Geochronology,geochemistry, Hf isotopic compositions and formation mechanism of radial mafic dikes in northern Tibet

A large mafic dike swarm is radially distributed in southern Qiangtang. Three typical samples were selected for geochronology, geochemistry, and Hf isotopic analysis. Zircon U–Pb dating indicates that the three dikes formed at 291 ± 2, 292 ± 3, and 300 ± 2 Ma. Whole-rock compositions show that the southern Qiangtang mafic dikes are alkaline, Fe + Ti rich, and exhibit relative enrichment in light rare-earth elements. The ratios of incompatible elements are similar to those of oceanic island and Emeishan basalts. Geochemical diagrams show that the dikes erupted in an intraplate environment. Zircon Hf isotopic data suggest that magma that produced the mafic dikes was derived from a depleted mantle source. The geochemical characteristics of the dikes approximate that of eruption products of a brief period of mantle plume activity (300–280 Ma). According to eight geologic maps of Qiangtang, the mafic dikes crop out over an area of 150 km from north to south and 500 km from east to west, radiating outward from Mayigangri. We conclude that the mafic dikes in southern Qiangtang are related to the combined effect of Permian plate motions and mantle plume activity, and the Mayigangri area overlies the hot spot. Furthermore, the mantle plume in southern Qiangtang may have propelled the closing of the Palaeo-Tethys Ocean.     

Petrogenesis of mid-Carboniferous volcanics and granitic intrusions from western Junggar Basin boreholes: geodynamic implications for the Central Asian Orogenic Belt in Northwest China

The western Junggar Basin is located on the southeastern margin of the West Junggar terrane, Northwest China. Its sedimentary fill, magma petrogenesis, tectonic setting, and formation ages are important for understanding the Carboniferous tectonic evolution and continental growth of the Junggar terrane and the Central Asian Orogenic Belt. This paper documents a set of new zircon secondary ion mass spectrometry U–Pb geochronological and Hf isotopic data and whole-rock elemental and Sr–Nd isotopic analytical results for the Carboniferous strata and associated intrusions obtained from boreholes in the western Junggar Basin. The Carboniferous strata comprise basaltic andesite, andesite, and dacite with minor pyroclastic rocks, intruded by granitic intrusions with zircon secondary ion mass spectrometry U–Pb ages of 327–324 Ma. The volcanic rocks are calc-alkaline and show low high ε_Nd(t) values (5.3–5.6) and initial ⁸⁷Sr/⁸⁶Sr (0.703561–0.703931), strong enrichment in LREEs, and some LILEs and depletion in Nb, Ta, and Ti. Furthermore, they also display high (La/Sm)_N (1.36–1.63), Zr/Nb, and La/Yb, variable Ba/La and Ba/Th and constant Th/Yb ratios. These geochemical data, together with low Sm/Yb (1.18–1.38) and La/Sm (2.11–2.53) ratios, suggest that these volcanic rocks were derived from a 5–8% partial melting of a mainly spinel Iherzolite-depleted mantle metasomatized by slab-derived fluids and melts of some sediments in an island-arc setting. In contrast, the granitic intrusions represent typical adakite geochemical features of high Sr and low Y and Yb contents, with no significant Eu anomalies, high Mg#, and depleted ε_Nd(t) (5.6–6.4) and ε_Hf(t) (13.7–16.2) isotopic compositions, suggesting their derivation from partial melting of hot subducted oceanic crust. In combination with the previous work, the West Junggar terrane and adjacent western Junggar Basin are interpreted as a Mariana-type arc system driven by northwestward subduction of the Junggar Ocean, possibly with a tectonic transition from normal to ridge subduction commencing ca. at 331–327 Ma.     

Geochemistry of Permian Mafic Igneous Rocks from the Napo‐Qinzhou Tectonic Belt in Southwest Guangxi,Southwest China: Implications for Arc‐Back Arc Basin Magmatic Evolution

The Napo-Qinzhou Tectonic Belt (NQTB) lies at the junction of the Yangtze, Cathaysia and Indochina (North Vietnam) Blocks, which is composed of five major lithotectonic subunits: the Qinzhou-Fangcheng Suture Zone (QFSZ), the Shiwandashan Basin (SB), the Pingxiang-Nanning Suture Zone (PNSZ), the Damingshan Block (DB) and the Babu-Lingma Suture Zone (BLSZ). On the basis of geochemical compositions, the Permian mafic igneous rocks can be divided into three distinct groups: (1) mafic igneous rocks (Group 1) from the Longjing region in the PNSZ and Hurun region in the BLSZ, which are characterized by intermediate Ti, P and Zr with low Ni and Cr contents; (2) mafic igneous rocks (Group 2) from the Naxiao and Chongzuo region in the DB, characterized by low-intermediate Ti, P and Zr with high Ni and Cr concentrations; and (3) mafic igneous rocks (Group 3) from the Siming region in the Jingxi carbonate platform of the northwestern margin of the NQTB, with intermediate-high Ti, P and Zr and low Ni and Cr contents. The Group 1 rocks yield a weighted mean 206Pb/238U age of 250.5±2.8 Ma and are geochemically similar to basalts occurring in back-arc basin settings. The Group 2 rocks exhibit geochemical features to those basalts in island arcs, whereas the Group 3 rocks show geochemical similarity to that of ocean island basalts. All three groups are characterized by relatively low εNd(t) values (–2.61 to +1.10) and high initial 87Sr/86Sr isotopic ratios (0.705309–0.707434), indicating that they were derived from a subduction-modified lithospheric mantle and experienced assimilation, fractional crystallization, and crustal contamination or mixing during magmatic evolution. Accordingly, we propose the existence of an arc-back arc basin system that developed along the NQTB at the border of SW Guangxi Province (SW China) and northern Vietnam, and it was formed by continued northwestward subduction of the Cathaysian (or Yunkai) Block under the Yangtze Block, and northeastward subduction of the Indochina Block beneath the Yangtze Block during Permian time.     

西昆仑库科西鲁克地区基性岩脉地球化学及构造意义

新疆库科西鲁克地区广泛发育基性岩脉,多呈岩墙、岩枝和小岩滴。基性岩脉岩石类型为辉长岩和辉绿岩。辉长岩属于碱性玄武岩,而辉绿岩属于过铝质碱性系列碱玄武岩与粗面玄武岩过渡型,其形成深度（浅成相）比辉长岩浅（中深成相）。区内基性岩脉形成于闭合边缘岛弧、活动陆缘造山带环境,是由幔源原生岩浆经过分异并同化混染地壳物质而形成,结晶分异是控制岩浆演化的主要因素。辉长岩中δEu具有弱的亏损。辉绿岩δEu为正异常,而C e均具弱亏损,成岩氧逸度较高,其成因与典型的I型花岗岩类相似,为壳幔混合型。辉长岩偏幔源,而辉绿岩偏壳源,可能为幔源岩浆上侵受围岩混染所致。辉长岩年龄（119 M a）要比辉绿岩年龄（46.1M a）老,辉长岩为冈底斯陆块向欧亚大陆板块碰撞拼贴过程中,逆冲挤压结束的标志,辉绿岩为大规模逆冲挤压剪切结束,青藏高原隆升初期拉张作用的产物。     

新疆北山地区两个含铜镍镁铁—超镁铁质岩体铂族元素地球化学研究

对北山地区坡一和罗东含铜镍的镁铁-超镁铁质岩体铂族元素研究表明,两个岩体的铂族元素(PGE)总量较低,PPGE较IPGE富集,原始地幔标准化模式呈正斜率,均较原始地幔亏损,具Ir和Rh的弱负异常。较低的Pd/Ir比值表明岩石主要受岩浆作用控制,后期热液作用影响不明显。两个岩体的原生岩浆均为MgO 含量较高的PGE不亏损的拉斑玄武质岩浆,较高的Cu/Pd、Ti/Pd比值表明岩浆在演化过程中发生了硫化物的熔离。罗东岩体早期矿物相（橄榄石、铬铁矿）的分离结晶作用对岩浆中的硫达到饱和具有重要的贡献,而坡一岩体该作用对硫化物熔离的贡献不明显。坡一和罗东岩体的R值表明两岩体均具有达到中型Ni矿床的潜力。     

Optical and infra‐red studies on high‐temperature Alkali Feldspars

In ternary feldspars of essentially one phase, calcium content has a dominant influence on the optic axial angle. In such feldspars and also in binary feldspars from solvsbergite rocks, variations of cooling histories do not significantly affect the optic axial angle. In ternary feldspars which are unmixed into two or three prominent phases, Al/Si ordering has an important effect on the 2V value. A recent suggestion of several writers that in feldspars the alkali structural site may be partially occupied by (H3O)+ ions is applied as a possible way to explain a correlation observed between petrographical features of the rocks and the optic axial angles of their feldspar phenocrysts.     

Geochemisty and tectonic setting of igneous rocks in the Glenrock Station area,N.S.W.

In the Upper Murray Valley, Victoria, Late Silurian, high‐Si igneous rocks, which are closely associated with alkalic, basaltic dykes, were emplaced at high crustal levels following the peak of the Benambran Orogeny, which deformed and metamorphosed the Wagga Zone in Late Ordovician‐Early Silurian times. These rocks, which are informally termed ‘the Upper Murray high‐Si magmatic suite’, include leucogranites, rhyolite dykes and flows, and ash‐flow tuffs characterised by the following features. They are transitional from mildly peraluminous to mildly metaluminous; they represent relatively anhydrous magmas, in which halides were important volatile constituents; they have high Si, total alkalies, Rb, Th, U, Nb, Sn and heavy rare earth elements; and they are relatively repleted in Mg, Ca, Sr, Eu, V, Cr and Ni. In these respects and in their post‐orogenic setting and close association with alkalic basalts, they resemble many post‐orogenic granitoids from elsewhere. Such granitoids appear to have formed as partial melts during crustal extension following major episodes of deformation and high‐Si magmatism. A residual granulitic crust, from which an earlier generation of granitoid magmas had been extracted, is argued to be the source rock‐type for these post‐orogenic magmas. Tectonic extension, affecting such a crust, was accompanied by deep fracturing and basaltic vol‐canism. Mantle‐derived, CO₂‐ and halide‐rich fluids moved into the residual crust, causing widespread metasomatism, and emplacement of basaltic magma caused temperatures to rise until melting took place and a second group of magmas was produced. This model explains most aspects of the trace and major element chemistry of post‐orogenic, high‐Si igneous rocks and, for the Upper Murray high‐Si suite it also provides an explanation for variations in trace elements and isotopic characteristics. Other processes, such as crystal fractionation, magma mixing, thermogravi‐tational diffusion, and separation and loss of a volatile phase, provide explanations for variations within individual units of the suite, but they do not explain overall variations or the highly fractionated nature of the suite.     

Phreatomagmatic boulder conglomerates at the tip of the ca 2772 Ma Black Range dolerite dyke,Pilbara Craton,Western Australia

Unusual volcanic conglomerates with a mixture of well-rounded granitic boulders (to 1.2 m diameter) derived from adjacent basement rocks, and smaller (1 – 10 cm) subspherical basaltic droplets with chilled margins occupy a linear zone along strike of the northern end of the Late Archaean Black Range dolerite dyke in the Pilbara Craton, Western Australia. The matrix of the volcanic conglomerates becomes more angular with decreasing grainsize and grades to rock flour, a trend opposite to that in sedimentary conglomerates. In other places, the matrix consists of chlorite that cuts through, and resorbs, granitic clasts, indicating an origin as volcanic melt. The volcanic conglomerates have peperitic contacts with immediately adjacent flows of the Mt Roe Basalt of the Fortescue Group. A welded volcanic tuff at the peperitic contact is dated at 2767 ± 3 Ma, within error of the 2772 ± 2 Ma Black Range dolerite dyke and the Mt Roe Basalt (2775 ± 10 Ma), confirming the contemporaneity of formation of these geological elements. Subsequent normal faulting has juxtaposed the higher level conglomerates down into their present exposure level along strike of the Black Range dolerite dyke. The linear zone of volcanic conglomerates is interpreted to represent a phreatomagmatic pebble dyke that formed immediately above, and as a result of intrusion of, the Black Range dolerite dyke. Interaction of magma with groundwater caused phreatomagmatic brecciation of the country rock, in situ milling of granitic boulders, incorporation of basaltic melt droplets, and the formation of a mixed matrix of devitrified volcanic glass and granitic material. This process was accompanied by along-strike epithermal Cu – Hg – Au mineralisation.     

Age and significance of voluminous mafic–ultramafic magmatic events in the Murchison Domain,Yilgarn Craton

Mafic–ultramafic rocks in structurally dismembered layered intrusions comprise approximately 40% by volume of greenstones in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton. Mafic–ultramafic rocks in the Murchison Domain may be divided into five components: (i) the ～2810 Ma Meeline Suite, which includes the large Windimurra Igneous Complex; (ii) the 2800 ± 6 Ma Boodanoo Suite, which includes the Narndee Igneous Complex; (iii) the 2792 ± 5 Ma Little Gap Suite; (iv) the ～2750 Ma Gnanagooragoo Igneous Complex; and (v) the 2735–2710 Ma Yalgowra Suite of layered gabbroic sills. The intrusions are typically layered, tabular bodies of gabbroic rock with ultramafic basal units which, in places, are more than 6 km thick and up to 2500 km² in areal extent. However, these are minimum dimensions as the intrusions have been dismembered by younger deformation. In the Windimurra and Narndee Igneous Complexes, discordant features and geochemical fractionation trends indicate multiple pulses of magma. These pulses produced several megacyclic units, each ～200 m thick. The suites are anhydrous except for the Boodanoo Suite, which contains a large volume of hornblende gabbro. They also host significant vanadium mineralisation, and at least minor Ni–Cu–PGE mineralisation. Collectively, the areal distribution, thickness and volume of mafic–ultramafic magma in these complexes is similar to that in the 2.06 Ga Bushveld Igneous Complex, and represents a major addition of mantle-derived magma to Murchison Domain crust over a 100 Ma period. All suites are demonstrably contemporaneous with packages of high-Mg tholeiitic lavas and/or felsic volcanic rocks in greenstone belts. The distribution, ages and compositions of the earlier mafic–ultramafic rocks are most consistent with genesis in a mantle plume setting.     

Petrogenesis of the ü?kap?l? Granitoid and its Mafic Enclaves in Elmal? Area (Ni?de, Central Anatolia, Turkey)

The Late Cretaceous ükapili Granitoid including mafic microgranular enclaves intruded into metapelitic and metabasic rocks, and overlain unconformably by Neogene ignimbrites in the Ni de area of Turkey. It is mostly granite and minor granodiorite in composition, whereas its enclaves are dominantly gabbro with a few diorites in composition. The ükapili Granitoid is composed mainly of quartz, K-feldspar, plagioclase, biotite, muscovite and minor amphibole while its enclaves contain mostly plagioclase, amphibole, minor pyroxene and biotite. The ükapili Granitoid has calcalkaline and peraluminous (A/CNK= 1.0-1.3) geochemical characteristics. It is characterized by high LILE/HFSE and LREE/HREE ratios ((La/Lu) N = 3-33), and has negative Ba, Ta, Nb and Eu anomalies, resembling those of collision granitoids. The ükapili Granitoid has relatively high 87Sr/86Sr (i) ratios (0.711189-0.716061) and low εNd (t) values (-5.13 to -7.13), confirming crustal melting. In contrast, the enclaves are tholeiitic and metaluminous, and slightly enriched in LILEs (K, Rb) and Th, and have negative Ta, Nb and Ti anomalies; propose that they were derived from a subduction-modified mantle source. Based on mineral and whole rock chemistry data, the ükapili granitoid is H-(hybrid) type, post-collision granitoid developed by mixing/mingling processes between crustal melts and mantle-derived mafic magmas.