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4.1 - Composition of the Continental Crust
... similar to factor ranges for Rtot and Rvis (Table S4 in Supporting Information S1). Only the average amounts of P and K, but barely, exceeded their respective average amounts for rocks of the upper continental crust (UCC; Rudnick & Gao, 2003; Table S5 in Supporting Information S1). The low relative amounts of most elements might have resulted from dilution by silica (not analyzed) especially in quartz in the weathered mature sedimentary rocks of the Colorado Plateau that provide numerous dust sources for UCRB dust (Reynolds et al., 2020). ...
... Bar plots of ratios of average trace elemental abundances in duston-snow (DOS; ALM matter) to respective Upper Continental Crust averages (UCC;Rudnick & Gao, 2003). Samples with anomalously high metal amounts were not used: Mo and Cu (106 and 215 ppm, respectively) in the WY16 Hoosier Pass sample; U, Th, and Y (71, 3,353, and 136 ppm, respectively) in the WY13 Grizzly Peak sample. ...
Atmospheric particulate matter (PM) as light‐absorbing particles (LAPs) deposited to snow cover can result in early onset and rapid snow melting, challenging management of downstream water resources. We identified LAPs in 38 snow samples (water years 2013–2016) from the mountainous Upper Colorado River basin by comparing among laboratory‐measured spectral reflectance, chemical, physical, and magnetic properties. Dust sample reflectance, averaged over the wavelength range of 0.35–2.50 μm, varied by a factor of 1.9 (range, 0.2300–0.4444) and was suppressed mainly by three components: (a) carbonaceous matter measured as total organic carbon (1.6–22.5 wt. %) including inferred black carbon, natural organic matter, and carbon‐based synthetic, black road‐tire‐wear particles, (b) dark rock and mineral particles, indicated by amounts of magnetite (0.11–0.37 wt. %) as their proxy, and (c) ferric oxide minerals identified by reflectance spectroscopy and magnetic properties. Fundamental compositional differences were associated with different iron oxide groups defined by dominant hematite, goethite, or magnetite. These differences in iron oxide mineralogy are attributed to temporally varying source‐area contributions implying strong interannual changes in regional source behavior, dust‐storm frequency, and (or) transport tracks. Observations of dust‐storm activity in the western U.S. and particle‐size averages for all samples (median, 25 μm) indicated that regional dust from deserts dominated mineral‐dust masses. Fugitive contaminants, nevertheless, contributed important amounts of LAPs from many types of anthropogenic sources.
... Interestingly, Au exhibited similar distribution pattern as Hg (AGM > CL > VG > LP > BS). Overall, the samples had Au concentrations greater than that of the mean composition of the upper continental crust (UCC; Rudnick and Gao 2004) followed by V (99.6%), Sc (81.41%), ...
... Percentage of the soil samples above the mean composition of upper continental crust (UC:Rudnick and Gao 2004) and Worldsoil average concentrations(Kabata-Pendias 2010) ...
Burkina Faso faces a major challenge of environmental degradation due to a booming of gold mining. To date, there is no available information regarding soil geochemical status of this rapidly transitioning land use from merely subsistence agriculture to market gardening and artisanal gold mining. Therefore, this case study investigates distribution of 13 potentially toxic elements in soil exposed to different land-uses. For that purpose, 226 topsoil samples were collected in a grid of 200 m × 200 m, and their pseudototal concentrations were determined by inductively coupled plasma mass spectrometer. Univariate statistics, multivariate and geostatistical techniques showed that chemical weathering of parent bedrocks contributed to La, Tl, Th, U and Ti distribution in the soil, whereas that of Au, Hg and Te were controlled by artisanal gold mining. Laterization of basaltic rocks appeared to be the main source of V, Ga and Sc. In contrast, spatial distribution of Sr and Ba might be attributed to application of inorganic fertilizers and agrochemicals and, to lesser degree, parent materials. The results of multivariate analyses were corroborated by the interpolated factor score maps. The high concentrations of V, Sc, Bi, Hg and Sb above the mean upper continental crust composition and world-average soil concentrations are likely to pose serious threats to human. The study showed that the soil geochemistry is affected by both geogenic and anthropogenic sources. Thus, understanding geochemical status of the soil is vital for developing sustainable agricultural practices and environmental protection schemes in the area.
