Joseph G. Arth

Generation of trondhjemitic-tonalitic liquids and Archean bimodal trondhjemite-basalt suites

Trondhjemitic and tonalitic liquids may form either by igneous differentiation of less silicic, more mafic liquids or by partial melting of rocks of basaltic composition. Low-Al 2 O 3 trondhjemitic-tonalitic liquids (defined as containing less than 15 percent Al 2 O 3 ) have formed in modern plate-tectonic environments by crystal fractionation of low-potassium ande-sitic liquid and in Precambrian environments by the partial melting of amphibolite and hornblende-bearing gabbro, in which process plagioclase is a residual phase and garnet and (or) hornblende are not. High-Al 2 O 3 trondhjemitic-tonalitic liquids (containing 15 percent or more of Al 2 O 3 ) are generated in both old and modern convergent and tensional tectonic environments, either by hornblende-controlled fractionation of hydrous basaltic liquid or by partial melting of metabasaltic rocks, in which process garnet and (or) hornblende are residual. A model for the origin of the andesite-free bimodal trondhjemite-basalt suites that are found in lower Archean gray gneiss complexes is based on a 1968 model of Green and Ringwood; it proposes (1) mantle upwelling and basaltic vol-canism to form a thick pile, (2) metamorphism of the lower parts of this pile to amphibolite, (3) partial melting of the amphibolite to yield trondhjemitic-tonalitic liquids, (4) ascent and extrusion or intrusion of these liquids into the upper crust before the fraction of melting of the parental amphibolite exceeds about 40 percent, (5) transformation of the residue of partial melting to anhydrous, refractory assemblages, and (6) continuation of mantle upwelling and basaltic volcanism as trondhjemitic-tonalitic liquids are being extruded.

Rare-earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondhjemitic-tonalitic magmas

New hornblende/groundmass partition coefficients for rare earths in a sodic, high-Al 2 O 3 dacite range from 0.9 for Ce to 4.5 to 6.2 for heavy rare earths, and the pattern is concave down with a distinct negative Eu anomaly. These coefficients indicate that hornblende may have been an important residual phase in the generation of high-Al 2 O 3 , heavy-rare-earth-depleted trondhjemites, tonalites, and equivalent dacites, implying that the magmas formed at a depth of less than 60 k.

The 1.7- to 1.8-b.y.-old trondhjemites of southwestern Colorado and northern New Mexico: Geochemistry and depths of genesis

Four trondhjemitic bodies — three of intrusive and one of extrusive origin — 1.7 to 1.8 b.y. in age occur in the Precambrian rocks of northern New Mexico and southwestern Colorado. These are the metamorphosed plutonic or hypabyssal trondhjemite of Rio Brazos, New Mexico, the interlayered quartzofeldspathic and metabasaltic metavolcanic Twilight Gneiss of the West Needle Mountains, Colorado, the syntectonic Pitts Meadow Granodiorite of the Black Canyon of the Gunnison River, Colorado, and the late syntectonic to posttectonic Kroenke Granodiorite of the Central Sawatch Range, Colorado. From south to north, over a distance of 235 km, the four rock units show systematic increases in average Al2O3 from 13.7 to 16.1 percent, in K2O from 1.5 to 2.6 percent, in Rb from 28 to 76 ppm, and in Sr from 101 to 547 ppm. Initial Sr87/Sr86 ratios are low — 0.7015 to 0.7027 — and suggest a mafic or ultramafic source. All four trondhjemite bodies have similar light rare-earth element (REE) contents. The trondhjemites of Rio Brazos and the Twilight Gneiss have relatively flat patterns (Ce/Yb 10) with low heavy rare earth content and small or no Eu anomalies. Whole-rock δO18 values for siliceous rocks of three of the bodies range from 5.8 to 8.0 per mil, although the Pitts Meadow Granodiorite gives values of 8.5 to 9.4 per mil. The parent magmas for these bodies were probably generated from a parental basaltic source, either by partial melting or fractional crystallization. Fractional crystallization mechanisms would operate at crustal levels where crystallization of plagioclase and clinopyroxene or hornblende would produce the Rio Brazos and Twilight magmas, and crystallization of hornblende, plagioclase, and biotite would produce the Kroenke and Pitts Meadows magmas. The preferred partial melting mechanism would produce the Rio Brazos and Twilight magmas at shallow depth (< 50 km), leaving a residue of plagioclase and clinopyroxene or amphibole; the Pitts Meadow magma at 50 to 60 km, where hornblende, garnet, clinopyroxene, and plagioclase would be residual; and the Kroenke magma at greater than 60 km leaving a residue of garnet and clinopyroxene. The magmas probably formed in a ridge-and-basin complex that lay between the early Precambrian craton to the north and the contemporaneous quartzite-rhyolite-tholeiite terrane to the south. A northward-dipping subduction zone can be postulated from the variation in compositions and inferred depths of melting, but complete modern analogues of similar setting are not known. A better tectonic analogue might be the Archean regimes, in which vertical motion is dominant and trondhjemitic magmas may have formed by melting at the base of foundering thick volcanic piles.

Genesis of Archean komatiites from Munro Township, Ontario: Trace-element evidence

Trace elements, particularly rare earths, determined in Archean komatiitic and tholeiitic rocks of Munro Township, Ontario, appear mainly to reflect the processes that produced the lavas, rather than the metamorphic processes. Pyroxenitic to basaltic komatiites may have originated by fractional crystallization of peridotitic komatiite liquid or by partial melting of plagioclase peridotite in the mantle. Peridotitic komatiite magmas may be partial melts of a residual mantle assemblage that was depleted in largeion lithophile (LIL) elements, possibly through earlier extraction of liquids. Picritic tholeiites may have originated from melting of garnet peridotite. A multistage melting model may relate both tholeiites and komatiites to a common source, perhaps in a rising mantle diapir. In this model tholeiites would be the first-formed liquids extracted whereas residual mantle might be further melted to produce komatiitic liquids.

Tin granites of Seward Peninsula, Alaska

Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO 2 , 72.5% to 76.6%; A1 2 O 3 , 12.7% to 14.3%; Na 2 O, 2.9% to 4.0%; K 2 O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe 2 O 3 + MgO ranges from 0.3% to 2.4%; and the K 2 O to Na 2 O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87 Sr/ 86 Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago. The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase.

Tonalites in Crustal Evolution

Tonalites, including trondhjemite as a variety, played three roles through geological time in the generation of Earth’s crust. Before about 2.9 Ga ago they were produced largely by simple partial melting of metabasalt to give the dominant part of Archaean grey gneiss terranes. These terranes are notably bimodal; andesitic rocks are rare. Tonalites played a crucial role in the generation of this protocontinental and oldest crust 3.7- 2.9 Ga ago in that they were the only low-density, high-SiO2 rocks produced directly from basaltic crust. In the enormous event giving the greenstone-granite terranes, mostly 2.8-2.6 Ga ago, tonalites formed in lesser but still important proportions by partial melting of metabasalt in the lower regions of down-buckled greenstone belts and by remobilization of older grey gneisses. Tectonism in the Archaean (3.9- 2.5 Ga ago) perhaps was controlled by small-cell convection (McKenzie & Weiss 1975). Little or no ophiolite or eclogite formed, and only minor andesite. Plate tectonics of modern type (involving large, rigid plates) commenced in the early Proterozoic. Uniformitarianism thus goes back one-half of the age of the earth. Tonalites compose about 5-10 % of crust generated in Proterozoic and Phanerozoic time at convergent oceanic-continental margins. They occur here as minor to prominent members of the compositionally continuous continental-margin batholiths. A simple model of generation of these batholiths is offered: mantle-derived mafic magma pools in the lower crust above a subduction zone reacts with and incorporates wall-rock components (Bowen 1922), and breaches its roof rocks as an initial diapir. This mantle magma also develops a gradient of partial melting in its wall rocks. This wall-rock melt accretes in the collapsed chamber and moves up the conduit broached by the initial diapir, the higher, less siliceous fractions of melting first, the lower, more siliceous (and further removed) fractions of melting last. The process gives in the optimum case a mafic-to-siliceous sequence of diorite or quartz diorite through tonalite or quartz monzodiorite to granodiorite and granite. The model implies that great masses of cumulate phases and refractory wall rock form the roots of continentalmargin batholiths, and that migmatites overlie that residuum and underlie the batholiths.

The Northeast Kingdom batholith, Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO<2 (48 to 77 wt.%). The batholith has calc-alkaline features, for example a Peacock index of 57, and values for K<2O/Na2O (<1), K/Rb (60–350), Zr/Hf (30–50), Nb/Ta (2–22), Hf/Ta (up to 10), and Rb/Zr (<2) in the range of plutonic rocks found in continental magmatic ares. Wide diversity and high values of minor- and trace-element ratios, including Th/Ta (0.5–22), Th/Yb (0–27), Ba/La (0–80), etc., are attributed to intracrustal contributions. Chondrite-normalized REE patterns of metaluminous and relatively mafic intrusives have slightly negative slopes (La/Ybcn<10) and negative Eu anomalies are small orabsent. The metaluminous to peraluminous inter-mediate plutons are relatively enriched in the light REE (La/Ybcn>40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80–164), Th/Ta (<9), Rb/Cs (7–40), K/Cs (0.1–0.5), Ce/Pb (0.5–4), high values of Rb/Sr (1–18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals.

Geochronology of archean gneisses in the Lake Helen area, Southwestern Big Horn Mountains, Wyoming

Abstract The RbSr and UPb methods were used to study gneisses in the 7 1 2 - minute Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite. A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial 87Sr/86Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source. A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The 87Sr/86Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.

A minimum age for high-grade metamorphism and granite intrusion in the Piedmont of the Potomac River gorge near Washington, D.C.

Near Great Falls on the Potomac River, metamorphosed Piedmont rocks of the Glenarm Group were intruded by small bodies of granite and pegmatite after the peak of high-grade metamorphism in the area. Muscovites from two of the feldspar-quartz-muscovite pegmatites were dated by the Rb-Sr method. They show ages of 469 ± 20 m.y. and 469 ± 12 m.y. The ages may represent the time of intrusion or cooling of the rocks below about 500 °C. The pegmatites crosscut the foliation of the Glenarm metasediments, and thus their age places a younger limit on the time of deposition of the series and of high-grade metamorphism in the area.

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex, Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379±5 m.y. and 381±4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende 40Ar/39Ar determinations from Passadumkeag River which indicate an age of 378±4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496±14 m.y., with an initial Sr isotopic ratio of 0.70262.Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399±8 m.y. for Whitney Cove, 388±6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396±32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the 207Pb/206Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381±3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data.Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement.