Assimilation Of Country Rocks Research Articles

Formation of PGM-Cu-Ni deposits in the process of evolution of flood-basalt magmatism in the Noril’sk region

The timing of the emplacement of ore-bearing melts in the process of evolution of flood-basalt magmatism in the Noril’sk District is discussed. The current models of ore formation consider the emplacement of ore-bearing intrusions either under the conditions of a closed magmatic system as a product of a self-dependent magmatic event, or under the conditions of an open magmatic system, where intrusions are parts of the conduits feeding lava flows. In both cases, the composition of the initial magma, the content of volatile components therein, and the contribution of country rock assimilation are important for the development of a genetic model. The relationships between lavas and intrusions are exemplified in the South Maslov intrusion, which cuts through the rocks of the Nadezhdinsky Formation. No geological evidence for links of lavas to intrusions has been established. Substantial difference in geochemistry (Ti contents, Gd/Yb and La/Sm ratios, etc.) of the tuff and lava sequence on the northern shore of Lake Lama and the Maslov intrusions are demonstrated. It is concluded that the Noril’sk deposits were formed as products of emplacement of self-dependent portion of magma in the post-lower Nadezhdinsky time. The melt composition determined from melt inclusions in olivine corresponds to high-Mg tholeiitic basalt (up to 7–8 wt % MgO) containing up to 1 wt % H2O and 0.3 wt % Cl and undersaturated with sulfur. The fluid regime of flood-basalt volcanism had no anomalous features—the fluid was aqueous-carbon dioxide. The melts of ore-bearing and barren intrusions had similar concentrations of volatile components. The distribution of major and trace elements in intrusive rocks of the contact zone with the lower part of the Nadezhdinsky Formation characterized by high (La/Sm)N ratio in comparison with gabbroic rocks (2.8–2.3 and 1.3–1.6, respectively), indicates that contamination of the initial melt only took place in a narrow (1 m) contact zone or did not develop at all. New data on isotopic compositions of Sr (87Sr/86Sr)251 = 0.7089 and Pb (206Pb/204Pb = 20.877–24.528 in anhydrite confirm that local assimilation did not play a substantial role in the formation of rock and ores. On the basis of chemical composition of ore-forming intrusions, their isotopic characteristics, and the composition of melt inclusions in olivine, it is suggested that the lower crustal rocks were a major source of ore-bearing magmas.

Magma chamber stability in arc and continental crust

The growth and thermo-mechanical stability of magma chambers in Earth's crust dictate the dynamics of volcanism at the surface, and the organization of volcanic plumbing at depth. We analyze a model of magma chamber evolution in which volumetric growth is governed by the mechanical focusing of rising dikes by the magma chamber, “magmatic lensing,” as well as melting and assimilation of country rock. This modeling framework emphasizes the two-way coupling between chamber stresses and thermal evolution with specific compositions of intruding magma and country rock. We consider as end member compositional scenarios a “wet” environment magma chamber, in which basalt with 2 wt.% H 2O intrudes an amphibolitic country rock, and a “dry” chamber consisting of anhydrous basalt intruding tonalitic country rocks. Magma chambers that erupt, freeze, or reach dynamic equilibrium in the crust occupy distinct regions of a parameter space that measures the relative importance of depth, chamber pressurization, wall rock viscoelastic rheology, and thermal viability. Lower crustal melt flux is the most important factor controlling chamber stability, but chamber depth and composition also help determine long-term dynamical behavior. In general, interactions between thermal and mechanical processes exert first-order control on chamber stability, defining four distinct regimes of magma chamber dynamics. In addition to thermally and mechanically unstable (freezing and eruptive) chambers, we find steady-state thermally viable chamber volumes are possible as well as a range of parameters for which chamber growth is roughly exponential in time and mechanically stable (no eruption occurs). Long-lived (> 1 Ma) chambers generally result from lower crustal melt flux values that range from ∼ 10 − 4 to ∼ 10 − 1 m 3/m 2/yr for 20 and 40 km deep chambers and both compositional end members used in this study. However chambers become considerably less stable in cool shallow environments, particularly with anhydrous compositions of magma and country rock. Model predictions in this framework suggest that a range of observed intrusive structures in Earth's crust may be the result of magma chambers in different, clearly defined dynamical regimes.

