Ultramafic Breccias from the Musa Valley, Eastern Papua

1961 ◽  
Vol 98 (1) ◽  
pp. 1-26 ◽  
Author(s):  
D. H. Green
Keyword(s):  
Ultramafic Rock ◽  
Country Rock ◽  
Parental Magma ◽  
Olivine Gabbro ◽  
Direct Crystallization ◽  
The Matrix ◽  
Fluid Environment ◽  
Valley Area ◽  
Rock Minerals ◽  
Layered Sequence

AbstractIn the Musa Valley area rocks of the Papuan Ultramafic Belt outcrop discontinuously over an area of 25 miles by 45 miles. The rocks are part of a folded layered sequence ranging from magnesian dunite and peridotite, chemically and petrographically typical of the alpine ultramafic suite, to olivine gabbro and bytownite gabbro. Agglomerate like breccias consisting of fragments of ultramafic rock in a variable matrix occur as irregular vent-like bodies in the peridotite and dunite and as horizontal sheets in a Pleistocene-Recent sedimentary sequence. The breccias are interpreted as vent and extrusive breccias resulting from the penetration, brecciation and local entrainment (fluidization) of peridotitic country rock by volcanic gases. Olivine alkali basalt was probably the parental magma responsible for the gaseous activity. The breccias show a sequence of stability fields for the assemblages mesh-texture serpentine and bastite (i.e. chrysotile and lizardite mixtures); antigorite; olivine and enstatite; and these are correlated qualitatively with variation in intensity of the brecciation process. Chemically the brecciation process was responsible for the addition of H2O, SiO2, CO2, Al2O3 and O2 and the removal of Fe and Mg from the fragmented rock. Minerals which are of direct crystallization in the matrix fluid environment include chalcedony, fibrous serpentine (chrysotile), magnesite, quartz, andradite, and garnierite. There is no evidence of the presence of an ultramafic, serpentine magma in the genesis of the breccias.

Open-system magma chamber process in the Freetown Complex of Sierra Leone: evidence from Zone 3

1995 ◽  
Vol 132 (3) ◽  
pp. 261-266 ◽  
Author(s):  
Christopher I. Chalokwu ◽  
Pamela J. Seney
Keyword(s):  
Sierra Leone ◽  
Magma Chamber ◽  
Parental Magma ◽  
Olivine Gabbro ◽  
Cryptic Variation ◽  
Anorthite Content ◽  
Mineral Compositions ◽  
Layered Complex ◽  
Layered Sequence

AbstractThe ˜ 7 km thick Freetown layered complex of Sierra Leone consists of four zones each composed of a cyclically layered sequence of troctolite, gabbro, olivine gabbro, gabbronorite, and anorthosite. The complex is thought by previous workers to have solidified in situ from a single parental magma without stratigraphic changes in mineral compositions. Evidence for cryptic variation is presented based on electron microprobe analyses of mega-unit Zone 3. Two reversals in olivine forsterite content, plagioclase anorthite content, and 100 Mg/(Mg Fe2) in clinopyroxene match the variations of Ni in olivine and Cr in clinopyroxene. These changes are consistent with a magma chamber that was open to periodic influxes of new magmas, and the mixing of new and fractionated resident magmas. Expansion of the magma chamber is thought to have occurred at 2000 m, corresponding to the level of a major influx.

The Tugtutôq older giant dyke complex: mineralogy and geochemistry of an alkali gabbro-augite-syenite-foyaite association in the Gardar Province of South Greenland

1985 ◽  
Vol 49 (354) ◽  
pp. 623-642 ◽  
Author(s):  
B. G. J. Upton ◽  
D. Stephenson ◽  
A. R. Martin
Keyword(s):  
Olivine Basalt ◽  
Side Wall ◽  
Parental Magma ◽  
Entire Group ◽  
Olivine Gabbro ◽  
Central Group ◽  
Alkali Olivine Basalt ◽  
Feti Oxide ◽  
Field Junction ◽  
Fluor Apatite

