Garnet is a common U-bearing mineral in various magmatic and metamorphic rocks with a high U–Pb closure temperature (> 850 °C), rendering it a potentially valuable U–Pb geochronometer. However, a ...high U (> 10 ppm) garnet reference material that suits both quadrupole and/or multi-collector inductively coupled plasma mass spectrometry (ICP-MS) is yet to be established. This study evaluates a potential reference material for in situ garnet U–Pb analysis with anomalously high U content from the Prairie Lake alkaline complex, Canada. The PL57 garnet, occurring in a calcite ijolite, has high TiO
2
(6.5–15.0 wt%, average 12.7 wt%) and Fe
2
O
3
(17.1–21.3 wt%) contents and is a member of the andradite (26–66 mol.%)-morimotoite (18–41 mol.%)-schorlomite (16–35 mol.%) solid solution series. Four samples were dated by U–Pb ID-TIMS to assess reproducibility. Twelve TIMS analyses produced concordant, equivalent results. Garnet PL57 yielded a concordant age of 1156.2 ± 1.2 Ma (2
σ
,
n
= 10, MSWD = 1.0), based on ten analyses with two results discarded due to possible mineral inclusions (if included, the concordia age is 1156.6 ± 1.8 Ma;
n
= 12, MSWD = 2.0). PL57 had 27–76 ppm (average 41 ppm) U with Th/U of 0.51–0.68 (average 0.63). The total common Pb content ranged from 0.4 to 3.9 pg (average 1.1 pg). Laser ablation coupled with ICP-MS and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging provide direct evidence that U is incorporated and homogeneously distributed within the garnet lattice rather than as defects or pore spaces. Published garnet samples and standards were then tested by calibrating the Willsboro, Mali, Qicun, and Tonglvshan garnet against PL57, which gave accurate ages within the recommended values. Case studies of garnet from the Archean Musselwhite orogenic gold deposit in Canada and the Cenozoic Changanchong and Habo skarn deposits in China yield reliable ages. This suggests that PL57 is a robust U–Pb isotope reference material. The limited variations of U and Pb isotopic ratios, together with the high U concentration and extremely low initial common Pb, make PL57 an ideal calibration and monitor reference material for in situ measurements.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Kimberlite sills emplaced in granite located near the town of Wemindji (Quebec, Canada) range from 2 cm to 1.2 m in thickness. The sills exhibit a wide variation in macroscopic appearance from ...fine-grained aphanitic dolomitic hypabyssal kimberlite to ilmenite/garnet macrocrystal hypabyssal kimberlite. Diatreme or crater facies rocks are not present. Multiple intrusions are present within the sills, and graded bedding and erosional features such as cross-bedding are common. The sills exhibit a wide range in their modal mineralogy with respect to the abundances of spinel, apatite, phlogopite and dolomite. Olivine is the dominant macrocryst, with an average composition of Fo
90
. Garnet macrocrysts are low chrome (2–3 wt. %) pyrope (G1/G9 garnet). Ilmenite occurs as rounded macrocrysts (7–13 wt. % MgO). Phlogopite microphenocrysts are Ti-poor and represent a solid solution between phlogopite and kinoshitalite end members. Spinel compositions mainly represent the Cr-poor members of the qandilite–ulvöspinel–magnetite series. The principle carbonate comprising the groundmass is dolomite, with lesser later-forming calcite. Accessory minerals include apatite, Sr-rich calcite, Nb-rich rutile, baddeleyite, monazite-(Ce) and barite. While some of these accessory minerals are atypical of kimberlites in general, it is expected that differentiation products of an evolved carbonate-rich kimberlite magma will crystallize these phases. The Wemindji kimberlites offer insight into the process of crystal fractionation and differentiation in evolved kimberlite magmas. The macroscopic textural features observed in the Wemindji sills are interpreted to represent flow differentiation of a mantle-derived, very fluid, low viscosity carbonate-rich kimberlite. The diverse modes and textural features result entirely from flow differentiation and multiple intrusions of different batches of genetically related kimberlite magma. The mineralogy of the Wemindji kimberlites has some similarities to that of the Wesselton and Benfontein calcite kimberlite sills but differs in detail with respect to dominant carbonate (i.e. dolomite versus calcite), and the character of the rare earth-bearing accessory minerals (i.e. monazite-(Ce) versus rare earth fluorocarbonates).
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Exploitable or potentially exploitable deposits of critical metals, such as rare-earth (REE) and high-field-strength elements (HFSE), are commonly associated with alkaline or peralkaline igneous ...rocks. However, the origin, transport and concentration of these metals in peralkaline systems remains poorly understood. This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China, to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution. Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions, with increasing REE and HFSE concentrations as a function of increasing Na-index Na#, defined as molar Na/(Na+Ca) ratios. The Na-amphiboles (i.e., arfvedsonite) and aegirine-augites can be subsequently altered, or breakdown, to form hydrothermal aegirine during late- or post-magmatic alteration. Representative compositions analyzed by in-situ LA-ICPMS show that the primary aegirine-augites have high and variable REE (2194–3627 ppm) and HFSE (4194–16,862 ppm) contents, suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites. Compared to the primary aegirine-augites, the presentative early replacement aegirine (Aeg-I, Na# = 0.91–0.94) has notably lower REE (1484–1972) and HFSE (4351–5621) contents. In contrast, the late hydrothermal aegirine (Aeg-II, Na# = 0.92–0.96) has significantly lower REE (317–456 ppm) and HFSE (6.44–72.2 ppm) contents. Given that the increasing Na# from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism, a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems. The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits. The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Melting and dehydration of subducted oceanic slabs dominate element recycling within the subduction factory, but the role of recycled oceanic crust in the source of intraplate magmas is poorly ...understood. In situ zircon O‐Hf isotope data from two Early Cretaceous alkaline A‐type granites that are genetically related to large‐scale extension of eastern China in the Late Mesozoic (circa 125 Ma) yield low δ18O (1.8 ± 0.3‰ to 5.1 ± 0.3‰, 2σ) and positive εHf(t) (1.5 ± 1.2 to 17 ± 1.2, 2σ). This suggests the contribution of altered oceanic crust and an enriched mantle component in the source region. Elevated initial 87Sr/86Sr and 206Pb/204Pb ratios, and εHf(t) whole rock values with relatively constant εNd(t) values beyond the normal mantle array, require a component that underwent seawater interaction in the source of protolith. The geochemical data require a complex source region for the alkaline A‐type granites in NE China involving more than 40% recycled oceanic crust. This altered oceanic crust beneath the Late Mesozoic lithospheric mantle likely represents remnants of multiple subduction and collision events between microblocks from the Late Paleozoic to Early Mesozoic in northeastern China. Recycling of subducted oceanic crust represents a novel exotic source for the origin of alkaline A‐type granites in intraplate extensional settings.
Key Points
Sr‐Nd‐Pb‐Hf isotopes of two associated alkaline A‐type granites suggest that the source region underwent seawater interaction
Low‐δ18O and positive‐εHf(t) values of primary zircons require a novel source of altered oceanic crust
The novel hybrid source is a new model for the genesis of A‐type granites and the role of recycled oceanic crust
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A unique occurrence of orbicular ijolite is hosted in a matrix of contemporaneous holocrystalline ijolite at the 1.1Ga Prairie Lake Carbonatite Complex (Marathon, Ontario, Canada), and is the only ...known occurrence of this textural type in a rock of ijolitic composition. This mineralogical and petrological study of this orbicular ijolite highlights many of the differences from other rare occurrences of orbicular rocks described from carbonatites, granites, diorites and lamprophyres. The orbicules occur along distinct, densely packed bands in equigranular nepheline-rich ijolite and range up to 6cm in diameter. Macroscopically, the orbicules show variability in the mineralogy of their cores. Detailed imaging of the cores shows evidence of quench textures. Radial outward zoning is common near the cores with concentric banding occurring toward the margins of the orbicules. The mineralogy of the orbicules consists of: nepheline; diopside; calcite; apatite; andradite–melanite garnet; titanite; Fe-rich phlogopite; titaniferous magnetite; perovskite; with secondary natrolite, calcite and cancrinite. The mineralogy of the host ijolite is similar to that of the orbicules. Mineral compositions from the orbicular ijolite and the host ijolite are similar. Within the orbicules, anhedral minerals are found occurring in a ‘matrix’ of garnet throughout the distinct concentric bands. The textures within the concentric bands of the orbicules are best described as annealing recrystallization textures. The rims of the orbicules form interlocking crystals with the host ijolite resulting in near-indistinguishable boundaries. The orbicules are interpreted to represent interaction of a partially-crystallized quenched ijolitic melt, which was in contact with a second pulse of consanguineous ijolite magma. Immersion in the latter resulted in sub-solidus diffusion and annealing recrystallization. Orbicular textures were produced from previously formed quenched ijolite, which was recrystallized producing the monominerallic concentric layers sequentially from the margins toward the center of the orbicule. This proposed model for the formation of orbicular ijolite from Prairie Lake highlights the complexities of these rock types, and supports previous models of magma mixing during the later stages of carbonatite emplacement and crystallization.