... Relating our results to the UCC [80] and other studies across Romania [81] and Europe [82], several noticeable aspects stand out for each category of elements. ...
... In our study, there is a clear enrichment in REE, especially in LREE (Fig 3). The samples from Ditrău River show an average ∑REE of 399.5 mg�kg -1 , and the samples from Jolotca show an average ∑REE of 420 mg�kg -1 , while the average ∑REE for the UCC is 148.1 mg�kg -1 [80]. ...
Ditrău Alkaline Massif is one of the few syenitic Massifs in Europe subjected to mining exploration in the past, located in the Eastern Carpathians, Romania. The heterogenous petrography includes acid to ultrabasic rocks such as syenites, hornblendites, and diorites, making it the defining feature of the Massif. In this study, we analyze the river bed sediments of two rivers, Ditrău and Jolotca, draining the Ditrău Alkaline Massif to determine their geochemical composition, with particular interest in Rare Earth Elements. The analysis was carried out with various analytical methods, including Inductive Coupled Plasma Mass Spectrometry, powder X-ray diffractometry, and electronic microscopy for mineralogical analysis to determine the presence of heavy minerals and quantify the concentration of Rare Earth Elements in the river sediment samples. The results indicate the existence of heavy minerals and Rare Earth Elements in bearing minerals such as Monazite and Epidote. High concentration values of Light Rare Earth Elements are identified, with values more than double compared to the Upper Continental Crust in some cases, of which stands out Cerium with 175.47 mg·kg⁻¹ and Lanthanum with 108.32 mg·kg⁻¹. Most samples share three main minerals: Quartz, K Feldspar, and Albite, while Diopside is only present in the Jolotca sediment samples, and Plagioclase exists in Ditrău samples. Moreover, many identified trace elements, such as Niobium, Tantalum, and Zirconium, indicate high enrichments, with samples’ mean value of 265.62 mg·kg⁻¹ for Zirconium and 200.24 mg·kg⁻¹ for Niobium. The sum of Rare Earth Elements identified in the analyzed river sediments is 385.01 mg·kg⁻¹ for Ditrău samples and 368.72 mg·kg⁻¹ for Jolotca, with Cerium being the most significant element. The La/Th and Hf distinction plots suggest a mixed felsic/basic source for the Ditrău area and an acidic source for the Jolotca area.
... The Nb depletion is also supported by the Nb/U ratios (Figure 17), which are clearly below the value of 47 ± 10 of MORB and OIB [128,129], i.e., of the oceanic basalts generated away from the influence of subduction processes. [130]. Data for Oceanic Island Basalts (OIBs) and Mid-Ocean Ridge Basalts (MORBs) are from [105,128]. ...
Post-collisional volcanism provides valuable insights into mantle dynamics, crustal processes, and mechanisms driving orogen uplift and collapse. This study presents geological, geochemical, and geochronological data for Ediacaran effusive and pyroclastic units from the Taghdout Volcanic Field (TVF) in the Siroua Window, Anti-Atlas Belt. Two eruptive cycles are identified based on volcanological and geochemical signatures. The first cycle comprises a diverse volcanic succession of basalts, basaltic andesites, andesites, dacites, and rhyolitic crystal-rich tuffs and ignimbrites, exhibiting arc calc-alkaline affinities. These mafic magmas were derived from a lithospheric mantle metasomatized by subduction-related fluids and are associated with the gravitational collapse of the Pan-African Orogen. The second cycle is marked by bimodal volcanism, featuring tholeiitic basalts sourced from the asthenospheric mantle and felsic intraplate magmas. These units display volcanological characteristics typical of facies models for continental basaltsuccessions and continental felsic volcanoes.s. Precise CA-ID-TIMS U-Pb zircon dating constrains the volcanic activity to 575–557 Ma, reflecting an 18-million-year period of lithospheric thinning, delamination, and asthenospheric upwelling. This progression marks the transition from orogen collapse to continental rifting, culminating in the breakup of the Rodinia supercontinent and the opening of the Iapetus Ocean. The TVF exemplifies the dynamic interplay between lithospheric and asthenospheric processes during post-collisional tectonic evolution.
... We model the Earth crust as a layer of constant mass density ρ = 2.7 g cm −3 , consisting of nuclei, whose abundances are given on the left hand side of table I [59]. ...
