Geology, geochemistry and origin of the banded and granite gneisses in the basement complex of the Ilesha area, southwestern Nigeria

The basement complex in the Ilesha area consists of two distinct units — the gneisses and the schists. The Ilesha Schist Belt is a back-arc basin where there has been a subduction of an ocean slab into the mantle. This was followed by partial melting of mantle and ocean sediments to generate a wet basaltic magma, as revealed by spidergrams and REE fractionation patterns for the rocks in this belt. In this environment, differentiation of the wet basaltic magma led to the emplacement of a set of rocks, which formed a proto-continent. These rocks were then eroded to generate a sedimentary sequence which was metamorphosed into banded gneiss from which the granite gneisses were derived. The banded gneiss, characterised by alternation of felsic and mafic bands, is composed of medium to very coarse plagioclase, hornblende, quartz and biotite. The granite gneiss, composed of biotite, K--feldspars, quartz and minor garnet, occurs in close association with the banded gneiss.Chemical evidence revealed that elements that are depleted in the banded gneiss are concentrated in the granite gneiss and vice-versa; suggesting a petrogenetic link between these rocks.The schists were deposited as sediments composed of quartz, muscovite, biotite and Fe oxides. These sediments were metamorphosed to form quartzite schists which were folded into the gneisses. After the emplacement of these rocks, there was transpressive tectonic activity in this schist belt, causing deformation of these rocks, and emplacement of the northeast-southwest Ifewara-Zungeru Fault System, which separates the Ilesha Schist Belt into two halves.     

Synkinematic granitoid magmatism of Western Sangilen,South-East Tuva

The problems of tectonic control of composition, size, and morphology of synkinematic crustal granitoids are discussed by the example of the Western Sangilen granites (South-East Tuva). Comparative analysis was performed for felsic bodies and massifs spatially confined to tectonic zone (Erzin shear zone): Erzin migmatite–granite complex (510–490 Ma), Matut granitoid massif (510–490 Ma), Bayankol polyphase gabbro-monzodiorite–granodiorite–granite massif (490–480 Ma), and the Nizhneulor Massif (480–470 Ma). It is shown that synkinematic felsic melts during the transition from collisional compression to transpression were formed at different crustal levels. An increase of shear component provided favorable conditions for the migration of felsic melts, increase of size and morphology of intrusive bodies from vein type to harploith (likely, loppoliths and laccoliths) and further to stocks. All kinematic granitoids of the Erzin tectonic zone are ascribed to the crustal S-type granites. Dispersion and average chemical composition of the synkinematic granites strongly depend on the degree of their “isolation” from protolith. From auto- and paraautochthonous granitoids to allochthonous granites, the compositional dispersion decreases and the chemical composition is displaced toward I-type magmatic rocks.     

碰撞造山带地壳深熔作用:来自纳米花岗岩包裹体的观测与实验约束

在碰撞造山带构造演化过程中,中下地壳深熔作用对于深部地壳物理性质与化学成分具有重要控制作用.作为深熔作用的"见证者",纳米花岗岩包裹体是寄主岩石部分熔融作用的产物,能够为确定陆壳岩石中天然熔体特征及分析熔融机制提供关键信息.在喜马拉雅东构造结南迦巴瓦岩群的代表性岩石单元（泥质片麻岩与长英质片麻岩）中,石榴石与锆石中常包含有典型的纳米花岗岩包裹体,其代表性子矿物组合为钾长石+斜长石+石英±黑云母,这是在黑云母脱水熔融过程中、寄主片麻岩中熔体被主要转熔矿物（如石榴石等）捕获所形成一类特殊包裹体.在观测基础上,采用高温高压与高温常压手段,对纳米花岗岩包裹体进行均一化实验并获得均一化玻璃质熔体.成分分析表明,均一化熔体成分以过铝质花岗岩为主,其主/微量元素特征能够有效反演部分熔融作用的演化过程.因此,纳米花岗岩包裹体的天然观测与实验研究对于确定天然熔体特征与深入剖析碰撞造山带的地壳深熔作用具有重要启示意义.      

Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa,and their bearing on granite genesis and the composition of the lower crust

The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue.Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr⁸⁷/Sr⁸⁶ ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition.The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.     

A HFSE- and REE-enriched ferrodiorite suite from the Bolangir Anorthosite Complex, Eastern Ghats Belt, India

The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D₂. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar, ) during a period of near-isobaric cooling from the igneous crystallization stage to the regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases have extremely ferrous compositions (X_Fe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO₂ 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO₂: 4.8-1.0 wt.%, P₂O₅: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through hybridization between the ferrodiorite and overlying felsic melts.     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam,northern Tibet

A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c. 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c. 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.     

Single zircon ages for felsic to intermediate rocks from the Pietersburg and Giyani greenstone belts and bordering granitoid orthogneisses, northern Kaapvaal Craton, South Africa

Previous models for the temporal evolution of greenstone belts and surrounding granitoid gneisses in the northern Kaapvaal Craton can be revised on the basis of new single zircon ages, obtained by conventional U---Pb dating and Pb---Pb evaporation. In the Pietersburg greenstone belt, zircons from a metaquartz porphyry of the Ysterberg Formation yielded an age of 2949.7±0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/−18 Ma. These data show felsic volcanism in this belt to have been coeval with felsic volcanism in the Murchison belt farther east, and the date of 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani greenstone belt has a zircon age of 3203.3±0.2 Ma, while a younger and cross-cutting feldspar porphyry has an emplacement age of 2874.1±0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giyani greenstones.Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in composition from tonalite to granite and texturally from well-layered to homogeneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constituted a basement to the Giyani greenstone sequence, while the younger gneisses are intrusive into the older gneiss assemblage and/or into the greenstones. The Giyani and Pietersburg belts probably define two separate crustal entities that were originally close together but were later displaced by strike-slip movement.     

燕山造山带后石湖山碱性环状杂岩体的成因与形成过程

后石湖山杂岩体是与垮塌破火山口有关的碱性环状杂岩体, 主要由呈环形分布的碱性火山岩、环状岩墙(斑状石英正长岩)、嵌套的中心复式岩株(晶洞碱长花岗岩和斑状碱长花岗岩)和锥状岩席(石英正长斑岩和花岗斑岩)组成.LA-ICPMS锆石U-Pb年代学分析表明, 斑状石英正长岩环状岩墙、石英正长斑岩和花岗斑岩锥状岩席的侵位年龄分别为119±3Ma、121±2Ma和121±2Ma.该环状杂岩体火山岩与侵入岩的形成年龄相近, 体现了它作为火山-侵入杂岩体的特征.斑状石英正长岩富碱(Na₂O+K₂O=10.0%~10.5%), K₂O含量较高(5.21%~5.42%), 具正的Eu异常(Eu/Eu*=1.05~1.40).碱长花岗岩和斑岩均具有富碱、高FeOtot/MgO、Ga/Al、Zr、Nb和REE值(Eu除外), 以及低Al₂O₃、CaO、MgO、Ba、Sr和Eu含量的特征, 都属于A型花岗岩质岩石.其中斑岩为铝质A型花岗岩, 具有高的初始岩浆温度(880~901℃).所有A型花岗质岩石均具有较富集的Nd同位素组成, ε_Nd(t)值变化于-13.9~-12.2之间.斑状石英正长岩是下地壳中-基性麻粒岩和片麻岩部分熔融产生的熔体与幔源玄武质岩浆混合, 后又发生单斜辉石分离结晶的产物; 碱长花岗岩源于上地壳长英质岩石部分熔融产生的熔体与幔源玄武质岩浆混合, 随后经历长石的分离结晶作用而成; 斑岩是受幔源岩浆底侵加热的上地壳长英质岩石的部分熔融产生的熔体, 并经历了长石的分离结晶作用而产生.该环状杂岩体的形成过程可以概括为: (1)火山爆炸性喷发形成大量的碱性火山熔岩和火山碎屑岩; (2)地下岩浆房空虚导致压力下降, 其顶板围岩失稳而沿火山口周围近直立的环状断裂垮塌, 形成塌陷的破火山口.与此同时, 下覆岩浆房的岩浆被动挤入环状断裂而形成斑状石英正长岩环状岩墙; (3)浅部地壳的长英质岩浆房过压, 促使其高温过碱质A型花岗质岩浆上升侵位形成了中心的斑状碱长花岗岩岩株, 这些岩浆的上涌导致上覆围岩产生倾角中-陡的、内倾的锥状裂隙, 为石英正长斑岩锥状岩席侵位提供了空间; (4)浅部岩浆房复活, 高温过碱质A型花岗质岩浆再度上升侵位形成被嵌套的晶洞碱长花岗岩岩株.同样, 这种岩浆的再度上侵导致上覆围岩产生了倾角较陡而内倾的锥状裂隙, 为花岗斑岩锥状岩席提供了侵位空间.后石湖山碱性环状杂岩体的形成是华北东部早白垩世与克拉通破坏相关的伸展构造体制下的产物, 这种构造体制可能与古太平洋板块的俯冲作用有关.      

Early Archaean Amîtsoq tonalites and granites of the Isukasia area,southern West Greenland: development of the oldest-known sial

Amphibolite facies early Archaean Amîtsoq gneisses envelop and intrude the c. 3,800 Ma Isua supracrustal belt, Isukasia area, southern West Greenland. Most of these gneisses are strongly deformed, but in a c. 75 km² augen of lower deformation, the Amîtsoq gneisses are seen to comprise predominantly 3,750–3,700 Ma tonalitic grey gneisses that were intruded first by thin bodies of mafic to dioritic composition, known collectively as the Inaluk dykes, and then by c. 3,600 Ma white gneisses and finally by sporadic c. 3,400 Ma pegmatitic gneiss sheets. The grey gneisses could have formed by partial melting of crust consisting predominantly of basic rocks. The Inaluk dykes are interpreted as strongly fractionated basic melts of mantle origin, contaminated by crustal material. The white gneisses consist mostly of medium grained granite and occur as lenses and anastomosing sheets throughout their host of grey gneisses with subordinate inclusions of supracrustal rocks. The white gneisses have chemistry compatible with formation by partial melting at depth of a source dominated by grey gneisses. The igneous chemistry, including REE abundances, of the grey gneisses and white gneisses has been modified to varying degrees by metasomatism and assimilation reactions during the crystallisation of the white gneisses and also during subsequent tectonometamorphic events. The white gneisses are evidence for considerable reworking by anatexis of sialic crust in the early Archaean, 150 to 100 Ma after its formation. The white gneisses and the pegmatitic gneisses show that granitic rocks s.s. were important in the earliest Archaean, and are further evidence of the diversity of the oldest-known sial.Previously at and the Geological Survey of Greenland, Øster Voldgade 10, 1350 Copenhagen K, Denmark     

冀东迁安地区太古代片麻杂岩的地球化学和演化

曹庄片麻杂岩包括35亿年的表壳岩及三个不同时期的复期正片麻岩。水厂片麻杂岩包括水厂表壳岩及侵入其中的淡花岗岩和紫苏花岗岩,后者年龄为26.5亿年。     

Finds of “Ancient” Zircons in Magmatic Rocks of the Shatak Complex (Southern Urals) and Their Petrogenic Implications

New data on the presence of “old” zircons in magmatic rocks of the Shatak Complex have been obtained. This suggests that the evolution of melts in intermediate magma chambers was controlled by the AFC process, in which the liquidus phases (olivine ± clinopyroxene) fractionated, the fluid phases accumulated in the apical part of the chamber, and the host rocks were partly assimilated. Hence, the “old” zircons were entrained from the Archaean–Paleoproterozoic granite–gneiss substrate and trapped by the melt during crustal contamination.     

Strata-bound scheelite mineralisation in skarns and gneisses from the Bindal area,Northern Norway

Scheelite in the Bindal area, Northern Norway, is developed in hornblende-biotite gneisses of volcano-sedimentary origin, especially in the zones of disseminated sulphides in the gneisses. Scheelite is present either in mobilised quartz veins or in skarnitised hornblende-biotite gneiss where limestone was in contact with these rocks. An exhalative-sedimentary origin of the mineralisation is proposed, on account of the strata-bound character and the lack of any direct relationship to the plutonic rocks of the Bindal massif.     

Partial melting of crustal rocks

Shales and graywackes were first metamorphosed at 650°C and then partially melted at 700 and 750°C at 2, 4, 6, and 8 kilobars in the presence of 0.75m NaCl−0.45m KCl and 0.225m CaCl₂−0.750m NaCl solutions. In experiments with shales,KK+Na ratio in the decreases with increasing pressure at 650 and 700°C; however, at 750°C this ratio is equal to 0.5 at all pressures investigated. This suggests that melts at 700°C and at 2 to 8 kilobars pressure may be affected metasomatically whereas melts at 700°C and in the same pressure range will not. Melt composition produced in the shale-KClNaCl experiments is granite at 2, 4 and 6 kilobars pressure, whereas the melt compositions in the shale-CaCl₂NaCl experiments range from quartz monzonite (2–5 kilobars) to granodiorite (above 5 kilobars). Experiments with graywacke-KClNaCl produced melts of trondhjemite composition at 2, 2.5, 4, 6, and 7.5 kilobars.

These results indicate that partial melting of crustal rocks such as metamorphosed shales and graywackes in the deeper parts of the crust can produce large volumes of granitic magmas ranging in composition from true granite to trondhjemite to quartz monzonite and granodiorite.     

Composition,sources, and mechanisms of origin of rare-metal granitoids in the Late Paleozoic Eastern Sayan zone of alkaline magmatism: A case study of the Ulaan Tolgoi massif

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.     

抚顺南部早前寒武纪变质杂岩的地质事件序列

抚顺南部早前寒武纪变质杂岩是华北克拉通北缘辽北-吉南早前寒武纪变质地块的一个重要组成部分,主要由浑南群石棚子组角闪岩相变质火山岩、火山碎屑岩及相伴生的沉积岩等表壳岩系和侵位于其中的石英闪长质片麻岩、英云闪长质-奥长花岗质-花岗闪长质(TTG)片麻岩和花岗闪长岩-二长花岗岩-钾长花岗岩岩石组合组成。LA-ICP-MS锆石U-Pb同位素分析结果显示,侵位于表壳岩中的石英闪长质片麻岩样品12LN39-3的岩浆结晶年龄为2571±7Ma,指示存在老于该年龄的表壳岩系。英云闪长质片麻岩样品12LN04-1和奥长花岗质片麻岩样品13LB49-3的岩浆结晶年龄分别为2544±4Ma和2550±10Ma,记录了一期重要的英云闪长质-奥长花岗质片麻岩侵位事件。斜长角闪岩(样品12LN25-2)的岩浆结晶的最小年龄为2530±5Ma,指示另一火山喷发阶段。晚期钾长花岗岩样品12LN01-1和奥长花岗质片麻岩样品12LN27-1分别侵位于2522±4Ma和2518±23Ma,说明它们的岩浆作用发生于同一时期。而采自于晚期未变形侵入体的石英闪长岩样品12LN30-2的岩浆结晶年龄为2496±18Ma,与上述表壳岩和深成侵入体的主要变质作用(2510~2470Ma)同期发生。这些年代学结果表明,抚顺南部地区新太古代大规模的铁镁质火山喷发作用在大于2571±7Ma已经发生,紧接着2571±7Ma发生石英闪长质岩浆侵位,在2550±10Ma~2544±4Ma之间发生英云闪长质-奥长花岗质岩浆侵位。接下来铁镁质火山再度喷发(~2530±5Ma),随后为钾长花岗岩和奥长花岗质岩浆的侵位(2522±4Ma~2518±23Ma)。晚期为角闪岩相变质作用时期(2510~2470Ma),伴随一定规模的石英闪长岩侵位。     

Contrasting stratified plutons exposed in tilt blocks, Eldorado Mountains, Colorado River Rift, NV, USA

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from 45–75 wt.% SiO₂. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate.

Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting.     

Early Palaeoproterozoic volcanism of the Karelian Craton: age,sources, and geodynamic setting

A combined study of major and trace elements, Nd isotopes, and U-Pb systematics has been conducted for the early Palaeoproterozoic (Sumian) volcanic rocks and granites localized in different portions of the Karelian Craton. SHRIMP dating of zircons from the Sumian basalts indicates an emplacement age of 2423 ± 31 Ma, which constrains the lower age boundary of the early Palaeoproterozoic sequence at the Karelian Craton. The early Palaeoproterozoic mafic volcanic rocks of the Karelian Craton show practically no lateral geochemical and isotope-geochemical variations. The rocks bear signs of crustal contamination, in particular Nb and Ti negative anomalies, light rare earth element (LREE) enrichment, and nonradiogenic Nd isotope composition. However, some correlations between incompatible element ratios suggest that the crustal signatures were mainly inherited from mantle sources metasomatized during a previous subduction event. En route to the surface, melts presumably experienced only insignificant contamination by crustal material. Felsic rocks do not define common trends with mafic rocks and were formed independently. They exhibit higher REE contents, large-ion lithophile element (LILE) enrichment, and extremely wide variations in Nd isotope composition, which clearly demonstrates a considerable contribution of heterogeneous basement to their formation. Geochemically, the felsic rocks of the Karelian Craton correspond to A2-type granites and were formed by melting of crustal rocks in an anorogenic setting. Their possible sources are Archaean sanukitoid-type granitoids and Archaean granite gneisses. The high Yb content and pronounced Eu anomaly imply that they were generated from a garnet-free pyroxene – plagioclase source at shallow depths. By the Palaeoproterozoic, the older Vodlozero block was colder than the Central Domain, which facilitated the development of the brittle deformations and faulting and, correspondingly, rapid magma ascent to the surface without melting of crustal rocks. This resulted in the absence of felsic rocks and the formation of more primitive basalts in this area.     

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into clots rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO₂, P₂O₅, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO₂, P₂O₅, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust.     

Redox Condition,Nature and Tectono-magmatic Environment of Granitoids and Granite gneisses from the Karbi Anglong Hills,Northeast India: Constraints from Magnetic Susceptibility and Biotite Geochemistry

The Karbi Anglong hills (erstwhile Mikir hills) in northeast India are detached and separated from the Meghalaya plateau by a NW-SE trending Kopili rift. The Karbi Anglong hills granitoids (KAHG) and its granite gneissic variants belong to Cambrian plutons formed during Pan-African orogenic cycle, which commonly intrude the basement granite gneisses and Shillong Group metasediments. The KAHG can be broadly classified into three major granitoid facies viz., coarse grained porphyritic granitoid, medium grained massive non-porphyritic granitoid, and granite gneiss, which share a common mineral assemblage of plagioclase-K-feldspar-quartz-biotite±hornblende-apatite-titanite-zircon-magnetite but differ greatly in mineral proportion and texture. Modal mineralogy of KAHG, granite gneiss and basement granite gneiss largely represents monzogranite and syenogranite. The magnetic susceptibility (MS) of the KAHG, granite gneiss and basement granite gneiss varies widely between 0.11×10^-3 and 43.144×10^-3 SI units, corresponding to ilmenite series (<3×10^-3 SI; reduced type) and magnetite series (>3×10^-3 SI; oxidized type) of granitoids respectively. The observed MS variations are most likely intrinsic to heterogeneous source regions, modal variations of orthomagnetic and ferromagnetic minerals, and tectonothermal and deformational processes that acted upon these rocks. The primary and re-equilibrated compositions of biotites from the KAHG, granite gneiss and basement granite gneiss suggest calcalkaline, metaluminous (I-type) nature of felsic host magma formed in a subduction or post-collisional to peraluminous (S-type) host magma originated in syn-collisional tectonic settings, which were evolved and stabilized between FMQ and NNO buffers typically corresponding to reducing and oxidising magma environments respectively.