•Orbicular ijolite hosted in a matrix of contemporaneous holocrystalline ijolite at the 1.1Ga Prairie Lake Carbonatite Complex•The orbicules represent interaction of a partially-crystallized quenched ijolitic melt, which was in contact with a second pulse of consanguineous ijolite magma.•Immersion in the latter resulted in sub-solidus diffusion and annealing of orbicules, in which previously formed quenched ijolite was recrystallized.•Highlights some of the intriguing features of carbonatitic/ijolitic magmatism, specifically magma mixing processes
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The Baerzhe Cretaceous (123.7 ± 0.9 Ma) peralkaline pluton underwent extensive fractional crystallization and extreme enrichment in incompatible elements including rare earth elements (REE) and the ...high field strength elements (HFSE). It is one of the largest resources of rare metals in China, containing approximately 100 million tons of ore with an average grade of 1.84 wt% ZrO2, 1.00 wt% REE2O3 (34% heavy rare-earth oxides), and 0.26 wt% Nb2O5. Zr, REE and Nb are mainly hosted by hydrothermal minerals, such as zircon, hingganite-(Y), monazite-(Ce), polycrase, pyrochlore, fergusonite and columbite. An integrated investigation of field geology, mineral textures and compositions of minerals and host granites was carried out to examine the evolution of the Baerzhe pluton and the roles of magmatic and hydrothermal processes in concentrating REE and HFSE. Most minerals in the intensively altered subsolvus granite show secondary textures or replaced pseudomorphs, such as the replacement of arfvedsonite by aegirine, zircon dissolution and reprecipitation, and the replacement of monazite-(Ce) and polycrase-(Y) by hingganite-(Y). Compositions of the key economic minerals and changes with respect to these alteration stages reveal evidence that hydrothermal alteration played a role in the mineralization of the pluton. In-situ analyses and element mappings suggest that large volume of metals were remobilized and redistributed during hydrothermal replacement, such as the replacing of monazite-(Ce) and polycrase by hingganite-(Y). It is suggested that subsolidus re-equilibration and hydrothermal alteration, in addition to magmatic fractionation, is critical for further concentrating REE and HSFE in peralkaline granitic systems.
•Enrichment of REE and HFSE in the Cretaceous Baerzhe peralkaline pluton.•Rare metals concentrated in the upper subsolvus granite with intensive alteration.•Remobilization and redistribution of rare metals during hydrothermal processes.•Subsolidus re-equilibration and alteration was critical to mineralization.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Kimberlites of Jurassic age occur in various parts of South Australia. Thirty-nine of these kimberlites, which are mostly new discoveries, were studied to characterize their structural setting, their ...petrography, and the composition of their constituent minerals. Although some of the kimberlites in South Australia occur on the Archean to Paleoproterozoic Gawler Block, most are part of a northwest-trending, semi-continuous kimberlite dike swarm located in the Adelaide Fold Belt. The kimberlites typically occur as dikes or sills, but diatremes are also present. In the Adelaide Fold Belt, diatremes are restricted to the hinge zones of regional-scale folds within thick sedimentary sequences of the Adelaidean Supergroup. Despite widespread and severe alteration, coherent and pyroclastic kimberlites can be readily distinguished. U-Pb and Sr/Nd isotopic compositions of groundmass perovskite indicate that all kimberlites belong to the same age group (177–197 Ma) and formed in a near-primitive mantle environment (87Sr/86Sr: 0.7038–0.7052, εNd: −0.07 to +2.97). However, the kimberlites in South Australia are compositionally diverse, and range from olivine-dominated varieties (macrocrystic kimberlites) to olivine-poor, phlogopite-dominated varieties (micaceous kimberlites). Macrocrystic kimberlites contain magnesium-rich groundmass phlogopite and spinel, and they are typically olivine macrocryst-rich. Micaceous kimberlites, in contrast, contain more iron- and titanium-rich groundmass phlogopite and less magnesian spinel, and olivine macrocrysts are rare or absent. Correlations between phlogopite and spinel compositions with modal abundances of olivine, indicate that the contrast between macrocrystic and micaceous kimberlites is primarily linked to the amount of mantle components that were incorporated into a compositionally uniform parental mafic silicate melt. We propose that assimilation of xenocrystic magnesite and incorporation of xenocrystic olivine from dunitic source rocks were the key processes that modified the parental silicate melt and created the unique hybrid (carbonate-silicate) character of kimberlites. Based on the composition of xenoliths and xenocrysts, the lithospheric mantle sampled by the South Australian kimberlites is relatively uniform, and extends to depths of 160–170 km, which is slightly below the diamond stability field. Only beneath the Eurelia area does the lithosphere appear thicker (>175 km), which is consistent with the presence of diamonds in some of the Eurelia kimberlites.
