Experimental constraints on the effects of pressure and H2O on the fractional crystallization of high-Mg island arc basalt

H₂O-undersaturated melting experiments of synthesized basalt (SiO₂ = 50.7 wt.%, MgO = 8.3 wt.%, Mg# = 60) were conducted at fO₂ corresponding to NNO+1 and NNO−1 to clarify the effects of pressure (2–7 kbar) and H₂O on fractional crystallization in island arcs. H₂O content was ranged from nominally anhydrous to 4.4 wt.%. Differentiation trends, namely the liquid lines of descent, change sensitively according to pressure-H₂O relations. Tholeiitic differentiation trends are reproduced with H₂O ≤ ∼2 wt.% in primary magma. With such quantities of H₂O, fractional crystallization is controlled by olivine + plagioclase at 2 kbar. Increasing the pressure from 2 to ≥4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore SiO₂ enrichment in the residual melts is suppressed. Increasing H₂O (≥ ∼2 wt.% in primary magma) stabilizes clinopyroxene relative to orthopyroxene and/or magnetite. Although the phase relations and proportions strongly depend on fO₂ and H₂O content, differentiation trends are always calc-alkaline.     

杭州泗岭铝质A型花岗岩的发现及其构造意义

初步研究表明,以往被认为是典型的S型花岗岩或改造-重熔型花岗岩的泗岭岩体应为铝质A型花岗岩。该岩体高硅、碱、钾而明显贫钙、镁;高(K2O Na2O)/CaO值(平均16.79)和AKI指数(平均0.92);FeO*/MgO比值大(平均13.95),高于M型、S型和I型花岗岩,而与世界A型花岗岩平均值(13.4)相近;岩石为弱碱性、准铝-弱过铝质,富含稀土元素、HFSE元素,高Rb、F和Nb,反映其具A型花岗岩的成分特征。泗岭岩体氧同位素δ18O为 8‰～ 9‰,反映岩浆主要起源于下地壳。该岩体侵位于拉张的构造环境,是晚白垩世早期一次重要的构造-岩浆热事件,同时也反映了区域性孝丰—三门湾大断裂与学川—湖州大断裂在晚白垩世早期处于拉伸阶段。     

Granitoid emplacement by multiple sheeting during Variscan dextral transpression: The Saint-Laurent – La Jonquera pluton (Eastern Pyrenees)

The structural study of the Saint-Laurent – La Jonquera pluton (Eastern Pyrenees), a Variscan composite laccolithic intrusion emplaced in metasedimentary and gneissic rocks of the Roc de Frausa dome, by means of the anisotropy of magnetic susceptibility (AMS) technique has allowed the determination of the nature and orientation of its magmatic fabrics. The magmatic foliation has a predominant NE–SW strike and the mean lineation is also NE–SW trending with a shallow plunge. A strain gradient is measured so that the tonalites to granodiorites that form the basal parts of the pluton, and are intruded into amphibolite-facies metamorphic rocks, recorded the highest anisotropies, whereas the monzogranites and leucogranites, emplaced into upper crustal, low-grade metamorphic rocks, are weakly deformed. These results point to the synkinematic sequential emplacement of multiple granitoid sheets, from less to more differentiated magmatic stages, during the Late Carboniferous D₂ event characterized by an E–W-trending dextral transpression. The magmatic foliation appears locally disturbed by the effects of two tectonic events. The first of them (D₃) produced mylonitization of granitoids along NW–SE retrograding shear zones and open folds in the host Ediacaran metasediments of the Roc de Frausa massif, likely during late Variscan times. Interference between D₂ and D₃ structures was responsible for the dome geometry of the whole Roc de Frausa massif. The second and last perturbation consisted of local southward tilting of the granitoids coupled to the Mesozoic–Cenozoic cover during the Alpine.     

Petrogenesis and geodynamic evolution of the Kajan Neogene subvolcanic rocks,Nain, Central Iran

Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO₂ (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO₂, P₂O₅, MnO, Al₂O₃ and Fe₂O₃ define linear trends with negative slopes against SiO₂, whilst Na₂O and K₂O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO₂, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (La_N/Yb_N = 4.5–6.4) with moderate HREE fractionation (Sm_N/Yb_N = 1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (< 400 ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the ⁸⁷Sr/⁸⁶Sr initial ratios vary from 0.704432 to 0.705989, and the ¹⁴³Nd/¹⁴⁴Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks.     