Above a critical dark matter-nucleus scattering cross section any terrestrial direct detection experiment loses sensitivity to dark matter, since the Earth crust, atmosphere, and potential shielding layers start to block off the dark matter particles. This critical cross section is commonly determined by describing the average energy loss of the dark matter particles analytically. However, this treatment overestimates the stopping power of the Earth crust. Therefore the obtained bounds should be considered as conservative. We perform Monte Carlo simulations to determine the precise value of the critical cross section for various direct detection experiments and compare them to other dark matter constraints in the low mass regime. In this region we find parameter space where typical underground and surface detectors are completely blind to dark matter. This "hole" in the parameter space can hardly be closed with an increase in the detector exposure. Dedicated surface or high-altitude experiments may be the only way to directly probe this part of the parameter space.
... REE concentrations were normalized to chondrite values (McDONOUGH & SUN, 1995) to detect any differentiation of the elements in earlier geological processes, and to Upper Continental Crust values (RUDNICK & GAO, 2014) to account for their subsequent distribution during weathering and pedogenesis. ...
The geochemical signature of weathering and pedogenesis in a temperate humid climate on two parent material types in the foothills of Medvednica Mt. was studied. Five soil profiles on Miocene marls and three sections of Plio-Quaternary (PlQ) proluvial sediments with overlying soil and weathered material were analyzed. The (clay) mineralogy of all profiles and sections had been determined in previous studies. The chemical composition of the samples was determined by inductively coupled plasma emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectroscopy (ICP-MS). Poorly ordered Fe and Mn oxides were determined in the PlQ sediment and overlying soil samples by atomic absorption spectroscopy (AAS), after oxalate dissolution in the dark. The concentrations and element ratios were used to determine element enrichment/mobility and intensity of chemical weathering, material provenance, and to compare the geochemical signatures with previously obtained clay mineralogy results. Trace elements in the Miocene marls, including rare earth elements (REE), indicate the continental origin of the marl siliciclastic component, while more scattered geochemical data of the PlQ sediments reflect their proluvial/torrential nature. The mass transfer coefficient (τ) for major elements and element ratios of the Miocene marl profiles indicate chemical weathering and pedogenesis of lower intensity. The geochemistry of these samples shows homogeneity within the profiles. In the geochemical signature of the PlQ sections, a chaotic proluvial deposition of the material is visible, as well as the heterogeneity with the overlying soil and weathered material. Overall, the geochemistry results largely support the clay mineralogy of the samples and demonstrate how a multiproxy approach can help test hypotheses about past environments and provide valuable additional information for complex paleoenvironmental studies.
... Crustal assimilation was limited, given their lower Nb/U and La/Sm ratios than those of the continental crust. The Nb/U ratios of Sailipu ultrapotassic volcanic rocks range from 1.06 to 2.02, which is much lower than those of the average continental crust (6.15; Rudnick and Gao, 2003). In addition, crustal assimilation could cause the La/Sm ratios of mantle-derived magmas to rapidly increase above 5 (Lassiter and DePaolo, 1997), but the La/Sm ratios of the Sailipu ultrapotassic volcanic rocks are 3.25-4.90, ...
Widely distributed Oligocene−Miocene ultrapotassic volcanic rocks in the Lhasa terrane of southern Tibet have been associated with the melting of the lithospheric mantle, plateau uplift, and porphyry Cu-Au mineralization. This study presents the mineral chemistry of olivine and clinopyroxene phenocrysts, whole-rock major and trace element data, and zircon U-Pb geochronological and Hf isotopic data for the Sailipu primitive ultrapotassic volcanic rocks. The Sailipu volcanic rocks exhibit high MgO (5.6−11.4 wt%), Cr (386−981 ppm), Co (22−43 ppm), and Ni (95−423 ppm) concentrations and have highly fractionated rare earth elements [REEs; (La/Yb)N = 23−73] and high-Fo (89.1−90.8) olivine phenocrysts containing elevated NiO (up to 0.59 wt%), which suggests a pyroxenitic mantle source that partially melted in the garnet stability