Syenite Bodies Research Articles

Evolução do segmento norte do orógeno Itabuna-Salvador-Curaçá: cronologia da acresção de arcos, colisão continental e escape de terrenos

U-Pb and Pb-evaporation zircon ages, together with whole-rock Sm-Nd and Pb-Pb data, are used to unravel the tectonic evolution of the northern segment of the Itabuna-Salvador-Curaca Orogen, northeastern Sao Francisco Craton. The results of this geochronologic study indicate that oceanic crust and island arc sequences accreted at 3.3 Ga, forming the Mundo Novo Greenstone Belt, and that between 2.2 - 2.12 Ga the Rio Itapicuru and Rio Capim Greenstone Belts were formed in a similar fashion. Age data and Nd isotopes also suggest that Andean-type continental margins are likely to have formed in the span of time 3.08 - 2.98 Ga, giving rise to the TTG belts of Retirolândia and Jacurici river, and to the Caraiba Complex between 2.69 - 2.63 Ga. Around 2109 Ma, continent-continent collision closed the Rio Itapicuru volcano-sedimentary basin, as inferred from the syntectonic emplacement of the Itareru Tonalite between the Rio Itapicuru Greenstone Belt to the east and the banded gneisses of the Serrinha Block to the west. This collision culminated in a major shift from frontal or oblique convergence to orogen-parallel block-escape about 2084 Ma ago and continued at least till 2039 Ma. During this event several elongated granitic and syenitic bodies were emplaced in the continental crust. The Itiuba Syenite, dated at 2084 Ma and stretching over 150 km east of the Caraiba Complex, is an example of one of these intrusive bodies.

Pan-African Alkali Granitoids from the Sør Rondane Mountains, East Antarctica

Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sør Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K 2O+Na 2O (7-13 wt%) and K 2O/Na 2O (1-2), low to intermediate Mg#, wide ranges of SiO 2 (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb) N and LREE/HREE, lower A/CNK, SREE and initial 87Sr/ 87Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K 2O, P 2O 5, TiO 2, Fe 2O 3/FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al 2O 3, Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sør Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Mühlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.

Evolution of the Amphibolite‐Granulite Facies Transition Exposed by the Vredefort Impact Structure, Kaapvaal Craton, South Africa

Geological mapping is integrated with high‐precision U‐Pb zircon and monazite geochronology to document in detail the nature and temporal evolution of the amphibolite‐granulite facies boundary exposed in the Vredefort impact structure of the central Kaapvaal Craton. Our study has revealed that the preimpact configuration of the amphibolite‐granulite facies boundary is best preserved in panels between impact‐related brittle faults and that past concepts of the boundary must be revised. A regional lithologic transition exists between amphibolite facies Outer Granite Gneiss (3.084–3.09 Ga) and a heterogeneous assemblage of granulite facies gneisses (ca. 3.1–3.3 Ga). Tabular deformed quartz syenite bodies, dated at $$3.084\pm .001$$ Ga, parallel the metamorphic transition and crosscut both amphibolite and granulite facies rocks. Although two episodes of late Archean deformation modified the boundary, it has retained its primary intrusive continuity since 3.08 Ga. Subsequent activity includes minor dolerite dikes, 2.2 Ga monazite growth, 2.02 Ga meteoritic impact, and episodic hydrothermal zircon growth events from 615 to 380 Ma. Our results suggest that the Vredefort amphibolite‐granulite facies transition can shed valuable insight into Archean crustal evolution and the nature of metamorphic boundaries in cratonic crust. The nature and paleodepth of the Vredefort metamorphic transition are compatible with the seismic characteristics of the unexposed crust of the central Kaapvaal Craton. The absence of major tectonic activity post–3.08 Ga in the Vredefort crystalline basement is consistent with the presence of a lithospheric mantle root that stabilized the central cratonic crust against subsequent disruption. The lithologic, metamorphic, and structural complexity of the transition zone between amphibolite and granulite facies rocks in the Vredefort crystalline basement may be typical of this metamorphic boundary in intracratonic crust.

Faults and their associated host rock deformation: Part I. Structure of small faults in a quartz–syenite body, southern Israel

We analyze pervasive and discontinuous deformation associated with small faults in a quartz–syenite body in southern Israel. The analysis includes detailed mapping, measurement of in-situ mechanical rock properties and microstructural study of the faults. The mapped faults have 1–100-m-long horizontal traces, consisting of linked, curved segments; the segmented nature of the faults is also apparent at the 1–10 mm scale. The observed deformation features are breccia, as well as intra- and inter-granular fractures; these features are accompanied by reduction of the Young modulus and uniaxial strength of the host rock. The deformation features are zoned from a central fault-core through a damage-zone to the protolith at distances of 0.05–0.06 the fault length. Shear strains up to 300% were calculated from measured marker lines displacements and distortion in proximity to the faults. We argue here that the fault-related deformation during fault propagation is manifested by highly localized deformation in a process zone having a width of 0.001–0.005 of the fault length (fault-related deformation due to subsequent slip along the existing faults is analyzed in Part II). The observed self-similarity of the discontinuities over five length orders of magnitude and the outstanding lack of tensile microcracks suggest fault initiation and growth as primary shear fractures.

U-Pb zircon ages from the Vaggeryd syenite and the adjacent Hagshult granite, southern Sweden

South of Lake Vättern a series of syenite bodies have intruded in an elongated area subsequently affected by the Protogine Zone deformation. The by far largest of these bodies, the Vaggeryd syenite, has been dated by the U-Pb zircon method together with the TIB granite immediately to the east of the syenite, in this paper temporarily called the Hagshult granite. The crystallization age of the syenite sample is determined to 1204±6 Ma. The possibility that the Vaggeryd syenite consists of magmas from two separate intrusion events is discussed. Since the Vaggeryd syenite is transected by the ductile deformation of the Protogine Zone, this deformation must be younger than c. 1205 Ma. The result from the Hagshult granite yields an intrusion age of 1673±19 Ma. The granite thus belongs to the youngest group of TIB intrusions.

A crustal-scale, orogen-parallel strike-slip fault in the Middle Urals: age, magnitude of displacement, and geodynamic significance

The kinematic evolution of an orogen-parallel strike-slip fault in the Middle Urals demonstrates that orogen-parallel mass transfer was an important, previously underestimated process during the syncollisional evolution of the Middle Urals. The Kyshtym strike-slip fault extends NNE, parallel and adjacent to the Main Uralian fault, which is the main suture of the Uralide orogen. The Kyshtym fault is interpreted as one of two conjugate strike-slip fault zones that have accomodated the longitudinal transfer of material along the margins of a rigid indenter belonging to the East European craton. The dextral Kyshtym shear zone was active under retrograde lower amphibolite to middle/lower greenschist facies conditions. Four metagranitic, muscovite-bearing mylonites yielded Rb–Sr internal mineral isochron ages of 247.5±2.9, 244.5±6.5, 240.0±1.4, and 240.4±2.3 Ma, whereas a biotite-rich sample, without muscovite, gave a mineral isochron age of 229.1±3.2 Ma. The results indicate almost complete Sr-isotopic reequilibration on the hand specimen scale during mylonitization. The muscovite ages are interpreted as deformation ages and demonstrate a Late Permian/Early Triassic age for the Kyshtym shear zone. The shear zone transects a pre-orogenic syenite intrusion of Ordovician age. A maximum shear strain of γ=7±3 is estimated from the shape of the ductily deformed syenite body in map view and from the length/width ratios of deformed amphibolite bodies in the country rock. This shear strain suggests a maximum displacement of 28±12 km for the ~4-km-thick Kyshtym shear zone. A younger brittle fault, oriented subparallel to the shear zone, accomplished an additional horizontal displacement of 15±3 km; thus, the total displacement along the fault system is 43±15 km.

