塔里木巴楚小海子正长岩杂岩体的岩石成因探讨

巴楚小海子正长岩杂岩体是二叠纪塔里木大火成岩省的重要组成部分.SIMS锆石U-Pb定年显示其形成于279.7±2.0Ma,与本区辉绿岩脉和石英正长斑岩岩脉近于同时侵位.根据矿物学特征,小海子正长岩体可分为铁橄榄石正长岩和角闪正长岩两类.前者主要由碱性长石、铁橄榄石、单斜辉石、角闪石和少量石英、斜长石组成,后者主要由碱性长石、角闪石、黑云母和少量的石英、斜长石组成.小海子正长岩体为铁质、碱性系列,轻稀土相对富集,重稀土亏损,具有明显的Eu正异常,无Nb、Ta负异常,相对低的(87Sr/86Sr);(0.7033 ～0.7038)和正的εNd(t)值(+3.1～+3.8),暗示它们来自亏损的地幔源区,没有地壳物质的加入.主微量和同位素地球化学分析,暗示巴楚小海子正长岩的母岩浆为碱性的幔源玄武质岩浆经橄榄石、单斜辉石分离结晶后的残余熔体,并且含有堆晶的碱性长石.这种含有碱性长石堆晶的熔体,在相对还原的条件下结晶,形成铁橄榄石正长岩;在相对氧化的条件下结晶,并经过不同程度斜长石的分离结晶形成角闪正长岩.     

Petrogenesis of the Sabongari alkaline complex,cameroon line (central Africa): Preliminary petrological and geochemical constraints

The petrography, mineral chemistry and geochemical features of the Sabongari alkaline complex are presented and discussed in this paper with the aim of constraining its petrogenesis and comparing it with other alkaline complexes of the Cameroon Line. The complex is mainly made up of felsic rocks: (i) granites predominate and include pyroxene–amphibole (the most abundant), amphibole–biotite, biotite and pyroxene types; (ii) syenites are subordinate and comprise amphibole–pyroxene and amphibole–biotite quartz syenites; (iii) pyroxene–amphibole–biotite trachyte and (iv) relatively abundant rhyolite. The minor basic and intermediate terms associated with felsic rocks consist of basanites, microdiorite and monzodioites. Two groups of pyroxene bearing rocks are distinguished: a basanite–trachyte–granite (Group 1) bimodal series (SiO₂ gap: 44 and 63 wt.%) and a basanite–microdiorite–monzodiorite–syenite–granite (Group 2) less pronounced bimodal series (reduced SiO₂ gap: 56–67 wt.%). Both are metaluminous to peralkaline whereas felsic rocks bare of pyroxene (Group 3) are metaluminous to peraluminous. The Group 1 basanite is SiO₂-undersaturated (modal analcite in the groundmass and 11.04 wt.% normative nepheline); its Ni (240 ppm) and Cr (450 ppm) contents, near mantle values, indicate its most primitive character. The Group 2 basanite is rather slightly SiO₂-saturated (1.56 wt.% normative hypersthene), a marker of its high crustal contamination (low Nb/Y-high Rb/Y). The La/Yb and Gd/Yb values of both basanites (1: 19.47 and 2.92; 2: 9.09 and 2.23) suggest their common parental magma composition, and their crystallization through two episodes of partial melting (2% and 3% respectively) of a lherzolite mantle source with <4% residual garnet. The effects of crustal contamination were selectively felt in the values of HFSE/LREE, LREE/LILE and LREE/HFSE ratios, known as indicators. Similar features have been recently obtained in the felsic lavas of the Cameroon Volcanic Line.     

Effects of melt fractional crystallization on Sr-Nd and Lu-Hf isotope systems: a case study of Triassic migmatite in the Sulu UHP terrane

The Weihai migmatite in the Sulu ultra-high-pressure (UHP) metamorphic terrane, eastern China, underwent partial melting in the Late Triassic during its exhumation. The primary partial melts experienced a decompressional fractional crystallization (DFC) process to produce plagioclase (Pl)-rich leucosome crystallized under eclogite to granulite facies conditions and K-feldspar (Kfs)-rich pegmatitic veins crystallized under amphibolite-facies conditions. In this study, our results demonstrate that the DFC process can cause decoupling between whole-rock Sr and Nd isotopes. The Pl-rich leucosome has ε_Nd(t) values (–10.4 to ?15.0) and initial (⁸⁷Sr/⁸⁶Sr) ratios (0.708173–0.712476) very similar to those of the melanosome, but the Kfs-rich pegmatitic veins have homogeneous ε_Nd(t) values (?14.8 to ?15.2) and significantly high initial (⁸⁷Sr/⁸⁶Sr) ratios (0.713882–0.716284). Our results also suggest that the DFC process can change zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf isotopic ratios, with no effect on ¹⁷⁶Hf/¹⁷⁷Hf ratios or ε_Hf (t) values. Zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios increase dramatically from the Pl-rich leucosome to the Kfs-rich pegmatitic veins, but zircon ¹⁷⁶Hf/¹⁷⁷Hf ratios (Pl-rich leucosomes = 0.282330 ± 0.000017; Kfs-rich pegmatitic veins = 0.282321 ± 0.000026) and ε_Hf (t) values (Pl-rich leucosomes = ?10.9 ± 0.6; Kfs-rich pegmatitic veins = ?11.6 ± 0.8) remain almost unchanged. We propose that the isotopic decoupling between the Pl-rich leucosome and Kfs-rich pegmatitic vein might be caused by melt fractional crystallization occurring too rapidly to allow complete equilibrium between them.     

