Origin of late Archean granite: geochemical evidence from the Vermilion Granitic Complex of northern Minnesota

The 2,700-Ma Vermilion Granitic Complex of northern Minnesota is a granite-migmatite terrane composed of supracrustal metasedimentary rocks, mafic rocks, tonalitic and granodioritic plutonic rocks, and granite. The metasedimentary rocks are predominantly graywacke, which has been regionally metamorphosed to garnet-sillimanite-muscovite-bearing biotite schist, and has locally undergone anatexis. The mafic rocks form early phases within the complex and are of two types: (1) basaltic amphibolite, and (2) monzodiorite and essexite rich in large ion lithophile elements (LILE). The members of the early plutonic suite form small bodies that intrude the metasedimentary rocks and mafic rocks, producing an early migmatite. The granite is of two distinct varieties: (1) white garnet-muscovite-biotite leucogranite (S-type; Chappell and White 1974) and (2) grayish-pink biotite-magnetite Lac La Croix Granite (I-type). The leucogranite occurs in the early migmatite and in paragneissic portions of the complex, whereas the Lac La Croix Granite is a late-stage intrusive phase that invades the early migmatite and metasediment (producing a late migmatite) and forms a batholith. This study focuses specifically on the origin of granite in the Vermilion Granitic Complex. Chemical mass-balance calculations suggest that the S-type two-mica leucogranite had a metagraywacke source, and that the I-type Lac La Croix Granite formed via partial fusion of calc-alkaline tonalitic material, which may have been similar to rocks of the early plutonic suite. This model is satisfactory for petrogenesis of similar Late Archean post-kinematic granites throughout the Canadian Shield.     

Geochemistry of an Island-Arc Plutonic Suite: the Uasilau-Yau Yau Intrusive Complex, New Britain, P.N.G

In the Uasilau-Yau Yau intrusive complex of central New Britain,Papua New Guinea, there is a compositional continuum in intrusiverock-types from gabbro to granodiorite and K-Ar mineral agesof the most mafic and most felsic components are not significantlydifferent (29?0.6 Ma versus 28.3?0.5 Ma, respectively). Tonaliteporphyry, the progenitor of porphyry copper mineralization inthe complex, represents a significantly younger intrusive eventat 24 Ma. Relatively calcic (An_95—50) plagioclase coresand salite to augite composition clinopyroxene are texturallyearly phases in the intrusive rocks. The main mafic mineral,calcic amphibole, generally has corroded clinopyroxene coresand may, like biotite, K-feldspar and quartz, generally be alate-stage, not a primary liquidus phase. Petrographic featuresindicate that the mafic minerals in the plutonic rocks crystallizedfrom melt, rather than being restite phases. The intrusive rocks cover an extensive silica range (45–75wt. per cent), do not exhibit simple straight-line variationon Harker diagrams for many elements (e.g. TiO₂, FeO, P₂O₅ andSr), and most are relatively depleted in incompatible traceelements (Rb, Zr, and REE). Major and trace element modellingsupports derivation of the complex by shallow level fractionalcrystallization dominated by removal of the phases calcic plagioclase,clinopyroxene, and magnetite from a parental magma closely resemblingrecent basaltic rocks in New Britain. The fact that the plutonicrocks are almost chemically indistinguishable from late Cainozoiccalc-alkaline volcanic rocks of New Britain supports fractionalcrystallization as a viable mechanism for generating these island-arcvolcanic rocks and indicates an analogous origin for the initialmagma. Granites, such as those of the Uasilau-Yau Yau intrusive complex,which are probably generated by partial melting of subductedoceanic crust or the overlying mantle, may be termed mantleor M-type granites. Documentation of the characteristics ofM-type versus normal I-type granites may enable the recognitionof M-type plutonic rocks in older, possibly more deeply erodedgeologic terrains. This would, by analogy to their volcanicequivalents, be very helpful in tectonic interpretations. Also,such plutonic rocks have known potential for Cu-Au mineralization.     