Crustal origin of Early Proterozoic syenites in the Congo Craton (Ntem Complex), South Cameroon

Two petrographically different syenite suites can be distinguished in the mostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites and a suite of clinopyroxene syenites. The syenites occur as discrete intrusions surrounded by Archean gneisses. Based on structural relationships the syenites are of late to post-orogenic origin. The Pb–Pb zircon evaporation ages indicate that the syenites intruded at ca. 2.3 Ga and thus postdate significantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin. The syenite suites do not show simple chemical trends, have highly variable trace element contents and display only a small variation in their major element contents, particularly in their SiO 2 contents. The isotope characteristics, including 143Nd/ 144Nd, 87Sr/ 86Sr and common Pb, point to a heterogeneous source or multiple sources for the syenites. The Sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87Sr/ 86Sr ratio of 0.690 to high values of 0.713. In contrast the initial 143Nd/ 144Nd ratios are more homogenous with εNd (2320) from −5.8 to −7.5. The Pb isotopes from leached K-feldspars indicate that the sources of the syenites had a history of U depletion and Th/U enrichment. The syenites show the same isotope characteristics as the surrounding gneiss terrane, thus the deep continental crust of the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source. In addition, inherited zircons from the syenites yield Pb—Pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses. In contrast to many other syenites worldwide, the syenites in the Ntem Complex do not contain juvenile mantle material, but are the sole products of crustal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust. The cause for the generation of these large syenite bodies in the Ntem Complex is most likely an episode of deep-seated thermal activity that was so far not known from this part of the Congo Craton.

The Santanápolis Syenite: Genesis and Evolution of Paleoproterozoic Shoshonitic Syenites in Northeastern Brazil

The Paleoproterozoic terrane of the state of Bahia, Brazil, is characterized by an important mag-matic event that resulted in the generation of large and numerous syenite complexes; the terrane constitutes one of the largest syenite provinces in the world. It includes the Itiuba (1,800km2), Santanápolis (180 km2), São Felix (32 km2), and Anuri (70 km2) complexes. These syenitic bodies of shoshonitic affiliation are silica saturated, metaluminous, and exhibit: (1) moderate to high K2O/Na2O ratios, (2) relatively high MgO contents (up to 3.0%); (3) high incompatible-element contents, particularly LILE and LREE; (4) high HREE/LREE fractionations; and (5) small Eu negative anomalies. Among these syenites, Santanápolis exhibits the greatest lithological diversity, with a porphyritic and a heterogranular facies, and diverse lithotypes that vary from leucosyenites to melanocratic syenites of cumulate origin, as well as enclaves and dikes. It intruded syntectonically at 2.1 Ga and shows magmatic flow structures and subsolidus recrystallization of quartz and alkali feldspars. The mineralogical assemblage is perthitic feldspar, diopside, hornblende, phlogopite-biotite, apatite, zircon, titanite, calcite, and monazite. Differentiation of the Santanópolis Syenite was controlled by flow segregation processes in an intermediate trachytic magma, with fractionation of early crystallized diopside, apatite, and plagioclase. The parental intermediate shoshonitic magma was extracted directly from an enriched, subduction-modified mantle characterized by anomalous concentrations of LREE and LILE elements, and isotopic parameters similar to the EMI-type.

Geochemistry of the Lower Protezoroic Granulite-Facies Grant Syenite Gneiss, Barra Velha, Santa Catarina State, Southern Brazil.

A syenite body was discovered and studied in the Santa Catarina Granulitic Complex, close to the beaches of Grant and Barra Velha. The quarry, located on km·91 of BR-101 highway, was sampled for geochemical whole rock studies and also of its individual minerals. The syenite body is in structural and metamorphic equilibrium with the containing hypersthene-bearing tonalitic and basaltic granulites. Major, trace and rare-earth elements analyses on rock samples indicate syenitic to quartz dioritic compositions, saturated in alumina and silica. They have high contents of K, Na, Sr and Ba and low contents of Zr and Ti. Total REE is high in chondrite-normalized paterns and indicates strong fractionation of the light REE, similar to those for the Itiúba Syenite from Bahia State and for the Linden Syenite from Minnesota, USA. Overall geochemistry, on the other hand, is remarkably similar to the Archean Cléricy Pluton from Canada. The REE are strongly concentrated in allanite, clinopyroxene and sphene strongly. A Sm-Nd mineral isochron indicates metamorphic crystallization at ca. 2.2 Ga. The syenite is depleted in HFSE, and this may indicated a subduction-zone tectonic environment of generation of the magma. The foregoing results are based on 12 whole rock chemical analyses for major and some trace elements, three whole rock REE analyses, two electron microprobe analyses on each of seven minerals and Sm/Nd isotopic determinations on five minerals.

Gamma-ray spectrometry and magnetometry of the alkaline rock area in Ampipal-Bhulbhule, Nepal Lesser Himalaya

Gamma-ray spectrometric and magnetometric survey conducted in the Ampipal-Bhulbhule area characterised by the distribution of syenitic gneisses hosted by the metasediments of the Kuncha Formation revealed that such a geophysical complex is effective in mapping the contact zones of syenitic bodies with the host rocks. Altogether 84 sites within the syenitic bodies and 32 sites in the host rocks were probed by the spectrometer for responses in U, Th and 40K., and total counts windows. The syenites always give readings distinctly higher than the host rocks. Within the main syenitic body, the maximum radiometric response is obtained at the central part. Several sites within the syenitic gneisses yield anomalies in equivalent Uranium content (eUmax = 303.5 ppm) and Thorium content (eTHmax=213.6 ppm) and also in K-channel counts. Across the zone of contact between the syenites and the host metasandstones, both the radiometric counts (especially K-counts) and the magnetic effect vary significantly. Prominent magnetic anomaly has been detected at Bhulbhule Khar and it is interpreted to be the effect of either a thick layer of basic or ultrabasic rock enclosed within the nepheline syenite, or an economic Fe-mineralisation residing in magnetite. Thus, magnetic method also is clearly effective for mapping the subsurface distribution of dyke rocks and also Fe-mineralised areas.

Grenville Magmatism in West Texas: Petrology and Geochemistry of the Red Bluff Granitic Suite

zone of regional extension, as suggested by recent geophysical and The Red Bluff granitic suite (RBGS) underlies part of the Franklin petrological studies. Mountains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120±35 Ma. The suite is predominantly metaluminous; it consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leucogranitic dikes, and late-stage peralkaline

Mineral Chemistry and Tectonic Significance of Neoproterozoic Ultrapotassic Plutonic Rocks from the Cachoeirinha-Salgueiro Fold Belt, Northeastern Brazil

The Terra Nova ultrapotassic igneous rocks of northeastern Brazil consist of two dike swarms (alkali-feldspar syenites to quartz syenites and alkali-feldspar granites) and one elongated E-W syenitic body (the Serra do Livramento pluton), which intruded metasediments of the Cachoeirinha-Salgueiro fold belt from 580 to 514 Ma. Mafic ultrapotassic syenite enclaves are recorded in the Serra do Livramento and Terra Nova shoshonitic plutons, both of which are cut by the dike swarms. Mineralogically, Terra Nova ultrapotassic hypabyssal rocks resemble shoshonitic lamprophyres. Pyroxene is present in all facies; the clinopyroxenes are zoned, SiO2 saturated, and Al2O3 poor (0.12 to 1.15%), and range from earlier diopside to late acmite. Amphiboles are characterized by high SiO2 and low Al2O3 (0.20 to 2.00%) and TiO2 (0.0 to 1.76%) contents; their compositions range from calcic to alkaline. The late amphiboles are riebeckite-arfvedsonites instead of K-richterites, as expected in ultrapotassic rocks, reflecting the e...