Late Cretaceous coeval acidic and basic magmatism,Karacaali Magmatic Complex,Central Anatolia,Turkey

The Central Anatolia Crystalline Complex (CACC) is characterized by Late Cretaceous high-temperature metamorphic rocks intruded by S-, I-, and A-type granitoids. Coeval basic plutonic and volcanic rocks also crop out in the complex. The NE–SW-trending Karacaali Magmatic Complex (KMC) represents a clear example of synchronous basic and acidic magmatic associations. We present new data on this coeval magmatism. The KMC plutonic rocks mainly consist of monzonite, granite, and gabbro, whereas the associated volcanic rocks are chiefly of basalt and rhyolite. All of the units have been cut by quartz, quartz-tourmaline, and calcite veins and by porphyritic leucogranite, aplitic, and basaltic dikes. The rhyolitic, basaltic, and gabbroic samples yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 69.1 ± 1.3, 58 ± 10, and 66.4 ± 1 million years, respectively; these data indicate that a younger multiphase basic magma was injected into a partially crystallized monzonitic magma chamber. The basic intrusions added heat to the system and gave rise to the re-fusion of the already crystallized parts of the monzonitic melt, forming the younger leucogranitic magma. The gradational contacts, cross-cutting relationships, trace element contents, trace element patterns, rare-earth element (REE) patterns, and ⁴⁰Ar/³⁹Ar geochronological data of the studied igneous suite clearly demonstrate that the acidic and basic rocks of the KMC were contemporaneous and are produced by partial melting of distinct sources rather than by fractional crystallization of a single source.     

新疆东天山镁铁-超镁铁岩体中橄榄石成分特征及其成因意义:以黄山东和图拉尔根为例

新疆东天山发育有十多个与铜镍硫化物成矿有关的镁铁-超镁铁杂岩体,对选取于黄山东和图拉尔根岩体岩体内不同岩性中的新鲜橄榄石进行了矿物化学成分测试。黄山东岩体角闪辉长岩相的橄榄角闪辉长岩中橄榄石Fo为67～73,超镁铁岩相的角闪(斜长)橄榄岩、角闪二辉橄榄岩和斜长单辉橄榄岩中橄榄石Fo为81～84,Ni含量为308～1762μg/g;图拉尔根岩体中橄榄石Fo为82～85,Ni含量变化范围为857～1818μg/g。利用最高Fo含量的橄榄石成分计算得到黄山东岩体母岩浆MgO为10.3%,图拉尔根岩体母岩浆MgO为11.47%。橄榄石分离结晶和硫化物熔离模拟结果显示,黄山东岩体发生了19%的橄榄石分离结晶,由橄榄石分离结晶导致硫化物熔离的程度约为0.3%,晶间硫化物熔浆的物质交换作用对黄山东橄榄石的成分有重要影响;图拉尔根岩体发生了9%左右的橄榄石结晶,由橄榄石结晶导致出熔的硫化物熔体大约为0.5%。     

Origin of galena and sphalerite mineralization in the Koru Volcanics,Biga Peninsula,Turkey: insights from MELTS modelling and SUPCRT calculations

Seismic and gravity data have disclosed correlations between the structure of the major folded belts with the surface of the basaltic layer and the Moho. The major structural elements of the Greater Caucasus province are reviewed, though with all too generalized maps for reference. These structures are reviewed in the light of their evolution in three stages: the first ending in the Medial-Late Jurassic, the second ending in the Middle Paleogene, and the third occupying the Oligocene-Neogene-Quaternary periods. A major synclinorium and anticlinorium of long duration are postulated. The granite layer is summarized in an isopachous map. This is correlated with a contour map of the Moho. Inheritances of earlier geosynclinal cycles (Hercynian). The primary geosyncline is related to planetary rifts. The thickest part of the crust is centered on the meganticlinorium in the Alpine structures of the Caucasus. Formation of folds, foredeeps, and folding after sedimentation is related to development of the granite layer during continental evolution. Deep-seated piercement of the granite into the cover in the axial portion is indicated. The observable seismicity is also correlated with evolution of the granite layer.--B. N. Cooper.     