新疆东准乌须克劳格岩体LA-ICP-MS锆石U-Pb年龄、Hf同位素特征及其地质意义

新疆东准噶尔琼河坝地区近年来找矿获得重大突破,发现了一系列重要的矿产,这些矿产基本上围绕该地区出露面积最大的乌须克劳格岩体分布。该岩体组成比较复杂,主要岩性为二长花岗岩和花岗闪长岩。采用LA-ICP-MS技术测得花岗闪长岩和二长花岗岩锆石~(206)Pb/~(238)U年龄分别为428.9±1.9Ma和427.2±1.7Ma,被解释为岩石的结晶年龄。2类岩石都属于过铝质-钙碱性系列花岗岩,具有较高的铝、钙含量,K_2O/Na_2O值普遍偏低,介于0.15~0.40之间;稀土元素配分曲线呈现右倾特征,稀土元素总量较低,轻稀土元素相对富集,负Eu异常不明显;在微量元素原始地幔标准化蛛网图上,高场强元素Th、Nb、Ta、P、Ti等相对亏损,大离子亲石元素Rb、Ba、U、K、Sr等相对富集,具有高Ba、Sr含量和高(La/Yb)_N、Sr/Y值,显示出高Ba-Sr花岗岩的特征;同时岩体具有高正ε_(Hf)(t)值和年轻的模式年龄。这些地球化学和同位素特征表明,晚志留世乌须克劳格岩体是大陆边缘弧环境下大洋板片熔融的产物。     

桂东南永安岩体锆石U-Pb测年、Hf同位素特征及其地质意义

永安岩体位于钦杭结合带南部,属桂东南十万大山-大容山复式岩体的一部分,其岩石类型为斑状堇青黑云花岗闪长岩。对永安岩体斑状堇青黑云花岗闪长岩进行了LA-ICPMS锆石U-Pb测年,获得其年龄为252. 8±3. 3Ma,为晚二叠世花岗岩。对永安岩体花岗岩进行的全岩地球化学分析结果表明,岩石富Al、高K,铝饱和指数A/CNK均大于1. 1,标准矿物分子刚玉均大于1%;岩石富集轻稀土元素,亏损重稀土元素,轻重稀土元素分馏明显,稀土元素的球粒陨石标准化配分曲线为轻稀土元素富集的右倾型,具弱负铕异常;岩石富集大离子亲石元素(Rb、Ba、K、Sr),亏损高场强元素(Nb、P、Ti、Ta)和Ba、Sr。花岗岩的锆石εHf(t)=-14～-8,tDM2=2. 3～1. 8 Ga。通过本文工作,结合前人关于永安岩体矿物学、全岩地球化学、Hf同位素和锆石微量元素的分析资料,认为永安岩体岩石为强过铝质S型壳源花岗岩,地幔组分没有提供成岩物质,但地幔底侵作用为成岩提供了热能,花岗岩形成于后碰撞阶段的拉张环境中。     

华北克拉通东缘金岭杂岩体岩浆源区及构造背景:来自岩相学、岩石地球化学及年代学的证据

金岭杂岩体由细粒黑云角闪闪长岩、中细粒黑云角闪闪长岩、中细粒黑云角闪二长闪长岩和细粒角闪二长闪长岩组成,是鲁西地区典型矽卡岩型富铁矿(金岭铁矿)控矿岩体。本次研究对细粒黑云角闪闪长岩和细粒角闪二长闪长岩进行了锆石LA‐ICP‐MS U‐Pb定年,其结果分别为129.2±3.2 Ma和132.8±1.2 Ma,表明该杂岩体的侵位时代为早白垩世。样品SiO2、K2O和Na2O含量分别介于54.17%~63.73%、1.92%~4.76%和3.10%~5.41%之间,K2O/Na2O为0.58~0.94,A/CNK为0.60~0.93,具有轻稀土富集,重稀土亏损的右倾型稀土配分模式,轻、重稀土分馏程度中等((La/Yb)N=9.94~23.49),Eu异常不明显(δEu=0.84~1.10),具中—弱负Ce异常(δCe=0.56~0.92)。样品以富集大离子亲石元素(Ba、K、U、Pb等)、亏损高场强元素(Nb、Ta、Ti)以及高Sr/Y为特征。金岭杂岩体为燕山晚期岩浆活动产物,属准铝质高钾钙碱性系列,岩浆主要来源于富集岩石圈地幔的部分熔融,并在岩浆上升侵位的过程中有地壳物质的同化混染。燕山晚期华北克拉通在古太平洋板块俯冲后后撤引起的板内伸展环境下,增厚陆壳减薄阶段,岩浆上侵就位形成金岭杂岩体。     

扬子陆块西缘江浪穹窿超基性岩的成因:锆石U-Pb定年、岩石地球化学及Sr-Nd同位素

江浪穹窿位于扬子陆块西缘,本文作者在穹窿南部新发现一套侵入于二叠系及志留系的超基性岩,岩石主要由蛇纹石(约60%)、橄榄石(约30%)和少量磁铁矿(约5%)、角闪石(约5%)组成。为探讨超基性岩的成因,本文进行了LA-ICP-MS锆石U-Pb定年、岩石地球化学及Sr-Nd同位素研究。定年结果表明,超基性岩中发育大量2427~430 Ma的捕获锆石,最年轻一组岩浆锆石²⁰⁶Pb/²³⁸U加权平均年龄为222.3±4.4 Ma(MSWD=1.9,n=6)。主微量元素分析显示岩石：(1)具有低的SiO₂含量(46.76%~39.07%)、高的Mg#值(82.3~74.0)与Cr、Co、Ni丰度;(2)稀土元素含量(ΣREE平均31.8 μg/g)与(La/Yb)_N值(5.26~1.38)偏低,稀土配分型式较为平坦,具有较弱的Ce负异常(Ce/Ce^*=0.80~0.67);(3)富集大离子亲石元素Rb、Ba和U,亏损高场强元素Zr和Hf;(4)(Th/Yb)_PM值(29.8~1.56)、(Th/Ta)_PM值(0.22~0.03)、(La/Nb)_PM值(1.91~0.39)及La/Sm值(5.88~1.11)较低。Sr-Nd同位素分析显示,超基性岩具有较低的(⁸⁷Sr/⁸⁶Sr)_i值(0.706872~0.702598)和高的ε_Nd(t)值(8.02~5.64),成分接近于亏损地幔和岛弧玄武岩,计算表明地壳物质的混染程度低于5%。结合前人研究成果,本文认为超基性岩结晶年龄为222.3 Ma,可能形成于古特提斯洋闭合阶段的岛弧背景;原始岩浆来自高度部分熔融的地幔源区,上升侵位过程中可能经历了铬铁矿与橄榄石的分离结晶作用。此外,捕获锆石的年龄谱反映江浪穹窿很可能存在太古宙—古元古代变质基底,并且具有Rodinia超大陆会聚—裂解以及泛非事件的地质年龄记录。     

Late Pan-African magmatism and crustal development in northeastern Egypt

The voluminous Pan-African calc-alkaline granitic suite of the Ras Gharib crustal segment in northeastern Eygpt provides a typical example of orogenic magmatism. The 552 ± 7 Ma-old granodiorite–adamellite and leucogranite suite is compositionally broad (58 to 77 wt.% SiO₂) and exhibits calc-alkaline geochemical trends and trace-element characteristics typical of the volcanic-arc granites. The rocks contain oligoclase, albite, K-feldspar, calcic amphibole, biotite, titanite, zircon, and magnetite. The suite exhibits typical features characteristic of I-type granites. We contend that the magma was formed by partial melting of a modified oceanic crust at an active continental margin during the late stage of the Pan-African orogeny. The process may have involved assimilation of Early Pan-African dioritic country rocks. The more felsic units were produced by progressive fractionation of that magma. The petrological–geochemical evidence suggest that the Pan-African crust in northeastern Egypt did not develop in an extensional tectonic regime, as proposed recently.