Feldspathoidal Syenites Research Articles

The Neoproterozoic Upper Ruvubu alkaline plutonic complex (Burundi) revisited: Large-scale syntectonic emplacement, magmatic differentiation and late-stage circulations of fluids

The Upper Ruvubu Alkaline Plutonic Complex (URAPC) in Burundi consists of three separate intrusions, each with a specific emplacement age and petrological composition. Three main units are recognized: an outer unit with silica-saturated plutonic rocks (from gabbro to granite), an inner unit with silica-undersaturated plutonic rocks (feldspathoidal syenite with subordinate feldspathoidal monzonite and ijolite) and a carbonatitic body in the subsoil, known by drilling. The URAPC is quite large in size (∼24 km long and up to 10 km wide). It is considered to have been intruded syntectonically in an overall extensional context, thanks to the kilometric shear zones that accommodated its emplacement. Radiometric ages from literature range from 748 to 705 Ma and point to structurally-controlled magmatic differentiation followed by long-lived circulations of late-stage fluids postdating the emplacement of a part of the undersaturated rocks and the carbonatites. In the north-western part of the outer unit, gabbro likely has been emplaced at a deeper structural level than the granite, which represents a more apical structural level of emplacement.This petrological, geochemical and isotopic (Sr-Nd-Hf) study concentrates on the processes that generated the URAPC: (i) fractional crystallization, evidenced by the chemical evolution trends of the major and trace elements, and by marked P, Ti and Ba anomalies in the trace element patterns; (ii) crustal assimilation/contamination, as shown by the wide range of Nd isotope compositions and the general increase of the Sr isotope ratios with increasing SiO2 contents, and (iii) late-magmatic/hydrothermal alteration inducing an increase of the Sr isotope composition without changing significantly the Nd isotope composition.The isotopic data are consistent with an asthenospheric mantle source, though less depleted than the Depleted Mantle (DM), contaminated by the Subcontinental Lithospheric Mantle (SCLM). The silicate and carbonate magmatic series are cogenetic. The outer unit is clearly more contaminated than the inner unit, whereas the carbonatitic body could have evolved by liquid immiscibility.The URAPC lies within East Africa’s Western Rift Valley, which is marked by 23 alkaline plutonic complexes. Their emplacement has been ascribed to reactivation of Proterozoic lithospheric weakness zones resulting from the breakup of the Neoproterozoic supercontinent Rodinia supercontinent.

Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities

The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow-level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar+biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz+hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behavior of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2<δ18O (vs. SMOW) <8.5‰ and -4.7<δ13C (vs. PDB) <-5.4‰ in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (δ18OZr=4−4.7‰, δ18OIlm=- 4.3−-1.5‰, respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high δ18O values (δ18O=8.6−21.8‰), show evidence of a medium- to low-temperature hydrothermal alteration event by an aqueous fluid. Calcitic veins in the carbonatite record another alteration event, outlined by the co-variation of δ18O and δ13C values (δ18O=16.3−24.7‰ and δ13C=-4.7−0.2‰), implying the involvement of a mixed H2O−CO2 fluid. As a whole, the circulation of fluids in the URAPC was initiated during magmatic emplacement and the geometry of this circulation was controlled by the syn-emplacement crustal scale shear zones. Element mobility, one expression of which being the mineralization features described here, follow the same scheme.

Petrography, geochemistry and U–Pb zircon age of the Matongo carbonatite Massif (Burundi): Implication for the Neoproterozoic geodynamic evolution of Central Africa

The Matongo carbonatite intrusion belongs to the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC), that is located in Burundi along the western branch of the East African Rift. Beside the Matongo carbonatite, the URAPC alkaline complex comprises feldspathoidal syenites, diorites, quartz-bearing syenites and granites.Three main facies have been recognized in the Matongo carbonatite: (1) Sövites represent the dominant facies. Two varieties have been recognized. A scarce coarse-grained sövite (sövite I), which is altered and poorly enriched in REE (4<ΣREE<8ppm), is encountered in highly fractured zones. A fine-grained sövite (sövite II), which is made of saccharoidal calcite, commonly associated with apatite, aegirine and amphibole, is abundant in the intrusion. Sövite II is enriched in LREE (442<ΣREE<1550ppm, 49<LaN/YbN<175). (2) Ferrocarbonatites, that form decimeter-wide veins crosscutting the sövites, are characterized by a LREE enriched patterns (225<ΣREE<1048ppm, 17<LaN/YbN<64). (3) K-feldspar and biotite-rich fenite facies (silicocarbonatites) have been recognized at the contact between the carbonatites and the country rock. They are likewise LREE-enriched (134<ΣREE<681ppm, 25<LaN/YbN<46). Additionaly, “late” hydrothermal MREE-rich carbonatite veinlets can be found in sövite I. They are characterized by moderate enrichment in REE (ΣREE=397ppm), with a MREE-humped pattern (LaN/YbN=3.7). The different facies represent the typical magmatic evolution of a carbonatite, while the silicocarbonatites are interpreted as resulting from the fenitisation of the country host-rocks. In addition, the most REE-depleted and fractionated facies, i.e. the coarse-grained sövite facies and the “late” calcite veinlets testify for hydrothermal processes that occurred after carbonatite emplacement and result from REE mobilization and redistribution.Large idiomorphic zircon crystals (megacrysts), found in the vicinity of the carbonatite can directly be related to the carbonatite evolution. They have been dated at 705.5±4.5Ma (U–Pb concordant age, LA–ICP–MS). Similar zircon megacrysts of the Lueshe carbonatite (DRCongo) have been dated and give a concordant age at 798.5±4.9Ma (U–Pb, LA–ICP–MS). Considering that an extensional tectonic regime occured at that time in Central Africa – what remains debated - both ages could relate to different stages of Rodinia breakup, with uprise of mantle-derived magmas along Palaeoproterozoic lithospheric zones of weakness.

Timing of Archean crust formation and cratonization in the Awsard-Tichla zone of the NW Reguibat Rise, West African Craton: A SHRIMP, Nd–Sr isotopes, and geochemical reconnaissance study

The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87Sr/86Sr(t) from 0.7003 to 0.7030, ɛNd(t) from +2 to +5, and Nd model ages in the range TCR=3.03–2.92Ga. Eight TTG gneisses yielded SHRIMP U–Pb zircon crystallisation ages between 3.04Ga and 2.92Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a TCR=3.09 and a zircon crystallisation age of 2.94Ga but also contains a 3.11Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03Ga and 3.01Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.

2.46 Ga kalsilite and nepheline syenites from the Awsard pluton, Reguibat Rise of the West African Craton, Morocco. Generation of extremely K-rich magmas at the Archean–Proterozoic transition

The Awsard pluton in the Moroccan part of the Reguibat Rise, West African Craton, contains the oldest kalsilite-bearing rocks discovered to date, with a SHRIMP zircon U/Pb age of 2.46±0.01Ga. The pluton is composed of nepheline syenites, kalsilite syenites and minor silica-saturated syenites, all with the same primitive Sr and Nd initial isotope compositions and Nd model age that cluster around 87Sr/86Sr2.46Ga≈0.7029, ɛNd2.46Ga≈−1.4, and TDM≈2.75Ga. Silica-saturated syenites are in fact contact fenites that grade into nepheline syenites, but the two feldspathoidal syenites are true magmatic rocks that crystallized from two coeval highly fractionated K-rich magmas with sharply different chemical compositions. Chemical, isotopic and textural evidence suggests that the two magmas originated by liquid immiscibility within an already fractionated alkaline potassic magma of asthenospheric origin that split in two phases, a nephelinitic melt rich in HFSE+REE, and a kalsilitic melt poor in HFSE+REE. The Awsard pluton, spatially associated to carbonatites and other alkaline rocks, does not mark a Late Archean fossil subduction zone but represents the first manifestation of a Paleoproterozoic alkaline province located in the Reguibat Rise, the full extent and importance of which is yet to be determined.

Geochemical constraints on petrogenesis of Late Cretaceous alkaline magmatism in east-central Anatolia (Hasancelebi–Basören, Malatya), Turkey

Late Cretaceous alkali magmatic rocks occur widely in the Hasancelebi and Basoren regions (Malatya). The Hasancelebi and Basoren intrusive rocks are mainly peralkaline and alkaline-oversaturated. The Hasancelebi intrusive rocks are made up of syenite to quartz monzonite. On the other hand the Basoren intrusive rocks mainly contain feldspathoidal syenites that are cut by feldspathoid-bearing dykes. The Hasancelebi intrusive rocks show comparable field, petrographic and geochemical characteristics with A-type rocks. All intrusive rocks show enrichment in LILE and LREE relative to HFSE. The Th/Yb versus Ta/Yb diagram indicates that all magmatic rocks formed from an enriched mantle source region(s). The parental magma for the Basoren rocks has a higher intraplate component than that from the Hasancelebi rocks which could be attributed to mantle source heterogeneity before collision in east-central Anatolia. Either delamination of the thermal boundary layer or slab-breakoff are likely mechanisms for the initiation of post-collisional magmatism in the Hasancelebi–Basoren areas.

PARAGENETIC TYPES OF CARBONATITE AS INDICATED BY THE DIVERSITY AND RELATIVE ABUNDANCES OF ASSOCIATED SILICATE ROCKS: EVIDENCE FROM A GLOBAL DATABASE

Data on the diversity and relative abundance of igneous rock types associated with carbonatite have been compiled for 477 occurrences, which represent 90% of the 527 occurrences of carbonatite known to the authors. The carbonatites have been subdivided into magmatic carbonatites (84%) and carbohydrothermal carbonatites (16%) on the basis of their mineralogy and their intrusive or extrusive characteristics. The carbohydrothermal carbonatites are associated with a variety of silicate rock-types, but nepheline syenite and syenite are the characteristic ones. For the magmatic carbonatites, 24% of these localities have no associated igneous silicate rocks, whereas a diverse range of silicate rock-types are recognized to be associated with the other 76%. The silicate rocks associated with these carbonatites are listed, quantified and described. We distinguish seven main associations with silicate rocks. These are (in decreasing order of abundance): 1) nephelinite–ijolite, 2) phonolite – feldspathoidal syenite, 3) trachyte–syenite, 4) melilitite–melilitolite, 5) lamprophyre, 6) kimberlite, 7) basanite – alkali gabbro. A significant proportion of the nephelinite–ijolite, melilitite–melilitolite and carbonatite only associations also contain ultramafic rocks, which are interpreted as cumulates. Extrusive carbonatite is found at forty-six localities; these are characterized by the high proportion that contain mantle debris (xenoliths and xenocrysts) and the occurrence of melilite-bearing silicate rocks in more than one third of them. The presence of mantle debris in some carbonatite occurrences and the absence of associated silicate rocks from others are taken as evidence of derivation of these carbonatites directly from the mantle. Most other carbonatites are considered to have been generated by differentiation from magmas represented by the associated silicate rocks that are the result of partial melting in a metasomatized lithosphere. Carbonatites can be generated in a number of ways, but the close spatial and temporal association with a broad spectrum of the silicate melts implies a close relationship with them. However, certain associated silicate rocks are not strictly consanguineous with the carbonatite, but are interpreted as products of independent melting events at shallower levels in the lithosphere.

The Alkaline-Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source

The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (ΔFMQ = +2–5 to −1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (ΔFMQ = −2) and intermediate a SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.

Affinites intra-oceaniques des metavolcanites de l'unite de la Boghen (ex-‘ante-Permien’ de Nouvelle-Caledonie, Pacifique sud-ouest). Consequences rocks of the Boghen Terrane (ex-‘ante-Permian’, New Caledonia, southwest Pacific). Palaeogeographic inferences) : Cluzel, D., Comptes Rendus - Academie des Sciences, Serie II: Sciences de la Terre et des Planetes, 1996, 323/8 (657–664)

The East Gondwana crustal fragments, namely southern India, Sri Lanka, Madagascar, East Antarctica and Western Australia, preserve evidence for a prominent Pan-African felsic magmatic event represented by feldspathoidal syenites, alkali granites and syenites emplaced proximal to fault lineaments. In Peninsular India, late or post-kinematic alkaline plutons ranging in age from 864 to 600 Ma characterize the Eastern Ghats belt, whereas a strong Pan-African imprint is provided by 550–750 Ma alkali granites and syenites in the southern granulite segment. In Sri Lanka, alkali granites occur in all major geologic units, with radiometric ages in the range of 550–760 Ma. In Madagascar, alkali granites and syenites with late Proterozoic to Palaeozoic ages are reported. Felsic magmatic activity in East Antarctica is characterized by post-tectonic granitic and syenitic plutons in Enderby Land and Queen Maud Land, showing an age range of 550–770 Ma. Pan-African felsic plutons in Western Australia define ages of 550–570 Ma. This paper summarizes the petrologic, geochemical and geochronologic characteristics of felsic plutons in the various East Gondwana fragments. Many of the intrusives represent anorogenic A-type magmas generated in rift-related environments of high heat flow and abundant volatile activity, correlative with an extensional tectonic regime and probably including melts generated from both upper mantle and lower crustal sources. This paper identifies a major Pan-African felsic magmatic province in East Gondwana, which, in association with their petrogenetic significance, is considered to be suggestive of a geodynamic signature of East Gondwana during the Pan-African period.

The felsic magmatic province in East Gondwana: implications for Pan-African tectonics

The East Gondwana crustal fragments, namely southern India, Sri Lanka, Madagascar, East Antarctica and Western Australia, preserve evidence for a prominent Pan-African felsic magmatic event represented by feldspathoidal syenites, alkali granites and syenites emplaced proximal to fault lineaments. In Peninsular India, late or post-kinematic alkaline plutons ranging in age from 864 to 600 Ma characterize the Eastern Ghats belt, whereas a strong Pan-African imprint is provided by 550–750 Ma alkali granites and syenites in the southern granulite segment. In Sri Lanka, alkali granites occur in all major geologic units, with radiometric ages in the range of 550–760 Ma. In Madagascar, alkali granites and syenites with late Proterozoic to Palaeozoic ages are reported. Felsic magmatic activity in East Antarctica is characterized by post-tectonic granitic and syenitic plutons in Enderby Land and Queen Maud Land, showing an age range of 550–770 Ma. Pan-African felsic plutons in Western Australia define ages of 550–570 Ma. This paper summarizes the petrologic, geochemical and geochronologic characteristics of felsic plutons in the various East Gondwana fragments. Many of the intrusives represent anorogenic A-type magmas generated in rift-related environments of high heat flow and abundant volatile activity, correlative with an extensional tectonic regime and probably including melts generated from both upper mantle and lower crustal sources. This paper identifies a major Pan-African felsic magmatic province in East Gondwana, which, in association with their petrogenetic significance, is considered to be suggestive of a geodynamic signature of East Gondwana during the Pan-African period.

Petrology of the alkaline magmatism from the Cretaceous North-Pyrenean Rift Zone (France and Spain)

The Cretaceous alkaline magmatism from the North-Pyrenean Rift Zone consists of a variety of effusive and intrusive rocks: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. An extensive survey of the magmatic mineralogy in the different rock-types is presented, and pyroxene and amphibole compositions are used to trace the magmatic differentiation. In alkali basalts and monchiquites, the composition of early pyroxenes (coloured Fe-rich xenocrysts and Cr-rich phenocryst cores) and amphiboles suggests fractionation in upper-mantle to lower-crustal conditions, whereas pyroxene phenocrysts rims and microphenocrysts are Al- and Ti-rich, consistent with crystallization at lower pressure. Modelling of major and trace elements suggests that trachytes and non-peralkaline feldspathoidal syenites could be extracted from mafic parents by 70-80% crystallisation of a gabbroic cumulate but the model fails to identify parental magmas of peralkaline nepheline syenites. The mantle source appears homogeneous in its REE contents over the Pyrenees and is shown to be LREE-enriched with a (La/Yb) N ratio close to 5

Petrology and geochemistry of the alkaline intrusion, Jombo Hill, Kenya

SummaryThe field occurrence and the known immiscibility between silicate and carbonate melts suggest the Jombo alkaline intrusion and Mrima carbonatite intrusion have come from the same magmatic source. However, only the ijolite series apparently originated from a magma body at Jombo hill intrusion, while the feldspathoidal syenite series appears to have resulted from metasomatic alteration of the country rock-sandstone. The K/Rb ratios seem to indicate a mantle origin for the ijolites and a crustal one for the feldspathoidal syenites. The feldspathoidal syenite series were presumably formed by alkali metasomatism, resulting from magmatic fluid infiltration through intergranular movement. It is suggested that the partitioning of K and Na components in the metasomatizing fluid and solid feldspar phases at different temperatures, was the cause of two rock types – albite nepheline syenite and orthoclase nepheline syenite.

The Tugtutoq Younger Giant Dyke Complex, South Greenland: Fractional Crystallization of Transitional Olivine Basalt Magma

The Younger Giant Dyke Complex consists of a set of massive coalescing dykes of Proterozoic age (c. 1170 Ma), resulting from intrusion of a suite of transitional olivine basaltic/hawaiitic magmas in a continental rift setting. The suite, compositionally related by low pressure (< 10 kb) olivine-plagioclase fractionation, is believed to have had a deeper level evolution dominated by pyroxene and possibly garnet fractionation. Slow cooling in situ of the interior parts of the dyke complex produced cumulitic suites. Those exposed range from gabbroic to syenitic; residual bodies of riebeckite granite and, very subordinate, feldspathoidal syenite were also generated. The basic magmas had notably low fO2 values, leading to delayed magnetite and clinopyroxene precipitation, relatively iron-rich differentiates and some residual liquids of pantelleritic composition. The basic magmas had high F/Cl values and are inferred to have had low H2O contents. They were also characterized by relatively high K/Rb and low 87Sr/86Sr values; these characteristics imply a mantle source with high F/Cl but depleted in Rb relative to K and Sr. Basaltic magmas responsible for (a) the preceding Older Giant Dyke Complex and (b) a suite of anorthositic xenoliths within the Younger Giant Dyke Complex, are inferred to have been derived from separate primary magma batches independent of those that yielded the Younger Giant Dyke Complex. The giant dykes are the highest-level representatives of a larger basic complex responsible for the extensive linear gravity ‘high’ in the Tugtutôq-Narssaq area.

Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico

Research Article| October 01, 1977 Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico DANIEL S. BARKER; DANIEL S. BARKER 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar LEON E. LONG; LEON E. LONG 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar G. KARL HOOPS; G. KARL HOOPS 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar FLOYD N. HODGES FLOYD N. HODGES 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Author and Article Information DANIEL S. BARKER 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 LEON E. LONG 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 G. KARL HOOPS 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 FLOYD N. HODGES 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1977) 88 (10): 1437–1446. https://doi.org/10.1130/0016-7606(1977)88<1437:PARIGO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DANIEL S. BARKER, LEON E. LONG, G. KARL HOOPS, FLOYD N. HODGES; Petrology and Rb-Sr isotope geochemistry of intrusions in the Diablo Plateau, northern Trans-Pecos magmatic province, Texas and New Mexico. GSA Bulletin 1977;; 88 (10): 1437–1446. doi: https://doi.org/10.1130/0016-7606(1977)88<1437:PARIGO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Shallow intrusive bodies in a 65-km-long belt consist of quartz-bearing and feldspathoidal syenite, trachyte, and phonolite, all within a narrow range of composition. K-Ar ages (35 ± 2 m.y.) and modal, whole-rock chemical, Rb-Sr isotopic, and mineralogic data suggest that all the intrusive rocks were derived from one magma reservoir.Nine intrusive rock types are divisible into two groups. The older group, commonly forming sills and laccoliths, is fine- to coarse-grained and has little or no flow structure, low Rb, high Sr, and initial 87Sr/86Sr ratios of 0.703 to 0.709. The younger group, tending to form discordant sheets, is finely porphyritic and has strongly developed flow structure, high Rb, low Sr, and initial strontium ratios of 0.705 to 0.712. The younger group shows more extreme compositional variation, including the most silica-oversaturated and the most silica-undersaturated rocks. Each group contains both quartz-bearing and feldspathoidal rock types.Fractional removal of plagioclase and biotite from mugearitic liquid progressively enriched the remaining liquid in sodium while depleting it in calcium, magnesium, and titanium. Later differentiation proceeded mostly by removal of alkali feldspar. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Feldspathoidal Rocks of the Red Hill Igneous Complex, New Hampshire

The Red Hill intrusive complex in the towns of Moultonboro and Sandwich, New Hampshire is a small elliptical stock of syenite, feldspathoidal syenite, and granite. It is divided into three distinct intrusive groups which are, from oldest to youngest, the Older Quartzose Group consisting of slightly oversaturated syenites occupying the crescent-shaped northwestern part of the intrusive, the Feldspathoidal Group consisting of undersaturated nepheline- and sodalite-bearing syenites which form a funnel-shaped intrusive body and a swarm of ring dikes or cone sheets, and the Younger Quartzose Group consisting of oversaturated syenites and granite occurring as crosscutting plugs. The compositions and textures of the Feldspathoidal Group rocks are believed to be the result of the intrusion of magma intermittently tapped from a subjacent body of fractionally crystallizing slightly undersaturated, chloride-rich syenitic magma. All the Feldspathoidal Group rocks cooled together under closed-system conditions, local equilibrium being maintained down to about 250° C. These rocks solidified at a depth of less than 6 or 7 km.

Neogene Peralkaline Igneous Activity in Eastern Peru

Research Article| August 01, 1971 Neogene Peralkaline Igneous Activity in Eastern Peru JAMES W STEWART JAMES W STEWART Institute de Investigations Geological, Casilla 10465, Santiago, Chile *Technical co-operation United Kingdom–Chile. Search for other works by this author on: GSW Google Scholar GSA Bulletin (1971) 82 (8): 2307–2312. https://doi.org/10.1130/0016-7606(1971)82[2307:NPIAIE]2.0.CO;2 Article history received: 05 Apr 1971 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES W STEWART; Neogene Peralkaline Igneous Activity in Eastern Peru. GSA Bulletin 1971;; 82 (8): 2307–2312. doi: https://doi.org/10.1130/0016-7606(1971)82[2307:NPIAIE]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Three volcanic necks which cut Tertiary sediments in the Amazon plain represent an apparently isolated igneous event which took place about 5 m.y. ago (K: Ar determinations on cryptoperthite). The principal rock types of these necks are unusual undersaturated alkaline varieties characterized by sodalite and melanite and containing cognate inclusions of jacupirangite. Alkali trachytes also occur.The existence of an underlying plutonic complex containing feldspathoidal syenite, alkali syenite and jacupirangite, is postulated. The presence of jacupirangite and melanite in the volcanic rocks increases the probability that carbonatite might be another constituent of the inferred subvolcanic complex.The intrusion of the phonolites was completely unrelated in time to the emplacement of the known alkaline complexes of Brazil. The phonolitic magma might have formed as a consequence of fusion near the eastern limit of a Benioff zone which dipped landward from the western edge of the South American continent. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.