Cumulus Olivine Research Articles

Petrogenesis and Metallogenesis of the Mazaertag Layered Intrusion in the Tarim Large Igneous Province, NW China

AbstractThe Mazaertag layered intrusion is located in the northwestern part of the Tarim large igneous province where several early Permian layered mafic‐ultramafic intrusions host important Fe‐Ti oxide deposits. The intrusion covers an area of ∼0.13 km2 and has a vertical stratigraphic thickness of at least 300 m. It consists chiefly of olivine clinopyroxenite, and is cut through by the nearby mafic‐ultramafic dykes. In this paper, we report new mineral chemistry data and whole‐rock chemical and isotopic compositions for the Mazaertag intrusion along with whole‐rock isotopic compositions for the nearby mafic dykes. The averaged compositions of cumulus olivine, clinopyroxene and intercumulus plagioclase within individual samples range from Fo71–73, Mg# = 76 to 79 and An65–75 but they do not define sustained reversals. The observed mineral compositions are consistent with the differentiation of a single batch of magma in a closed system. Rocks of the Mazaertag intrusion are characterized by enrichment in light REE relative to heavy REE, positive Nb and Ta anomalies and a small range of age‐corrected ∊Nd(t) (−0.1 to +0.9) and initial 87Sr/86Sr values (0.7044 to 0.7068). The slightly lower ∊Nd(t), initial 206Pb/204Pb and higher initial 87Sr/86Sr values of the intrusion compared to those of the least contaminated dykes [∊Nd(t) = +2.8 to +3.4; (206Pb/204Pb), = 18.516–18.521; (87Sr/86Sr)1 = 0.7038–0.7041] imply that the Mazaertag magma was subjected to small to modest degrees of contamination by the upper crust. The Sr‐Nd isotopic compositions of the least contaminated dykes are consistent with derivation from a FOZO‐like mantle source. The parental magma of the Mazaertag intrusion, estimated from clinopyroxene compositions using mineral‐melt partition coefficients, has trace element compositions similar to some of the most primitive mafic dykes in the same area. This suggests that the Mazaertag intrusion and mafic dykes shared a similar mantle source. Therefore, the parental magma of the Mazaertag intrusion was interpreted to have originated from a mantle plume. Based on the Cr2O3 contents in titanomagnetite and less‐evolved characteristics of the Mazaertag intrusion compared to the Wajilitag Fe‐Ti oxide deposit in Bachu, it is speculated that there might not be a potential to find economic Fe‐Ti oxide mineralization in the intrusion.

Magmatic structure and geochemistry of the Luanga Mafic–Ultramafic Complex: Further constraints for the PGE-mineralized magmatism in Carajás, Brazil

The Luanga Complex is part of the Serra Leste Magmatic Suite, a cluster of PGE-mineralized mafic–ultramafic intrusions located in the northeastern portion of the Carajás Mineral Province. The Luanga Complex is a medium-sized layered intrusion consisting of three main zones: i. the lower Ultramafic Zone comprising ultramafic adcumulates (peridotite), ii. the Transition Zone comprising interlayered ultramafic and mafic cumulates (harzburgite, orthopyroxenite and norite) and iii. the upper Mafic Zone comprising a monotonous sequence of mafic cumulates (norite) with minor orthopyroxenite layers. Several PGE-mineralized zones occur in the Transition Zone but the bulk of the PGE resources are hosted within a 10–50meter thick interval of disseminated sulfides at the contact of the Ultramafic and Transition Zones. The compositional range of cumulus olivine (Fo78.9–86.4) is comparable to those reported for layered intrusions originated from moderate primitive parental magmas. Mantle normalized alteration-resistant trace element patterns of noritic rocks are fractionated, as indicated by relative enrichment in LREE and Th, with negative Nb and Ta anomalies, suggesting assimilation of older continental crust. Ni contents in olivine in the Luanga Complex (up to 7500ppm) stand among the highest values reported in layered intrusions globally. The highest Ni contents in olivine in the Luanga Complex occur in distinctively PGE enriched (Pt+Pd>1ppm) intervals of the Transition Zone, in both sulfide-poor and sulfide bearing (1–3vol.%) rocks. The origin of the PGE- and Ni-rich parental magma of the Luanga Complex is discussed considering the upgrading of magmas through dissolution of previously formed Ni-rich sulfide melts. Our results suggest that high Ni contents in olivine and/or orthopyroxene provide an additional exploration tool for Ni–PGE deposits, particularly useful for target selection in large magmatic provinces.

Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

Mafic–ultramafic cumulates enclosed in gabbroic-dioritic rocks form part of the Mesozoic Ditrău Alkaline Massif in the Eastern Carpathians, Romania. The poikilitic olivine- and pyroxene-rich and nearly mono mineralic hornblendite rocks display typical cumulate textures with early crystallised olivine (Fo75–73), diopside and augite. In the early stages of their genesis the amphibole was intercumulus whilst in later stages it acquired cumulus status as the fractionating magma evolved. Using major and trace element compositions of minerals and whole-rock samples the origin of these cumulates is determined and the parental magma composition and depth of emplacement are calculated.Cumulus clinopyroxene has more primitive composition than intercumulus amphibole suggesting closed system fractionation for the evolution of poikilitic olivine- and pyroxene-rich cumulates. The evolution of the amphibole-rich mesocumulates is more clearly the result of closed system crystallisation dominated by the precipitation of clinopyroxene and amphibole cumulus crystals.Lamprophyre dykes of the Ditrău Alkaline Massif are proposed to reflect multiple basanitic parental magma batches from which the cumulus olivine and clinopyroxene crystallised. Relative to these dykes the calculated equilibrium melts for intercumulus amphibole in the cumulates was more primitive whilst that for the cumulus amphibole was more evolved. The calculated crystallisation temperature and pressure of ~1000–1050°C and ~0.7GPa, based on the composition of the amphiboles, indicate crystallisation at lower crustal depths. Rare earth element compositions are consistent with an intra-plate tectonic setting.

High tenor Ni–PGE sulfide mineralization in the South Manasan ultramafic intrusion, Paleoproterozoic Thompson Nickel Belt, Manitoba, Canada

The South Manasan ultramafic intrusion, located in the Early Proterozoic Thompson Nickel Belt (TNB), contains Ni and platinum group element (PGE) mineralization in the form of disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7wt.% and total precious metals range from 0 to 1.3ppm Pt+Pd+Au. In 100% sulfide the concentration of Ni ranges from 11 to 39wt.%, and the concentration of Pt+Pd+Au ranges from 8 to 27ppm. The intrusion is a steeply dipping, boudinaged, sill-like body with a strike length of approximately 1200m, an average width of 125m, and a depth extent of >1000m. The intrusion is composed of ~25% fresh dunite, ~50% serpentine-altered dunite and ~25% tectonized and carbonate altered dunite. The most intense alteration is found near the margin of the intrusion where it is in contact with Pipe Formation metasedimentary rocks of the Ospwagan Group. The sulfide assemblage in fresh dunite is interstitial between cumulus olivine and is composed predominantly of pentlandite with accessory pyrite with symplectic intergrowths of the two same sulfides. The interstitial sulfide assemblage displays a primary magmatic position between olivine grains, but the sulfides are penetrated by platy intergrowths of chlorite and serpentine. In contrast, the most intensely carbonate altered dunite contains irregular patches of pyrrhotite and pentlandite.Representative drill core samples through the South Manasan Intrusion have been used to establish variations in whole rock geochemistry and mineralogy; these variations coupled with the stratigraphy of the host sedimentary rocks are consistent with magmatic differentiation in a sill-like body which was serpentinized and then tilted upright such that tops now face west. Intrusion of the original komatiitic magma and assimilation of sulfur from Ospwagan Group sedimentary rocks triggered sulfide saturation and formation of an immiscible sulfide melt. The early crystallization of olivine inhibited the sulfide melt from settling to the bottom of the magma column and, as a consequence, the sulfides now have a primary interstitial magmatic texture. Calculated Ni and PGE tenor values indicate an R factor of 500–2500. The current sulfide association dominated by pentlandite≫pyrite>chalcopyrite has a mineral paragenesis that is consistent with crystallization from a monosulfide solid solution which cooled down to pentlandite and pyrite. The subsequent processes of serpentinization, deformation and carbonate alteration modified the primary sulfide assemblages to pentlandite and pyrrhotite. The alteration of the dunite and serpentine to talc-carbonate occurred by open-system hydrothermal processes that produced a reduction in metal tenor through a dilution process accompanying the addition of carbonate and water.It is proposed that the enrichment of the sulfide assemblage at South Manasan in Ni and PGEs is a consequence of a primary magmatic process involving a high-R factor and that the effects of later overprinting processes (alteration and deformation) are not responsible for the high-tenor sulfide association. The study has important exploration implications, as it helps to establish the characteristics of a new type of Ni–PGE mineralization in the TNB, and thus provides a basis on which to better understand the formation of other low-grade ultramafic-hosted nickel deposits.