Trace elements behavior in granite genesis: A case study The calc-alkaline plutonic association from the Querigut complex (Pyrénées,France)

The different granitoids of the zoned Querigut complex (Hercynian Pyrenees) are associated with a series of basic to intermediate rocks ranging from hornblende-bearing peridotites to quartz-diorites. The whole complex appears as a calc-alkaline plutonic suite typical of orogenic zones. The distribution of lanthanides and other trace elements amongst coexisting minerals indicate they are essentially held by accessory phases, particularly in granitoids. This restricts the use of those elements in the calculation of petrogenetic models for acidic plutonic rocks. Magmatic differentiation, mainly by hornblende + plagioclase fractionation, can produce the basic series. This differentiation cannot directly produce the different granitoids, which require a preponderant contribution of crustal melts. The sequence of different granitoids can be explained either by an heterogeneity in the source region, or by magmatic differentiation. The most plausible interpretation of the whole complex calls for the emplacement of a mantle-derived magma into a wet, anatectic continental crust, with interactions between basic rocks and the soproduced acidic melts.     

Magmagenesis in a subduction-related post-collisional volcanic arc segment: the Ukrainian Carpathians

Calc-alkaline magmatism in the south-west Ukraine occurred between 13.8 and 9.1 Ma and formed an integral part of the Neogene subduction-related post-collisional Carpathian volcanic arc. Eruptions occurred contemporaneously in two parallel arcs (here termed Outer Arc and Inner Arc) in the Ukrainian part of the Carpathians. Outer Arc rocks, mainly andesites, are characterized by LILE enrichment (e.g. K and Pb), Nb depletion, low compatible trace element abundances, high ⁸⁷Sr/⁸⁶Sr, high δ¹⁸O and low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios (0.7085–0.7095, 7.01–8.53, 0.51230–0.51245, respectively). Inner Arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas. They also show low compatible element abundances but have lower ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and higher ¹⁴³Nd/¹⁴⁴Nd ratios (0.7060–0.7085, 6.15–6.64, 0.5125–0.5126, respectively) than Outer Arc rocks. Both high-Nb and low-Nb lithologies are present in the Inner Arc. Based on the LILE enrichment (especially Pb), a higher fluid flux is suggested for the Outer Arc magmas compared with those of the Inner Arc.

Combined trace element and Sr–Nd–O isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex. Compositional differences between the Inner and Outer Arcs were produced by introduction of variable proportions of slab-derived sediments and fluids into a heterogeneous mantle wedge, and by different extents of upper crustal contamination. Degrees of magmatic fractionation also differed between the two arcs. The most primitive magmas belong to the Inner Arc. Isotopic modelling shows that they can be produced by adding 3–8% subducted terrigenous flysch sediments to the local mantle wedge source. Up to 5% upper crustal contamination has been modelled for fractionated products of the Inner Arc. The geochemical features of Outer Arc rocks suggest that they were generated from mantle wedge melts similar to the Inner Arc primitive magmas, but were strongly affected by both source enrichment and upper crustal contamination. Assimilation of 10–20% bulk upper crust is required in the AFC modelling, assuming an Inner Arc parental magma. We suggest that magmagenesis is closely related to the complex geotectonic evolution of the Carpathian area. Several tectonic and kinematic factors are significant: (1) hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes; (2) thermal disturbance caused by ascent of hot asthenospheric mantle during the back-arc opening of the Pannonian Basin; (3) clockwise translational movements of the Intracarpathian terranes, which facilitated eruption of the magmas.     

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.     

Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith

Calc-alkaline plutonic rocks, intruded at 3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm----Nd isotopic systematics of the 3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and Sm---Nd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.     

Composition and geodynamic conditions of formation of the intrusive rocks of the Gromadnen-Vurguveem peridotite-gabbro massif,western Chukotka

The Gromadnen-Vurguveem peridotite-gabbro massif is confined to one of the largest ophiolite complex of western Chukotka and composed mainly of intrusive rocks. This paper reports the first comprehensive compositional data for its plutonic rocks (petrochemistry, geochemistry, and compositions of minerals). In terms of petrography, two groups of rocks can be distinguished in the Gromadnen-Vurguveem peridotite-gabbro massif. The first group includes leucocratic gabbroids (mostly gabbronorites), composing most of the massif. The second group includes olivine-bearing cumulate rocks: olivine gabbros, troctolites, plagioclase-bearing dunites, and amphibolized wehrlites. The major element variations in these rocks suggest their affiliation to low-titanium, low-potassium, and high-alumina plutonic derivatives of island-arc magmatism. According to geochemical characteristics (distribution of REEs and indicator incompatible elements), the gabbroids of the first group are akin to both island-arc tholeiites and boninites. The olivine-bearing rocks of the second groups show boninitic affinity. Based on these observations, it was concluded that the intrusive complex of the Gromadnen-Vurguveem massif was formed during an early stage of the development of an ensimatic island arc.     

Hydrothermal fluids responsible for the formation of precious minerals in the Nigerian Younger Granite Province

Preliminary investigations in the Younger Granite Province of Nigeria have revealed that precious and semi-precious minerals like rubies, sapphires, emeralds, aquamarine, zircon and fluorite can be found in the region. The gem minerals are shown to have been produced either by direct deposition along fissures, veins and greisens by hydrothermal fluids or as a result of hydrothermal fluids reacting with wall-rocks. These wall rocks are either biotite granites from which the hydrothermal fluids originated or basement rocks or any other rocks which the biotite granites intrude and their residual hydrothermal fluids have invaded. The hydrothermal fluids appear to have been rich in alkalis (Na⁺, K⁺, etc.), rare elements (Be, Zr, F, REE, etc.) and siliceous. As these fluids rose through fractures and channel ways through the rocks, they either deposited the gem minerals in the fractures at the appropriate stability conditions or reacted with the wall-rocks producing the gem minerals at the expense of elements like Ca and A1 in the minerals of these rocks.     

The mafic and ultramafic rocks of the Mocksville complex of Central North Carolina: Petrogenetic and tectonic implications

Located in the northwestern part of the Charlotte terrane of the Carolina zone in central North Carolina, the Mocksville complex is a tabular body which trends NE-SW and covers an area of approximately 500?km². It consists of late Proterozoic to early Paleozoic, moderately metamorphosed and variably deformed, mainly plutonic ultramafic, mafic and felsic rocks. The ultramafic rocks are pyroxenites, wehrlites, and hornblendites; the mafic rocks are metagabbros and amphibolites; and, the felsic rocks are granites and diorites. Field, geochemical, and geothermobarometry studies suggest that the igneous and metaigneous rocks of the Mocksville complex are likely to be genetically related, formed by calc-alkaline differentiation of mafic magma, and originated in a moderate pressure environment (~8?kbar). Based mainly on the study of volcanic rocks, the terranes of the Carolina zone have been interpreted as magmatic arc terranes in most tectonic models concerning the evolution of the southern Appalachian orogen. The geochemical features of the mafic and ultramafic plutonic rocks of the Mocksville complex corroborate the arc origin of the Charlotte terrane.     

Late Neoarchean thick-skinned thrusting and Paleoproterozoic reworking in the MacQuoid supracrustal belt and Cross Bay plutonic complex,western Churchill Province,Nunavut, Canada

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks.The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.     

热水沉积岩岩石学特征:以内蒙古二连盆地白音查干凹陷下白垩统腾格尔组为例*

内蒙古二连盆地白音查干凹陷下白垩统腾格尔组—都红木组发育1套热水沉积岩。通过岩心观察、显微镜下鉴定、扫描电镜、全岩X衍射和电子探针分析等手段,对研究区热水沉积岩的矿物组成、结构与构造特征、矿物组合关系进行系统研究,在此基础上进行岩石分类与命名。研究表明: 研究区热水沉积岩在岩心上表现为深灰色、灰色和灰褐色;其矿物组分复杂多样,以白云石和沸石(钠沸石和方沸石)为主要造岩矿物,黄铁矿、菱镁矿、菱铁矿、水镁铁石及重晶石为次要矿物,混有黏土矿物、石英、钾长石和斜长石等泥质陆源碎屑;热水沉积岩结构构造特征复杂多样,根据结构特征可以划分为热水内碎屑结构、泥晶结构和团块结构3种,依据构造特征可以划分为纹层状和条带状构造、网脉状构造、同生变形构造、角砾状构造、蝌蚪状构造、星散状构造、块状构造和韵律性构造8种构造类型;以主要的热水沉积矿物白云石、沸石及陆源的泥质物(黏土矿物、石英及长石)为三端元,以90%、75%、50%、25%、10%为界限,将研究区热水沉积岩划分为泥质白云岩、含沸石白云质泥岩及沸石岩,前两者是研究区的主要岩石类型。本区热水沉积岩岩石学特征的研究可为其他地区的热水沉积岩及其沉积模式研究奠定基础,可以进一步丰富现今的沉积学理论。     

The geology and geochronology of the Annandagstoppane granite,Western Dronning Maud Land,Antarctica

The Annandagstoppane Granite is exposed at three nunataks in Western Dronning Maud Land, Antarctica. It comprises medium- to coarse-grained granite crosscut by veins of pegmatite and graphic granite and has many S-type characteristics such as containing normative corundum greater than 1.1%, molecular Al₂O₃/(CaO+K₂O+Na₂O) greater than 1.1 and very little zircon. Hydrothermal alteration in the Granite is variably developed and has affected only certain minerals in any phase. R-Sr and Pb whole rock and mineral isotopic data suggest: 1) that Sr isotopes within it were nearly homogenized on a whole rock scale about 2823 Ma ago by this hydrothermal alteration; 2) that the Pb isotopic system was also disturbed at that time, and 3) that the Granite may have been was emplaced sometime during the interval 3115 Ma to 2945 Ma ago. The Granite was probably intruded by the Annandagstoppane Gabbro about 1200 Ma ago, resetting the Rb-Sr system in biotite. The Annandagstoppane Granite may form part of a basement complex to the Proterozoic sedimentary, volcanic and mafic igneous rocks exposed to the east in the Ahlmannryggen and the Borgmassivet. Its chemical composition and geologic history appears to be unique in Antarctica and in the Kaapvaal Craton of Southern Africa, consistent with the possibility that the Annandagstoppane Granite is part of a crustal fragment that joined Antarctica relatively late in the history of that continent.     

Li,F and Rb contents and Ba/Rb and Rb/Sr ratios as indicators of postmagmatic alteration and mineralization in the granitic rocks of the Banke and Ririwai Younger Granite complexes,Northern Nigeria

The Banke and Ririwai complexes have plutonic phases of igneous activity composed mainly of granitic rocks. These granitic ring complexes are associated with Sn-Nb mineralization and are characterized by high Li, F and Rb contents and Rb/Sr ratios, and low Ba and Sr contents and Ba/Rb ratios. — The altered and mineralized granites have variable Rb/Sr and Ba/Rb ratios differing significantly from those of fresh rocks. These ratios as well as the Li, F and Rb concentrations are good indicators of granitic rocks associated with postmagmatic alteration and mineralization providing valuable tools for Sn-Nb exploration within the Nigerian Younger Granite province.     

Genetic Aspects of the Manto-type Copper Deposits Based on Geochemical Studies of North Chilean Deposits

Recent studies on mineralogy, geochronology, fluid inclusion and stable isotope (Pb, Os, S, C, O, Sr) characteristics were reviewed to determine constraints for genetic models of the Chilean manto‐type copper deposits. The Chilean manto‐type deposits are divided into the two geologic categories of the northern areas (Arica–Iquique, Tocopilla–Taltal) and the central areas (Copiapó, La Serena, Santiago). The former is distributed in the coastal range composed of Jurassic andesite‐dominated volcano‐sedimentary piles and younger plutonic intrusions, and yields chalcocite (‐digenite) and bornite as the principal hypogene copper sulfides. The latter is hosted mostly in Lower Cretaceous volcano‐sedimentary sequences, and has chalcopyrite‐rich mineral associations. The fluid inclusion data indicate that the primary copper mineralization was commonly generated in the temperature range 150–360°C under low‐pressure conditions near the boiling curve, mediated with relatively saline brines. Generally, homogeneous Pb and S isotope compositions for primary copper minerals imply direct magma source or leaching of igneous rocks. Pb and Os isotope data published for some deposits, however, suggest that ore‐forming metals were derived mainly from the volcano‐sedimentary host rocks. The noticeably negative isotope ratios of primary sulfide sulfur and hydrothermal calcite carbon of some central area deposits indicate influx of sedimentary rock components, and the high ⁸⁷Sr/⁸⁶Sr initial ratios of hydrothermal calcite from the Tocopilla–Taltal area deposits imply contribution of the contemporaneous seawater or marine carbonates. These isotopic constraints imply a formation mechanism in which the Chilean manto‐type copper deposits formed epigenetically in the process of hydrothermal interaction of non‐magmatic surface‐derived brine with the volcano‐sedimentary host rocks, which is inferred to have been induced by a deep‐seated plutonic complex as the possible heat source.     

Decoupling of paired elements,crossover REE patterns,and mirrored spider diagrams: Fingerprinting liquid immiscibility in the Tapira alkaline–carbonatite complex,SE Brazil

Tapira is an alkaline silicate–carbonatite complex belonging to the kamafugite-carbonatite association in the Late-Cretaceous Alto Paranaíba Igneous Province (APIP). It is dominated by coarse-grained plutonic rocks (bebedourite – a phlogopite-, apatite-, and perovskite-rich clinopyroxenite – with subordinated dunites, wehrlites, carbonatites and phoscorites). The plutonic rocks are crosscut by fine-grained ultramafic alkaline rocks (phlogopite picrites, bebedouritic dikes) and fine-grained carbonatites. Both types of dike-rocks show petrographic evidence of the coexistence of immiscible silicate and carbonatite liquids, such as carbonate ocelli present in the silicate rocks and, more rarely, silicate ocelli within carbonatites. A detailed geochemical study of the rock types in the complex, with emphasis on the fine-grained varieties, showed that whilst some rocks may be related to each other through crystal fractionation (e.g. phlogopite picrites and bebedouritic dikes), others display anomalous trace-element behaviour that cannot be readily explained by the fractionation of a particular phase or combination of phases. We interpret such anomalous geochemical signatures as produced by silicate–carbonate liquid immiscibility, on the basis of available experimental data on partition coefficients between coexisting immiscible liquids. The immiscibility signatures comprise: (a) decoupling of geochemical pairs, such as Nb–Ta and Zr–Hf; (b) rotation of REE patterns, which cross over the patterns of the primitive liquids; and (c) matching and opposite enrichment-depletion trace elements relationships in spider diagrams of conjugate immiscible liquids. We suggest that, once established, such geochemical signatures are very difficult to erase during the subsequent petrogenetic evolution processes, which may result in superimposed conflicting signatures.     

阴极发光仪在变质岩和花岗岩类岩石中的应用

利用阴极发光仪对一些前寒武纪变质岩(河北,内蒙等地)和西藏及其它地区的后寒武变质岩,以及不同时代和成因的花岗岩类岩石的样品进行了观察。阴极发光与某些过渡元素的含量有关,可以揭示岩石的构造,间接地反映矿物化学成分的特点。本文介绍了阴极发光在变质岩和花岗岩类岩石的以下几个方面的应用:1) 间接地确定变质级;2) 揭示原岩的变余构造,3) 鉴别细粒和发光矿物;4) 研究矿物之间的反应;5) 重建变形构造。     

Zircons of igneous rocks at the Erdenetuin-Obo porphyry Cu-Mo deposit (Mongolia): U-Pb dating and petrological implications

The Erdenetuin-Obo porphyry Cu-Mo deposit was formed at the final stage of development of magmatic activity occasionally manifested in the Late Permian-Early Triassic in the period of at least 40 Ma. Early plutonic (host) and late ore-bearing porphyry intrusive complexes were formed in that period. The plutonic complex is multiphase, while the porphyry complex is polyrhythmical and multiphase within rhythms. The obtained data on the U-Pb isotopic composition (SHRIMP II) of zircons from unaltered rocks of the ore field are discussed: gabbro, diorite, and granodiorite of the plutonic complex and granodiorite-porphyry I and II of the first and second rhythms of the ore-bearing complex, respectively. Zircons of different age levels and genotypes were identified in the course of performed investigations. Gabbro are dominated by postmagmatic (superimposed) zircons with the datings of 239–225 Ma. The age of xenogenic zircon brought out from the basement rocks is estimated at 1146 ± 11 Ma. Zircons occur as magmatic and postmagmatic (superimposed) minerals dated 252–247, 244–233 Ma in diorite and 244–242, 239–224 Ma in granodiorite. The ages of postmagmatic zircons from diorite are partially overlapped by datings of magmatic zircons from granodiorite and granodiorite-porphyry. In the porphyry complex, the datings of magmatic zircons are 240–234 and 222–220 Ma in granodiorite-porphyries I and II, respectively. There are also inherited zircons with datings coinciding with those of magmatic zircons from precursor intrusive rocks. Datings of such zircons are 249–241 and 257–231 Ma for granodiorite-porphyries I and II, respectively. As a whole, zircon datings in all studied igneous rocks forming a virtually uninterrupted range in the period of 257–220 Ma allow us to suggest the relation of the ore magmatic system to the long-living constantly active deep source occasionally delivering melt to the upper levels.     

丰宁变质磷矿深成变质作用及其成矿问题

丰宁变质磷矿产于华北地台北部基底变质岩系之中。含磷灰石的角闪岩类似层状矿层已被卷入基底塑性褶皱之内,并且经历了角闪岩相至角闪石麻粒岩亚相变质作用和混合岩化(局部熔融)作用。从其角闪岩类岩石含V、Ti、P、Fe元素推测磷质矿源是与基底基性岩浆岩经变质成因有关。     

Geo–tectonic Position of Tin Polymetallic Mineralization Zone in the Southern Da Hinggan Mountains Area, Inner Mongolia, China: An Introduction to This Special Issue

Abstract: As a part of the main activities of Japan‐China technical cooperation project, a test survey area, approximately 5,000 km², was established for the implement of its geological and geochemical research program. A major mineralization zone called Huanggang–Ganzhuermiao–Wulanhaote Sn‐Cu polymetallic mineralization zone is recognized in the southern Da Hinggan Mountains area. The southern half of this zone is known as the sole Sn‐mineralization zone in North China. The survey area lies in this prominent zone. As the most of the papers presented in this issue have concerns to the geology and mineralization in this survey area, this report was prepared to introduce geo‐tectonic situation of the Sn‐Cu polymetallic mineralization zone in the Inner Mongolia orogenic belt. The belt is divided into four tectonic facies (from NW to SE); I: Wuliyasitai volcano‐plutonic zone, II: Hegenshan ophiolite mélange zone, III: Sunitezuoqi volcano‐plutonic zone, IV: Wenduermiao ophiolite mélange zone. The subject Sn‐Cu polymetallic mineralization zone is situated in the southeastern part of the Sunitezuoqi magmatic zone. About this Sunitezuoqi magmatic zone, three geo‐tectonic characteristics are pointed out. In late Carboniferous to early Permian period, subduction of Hegenshan oceanic crust occurred, which accelerated volcano‐plutonic activities and brought about basic to intermediate volcanic rocks of tholeiitic to calc‐alkaline series represented by Dashizhai Group in the Sunitezuoqi magmatic zone. Late Jurassic to early Cretaceous acidic rocks representing the most culminated volcanism and plutonism in Mesozoic era in the Da Hinggan Moutains area are distributed very extensively in and around the Sn‐Cu polymetallic mineralization zone. The Proterozoic metamorphic basement rocks called Xilinhaote complex are distributed close to the mineralized area in the Sunitezuoqi magmatic zone. Although the real mineralization was known associated with Mesozoic acidic to intermediate volcano‐plutonic activities, it is thought that the lower Permian Dashizhai volcanic rocks and pre‐Cambrian basement rocks might have played certain significant role in the process respectively of extraction of elements and formation of the magma favorable for such mineralization in the Sunitezuoqi magmatic zone. It would be necessary to give further considerations to these three geological units in relation to the Sn‐Cu polymetallic mineralization.     

Geochemistry and geochronology of the Neoproterozoic Pan-African Transcaucasian Massif (Republic of Georgia) and implications for island arc evolution of the late Precambrian Arabian–Nubian Shield

The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNd_int = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between  750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia.