Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite,India:Implications for petrogenesis

The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory).     

新疆北部乌恰沟地区镁铁质侵入岩的年代学、地球化学和Sr-Nd-Pb同位素组成：对地幔源区特征和深部过程的约束

阿尔泰造山带东段的乌恰沟地区发育大量镁铁质侵入岩(岩脉或小岩体),包括闪长玢岩和具堆积结构的角闪辉长岩和橄榄辉长岩。锆石SHRIMP U-Ph同位素定年结果说明这些镁铁质岩浆的侵位时间为257．4±5．3Ma,与峨眉山玄武岩相近(259±3Ma)。在微量元素原始地幔标准化图上,它们共同的特征是富集大离子亲石元素,亏损Nb和Ta,具K和Pb的正异常,LREE相对HREE富集,即具弧火山岩的地球化学特征。同时它们之间还存在互补的地球化学特征,如闪长玢岩存在Ba、Sr、Eu、P和Ti的负异常,橄榄辉长岩具有Ba、Sr、Eu、P的正异常,而角闪辉长岩具有Ti的正异常。如果扣除围岩混染和后期流体交代的影响,这些岩石的Sr-Nd-Pb同位素组成基本一致,因此闪长玢岩和两种辉长岩分别代表同源岩浆因结晶分离作用形成的演化岩浆和堆晶岩。乌恰沟晚二叠世镁铁质侵入岩和新疆北部早二叠世与铜镍矿有关的基性-超基性杂岩体都具有与弧火山岩相似的地球化学特征,反映了被俯冲改造的陆下岩石圈地幔的地球化学特征。但是与后者相比,前者的Mg~#值、ε_(Nd)(t)值和La/Nb比明显偏低,暗示存在软流圈地幔来源物质的显著贡献。综合阿尔泰乌恰沟地区镁铁质侵入岩的年代学、地球化学和同位素组成特征,我们推测该区在晚二叠世存在地幔柱与岩石圈地幔之间的相互作用。     

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO₂ (0.18–0.57 ​wt.%), P₂O₅ (<0.05 ​wt.%), K₂O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu1 ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)_N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)_N ​= ​0.10–0.27 and (La/Yb)_N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.     

东天山卡拉塔格地区镁铁质岩体年代学、岩石地球化学研究

东天山大南湖岛弧带北缘卡拉塔格地区发育一系列镁铁质岩体,出露面积1km2。岩体岩相较为单一,主要岩性有辉长岩、橄榄辉长岩、橄长岩、淡色辉长岩和辉绿岩。本文利用LA-ICP-MS测得2个辉长岩的锆石U-Pb年龄分别为282. 2±0. 6Ma和278. 3±0. 6Ma;与吐哈盆地玄武岩及东天山地区铜镍硫化物矿床时代一致。岩石地球化学显示相对低Fe、高Al、Ca、碱、Ti,富集Rb、Ba、Sr、K,亏损Th、Nb、Ta的特征。岩石的(~(87)Sr/~(86)Sr)_i=0. 70313～0. 70461,εNd(t)=+6. 22～+8. 64,(~(206)Pb/~(204)Pb)_i=17. 68～18. 103,(~(207)Pb/~(204)Pb)_i=15. 443～15. 536,(~(208)Pb/~(204)Pb)_i=37. 423～37. 801,表现出高Nd、低Sr、低Pb,且变化范围较窄的特征。综合研究表明,该地区镁铁质岩体的岩浆动力学背景为后碰撞伸展环境;岩浆源区为俯冲板片交代的岩石圈地幔,并被软流圈物质混染;同位素和微量元素模拟显示母岩浆在源区遭受了5%～15%的板片流体交代,在上升过程中遭受了5%的上地壳物质混染。卡拉塔格地区镁铁质岩体与东天山二叠纪镁铁-超镁铁质岩体和吐哈盆地玄武岩形成于同一构造岩浆系统,在剥蚀较浅的大南湖岛弧带具有形成铜镍硫化物矿床的潜力。     

Geochemistry of Cretaceous mafic rocks from the Lower Yangtze region, eastern China: Characteristics and evolution of the lithospheric mantle

Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO₂ contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic ²⁰⁷Pb/²⁰⁴Pb(t) (15.41–15.65) and ²⁰⁸Pb/²⁰⁴Pb(t) (37.66–38.51) ratios at given ²⁰⁶Pb/²⁰⁴Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with ⁸⁷Sr/⁸⁶Sr(t) ranging from 0.7056 to 0.7071 and ε_Nd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record ⁸⁷Sr/⁸⁶Sr(t) ratios of between 0.7044 and 0.7050 and ε_Nd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.     

