Showing papers on "Partial melting published in 1976"

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

Abstract: 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.

Experimental generation of cordierite-or garnet-bearing granitic liquids from a pelitic composition

Abstract: High-pressure experimental melting studies on a pelitic composition with 2 and 5 percent by weight of added water demonstrate that cordierite, quartz, biotite, sillimanite, and plagioclase are important residual phases coexisting with granitic liquid at 4 kb and are joined by garnet at 7 kb. At 10 kb, garnet quartz, biotite, sillimanite, and plagioclase constitute the residuum coexisting with granitic liquid. Compositions of residual phases and their Mg-Fe partition relationships compare closely with phases from cordierite- and garnet-bearing granitic rocks in eastern Australia and strongly support geochemical arguments that this particular granitic suite is derived by partial melting of pelitic sediments. Thus, the cordierite-bearing granitic rocks were probably derived by partial melting of pelite at >25-km depth, whereas the garnet-bearing granites were derived at ≿25-km depth.

Rare Earth data and petrogenesis of andesite from the North Chilean Andes

Abstract: Abundances of 9 Rare-Earth Elements are reported for 21 andesite and dacite samples from a small group of volcanoes in the North Chilean segment of the Andean plate margin. The data are correlated with major and trace element data (Roobol et al., in press) and are used to critically assess several models proposed to account for the origin of andesite. A three-stage model for andesite petrogenesis is proposed. This involves: (i) initiation of melting at the subduction zone and rise of magmas into the overlying mantle-wedge, which thereby becomes richer in garnet-pyroxene components and large-ion lithophile (LIL) elements, (ii) partial melting of this LIL-enriched garnet pyroxenite material to form andesitic magmas within the mantle wedge, (iii) minor crystal fractionation during rise through the crust.

Does CO2 cause partial melting in the low-velocity layer of the mantle?

Abstract: Recent analyses of fluid inclusions in peridotite minerals suggest that CO 2 is a dominant volatile species in the upper mantle. In a CO 2 -bearing oceanic mantle, the low-velocity zone (LVZ) can be explained by a large decrease in the peridotite solidus temperature at a depth of about 90 km, causing melting by intersection with a geotherm. This decrease in the solidus temperature has been found in the system CaO-MgO-SiO 2 -CO 2 and results from a change in partial melt composition along the solidus from enstatite-normative at pressures less than 26 kb to larnite-normative (melilititic) at greater pressure. Although these liquids dissolve up to 20 wt percent CO 2 , they are silicate liquids containing at least 30 percent SiO 2 . These silica levels are appropriate for kimberlitic liquids, but the liquids are more calcic than typical kimberlites. At depths of less than 90 km in suboceanic mantle, CO 2 may be present in carbonate minerals or in vapor, depending upon the geotherm, but cannot be in solution in silicate peridotite minerals. Beneath continents, CO 2 will be present in carbonate minerals, and the mantle will not melt at least to depths of 120 km.

Rare earth element evidence for differentiation of McMurdo volcanics, Ross Island, Antarctica

Abstract: Eighteen samples of the McMurdo volcanics on Ross Island, Antarctica consisting of basanitoid, trachybasalt and phonolite have been analyzed for rare earth elements (REE) in order to determine the details of differentiation using quantitative trace element modeling. The basanitoids have REE patterns similar to those for alkali basalts or nephelinites from ocean islands. Since there is no correlation between REE and silica contents among five basanitoids, some of the variability in the REE contents must be related to the extent of partial melting, variation in the residual mineralogies of the mantle during melting, or to inhomogeneities in the REE composition of the mantle.

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

Abstract: 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.

Petrogenesis of migmatites in the Huntly-Portsoy area, north-east Scotland

Abstract: SUMMARY. Migmatites are described from the Sillimanite-potash-feldspar Zone of the aureole around the Newer Basic suite of synorogenic intrusions. The lowest-grade migmatites are trondhjemitoid (characterized by the assemblage quartz-plagioclase-biotite) or muscovite-granitoid (quartz-plagioclase potash-feldspar-muscovite-sillimanite-biotite). With increasing grade, a transition occurs to cordierite-granitoid assemblages (quartz-plagioclase-potash-feldspar-cordierite-garnet-sillimanitebiotite), which persist to the highest grades observed, where there are also noritoid migmatites (quartzplagioclase~)rthopyroxene-cordierite-biotite). The trondhjemitoids are texturally simple because the minerals did not undergo dehydration reactions. Textural immaturity and consistently cotectic modal compositions indicate that their leucosomes originated as melts. Scatter of plagioclase compositions suggests that the partial melting occurred in small closed systems. The other migmatites have more fusible compositions, so it is deduced that they also underwent partial melting. Retrograde reaction textures are used to infer the sequence of reactions, involving muscovite and biotite, by which melting proceeded during prograde evolution. Whereas the fugacity of water probably varied among spatially associated trondhjemitoid leucosomes, in the muscovite-granitoids it was constrained to an approximately constant value, at given pressure and temperature, by the buffering effect of the mineral assemblage. IT is widely believed that large-scale occurrences of migmatites in rocks of high metamorphic grade are due to partial melting in situ (e.g. Winkler, 1967; Turner, I968), but there are still only a few areas in which this hypothesis has been tested against evidence from the rocks themselves (Mehnert, I968). The area around Huntly (Aberdeenshire) and Portsoy (Banffshire) is auspicious for this purpose because it contains a variety of migmatites in a small area that have suffered little subsequent change. The area was mapped by Read (I923) and is part of the 'sillimanite-ove rprinted' Dalradian terrain for which Chinner (I966) has already discussed the possibility of melting. The migmatites are confined to, and abundant in, the Sillimanitepotash-feldspar Zone, which represents the highest metamorphic grade of the area (Ashworth, I975). This zone is the inner part of the thermal aureole of the Newer Basic igneous masses. These include gabbros, norites, and allied rocks, which were

Petrogenesis of McKinney (Snake River) olivine tholeiite in light of rare-earth element and Cr/Ni distributions

Abstract: Samples of McKinney Basalt, including a pillow glass, are characterized by light rare-earth element–enriched abundance patterns with small positive Eu anomalies (relative to chondrites) and high Cr/Ni ratios. In particular, the positive Eu anomaly observed in the pillow glass is considered to be characteristic of the McKinney parental magma. These compositional features are apparently inconsistent with derivation of McKinney magma by partial fusion of garnet- or plagioclase-bearing Iherzolite or clinopyroxenite or by crystal fractionation of likely liquidus phases at high or low pressure. Rather, partial melting calculations show that fusion of spinel peridotite or aluminous clinopyroxene peridotite can yield liquids with the rare-earth element patterns and Cr/Ni ratios of McKinney Basalt. An essential feature of the favored models is a relatively large contribution of clinopyroxene to the melt. This result suggests an origin of McKinney magma by fusion of mantle Iherzolite at pressures between the stability fields of plagioclase peridotite (low P ) and garnet peridotite or garnet clinopyroxenite (high P ), that is, at depths within the continental lithosphere in this region.

Trace-element variations at Summer Coon volcano, San Juan Mountains, Colorado, and the origin of continental-interior andesite

Abstract: The Oligocene Summer Coon center, an eroded continental-interior volcano of the eastern San Juan Mountains, Colorado, was the source of magmas ranging in composition from basaltic andesite to rhyolite. Previous Pb and Sr isotope studies indicate derivation of the magmas from an isotopically homogeneous source. This study presents new data for rare-earth elements (REE), U, Th, Ba, Sr, Rb, and Ni from 10 samples of the Summer Coon sequence. Alkali elements are high in all rocks; as SiO 2 increases, Ba increases from 900 to 2,000 ppm, Rb increases from 35 to 90 ppm, Sr decreases from 900 to 350 ppm, K/Rb decreases slightly, Ba/Sr increases, U increases from 0.5 to 2.5 ppm, and Th increases from 2 to 7 ppm. Chondrite-normalized REE patterns are strongly fractionated in comparison with oceanic-arc andesite-dacite sequences. La is 80 to 120 times chondritic abundance, but Yb and Lu are less than 10 times chondritic abundance. Small negative Eu anomalies characterize the rhyolites. Nickel in the andesites is 40 to 70 ppm. The origin of the andesite is interpreted in terms of nonmodal partial melting of a trace-element—enriched garnet-bearing source, possibly subducted crust that has converted to eclogite. Rhyodacite and rhyolite are interpreted as low-pressure crystal-fractionation products of silicic andesite, in which crystallizing phases are hornblende rich in REE and plagioclase.

Rare earth element geochemistry of potassic lavas from the Birunga and Toro-Ankole regions of Uganda, Africa

Abstract: Neutron activation determination of La, Ce, Sm, Eu, Tb, Yb, Lu, Ta, Hf, Sc, Co and Th in potassic lavas from the Birunga and Toro-Ankole regions show that the rocks are characterized by high rare earth element (REE) contents (161–754 ppm) and form two groups based upon differing La/Yb ratios. One group is made up of katungite, ugandite and mafurite with La/Yb =146–312, and the other of rocks of the leucitite and phonolitic tephrite series, La/Yb =30–56. The trace element content of the ugandite group is similar to that of kimberlites. The data do not indicate any trends of differentiation or simple relationships between the two groups of rocks, although katungite is unlikely to be parental to rocks of lower La/Yb ratios. It is unlikely that in terms of La/Yb ratios that partial melting of mica-garnet-lherzolite mantle can form katungite because of the very small amounts of partial melting required (0.2%), although the La/Yb ratios of 150–200 (ugandites, mafurites) and 30–60 (leucitites, phonolitic tephrites) can be accounted for by 0.3–1.5% and 1–9% melting respectively, if the REE are then concentrated without further La and Yb fractionation. Partial melting of mantle which has been metasomatized by alkaline earths and REE bearing fluids or mixing of carbonatite and nephelenite are also compatable with the observed geochemistry of the lavas. It is considered that gas transfer processes which selectively enrich the light REE may have obscured REE evidence pertaining to early partial melting and/or differentiation processes and therefore that REE geochemistry is of little use in determining the petrogenetic processes involved in the formation of potassic lavas.

Major, minor, and trace element compositions of peridotitic and basaltic komatiites from the precambrian crust of Southern Africa

Abstract: Major and trace element compositional data are reported for nine mafic and ultramafic rock samples from the Barberton greenstone belt. Rocks from this province are among the oldest fragments of the Earth's crust (∼3.5 b.y.). The data are consistent with an oceanic crust related origin for these rocks. The high abundances of Ni in these samples make their origin by fractional crystallization of a primitive magma unlikely but are consistent with their generation by partial melting of an upper mantle source. The basaltic samples from the Komati formation can be related by small degrees of partial melting of a primitive upper mantle source to the peridotitic komatiite which probably derived from much more extensive partial melting of a similar source. REE and especially Ni abundances limit the proportion of olivine that is permitted in the residue.

Mare basalt genesis - A cumulate-remelting model

Abstract: In the framework of the cumulative-remelting model of lunar evolution, high-Ti basalts can be produced by extensive melting of a late-stage cumulate consisting of ilmenite-plagioclase-clinopyroxene-olivine at shallow depths immediately beneath about 60 km thick plagioclase-rich crust. Low-Ti mare basalts can be derived by small degrees of partial melting of an earlier cumulate containing mainly olivine-clinopyroxene-orthopyroxene with minor plagioclase or spinel at greater depth.

Neogene and quaternary volcanism of the Bijar Area (Western Iran)

Abstract: In the Bijar region (Western Iran) two distinct volcanic cycles have been recognized. The first, of Upper Miocene age, consists of high-K cale-alkaline volcanic rocks interpreted as final products of the cale-alkaline Tertiary phase of central Iran. The second volcanic cycle, mostly of Pleistocene age (0.5–1.3 m.v.) consists of undersaturated, mainly potassic, alkaline products. As the lavas of this last phase are slightly fractionated, the chemical differences shown by these rocks have been interpreted as primitive features related to the physical conditions governing the partial melting in the mantle and/or the mantle heterogeneity. In a volcanic center (Sarajukh volcano) contemporaneous basic and acid magmas have been found, and interpreted as derived from two different and independent sources. The alkaline basic volcanism is considered as an expression of disjunctive processes that have affected the western margin of the Iranian plate after the Pliocene.

Chemical variance in deep ocean basalts

Abstract: Multiple regression analysis was used to determine the relative importance of crystal fractionation, partial melting, and seawater alteration on the concentrations of 28 elements in deep ocean basalt. The components Al2O3, P2O5, and H2O were used as independent variables to represent each of these processes, and the computerderived equations show a high level of significance for all 28 elements. The Al2O3, total Fe, and TiO2 of oceanic basalt vary greatly in any given region of the oceanic crust due to small-scale crystal settling. The oceanic crust is enriched in large-ion-lithophile (LIL) elements with time by off-ridge volcanism. With the notable exception of Zr and Hf, seawater alteration probably affects the concentration of LIL elements, but much less than partial melting. Seawater alteration leaches significant amounts of Si and Ca from oceanic basalt while enriching it in K, Li, and B.

Gravity anomalies and granite emplacement in west-central Colorado

Abstract: A major negative gravity anomaly with a minimum of —325 mgal and an amplitude of —70 mgal occurs in west-central Colorado between Aspen and Gunnison. The gravity minimum is closely associated spatially with Late Cretaceous to Oligocene granitic rocks and continues along the Colorado mineral belt to the northeast. Gradients indicate that the source of the negative anomaly is in the upper crust, but part of the negative anomaly is attributed to crustal thickening that is a result of isostatic compensation. The most plausible explanation for the negative gravity anomaly is that most of it is caused by a granite batholith 8 to 25 km thick and that the numerous granitic stocks in the area are protrusions from this batholith, so that the mineral belt occurs along the roof zone of the batholith. Although the stocks appear to have been emplaced primarily by stoping, the gravity effect of the sloped material must be at the base of the crust or dispersed, because the gravity effect is minimal. Temperatures in the lower crust may be high enough for the granite to have been formed by partial melting. The postulated batholith is a major crustal feature that cuts obliquely across many Laramide structural trends.