On an Occurrence of Fayalite Quartz-Monzonite in the Basement Complex around Bauchi, Northern Nigeria

1961 ◽  
Vol 98 (6) ◽  
pp. 473-482 ◽  
Author(s):  
M. O. Oyawoye
Keyword(s):  
Linear Structure ◽  
Northern Nigeria ◽  
Basement Complex ◽  
Green Colour ◽  
Quartz Monzonite ◽  
Ring Complexes ◽  
High Level ◽  
Mineral Constituents ◽  
Ring Dyke

AbstractAn unusual fayalite-bearing quartz-monzonite occurs around Bauchi in Northern Nigeria and is distinguished from the high-level fayalite granites of the Newer Granites ring complexes by the presence of a linear structure conformable with those in the surrounding rocks, the gradational nature of its contacts, the extreme coarseness of its mineral constituents and the abundance of myrmekite. It is similar to them in its distinctive green colour and in the high FeO: Fe2O3 ratio. The rock is believed to be definitely older than the rocks of the ring-dyke complexes.

Structure and petrogenesis of a mixed-magma ring dyke in the Peruvian Coastal Batholith: eruptions from a zoned magma chamber

1988 ◽  
Vol 79 (2-3) ◽  
pp. 87-104 ◽  
Author(s):  
M. Andrew Bussell
Keyword(s):  
Magma Chamber ◽  
Present Level ◽  
Ring Fault ◽  
Brittle Ductile Transition ◽  
Erosion Level ◽  
Ring Complex ◽  
Ring Structures ◽  
Transient Events ◽  
Flat Roofs ◽  
Ring Dyke

ABSTRACTRing complex granites of the Peruvian Batholith are tabular bodies with flat roofs emplaced by cauldron subsidence. Marginal precursory ring dykes extend upwards above roof level and a typical intrusion is “H”-shaped in cross-section. Advance of magma by repeated subsidence would give a ladder-shaped profile for such intrusions above the brittle-ductile transition. Close relationships exist between intrusion geometry, emplacement process and petrogenetic evolution. Initially a granodioritic magma chamber lay beneath the present erosion level, trapping a rising mass of dioritic magma. Expansion of granodioritic liquid resulted in the injection of microgranite and tuffisite cone sheets accompanied by roof uplift within a ring fault. Next, during subsidence within the ring fault, liquids from deeper levels in the underlying chamber rose by stoping along the outer margin of the fault to form a ring dyke. Prior to intrusion, this liquid was vertically zoned from rhyodacite downwards to diorite and these liquids became partially mixed during emplacement. Finally, granodioritic magma rose to the present level by subsidence of a roof slab bounded by the ring fault. The precursory ring structures preserve evidence of significant but transient events in the underlying chamber. Liquid differentiation may be significant in the evolution of many large plutons.

scholarly journals The South Qôroq Centre nepheline syenites, South Greenland. Petrology, felsic mineralogy and petrogenesis

1976 ◽  
Vol 118 ◽  
pp. 1-55
Author(s):  
D Stephenson
Keyword(s):  
Phase Equilibria ◽  
Trace Element ◽  
Experimental System ◽  
Alkali Feldspar ◽  
Continuous Series ◽  
Small Range ◽  
The South ◽  
Outward Diffusion ◽  
Co Precipitation ◽  
Ring Dyke

The South Qoroq Centre is one of four high-level, major intrusive centres comprising the Igaliko Nepheline Syenite Complex. Three elliptical stocks of foyaite were emplaced in fairly rapid succession by ring fracture and block subsidence, followed by a partial ring· dyke of augite syenite. Inward-dipping microsyenite sheets appear to be associated with the ring-dyke; and four earlier, satellitic stocks occur around the periphery of the centre. Petrographic and mineralogical data show that the intrusions become successively less differentiated with time. Felsic mineral phases (alkali feldspar, nepheline and sodalite) constitute over 80% of most rocks from the centre. Electron-microprobe analyses demonstrate the major role of felsics in the fractionation of the magma and, together with estimates of feldspar structural state from 2V measurements, give indications of the history and conditions of crystallisation. Nepheline compositions fall within a small range of decreasing Si content, but are outside the Morozewicz-Buerger convergence field and are not affected by sub-solidus alkali exchange. Feldspars form a continuous series from Or1Ab67An32 to a K-enriched alkali feldspar Or72Ab28An0. From comparison with other rock suites, this extended feldspar trend seems to be associated with the co-precipitation of nepheline, and increasing peralkalinity of the magma. Major and trace element analyses of the rocks, made by X-ray fluorescence, give variation trends which may be interpreted mainly in terms of fractionation of feldspar and the ferromagnesian phases. In particular, trace element distributions are highly characteristic of fractional crystallisation series, but may not be compatible with fractional melting. Analyses of rocks with co-existing felsic phases compare favourably with phase equilibria in the experimental system Q-ne-ks at 1kb. It is suggested that the centre evolved from an underlying differentiated magma chamber, formed by crystal fractionation and accumulation. Successively lower portions of the chamber were tapped, producing batches of fractionated magma. Later stages of crystallisation were influenced by a build-up in volatiles consisting of H2O, CO2, Cl and F, and post-emplacement differentiation was implemented mainly by outward diffusion of these volatiles together with alkalis under a thermal diffusion gradient. Temperatures of crystallisation deduced from the nepheline geothermometer (Hamilton, 1961) and from phase equilibria in the Ab-Or-H2O system are in reasonable agreement. Assuming a PH2O of about 1 kb, the foyaite feldspars crystallised at about 850°C and augite syenite feldspars slightly higher. Nephelines commenced crystallisation within the range 900-850°C but stabilised at 775-700°C irrespective of rock-type. Physico-chemical conditions during recrystallisation attributable to the later Igdlerfigssalik Centre are inferred from textural, geochemical and mineralogical changes. The recrystallised rocks provide evidence for the behaviour of trace elements during the initial stages of remelting under hydrous conditions in an open system.

On the Differentiation of the Glen More Ring-dyke, Mull

1938 ◽  
Vol 75 (4) ◽  
pp. 145-160 ◽  
Author(s):  
Cath Koomans ◽  
Ph. H. Kuenen
Keyword(s):  
Iron Ore ◽  
Gradual Transition ◽  
Variation Diagram ◽  
Normal Series ◽  
Magma Series ◽  
Ring Dyke

SummaryNew analyses are given of the Glen More Ring-dyke. The basic end of the variation diagram is found to differ considerably from that of the Normal Magma Series. The magma variation is explained by gravitative sinking of augite and iron-ore (and plagioclase), the olivine of the Normal Series playing only a subordinate part. Various objections are brought forward to the explanation of the magmas by hybridization. The peculiar microscopic aspect of the intermediate rocks is tentatively attributed to pneumatolytic emanations from the lower reaches of the column. It is shown that a gradual transition from basic to acid already existed before the final consolidation of the magma took place.

The Structure of the Granophyric Quartz-Dolerite Intrusion of Centre 2, Ardnamurchan and the Problem of Net-Veining

1954 ◽  
Vol 91 (4) ◽  
pp. 293-307 ◽  
Author(s):  
M. K. Wells
Keyword(s):  
Outer Wall ◽  
External Pressure ◽  
Residual Magma ◽  
Ring Dyke

AbstractThe Quartz-dolerite intrusion of Sgùrr nam Meann, Ardnamurchan, is shown to be a ring-dyke which has sill apophyses extending from its outer wall. This is a form of intrusion which has not been recognized before, and its field relations are described in detail. The quartz-dolerite is extensively veined by acid rocks which are ubiquitous in all parts of the intrusion, and most abundant in the sills. From the evidence of the distribution and nature of the veining it is concluded that the acid and basic components of the intrusion must have been ernplaced simultaneously. It is suggested that a gas-charged, acid residual magma may have separated from a crystallizing basic parent under the influence of sudden reductions of external pressure acting upon the system.

Intrusive history of the Slieve Gullion ring dyke, Ireland: implications for the internal structure of silicic sub-caldera magma chambers

2004 ◽  
Vol 68 (5) ◽  
pp. 725-738 ◽  
Author(s):  
S. McDonnell ◽  
V. R. Troll ◽  
C. H. Emeleus ◽  
I. G. Meighan ◽  
D. Brock ◽  
...  
Keyword(s):  
Granitic Magma ◽  
Caldera Collapse ◽  
Magma Chambers ◽  
Magma Body ◽  
Rock Types ◽  
Geochemical Variations ◽  
History Of ◽  
Complex Relationships ◽  
Chemical Groups ◽  
Ring Dyke

AbstractThe Palaeogene Slieve Gullion Igneous Complex comprises a layered central intrusion surrounded by a slightly older ring dyke. The ring dyke contains two major intrusive rock types. About 70% of the ring dyke is occupied by porphyritic granophyre and 30% by porphyritic felsite. Locally complex relationships between the two lithologies are observed. Major and trace element compositions suggest that there are two distinct chemical groups within each lithology: a Si-rich felsite, concentrated in a ~1 m wide zone at the outer margins of the dyke which grades into a less Si-rich felsite towards the interior. Similarly, a Si-rich granophyre, concentrated in the centre of the intrusion grades outwards into a Si-poor granophyre facies.These rock relationships and geochemical variations suggest that a complex magma chamber hosted a stratified granitic magma body and various wall/floor magma facies. Low density, high-Si felsite magma from the top of the chamber was tapped first, followed by less Si-rich felsite magma as evacuation proceeded. The granophyres probably originate from the chamber walls/floor, representing more mushy equivalents of the felsite magma. Little granophyre magma was tapped during the early stages of the evacuation sequence. As evacuation continued, probably aided by trap-door caldera collapse, the ‘granophyre magmas’ intruded the already emplaced and slightly cooled felsite, forming the complexly zoned structure of the Slieve Gullion ring intrusion.

An audiomagnetotelluric study of the Meugueur-Meugueur ring structure, Aïr, Niger: ring dyke or cone sheet?

1996 ◽  
Vol 34 (4) ◽  
pp. 229-236 ◽  
Author(s):  
Michel Ritz ◽  
William L. Brown ◽  
Christian Moreau ◽  
Daniel Ohnenstetter
Keyword(s):  
Ring Structure ◽  
Ring Dyke
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