山东郯城神泉钠质火山岩的年代学与地球化学——对源区地幔性状与岩石成因的启示

山东神泉钠质火山岩出露于郯城李庄东侧,构造上位于沂沭断裂带南段,构成一盾状火山机构,主体岩性为粗安岩。锆石LA-ICP-MS U-Pb定年结果显示,其成岩年龄为96.5±1.4 Ma,较之区内广泛分布的钾质火山岩形成年龄晚,指示沂沭断裂带及邻区中生代火山岩总体上具有由钾质向钠质演变的趋势。在化学组成上,神泉火山岩具有富碱(Na2O+K2O=10.34%~11.95%)和明显富钠(Na2O/K2O=1.76~3.92)的特征,按多种方法判别均可归为典型的钠质火山岩。该火山岩稀土元素总量较高(ΣREE=411.3×10-6~456.3×10-6),富集轻稀土元素,LREE/HREE=19.53~21.10,(La/Yb)N=29.69~33.26,缺乏明显的铕异常(δEu=0.77~0.82),稀土元素配分型式呈明显的右倾型。岩石富Rb、Ba、Th等大离子亲石元素,亏损Ti、Nb、Ta等高场强元素,在微量元素蛛网图上显示一定的Pb负异常。神泉火山岩具有富集的Sr-Nd同位素特征,ISr=0.706 8~0.707 7,εNd(t)=-16.66~-16.82,与山东中生代镁铁质岩石的Sr-Nd同位素组成类似,表明其应起源于富集的岩石圈地幔。综合分析表明,神泉火山岩源区极可能遭受了因扬子板块深俯冲和华北克拉通岩石圈拆沉等多次富集事件的影响,至晚白垩世,随着郯庐断裂引张的加剧,诱发了软流圈地幔上涌,并对先期形成的富集地幔进一步改造,同时在断裂减压作用的影响下,促使了这一经叠次改造的富集地幔的部分熔融,由此产生的岩浆经进一步分异演化,最终导致了神泉钠质火山岩的形成。     

塔里木瓦吉里塔格超镁铁质隐爆角砾岩铂族元素和微量元素地球化学特征及其岩石成因探讨

本文对塔里木盆地二叠纪大火成岩省中瓦吉里塔格地区超镁铁质隐爆角砾岩进行了铂族元素(PGE)和主、微量及稀土元素分析和研究。结果显示,隐爆角砾岩中Os、Ir、Ru、Rh、Pt和Pd含量分别为0.36×10-9～1.08×10-9、0.23×10-9～0.44×10-9、0.29×10-9～0.92×10-9、0.11×10-9～0.18×10-9、1.88×10-9～3.16×10-9和1.39×10-9～3.52×10-9,均低于原始地幔,与夏威夷苦橄岩相似。该岩石的Pd/Ir比值在3.6～11.9之间,PGE分配模式呈一条正倾斜的曲线,表现出一定程度的分异,具有非俯冲背景下产生的基性-超基性岩的PGE配分特点。略高于原始地幔但变化较小的Cu/Pd比值(5.1×104～12.1×104)表明其岩浆在上升侵位过程中并没有发生明显的饱和硫化物熔离作用,而岩浆源区在部分熔融过程中可能有少量残留的硫化物存在。隐爆角砾岩全岩的IPGE元素与MgO之间基本上呈正相关,而PPGE元素与MgO之间则略成负相关或无明显相关性,指示PGE的分异主要受到橄榄石结晶分异作用的控制。地球化学特征显示隐爆角砾岩的稀土元素总量高度富集(964.1×10-6～1299×10-6)和轻、重稀土强烈分馏((La/Yb)N=45.88～64.90),且微量元素蛛网图上大离子亲石元素富集和Nb、Ta的轻微亏损以及Zr、Hf的明显贫化,表明岩石可能遭受一定程度的地幔交代作用影响。但是,角砾与胶结物具有相近的PGE特征表明交代作用对PGE的影响并不大,暗示PGE可能主要赋存于禁锢在硅酸盐矿物内的硫化物包裹体中。     

新疆谢米斯台布拉特矿化次火山岩的年代学、地球化学特征及其地质、成矿意义

布拉特矿化次火山岩出露于谢米斯台中部,岩性主要为英安斑岩和流纹斑岩。本文通过锆石U-Pb年代学、岩石地球化学和Sr-Nd-Hf同位素等研究,探讨其形成的构造环境、成因及成矿意义。结果表明,英安斑岩形成年龄为434.9±2.3Ma,属高钾钙碱性-钾玄岩系列准铝质-强过铝质岩石;岩石稀土总量中等(∑REE=97.9×10~(-6)～107×10~(-6)),富集轻稀土且轻重稀土分异明显((La/Yb)_N=7.93～9.95),具较弱铕负异常(δEu=0.78～0.90),相对富集Rb、Th、U、K等元素,亏损Nb、Ta、P、Ti等元素;岩石具有低的(~(87)Sr/~(86)Sr)_i值(0.7036～0.7043)和正的ε_(Nd)(t)值(+2.64～+5.01),t_(DM1)(Nd)=750～930Ma。流纹斑岩形成年龄为423.2±0.9Ma,属高钾钙碱性-钾玄岩系列准铝质-强过铝质岩石;稀土总量中等(∑REE=65.3×10~(-6)～127×10~(-6)),富集轻稀土且轻重稀土分异明显((La/Yb)_N=6.82～8.24),具较强铕负异常(δEu=0.51～0.71),相对富集Rb、Th、U、K等元素,亏损Nb、Ta、P、Ti等元素;岩石具有低的(~(87)Sr/~(86)Sr)_i值(0.7022～0.7038)和正的ε_(Nd)(t)值(+2.86～+5.78),t_(DM1)(Nd)=680～940Ma;锆石ε_(Hf)(t)=+9.8～+14.9,t_(DM2)(Hf)=456～783Ma。综合研究表明,布拉特矿化英安斑岩和流纹斑岩是不同岩浆演化结晶的产物,二者都形成于活动大陆边缘弧环境,可能是新生下地壳部分熔融形成的花岗质岩浆与部分源自地幔楔的玄武质岩浆发生混合、向上运移、冷凝结晶的产物。较晚形成的流纹斑岩岩浆形成过程中新生下地壳部分熔融的比例有所降低,熔出的岩浆相对更偏酸性;谢米斯台地区以志留纪-早泥盆世火山岩、次火山岩和中酸性侵入体为代表的岩浆岩带为一个主体形成于早古生代的陆缘弧岩浆岩带,是形成和寻找斑岩型铜矿的有利地区。     

南天山北缘榆树沟变质基性超基性岩的地球化学及其成因机制

榆树沟变质基性－超基性岩带出露于塔里木板块与哈萨克斯坦板块之间的南天山北缘，主要由变质橄榄岩和变质基性岩组成。变质橄榄岩富相容元素Cr、Co和Ni，贫不相容元素，太离子亲石元素Ba、Rb和Sr含量较低，与世界典型蛇绿岩相似，代表了地幔残留物特征。REE分布模式为LREE亏损型，REE含量小于或等于2．5倍球粒陨石，类似于阿尔卑斯型变质橄榄岩，显示榆树沟的变质橄榄岩是原始地幔岩部分熔融萃取出玄武岩后的残留物。变质基性岩绝大部分为LREE亏损型，类似于N－MORB。所有样品均以富集Nb和Ta、高场强元素不分异，以及微量元素含量低为特征，批示岩浆源区总体上类似于MORB，Nb、Ta富集可能与OIB型源区有关，Nd、Sr同位素特征也显示其具有OIB型源区特征。综合分析认为，榆对沟变质基性岩石的岩浆可能经历了两个阶段的演化过程，即上地幔底部或下地幔顶部的OIB型原始岩浆形成阶段和软流圈地幔亏损阶段。     

Geochronology and petrogenesis of the mafic dykes from the Purang ophiolite:Implications for evolution of the western Yarlung-Tsangpo suture zone,southwestern Tibet

The>2000 km Indus-Yarlung Tsangpo suture zone(IYSZ)is composed of the Neo-tethys oceanic remnants,flysch units and related continental rocks,which has been regarded as the boundary between the Eurasian and Indian terranes.Among the ophiolitic complexes,the Purang ophiolite is the biggest massif in the IYSZ,and many studies have been conducted on this ophiolite.However,previous studies have mainly focused on harzburgite,clinopyroxenite and dunite.Field observations show that mafic dykes were emplaced within the Purang ophiolite.However,petrogenetic evolutions of those mafic dykes are poorly understood.In this study,we present new LA-ICP-MS zircon U-Pb dating results,whole-rock geochemistry and Sr-Nd-Hf isotope analyses for microgabbro,gabbro and dolerite dykes from the Purang ophiolite of the southwestern IYSZ,respectively.Three samples yielded zircon U-Pb ages of144.2±2.1 Ma.127.9±2.3 Ma and 126.5±0.42 Ma,suggesting two different phases of magmatic activities distinctly.Whole-rock geochemical results suggest that the gabbro samples show alkaline features marked by enrichments of light rare earth elements(LREE)and large-ion lithophile elements(LILE),as well as Nb-Ta elements,suggesting an oceanic island basalt-like(OIB-like)geochemical affinity.However,the dolerite and microgabbro samples demonstrate sub-alkaline characteristics with normal mid-oceanic ridge basalt-like(N-MORB-like)geochemical features.Three distinct mafic dykes show significant Rb element depletion.The geochemical data and Sr-Nd-Hf isotopic features suggest that the microgabbro and gabbro rocks were derived from a depleted mantle that had been metasomatized by partial melts of sediments and enriched slab-derived fluids.The dolerite was also originated from a depleted mantle marked by significantly depleted Sr-Nd-Hf compositions,which was not influenced by enriched slab-derived fluids and sediments contamination during subsequent evolution.The isotope and geochemical data and tectonic diagrams suggest a tectonic transition from a within-plate to a midoceanic ridge basalt-like(MORB-like)setting during the period from ca.144 Ma to 127 Ma.Combined with regional background and this study,we propose that these mafic dykes were formed in an oceanic back-arc basin setting.Additionally,integrated with previous studies,we suggest that the geodynamic evolution of the southwestern and central parts of the Neo-Tethys oceanic basin is comparable in Early Cretaceous.     

Geochronology and paleomagnetism of mafic igneous rocks in the Olenek Uplift, northern Siberia: Implications for Mesoproterozoic supercontinents and paleogeography

We present a new, reliably dated Mesoproterozoic paleopole for Siberia, based on a combined geochronological and paleomagnetic study of mafic rocks within the Mesoproterozoic Sololi Group of the Olenek Uplift in northern Siberia. Ion microprobe (SHRIMP) U–Pb analysis yields crystallisation ages of 2036 ± 11 Ma for zircon from a basement granite and 1473 ± 24 Ma for baddeleyite from a large dolerite sill within the Kyutingde Formation. The baddeleyite result indicates that the lower Sololi Group is significantly older than was suggested by previous K–Ar results. Paleomagnetic analysis of the dolerite sill and related mafic intrusive rocks yields a paleopole at 33.6°N, 253.1°E, A₉₅ = 10.4°. A positive baked-contact test between the Kyutingde sill and sedimentary country rocks shows that the magnetisation is primary. Comparison of this paleopole with coeval results for Laurentia provides a revised reconstruction between Siberia and Laurentia, and implies that these two continents were parts of a single Mesoproterozoic supercontinent since at least 1473 Ma. We argue that Siberia, Laurentia, and Baltica belonged to the same supercontinent between 1473 Ma and mid-Neoproterozoic time.     