AbstractThe Older Giant Dyke Complex is a differentiated alkaline intrusion of Proterozoic age (1154±16 Ma) and is the earliest of the late Gardar intrusions in the Tugtutôq-Ilímaussaq region. The dyke is approximately 20 km long by 0·5 km broad and comprises (i) marginal ‘border group’ rocks of alkali olivine gabbro, grading inwards to ferro-syenogabbro and (ii) an axial ‘central group’ of salic rocks ranging from augite syenite in the WSW to sodalite foyaite in the ENE.Chilled margins contain plagioclase (An53), olivine (Fo53), magnetite, ilmenite, and apatite as liquidus phases and later-crystallized augite (Di69Hd27Ac4) and biotite (Annite32). The coexisting Fe-Ti oxides indicate fO2 and T values just below the synthetic QFM buffer curve. In the border group, plagioclase cores zone into anorthoclase and soda-sanidine rims, olivines reach Fo16, pyroxenes Di32Hd59Ac9, and biotites Annite86. Interstitial pargasitic amphibole appears close to the innermost margins. In the central group, feldspars are all perthitic alkali feldspars and nepheline becomes a major, early crystallizing phase. Olivines range from Fo10-Fo4, in the augite-syenites where they coexist with ferro-salites Di50Hd47Ac3, but olivine is absent from foyaitic assemblages in which the pyroxenes range through aegirine-augite to pure aegirine. Interstitial amphiboles range from ferro-pargasite or hastingsite to katophorite and thence towards arfvedsonite, but are absent from the most differentiated rocks, whereas biotite occurs throughout the entire group in the range Annite71-Annite100.The parental magma, represented by the chilled margins, was a relatively anhydrous alkali olivine-basalt with an initial 87Sr/86Sr ratio of 0.70326. Its high Ti, P, Ba, and F contents are inferred to be features inherited from a primary magma, derived from the mantle as a small partial melt fraction which involved significant amounts of fluor-apatite and phlogopite. While all lithologies are considered as differentiates from this parental magma, there is both a well-defined field junction and a compositional hiatus between the border group and the central group rocks. Mineralogical considerations and REE patterns suggest that the later, more salic (?benmoreitic) magma from which the central group crystallized, related to the parental magma by ol-fsp-ap-FeTi oxide fractionation. Congelation in both border group and central group occurred by side-wall crystallization, but the salic magma became compositionally stratified, with upward concentration of alkalis and volatiles to produce a phonolitic upper facies which is preserved at the ENE end of the intrusion owing to subsequent axial tilting.

The Samapleu mafic-ultramafic intrusion and its Ni-Cu-PGE mineralization: an Eburnean (2.09 Ga) feeder dyke to the Yacouba layered complex (Man Archean craton, western Ivory Coast)

2014 ◽  
Vol 185 (6) ◽  
pp. 393-411 ◽  
Author(s):  
Franck Gouedji ◽  
Christian Picard ◽  
Yacouba Coulibaly ◽  
Marc-Antoine Audet ◽  
Thierry Auge ◽  
...  
Keyword(s):  
Hybrid Zone ◽  
Ivory Coast ◽  
Country Rock ◽  
Parental Magma ◽  
Massive Sulfide ◽  
Contact Metamorphism ◽  
Sulfide Deposit ◽  
Sulfide Liquid ◽  
Melt Composition ◽  
Layered Complex

Abstract The Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths > 500 m. Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion. The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile. The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust. These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).

The Jurassic gabbroic intrusions of Utpostane and Muren: insights into Karoo-related plutonism in Dronning Maud Land, Antarctica

2003 ◽  
Vol 15 (2) ◽  
pp. 283-301 ◽  
Author(s):  
SAKU K. VUORI ◽  
ARTO V. LUTTINEN
Keyword(s):  
Incompatible Element ◽  
Parental Magma ◽  
Olivine Gabbro ◽  
Flood Basalts ◽  
Continental Flood Basalts ◽  
Dronning Maud Land ◽  
Magma Pulses ◽  
Parental Melts ◽  
Inclined Sheet ◽  
Olivine Gabbronorite

Middle Jurassic continental flood basalts of Vestfjella, western Dronning Maud Land are cut by gabbroic intrusions that represent rare exposures of Karoo-related mafic plutons in Antarctica. The gabbros and numerous associated dolerites indicate high magmatic activity along the continental margin of Dronning Maud Land during the break-up of Gondwana. The scattered nunataks of Utpostane (∼25 km2) are dominated by olivine gabbronorite and olivine mela-gabbronorite, which can be grouped into four zones. The Utpostane intrusion exhibits inclined sheet-like geometry, is at least ∼3 km thick and shows moderate enrichment of incompatible element contents from its base towards the exposed roof contact (e.g. Zr from ∼15 to ∼60 ppm). High MgO contents (∼8–36 wt%) indicate that the parental magma of Utpostane was more primitive than a typical Karoo tholeiite (MgO ∼6 wt%). At Muren, gabbroic outcrops record a cross section of a ∼1.3 km thick inclined sheet-like intrusion. The intrusion can be divided to two main units, the upper and lower zones, which are dominated by olivine gabbro and gabbronorite, respectively. Parental melts of the geochemically differentiated upper zone and the homogeneous lower zone of Muren were typical low-MgO Karoo tholeiites, but they were chemically distinct and were emplaced as separate magma pulses.