Geochemical and isotopic studies of the Lady of the Lake Intrusion and associated tobacco root Batholith: Constraints on the genetic relation between Cretaceous mafic and silicic magmatism in Southwestern Montana

Small volumes of alkalic mafic intrusions are spatially associated with Cretaceous to Early Tertiary granodioritic to granitic intrusions in the batholithic province of southwestern Montana. The mafic rocks generally occur near the contacts of the Boulder, Pioneer, and Tobacco Root Batholiths with country rocks, but their genetic relation with the batholiths is uncertain. The Lady of the Lake Intrusion is a small layered body composed of melagabbro and gabbro that occurs along the south-central margin of the Tobacco Root Batholith near its contact with Archean country rocks. A diorite unit, spatially distinct from the granodiorite/quartz monzonite of the Batholith intrudes the gabbroic rocks of the Lady of the Lake Intrusion. Zircon crystals from the melagabbro and diorite units give U–Pb ages that are very similar to that of the Tobacco Root Batholith at 74.88±0.17 Ma and 76.24±0.08 Ma, respectively. Mineral chemistry, whole rock major and trace element compositions, and oxygen and sulfur isotope ratios have been utilized to evaluate the genetic relation between the Lady of the Lake Intrusion, the diorite, and the Tobacco Root Batholith. No significant variation in the composition of clinopyroxene is observed in different rock units of the Lady of the Lake Intrusion. Minor olivine with Fo64 in the melagabbro unit is interpreted to represent early crystallization in the base of the intrusion. Whole rock major and trace element compositions, as well as results from modeling using the MELTS program, are consistent with the premise that the diorite was produced by fractional crystallization of the same magma that was parental to the gabbros of the Lady of the Lake Intrusion. Both whole rock chemistry and oxygen isotopes support the interpretation that the parental magma was an uncontaminated mantle-derived basaltic magma. In contrast, trace element and oxygen isotopes indicate that the quartz monzonitic and granodioritic rocks of the Tobacco Root Batholith and the gabbroic rocks of the Lady of the Lake Intrusion are not related to the same parental magma. Elevated d18O values of quartz (9–10‰) and feldspar (8–9‰) from the Batholith are consistent with an origin involving partial melting of Archean and Proterozoic country rocks that was initiated by heat supplied by basaltic magma, similar to that which was parental to the Lady of the Lake Intrusion. d34S values of the melagabbro range from 0.1 to 0.7‰, and support the premise that the mantle-derived parental magma was uncontaminated. In contrast, d34S values of the gabbro and diorite units are elevated (0.6 to 6.5‰), and indicate that assimilation of country rock S occurred, primarily via volatile transfer. Sulfur contents of the Lady of the Lake Intrusion are low (<1000 ppm) and no evidence suggests that S was transferred to crustal melts that produced the Tobacco Root Batholith.

Petrology and geochemistry of nepheline syenite and related carbonate-rich rocks in the Pan-African Dahomeyide orogen, southeastern Ghana, West Africa

Nepheline syenite and carbonate-rich rocks known as the Kpong complex occur as deformed layers within the Pan-African Dahomeyide suture zone, in southeastern Ghana. The dominant phases in the nepheline syenite are nepheline and feldspar, whereas the carbonate-rich rock is dominated modally by Sr-rich calcite and annitic biotite. Biotite, calcite and nepheline have similar compositions in the nepheline syenite and the carbonate-rich rock. The rocks of the Kpong complex are enriched in alkalis, Ba, Sr, and Nb, have high LREE/HREE ratios, and show large depletions in Zr and Hf relative to primitive mantle. These features are characteristic of carbonatitic and related alkaline rocks worldwide. Whole-rock major elements in the Kpong complex with respect to SiO 2 in bivariate plots show coherent variations with linear trends. However, based on the low Mg number, low Cr and Ni contents in whole rocks, and on overall high TiO 2, FeO, rocks of the Kpong complex appear to have crystallized from an evolved parent, rather than primary mantle-derived magma. The ubiquitous disaggregated enclaves within the carbonate-rich horizons, the linear compositional arrays defined in binary variation diagrams, and overlaps in major modal mineral compositions all suggest magma assimilation of country rock was involved in the evolution of the Kpong complex.

Mingling of carbonate and silicate magmas under turbulent flow conditions: Evidence from rock textures and mineral chemistry in sub-volcanic carbonatite dykes, Chagatai, Uzbekistan

The Triassic Chagatai Complex, Uzbekistan, comprises explosive pipes and dykes, dominantly of silicocarbonatite composition, with cross-cutting relationships indicating multi-stage emplacement. Although the dykes have been reported as diamond-bearing, they have not previously undergone detailed investigation in terms of their mineral chemistry or rock texture. The xenolith-rich dykes contain irregularly-shaped microscopic magmatic enclaves of silicate composition within carbonatite magma and corroded microphenocrysts with crystal overgrowths that record synmagmatic geochemical disequilibrium. Quench crystals of apatite and aegirine, and anhedral baryte, which formed after corrosion of apatite and magnetite microphenocrysts but prior to formation of crystal overgrowths and mantles, indicate contemporaneous rapid undercooling. The anhedral baryte formed as a by-product of an oxidising hydrous reaction from Ba-rich biotite and pyrite to chlorite. The rock and microphenocryst textures suggest that mingling between two magmas occurred and a post-mingling mineral assemblage, including baryte, crystallised in a partially hybridised heterogeneous magma. An initial carbonatite mineral assemblage is identified as calcite + magnetite + apatite ± augite ± barium-rich biotite ± melilite ± pyrite. Changes in mineral chemistry of the carbonatite assemblage that are contemporaneous with the disequilibrium reaction textures suggest addition of a hydrous, Na–Si–Al-rich magma, and the mineral assemblage in the magmatic enclaves is similar to that of trachyte dykes in the Chagatai Complex. Using primarily rock textures and mineral chemistry, supported by mass balance calculations and isotope data, the silicate material is interpreted as a hydrous trachyte magma that had assimilated upper crustal material. The trachyte magma was entrained by carbonatite that was rapidly and turbulently ascending through the crust, shortly before emplacement as silicocarbonatite. The interpretation of magma mingling textures in the Chagatai Complex are unique amongst reported carbonatite occurrences: previously reported carbonate–silicate magma systems either formed globular textures or were interpreted as the products of assimilation of country rock only.

Stable isotope, fluid inclusion, and mineral chemistry constraints on contamination and hydrothermal alteration in the Uitkomst Complex, South Africa

The Uitkomst Complex is a satellite intrusion of the Bushveld Complex and contains the only mine currently producing Ni as the main product in South Africa. Previously reported sulfur isotopic data suggest that externally derived sulfur was important for the generation of sulfide mineralization in harzburgite units that comprise the lower portion of the Complex. We have determined oxygen isotopic compositions of minerals and variations in pyroxenes and plagioclase compositions, and analyzed fluid inclusions in quartz from highly altered zones in order to evaluate the importance of country rock assimilation and hydrothermal processes in the evolution of the complex. Oxygen isotopic compositions of pyroxene and olivine from the sulfide-bearing harzburgite in the Complex range from 5.2 to 5.9‰ and suggest that bulk assimilation of high- 18O country rocks by parental magmas did not occur or was minimal. Sulfur transfer to magma was accomplished via a fluid which did not perturb the oxygen isotope composition of the magma. Harzburgite, pyroxenite, and gabbronorite that overlie the ore-bearing harzburgites contain minor amounts of sulfide minerals that are characterized by mantle-like δ 34S values of 0 ± 2‰. Low abundances of sulfides in these units are consistent with segregation of cotectic proportions. Hydrothermal alteration is pronounced in the upper and lower marginal zones of the Complex. Secondary quartz and albite are common in the upper portion of the Main Gabbronorite unit. Measurements of fluid inclusion in quartz indicate that alteration took place at ~ 500 °C and involved a moderately saline fluid of 20 to 25 eq. wt.% NaCl. Oxygen and hydrogen isotopes of secondary silicate minerals are most consistent with the involvement of an evolved, long path-length meteoric water that infiltrated the upper and lower contacts zones of the Complex.