•South Australian kimberlites range from olivine macrocrystic to micaceous varieties.•Formation of diatremes is restricted to hinge zones of large-scale folds.•Groundmass mineral composition correlates with olivine xenocryst content.•Kimberlite composition is controlled by magnesite assimilation in mafic parental melt.•Lithosphere thickness variations can account for local presence of diamonds.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Paleo-to Neoarchean granitoid gneisses (ca. 3.30 to 2.49 Ga) are well preserved in the Western Superior Craton. Protoliths of these gneisses are mainly I-type granitoids characterized by high Sr/Y ...and La/Yb ratios and low Mg#, consistent with Archean tonalite-trondhjemite-granodiorites. Zircons from granitoid gneisses commonly contain three growth phases: inherited cores (zircon I), magmatic rims (zircon II) and outer rims that have undergone Pb-loss (zircon III). The 3.12 Ga to 2.86 Ga zircon I represent early crustal material, that was captured in younger zircons; zircon II preserve crustal re-working and younger crustal additions that are constrained between 2.85 to 2.72 and 2.69 to 2.65 Ga.
Zircon II contains both positive and negative εHf(t) values (−6.3 to +8.1), with both depleted-mantle and older crustal signatures. Half of the magmatic rims (II) are characterized by depleted mantle signatures with positive εHf(t) values representing juvenile crust-forming events, whereas the other half are characterized by recycled crustal signatures with negative εHf(t) values. εHf(t) results show that the North Caribou and the Island Lake terranes and the northern Uchi domain are isotopically more enriched than the southern Uchi, English River, Wabigoon and Winnipeg River terranes, suggesting the northern Uchi margin represents a major terrane boundary.
Based on mass balance calculations, large volumes of juvenile material at circa 3.0 Ga mixed with smaller amounts of older crust. The vast majority of the granites were derived from a source with about 50% mantle material during the peak crust formation events after 2.8 Ga. The decline in the volume of felsic magmatism in the later Archean is coeval with a reduced supply of both heat and material from depleted mantle sources. Combined with previously published geochemical, geochronological and isotopic data, this suggests an evolution in felsic magma sources consistent with crustal thickening.
