Analyses Of Igneous Rocks Research Articles

Deep formation of Earth’s earliest continental crust consistent with subduction

About four billion years ago, Earth’s outer layer is thought to have been composed mostly of a 25- to 50-km-thick basaltic crust that differentiated to form the oldest stable continental crust. However, the tectonic processes responsible for the formation of this continental material remain controversial. Suggested explanations include convergent plate boundary processes akin to subduction operating today and a variety of relatively shallow (<50 km) non-plate-tectonic intracrustal mechanisms. Here we perform high-pressure–temperature melting experiments on an oceanic plateau analogue for the early basaltic crust and show that magmas with the composition of the early continental crust cannot form at pressures <1.4 GPa (~50 km depth). This suggests that Eoarchaean continental magmas are formed in deep (>50 km) subduction-like environments. Our results support previous Eoarchaean field evidence and analyses of igneous rocks that date to 4.0–3.6 billion years ago, which are consistent with subduction-like processes and suggest a primitive type of plate tectonics operated as long as 4 billion years ago on early Earth.

Preserved intercratonic lithosphere reveals Proterozoic assembly of Australia

Abstract The Proterozoic assembly of Australia, the understanding of which is critical for reconstructing Proterozoic supercontinents, involved amalgamation of the West Australian (WAC), North Australian (NAC), and South Australian cratons (SAC). However, the basement between these Archean to early Proterozoic lithospheric blocks is mostly buried beneath younger basins; hence, its composition and age and the timing of Proterozoic assembly remain uncertain. In situ zircon U-Pb-O-Hf analyses of igneous rocks from drillholes that intersected basement beneath the northwestern Canning Basin reveal the presence of a substantial domain of juvenile Proterozoic lithosphere, the Percival Lakes province, between the WAC and NAC. Although isotopically distinct from the neighboring WAC and NAC, the Percival Lakes province is strikingly similar to other juvenile Proterozoic tectonic elements between the WAC, NAC, and SAC. Combining isotope and seismic data, we interpret the Percival Lakes province as part of an ~1700 × 400 km Proterozoic lithospheric domain that lacks evidence of Archean provenance but consists mainly of reworked remnants of Mesoproterozoic oceanic crust that survived WAC-NAC-SAC convergence. The apparent absence of Archean lithosphere between the cratons implies they never directly collided or that complete collision was prevented by impingement of three-dimensional promontories in the converging lithospheric blocks. Instead, the Percival Lakes province and other Proterozoic elements between the WAC, NAC, and SAC consist of oceanic lithosphere extracted from Earth's mantle in the Proterozoic. Our results imply that WAC-NAC convergence was younger than Columbia amalgamation at ca. 1.8 Ga and that Proterozoic Australia formed during the earliest phases of Rodinia assembly at ca. 1.3 Ga.

ДЕФЕКТЫ МАГМОФОРМАЦИОННОГО АНАЛИЗА

The formation analysis of igneous rocks is a great achievement of the Soviet science in terms of classifying natural communities of magmatic rocks for the purpose of forecasting and prospecting of associated deposits. But due to the complexity of the problem itself, the uncertainty of boundaries between formational types in the practice of geological works, there are numerous defects that were clearly observed during the regional and interregional correlation of magmatic complexes of the Altai-Sayan region and Yenisey Ridge. The reasons and consequences of these defects, ways of their correction are analyzed.

Crystallization of oxidized, moderately hydrous arc basalt at mid- to lower-crustal pressures: implications for andesite genesis

This study focuses on the production of convergent margin calc-alkaline andesites by crystallization–differentiation of basaltic magmas in the lower to middle crust. Previous experimental studies show that dry, reduced, subalkaline basalts differentiate to tholeiitic (high Fe/Mg) daughter liquids, but the influences of H2O and oxidation on differentiation are less well established. Accordingly, we performed crystallization experiments at controlled oxidized fO2 (Re–ReO2 ≈ ΔNi–NiO + 2) on a relatively magnesian basalt (8.7 wt% MgO) typical of mafic magmas erupted in the Cascades near Mount Rainier, Washington. The basalt was synthesized with 2 wt% H2O and run at 900, 700, and 400 MPa and 1,200 to 950 °C. A broadly clinopyroxenitic crystallization interval dominates near the liquidus at 900 and 700 MPa, consisting of augite + olivine + orthopyroxene + Cr-spinel (in decreasing abundance). With decreasing temperature, plagioclase crystallizes, Fe–Ti-oxide replaces spinel, olivine dissolves, and finally amphibole appears, producing gabbroic and then amphibole gabbroic crystallization stages. Enhanced plagioclase stability at lower pressure narrows the clinopyroxenitic interval and brings the gabbroic interval toward the liquidus. Liquids at 900 MPa track along Miyashiro’s (Am J Sci 274(4):321–355, 1974) tholeiitic versus calc-alkaline boundary, whereas those at 700 and 400 MPa become calc-alkaline at silica contents ≥56 wt%. This difference is chiefly due to higher temperature appearance of magnetite (versus spinel) at lower pressures. Although the evolved liquids are similar in many respects to common calc-alkaline andesites, the 900 and 700 MPa liquids differ in having low CaO concentrations due to early and abundant crystallization of augite, with the result that those liquids become peraluminous (ASI: molar Al/(Na + K + 2Ca) > 1) at ≥61 wt% SiO2, similar to liquids reported in other studies of the high-pressure crystallization of hydrous basalts (Muntener and Ulmer in Geophys Res Lett 33(21):L21308, 2006). The lower-pressure liquids (400 MPa) have this same trait, but to a lesser extent due to more abundant near-liquidus plagioclase crystallization. A compilation of >6,500 analyses of igneous rocks from the Cascades and the Sierra Nevada batholith, representative of convergent margin (arc) magmas, shows that ASI increases continuously and linearly with SiO2 from basalts to rhyolites or granites and that arc magmas are not commonly peraluminous until SiO2 exceeds 69 wt%. These relations are consistent with plagioclase accompanying mafic silicates over nearly all the range of crystallization (or remelting). The scarcity of natural peraluminous andesites shows that progressive crystallization–differentiation of primitive basalts in the deep crust, producing early clinopyroxenitic cumulates and evolved liquids, does not dominate the creation of intermediate arc magmas or of the continental crust. Instead, mid- to upper-crustal differentiation and/or open-system processes are critical to the production of intermediate arc magmas. Primary among the open-system processes may be extraction of highly evolved (granitic, rhyolitic) liquids at advanced degrees of basalt solidification (or incipient partial melting of predecessor gabbroic intrusions) and mixing of such liquids into replenishing basalts. Furthermore, if the andesitic-composition continents derived from basaltic sources, the arc ASI–SiO2 relation shows that the mafic component returned to the mantle was gabbroic in composition, not pyroxenitic.

Origin of Geochemical Variability by Arc-Continent Collision in the Biru Area, Southern Sulawesi (Indonesia)

Analyses of igneous rocks from the Eocene calc-alkaline and Miocene geochemistry can be modelled by varying contributions potassic volcanic arc in southwest Sulawesi indicate that magmas from the mantle wedge, fluid from the subducted slab, became more heterogeneous in their trace element and Pb–Sr–Nd and the presence or absence of a subducted sedimentary isotopic signature following the collision of the Buton microcontinent component (White & Patchett, 1984; Gill & Williams, with the arc at >15 Ma. Isotopic ratios become more ‘continental’ 1990; Woodhead & Johnson, 1993; Kepezhinskas et al., 4 my after the collisional event ( Sr/Sr Ζ0·7085, Nd/ 1997; Peate et al., 1997; Alves et al., 1999; Hoogewerff, Nd [0·5125, Pb/Pb Ζ 19·2, Pb/Pb Ζ 15·73, 1999; Pearce et al., 1999; Eiler et al., 2000). Despite Pb/Pb Ζ39·4). As the overriding plate consists of young agreement on the importance of these components, there Sundaland crust, whereas the subducted sediment is likely to is still some dispute about the importance of an ocean have been shed from a compositionally distinct microcontinent of island basalt (OIB) component in the wedge (Stolz et al., Australian derivation, we can be certain that the continental isotopic 1990; Edwards et al., 1991) and the implied role of signature reflects subduction of continental material rather than residual rutile (Ryerson & Watson, 1987; Foley et al., crustal contamination. The isotopic compositions of the magmas 2000). If an arc is built upon continental crust, or can be explained by the melting of a mixed mantle wedge, consisting overlain by a thick layer of sediment, there is a potential of fluid-fluxed and sediment-modified MORB mantle. In this model, contribution from the upper plate via assimilation– the maximum amount of sediment added to the mantle source is fractional crystallization (AFC) processes, which can 10%. sometimes be difficult to distinguish from the signature of subducted sediment (Davidson, 1987; Thirlwall et al., 1996; Gasparon & Varne, 1998). The only unambiguous way to distinguish between subducted sediment and

Resumen cronoestratigráfico de las rocas ígneas de Costa Rica basado en dataciones radiométricas

Based on a recompilation of about 160 K-Ar and U-Th analyses of igneous rocks and their stratigraphic relationships, a synthesis of the magmatic evolution of Costa Rica is presented. The igneous rocks of Jurassic to Oligocene age belong principally to the tholeiitic series. Widespread calc-alkaline volcanism started in the late Miocene and culminated during the Pleistocene in the northern and central part of Costa Rica with the eruption of large volumes of andesitic to rhyolitic ignimbrites and the development of the stratovolcanoes of the Cordillera de Guanacaste and Cordillera Central; some of these volcanoes are still active. Alkaline rocks are subordinate and concentrated on the Caribbean side of Costa Rica. Plutonic intrusions, mainly of late Miocene age, are frequent in the southern part of Costa Rica. The histogram of the available data indicates some periods of more intensive volcanic activity.

Occurrence of negative open variances in ternary systems

Parameters of the hypothetical open arrays were computed for a total of 20 published sets of chemical analyses of igneous rocks in oxide, mole, cation, and cation weight-percentage forms. Negative open variances in the full (11 variables) data sets were eliminated by using a linear transformation of the raw data. For the 13 variables (11 oxides, the sum of the alkalies and total iron as FeO) 286 different simple ternary combinations can be formed. The CaO-Na2O-K2O (CNK) and total iron-serum of the alkalies-MgO (FMA) combinations are of frequent use in depicting “differentiation trends” within a series of igneous rocks. In each of the 20 data sets the estimated open variances of Na2O in the CNK and total iron in the FMA combinations are negative. Elimination of negative open variances in the full data array does not eliminate negative open variances in these ternary combinations. Greater than 50 percent of the 29,172 ternary combinations examined contain a negative open variance.

Published Analyses of Igneous Rocks

THE most comprehensive published collection of analyses of igneous rocks1 contains more than 7,600 entries considered “superior” by the author. One of us (R. W. Le M.) has a comparable file of 16,050 published analyses, and F. C. has a file of 10,494 published analyses of Cainozoic volcanic rocks alone. Comparison of our files, referred to below as the C and L files, provides some basis for estimating how much information of this type has now been published.

Om spektrokemisk analys av sediment och bergarter

Abstract A brief survey is given of the laboratory studies on analytical technique by means of combined chemical and spectrographic methods for the determination of major and minor constituents in sediments and igneous rocks. For details the reader is referred to a paper: On the spectrochemical analysis of igneous rocks, sediments, and ores by S. Landergren and W. Muld. Microchimica Acta 1955 (in press).

Chemical Analyses of Igneous and Metamorphic Rocks of Denmark, the Faeroes and Greenland. (To and including 1940)

The past ten years or so have seen the publication of collections of chemical rock analyses which, as a result of their clear form of set-up, in many ways faciliate the comparative study of the chemistry of the rocks and their mutual relationships. Among these publications there are: P. Niggli, F. De Quervain &amp; R. U. Wintherthalter: Chemismus schweizerischer Gesteine. Bern 1930, and the analyses publish ed by the Geological Survey of Great Brita in: Chemical Analyses of Igneous Rocks, Metamorphic Rocks and Minerals. London 1931. Similar publications are available from two neighbouring countries, i.e. from Sweden: W. Larsson: Chemical Analyses of Swedish Rocks (Bull. Geol. Inst., Uppsala 1932) and from Finland : L. Lokka: Neuere Chemische Analysen von Finnisch en Gesteinen (Bull. Comm. Geol. de Finlande No. 105. Helsingfors 1934).

The frequency-distribution of igneous rocks. I. Frequency-distribution of the major oxides in analyses of igneous rocks

The quantitative distribution of igneous rocks has recently assumed a place of importance in petrogenic discussions. This is due largely to the influence of Daly, who made a survey of the areal distribution of rock types as shown by their outcrops on published maps, and reached the well-known conclusion that intrusions are overwhelmingly granitic in composition, and that extrusions arc largely basaltic. If further research should firmly establish this result, it will unquestionably be a fact of the first order of importance of which any general theory of petrogenesis must take cognizance.

Chemical Analyses of Igneous Rocks . Published from 1884 to 1913 inclusive. With a critical discussion of the character and use of analyses. By Henry Stephens Washington. U. S. Geological Survey, Professional Paper 99, Washington, 1917.

<i>Chemical Analyses of Igneous Rocks</i> . Published from 1884 to 1913 inclusive. With a critical discussion of the character and use of analyses. By Henry Stephens Washington. U. S. Geological Survey, Professional Paper 99, Washington, 1917.

Chemical Analyses of Igneous Rocks Published from 1884 to 1900, with a critical discussion of the character and use of analyses . By Henry Stephens Washington. United States Geological Survey, Professional Paper No. 14. Washington, Government Printing Office. 1903. Pp. 495.

<i>Chemical Analyses of Igneous Rocks Published from 1884 to 1900, with a critical discussion of the character and use of analyses</i> . By Henry Stephens Washington. United States Geological Survey, Professional Paper No. 14. Washington, Government Printing Office. 1903. Pp. 495.

X.—Diagram of Composition of Igneous Rocks

Recent studies about the average chemical composition of larger numbers of igneous rocks in the aggregate have shown that figures obtained from any one hundred or more samples are very similar, in fact practically equal. (See A. Harker, “On the Average Composition of British Igneous Rocks”: Geol. Mag., No. V, May, 1899.) This will be found also the case when comparing the following average which I calculated from the analysis of igneous rocks compiled by Roth in his “Petrographie der plutonischen Gesteine.”