Significance of epidote in orbicular diorite from the Grenville Front zone, eastern Labrador

1991 ◽  
Vol 55 (379) ◽  
pp. 173-181 ◽  
Author(s):  
J. Victor Owen
Keyword(s):  
Boundary Layers ◽  
Shell Structures ◽  
Direct Crystallization ◽  
Sodic Plagioclase ◽  
Mutual Contact ◽  
The Matrix ◽  
History Of ◽  
Unidentified Source ◽  
Water Saturated ◽  
Front Zone

AbstractOrbicules in diorite from the Grenville Front zone of eastern Labrador are defined by shell structures alternately enriched and depleted in biotite, epidote and magnetite. Hornblende occurs locally in orbicule cores and the matrix, but not in the shells. The shells enclose plagioclase-rich (An40–45), leucodioritic cores containing biotite, epidote, magnetite and/or hornblende-bearing mafic clots. The matrix of the orbicules is mineralogically-similar to the orbicule cores, but is mesocratic, and contains relatively sodic plagioclase and accessory quartz and K-feldspar. In places, hornblende contains quartz oikocrysts, implying the resorption of early-formed clinopyroxene, and is rimmed by biotite and epidote. The latter phases also occur as inclusions in quartz-free hornblende interpreted to have crystallized directly from the magma. Epidote has a pistacite content of 21 to 26 and occurs as (1) tiny, idiomorphic crystals (‘epidote I’) enclosed by plagioclase or hornblende, and (2) relatively large (to 1 mm) grains with vermicular textures (‘epidote II’), particularly where in mutual contact with biotite (or hornblende) and plagioclase. These microstructures suggest that epidote is a magmatic phase which formed by direct crystallization from the magma, and by reaction of previously-formed minerals with the magma.The following approximate paragenetic sequence has been inferred for orbicule cores and the matrix: clinopyroxene (clinopyroxene resorbed [→ poikilitic hornblende]), epidote I, Ca-Na plagioclase, biotite, hornblende (biotite and/or hornblende ± plagioclase resorbed [→ epidote II]), quartz + K-feldspar. Biotite compositions became progressively more Fe-rich during crystallization (XMg ⋍ 0.6 → 0.4), and the first-formed plagioclase (inclusions in quartz-free hornblende in orbicule cores) is more calcic (An51) than the last (matrix grains: An35).The appearance of epidote early in the crystallization history of the diorite testifies to elevated PH2O and PTotal (PT). The most aluminous hornblende indicates maximum PT of 5 to 6 kbar. Orbicule shell structures are interpreted to have crystallized from supercooled boundary layers enclosing water-saturated globules within the dioritic magma. Although sufficient to suppress the formation of hornblende in the shell structures, the extent of magma supercooling did not permit the development of comb layering in the orbicules. Supercooling is attributed to an influx into the magma of water from an unidentified source.

Kûngnât, revisited. A review of five decades of research into an alkaline complex in South Greenland, with new trace-element and Nd isotopic data

2013 ◽  
Vol 77 (4) ◽  
pp. 523-550 ◽  
Author(s):  
B. G. J. Upton ◽  
R. Macdonald ◽  
N. Odling ◽  
O. T. Rämö ◽  
B. Bagiński
Keyword(s):  
Crustal Contamination ◽  
Alkali Feldspar ◽  
Olivine Gabbro ◽  
Alkaline Complex ◽  
Nd Isotope ◽  
Modal Layering ◽  
Nearly Continuous ◽  
Lithospheric Mantle Source ◽  
Ring Dyke ◽  
Layered Sequence

AbstractThe Kûngnât Complex (1275±1.8 Ma) in the Gardar Alkaline Province, South Greenland, cuts Archaean gneisses and comprises two intersecting syenitic stocks and a gabbroic ring-dyke. The magmas, with increasingly more primitive compositions, were emplaced successively by ring-faulting and roof stoping. The syenites are orthocumulates (cumulus alkali feldspar, olivine, pyroxene, titanomagnetite and apatite; intercumulus phases include alkali amphibole, biotite, quartz and calcite). In the well dissected earlier stock, a 2.2 km-thick layered sequence displays graded modal layering, feldspar lamination and cryptic layering. Modal layering in both stocks is directed mainly inwards at 35° – 50°. Heterogeneous nucleation of the cumulus assemblage, close to steep thermal boundary layers, is inferred. The modal layering is ascribed primarily to gravitational sorting aided by the large density differential between a) feldspar and b) Fe-rich silicates and oxides. Episodic collapse of cumulus + melt slurries contributed to inward-dipping crystal pediments on the chamber floors. The Ring-Dyke (up to 100 m wide) is nearly continuous through 360°. Kûngnât exhibits a compositional nearcontinuum from olivine gabbro through syenite intermediaries to alkali granite, ascribed to protracted assimilation/fractional crystallization processes. The most radiogenic Nd isotope data from Kûngnât (εNdi values between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment in LREE by crustal contamination of the magmas.

Crystallization from stratified magmas in the Honningsvåg Intrusive Suite, northern norway: a reappraisal

1996 ◽  
Vol 60 (398) ◽  
pp. 41-51 ◽  
Author(s):  
Christian Tegner ◽  
Brian Robins ◽  
Henning S. Sørensen
Keyword(s):  
Magma Chamber ◽  
Country Rock ◽  
New Mineral ◽  
Olivine Gabbro ◽  
Cyclic Unit ◽  
Mineral Data ◽  
Compositional Changes ◽  
Intrusive Suite ◽  
Double Diffusive ◽  
Chamber Floor

AbstractWedge-shaped layers of ultramafic and mafic cumulates in Intrusion II of the Caledonian Honningsvåg Intrusive Suite suggest crystallization on an inclined magma chamber floor from a compositionally-zoned and density-stratified magma.Cyclic unit 8 (140–100 m thick) consists of a distally-thinning olivine gabbro (denoted paoC) macrolayer overlain by a distally-thickening gabbronorite, pahC. New mineral data in four traverses across cyclic unit 8 show systematic compositional changes; the Mg# of the mafic phases decreases upwards through the unit and distally, both along the base and along the paoC/pahC interface.A crystallization model based on an effectively continuously-zoned magma chamber with numerous, relatively thin, double-diffusive magma layers is proposed. Differential migration of horizontal isopleths (e.g. Mg# and aSiO2) in response to fractional crystallization and assimilation of country rock can explain the variations in the Mg# of the cumulates.

The influence of country-rock assimilation and silicate to sulfide ratios (R factor) on the genesis of the Dunka Road Cu – Ni – platinum-group element deposit, Duluth Complex, Minnesota

1997 ◽  
Vol 34 (4) ◽  
pp. 375-389 ◽  
Author(s):  
Robert D. Thériault ◽  
Sarah-Jane Barnes ◽  
Mark J. Severson
Keyword(s):  
Platinum Group Element ◽  
Country Rock ◽  
Metal Sulfide ◽  
Parental Magma ◽  
Group Element ◽  
Duluth Complex ◽  
Metasedimentary Rocks ◽  
R Factor ◽  
Platinum Group ◽  
R Factors

The Dunka Road deposit is one of several Cu – Ni – platinum-group element (PGE) sulfide occurrences found along the northwestern margin of the Duluth Complex, where the host troctolitic rocks are in contact with metasedimentary rocks of the Animikie Group. Magma contamination through assimilation of sulfidic argillaceous country rocks is generally recognized as having played a key role in the genesis of the mineralization. Three main types of disseminated sulfide mineralization have been identified within the Dunka Road deposit: (i) norite-hosted sulfides, (ii) troctolite-hosted sulfides, and (iii) PGE-rich sulfide horizons. The norite-hosted sulfides are found either adjacent to country-rock xenoliths or near the basal contact. The troctolite-hosted sulfides form the bulk of the deposit, and occur throughout the lower 250 m of the intrusion. The PGE-rich sulfide horizons are typically localized directly beneath ultramafic layers. The composition of the different types of sulfide occurrences is modelled using Cu/Pd ratios. It is shown that each type results from the interplay of two main parameters, namely the degree of magma contamination and the silicate magma to sulfide melt ratio (R factor). The norite-hosted sulfides formed at low R factors and high degrees of contamination, as expressed by their PGE-depleted nature, low Se/S ratios, and elevated content in pyrrhotite and arsenide minerals. The troctolite-hosted sulfides formed at moderate R factors and small degrees of contamination, as shown by their moderate PGE content and mantle-like Se/S ratios. Finally, the PGE-rich sulfide horizons are modelled using elevated R factors from an uncontaminated parental magma, which is substantiated by their elevated noble metal content and Se/S ratios, and low pyrrhotite to precious metal sulfide ratio.

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