新疆坡北镁铁-超镁铁杂岩体中橄榄石成分特征及其成因意义

橄榄石的矿物化学成分可以很好的反映母岩浆成分、岩浆分异结晶及硫化物熔离作用等信息.坡北镁铁-超镁铁杂岩体中橄榄石电子探针成分分析表明:橄榄石均为贵橄榄石,其化学成分与寄主岩石类型密切相关,其中橄榄辉长岩中橄榄石的Fo值在74～81之间,橄长岩或橄榄单斜辉石岩中橄榄石Fo=75～82.7,异剥橄榄岩或橄榄岩中橄榄石Fo值为83.6～86.3,暗示橄榄岩结晶早于橄榄辉长岩.橄榄石Ni含量变化范围为(174～2281)×10-6.利用最高Fo含量的橄榄石成分计算得到坡北岩体母岩浆MgO=14.38％,为高镁玄武岩浆.橄榄石分离结晶和硫化物熔离模拟显示:坡北岩体主要发生了～25％的橄榄石分离结晶,在橄榄石结晶程度为2％～4％的过程中,发生过硫化物熔离作用,但熔离程度较低(～0.2％).总体来看,该区成矿远景不佳.     

Effect of the Dead Sea–Red Sea canal modelling on the prediction of the Dead Sea conditions

A project to link the Dead Sea to the Red Sea via a canal is undergoing extensive study. In previous works, a generalized mathematical model describing the state of the Dead Sea and a simulation model to implement it have been developed. The model is extended to include the proposed canal project and investigates two alternative modelling canal scenarios: (1) introducing the canal water inflow into the bottom layer or (2) the top layer of the sea. The predicted general effects of the canal are the restoration of the water level of the sea to pre‐1970s level; an increase in the total evaporation rate and a decrease in the top layer salinity. Implementing scenario 1, the model predicts that: the water level of the Dead Sea will exceed the desired level design value and therefore shorter filling time can be used; seasonal stratification will persist; total evaporation rate will increase Modestly; there will a small decrease in the salinity of the top layer but a substantial decrease in the salinity of the bottom layer, which will hurt industries severely; there will be a continuation of seasonal crystallization of aragonite and gypsum. Implementing scenario 2 the model predicts that: the water level of the Dead Sea will be maintained at the desired level design value; stratification will be re‐established, with the formation of a permanent two‐layer system; there will be a substantial increase in the total evaporation rate; the salinity of the top layer will decrease significantly but there will be continuous slower salinity increase in the bottom layer; the crystallization of aragonite will cease, but seasonal gypsum crystallization can be expected to continue as soon as the filling period ends and the canal shifts into normal operation. Copyright © 2003 John Wiley & Sons, Ltd.     

A study of rare earth element (REE)–SiO2 variations in felsic liquids generated by basalt fractionation and amphibolite melting: a potential test for discriminating between the two different processes

The origin of felsic magmas (>63% SiO₂) in intra-oceanic arc settings is still a matter of debate. Two very different processes are currently invoked to explain their origin. These include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite. Because both fractionation and melting can lead to similar major element, trace element and isotopic characteristics in felsic magmas, such lines of evidence have been generally unsuccessful in discriminating between the two processes. A commonly under-appreciated aspect of rare earth element (REE) solid–liquid partitioning behavior is that D _REE for most common igneous minerals (especially hornblende) increase significantly with increasing liquid SiO₂ contents. For some minerals (e.g., hornblende and augite), REE partitioning can change from incomptatible (D < 1) at low liquid SiO₂ to compatible (D > 1) at high liquid SiO₂. When this behavior is incorporated into carefully constrained mass-balance models for mafic (basaltic) amphibolite melting, intermediate (andesitic) amphibolite melting, lower or mid to upper crustal hornblende-present basalt fractionation, and mid to upper crustal hornblende-absent basalt fractionation the following general predictions emerge for felsic magmas (e.g., ∼63 to 76% SiO₂). Partial melting of either mafic or intermediate amphibolite should, regardless of the type of melting (equilibrium, fractional, accumulated fractional) yield REE abundances that remain essentially constant and then decrease, or steadily decrease with increasing liquid SiO₂ content. At high liquid SiO₂ contents LREE abundances should be slightly enriched to slightly depleted (i.e., C _l/C _o ∼ 2 to 0.2) while HREE abundances should be slightly depleted (C _l/C _o ∼ 1 to 0.2). Lower crustal hornblende-bearing basalt fractionation should yield roughly constant REE abundances with increasing liquid SiO₂ and exhibit only slight enrichment (C _l/C _o ∼ 1.2). Mid to upper crustal hornblende-bearing basalt fractionation should yield steadily increasing LREE abundances but constant and then decreasing HREE abundances. At high liquid SiO₂ contents LREE abundances may range from non-enriched to highly enriched (C _l/C _o ∼ 1 to 5) while HREE abundances are generally non-enriched to only slightly enriched (C _l/C _o ∼ 1 to 2). Hornblende-absent basalt fractionation should yield steadily increasing REE abundances with increasing liquid SiO₂ contents. At high SiO₂ contents both LREE and HREE are highly enriched (C _l/C _o ∼ 3 to 4). It is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra-oceanic arc environments where continental crust is absent. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.