field. Their high K2O contents (4.8−8.0 wt%) and global subduction sediment-like trace element patterns suggest that the metasomatic agents, which reacted with mantle peridotites to form phlogopite-bearing pyroxenites, were dominantly derived from the melting of subducted continental sediments. Their high whole-rock Ba/La and Th/Nd ratios are consistent with this hypothesis. The Sailipu ultrapotassic volcanic rocks also exhibit low initial 176Hf/177Hf ratios that resemble those of Himalayan leucogranites, and high Ca contents in olivine phenocrysts, which is consistent with contributions from the subducted carbonate-rich sedimentary strata on top of the thinned Greater Indian continental crust. The zircon U-Pb chronological data yielded concordant ages of 24.33 ± 0.19 Ma, 21.20 ± 0.62 Ma, and 17.05 ± 0.31 Ma for different exposures of the Sailipu volcanic rocks, which establishes a maximum age of ca. 24 Ma for these rocks. The northwest−southeast spatial distribution and the southeastward decrease in age (80°E−90°E) suggest west-to-east tearing of the thinned Greater Indian slab, which caused asthenospheric upwelling and melting of the Tibetan lithospheric mantle. Geothermometric calculations show relatively high primary magma temperatures (∼1250 °C) that are consistent with asthenospheric upwelling. We propose a mechanism that could genetically link the coeval Cu-Au ore-forming granitoids with the ultrapotassic magmatism of the Gangdese belt. The ultrapotassic rocks supply a large-volume of external magmatic volatiles, particularly H2O, which could trigger melting of the Tibetan lower crust and lead to the generation of the ore-forming granitoids and the establishment of oxidizing conditions for porphyry deposits. The oxygen fugacity (log ƒO2 values of ΔFMQ) of the primitive Sailipu ultrapotassic volcanic rocks (ΔFMQ = 0.48 ± 0.51 based on the Dol/melt V oxybarometer and ΔFMQ = 0.33 ± 1.19 according to the magmatic zircon U-Ce-Ti oxybarometer) is slightly lower than that of porphyry Cu-Au ore-forming granitoids in the eastern Gangdese (ΔFMQ = +0.8 to +2.9), which suggests that the direct injection of ultrapotassic melts into ore-forming granitoids played a limited role in changing oxygen fugacity, but more oxidized fluids/volatiles exsolved from these ultrapotassic melts may have facilitated the remelting of sulfide-bearing lower crust and/or directly scavenged sulfides from the mush-state reservoirs of the ore-forming granitoids in the middle−upper crust.
... The basaltic rocks have relatively low loss-on-ignition values (LOI, mostly lower than 0.6 wt.%), suggesting limited alteration. Moreover, the good correlations between fluid-mobile elements (e.g., Cs, U, Sr and Pb) and Nb (Rudnick & Gao, 2003), the basalts have lower Th/Nb (0.06-0.08) and higher Nb/Pb ratios (18-33), and their negative Pb as well as positive Nb and Ta anomalies make them compositionally analogous to OIB (Figure 3; Sun & McDonough, 1989). Their radiogenic Nd-Hf isotopic compositions (0.51285-0.51295 ...
It remains uncertain whether a stagnant slab in the mantle transition zone can affect the asthenospheric mantle beyond its leading edge. To address this question, we investigated Cenozoic alkaline basalts from the Dariganga volcanic field (DVF) in southeastern Mongolia. The DVF is located west of North–South Gravity Lineament (NSGL) in Eastern China, which is spatially coincident with the seismically detected stagnant Pacific slab front. Basalts from the DVF consist of nephelinite, basanite and alkali olivine basalt. These rocks have relatively high Nb/U (average = 58) and Nb/La (>1) ratios and radiogenic Nd–Hf isotopic compositions. They also have high Ca/Al (0.60–1.13), Zn/FeT (13.5–16.5), and FeO/MnO (77–112) ratios as well as low δ²⁶Mg (−0.42‰ to −0.26‰) values, reflecting an asthenospheric mantle source modified by carbonated eclogite‐derived melts. Pb–Nd–Hf isotope characteristics indicate that the carbonated eclogite‐derived melts likely originated from the stagnant Pacific slab. Although Cenozoic basalts from both the east of the NSGL (ENSGL) and DVF domains exhibit light δ²⁶Mg values, basalts from the ENSGL nevertheless have lower δ²⁶Mg values than those in the DVF domain. This suggests a gradual westward decline in the amount of carbonated melts/fluids derived from the stagnant Pacific slab. This variation trend, combined with a more fertile and oxidized asthenospheric mantle toward the ENSGL, indicates that the stagnant slab has affected the mantle and created a compositional aureole beyond its leading edge, which substantially contributed to the formation of the alkaline basalts in the DVF.
The Jiaojia gold deposit in the Jiaodong Peninsula, located in the northwestern part of the Jiaodong gold province in eastern China, has a gold reserve of over 300 t. Gold mineralization in Jiaojia deposit occurred in three stages: (1) The Pyrite–Quartz–Sericite Stage (Stage I) developed primary minerals that included quartz, sericite, and a small amount of anhedral pyrite, appearing as disseminations within milky quartz and foliated sericite. (2) The Quartz–Pyrite Stage (Stage II) developed quartz that appears smoky gray and pyrite that appears with a euhedral cubic morphology, with crystal faces oriented in a longitudinal pattern. Native gold occurs as fracture filling in pyrite. (3) The Quartz–Polymetallic Sulfides Stage (Stage III) developed polymetallic sulfides, including pyrite, chalcopyrite, galena, sphalerite, and magnetite. Native gold filled the pyrite fractures and was enclosed within the pyrite. (4) The Quartz–Carbonate Stage (Stage IV) developed the main minerals of quartz and carbonate, with scattered occurrences of pyrite. In situ geochemical analysis of pyrite, the main gold-carrying mineral from mineralization Stages I to III in the Jiaojia gold deposit, was conducted, including major element, trace element, and sulfur isotope analyses. The δ34S values of Jiaojia pyrite range from 4.5 to 8.0‰. Pyrite in Stage I (Py I) has δ34S values ranging from 4.5 to 7.4‰, with an average of 6.4‰. Pyrite in the Stage II (Py II) has δ34S values ranging from 5.9 to 8.0‰, with an average of 6.8‰. Pyrite in Stage III (Py III) has δ34S values ranging from 6.4 to 7.9‰, with an average of 7.4‰. Combined with the C-D-O-He isotopes, the ore-forming fluids of the Jiaojia gold deposit likely originated from subducted oceanic plate-related metasomatized mantle. The Co/Ni ratios of Jiaojia pyrite range from 0.50 to 1.47 in Stage I, 0.27 to 1.69 in Stage II, and 0.58 to 295 in Stage III. The Cu/Au ratios in the Jiaojia pyrite in all mineralization stages were >1. These geochemical features imply that the ore-forming fluids of the Jiaojia gold deposit were in a medium- to low-temperature reducing environment, with temperatures gradually decreasing from ore Stages I to III. The increase in Co and As in the pyrite of Stage III implies that gold precipitation resulted from fluid immiscibility caused by a decrease in pressure and temperature and an increase in the oxygen fugacity of the ore-forming fluid.
The Bainiuchang Ag-polymetallic ore deposit, located in southeastern Yunnan, China, is one of the region’s largest deposits. However, the hypabyssal granite porphyry within this mining area has yet to be comprehensively investigated. In this study, we conducted geochemical, geochronological, whole-rock Sr–Nd isotope, and zircon Hf isotope analyses on granite porphyry samples collected from the Bainiuchang deposit. The results indicate that the granite porphyry formed between 87.5 and 87.4 Ma in the Late Yanshanian period. Geochemically, the granite is strongly peraluminous, with high silica and alkali contents consistent with S-type granite characteristics. The granite porphyry is enriched in large-ion lithophile elements (Rb, Th, U, and K) and is relatively depleted in Ba and Sr. The initial 87Sr/86Sr ratios are high (0.71392–0.71585), accompanied by low εNd(t) values (− 8.9 to − 8.2). The zircons exhibited similarly low εHf(t) values (− 9.31 to − 3.6). These data suggest that the porphyry-forming magma originated from a continental crustal source. The two-stage Hf and Nd model ages are estimated at 1534–1216 Ma and 1615–1561 Ma, respectively. Thus, the granite porphyry likely formed under a strike-slip extensional setting in the Late Yanshanian period and resulted from the re-melting of Proterozoic basement metagreywackes. This porphyry shares a similar magmatic origin with concealed granite bodies within the deposit and is associated with structural reactivation during the Yanshanian. The findings of this study provide valuable insights into the tectonomagmatic mineralization processes in the Bainiuchang area.
The ever-expanding universe is infinite in space and time and is believed to have come into existence, sometimes around 13.787 ± 0.020 billion years ago. The Sun, which is one of the billions of stars in our Milky Way, is orbited by groups of celestial objects, such as eight numbers of planets, nine numbers of dwarf planets, countless asteroids, planetoids or minor planets, comets, and centaurs, is the main source of energy. The Milky Way is a part of several galaxies in the universe. The planets, the dwarf planets, and the smaller solar system bodies (SSSBs) are orbited by natural satellites, referred to as moons. Combined, they form a solar system, a part of billions of galaxies in the universe. The Earth, located at about 150 million kilometres (i.e., 1 AU) from the Sun, is one of eight planets, including Mercury, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. The Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus are the recognized dwarf planets. The comets, asteroids, planetoids or minor planets, and centaurs are considered small solar system bodies. Each planet, except Venus, has its own natural satellites called the Moon. The interplanetary medium of the universe consists of the interplanetary dust, the solar magnetic field, cosmic rays, and solar wind. The outer space of the solar system, beyond the radius of 30 AU from the Sun, is known as the Trans-Neptunian region (TNR), where mainly dwarf planets and asteroids are located. According to the distance from the Sun, this region can be categorized into three larger groups: the Kuiper belt region (KBR), the Scattered Disc Zone, and the Oort Cloud. Mostly unexplored, the region beyond 150–250 AU, orbiting the Sun, is called an extremely scattered disc, referred to as the “third zone of the Solar System”, and objects such as sednoids are located in this area. There exist different hypotheses and theories about the origin and evolution of planetary systems; the nebular theory is the most accepted so far. The universe and planetary system evolved from the very early universe to the Dark Ages when large-scale structures emerged, and gradually, the present-day universe appeared. Our planet Earth is also evolved along with the solar system sometimes around 4.6 billion years ago. It rotates around the Sun, has one natural satellite—the Moon, and is the only planet having a biosphere with conditions suitable for the survival of life. It comprises three layers, the outer crust, middle mantle, and central core, and has its atmosphere, magnetic and gravitational field.
Subduction zones are pivotal in understanding the interaction between Earth's surface and deep materials. The markedly diverse Mo isotopic compositions observed in arc magmas provide a valuable opportunity to explore the material cycling processes within subduction zones. Arc magma sources altered by slab fluids exhibit heavy Mo isotopes, while the origin of arc lavas with light Mo isotopes remains contentious, which is hypothesized to originate from two potential sources: (1) the dehydrated oceanic crust and (2) subducting sediments. Although the former has been extensively recognized, the latter still poses an enigma. Here, we present the Mo-Sr-Nd-Hf isotopic and elemental data of Jiang Tso andesites in the central Tibetan Plateau to elucidate the chemical compositions of sediment melts. These andesites show elevated Mg# values, along with trace element characteristics reminiscent of typical sediment melts. Their Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.710260–0.710671, εNd(t) = –10.63 to –8.97, and εHf(t) = –9.30 to –7.95) closely resemble those of contemporaneous sediments in the central Tibetan Plateau. They exhibit higher Ce/Mo ratios (396–587) and lower δ98/95Mo values (−1.62‰ to −0.69‰) compared to the depleted mantle and most arc lavas, suggesting a more plausible explanation lies in the involvement of dehydrated subducting sediments rather than dehydrated oceanic crust in the source. Our findings, integrated with existing research, suggest that the Mo isotopes of arc magmas, in conjunction with trace elemental ratios, can preliminarily constrain the different subduction components (fluid or melt) in their sources. In addition, arc rocks with extremely light Mo isotopes may not be exclusively derived from subducted oceanic crust. Instead, they could originate from dehydrated sediment residues that enter the sub-arc mantle. Sediments with light Mo isotopes that are subducted into and preserved within the continental lithospheric mantle are likely to form localized reservoir characterized by light Mo isotopic signatures. This reservoir could play a crucial role in reconciling the discrepancies in Mo isotopic compositions between the continental crust and the depleted mantle.
Impacts of the acid mine drainage (AMD) remediation are investigated on the largest gold mine in Latin America, located in the Dominican Republic. Geochemical analysis of suspended matter in water performed in 2022 on water bodies located downstream to the mine, namely, the Margajita River and Lake Hatillo, are compared with analyses made in 2007, before the AMD remediation. The results for the Margajita River show a strong decrease in heavy metal and metalloid concentrations in the dissolved phase for Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Cd, Sb, and Pb (between 89.1 and 99.9%) which is correlated with a pH neutralization (from a pH of 2 to 7.5). However, the concentrations of Cd and sulfate in the dissolved fraction remain higher than their environmental standard values, and a high concentration of K is also observed. High contributions in terms of mass are identified for Cs, Tl, Cd, and Li in the Margajita River. Concentrations of Al, Fe, Ni, Zn, and As in suspended sediments are higher than the environmental standard values in 2022. Modeling of the chemical speciation shows that the higher pH leads to the formation of suspended particles in the form of hydroxides and sulfates. The concentrations in suspended sediments are highly correlated between Margajita River and Lake Hatillo in 2022 for Fe, Cd, Li, and Ca (R² between 0.9 and 0.99 with p ≤ 0.05). In Lake Hatillo, principal component analysis shows that invariant elements Fe, P, Ti, Zr, and Al are correlated, as they precipitate between the inlet and outlet of the lake.
Freshwater ecosystems, including high-altitude lakes, can be affected by trace metal pollution derived from a mix of natural sources and anthropogenic activities. These pollutants often collect in surface sediments, with notable concentrations in the deeper areas of lakes. To evaluate the environmental risk associated with metal contaminated sediment in Rara Lake, southern Himalaya, surface sediment samples were systematically collected in November 2018, with a subsequent specific emphasis on determinations of trace element concentrations. Subsequent analysis revealed nine elements exhibiting a descending mean concentrations order: iron (Fe) > manganese (Mn) > chromium (Cr) > rubidium (Rb) > nickel (Ni) > strontium (Sr) > cobalt (Co) > copper (Cu) > cadmium (Cd), of 7205.55 mg kg⁻¹, 2290.34 mg kg⁻¹, 176.29 mg kg⁻¹, 153.78 mg kg⁻¹, 51.86 mg kg⁻¹, 44.61 mg kg⁻¹, 38.89 mg kg⁻¹, 29.11 mg kg⁻¹, and 0.10 mg kg⁻¹, respectively. Comparisons to sediment quality guidelines highlight that Mn, Cr, Cu, and Cd as significant threats to the aquatic ecosystem in Rara Lake. To assess the impact of metal pollution, enrichment factor (EF), geo-accumulation index (Igeo), pollution load index (PLI), and contamination factor (CF) were computed. All metals (except Cd) had Igeo value exceeding 5, displaying strong contamination. EF values for Mn, Cr, Co, and Ni metals were > 10, indicating severe effects of anthropogenic influences. CF and PLI values also indicated significant pollution for most of the investigated sites. Elevated trace element concentrations have the potential to adversely affect water, sediment, and aquatic life, also potentially impacting nutrient cycling and microbial activity. This study enhances our understanding of the metal compositions within Rara Lake sediments and provides a basis for more effective lake management and pollution control strategies. Urgent action by regional governing bodies is crucial to address the early stages of metals pollution, including identification and controlling of pollution sources, by appropriate regulations, optimizing industrial practices, and remediating existing pollution to prevent further contamination and protect the lake ecosystem.
Post-collisional ultrapotassic volcanic rocks (UPVRs) in the Tethyan domain archive crucial information about the composition and evolution of the lithospheric mantle, offering valuable insights into deep geodynamic processes. The Tibetan Plateau, particularly the Lhasa Terrane in its southern part, remains a subject of debate regarding the genesis and geodynamic mechanisms of the UPVRs. This study investigates geochronology, mineralogy, and whole-rock geochemistry of the UPVRs in the Majiang area of Lhasa Terrane, southern Tibet. Zircon Usingle bondPb dating reveals that these rocks formed at ca. 21 Ma. Petrologically, the Majiang volcanic rocks are classified as trachyandesites, while geochemically they qualify as ultrapotassic rocks (characterized by K2O > 3 wt%, MgO > 3 wt% and K2O/Na2O > 2). They exhibit a broad range of SiO2 contents (45.85–57.94 wt%) and high Mg# values (57.7–72.3). Elevated trace element concentrations, marked by enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), suggest a mantle source influenced by fluid/melt interactions. The presence of phlogopite phenocrysts in the Majiang UPVRs suggests high magma temperatures (1116 ± 15 °C) and low crystallization pressures (1.3 ± 0.3 GPa). Trace element geochemical modeling indicates that the Majiang UPVRs likely originate from a garnet stable mantle domain and experience rapid magma ascent after their formation. The Majiang UPVRs exhibit high Th/U (8.97–19.08) and Th/La (2.27–2.45) ratios, low Hf/Sm (0.52–0.64) ratios, and enriched Srsingle bondNd isotopes (87Sr/86Sri = 0.717286–0.721658, εNd(t) = −9.55 - -9.28). These geochemical signatures suggest derivation from an enriched lithospheric mantle metasomatized by oceanic sediments. Integration of published data reveals that the UPVRs in both the eastern and western Lhasa Terrane share close geochemical affinities, suggesting a common mantle source. However, limited UPVR exposures in the eastern Lhasa Terrane and slightly more depleted Srsingle bondNd isotopes in the Majiang UPVRs compared to those in the western Lhasa Terrane imply distinct Miocene geodynamic processes. Lithospheric convective thinning caused by post-collisional extension and mantle upwelling likely occurred more extensively in the eastern Lhasa Terrane. This process may have led to the gradual replacement of ancient lithospheric mantle by juvenile material in the east, contrasting with the more localized effects in the western Lhasa Terrane.
