Fred Barker

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 geochemical nature of the Archean Ancient Gneiss Complex and Granodiorite Suite, Swaziland: a preliminary study

Abstract The Ancient Gneiss Complex (AGC) of Swaziland, an Archean gray gneiss complex, lies southeast and south of the Barberton greenstone belt and includes the most structurally complex and highly metamorphosed portions of the eastern Kaapvaal craton. The AGC is not precisely dated but apparently is older than 3.4 Ga. The AGC consists of three major units: (a) a bimodal suite of closely interlayered siliceous, low-K gneisses and metabasalt; (b) homogeneous tonalite gneiss; and (c) interlayered siliceous microcline gneiss, metabasalt, and minor metasedimentary rocks — termed the metamorphite suite. A geologically younger gabbro-diorite-tonalite-trondhjemite suite, the Granodiorite Suite, is spatially associated with the AGC and intrusive into it. The bimodal suite consists largely of two types of low-K siliceous gneiss: one has SiO2 14%, low Rb/Sr ratios, and depleted heavy rare earth elements (REEs); the other has SiO2 > 75%, Al2O3 The siliceous gneisses of the metamorphic suite show low Al2O, K2O/Na2O ratios of about 1, high Rb/Sr ratios, moderate REE abundances and negative Eu anomalies. K/Rb ratios of siliceous gneisses of the bimodal suite are very low (∼130); of the tonalitic gneiss, low (∼225); of the siliceous gneiss of the metamorphite suite, moderate (∼300); and of the Granodiorite Suite, high (∼400). Rocks of the AGC differ geochemically in several ways from the siliceous volcanic and hypabyssal rocks of the Upper Onverwacht Group and from the diapirs of tonalite and trondhjemite that intrude the Swaziland Group.

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.

The 50 Ma granodiorite of the eastern Gulf of Alaska: Melting in an accretionary prism in the forearc

The generation of granitic rocks by melting of flyschoid sediments in an accretionary prism is addressed in this study of 50 Ma silicic igneous rocks in the Gulf of Alaska, near Cordova, Alaska. Plutons of relatively homogeneous biotite and biotite-hornblende granodiorite and minor tonalite and granite are scattered through the Paleocene and Eocene Orca Group. Two masses of cointrusive gabbro and minor dacite dikes also were intruded here. The Orca Group consists of flysch, (quartzofeldspathic graywacke and argillite of turbidite or deep-sea fan origin) and of minor basaltic rocks and pelagic sedimentary rocks. The Orca represents the youngest and structurally lowest part of a late Cretaceous to Eocene composite accretionary prism that extends for 2100 km along the Gulf of Alaska. The plutons are 5–150 km2 in plan and represent less than perhaps 5% of the total volume of this part of the prism. These granitic rocks are unusual in that they were emplaced in a forearc environment during the last stages of deformation of the prism. The three intrusive bodies chosen for study (the McKinley Peak, Rude River, and Sheep Bay plutons) show a range of chemical and initial isotopic compositions (SiO2 = 66.3–71.3%, Na2O = 2.8–3.6%, K2O = 1.8–3.0%, eNd = +2.1 to −3.3, 87Sr/86Sr = 0.7051–0.7067, 206Pb/204Pb = 19.04–19.20, 207Pb/204Pb = 15.60–15.66, and 208Pb/204Pb = 38.59–38.85). Relative to the other two plutons, the McKinley Peak pluton generally shows slightly lower K2O, higher Al2O3, higher eNd, and lower 87Sr/86Sr ratios. All three plutons, however, have similar, well-defined minor and trace element abundances characterized by relative enrichment in light rare earth elements and depletion in high field strength elements. The granodiorites and flysch of the Orca Group show overlapping elemental and isotopic compositions. The only clearly defined chemical differences between the flysch and the granodiorites are weak negative Eu anomalies in the granodiorites and slightly lower Ca and higher Na contents in the flysch. The Nd and Sr isotopic compositions of the Rude River and Sheep Bay plutons completely overlap those of the flysch. The McKinley Peak pluton, however, shows discretely higher eNd slightly lower 87Sr/86Sr values than those of the flysch. The Pb isotopic compositions of the flysch and the Rude River pluton also overlap, but Pb of the other two plutons is slightly less radiogenic. Our chemical data, modeling, and comparison with Conrad et al.s (1988) melting experiments of graywacke indicate that the granodiorite orginated by large fractions (65–90%) of melting of the Orca Group graywacke and argillite. Plagioclase, pyroxene(s), and biotite(?) were residual to melting at about 850°–950°C and at low H2O activities. The distinct chemical and isotopic compositions of the McKinley Peak pluton probably result from variations in the character of the flysch at depth in the prism, rather than from mixing between melts of the flysch and mafic magmas injected into the prism itself. However, basaltic magmas were injected into the accretionary pile, as evidenced by the coeval gabbroic plutons, and these apparently provided the heat necessary for crustal melting. The mafic magmas probably originated from the subjacent oceanic Kula plate. We suggest that the subducting Kula “plate” consisted of several small plates, much as the modern Juan de Fuca and nearby smaller plates lie at the margin of the Pacific plate. Basaltic magmas produced along the boundaries of such small plates were injected for more than 12 m.y., first into the Orca Group deep-sea fans and later into the accretionary prism. Accretionary prisms have been an important, but little discussed, source of granitic magmas since Archean time. Their emplacement as complete sections of lower to upper crust means that any basaltic magma coming up from the mantle will impinge upon and tend to melt them. Furthermore, many prisms, especially those bearing high proportions of quartzofeldspathic graywacke, are fertile in granitic melts. These Alaskan granodiorites do not fit into the alphabetical classification of Australian workers. Being melts of sedimentary rocks, they should have S-type character. Because the source flysch is quartzofeldspathic and of arc origin, however, the granodiorite shows I-type character. Our results also highlight a problem with Pearce et al.s (1984) and Harris et al.s (1986) purportedly tectonic-discriminant plots for granitic rocks. These diagrams classify our granodiorites as “volcanic arc granite” and reflect their source rocks rather than their tectonic environment of origin.

Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Abstract Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwoods experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.

Geochemical investigation of Archaean Bimodal and Dwalile metamorphic suites, Ancient Gneiss Complex, Swaziland

The bimodal suite (BMS) comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites. Based on geochemical parameters three main groups of siliceous gneiss are recognized: (i) SiO2 14%, and fractionated light rare-earth element (REE) and flat heavy REE patterns; (ii) SiO2 and Al2O3 contents similar to (i) but with strongly fractionated REE patterns with steep heavy REE slopes; (iii) SiO2 > 73%, Al2O3 < 14%, Zr ∼ 500 ppm and high contents of total REE having fractionated light REE and flat heavy REE patterns with large negative Eu anomalies. The interlayered amphibolites have major element abundances similar to those of basaltic komatiites, Mg-tholeiites and Fe-rich tholeiites. The former have gently sloping REE patterns, whereas the Mg-tholeiites have non-uniform REE patterns ranging from flat (∼ 10 times chondrite) to strongly light REE-enriched. The Fe-rich amphibolites have flat REE patterns at 20–30 times chondrite. The Dwalile metamorphic suite, which is preserved in the keels of synforms within the BMS, includes peridotitic komatiites that have depleted light REE patterns similar to those of compositionally similar volcanics in the Onverwacht Group, Barberton, basaltic komatiites and tholeiites. The basaltic komatiites have REE patterns parallel to those of the BMS basaltic komatiites but with lower total REE contents. The Dwalile tholeiites have flat REE patterns. The basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source. The Fe-rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation.

Carbon Isotopes in Pelites of the Precambrian Uncompahgre Formation, Needle Mountains, Colorado

Carbon isotopic ratios and weight percentages of carbon were measured in 15 samples of slate, phyllite, and schist of the approximately 1500- to 1600-m.y.-old Uncompahgre Formation of the Needle Mountains, southwestern Colorado. Rocks with less than 1 percent total carbon, all of which is reduced, have δC 13 values of −23 to −28 per mil, whereas those with 1 to 6.4 percent carbon have δC 13 from −29 to −31 per mil. In general, the slates and phyllites contain more carbon and isotopically lighter carbon than do the schists of higher metamorphic rank. Increasing loss of C 12 -enriched methane with increasing intensity of metamorphism is suggested to account for these differences.

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.

Geochemical Characteristics and Origin of the Lebowa Granite Suite, Bushveld Complex

The ∼ 2052-Ma Lebowa Granite Suite (LGS) represents the culminating phase of an Early Proterozoic magmatic cycle in the Central Transvaal area of the Kaapvaal Province. Following extrusion of at least 200,000 km3 of intermediate to acid volcanics (Rooiberg Felsite), mafic and ultramafic magmas intruded at 2065 Ma to form the Rustenburg Layered Suite (RLS). The LGS includes the Nebo, Makhutso, Bobbejaankop, Lease, and Klipkloof granites. The Nebo Granite intruded the Rooiberg Felsite as sheets up to 4 km thick above the RLS. Smaller stocks of the other granites crosscut the Nebo. We determined major-and trace-element compositions and oxygen, Rb-Sr, and Sm-Nd isotope ratios for samples of: Nebo Granite; Rooiberg Felsite; granophyre and granophyric granite; Makhutso, Bobbejaankop, and Lease granites; and feldspar porphyry from areas throughout the exposed area of the LGS (Dennilton, Verena-Balmoral, Enkeldoorn, Sekhukhune Plateau, Zaaiplaats-Potgeitersrus, and Western Transvaal). Coherent floor-to-roof geoch...