A new pyroxene fractionation trend from a layered basic intrusion

Nature ◽  
1981 ◽  
Vol 290 (5804) ◽  
pp. 325-326 ◽  
Author(s):  
J. R. Wilson ◽  
K. H. Esbensen ◽  
P. Thy
Keyword(s):  
Fractionation Trend

Variable modes of formation for tonalite-trondhjemite-granodiorite-diorite (TTG)-related porphyry-type Cu ± Au deposits in the Neoarchean southern Abitibi subprovince (Canada): Evidence from petrochronology and oxybarometry

2021 ◽  
Author(s):  
Xuyang Meng ◽  
Jeremy P Richards ◽  
Daniel J Kontak ◽  
Adam C Simon ◽  
Jackie M Kleinsasser ◽  
...  
Keyword(s):  
Crustal Contamination ◽  
Edge Structure ◽  
Isotopic Signatures ◽  
Porphyry Cu ◽  
Abitibi Subprovince ◽  
Local Restriction ◽  
Fractionation Trend ◽  
Magma Crystallization ◽  
X Ray Absorption

Abstract Most known porphyry Cu ± Au deposits are associated with moderately oxidized and sulfur-rich, calc-alkaline to mildly alkalic arc-related magmas in the Phanerozoic. In contrast, sodium-enriched tonalite-trondhjemite-granodiorite-diorite (TTG) magmas predominant in the Archean are hypothesized to be unoxidized and sulfur-poor, which together preclude porphyry Cu deposit formation. Here, we test this hypothesis by interrogating the causative magmas for the ~2.7 Ga TTG-related Côté Gold, St-Jude, and Clifford porphyry-type Cu ± Au deposit settings in the Neoarchean southern Abitibi subprovince. New and previously published geochronological results constrain the age of emplacement of the causative magmas at ~2.74 Ga, ~2.70 Ga, and ~2.69 Ga, respectively. The dioritic and trondhjemitic magmas associated with Côté Gold and St-Jude evolved along a plagioclase-dominated fractionation trend, in contrast to amphibole-dominated fractionation for tonalitic magma at Clifford. Analyses of zircon grains from the Côté Gold, St-Jude, and Clifford igneous rocks yielded εHf(t) ± SD values of 4.5 ± 0.3, 4.2 ± 0.6, and 4.3 ± 0.4, and δ18O ± SD values of 5.40 ± 0.11 ‰, 3.91 ± 0.13 ‰, and 4.83 ± 0.12 ‰, respectively. These isotopic signatures indicate that although these magmas are mantle-sourced with minimal crustal contamination, for the St-Jude and Clifford settings the magmas or their sources may have undergone variable alteration by heated seawater or meteoric fluids. Primary barometric minerals (i.e., zircon, amphibole, apatite, and magnetite-ilmenite) that survived variable alteration and metamorphism (up to greenschist facies) were used for estimating fO2 of the causative magmas. Estimation of magmatic fO2 values, reported relative to the fayalite-magnetite-quartz buffer as ΔFMQ, using zircon geochemistry indicate that the fO2 values of the St-Jude, Côté Gold, and Clifford magmas increase from ΔFMQ -0.3 ± 0.6, ΔFMQ +0.8 ± 0.4, to ΔFMQ +1.2 ± 0.4, respectively. In contrast, amphibole chemistry yielded systematically higher fO2 values of ΔFMQ +1.6 ± 0.3 and ΔFMQ +2.6 ± 0.1 for Côté Gold and Clifford, respectively, which are consistent with previous studies that indicate amphibole may overestimate the fO2 of intrusive rocks by up to one log unit. Micro X-ray absorption near edge structure (μ-XANES) spectrometric determination of sulfur (i.e., S6+/ΣS) in primary apatite yielded ≥ΔFMQ -0.3 and ΔFMQ +1.4–1.8 for the St-Jude and Clifford, respectively. The magnetite-ilmenite mineral pairs from the Clifford tonalite yielded ΔFMQ +3.3 ± 1.3 at equilibrium temperatures of 634 ± 21 °C, recording the redox state of the late stage of magma crystallization. Electron probe microanalyses revealed that apatite grains from Clifford are enriched in S (up to 0.1 wt. %) relative to those of Côté Gold and St-Jude (below the detection limit), which is attributed to either relatively oxidized or sulfur-rich features of the Clifford tonalite. We interpret these results to indicate the deposits at Côté Gold and Clifford formed from mildly (~ΔFMQ +0.8 ± 0.4) to moderately (~ΔFMQ +1.5) oxidized magmas where voluminous early sulfide saturation was probably limited, whereas the St-Jude deposit represents a rare case whereby the ingress of externally derived hydrothermal fluids facilitated metal fertility in a relatively reduced magma chamber (~ΔFMQ +0). Furthermore, we conclude that variable modes of formation for these deposits and, in addition, the apparent rarity of porphyry-type Cu-Au deposits in the Archean may be attributed to either local restriction of favorable metallogenic conditions, and/or preservation, or an exploration bias.