甘肃北山地区煌斑岩的年代学和地球化学及其壳幔作用过程讨论

柳园地区下二叠统中发育多条煌斑岩脉，并且在空间上和二叠纪玄武岩、辉长岩和超镁铁岩关系密切。煌斑岩K-Ar、Ar-Ar定年测定表明岩脉侵入时间在220～240Ma之间。地球化学表现为富集大离子亲石元素和轻稀土，亏损高场强元素（Ta-Nb-Ti、Zr、Hf）的特征。Sr-Nd同位素以高的（^87Sr／^86Sr）i 0．708616～0．708744，较低的εNd（t）-3．89～-2．14为特征。Nb／Ta（16．16～19．14，相对于原始地幔的17．5）、Zr／Hf（42．14～43．95，相对于原始地幔的36．7）以及Ta．Nd．Ti的亏损，显示源区可能和早期俯冲洋壳或造山带根部拆沉组份脱水、富集流体交代岩石圈地幔有关。本区煌斑岩为二叠纪裂谷发展到后期，富集流体上涌，交代岩石圈地幔部分熔融，并沿断裂侵入的产物；是二叠纪开始的裂谷作用和火山活动晚期阶段产物。     

北山南部早二叠世A型流纹岩地球化学特征及其地球动力学意义

北山南部晚古生代构造背景争议已久,该区下二叠统普遍发育一套流纹岩系,但尚未对其开展系统的年代学与地球化学研究。本文首次对该区下二叠统5条代表性剖面中的流纹岩进行了元素地球化学、锆石U-Pb定年、全岩Sr-Nd及锆石Hf同位素等研究。U-Pb年龄介于294. 7～272. 7Ma之间,结合古生物资料,明确其喷发时代为早二叠世。该区早二叠世流纹岩大多属高钾钙碱性系列,呈钙碱-碱钙质,为准铝-过铝质岩石。岩石主量元素Si、K、Na、Fe含量高,贫Ca、Mg;微量元素富集Ga、Rb、Zr、Hf、Y,亏损Ba、Sr等;稀土元素存在明显负Eu异常;呈现高Fe OT/MgO与10000×Ga/Al比值,这些特征表明该区早二叠世流纹岩为A型流纹岩。独山剖面流纹岩锆石具有高的εHf(t)值(+11. 4～+16. 7),Hf模式年龄(tDM2)为231～573Ma;干泉剖面流纹岩锆石εHf(t)值为-8. 8～+8. 9,Hf模式年龄(tDM2)为802Ma～1961Ma;珊瑚井和俞井子剖面中的流纹岩具有低的εNd(t)值(-4. 21～-2. 92),高的(87Sr/86Sr)i值(0. 7108～0. 7223),Nd亏损地幔模式年龄(tDM)为1256～1386Ma,其锆石εHf(t)为正值(+1. 3～+17. 0)(tDM2为691～1797Ma);沙红山南剖面流纹岩εNd(t)值(-2. 00～-1. 81)较低,Nd亏损地幔模式年龄(tDM)为1207～1222Ma,锆石εHf(t)值介于+1. 6～+10. 7(tDM2为623～2576Ma)。地球化学特征表明独山剖面的流纹质岩浆为古生代新生地壳的重熔,其它剖面酸性火山岩岩浆则主要为中新元古代地壳物质熔融所形成,揭示了北山南部早二叠世岩石圈伸展减薄的地球动力学背景,结合该区构造岩浆事件序列、地层学以及沉积学等其它地质学证据,认为北山南部早二叠世为裂谷环境。     

藏南米拉山地区中新世埃达克质岩石的年代学、地球化学和成因

中新世是青藏高原隆升、增厚的重要时期,并且在这一时期内拉萨地块广泛发育碰撞后岩浆岩。本文对南拉萨地块米拉山地区的钙碱性钾质火山岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素的测定与系统研究。米拉山中新世火山岩为粗面英安岩、英安岩和流纹岩（SiO₂=59.89%~71.78%）。锆石U-Pb定年结果为16.1±0.2Ma~20.4±0.3Ma,表明其喷发时代为中新世。岩石具有较高的Al₂O₃含量（13.54%~16.31%）,低MgO（0.46%~1.95%）、高Sr（388×10^-6~804×10^-6）、低Y（6.55×10^-6~11.20×10^-6）和Yb（0.70×10^-6~1.07×10^-6）的特征,具有较高的Sr/Y值（51~80）、低相容元素（Cr=4.26×10^-6~32.53×10^-6,Ni=4.16×10^-6~25.75×10^-6）和弱Eu负异常。岩石具有轻稀土元素和Rb、Th、U、K等元素富集、重稀土元素和高场强元素Nb、Ta、Ti亏损的特征。米拉山中新世火山岩显示出埃达克质岩石的地球化学特征,可能来自于的镁铁质加厚下地壳的部分熔融,推测下地壳源区是石榴石角闪岩。锆石ε_Hf（t）值为+2.2~+7.8,表明源区为新生地壳物质,有俯冲板片熔体加入。米拉山中新世火山岩的喷发时代与米拉山断裂活动时间一致,二者可能同为拉萨地块岩石圈拆沉的结果。

     

胶东青山群基性火山岩的Ar-Ar年代学和地球化学特征: 对华北克拉通破坏过程的启示

青山群火山岩是华北克拉通破坏期间最具代表性的地幔或地壳熔融产物,记录了华北深部地质演化的重要信息。本文对胶东青山群基性火山岩进行了40Ar/39Ar定年和岩石地球化学分析,结合前人报道的胶东青山群酸性火山岩资料,发现:(1)基性火山岩喷发年龄为122～113Ma,早于青山群酸性火山岩(110～98Ma);(2)基性和酸性火山岩显示了不同的元素和同位素地球化学特征。岩石成因分析表明,基性火山岩为交代富集地幔部分熔融作用的产物,而酸性火山岩为古老下地壳和中生代底侵岩浆的熔融产物(Ling et al.,2009)。因此,胶东地区青山群火山岩记录了岩浆熔融源区从地幔向下地壳的转变。这与长时间尺度的岩石圈减薄过程中热能由地幔向地壳传递过程相吻合,而不同于地壳拆沉作用所预测的岩浆演化趋势。     

胶东金矿区高钾-钾质脉岩地球化学与俯冲-壳幔作用研究

胶东金矿区与金矿成矿伴生的脉岩为一套高钾 -钾质脉岩。根据地质、岩相学及与金矿化的时空关系 ,将主要岩石类型划分为煌斑岩、安山玢岩、英安玢岩类 ,它们分别形成与金矿化早期、同期和晚期 ;主要元素成分以富碱高钾、低钛为特征 ,成分变异具同源岩浆结晶分异演化的一般规律 ,早期以辉石和橄榄石、中晚期以角闪石和斜长石为主的矿物相分离结晶控制岩浆的演化 ;岩石明显富 Ba、Sr、Rb、K、L REE等大离子元素、强烈亏损 Cr、Ni及相对亏损 Th、Nb、Ti、Y等高场强元素元素 ,体现初始岩浆起源于富集地幔源区 ;大量的挥发组分及强烈的富集 Ba、Sr及低的 Sr/ Nd比值 ,指示源区为以俯冲陆源沉积为主、有玄武质洋壳参与的脱水、脱气等作用交代早期地幔楔形成的富集地幔源 ,初始岩浆是在中生代印支期形成的富集地幔向燕山期亏损方向演化的特定阶段发生低程度熔融形成 (熔融程度为 6 %～ 8% ) ;就位环境的不同是导致石英脉岩型与蚀变岩型矿区脉岩成岩作用有一定差别的主要原因     

Geochronology and geochemistry of mafic dykes in the Helanshan complex: Implications for Mesozoic tectonics in the North China Craton

The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.     

古亚洲洋对华北陆缘岩石圈的改造作用：来自于西山南大岭组中基性火山岩的地球化学证据

北京西山南大岭组中基性火山岩都表现出LREE富集的右倾平滑稀土配分模式,具有富集LILE(如Ba、K),亏损Nb-Ta和Th-U的微量元素特征,Sr同位素组成中等富集,Nd同位素组成变化较大.根据其元素地球化学和同位素特征可以划分两组岩石Ⅰ组火山岩主要为亚碱性玄武岩,高Ti(TiO2＞1.7%)、P(P2O5＞0.7%),Ⅱ组火山岩主要由安山岩和亚碱性玄武岩组成,低Ti(TiO2＜1.3%)、P(P2O5＜0.5%);Ⅰ组火山岩总体上较Ⅱ组火山岩高相容元素(如Cr、Ni)、REE和HFSE,二者化学成分上渐变演化趋势不明显;Ⅰ组火山岩Sr-Nd同位素组成(87Sr/86Sr(t)=0.705939～0.706057,εNd(t)=-7.4～-7.5)与Ⅱ组火山岩Sr-Nd同位素组成(87Sr/86Sr(t)=0.705822～0.706697,εNdd(t)=-12.0～-13.5)有明显的差别,以上事实说明两组岩石可能来源于不同的地幔源区.对比于周缘地区中生代基性火山岩特征,西山地区南大岭组两组火山岩的Sr-Nd同位素组成和Nb/La、Hf/Sm比值都介于华北陆块内部和兴蒙造山带之间,反映了其熔融地幔源区继承了华北陆块内部的EMI型地幔特征外,还很可能受到俯冲板片交代作用的影响,暗示了古亚洲洋板块消减过程对华北陆缘岩石圈地幔的改造作用.其相对Ba-La明显的Th-U亏损,可能暗示早期有古老下地壳组分再循环到地幔源区.结合南大岭组中基性火山岩沿断裂带局限分布特点和区域早中生代构造-热年代学格架,我们认为南大岭组中基性火山岩形成于陆内伸展环境,即深大断裂带再次活动,导致软流圈上隆,从而诱发俯冲交代改造的古老岩石圈地幔减压熔融而成.     

Geochronology and geochemistry of Middle Devonian mafic dykes in the East Kunlun orogenic belt,Northern Tibet Plateau: Implications for the transition from Prototethys to Paleotethys orogeny

The tectonic transition from Prototethys to Paleotethys orogeny in the East Kunlun orogenic belt is not completely clear, and is a major unresolved geologic issue in Northern Tibet Plateau. Here, we present zircon geochronology, whole-rock elemental and zircon Hf isotopic geochemistry for newly discovered mafic dykes in the East Kunlun orogenic belt, to provide constraints on this issue. The studied mafic dykes are hornblende gabbros, consisting of hornblende (60–65 vol.%), plagioclase (15–25 vol.%) and augite and biotite (0–5 vol.%). LA–ICP–MS zircon U–Pb dating shows that these mafic dykes were emplaced at about 393 Ma. All the mafic dykes are characterized by high contents of CaO (8.82–11.48 wt.%), MgO (9.07–11.39 wt.%), V (275–336 ppm), Cr (370–467 ppm) and Ni (78.3–120 ppm), with high Mg# (63–67), flat CI-normalized REE distribution and depleted ?Hf(t) values (2.03–5.35), showing tholeiitic affinities and geochemical characteristics similar to those of mid-ocean ridge basalts. They were derived from low degree (about 5–15%) partial melting of a fertile spinel lherzolite source, which have been metasomatized by fluids introduced to the mantle by former subducted slab. The geologic–petrologic evidence suggests that the mafic dykes were emplaced in a shift tectonic setting related to continental rifting, which was caused by the extensional collapse related to the lithospheric thinning after the Prototethys orogeny. The delamination-induced thermal disturbance and extensional decompression triggered partial melting of the mantle and the emplacement of the mafic dykes. Combined with previous work, we propose that the Middle Devonian mafic dykes may be the early magmatic response to the transition from Prototethys to Paleotethys marking the opening of the Paleotethys in the East Kunlun orogenic belt.     

Geochronological and geochemical study of mafic dykes from the northwest Chinese Altai: Implications for petrogenesis and tectonic evolution

Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean ²⁰⁶Pb/²³⁸U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO₂ contents (45.2–52.7 wt.%) and total alkaline (K₂O + Na₂O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/Yb_N = 0.86–1.1), together with their high ε_Nd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/Yb_N = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low ε_Nd(t) values (+ 3.4 to + 5.4) and high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give ε_Hf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai.