Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar

The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km². It has an ⁴⁰Ar/³⁹Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO₂ (2·2wt %) and total iron (14 wt % as Fe₂O₃), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar     

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, Fe–Ti oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSr–Nd–Pb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The Nd–Sr–Pbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotite–spinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany     

Age and Tectonic Significance of the Mafic Dyke Swarm in the Kuruktag Region, Xinjiang

The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar / 36Ar-40K/ 36Ar isochron age of 282 Ma with the initial 40Ar/ 36Ar = 507. The characteristics of the major, trace and rare earth elements of the dyke rocks indicate that they belong to the calc-alkaline series and were emplaced in extensional structures at an intermediate depth of the crust. This shows that there was a large-scale extension in the Permian Period, which might be associated with the post-collisional extension of the ancient Tianshan Mountains.     

Petrogenesis of Corundum-Bearing Mafic Rock in the Horoman Peridotite Complex, Japan

A corundum-bearing Type II mafic rock, within the Horoman peridotite,Japan, was petrologically examined in detail to obtain the P–Tpaths of the mafic rock as well as of the host peridotite. Ofall the mafic rocks documented from the Horoman complex, onlythe corundum-bearing mafic rock has preserved, at least partly,its high-pressure mineralogy; all of the others have been completelyrecrystallized at low pressures. The Type II mafic rocks wereinitially formed at <1·0 GPa as cumulates of olivine,plagioclase and clinopyroxene. Corundum was then formed by metamorphismand/or partial melting of the Type II protolith at higher pressures(>1·5 GPa) than the initial condition of formation.Corundum reacted with clinopyroxene during exhumation of theHoroman peridotite down to the plagioclase stability field.The field and petrographical observations of the Type II maficrocks (± corundum) coupled with published isotopic datasuggest a complicated spiral-like P–T trajectory for theHoroman peridotite. The Type II protolith was formed at lowpressure within the peridotite at the time of initial formationof the Horoman peridotite as a residue from primitive mantleat     

新疆克拉玛依北西西向岩墙群侵位的磁组构证据

准噶尔盆地西北缘克拉玛依地区广泛发育二叠纪中基性岩墙群,该区岩墙被认为是碰撞后伸展的典型产物.本文根据岩墙宏观野外特征和镜下显微构造分析(岩墙分支、捕虏体、矿物线理和定向斑晶)得出研究区北西西-南东东向岩墙侵位方向由北西西向南东东侵位.通过对该区岩墙边界采样后进行磁化率各向异性测量得到磁组构的最大磁化率长轴优势方位分布图和磁组构各向异性特征分析,进一步指示其岩浆以一定仰角由北西西向南东东侵位.结合克拉玛依地区岩墙群与达拉布特深大断裂的时空分布特征推测该区北西西向岩墙岩浆来源于北西侧的达拉布特断裂,岩浆沿着达拉布特深大断裂上侵到先存裂隙而形成岩墙.     

Age and Geochemical Characteristics of a Mafic Dyke Swarm in the Archaean Vestfold Block, Antarctica: Inferences about Proterozoic Dyke Emplacement in Gondwana

Archaean gneisses in the Vestfold Block, Antarctica are cutby abundant tholeiite and rare alkaline dykes. At least fivegenerations of dykes have been recognized on the basis of intrusiverelationships, petrography and geochemistry. Rb-Sr isotopicdata indicate that intrusion of the tholeiites occurred overa period of c. 1000 Ma, during three clearly defined events(viz. c. 2400, 1800, and 1400 Ma). Dykes in the southwesternpart of the Vestfold Block were recrystallized during a lateProterozoic (c. 1000-1100 Ma) high-grade event. Mineral-wholerock Rb-Sr isotopic data show that the terrain was also variablyaffected by an even younger thermal event c. 500 Ma ago whichis correlated with the Pan-African Orogeny. Isotopic, major element and trace element data suggest thatthe tholeiite dyke suites were derived by varying degrees ofpartial melting of compositionally distinct, in some cases ratherheterogeneous subcontinental mantle source regions, combinedwith variations in the extent and nature of crystal fractionation.Extensive crustal contamination does not appear to have playeda significant role in determining compositional trends withinthe different suites, although minor contamination cannot bediscounted. The oldest dykes, a diverse group of high-Mg tholeiites,and the youngest, c. 1400 Ma tholeiites, appear to have beenderived from isotopically similar source regions, which werenevertheless characterized by quite different abundances ofhighly incompatible elements (Pb, Rb, Ba, Th, K, La, and Ce).1800 Ma tholeiites were derived from a more radiogenic mantlesource region, characterized by long-term enrichment in incompatibleelements. Such enrichment is interpreted to be a consequenceof metasomatism of their mantle source region. 2400 Ma high-Mg dykes in Enderby Land have virtually identicalchemical and isotopic compositions to those in the VestfoldBlock, indicating generation from a similar source region; however,1200 Ma tholeiites were apparently derived from a more radiogenicsource region like that of the c. 1800 Ma Vestfold Block dykes.Hence, both Sr isotopic and geochemical data indicate that theancient subcontinental lithospheric mantle beneath Gondwanawas extremely heterogeneous.     

Petrology and Petrogenesis of the Monchique Alkaline Complex, Southern Portugal

Monchique is a sizeable subvolcanic ?laccolith, unusual amongalkaline complexes in invading sediments and lacking apparentconnection with either rifting or orogeny; it may however relateto the opening of the North Atlantic(age = 76 m.y.). The intrusionis predominantly miaskitic syenite, varying irregularly fromfoyaite to pulaskite with fine-grained nepheline-poor marginsbut showing no rhythmic or cryptic layering. Minor rock-typesinclude early masses of olivine-free kaersutite—theralites(berondrites) and essexites, bodies of igneous breccias, maligniteand agpaite, veins of foyaite—pegmatite and shonkinite,and dykes of lamprophyres and peralkaline tinguaite. Coevaldykes outside the main intrusion include quartz—trachytes,normal (olivine-bearing) basanites and amphibole—picrites. The whole suite may have derived from a basanitic parent undermoderately oxidizing conditions. Geochemistry is apparentlycontinuous along the trend Berondrite—Essexite—Foyaite(Malignite)—Pulaskite—Quartz trachyte. As far asFoyaite this parallels the normal oceanic basanite—phonolitetrend with major and trace elements and minerals (except olivine)behaving as in normal fractionation; the absence of nepheline—monzonites,creating a Daly Gap, may merely reflect high fractionation efficiency.The apparent evolution across the thermal barrier in Ne—Ks—Qtz,with a reversal in some major element trends, however, can beexplained neither by fractionation nor country-rock assimilation.The enigmatic pulaskites cannot be related directly to the foyaitesbut might have formed from the same parent under lower pressureconditions; they themselves fractionated to the peralkalinetinguaites. The quartz—trachytes probably originated wherefoyaite magmas lost alkalis to the siliceous country-rocks,became oversaturated, and then fractionated feldspars. Liquidimmiscibility might explain some anomalous monzonitic rocksbut otherwise contributed little to the evolution of the complex.     

华北克拉通阜平杂岩的深熔和混合岩化作用

华北克拉通的阜平杂岩长英质岩石中常产出显著的浅色体、岩脉和花岗岩侵入体,并形成广泛的混合岩化作用。通过矿物自形晶的形成、黑云母向角闪石的转换和大量钠长石净边的出现以及其它与熔体活动有关结构的分析,浅色脉体和混合岩化作用的发生与外来熔体的注入有关。在长英质片麻岩中可出现明显的熔体注入,在一些不易片理化的岩石如石英岩中亦可形成浸染状熔体渗入。熔体汇集可形成浅色体、岩脉,直至花岗岩侵入体。而深熔作用本身形成熔体的作用在本区几乎可以忽略不计。在遭受渗透式混合岩化作用的过程中,岩石成分发生了改变,形成开放系统。随着渗透熔体的结晶,可形成一些岩浆锆石,在副片麻岩中则很容易被当作碎屑锆石。     

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa_83–93), ferroaugite (Di₅₀Hd₄₅Ac₅–Di₅Hd₉₀Ac₅)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa₉₃), and acmitic-hedenbergite (Di₅Hd₅₀Ac₅–Ac₅₀Hd₅₀).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10^–15 to 10^–23–10^–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.     

Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Querigut Complex, French Pyrenees

The Quérigut mafic–felsic rock association comprisestwo main magma series. The first is felsic comprising a granodiorite–tonalite,a monzogranite and a biotite granite. The second is intermediateto ultramafic, forming small diorite and gabbro intrusions associatedwith hornblendites and olivine hornblendites. A U–Pb zirconage of 307 ± 2 Ma was obtained from the granodiorite–tonalites.Contact metamorphic minerals in the thermal aureole providea maximum emplacement pressure of between 260 and 270 MPa. Petrographiccharacteristics of the mafic and ultramafic rocks suggest crystallizationat <300 MPa, demonstrating that mantle-derived magmas ascendedto shallow levels in the Pyrenean crust during Variscan times.The ultramafic rocks are the most isotopically primitive components,with textural and geochemical features of cumulates from hydrousbasaltic magmas. None of the mafic to ultramafic rocks havedepleted mantle isotope signatures, indicating crustal contaminationor derivation from enriched mantle. Origins for the dioritesinclude accumulation from granodiorite–tonalite magma,derivatives from mafic magmas, or hybrids. The granitic rockswere formed from broadly Proterozoic meta-igneous crustal protoliths.The isotopic signatures, mineralogy and geochemistry of thegranodiorite–tonalites and monzogranites suggest crystallizationfrom different magmas with similar time-integrated Rb/Sr andSm/Nd isotope ratios, or that the granodiorite–tonalitesare cumulates from a granodioritic to monzogranitic parent.The biotite granite differs from the other felsic rocks, representinga separate magma batch. Ages for Quérigut and other Pyreneangranitoids show that post-collisional wrenching in this partof the Variscides was under way by 310 Ma. KEY WORDS: Variscan orogeny; Pyrenees; Quérigut complex; epizonal magmatism; post-thickening; mafic–felsic association     

Open-System, Sub-Volcanic Magmatic Evolution: Constraints on the Petrogenesis of the Mount Brome Alkaline Complex, Canada

The Mount Brome alkaline complex of southern Quebec (Canada)comprises gabbroic to silica-oversaturated and -undersaturatedfelsic rocks which have variable initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios: 0.70345–0.70431 and 0.51270–0.51258, respectively,in mafic lithologies (gabbro, nepheline diorite, and biotitemonzodiorite); 0.70353–0.70403 and 0.51270–0.51263in silica-undersaturated felsic units (pulaskite and nephelinesyenite); and 0.7051–0.7115 and 0.51262–0.51259in silica-oversaturated nordmarkite. Negatively correlated ⁸⁷Sr/⁸⁶Srvs. ¹⁴³Nd/¹⁴⁴Nd trends for the various rock types appear toconverge at the composition ⁸⁷Sr/⁸⁶Sr = 0.70345 and ¹⁴³Nd/¹⁴⁴Nd= 0.51270 (_Nd = 4.3). This suggests that all rock types sharea common parental magma composition, either through derivationfrom a single batch of liquid, or (more probably) through twoepisodes of melting of the same depleted mantle source region.Delta ¹⁸O ranges from + 5.5 to + 8% and is broadly correlatedwith radiogenic isotopes and bulk composition. Isotopic, and major and trace element compositions suggest thatfractional crystallization (FC) of the parental magma at deeplevels produced evolved magmas, apparently without significantcrustal contamination [FC without assimilation (A)]. Duringascent and emplacement into the upper crust, these magmas thenevolved by simultaneous crystal fractionation and country-rockassimilation (AFC). Within both basic and felsic rocks thereis a clear relationship between silica saturation and degreeof contamination. Indeed, the oversaturated nordmarkites owetheir origin to assimilation of siliceous country rocks by acritically undersaturated magma. More generally, it seems likelythat this type of process is the normal mode of origin for coexistingquartz and nepheline syenites in many sub-volcanic alkalineigneous complexes. Additionally, such complexes would be likelyto develop by punctuated FC and AFC processes throughout theirpetrogenetic history.     

北大巴山超基性、基性岩墙和碱质火山杂岩形成时代的新证据及其地质意义

北大巴山早古生代地层广泛出露一套超基性、基性岩墙和碱质火山杂岩(包括碱性玄武岩和粗面岩),为研究北大巴山早古生代构造演化提供了重要的载体。本文通过利用锆石U-Pb定年和金云母~(40)Ar/~(39)Ar同位素定年方法对它们的形成时代进行了系统研究,结合地球化学特征探讨它们的成因及构造意义。通过对岚皋县和镇坪县的两个辉绿岩墙开展锆石U-Pb定年,分别获得它们的年龄为399±1Ma和451±4 Ma,是研究区目前已报道的有关超基性、基性岩墙最年轻和最老的就位时代。地球化学分析结果表明,它们的主量元素与区内其他基性岩墙位于同一演化趋势线上,微量和稀土元素地球化学特征相似,类似于OIB的特征,表明它们来自相同的地幔源区。因此,研究区内基性岩墙侵入事件最早开始于晚奥陶世中期(约450Ma),结束于早泥盆世晚期(约400 Ma),期间经历了多次岩浆侵入活动。对与碱性玄武岩共生的火山碎屑岩中金云母晶屑进行~(40)Ar/~(39)Ar同位素定年,获得年龄446±3Ma,代表了火山喷发时间,表明碱性玄武质岩浆喷发活动与基性岩浆侵入事件大致同时发生。对粗面岩进行SHRIMP锆石U-Pb定年虽未获得准确的岩浆结晶年龄,但较年轻锆石年龄(165±3Ma和229±2Ma)的存在,暗示其形成时代可能属于中生代。系统的年代学研究表明,北大巴山地区早古生代地层中的超基性、基性岩墙和碱性玄武质火山杂岩为同期岩浆活动的产物,最早开始于450Ma,经历了多期火山喷发和岩浆侵入活动(450~400 Ma),结束于早泥盆世晚期(~400Ma)。而粗面岩与上述岩石不是同一期岩浆活动产物,研究区不存在双峰式火成岩组合,它们可能形成于中生代,属于南秦岭中生代岩浆活动的产物,为南秦岭、北大巴山中生代成矿作用提供物源。     

The Role of Water Retention in the Anatexis of Metapelites in the Bushveld Complex Aureole, South Africa: an Experimental Study

Highly restitic metapelites occur at the contact of the RustenburgLayered Suite (Bushveld Complex). On the basis of previous experimentalstudies, the high (     

Possible Non-melted Remnants of Subducted Lithosphere: Experimental and Geochemical Evidence from Corundum-Bearing Mafic Rocks in the Horoman Peridotite Complex, Japan

We report experimental results and whole-rock trace-elementcharacteristics of a corundum-bearing mafic rock from the Horomanperidotite complex, Japan. Coronitic textures around corundumin the sample suggest that corundum was not stable in maficrock compositions during the late-stage P–T conditionsrecorded in the complex (P < 1 GPa, T < 800°C). Basedon the experimental results, corundum is stable in aluminousmafic compositions at pressures of 2–3 GPa under dry conditions,suggesting that the corundum-bearing mineral assemblages developedunder upper-mantle conditions, probably within the surroundingperidotite. Variations in the trace-element compositions ofthe corundum-bearing mafic rock and related rocks can be controlledby modal variations of plagioclase, clinopyroxene and olivine,suggesting that they formed as gabbroic rocks at low-pressureconditions, and that the corundum-bearing mafic rock was derivedfrom a plagioclase-rich protolith. A complex P–T trajectory,involving metamorphism of the plagioclase-rich protolith ata pressure higher than that at which it was first formed, isneeded to explain the origin of the corundum-bearing mafic rocks.They show no evidence for partial melting after their formationas low-pressure cumulates. The Horoman complex is an exampleof a large peridotite body containing possible remnants of subductedoceanic lithosphere still retaining their original geochemicalsignatures without chemical modification during subduction andexhumation. KEY WORDS: Horoman; mafic rock; corundum; experiment; P–T history; recycling     

西昆仑塔什库尔干苦子干碱性杂岩体的成因及其构造意义

苦子干碱性杂岩体是塔什库尔干新生代碱性岩带的主要组成之一,主要由霓辉正长岩、石英霓辉正长岩和碱性花岗岩组成。霓辉正长岩、石英霓辉正长岩和碱性花岗岩的SiO2含量差别较大,分别为50．26％～54．11％、59．74％～60．43oA、69．59％～72．13％;霓辉正长岩与石英霓辉正长岩的里特曼指数相近,分别为7．83～10．97、8．51～9．05,明显不同于碱性花岗岩（2．49～3．71）。这3种碱性岩石都表现出富集轻稀土,以及K、Rb、Sr、Ba等大离子亲石元素（LILE）,亏损Nb、Ta、Ti等高场强元素（HFSE）的特征。其中异常高的Sr、Ba和∑REE含量暗示可能还有富集地幔物质的加入。利用原位锆石LA—ICP—MS同位素测试方法,获得石英霓辉正长岩和碱性花岗岩的锆石U—Pb年龄分别为10．9±0．1Ma和11．9土0．4Ma,代表了岩石的结晶年龄;石英霓辉正长岩锆石εHf（t）值为-9．78～3．24,碱性花岗岩中锆石εHf（t）值为-13．54～5．78,均显示岩浆源区具有壳幔混合的特点。依据这些资料,并结合区域地质研究的综合分析,笔者初步提出西昆仑塔什库尔干苦子干碱性杂岩体的形成是由于青藏高原在～25Ma期间主体拆沉作用发生后,造成软流圈物质向北逃逸,并受到北面塔里木克拉通岩石圈山根阻挡而上涌,进而发生壳幔混合作用的产物。因此,该杂岩体是青藏高原大规模拆沉后浆活动的远程效应,且标志着研究区在～11Ma期间已处于伸展构造背景。     

Paleomagnetic Evidence for Tectonic Setting of Paleoproterozoic Dyke Swarms in the North China Craton,China

正In order to investigate the tectonic setting of 1.77-1.78Ga dyke swarms emplaced into the central North China Craton(NCC),we carried out a paleomagnetic and magnetic fabric study on the well geochronologically     

Ore-Controlling Structures of the Heishan Iron Deposit, Damiao Anorthosite Complex in North China Craton and Ore-Prospecting

Please refer to the attachment(s) for more details.     

湘中紫云山岩体暗色微粒包体的成因:岩相学、全岩及矿物地球化学证据

暗色微粒包体广泛分布于湘中紫云山岩体中的似斑状角闪石黑云母花岗闪长岩中,但其研究程度较低.对具有火成结构的暗色微粒包体及其寄主岩进行了岩相学、全岩及长石、辉石、黑云母的矿物地球化学研究,探讨其岩石成因及构造意义.寄主岩的全岩主量、微粒元素较为均一,而暗色微粒包体变化较大,且后者相对贫SiO₂而富Na₂O,但总体上二者均具有准铝质、钙碱性、镁质的特征,均富集轻稀土和大离子亲石元素,而亏损重稀土和高场强元素.寄主岩和暗色微粒包体的斜长石、辉石和黑云母均分别属于中长石、次透辉石－低铁次透辉石和铁质黑云母的范畴,显示相似的矿物地球化学特征.详尽的岩相学和地球化学特征表明,寄主岩属于I型和ACG型花岗岩,具有明显壳幔混合的特点;而暗色微粒包体形成时处于液态并具有流动性,与寄主岩间存在明显的机械和化学混合作用,并具有早期为骤冷快速结晶、晚期缓慢结晶这两期过程.因此,紫云山岩体中出现大量暗色微粒包体,是印支晚期湘中地区在强烈挤压之后的松弛阶段,由于软流圈物质上涌,并与其诱发的壳源酸性岩浆混合作用的产物.      

Isotopic Composition of Strontium in Rocks of the Fen Alkaline Complex, South Norway

The isotopic composition of Sr and the abundances of Rb andSr have been determined in the alkaline rocks of the Fen Complex,South Norway. The ⁸⁷Sr/⁸⁸Sr ratios range from as low as 0.703in carbonatite (s?vite) to as high as 0.710 in rauhaugite. Thewhole rock analyses do not plot on a Nicolaysen diagram as anisochron. Calculation of the initial isotopic composition ofSr in the rocks at the time of intrusion of the complex, approximately550 m.y. ago, shows that the rocks are not simply related asthe differentiation products of a single magma, since they donot possess similar initial ⁸⁷Sr/⁸⁶Sr ratios. The lowest initialratio is seen in the s?vites and it is concluded that this rockrepresents the magma from which the other rocks of the complexwere derived. Production of hybrid rocks by bulk assimilationof granite gneiss in carbonatite is inadequate to account forthe observed Rb/Sr ratios. It is envisaged that selective concentrationof granitic Sr of high ⁸⁷Sr/⁸⁶Sr ratio together with loss orgain of variable amounts of Rb has taken place. The processis closely connected with the fenitization observed at the marginsof the complex and is of a metasomatic nature. Thus, the genesisof rocks of the melteigite-ijolite-urtite series may be consideredto be a rheomorphic process. The ⁸⁷Sr/⁸⁶Sr ratios observed inr?dberg and rauhaugite can be explained in terms of the metasomaticalteration of damtjernite.     

2.2Ga Subduction-Related Mafic Magmatic Rocks in the Kongling Complex: Evidence for the Assembly of the Columbia Supercontinent

<正>Objective Petrogenesis of the Paleoproterozoic mafic dikes and their tectonic implications are of great significance to the tectonic evolution of the Yangtze craton as well as the paleoposition of the Yangtze craton relative to the Columbia supercontinent.Till now,intensive studies have