Mode of Occurrence of Nepheline Syenites in the Gorkha-Ampipal Area, Central Nepal Lesser Himalaya

In the Gorkha-Ampipal area, low-grade metamorphic rocks of the Kuncha Formation are delimited in the north by the Masel Thrust. The Kuncha Formation is characterised by doubly-plunging, en-echelon types of noncylindrical folds which are 2 to 20 km long (essentially in NW-SE direction), and have wavelengths of a few km. Mineral and stretching lineations are gently plunging due NNE or SSW. The hanging wall of the Masel Thrust is represented by garnet-biotite schists and gneisses. The schists and gneisses make up a steeply northward dipping homocline. In contrast to the rocks of the footwall, they are generally gently dipping and constitute several mesoscopic folds. Further north, the homocline is discordantly overlain by the intensely deformed unit of phyllites, graphitic schists, marbles, crystalline limestones, and calcareous quartzites. The Main Central Thrust sharply overrides the latter unit and brings with it gently northward dipping kyanite-garnet-biotite schists, quartzites, feldspathic schists, and mylonitic gneisses.
 There are several nepheline syenite intrusive bodies in the Kuncha Formation in the vicinity of the villages Harmi Bhnnjyang, Ampipal, Chanp Bhanjyang, Bhulbhule Khar, and Luintel Bhanjyang. Two separate bodies are also encountered at the confluence of the Masel Khola and the Daraundi Khola. The nepheline syenite bodies observed in the study area vary widely in their shape, size, and orientation. The largest pluton is observed in the vicinity of the villages Ampipal and Chanp Bhanjyang. It is about 7.5 km long in NNE-SSW direction and about 2 km wide. The second largest body is observed between the villages Bandre and Luintel Bhanjyang. It is about 2.5 km long approximately in east-west direction and 300 m wide. Numerous other smaller bodies ranging in size from hundreds of m to a few cm also occur in the region. The nephelinesyenites show sharp and irregular contacts with the country rock, they are crosscut by numerous dykes, and occasionally the effect of contact metamorphism is also observed in the country rock.
 The northeastern part of the largest nepheline syenite pluton (which occurs between Ampipal and Chanp Bhanjyang) is covered by about 500 m thick band of impure marbles. Rare, thin alternations of impure marble with phyllite as well as large (more than 10 m in diameter) scattered marble boulders areseen on the slopes NE of Chanp Bhanjyang, N of Bhulbhule Khar, at the saddle of Lagamkot, and at Khanigaun. The secondary mineralisation in the marbles is represented by magnetite, actinolite, biotite, and chlorite. There exist a few old iron mine workings in the magnetite mineralisation zones. Similar minerals are also seen in the nepheline syenite suggesting a direct relationship between the mineralisation in the nepheline syenite and the marbles. Generally, the nepheline syenite bodies exhibit the same trends of foliation and lineation as those of the country rock, and therefore, they must be intruded before the development of the secondary structures.
 There are a few hot springs at Bhulbhule Khar, which contain a high amount of H2S gas and sulphur, and are coming through the nepheline syenite. The development of copper as well as other secondary ore minerals and several generations of veins in the country rock, and the presence of hot springs probably indicate a continued hydrothermal activity in that area up to the recent times.