Role of magmatic and fluid concentrating in formation of platinum mineralization in the Lower Zone and Platreef as follows from composition of phlogopite, cumulus silicates, and sulfide melt, the northern limb of Bushveld Complex

As follows from the results of new geological exploration, the thick sequence of primitive cumulates of the Lower Zone underlies Platreef in separate magmatic depressions of the northern limb of the Bushveld Complex. In the Turfspruit area, the Lower Zone is separated from the overlying Platreef by an interval of contact metasedimentary rocks with sills of fine-grained norite and plagioclase orthopyroxenite of the Marginal Zone. The magmatic stratigraphy of Platreef in Turfspruit and its western plunge, where it is slightly contaminated with sedimentary rocks, is close to the section of Critical Zone in the Bushveld. The Lower Zone and Platreef contain sulfide mineralization enriched in PGE in particular units-reefs-throughout the section. The main reef can be correlated with the Merensky Reef. The objective of this study was to estimate the relationships between fluid and magmatic PGE concentrating in the reefs localized in the upper part of Platreef near its unconformable contact with the Main Zone. It is shown that Ni partitioning between cumulus olivine and orthopyroxene, on the one hand, and coexisting sulfide liquid, on the other, indicates their equilibrium crystallization in the Lower Zone and Platreef at close redox conditions. The composition of sulfide liquid was recalculated as sulfide tenor on the basis of bulk Ni, Cu, and S concentrations in rock. In contrast, Ni partitioning between phlogopite and sulfide melt does not provide evidence in favor of their equilibrium crystallization, although Mg # of phlogopite from the intercumulus assemblage is correlated with Mg # of cumulus mineral throughout the section. The results of phlogopite microprobing throughout the section show that the highest F and Cl contents are characteristic of highly evolved rocks, including both the PGE-bearing plagioclase pyroxenite from reefs and barren norite from the Marginal Zone. We have arrived at the conclusion that enrichment in volatile components is the attendant rather than necessary condition of PGE mineralization formation in reefs, whereas the partitioning of metals between silicate and sulfide melts is the main ore-controlling factor.

Metamorphic modifications of the Muremera mafic–ultramafic intrusions, eastern Burundi, and their effect on chromite compositions

The Muremera mafic–ultramafic intrusions were emplaced into metasedimentary rocks of the Karagwe–Ankole Belt in eastern Burundi, as part of the Mesoproterozoic Kibaran tectonomagmatic event. Igneous minerals of the Muremera intrusions have been partly altered to hydrous and carbonated metamorphic assemblages, although in most cases, the original igneous textures are well-preserved. Rounded, subhedral cumulus olivine has been partially and pseudomorphically replaced by lizardite–magnetite mesh-rim and lizardite–brucite mesh-centre assemblages, while anhedral interstitial plagioclase has been replaced by chlorite–tremolite. A later and localized event results in prograde alteration to antigorite–magnetite–chlorite–talc–carbonate and talc–carbonate–chlorite assemblages. The rocks are inferred to have undergone at least three separate metamorphic/alteration events resulting in: AS1 – an early alteration assemblage (mesh-rim lizardite–magnetite) characterized by very low fluid/rock ratios and widespread distribution; AS2 – a later, widespread low-temperature retrogressive (mesh-centre lizardite–brucite) assemblage associated with abundant close-spaced parallel veins; AS3 – later, prograde (antigorite–magnetite) and AT4 (talc–chlorite–carbonate) assemblages associated with more localized shearing and higher fluid/rock ratios. The AS1 assemblage most likely represents deuteric alteration that occurred soon after intrusion and cooling. The AS2 assemblage may relate to a continuation of this cooling, or may be correlated with the regional upright D2 folding event, while the AS3 and AT4 alteration assemblages are most likely correlated with the N–S oriented D3 faulting episode linked to the distal East African Orogeny. Euhedral to subhedral chromite grains are essentially unaltered where enclosed in primary unaltered olivine, pyroxene or plagioclase, as well as in AS1 lizardite–magnetite and AS2 lizardite–brucite altered olivine or pyroxene. In samples which show alteration to AS3 antigorite–magnetite and AT4 talc–carbonate–chlorite assemblages, the chromite grains are zoned to Mg–Al-poor ferritchromite rims. The width and the composition of these alteration rims are related to the degree of alteration of the silicate assemblages. However, it is concluded that chromite found in rocks that have avoided late/localized AS3–AT4 alteration preserve their magmatic to late-magmatic chemical and textural characteristics, and can therefore be used in regional geological studies and exploration.