Distribution and evolution of uranium in the oceanic lithosphere

1979 ◽  
Vol 291 (1381) ◽  
pp. 423-431 ◽  
Keyword(s):  
Sea Water ◽  
Oceanic Lithosphere ◽  
Uranium Content ◽  
Major And Trace Elements ◽  
Ultramafic Rocks ◽  
Layer 2 ◽  
Secondary Enrichment ◽  
Layer 4 ◽  
Mid Atlantic Ridge ◽  
Fractionation Trend

Induced fission track techniques permit us to determine quantitatively the microscopic distribution of uranium in rocks, in their constituent minerals, and in percolating fluids. Both primary magmatic variations and secondary mobilization of uranium can be discerned. Concentrations of uranium in phenocrysts and fresh glasses of oceanic basalts and gabbros are very low (2-80 parts/10 9 ) and are comparable to concentrations in the same minerals of the associated ultramafic rocks. Variations with depth in D.S.D.P. holes show several distinct cyclic variations of uranium, accompanied by parallel trends in some major and trace elements. In Hole 332B (mid-Atlantic ridge, 36 °N), uranium and other elements can be shown to fall into two distinct groupings, each group following its own characteristic fractionation trend, suggesting that two distinct magmas differentiated independently beneath the median valley, the two magmas alternating in their contribution to the formation of oceanic layer 2. Earlier investigations of the uranium distribution in surface pillows and other dredged rocks exposed to sea water had shown that, owing to halmyrolysis, the uranium concentration increases systematically with distance from the axis of a midoceanic ridge. Subsequent investigations on rocks drilled from horizons deeper into oceanic layer 2 indicate that secondary enrichment or redistribution of uranium is confined to specific zones of altered basalt, near fractures, pillow and flow margins, and especially along horizontal planes of breccias and sediments in between massive flow where convective water circulation is thought to occur. Ultramafic rocks from the base of layer 3 and top of layer 4 are also enriched in uranium when hydrated by sea water during the process of serpentinization. A combination of these processes may double the uranium content of an oceanic lithospheric plate between the time of its formation and its eventual subduction.

scholarly journals Silicate-Carbonate Liquid Immiscibility: Insights from the Crevier Alkaline Intrusion (Quebec)

2020 ◽  
Vol 61 (3) ◽  
Author(s):  
Pierre-Arthur Groulier ◽  
François Turlin ◽  
Anne-Sylvie André-Mayer ◽  
Daniel Ohnenstetter ◽  
Alexandre Crépon ◽  
...  
Keyword(s):  
Nepheline Syenite ◽  
Parental Magma ◽  
Liquid Immiscibility ◽  
Grenville Province ◽  
Pyrochlore Group ◽  
Alkaline Intrusion ◽  
Fractionation Trend ◽  
Zircon Thermometry ◽  
New Generation ◽  
Silicate Carbonate

Abstract This contribution explores the petrogenetic relationships between silicate and carbonatitic rocks in the Crevier Alkaline Intrusion (CAI, Québec, Canada). The CAI is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated peralkaline rocks from ijolite to nepheline syenite and carbonatites. Petrogenetic relationships between different undersaturated alkaline igneous rocks, carbonate-bearing and carbonate-free nepheline syenite and carbonatites observed in the CAI suggest that (1) carbonate-bearing and carbonate-free silicate rocks are comagmatic with carbonatite, and that (2) both silicate and carbonatitic liquids are fractionated from an ijolitic parental magma that has undergone liquid immiscibility. One of the observed facies is characterized by spectacular ocelli of carbonate-bearing nepheline syenite in a matrix of carbonatite. The younger nepheline syenite facies can be divided into two groups based on the presence or absence of magmatic carbonates. Both groups are characterized by the presence of pyrochlore-group minerals that carry the Nb–Ta mineralization. We specifically use accessory minerals such as zircon, pyrochlore and apatite to constrain the temporal and physicochemical parameters of the immiscibility process. By coupling (1) mineral textures, (2) trace elements, (3) Ti-in-zircon thermometry, and (4) oxygen isotope compositions, we have traced the crystallization of zircon before, during and after the immiscibility process. The results allowed us to constrain the minimum temperature of this process at ∼815–865°C. In addition, two magmatic populations of pyrochlore are identified through their petrographic and geochemical characteristics within the younger nepheline syenite facies. Pyrochlore from the earlier ocelli facies of carbonate-bearing nepheline syenite follow a Nb–Ta differentiation trend, whereas pyrochlore from the younger carbonate-free nepheline syenite follow a more classical Nb–Ti trend. Following the complete immiscibility between the silicate and carbonatitic liquids, the fractionation between Nb and Ta stopped while a new generation of Nb-rich pyrochlore grew, displaying a more classical Nb–Ti fractionation trend and a higher Nb/Ta ratio in the nepheline syenite.

scholarly journals Pseudo-fractionation trend of the Fiskenæsset anorthosite complex, southern West Greenland

1975 ◽  
Vol 75 ◽  
pp. 77-80
Author(s):  
J.S Myers
Keyword(s):  
Volcanic Rocks ◽  
Granulite Facies ◽  
West Greenland ◽  
Regional Deformation ◽  
Gneiss Complex ◽  
Enormous Amount ◽  
Fractionation Trend ◽  
Anorthosite Complex

The Fiskenæsset anorthosite complex is a sheet of layered igneous cumulates, 350-400 m thick, which was intruded into volcanic rocks, now amphibolites (Escher & Myers, this report). It was later disrupted by the intrusion of an enormous amount of granitoid material, mainly as sheets during regional deformation, and was metamorphosed in amphibolite and hornblende granulite facies about 2850 m.y. ago (Black et al., 1973). Fragments of similar rocks are widespread throughout the Archaean gneiss complex of Greenland, although the name Fiskenæsset complex is limited to the occurrences in the Fiskenæsset region (Bridgwater et al., in press).

Massive sulfide deposits of the Noranda area, Quebec. II. The Aldermac mine

1991 ◽  
Vol 28 (9) ◽  
pp. 1301-1327 ◽  
Author(s):  
T. J. Barrett ◽  
S. Cattalani ◽  
F. Chartrand ◽  
P. Jones
Keyword(s):  
Volcanic Rocks ◽  
Parental Magma ◽  
Mass Gain ◽  
Massive Sulfide ◽  
Sulfide Deposit ◽  
Calc Alkaline ◽  
Sulfide Deposits ◽  
Massive Sulfide Deposits ◽  
Fractionation Trend ◽  
Felsic Volcanic

The original Aldermac mine near Noranda contained several Cu–Zn massive sulfide lenses hosted by felsic to mafic volcanic rocks of the late Archean Blake River Group. The original Nos. 3–6 orebodies, which consisted of massive pyrite, with lesser magnetite, pyrrhotite, chalcopyrite, and sphalerite, contained 1.87 Mt of Cu–Zn ore that averaged 1.47% Cu (Zn was not recovered). The orebodies occurred within felsic breccias and tuffs up to 100 m thick that are stratigraphically overlain by an extensive dome of mainly massive rhyolite and rhyodacite (up to 250 m thick and at least 550 m across). Most of the volcanic rocks that laterally flank and overlie the felsic dome are dacitic to andesitic flows, breccia, and tuff, with minor rhyolites, and associated subvolcanic sills of quartz-feldspar porphyry and gabbro.The new massive sulfide deposit, discovered in 1988, lies 150–200 m east of the mined-out orebodies, at a similar stratigraphic level within altered felsic breccia and tuff. The sulfides are mainly in the No. 8 lens, which contains 1.0 Mt at an average grade of 1.54% Cu, 4.12% Zn, 31.2 g/t Ag, and 0.48 g/t Au. Pyrite forms porphyroblastic megacrysts in a groundmass of pyrrhotite, sphalerite, magnetite, and chalcopyrite. A funnel-shaped, chloritized stockwork zone underlies the No. 8 lens and contains Cu-stringer mineralization. The No. 8 lens appears to be zoned, with overall decreasing Cu:Zn ratios from the core to the fringes of the lens. Massive sulfides in this lens have high Ag, Cd, and Hg contents relative to other massive sulfide deposits near Noranda.Ti versus Zr trends for least-altered Aldermac volcanic rocks indicate a more or less continuous magmatic fractionation trend ranging from high-Ti andesite to andesite, dacite, rhyodacite, and two distinct rhyolites (A and B). Most volcanic rocks were derived from a common parental magma that was transitional between tholeiitic and calc-alkaline compositions, as indicated by Ti–Y–Zr–Nb data and rare-earth-element distributions.Ti versus Zr trends in altered volcanic rocks indicate that silicification (mass gain) has affected some of the andesitic to rhyodacitic rocks, whereas chloritization (mass loss) has affected many of the rhyolitic rocks. Intermediate to mafic volcanic rocks above and lateral to the felsic dome are commonly silicified, possibly the result of hydrothermally remobilized silica derived from underlying felsic volcanic rocks.The orebodies appear to have formed at an eruptive hiatus between mafic → felsic and felsic → mafic cycles, during explosive activity and accumulation of felsic breccia and tuff. Ore was deposited mainly within a felsic fragmental sequence (rhyolite A), but before emplacement of the dome of rhyolite B. In compositionally diverse volcanic terrains, the contact between successive mafic–felsic and felsic–mafic cycles may be a good exploration target, in particular specific geochemical contacts within the felsic stratigraphy.

scholarly journals The significance of the kakortokite in the evolution of the Ilímaussaq intrusion, South Greenland

1970 ◽  
Vol 89 ◽  
pp. 1-193
Author(s):  
J Ferguson
Keyword(s):  
Major Element ◽  
Residual Liquid ◽  
Rock Types ◽  
Field Evidence ◽  
Intrusive Rocks ◽  
Petrology And Geochemistry ◽  
Fractionation Trend ◽  
Quantitative Analyses ◽  
Mineralogical Investigation ◽  
Rock Specimens

The feldspars of the intrusive rocks of Ilímaussaq were subjected to a fairly detailed mineralogical investigation. The K-feldspars from the early augite syenite have an average obliquity of 0.61 compared with the corresponding value of 0.90 for the feldspars of the remaining rock types. In addition, the obliquity values within the augite syenite group show a mildly contrasting variation related to the grainsize of the rock. The co-existing augites and olivines in the augite syenite vary sympathetically with the obliquity values. The structural states and compositions of the Na-feldspars in the major rock groups of the Intrusion indicate that they are all low temperature albites. Most of the rock types in the Intrusion develop rhythmic layering. Petrofabric analyses of the kakortokite and lujavrites indicate that the cumulus minerals show a marked planarism and, in some cases, lineation. Six new major element rock analyses of kakortokite together with pre-existing published analyses have been presented in ternary diagrams. In addition, quantitative analyses were carried out on 90 whole rock specimens for the following elements: Na, K, Rb, Li, Cs, Ca, Sr, Ba, Ti, Zr, Nb, Sn, Be, La and Ga. Further results include semi-quantitative analyses for Bi, Pb, Zn, Sb, Sn, Y, Be, Mo, Nb, Cu, Ni, La, Ag, Co and V on 100 kakortokite rocks and 40 minerals. The fractionation trend displayed by the K: Rb ratio shows a remarkably strong correlation with the chronology as determined from the field evidence. Other ratios used include Sr: Ca, Ba: Rb, Ti: Zr, Ti: Nb and Zr: Nb. In general, these ratios conform to established behaviour patterns during fractionation; an exception is the Zr: Nb ratio which increases with fractionation. Furthermore, the kakortokite sequence displays a generally progressive variation of these ratios with height, suggesting that they crystallized from the floor upwards. On the basis of field evidence, petrology and geochemistry it would appear that the earliest magma of the Intrusion had an augite syenitic composition. It is thought that this original augite syenitic magma differentiated along an undersaturated trend and that the heterogeneous syenite represents the roof-development of the innermost differentiate of the augite syenite. It is postulated that by the addition of volatiles the inner augite syenitic magma was converted to a magma of peralkaline composition. The agpaitic (per-alkaline) magma then underwent crystallization which took place from the roof downwards with simultaneous gravity accumulation of the heavier minerals on the floor of the Intrusion. This gave rise to both the sheeted occurrences of sodalite foyaite and naujaite, essentially products of the flotation of sodalite and intercumulus crystallization, and the markedly rhythmically layered kakortokite, a product of gravity accumulation. Finally, a residual liquid, rich in volatiles, was trapped between the downward crystallizing naujaite and the gravity accumulated kakortokite. As a result of faulting and/or slumping the residual mush was injected into the overlying brecciated rocks where it crystallized to form the fissile lujavrites. The origin of the alkali granite is problematical, but it would appear to predate the agpaites and post-date the augite syenite and heterogeneous syenite.

Accretion of fast-spread lower oceanic crust: drill core GT1 from the ICDP Oman Drilling Project

2021 ◽  
Author(s):  
Dominik Mock ◽  
David Axford Neave ◽  
Samuel Müller ◽  
Dieter Garbe-Schönberg ◽  
Benoit Ildefonse ◽  
...  
Keyword(s):  
Transition Zone ◽  
Oceanic Crust ◽  
Upper Mantle ◽  
Planetary Science ◽  
Drill Core ◽  
Thin Sections ◽  
The Core ◽  
Molar Basis ◽  
Fractionation Trend ◽  
Drilling Project

<p>The Sumail Ophiolite at the northeastern coast of the Sultanate of Oman provides an ideal field laboratory for studies on fast-spread oceanic crust on land. Based on numerous campaigns in the past, the Oman Drilling Project (OmanDP) of the International Continental Scientific Drilling Program (ICDP) obtained nine 300 to 400 m long drill cores covering sections from the upper mantle to the dyke/gabbro transition zone. Drill core GT1 is located in the layered gabbros between ~1200 and ~800 m above the Moho transition zone (m.a.M.) and comprises of modally layered gabbro with cm-scale coherent bands of troctolite, anorthosite, and wehrlite. We prepared thin-sections with a small average spacing of <2 m and analyzed them by petrological, microstructural and geochemical methods. Clinopyroxene reveals Mg# (where Mg# = Mg/Mg+Fe x 100; molar basis) between 74 and 86, with some heavily altered olivine relicts between 70 and 83, and Ca# (where Ca# = Ca/Ca+Na x 100; molar basis) of plagioclase range from 68 to 87. The plots of these data show clear and consistently decreasing trends from the base of the drill core up section to a crustal height of 1070 m.a.M. where all fractionation indices show significant minima. Above 1070 m.a.M., the indices increase to their maxima. Clinopyroxene shows core/rim zonation in Mg# and TiO<sub>2</sub> content with more primitive core compositions. However, distinct zonation is only observed above the minima mentioned above. Besides this general fractionation trend from the core base to 1070 m.a.M., individual fractionation trends on the scale of several decameters can be defined along the core (e.g., 820 to 895, 890 to 970, and 1085 to 1110 m.a.M.). As a quantifier of the plagioclase fabric symmetry, we used the BA index which ranges from 0 for a purely foliated to 1 for a purely lineated fabric. We found that the rock fabric changes parallel the observed fractionation trend with significant lineation at the base of the core and evolving towards almost purely foliated fabrics up section to 1070 m.a.M., indicating either an intense compaction or weaker shearing, or both at 1070 m.a.M. A possible scenario creating the observed trends is an evolved melt entering the more primitive crystal mush at 1070 m.a.M.  and crystallizing primary phases with significantly more evolved compositions. In such an environment, where the liquid/solid ratio is increased, minerals may be more sensitive to compaction and less affected by shearing which is possibly induced by convection of the upper mantle. Magmatic deformation would therefore lead to a strong foliation with only a limited lineation component. Moreover, we interpret the observed decameter-scale fractionation trends, also being accompanied by slight changes in the fabric, as results of individual magma reservoirs crystallizing in-situ and leading to the accretion of the lower gabbros in Oman (e.g., [1]).</p><p> </p><p>[1] Kelemen, P. B., Koga, K., & Shimizu, N. (1997). Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman ophiolite: Implications for the origin of the oceanic lower crust. Earth and Planetary Science Letters, 146(3-4), 475-488.</p>

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