Isotopic evidence on the origin of compositional layering in an epizonal magma body

A detailed isotopic study of the Oligocene age (36 Ma), alkaline composition Organ Needle pluton in south-central New Mexico was undertaken to test models for the generation of compositional layering in silicic, epizonal magma bodies. The pluton is isotopically heterogeneous with its alkali feldspar granite composition cap (73–76% SiO 2) having lower initial ϵ Nd and higher 87 Sr 86 Sr ratio than the underlying main syenite (−5 vs. −2 and ∼ 0.709 vs. ∼ 0.706, respectively). Both lithologies have isotopic compositions significantly different from those of the Precambrian granite wall-rock ( ϵ Nd ∼ −12.1 and 87 Sr 86 Sr ∼ 0.784 at 36 Ma). The isotope data indicate that none of the lithologies of the pluton represent the products solely of roof or wall-rock melting, and that the capping granite could not have been derived in a closed-system from differentiation of the underlying syenitic magma. However, the capping granite has isotopic compositions similar to those of a chemically heterogeneous inequigranular syenite found along the margin of the pluton at its deepest exposed level (4–6 km paleodepth). Field observations, and new 40 Ar 39 Ar age determinations, confirm that this lithology was comagmatic with the remainder of the pluton. We conclude that the capping granitic magma was derived from buoyant ϵ Nd ∼ −5 magma rising along the margins of the magma chamber and that the inequigranular syenite preserves a remnant of this sidewall magma. The main syenite body was encapsulated in this marginal, lower ϵ Nd, magma, while undergoing closed-system differentiation, most likely from a mafic progenitor. The ϵ Nd = −5 magma may represent mafic magma initially equivalent isotopically to the main syenite but which subsequently assimilated Precambrian wall-rock (∼ 10% by mass) at the base of the magma system.

Rare metal mineralization at the Salpeterkop carbonatite complex, Western Cape Province, South Africa

The Salpeterkop complex is an eroded central volcano of Late Cretaceous (66 Ma) age with a sediment-filled crater remnant 1 km in diameter associated with radial and concentric carbonatite/K-trachyte dykes as well as K-trachyte, breccia and olivine melilitite plugs. The sedimentary country rocks have been domed. Geochemical anomalies for Ti, Y, Zr, Nb, Th, V, Zn and Mo, based on a core-drilling programme, were found to outline a mineralized zone coinciding with the crater rim, especially on the western and southern sides. The rim rocks are volcanic breccias intimately mixed and veined by decomposed, silicified and ferruginized carbonatite and carbonatitic tuff. A large variety of very fine-grained minerals have been identified in a groundmass consisting mainly of goethite and chalcedony. The principal Nb ore mineral is niobian brookite occurring as microscopic disseminated grains of several morphological types. High-Nb (12% Nb 2O 5) and low-Nb (8% Nb 2O 5) brookites have been distinguished. Some of them contain inclusions of ilmenorutile. Delicate brookite crystal aggregates crystallized in situ. No discrete V and Zn minerals could be identified but Th occurs as thorianite and thorite, Zr occurs in eudialyte and an unidentified KNb silicate and Y occurs as xenotime. Nepheline and eudialyte are probably xenocrysts from an unexposed body of nepheline syenite. It is concluded that mineralization took place in three overlapping stages: 1. i)primary magmatic, with concentration of Nb, Zr, Th, V in both carbonatite and associated silicate rocks; 2. ii)mesothermal, with oxidation and introduction of Fe, Ti, REE, Mo and F; 3. iii)epithermal, with hydration and dissolution of carbonates, removal of Ca and Mg, redistribution of Y, Ti, Nb, Zn and deposition of Si, Ba, Mn and other mobile elements. The suggestion is made that the preservation and extent of the third stage, which may be linked to fumarolic activity, distinguishes the multi-element mineralization at Salpeterkop from other carbonatite complexes. Secondary alteration in the weathered zone merged with the third stage, but is less important.

Early Cambrian carbonatite in Antarctica

Nepheline syenite bodies intrude the basement, Koettlitz Group, gneisses of the Transant-arctic Mountains of South Victoria Land, Antarctica. A calcite–fluorite–biotite–alkali feldspar dyke has mineralogical, trace element, stable isotope, and radiogenic isotope characteristics similar to the host syenite and distinct from the country rock, Koettlitz Group, marble. The dyke is interpreted as the first carbonatite to be described from Antarctica. Zircon in the carbonatite has been dated by U–Pb techniques as 531±5.5 Ma, early Cambrian. This age occurs within the range of metamorphism, deformation and subduction-related granitoid intrusive events associated with the Ross Orogeny. Whether the nepheline syenite and carbonatite igneous activity occurred strictly contemporaneously with plate convergence and granitoid genesis or marks an extensional interlude, awaits the detailed unravelling of tectonic events along this segment of the Neoproterozoic to early Cambrian margin of Antarctica.

Mineralogy of the Lake zone, Thor Lake rare-metals deposit, N.W.T., Canada

The Proterozoic (ca. 2.1 Ga) Blatchford Lake suite hosts significant concentrations of rare metals at the core of a peralkaline granite–syenite pluton. After emplacement of the Grace Lake Granite and the Thor Lake Syenite within it, bodies of nepheline syenite, ijolite, and urtite were intruded beneath the present-day Lake zone. This is the largest of five zones of mineralization and lies close to the apex of the Thor Lake Syenite, a region which was then subjected to albitization, microclinization, and, finally, rare-metal mineralization. The underlying silica-undersaturated rocks contain clots of rare-metal-bearing minerals, including cerianite-(Ce), britholite-(Ce), thorite, and calcium catapleiite, interstitial to nepheline and aegirine. The Lake zone itself contains major quantities of Zr (in zircon), Nb (in ferrocolumbite, pyrochlore group minerals, aeschynite group minerals, and fergusonite-(Y)), and Ce (in allanite-(Ce), monazite-(Ce), and bastnäsite group minerals). Lesser amounts of Ta, Y, heavy rare earth elements, U, Th, and Ga are also present, mainly as minor components of rare-metal-bearing minerals. Electron microprobe analyses of the major rare-metal-bearing minerals are presented.

Mechanisms of differentiation in shallow mafic alkaline intrusions, as illustrated in the Big Bend area, western Texas

Syenitic bodies are a common feature in alkaline sills and laccoliths that range in composition from syenogabbroic to syenodioritic. The syenitic bodies are generally accepted to be the result of in-situ differentiation. Such bodies are usually called segregations, but relatively little discussion is given to the actual ways they may form. Subhorizontal sheets or layers are most common, although rounded ocelli and vertically elongated cylindroidal forms are also common. These are all systematically arranged in the intrusions. Such features are found in at least ten sills in the Big Bend area of Texas, and are reported as well in Montana, Utah, Australia, Sakhalin, Scotland and New Zealand. Detailed chemical and mineralogical analyses demonstrate crystal fractionation very convincingly for the generation of syenitic liquids that form the bodies. The analyses also allow the calculation of viscosities, cooling time and mineral settling rates in the sill studied most thoroughly. These factors indicate, or are compatible with, proposed mechanisms for the formation of syenite bodies. In many intrusions there is a bimodal distribution of syenite compositions; one type is relatively plagioclase rich and the other type is plagioclase poor. Mass balance calculations on several intrusions show that a residual liquid of plagioclase-rich composition will form after about 30% of the parent magma has crystallized, whereas a residual liquid of plagioclase-poor composition will form only after about 50% of the original magma has crystallized. The several mechanisms by which plagioclase-rich residual liquids are aggregated into the different forms of syenite bodies include localized crystal settling and sagging of a crystaline framework on a scale of tens of centimeters to a meter, within an upper solidification front; formation of cylindroidal columns of syenitic differentiate in a crystal mush within a lower solidification front, and diapiric rise of such masses. After the amount of magma crystallized reaches 50% or more, rupture of a rigid crystalline framework occurs through the processes of contraction upon cooling, shearing caused by deformation, and expansion of bubbles in the interstitial residual liquid, while plagioclase-poor differentiate flows into spaces created in these processes. Important for the effectiveness of these processes is the buildup of volatiles in the residual liquids, which results from crystallization of anhydrous minerals, that causes second boiling and lowering of the viscosity of the liquids.