Reply to Comments by Fergus G. F. Gibb and C. Michael B. Henderson on 'Mafic-Ultramafic Sills: New Insights from M- and S-shaped Mineral and Whole-rock Compositional Profiles'

The comments by Fergus Gibb and Michael Henderson on a recent and some previous publications are most welcome and I am glad of the opportunity to clarify some of the issues they raise. Recent studies of chemical zonation in mafic dykes (Chistyakova & Latypov, 2009a, 2009b, 2010, 2011, 2012), dolerite sills (Latypov & Egorova, 2012; Egorova & Latypov, 2013) and layered intrusions (Latypov et al., 2011; Egorova & Latypov, 2012a, 2012b) have provided several new and important constraints on the initial stages in the development of intrusive bodies. In particular, we have found that the An content of plagioclase cores in the marginal zones of mafic sills and layered intrusions systematically increases inwards, indicating that these zones were not formed from a single pulse of parental magma of uniform composition. Rather, they resulted from an early stage of filling when the inflowing magma became increasingly more primitive. This and other findings resulted in an abandonment of Soret differentiation, a hypothesis that I had supported earlier for the origin of marginal reversals (Latypov, 2003a, 2003b), in favour of the ‘leading-edge fractionated magma’ hypothesis (see Latypov, 2014). This is quite an old concept in petrology that implies the initial emplacement of batches of evolved magma produced by three principal mechanisms: (1) the early stages of fractional melting without complete mixing in the mantle source; (2) magma stratification in a subjacent chamber; (3) fractional crystallization of magma while flowing through conduits. In accepting this hypothesis as an explanation for marginal reversals I referred to Morse (1979, 1981, 2008), who has been supporting this idea for many years for layered intrusions. I fully agree, however, that it was a grave omission not to mention the work of Gibb and Henderson, who proposed essentially the same process for mafic^ultramafic sills (Gibb & Henderson, 1978, 2006; Henderson & Gibb, 1987; Henderson et al., 2000). References to their publications would certainly have made our case even stronger. It should be noted that the conclusion about the formation of basal reversals by inflowing magma that gradually becomes more primitive is not trivial. No absolutely unequivocal evidence for this process has been forthcoming from petrological studies (including those by Gibb and Henderson) of mafic^ultramafic sills owing to their enrichment in olivine. This is because the primary composition of olivine is commonly altered to varying degree by reaction with interstitial melt, making this mineral unreliable for deciphering igneous processes. It is therefore understandable that most workers considered such sills as having formed from a single pulse of magma followed by gravitational accumulation of olivine (and/or pyroxene), either already present in the magma at the time of emplacement (Marsh, 1996; Ubide et al., 2012) or that grew in the intrusion itself (Frenkel’ et al., 1989; Worster et al., 1993; Ariskin & Yaroshevsky, 2006; Lo¤ pez-Moro et al., 2007). This crystallization model suggests that an upward increase in whole-rock MgO in basal reversals is a consequence of a decreasing solidification rate that ensures an increase in the amount of olivine able to settle to the bottom of the magma. In other words, basal reversals are considered to be mixtures of melt and cumulus olivine

Mineral chemistry and petrology of highly magnesian ultramafic cumulates from the Sarve-Abad (Sawlava) ophiolites (Kurdistan, NW Iran): New evidence for boninitic magmatism in intra-oceanic fore-arc setting in the Neo-Tethys between Arabia and Iran