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•This data, along with the Hf, suggests that the terranes have a shared history.•Lu–Hf isotopes suggest a combination of depleted mantle and an older source.•The mantle: crust interaction ratio was quantified as 5:5 base on Lu–Hf isotope.•An overthickened crust likely acted as the driver for crustal evolution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The time and processes of hydrothermal mineralization are long-standing problems in geology. This work addresses these questions with reference to the Maoniuping giant rare earth elements (REE) ...deposit (southwest China), which has rare earth oxides (REO) reserves of 3.17 million tons with an average grade of 2.95 wt%. Bastnäsite is the dominant economic mineral, occurring as four distinct paragenetic types in the Maoniuping syenite-carbonatite complex: (1) primary euhedral bastnäsite (type-A) in syenite, with isolated melt inclusions; (2) macro-crystalline tabular euhedral bastnäsite (type-B) in pegmatitic dikes, with a diverse variety of fluid inclusions; (3) fine-grained, anhedral veinlet-disseminated bastnäsite (type-C) in syenite; and (4) coarse-grained anhedral bastnäsite (type-D) in carbonatite dikes, occurring as veinlets or interstitial to calcite, fluorite, and barite. From the paragenetic and compositional variations, it is inferred that type-A bastnäsite is of primary magmatic origin, whereas the other three types have characteristics of hydrothermal origins. In situ LA-ICP-MS U-Pb geochronology of the four types of bastnäsite results in lower intercept ages of 28.2 ± 0.5 Ma (n = 95, MSWD = 5.10), 27.8 ± 0.4 Ma (n = 43, MSWD = 0.73), 26.8 ± 0.7 Ma (n = 50, MSWD = 0.83), and 25.8 ± 0.7 Ma (n = 55, MSWD = 1.70), respectively, which are consistent with the weighted average 206Pb/238U and 208Pb/232Th ages by 207Pb-correction method. Compositional variations of clinopyroxene and apatite from the associated syenite, pegmatitic and carbonatitic dikes indicate a genetic relationship of the Maoniuping alkaline complex. The compositions of clinopyroxene range from Ae44-67Di14-18Hd17-41 in pegmatitic dikes, Ae43-66Di6-20Hd21-38 in carbonatitic dikes to Ae68-90Di0-3Hd10-30 in syenite. Apatites in the pegmatitic and carbonatitic dikes have similar compositions with higher F, total REE, and Sr, and lower CaO contents than those in the syenite, which suggests a cogenetic origin for the associated pegmatite and carbonatite. Clinopyroxene and apatite compositions suggest that the pegmatitic melt might differentiate directly from the initial carbonatitic melt rather than the syenitic magma. The bastnäsite U-Pb geochronology and minerals data indicate continuous magmatic-hydrothermal evolution for the REE mineralization in the Maoniuping alkaline complex.
Recent discoveries of kimberlites in North America have revealed that different processes are involved in the generation of kimberlite magma. A multi-disciplinary approach combining mineralogical, ...petrological, geochemical, and geochronological methods is used to classify the kimberlites, investigate possible sources of magma and evaluate current tectonic models proposed for the generation of kimberlite magma. The two main study areas are 1) the diamond-poor Churchill kimberlite field (Nunavut); and 2) the highly diamondiferous Lac de Gras kimberlite field (NWT). The Attawapiskat kimberlite field, the Kirkland Lake kimberlite field and the Timiskaming kimberlite field (Ontario) are also included in this study.
The 55-56 Ma Diavik kimberlite cluster (NWT) have been classified as resedimented volcaniclastic > olivine-bearing volcaniclastic > mud-bearing volcaniclastic > macrocrystic oxide-bearing hypabyssal kimberlite > calcite oxide hypabyssal kimberlite > tuffisitic kimberlite breccia. Geochemical features of Diavik kimberlites include: 1) LREE enrichment, 2) large intra-field range in REE content, and 3) highly diamondiferous kimberlites at Diavik with primitive geochemical signatures.
The Churchill kimberlites are classified as sparsely macrocrystic, oxide-rich calcite evolved hypabyssal kimberlite and macrocrystic oxide-rich monticellite phlogopite hypabyssal kimberlite. Electron microprobe analyses of olivine, phlogopite, spinel and perovskite support this petrographical classification. Twenty-seven precise U-Pb perovskite and Rb-Sr phlogopite emplacement ages indicate that magmatism spans ~45 million years (225-170 Ma).
The crystallization ages and the Sr and Nd isotopic compositions of groundmass perovskite from a well-established, SE-trending Triassic-Jurassic corridor of kimberlite magmatism in Eastern North America (ENA) were determined to investigate the origin of this magmatism. The Sr isotopic results indicate that the Churchill (0.7032-0.7036) and Attawapiskat kimberlites (0.7049-0.7042) have unique isotopic compositions, while Kirkland Lake/Timiskaming perovskite have a larger range of 87Sr/86Sr ratios. This implies the derivation of kimberlite magma from two distinct sources in the mantle, a depleted MORB mantle source and a kimberlite magma with a Bulk Silicate Earth signature. The pattern of increasing 87Sr/86Srinitial with younging of kimberlite magmatism along the ~2000 km corridor of continuous Triassic/Jurassic magmatism could be explained from either a single or multiple hotspot track(s), responsible for the addition of heat required to generate small volume mantle melting of a kimberlite source.
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