Late proterozoic thrust tectonics, high-pressure metamorphism and uranium mineralization in the Domes Area, Lufilian Arc, Northwestern Zambia

Abstract The following main lithostratigraphic units have been distinguished in the Domes Area. The Kibaran basement complex composed of gneisses, migmatites with amphibolite bands and metagranites is exposed in dome structures; metamorphic features of Kibaran age have been almost completely obliterated by extensive Lufilian reactivation. The post-Kibaran cover sequence is subdivided into the Lower Roan Group consisting of well-preserved quartzites with high Mg content, talc-bearing, extremely foliated schists intercalated with pseudo-conglomerates of tectonic origin and the Upper Roan Group including dolomitic marbles with rare stromatolites, metapelites and a sequence of detrital metasediments, with local volcano-sedimentary components and interlayered banded ironstones. The sediments of the Lower Roan Group are interpreted as continental to lagoonal-evaporitic deposits partly converted into the talc-kyanite + garnet assemblage characteristic of “white schists”. The dolomites and metapelites of the Upper Roan Group are attributed to a carbonate platform sequence progressively subsiding under terrigenous deposits, whilst the detrital metasediments and BIF may be interpreted as a basinal sequence, probably deposited on oceanic crust grading laterally into marbles. Metagabbros and metabasalts are considered as remnants of an ocean-floor-type crustal unit probably related to small basins. Alkaline stocks of Silurian age intruded the post-Kibaran cover. Significant ancestral tectonic discontinuities promoted the development of a nappe pile that underwent high-pressure metamorphism during the Lufilian orogeny and all lithostratigraphic units. RbSr and KAr and UPb data indicate an age of 700 Ma for the highest grade metamorphism and 500 Ma for blocking of the KAr and RbSr system in micas, corresponding to the time when the temperature dropped below 350°–400°C and to an age of about 400 Ma for the emplacement of hypabyssal syenitic bodies. A first phase of crustal shortening by decoupling of basement and cover slices along shallow shear zones has been recognized. Fluid-rich tectonic slabs of cover sediments were thus able to transport fluids into the anhydrous metamorphic basement or mafic units. During the subsequent metamorphic re-equilibration stage of high pressure, pre-existing thrusts horizons were converted into recrystallized mylonites. Due to uplift, rocks were re-equilibrated into assemblages compatible with lower pressures and slightly lower temperatures. This stage occurs under a decompressional (nearly adiabatic) regime, with Pfluid≈Plithostatic. It is accompanied by metasomatic development of minerals, activated by injection of hot fluids. New or reactivated shear zones and mylonitic belts were the preferred conduits of fluids. The most evident regional-scale effect of these processes is the intense metasomatic scapolitization of formerly plagioclase-rich lithologies. Uraninite mineralization can probably be assigned to the beginning of the decompressional stage. A third regional deformation phase characterized by open folds and local foliation is not accompanied by significant growth of new minerals. However, pitchblende mineralization can be ascribed to this phase as late-stage, short-range remobilization of previously existing deposits. Finally, shallow alkaline massifs were emplaced when the level of the Domes Area now exposed was already subjected to exchange with meteoric circuits, activated by residual geothermal gradients generally related to intrusions or rifting. Most of the superficial U-showings with U-oxidation products were probably generated during this relatively recent phase.

U-Pb zircon age of the Görbjörnarp syenite in Skåne, southern Sweden

Abstract Zircons were separated from a syenite body occurring along the Protogine Zone near Görbjörnarp in northeastern Skåne, southern Sweden. U-Pb analyes of the zircons yield an almost concordant age of 1204+14 -8 (2σ) Ma. The MSWD is 0.740. The obtained age is interpreted as the intrusion age of the syenite. It is within its limits of error identical to upper intercept U-Pb zircon ages of the Vaggeryd and Önnestad syenites and the Gumlösa-Glimakra granite also found aligned to the Protogine Zone.