The Sarve-Abad (Sawlava) ophiolitic complex consists of several tectonically dismembered ophiolitic sequences. They are located along the Main Zagros Thrust Zone, which marks the ophiolitic suture between the Arabian and Sanandaj–Sirjan continental blocks. They represent a portion of the southern Neo-Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Sarve-Abad ophiolites include cumulitic lherzolites bearing minor dunite and chromitite lenses in places. The main rock-forming minerals in ultramafic cumulates are cumulus olivine and inter-cumulus clinopyroxene and orthopyroxene. Minor (<5%) chromian spinel occurs as both cumulus and inter-cumulus phases.Cr#, Mg# and TiO2, Cr2O3, and Al2O3 concentrations of chromian spinel from ultramafic cumulates and chromitites plot in the forearc and boninite spinel fields, respectively. Clinopyroxene has very high Mg# and low TiO2 contents. Calculated TiO2 and Al2O3 compositions and Mg# in the parental melt that was in equilibrium with chromian spinel and olivine are consistent with supra-subduction zone-type compositions. Whole-rock geochemistry of the ultramafic cumulates is characterized by very low incompatible element content and a general enrichment in Th with respect to Ta and Nb. Chondrite-normalized REE patterns show different trends with either (La/Sm)N<1 and (Sm/Yb)N<1 or (La/Sm)N>1 and (Sm/Yb)N<1 (U-shaped pattern). Both these patterns are compatible with boninitic-type parental melts. Accordingly, petrogenetic modeling using REE composition indicates that Sarve-Abad ultramafic cumulates may have formed by small degrees (5–15%) of fractional crystallization from typical boninitic melts characterized by either light REE/medium REE depletion or enrichment.Mineral chemistry and whole-rock chemistry clearly indicate that the Sarve-Abad ultramafic cumulates and chromitites record an episode of boninitic magmatism that occurred within the southern Neo-Tethys Ocean during the Late Cretaceous. Boninitic melts in the Sarve-Abad ophiolites were formed by partial melting of depleted peridotite which made up the residual mantle after MORB-type melt extraction. This was subsequently enriched with light REE and large ion lithophile elements by subduction-derived fluids. It is therefore suggested that this boninitic magmatism was generated in the forearc sector of a short-lived intra-oceanic arc that was located southward with respect to the “Andean-type” subduction below the Sanandaj–Sirjan continental margin.

Mineralogy, geochemistry and petrogenesis of the Khopoli mafic intrusion, Deccan Traps, India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (eNdt = −9.0 to −12.7; 87Sr/86Sr = 0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.

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

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 (εNdivalues between –3.3 and –1.0) point to a lithospheric mantle source, whereas the most unradiogenic values imply enrichment inLREEby crustal contamination of the magmas.

Petrology and Geochemistry of the Crust–Mantle Boundary in a Nascent Arc, Massif du Sud Ophiolite, New Caledonia, SW Pacific

The Massif du Sud ophiolite, New Caledonia, SW Pacific, is one of the largest exposed ultramafic bodies on Earth. The ophiolite consists of a mantle section of ultra-depleted tectonite harzburgite, overlain by a large dunite zone, which is separated by a transition zone from the gabbros at the top of the massif. Profiles through the stratigraphy of the mantle section show complex geochemical and petrological variations. The harzburgites are characterized by a high degree of partial melting and document a complex evolution from mantle exhumation towards a supra-subduction zone environment. Olivine and spinel compositions suggest that the harzburgites are residual after boninite-like melt extraction at 5–10 kbar. The lower dunite zone is analogous to the replacive dunite channels in the harzburgite, in that it is formed by pyroxene consumption. In contrast, the upper dunite zone has chemical and textural characteristics of cumulus olivine crystallized from primitive mafic melts. The upper dunite zone grades into a transition zone with pyroxenite cumulates that are intruded by gabbro sills. The crystallization sequence and mineral compositions indicate that pyroxenites and gabbros formed from hydrous, oxidized primitive basaltic magmas at ∼1250°C and 2–4 kbar. Their geochemistry indicates that these parental melts are transitional between boninites and primitive arc magmas and carry a fore-arc basalt signature. The Massif du Sud therefore represents a crust–mantle section in a nascent arc. The cryptic transition between residual mantle rocks and crustal cumulates highlights the difficulty of estimating the thickness and average composition of an arc crust by seismic methods.

Mafic forearc cumulates and associated rocks in the central high-pressure belt of the Acatlán Complex of southern México: geochemical constraints

The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO2 (38–52 wt.%) and high Mg# (∼48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ϵ Nd(t) = 5.3–8.5 and 147Sm/144Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.

Textures and mineral compositions of the Xinjie layered intrusion, SW China: Implications for the origin of magnetite and fractionation process of Fe-Ti-rich basaltic magmas

The Xinjie layered intrusion in the Panxi region, SW China, hosts both Fe-Ti oxide and platinum-group element (PGE) sulfide mineralization. The intrusion can be divided, from the base upward, into Units I, II and III, in terms of mineral assemblages. Units I and II are mainly composed of wehrlite and clinopyroxenite, whereas Unit III is mainly composed of gabbro. PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in Unit III. An ilmenite-rich layer occurs at the top of Unit I. Fe-Ti oxides include magnetite and ilmenite. Small amounts of cumulus and intercumulus magnetite occur in Units I and II. Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene. They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%, indicating that they are likely an early crystallized phase from magmas. Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase. Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite. The intercumulus magnetite may have crystallized from the trapped liquid. Large amounts of magnetite in Unit III contains Cr2O3 (<0.28 wt.%) much lower than magnetite in Units I and II. The magnetite in Unit III is proposed to be accumulated from a Fe-Ti-rich melt. The Fe-Ti-rich melt is estimated to contain 35.9 wt.% of SiO2, 26.9 wt.% of FeOt, 8.2 wt.% of TiO2, 13.2 wt.% of CaO, 8.3 wt.% of MgO, 5.5 wt.% of Al2O3 and 1.0 wt.% of P2O5. The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions. Paired non-reactive microstructures, granophyre pockets and ilmenite-rich intergrowths, are representative of Si-rich melt and Fe-Ti-rich melt, and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma.

The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?

Research Article| January 01, 2013 The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both? Bénédicte Abily; Bénédicte Abily Toulouse University, CNRS–UMR 5563, GET, OMP, 31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Georges Ceuleneer Georges Ceuleneer Toulouse University, CNRS–UMR 5563, GET, OMP, 31400 Toulouse, France Search for other works by this author on: GSW Google Scholar Author and Article Information Bénédicte Abily Toulouse University, CNRS–UMR 5563, GET, OMP, 31400 Toulouse, France Georges Ceuleneer Toulouse University, CNRS–UMR 5563, GET, OMP, 31400 Toulouse, France Publisher: Geological Society of America Received: 27 Feb 2012 Revision Received: 30 Jun 2012 Accepted: 16 Jul 2012 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2012 Geological Society of America Geology (2013) 41 (1): 67–70. https://doi.org/10.1130/G33351.1 Article history Received: 27 Feb 2012 Revision Received: 30 Jun 2012 Accepted: 16 Jul 2012 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Bénédicte Abily, Georges Ceuleneer; The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?. Geology 2013;; 41 (1): 67–70. doi: https://doi.org/10.1130/G33351.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The contact between mantle peridotite and gabbro from the lower oceanic crust is usually underlined by a horizon of dunite. The origin of this dunitic transition zone (DTZ) is still debated. It is viewed either as a pile of cumulus olivine from high-MgO melts, or as former mantle peridotite pervasively percolated by melts undersaturated with pyroxene (e.g., as mid-oceanic ridge basalts [MORBs] at low pressure), and transformed into dunite. We show that the two hypotheses are not mutually exclusive, although they do not account for the same parts of the DTZ. We determined a petrological profile through a 330-m-thick DTZ that developed at the top of a mantle diapir in the Oman ophiolite. The lowermost 280 m have a reactional origin: olivine and Cr-spinel record the complex percolation and interaction history between mantle peridotite and MORB. In the uppermost 50 m, chemical trends become consistent with a cumulus origin of the dunite, olivine crystallization being a prelude to the crystallization of the overlying gabbros. The DTZ develops largely in response to melt-rock reaction, consistent with the “reactive filter” hypothesis, but the proportion of cumulate dunite is high enough to require parent melts with a significantly higher Mg content than the most primitive MORB erupted on the seafloor. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High-Mg low-Ni olivine cumulates from a Pan-African accretionary belt in southern India: Implications for the genesis of volatile-rich high-Mg melts in suprasubduction setting

We report petrologic, geochemical, fluid inclusion and stable isotopic data from an unusual spinel dunite occurring within metasedimentary units of the Late Neoproterozoic–Cambrian accretionary belt in the southernmost part of the Southern Granulite Terrain in India. The volatile-rich dunite is mainly composed of cumulus olivine with intercumulus ilmenite, phlogopite, spinel, graphite, Ni-bearing pyrrhotite and calcite. The olivine is Mg-rich (Fo∼96) and unusually poor in Ni. Spinel, even though a minor component, is Mg–Al type (Mg#∼0.92) and is depleted in Cr (Cr#<0.01). Phlogopite is highly magnesian (Mg# around 0.95–0.97) with high K/(K+Na) ratios of 0.93–1.00 similar to phlogopite reported from mantle-derived peridotites. Ilmenite is dominantly geikielite with Mg# ranging from 0.56 to 0.62. The calcite in this rock exhibits LREE-enriched nature with a pronounced negative Eu anomaly. Carbon stable isotope analyses on the calcite and graphite from the dunite yielded δ13C compositions comparable with mantle values. Raman peaks of matrix and inclusion graphites show highly crystalline character. The data reported in this study suggest formation of the spinel dunite from an Mg-rich melt enriched in C–O–H–S volatiles with low Ni and Cr. The silica activity appears to have been too low for the crystallization of plagioclase, pyroxenes and amphiboles. The formation of such a melt depleted in Cr, Ni and Fe suggests an early removal of Fe–Ni sulfide and Cr-rich spinel resulting in the depletion of Cr, Fe and Ni and enrichment of Mg and Al in the residual melts from which the high-Mg olivine and Mg–Al spinel cyrstallized. Fe and Ni partitioned to sulfide phase rather than olivine in shallow upper mantle/deep lower crustal levels under high sulfur fugacity. The spinel dunite of present study, together with similar ultramafic cumulate rocks occurring as dismembered units in other localities of the Neoproterozoic–Cambrian accretionary belt in southern India probably represents partial melting under the presence of volatiles in a mantle wedge within a suprasubduction setting.

Petrogenetic implications of mineral chemical data for the Permian Baima igneous complex, SW China

The Late Permian Fe-Ti oxide ore-bearing Baima igneous complex (BIC) is one of three gabbro-granitoid complexes with the Emeishan large igneous province. Mineral compositions are determined for the BIC layered gabbro in order to constrain subsolidus and magma chamber processes. The averaged compositions of cumulus olivine, clinopyroxene and plagioclase within individual samples range from Fo65-76, Mg# = 75 to 82 and An49-64 but they are not correlative. The observed mineral compositions are consistent with those modeled using the pHMELTS program. Highly variable magnetite compositions are consistent with extensive subsolidus re-equilibration and exsolution. The occurrence of reversely-zoned granular olivine in Fe-Ti oxide ores is a manifestation of Mg transfer between Fe-Ti oxides and olivine at relatively high (<1150 °C) subsolidus temperatures. The primary oxide is inferred to be an aluminous titanomagnetite. Similar to other layered intrusions in the region, the gabbroic unit of the BIC displays Zr depletion which is consistent with loss of a residual liquid during magma differentiation. If the most Zr-rich syenites of the complex are taken into account, the Zr budgets between the combined gabbro-syenite and the basalts are similar. This indicates that the BIC most likely represents a closed system in terms of magma extraction.

Chromite symplectites in Mg-suite troctolite 76535 as evidence for infiltration metasomatism of a lunar layered intrusion

Despite the very low chromium concentrations in its cumulus olivine (∼140ppm), lunar troctolite 76535 contains large amounts of Cr sporadically, but highly concentrated, in symplectite assemblages consisting of Mg–Al-chromite and two pyroxenes. Previously proposed symplectite formation mechanisms include crystallization of trapped interstitial melt, diffusion of Cr from cumulus olivine, and/or remobilization of cumulus chromite grains. These mechanisms would imply that the highly Cr-depleted nature of Mg-suite parental magmas and their source materials inferred from cumulus olivine may be illusory. We have conducted a detailed petrologic and textural study of symplectites, as well as chromite veins, intercumulus assemblages, olivine-hosted melt inclusions and clinopyroxene-troilite veins in 76535 with the goals of constraining the origin of the symplectites, and the degree of Cr-depletion in Mg-suite magmas relative to other lunar basalts. Orthopyroxene and clinopyroxene in melt inclusions are depleted in Cr relative to their symplectite counterparts, averaging 900 and 1200ppm vs. 7400 and 8100ppm Cr2O3, respectively. Olivine in contact with symplectite assemblages may exhibit a diffusion profile of Cr going into olivine, whereas olivine boundaries away from symplectites show no diffusion profile. There is also a distinct lack of primary chromite as inclusions in cumulus phases and melt inclusions. Multiple textural observations, melt inclusion chemistry, and modeling of chromite–olivine equilibrium rule out previously proposed symplectite formation mechanisms, and strongly suggest that chromite was not a primary crystallization product of the 76535 parental magma. Accordingly, the post-cumulus addition of Cr and Fe is required to produce the symplectites. After considering multiple models, the addition of Cr and Fe to 76535 via infiltration metasomatism by an exogenous chromite-saturated melt is the model most consistent with multiple textural and geochemical observations. Failure of models that call upon Cr diffusion out of olivine grains imply that the observed Cr-depleted nature of olivine observed in many Mg-suite lithologies is a primary feature of the Cr-depleted nature of the Mg-suite parental magmas and their source materials. This substantial depletion of Cr in the magma relative to mare basalt magmas still requires a satisfactory explanation in order to be consistent with Mg-suite petrogenetic models and currently accepted bulk-Moon compositions. Additionally, if the intimate interaction of migrating melts with early lunar crustal lithologies was a widespread phenomenon after LMO solidification, it provides another mechanism by which to reset or delay closure of radiogenic isotopic systems and explain the Mg-suite-ferroan anorthosite age overlap.

On the origin of multi‐layer coronas between olivine and plagioclase at the gabbro–granulite transition, Valle Fértil–La Huerta Ranges, San Juan Province, Argentina

AbstractTroctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at &lt;840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at &lt;770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.

High-precision U–Pb age and geochemistry of the mineralized (Ni–Cu–Co) Suwar intrusion, YemenThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision U–Pb isotope dilution – thermal ionization mass spectrometry (ID–TIMS) geochronology on chemically abraded zircon grains from a noritic gabbro of the Ni-bearing Suwar mafic–ultramafic layered complex, northwestern Yemen, gives a mean 206Pb/238U age of 638.46 ± 0.73 Ma (2σ; MSWD = 1.4). At Wadi Qutabah, ∼30 km to the north, a similar mafic sample has an identical age of 638.58 ± 0.51 Ma (2σ; MSWD = 0.32), which supports the possibility of there being a single, large intrusive complex with an estimated areal extent of ∼250 km2. This is supported by geochemical data of samples from each locality, which are postkinematic, gabbroic rocks that contain variable amounts of cumulus olivine, plagioclase, orthopyroxene, and ilmenite with intercumulus augite, hornblende, and Ni-sulphides. Straight rare Earth element (REE) patterns, Ba/La ∼30, Rb/Ba ∼0.04, and negative primitive-mantle-normalized P anomalies resemble EM1 (Enriched Mantle 1) of oceanic island basalts and Archean subcontinental mantle lithosphere. The mineralogy and magmatic/tectonic discrimination diagrams suggest within-plate continental tholeiitic and noritic cumulates typical of a rift setting for both intrusions. The complex intrudes retrograded, amphibolite-facies paragneiss of the Pan-African Afif lithotectonic terrane, and is generally undeformed and unaltered, and, therefore, unaffected by the Pan-African orogeny. Emplacement of the 639 Ma complex occurred during a tensional tectonic regime on the Arabian Peninsula and marks the time of proto-Iapetan rifting. The estimated size of the intrusion, its noritic composition, and Archean subcontinental lithospheric mantle signature and its position in thin Proterozoic lithosphere abutting Archean cratonic rocks, give it the key characteristics of known, mostly Proterozoic, intrusions that host world-class Ni–Cu–Co ore deposits.