Geochemical constraints on the origin of sulfide mineralization in the Duke Island Complex, southeastern Alaska

Magmatic Cu‐Ni‐PGE sulfide mineralization associated with mafic to ultramafic igneous rocks is rare in convergent plate settings. However, recent exploration in the Duke Island Complex of southeastern Alaska has detected horizons of sulfide mineralization primarily in the olivine clinopyroxenite unit. The composition of the sulfides, recalculated to 100% sulfide, averages 0.48% Ni and 1.4% Cu. The Duke Island Complex was generated by the emplacement of multiple pulses of variably fractionated magma ranging in composition from picrite to ankaramite that originated in deeper staging chambers. The presence of sulfides in the complex indicates that fO2 conditions were more reduced than those commonly ascribed to arc‐related magmas. The mass of sulfide mineralization together with the sulfur isotopic values indicates that external sulfur was added to the parent magmas at staging chambers. Os isotopic data from sulfides and C isotopic compositions of graphite in the olivine clinopyroxenites also indicate that the parental magmas were contaminated by crustal material. However, oxygen isotopic values of clinopyroxene together with the REE data suggest that bulk assimilation of country rocks has not occurred and the contamination was selective, involving C‐ and S‐bearing fluids or graphitic sulfides. The observed enrichment in Cu and S isotopic values of the sulfides is best explained by assimilation of sulfides from metal‐bearing rock types such as black shales or volcanogenic massive sulfide deposits that occur in terranes above the mantle wedge.

Repeated recharge, assimilation, and hybridization in magmas erupted from El Chichón as recorded by plagioclase and amphibole phenocrysts

Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichón record multiple cycles of country rock assimilation, magma injection, hybridization, and mixing. Laser ablation ICP-MS and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning. These compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section (e.g. sieved or patchy regions). Amphiboles are frequently zoned in Al and Si, and, in two magmas, may have clinopyroxene rims. Interestingly, most plagioclase show multiple and repeated zoning patterns. Moreover, all magmas contain all zoning patterns and textures, and crystals with substantially different sequences of zones occur within mm of one another. The most reasonable explanation for the origin of these textures is a frequently recharged chamber. Plagioclase zones with increasing anorthite contents ( An) and decreasing 87Sr/ 86Sr ratios record injection by a hotter, possibly wetter, and more primitive magma (lower 87Sr/ 86Sr ratio). Zones with decreasing An and increasing 87Sr/ 86Sr ratios record assimilation of country rock and/or hybridization of the host and injected magmas; injection of hot magma may provide the heat for country rock assimilation. Changes in An without corresponding changes in 87Sr/ 86Sr ratio likely record slight variations in pressure or temperature during crystallization, or the far-field thermal effects of magma injection. Variations in 87Sr/ 86Sr ratio unaccompanied by Anzoning record assimilation of country rock. Amphibole zoning patterns also record periodic heating events; amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field. Bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed. Strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than ~ 500 years, thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval. Long-term compositional and isotopic homogeneity indicate that there is a balance between recharge, assimilation, and crystallization.

Re–Os and O isotopic variations in magnetite from the contact zone of the Duluth Complex and the Biwabik Iron Formation, northeastern Minnesota

Osmium and oxygen isotopic analyses of magnetite from the South Kawishiwi Intrusion (SKI) of the Duluth Complex and country rocks of the Biwabik Iron Formation (BIF) were undertaken to evaluate the potential importance of country rock assimilation in the genesis of PGE enrichment in the intrusive rocks, and to track the response of the Re–Os isotopic system to contact metamorphism of the iron formation. Oxygen isotopic values of magnetite from contact metamorphosed Biwabik Iron Formation and magnetite-rich oxide assemblages from highly contaminated troctolitic rocks range from 7.0 to 13.7‰, distinctly higher than oxide minerals from less strongly contaminated troctolitic to melatroctolitic rocks of the SKI that range from 2.3 to 6.9‰. 187Re/ 188Os ratios of metamorphosed BIF tend to be much lower than values from low-grade, weakly recrystallized BIF. Only one sample of BIF from the contact aureole shows the type of Os isotope ratios expected as a result of mixing with a chondritic fluid at 1.1 Ga, the time of emplacement of the SKI. Both low- 18O and high- 18O magnetite populations from the SKI are characterized by 187Re/ 188Os values less than 7.2 and Os isotopic ratios that plot above a 1.878 Ga reference isochron for the low-grade BIF samples. Although elevated γ Os values are consistent with the premise that the SKI was contaminated by crustal material, we suggest that the Os isotopic compositions reflect a later lowering of Re/Os ratios related to interaction with a fluid, potentially that which was involved in the locally extensive serpentinization of the melatroctolitic rocks. Oxygen and H isotopic values of serpentine and serpentinized melatroctolites indicate that the fluids responsible for hydrothermal alteration ( T ~ 300 °C) were low in 18O and characterized by a large meteoric water component. Magnetite δ 18O values record less than a 15% approach to equilibrium with the fluid involved in the serpentinization process, suggesting that perturbations to the Os isotopic system under hydrothermal conditions may occur in the absence of significant shifts in magnetite δ 18O values.

Sulfide saturation mechanisms in gabbroic intrusions in the Nipigon Embayment

The Seagull and Kitto intrusions in the Nipigon Embayment of northwestern Ontario were studied to gain an understanding of the processes involved in sulfur saturation of the mafic to ultramafic magmas leading to the formation of platinum group element (PGE) concentrations. Profiles of sulfur, copper, nickel, gold, palladium, and platinum concentrations as a function of depth revealed that sulfur saturation occurred at the base of the Seagull intrusion. A higher grade horizon occurs well above the base of the intrusion, suggesting that a reef-type of process was significant here and possibly in the Kitto intrusion, as well. Olivine compositions indicate that, in both cases, the parental magmas were undersaturated with respect to sulfur. Sulfur, neodymium–samarium, and rubidium–strontium isotopic data suggest that assimilation of country rock and sulfide played a role, especially in the formation of basal concentrations of PGEs.

Mid-Pleistocene silicic tephra beds in the Auckland region, New Zealand: Their correlation and origins based on the trace element analyses of single glass shards

The trace element composition of individual glass shards from twelve distinct mid-Pleistocene silicic tephra beds from the Auckland region, New Zealand, has been determined by laser ablation (LA) ICP-MS analysis. This is the first application of such data to tephra correlation studies in New Zealand. These analyses support earlier major-element based correlations between three surface sections (Oruarangi and Okariha at Port Waikato, and St Kentigern's at east Auckland) and a long core (Patiki Road, west Auckland). The correlations provide a higher resolution record of Taupo Volcanic Zone (TVZ) activity than is preserved in more proximal deposits, where burial and erosion obscure much of the volcanic record. A combination of major and trace element analysis enables the correlation of three units exposed in the Auckland region with their proximal equivalents (ignimbrites) from the TVZ, namely deposits associated with (1) the Ongatiti ignimbrite (1.21±0.04 Ma), (2) an un-named, post-Ahuroa/pre-Unit E ignimbrite (sample UT1274) from the Rocky Hill section, King Country and (3) the Unit E ignimbrite (∼1 Ma). Combined major and trace element data show a marked evolution in magma chemistry, with the large Ongatiti and Unit E ignimbrite eruptions occurring after a change in composition caused by replenishment/reactivation of the magma chamber. Changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase-dominated assemblage. Trace element data and the wider dispersal of the silicic tephras in the Auckland region suggests that they are all sourced from the Mangakino Volcanic Centre (MVC) in the TVZ. This study illustrates for the first time in a New Zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry.

Oxygen isotopic studies of the interaction between xenoliths and mafic magma, Voisey’s Bay Intrusion, Labrador, Canada

Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2–10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ 18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7–12.3‰), orthoclase (9.5–9.8‰), biotite (5.2–8.7‰), garnet (8.3–10.8‰), pyroxene (8.0–10.1‰), and quartz (9.6–14.0). The δ 18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ 18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ 18O values are significantly lower than expected through melt loss alone. The relatively low δ 18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ 18O value of ∼5.5‰ to 6.0‰. The high- 18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths ( δ 18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ 18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high- 18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.

Rb-Sr isotopic study of the Hwacheon granite in northern Gyeonggi massif, Korea: A case of spurious Rb-Sr whole rock age

The Yonghwasan pluton of the Hwacheon granite intruding the Precambrian metamorphic rocks of the northern Gyeonggi massif gives two apparent Rb-Sr whole rock isochrons depending on lithology: 177±11 Ma with an initial87Sr/86Sr of 0.71538±0.00031 for mica granite and 134±10 Ma with an initial87Sr/86Sr of 0.72183±0.00096 for garnet two-mica granite. However, the whole rock age for the garnet two-mica granite is much younger than mineral ages: Rb-Sr biotite (146.2±2.2 Ma) and muscovite (160.2±1.6 Ma), and reported CHIME monazite and K−Ar muscovite. Circumstantial evidences suggest that the young whole rock isochron may be the result from fortuitous alignment of data due to assimilation of country rocks, and thus meaningless, rather than resetting of whole rock Sr isotope ratios by later events such as post-emplacement heating and fluid infiltration. Considering the above, the best Rb-Sr whole rock age of the Yonghwasan pluton is estimated to be 172.0±6.8 Ma. The isotopic ages of this and previous studies for the garnet two-mica granite form a near linear cooling curve with a cooling rate of about 14.5°C/Ma.

Origin of chromitites in layered intrusions: Evidence from chromite-hosted melt inclusions from the Stillwater Complex

Chromitites from layered mafic intrusions are of great economic importance, yet the origin of these deposits remains enigmatic. We describe multiphase silicate inclusions trapped within chromite grains from the G chromitite seam of the Stillwater Complex, Montana, United States. These inclusions are interpreted to represent melt trapped during chromite growth and hence provide information on chromitite formation. Most reheated inclusions have variable quench textures and chemical compositions that are consistent with variable degrees of mixing between a high-Mg basaltic parental magma and a Na-rich trondhjemitic melt. The trondhjemite is suggested to derive from partial melting of mafic or metasedimentary country rocks. Based on these inclusions, we outline a model for chromitite formation involving ponding of a new pulse of primitive magma at the roof of the Stillwater magma chamber, followed by localized partial melting and assimilation of the country rock. The newly formed hybrid melts become oversaturated in chromite, leading to extensive chromite crystallization. Chromitite horizons are proposed to form from dense chromite-rich plumes that periodically sink down from the roof zone to settle out as layers at the basal cumulate mush zone. Numerous radiogenic isotope studies, together with the widespread occurrence of similar multiphase inclusions in chromite from other cumulate complexes, indicate that assimilation of country rock by primitive magma may be a critical mechanism for forming chromitites in many layered intrusions.

Petrology and geochemistry of the Shuiquangou syenitic complex, northern margin of the North China Craton

The Palaeozoic Shuiquangou complex is a major syenitic intrusion along the northern margin of the North China Craton that hosts a number of gold deposits. Despite the heterogeneity of the complex in terms of texture and modal mineralogy, mineral compositions, whole-rock chemistry as well as radiogenic isotope compositions show a well-constrained magmatic evolutionary trend. The whole-rock major element and REE trends are broadly consistent with fractional crystallization dominated by clinopyroxene, amphibole and accessory minerals. Initial Nd–Sr isotopic compositions suggest that the parent magmas to the complex were ultimately derived from an enriched mantle with relatively high initial 87 Sr/ 86 Sr ratios and negative ϵ Nd values. The enrichment of the mantle source was probably inherited from an older lithospheric mantle beneath the North China Craton. Sr isotopic data indicate that the Rb–Sr system was disturbed by later alteration processes, whereas Nd isotopes are essentially unaffected and exhibit a broad trend consistent with simultaneous fractional crystallization and country-rock assimilation (AFC). Quantitative modelling indicates that a very high bulk distribution coefficient for Nd and low ratios of assimilation to crystallization are needed to produce the observed Nd isotope evolutionary trend.

Mineral inventory of continuously erupting basaltic andesites at Arenal volcano, Costa Rica: implications for interpreting monotonous, crystal-rich, mafic arc stratigraphies

Except for the first 2 years since July 29, 1968, Arenal volcano has continuously erupted compositionally monotonous and phenocryst-rich (∼35%) basaltic andesites composed of plagioclase (plag), orthopyroxene (opx), clinopyroxene (cpx), spinel±olivine. Detailed textural and compositional analyses of phenocrysts, mineral inclusions, and microlites reveal comparable complexities in any given sample and identify mineral components that require a minimum of four crystallization environments. We suggest three distinct crystallization environments crystallized low Mg# (<78) silicate phases from andesitic magma but at different physical conditions, such as variable pressure of crystallization and water conditions. The dominant environment, i.e., the one which accounts for the majority of minerals and overprinted all other assemblages near rims of phenocrysts, cocrystallized clinopyroxene (Mg# ∼71–78), orthopyroxene (Mg# ∼71–78), titanomagnetite and plagioclase (An 60 to An 85). The second environment cocrystallized clinopyroxene (Mg# 71–78), olivine (<Fo 78), titanomagnetite, and very high An (∼90) plagioclase, while the third cocrystallized clinopyroxene (Mg# 71–78) with high (>7) Al/Ti and high (>4 wt.%) Al 2O 3, titanomagnetite with considerable Al 2O 3 (10–18 wt.%) and possibly olivine but appears to lack plagioclase. A fourth crystallization environment is characterized by clinopyroxene (e.g., Mg#=∼78–85; Cr 2O 3=0.15–0.7 wt.%), Al-, Cr-rich spinel, olivine (∼Fo 80), and in some circumstances high-An (>80) plagioclase. This assemblage seems to record mafic inputs into the Arenal system and crystallization at high to low pressures. Single crystals cannot be completely classified as xenocrysts, antecrysts (cognate crystals), or phenocrysts, because they often contain different parts each representing a different crystallization environment and thus belong to different categories. Bulk compositions are mostly too mafic to have crystallized the bulk of ferromagnesian minerals and thus likely do not represent liquid compositions. On the other hand, they are the cumulative products of multiple mixing events assembling melts and minerals from a variety of sources. The driving force for this multistage mixing evolution to generate erupting basaltic andesites is thought to be the ascent of mafic magma from lower crustal levels to subvolcanic depths which at the same time may also go through compositional modification by fractionation and assimilation of country rocks. Thus, mafic magmas become basaltic andesite through mixing, fractionation and assimilation by the time they arrive at subvolcanic depths. We infer new increments of basaltic andesite are supplied nearly continuously to the subvolcanic reservoir concurrently to the current eruption and that these new increments are blended into the residing, subvolcanic magma. Thus, the compositional monotony is mostly the product of repetitious production of very similar basaltic andesite. Furthermore, we propose that this quasi-constant supply of small increments of magma is the fundamental cause for small-scale, decade-long continuous volcanic activity; that is, the current eruption of Arenal is flux-controlled by inputs of mantle magmas.

Petrology of the Renard igneous bodies: host rocks for diamond in the northern Otish Mountains region, Quebec

The Renard igneous bodies were discovered in late 2001 as part of a regional diamond exploration program launched by Ashton Mining of Canada and SOQUEM. Nine bodies have been discovered within a 2-km-diameter area, and are comprised of root zone to lower diatreme facies rocks including kimberlitic breccia, olivine macrocrystic hypabyssal material, and brecciated country rock with minor amounts of kimberlitic material. Many mineralogical and petrographic features are common to both kimberlite and melnoite, and strict assignment of the rocks as kimberlite is not possible with these criteria alone. Whole rock trace element compositions suggest a closer affinity to Group I kimberlite, with derivation from a garnet-bearing mantle. Exceptions to conventional classification of the rocks along petrographic or mineralogical lines may be due in part to assimilation of felsic country rock into the Renard magmas at the time of emplacement. The Renard magmas were emplaced into northeastern Laurentia at 630 Ma, when the supercontinent was undergoing a change from convergent margin magmatism to rifting, the latter being associated ultimately with the opening of the Iapetus ocean.

Sulfur Isotope Study of Precambrian Basement and Mesozoic Intrusive Rocks in the Southwestern Part of Ryeongnam Massif, Korea

Abstract. Isotope composition of whole rock sulfur has been measured on 14 schists, 10 gneisses, 7 gabbroids, 7 granitoids and 2 sedimentary rocks, with of 9 sulfide (pyrite) sulfurs in gabbros and granitoids, from the southwestern part of the Ryeongnam Massif, Korea. The δ34S values of schists range from ‐4.6 to +6.1 % (average +0.9 %), those of gneisses from ‐4.0 to +0.8 % (‐1.9%), those of gabbroids from ‐2.3 to +3.7 % (+1.0 %), and those of granitoids from ‐5.9 to +3.2 % (‐1.9 %). The δ34S values of pyrite separated from gabbros and granitoids show rather heavier values ranging from +3.1 to +9.4 % with an average of+5.8%.Though the δ34S values of whole rock sulfur give wide range of ‐5.9 to +6.1 %, the average of about ‐0.5 % is close to the mantle value. The granitoids sampled at the central parts of intrusive bodies or at the contacts with other plutonic rocks tend to show positive values, while those sampled near the boundary with basement rocks such as granitic gneiss and por‐phyroblastic gneiss show negative values. Though the reason of this tendency is not clear at present, the δ34S values of some granitoids in this area seem to represent possible influence by the assimilation of country rocks, particularly of gneisses.Average isotopic compositions of ore sulfur from individual metal deposits in the studied area are summarized to have a range of+1.0 to +7.8 % with an average value of+3.2 %. The values are consistent with the previous finding that the ore sulfur isotopic values of the Ryeongnam Massif are the lowest among the four tectonic belts in Korea; Gyeonggi Massif, Ogcheon Belt, Ryeongnam Massif, and Gyeongsang Basin. This feature may reflect the isotopic compositions of plutonic rocks and basements in this area, which are characterized by relatively low values around zero permil.

Contaminating the lower crust in the Oman ophiolite

Contamination of magmas, via assimilation of country rock, is widely recognized as an important process in the thermal and chemical evolution of magmas emplaced into continental crust. Its importance at mid-ocean ridges has, however, received much less consideration. Lavas from the fast-spreading East Pacific Rise show evidence for contamination through an enrichment in seawater-derived chlorine that is not observed in basalts from slow-spreading ridges. In this study magmatic amphiboles in plutonic rocks from the Oman ophiolite—an intermediate- to fast-spreading ridge analogue—have been analyzed to determine the extent of this contamination in the lower oceanic crust. These data suggest an enrichment in chlorine throughout much of the lower ocean crust. If this enrichment is representative of lower oceanic crust formed at typical intermediate- to fast-spreading ridges, assimilation may be a more important process at mid-ocean ridges than previously recognized.

Spatial and temporal variations in the geochemistry of komatiites and komatiitic basalts in the Abitibi greenstone belt

Komatiites and komatiitic basalts in the Abitibi greenstone belt exhibit pronounced spatial and temporal variations in major and trace element geochemistry that provide important constraints on their petrogenesis and tectonic setting. Ti-depleted komatiites exhibit high Al 2O 3/TiO 2 (25–35) and low [Gd/Yb] MN (∼0.6–0.8), suggesting derivation by (1) dynamic melting of a refractory source leaving a refractory harzburgite residue, similar to that which produced Gorgona picrites, or (2) relatively high degree melting of a garnet-rich source leaving a peridotite residue. Al-depleted–Ti-enriched komatiites exhibit low Al 2O 3/TiO 2 (6–14) and high [Gd/Yb] MN (∼1.2–2.0), suggesting derivation by partial melting of a garnet peridotite source leaving a garnet-bearing residue. Al-undepleted komatiites exhibit moderate Al 2O 3/TiO 2 (15–25) and moderate [Gd/Yb] MN (∼0.8–1.2), suggesting derivation by shallower or higher-degree partial melting of a garnet peridotite source leaving a garnet-free residue. Those in the 2750–2735 Ma Pacaud assemblage are all Ti-depleted komatiites, those in the 2725–2720 Ma Stoughton–Roquemaure assemblage are predominantly Al-depleted–Ti-enriched komatiites with lesser Al-undepleted komatiites, those in the 2718–2710 Ma Kidd–Munro assemblage are predominantly Al-undepleted komatiites with lesser Al-depleted–Ti-enriched komatiites, and those in the 2710–2703 Ma Tisdale assemblage are predominantly Al-undepleted komatiites. Some of the komatiites in the younger assemblages (Kidd–Munro and Tisdale) are enriched in highly incompatible lithophile elements, exhibit low [Nb/Th] MN, and felsic-intermediate rocks in these assemblages contain inherited zircons, suggesting contamination by upper crustal rocks or sediments derived from upper crustal rocks. The high Mg contents of the komatiites suggest that they formed in mantle-derived plumes. The number of plumes required to generate the komatiitic magmatism in the Abitibi greenstone belt is not clear; it may be possible to derive all of the different magma types from different parts of a single plume at different times, but the long duration of komatiitic magmatism (≥47 My) suggests that multiple plumes are more likely. In any case, the temporal changes in composition are consistent with a decreasing influence of garnet in the source region of the plume(s) with time. There are several possible mechanisms to account of these variations, none of which are mutually exclusive: (1) a decrease in the depth of melt extraction as the plume(s) ascended; (2) a decrease in the amount of magma derived from deeper levels in the plume relative to that derived from shallower levels in the plume, possibly representing a change in plume morphology with time, for example as it was dragged by the overlying plate; (3) a decrease in the rate of plume ascent due to interaction with subducting slabs; or (4) a decrease in the thickness of the lithosphere during plume ascent, perhaps related to lithospheric thinning by the rising plume(s). The restriction of contamination to individual flow units or individual parts of flow units in the Kidd–Munro and Tisdale assemblages suggests that crustal contamination occurred locally, via assimilation of country rocks during emplacement.