６２６±２５Ｍａ。从稀土元素、岩石化学和εＮｄ（Ｔ）＝十８．４等综合分析，蛇绿岩来自亏损地幔岩浆，纯属洋中脊产物。     

Crystallization conditions in the Upper Pollara magma chamber, Salina Island, Southern Tyrrhenian Sea

Summary Pyroclastites erupted from the Upper Pollara magma chamber (13 ka, Salina Island, Aeolian Archipelago) resulted from mingling and mixing of rhyolitic and andesitic magmas. An experimental study has been conducted on the rhyolitic end-member to constrain the pre-eruptive conditions of the magma. In order to check for the role of mixing on the equilibrium phase assemblage, three different starting compositions, corresponding to three different mixing degrees, have been used. The crystallization experiments were conducted at two different oxygen fugacities and at variable temperature and fluid contents. The results indicate that the natural mineralogical assemblage can only be reproduced from a composition showing a certain degree of mixing. Assuming a pressure of 200 MPa (generally accepted for the Aeolian Islands), the pre-eruptive temperature of the magmas is estimated between 755 and 800 °C and the water content of the melt was higher than 4–4.5 wt.%. The Upper Pollara magma crystallized at relatively high fO₂ (ΔlogfO₂ = Ni–NiO + 1 log unit), compared to rhyolitic magmas from Lipari and Vulcano. As this difference has not been observed for the most primitive magmas the difference in fO₂ could be related to different degassing processes operating in Salina and Lipari – Vulcano magmas.     

Lamprophyre or Lamproite Dyke in the SW Tarim Block?--Discussion on the Petrogenesis of These Rocks and Their Source Region

The mineralogical, petrological and geochemical studies on Keliyang (克里阳) potassic dykes have been carried out to understand their rock types, the petrogenesis and the nature of their mantle sources. They are potassic lamprophyre, not lamproites as the previous researchers believed. In this study, the whole-rock major and trace element compositions of another 6 lamproite dykes recently discovered are reported. Major elements were determined by X-ray fluorescence spectrometry (XRF) techniques, while REE and trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS). They can be classified into phlogopite-diopside lamprophyre, leucite-diopside lamprophyre and granular carbonatite-bearing diopside lamprophyre on the basis of their mineral components. They are all characterized by relatively low SiO2 (41.31% -44. 84%), TiO2 (0.75% -0.86%) and high MgO (7.30%-11.33%), K2O (4.01%-6.01%) concentrations with K2O/Na2O ratios of 2. 77-12.49. In addition, they display enrichment in large-ion iithophile elements (LILEs, e. g., Rb, Sr,Ba) and LREE, but a relative depletion in high-field-strength elements (HFSEs, e.g. , Nb, Ta, Zr, Hf and Ti). They display similar chondrite-normalized REE patterns with slight negative Eu anomalies (δEu=0.64-0. 82), and high initial 87Sr/86Sr ratios, which resemble those of high K/Ti and low-Ti potassic magmas formed in subduction-related settings. Consequently, we suggest that the parental magma was generated by partial melting of the phlogopite-amphibole- bearing garnet lherzolite within the lithospheric mantle that might have been metasomatized by a potassium-bearing fluid released from a subduction oceanic crust.     

新疆某地红宝石和蓝宝石矿物学研究

本文系统地研究了新疆某地红宝石-蓝宝石矿床,基本查明了该矿床中红宝石和蓝宝石矿床的粒度、形态、双晶、裂纹、包裹体形态和成分,以及硬度、密度、多色性、吸收性、Ne和No值;晶胞参数、化学成分等特征,从而为该类矿床的开发利用提供了重要信息。并根据该矿红、蓝宝石的物理性质、光学性质和化学成分上的不同,进一步探讨了呈色机理。     

喀拉通克Ⅰ号硫化铜镍矿床地球化学特征及矿化富集规律探讨

喀拉通克I号铜镍矿床是与基性杂岩体有关的岩浆硫化物矿床.矿床∑REE和稀土配分模式与国内同类矿床相似.铜、镍、钴等主要元素和金、银、铂等贵重金属元素赋存规律明显,与矿化富集规律计算机模拟结果吻合.岩石模式年龄等数项测试分析.为矿床的形成(物理化学条件分析)提供了较合理的依据.     

西昆仑克里阳地区钾镁煌斑岩岩石学特征及其意义

对西昆仑克里阳钾镁煌斑岩的岩石学特征及源区性质进行了详细研究 ,并同国内外钾镁煌斑岩及其他超钾质岩石作了对比。该钾镁煌斑岩富 Al2 O3、贫 Ti O2 ,不相容元素相对亏损 ,为不含金刚石类型。其源区富集程度不高 ,交代作用较弱 ,且起源深度较浅 ( >1 1 5km)。高的 C、Sr、Pb同位素组成表明该钾镁煌斑岩可能受到了壳源物质的混染。这些特征暗示西昆仑地区有其他含金刚石源岩的存在。