The Problem of the Association of Acid and Basic Rocks in Central Complexes

1931 ◽  
Vol 68 (6) ◽  
pp. 241-255 ◽  
Author(s):  
Arthur Holmes
Keyword(s):  
North America ◽  
Igneous Rocks ◽  
Iron Ores ◽  
Residual Magma ◽  
Crystallization Differentiation ◽  
Igneous Complexes ◽  
Basaltic Magmas ◽  
Parent Stock ◽  
Mechanical Separation ◽  
Basic Rocks

One of the outstanding problems of petrogenesis at the present time is that offered by the remarkable association of sharply contrasted acid and basic rocks (e.g. granite-gabbro; granophyredolerite; pitchstone-tholeiite; and rhyolite-basalt) in igneous complexes such as those of the British Tertiary Province and those of the great lopoliths of Duluth and Sudbury in North America and the Bushveld in South Africa. The contrast was first recognized in the lavas of Iceland by Bunsen (1) and it led him to the conception that two fundamental magmas, respectively acid and basic, were concerned in the genesis of the igneous rocks of Iceland and similar provinces elsewhere. Bunsen's view, however, has had little influence in the development of petrological philosophy. The petrologists of the Geological Survey in this country, and Bowen and others in North America, have assembled a very weighty and reasonable mass of field and laboratory evidence supporting the hypothesis that the acid rocks are residual products arising from the crystallization-differentiation of basaltic magmas. In the recently published Ardnamurchan Memoir (2), for example, it is claimed that as a result of the early extraction from the Plateau Magma of olivine, pyroxenes, basic plagioclase, and iron ores, the residual magma would reach a composition “which would find expression as quartz-doleritic rocks with an acid mesostasis capable of mechanical separation and a separate existence as acid lavas or intrusions” (p. 95). Bunsen's two magmas are thus regarded as successive products from a single parent stock.

A mechanism forming silicic segregations from basaltic magma discovered in igneous rocks of Western Sicily

1983 ◽  
Vol 120 (4) ◽  
pp. 321-329 ◽  
Author(s):  
G. Lucido
Keyword(s):  
Basaltic Magma ◽  
Liquid Immiscibility ◽  
Igneous Rocks ◽  
Silicate Liquid ◽  
Basaltic Magmas ◽  
Western Sicily ◽  
Basic Rocks ◽  
Silicate Liquid Immiscibility ◽  
Textural Evidence

Summary. Mechanisms forming silicic segregations from basaltic magmas are considered of primary importance when dealing with magmatic problems. However, the processes which give rise to silicic segregations from basaltic magmas are so far obscure. Fortunately, the discovery of spheroidal felsic masses in some basic rocks of Western Sicily throws light on this subject. To clarify the relationships between felsic and basic fractions particular attention has been paid to the interactions which occurred at their contact. Textural evidence indicates that the accretion mechanism of the Sicilian felsic segregations tends to obliterate the silicate liquid immiscibility effects and suggests that the formation of silicic segregations is a consequence of liquid unmixing phenomena.

scholarly journals The system CaO-MgO-FeO-SiO2 and its bearing on the origin of ultrabasic and basic rocks

1960 ◽  
Vol 32 (249) ◽  
pp. 459-470 ◽  
Author(s):  
Peter J. Wyllie
Keyword(s):  
Experimental Data ◽  
Temperature Interval ◽  
Wide Temperature Range ◽  
Multicomponent System ◽  
Small Temperature ◽  
Experimental Procedure ◽  
Temperature Range ◽  
A Value ◽  
Basaltic Magmas ◽  
Basic Rocks

SummaryExperimental data in the system CaO-MgO-FeO-SiO2 suggest that there may be a plateau on the liquidus and solidus of the multicomponent system basalt-peridotite. If this is so, fusion of peridotite would produce only basaltie magmas over a wide temperature range; when the temperature reached a value such that the liquid crossed the threshold of the plateau, there would be a rapid increase in the amount of fusion for small temperature increases, with the formation of picritic magmas; basaltic magmas containing suspended forsteritic olivine crystals could dissolve them if the temperature rose slightly above that of the plateau threshold; a high proportion of a picritic magma would crystallize in a small temperature interval, with the precipitation of forsteritic olivine that was only slightly zoned. These possibilities are compared with current theories, and it is concluded that several petrological axioms may require critical re-examination. An experimental procedure is outlined to determine the shape of the liquidus and solidus in the basalt-peridotite system.

The Idea of Contrasted Differentiation

1936 ◽  
Vol 73 (5) ◽  
pp. 228-238 ◽  
Author(s):  
Arthur Holmes
Keyword(s):  
Basaltic Magma ◽  
Necessary Condition ◽  
Residual Liquid ◽  
Continuous Separation ◽  
Intermediate Composition ◽  
Rock Types ◽  
Crystallization Differentiation ◽  
Intermediate Layers ◽  
Intermediate Rock ◽  
Basic Rocks

As early as 1915, it was made clear by Bowen that, given continuous separation of crystals from the successive residual liquids of a consolidating magma which was initially basaltic in composition, the inevitable result would be a gabbroic rock in depth, a granitic one above, and “various intermediate types in the intermediate layers”. As a necessary condition to the evolution of a magmatic residuum of granitic composition, this process of progressive crystallization differentiation involves the generation of intermediate rock-types from residual liquids of intermediate composition. Certain investigators, however, led by the authors of the Mull memoir (1924), have entirely overlooked the necessity of this condition. Faced with the association of contrasted acid and basic rocks, which is a conspicuous characteristic of many continental central complexes, they have attempted to account for the facts by introducing a modified conception of crystallization differentiation for which Nockolds has recently proposed the name contrasted differentiation. According to this conception, the residual liquid of consolidating basaltic magma is of granitic composition and can be separated in bulk from a crystalline gabbroic phase.

Morphology and geochemistry of zircons from late Mesozoic igneous complexes in coastal SE China: implications for petrogenesis

2002 ◽  
Vol 66 (2) ◽  
pp. 235-251 ◽  
Author(s):  
X. Wang ◽  
W. L. Griffin ◽  
S. Y. O’Reilly ◽  
X. M. Zhou ◽  
X. S. Xu ◽  
...  
Keyword(s):  
Growth Stages ◽  
Continental Margins ◽  
Calc Alkaline ◽  
Mafic Rocks ◽  
Late Mesozoic ◽  
Igneous Complexes ◽  
Basaltic Magmas ◽  
Morphological Indices ◽  
Active Continental Margins ◽  
Se China

AbstractThe Pingtan and Tonglu igneous complexes in SE China are typical of the calc-alkaline series developed at active continental margins. These two complexes are dominated by felsic rocks, temporally and spatially associated with minor mafic rocks. Morphological and trace-element studies of zircon populations in rocks from each of these complexes show that the zircon populations may be divided into 3–4 distinct growth stages, characterized by different distributions of morphological indices (Ipr, Ipy and Iel), and different contents of the substituting elements (Hf, U, Th, Y and P). The four growth stages recognized in the zircons are believed to have formed successively in the magma chamber, during the emplacement, and in the early and later stages of magma consolidation, respectively. All four stages are recognized in the plutonic Pingtan complex, whereas the stages 3 and 4 are less developed in the volcanic/subvolcanic Tonglu complex. Based on the chemistry and morphology of the different zircon populations of the Pingtan and Tonglu complexes, it is suggested that basaltic magmas underplating at the boundary between crust and mantle caused partial melting of the mid–lower crust and produced granitoid magmas. Subsequently, mixing between magmas was important.

Paleomagnetism of igneous rocks from the Belcher Islands, Northwest Territories, Canada

1980 ◽  
Vol 17 (7) ◽  
pp. 807-822 ◽  
Author(s):  
P. W. Schmidt
Keyword(s):  
North America ◽  
Northwest Territories ◽  
Igneous Rock ◽  
Pole Position ◽  
Igneous Rocks ◽  
Hudson Bay ◽  
Plate Model ◽  
Polar Wander ◽  
Apparent Polar Wander Path ◽  
Paleomagnetic Pole

Paleomagnetic results from igneous rock units on the Belcher Islands, Hudson Bay, are described. Fold tests for all units studied, as well as a contact test for the intrusive bodies, indicate that both primary (initial), and secondary (post-folding) magnetizations are present.The paleomagnetic pole position from primary directions of the oldest unit studied, the Eskimo volcanics, is situated at 40°S, 002°E (A95 = 12°) and is similar to that derived from equivalent volcanics on the mainland. The younger volcanic unit studied, the Flaherty volcanics, yielded a pole position from primary directions at 0°, 244°E (A95 = 7°). The Haig intrusions, associated with these younger volcanics, yields an almost identical pole position at 1°N, 247°E (A95 = 6°), being derived from directions which are shown to be not only pre–folding but also date from initial cooling. The Eskimo volcanics, which have been more deeply buried than the Flaherty (upper) volcanics, carry substantial components of secondary (post-folding) magnetization which yield a pole position at 19°N, 243°E (A95 = 15°), about 20° north of the pole positions derived from the youngest units.It is argued that the apparent polar wander path (APWP) constructed for the Belcher Islands is representative of the mainland Ungava Craton. Comparison with the equivalent APWP from elsewhere in North America shows that the two APWP's are at variance. Although a two-plate model could be advanced, perhaps a more conservative interpretation is to extend the existing North American APWP eastward to include the Belcher–Ungava APWP, that is, to favour a one-plate model.

The geochemistry of the Devonian lavas of the northern Lorne Plateau, Scotland

1974 ◽  
Vol 39 (306) ◽  
pp. 621-640 ◽  
Author(s):  
D. R. Groome ◽  
A. Hall
Keyword(s):  
Volcanic Rocks ◽  
Parent Magma ◽  
Igneous Rocks ◽  
Continuous Series ◽  
Chemical Analyses ◽  
Scottish Highlands ◽  
Igneous Activity ◽  
Common Parent ◽  
Short Time ◽  
Basic Rocks

SummaryChemical analyses of the Lorne volcanic rocks show that the basalts and andesites are a closely related suite of lavas, notably rich in alkalis, especially potassium. The rhyolites of the Lorne area do not appear to form a continuous series with the more basic rocks. The various lava types are compared with the plutonic rocks of the area, and a relationship is suggested between the basic lavas and appinitic intrusions. The acid lavas are not comparable to granites in the area.The late orogenic igneous activity in this part of the Caledonides therefore appears to involve three generations of magma, produced separately but within a short time of one another. The assumption that all the late Caledonian igneous rocks of the Scottish Highlands are differentiates of a common parent magma is not justified.

Chlorites in a spectrum of igneous rocks: mineral chemistry and paragenesis

1995 ◽  
Vol 59 (394) ◽  
pp. 129-141 ◽  
Author(s):  
Abdel-Fattah M. Abdel-Rahman
Keyword(s):  
Structural Type ◽  
Mineral Chemistry ◽  
Igneous Rocks ◽  
The Other ◽  
High Alumina ◽  
X Ray Diffraction ◽  
Calc Alkaline ◽  
Peralkaline Granite ◽  
Igneous Complexes ◽  
Data Points

AbstractThe chlorite data presented are from four igneous complexes covering the compositional spectrum of igneous rocks (gabbro to granite) of orogenic and anorogenic settings. The four igneous complexes are; early orogenic gabbro-diorite-tonalite (D-T) suite, late orogenic granodiorite-adamellite (G-A) suite (both are calc-alkaline suites), high-alumina trondhjemite (TR), and anorogenic peralkaline granite (PGR).Chlorites in these igneous rocks show characteristic compositional fields. The Mg vs Fe plot provides the best discriminant, as data points define three compositionally different groups. Phases in the PGR are Fe-rich, siliceous, interlayered chlorite-smectite (Fe/Mg = 8.6), and differ significantly from those in the calc-alkaline D-T and G-A rocks which are Mg-rich chlorites (Fe/Mg = 0.6–0.8). The X-ray diffraction data for the peralkaline granite samples show superlattice reflections at approximately 31 Å (air-dried) and 34 Å (ethylene glycollated), thus suggesting the presence of an expandable (smectite-like) component in this interlayered (chlorite-smectite) phyllosilicate phase. Chlorites in the peraluminous TR rocks contain Fe/Mg values intermediate between the other two types (Fe/Mg = 1.3). Tetrahedral Al (AlZ) values are remarkably low (0–0.5) in phyllosilicates in the PGR, but vary from 1.9–2.5 in chlorites from the other suites. Yet, these chlorite groups with their generally low AlZ values are distinct from the more stable (type IIb) metamorphic chlorites. Sedimentary chlorites are somewhat similar, in their low AlZ values and metastable structural type, to chlorites in igneous rocks.In the calc-alkaline rocks, chlorite may have been formed at the expense of both biotite [biotite + 3M + 3H2O = chlorite + A], and calcic amphibole [2 Ca-amphibole + 6H2O + 5O2 + 1.8Al = 1 chlorite + 8SiO2 + A], where M = Fe, Mg, Al, and A = K, Na, Ca. The alteration of alkali amphibole in the peralkaline rocks may have produced interlayered chlorite-smectite via this reaction; [1 Na-amphibole + 7H2O + 2.5O2 + M = 1 chlorite-smectite + A]. The presence of such interlayered chlorite-smectite which typically form at low T (150–200°C) suggests that the region was not affected by any major reheating events, which is consistent with the nature of the feldspars.

Partial melting of semipelite and the development of marginal breccias around a late Caledonian minor intrusion in the Grampian Highlands of Scotland

1983 ◽  
Vol 120 (1) ◽  
pp. 37-49 ◽  
Author(s):  
I. M. Platten
Keyword(s):  
Partial Melting ◽  
Contact Zone ◽  
Igneous Rock ◽  
Small Volume ◽  
Country Rock ◽  
Igneous Rocks ◽  
Minor Intrusion ◽  
Water Saturated ◽  
Basic Rocks ◽  
Quartz Grains

SummaryA water saturated melt was developed in the semipelitic Leven Schist in contact with the kentallenitic basic rocks of the Barnamuc intrusion. The partly melted rock consisted of a small volume of granitic melt with relict refractory quartz grains and layers of aluminosilicates. This material could be mobilized and mixed with phenocryst-bearing magma. Sheets of country rock breccia are associated with the mobilization and veins of breccia and mobilized hornfels invade the intrusion. Sheets of igneous rock occur along the contact zone and their emplacement is related to the mobilization of the hornfels. Loss of cohesion between solid country rock and crystallized outer parts of the plug leads to collapse and removal of early formed igneous rocks, accounting for the lack of chilled margins in some intrusions.

Reconnaissance paleomagnetic study of igneous rocks from the eastern sector of the Labrador Trough

1985 ◽  
Vol 22 (11) ◽  
pp. 1561-1570
Author(s):  
Maurice K.-Seguin ◽  
Thomas Clark
Keyword(s):  
North America ◽  
Igneous Rocks ◽  
Polar Wander ◽  
Thermal Demagnetization ◽  
Eastern Sector ◽  
Paleomagnetic Study ◽  
Component C ◽  
The Mean ◽  
Three Components

A paleomagnetic study has been carried out on 75 samples of metagabbro and metaperidotite from 14 sites in the Labrador Trough. The sites were distributed over a distance of 500 km and are situated in sills intruded into formation near the top of the trough stratigraphic column. Four components (A, B, C, and D) were obtained after alternating field (AF) and thermal demagnetization. The mean directions of magnetization are A. 143°, +33°. α95 = 14.4°. k = 15.4, 8 sites: B, 69°, +51°, α95 = 10.3d, k = 22.9, 10 sites: C. 278°. +15°, α95 = 7.7°, k = 28, 14 sites; and D, 200°, +16°. α95 = 13.3°, k = 16.2, 9 sites. All components are secondary. The fold test is negative, and consequently no primary magnetization has been preserved in the isolated components. Component C may have erased all others in zones of highest metamorphism. The paleopoles obtained for components D, A, and C are, respectively, 270°E. 23°S (δP = 10°. δm = 18°); 327°E, 9°S (δp = 11°. δm = 20°); and I9°E, 11°S (antipole) (δp = 10°. δm = 17°). The paleopole positions obtained for these three components are compared with me existing late Aphebian and early Paleohelikian apparent polar wander (APW) path for North America and fall in the 1750–1850 Ma interval.

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