This chapter reviews the main geochemical features of the subcontinental lithospheric mantle and discusses their implications for the formation and evolution of this domain. Extraction of the pertinent geochemical signals, unclouded by complexities related to source composition and emplacement processes, is discussed. The main compositional differences between the cratonic and non-cratonic lithosphere are described, and compared with models of melt extraction and melt-rock reaction. Implications for how the mechanisms of lithosphere formation, modification and destruction have changed over time, and the geodynamic significance of these changes, are considered. The chapter ends by suggesting future avenues of research.
Key points:
• The highly refractory composition of the cratonic lithospheric mantle constrasts markedly with the more fertile and variable composition of the non-cratonic lithospheric domain, likely reflecting secular cooling and resulting changes in tectonic styles.
• Compositions of cratonic xenoliths and inclusions in diamond demand onset of melting at pressures ≥5 GPa, implying a thermal plume, in which melting proceeded beyond the garnet-out reaction, rather than a ridge environment.
• In the cratonic lithosphere, the contrast between dominantly harzburgitic diamond inclusions and dominantly lherzolitic xenoliths testifies to massive overprinting by a wide range of melts and fluids, resulting over time in lithospheric thinning and sometimes decratonization.
• The similar conditions of formation of komatiites and cratonic lithospheric mantle at excess mantle potential temperatures and high melt fractions suggest a genetic link that is obscured by subsolidus and melt-rock reactions.
• In the non-cratonic lithosphere, incompatible element enrichment of refractory lithologies is ubiquitous, with Sr and Nd isotope evidence arguing that this enrichment was mostly ancient.
• The absence of fertile lherzolites with unradiogenic Os isotope compositions argues that lherzolites are not formed by recent refertilization of ancient depleted harzburgites, because Os-poor melts cannot significantly alter the Os isotope composition of Os-rich peridotites.
• The isotopic and major and trace element signatures of non-cratonic peridotites may imply high degrees of melting followed by auto-refertilization in Proterozoic mantle plumes.
• Open questions concern the geodynamic regime associated with the transition from cratonic to non-cratonic lithosphere, the mechanisms of lithosphere preservation vs. destruction, and the relationship between the subcontinental lithosphere and the convecting mantle.
The Don terrane, which is extensively reworked by metamorphism and granitoid intrusions, is part of the Volga–Don orogen stretching along the eastern margin of the Sarmatian segment of the East European Craton. The terrane consists of gneiss-granites of the Pavlovsk complex, metavolcanic rocks, ranging from basaltic andesites to dacites (amphibolites and gneisses), and metasedimentary rocks (marbles and calc-silicate rocks) of the Don Group. The volcanic rocks are typically enriched in LILE and LREE and show negative HFSE anomalies, indicating fluid-assisted melting of the mantle wedge in a subduction zone. The Nd isotopic composition (εNd2200 = –1.2 to +3.4, model age 2180–2550 Ma) and Hf isotopic composition (εHf = –4.3 to +3.3, model age 2290–2640 Ma) indicate an enriched mantle or a mixed crustal–mantle source of the parental melts of the volcanics and a juvenile source for sediments of the Don Group. The U–Pb zircon metamorphic age of the gneisses and amphibolites is 2047 ± 7 Ma, and that of the thermal effect of the granitoid batholith on the host rocks is 2060 ± 4 Ma. According to isotope geochemical and geochronological data, the Don Group is underlain by Archean rocks, contains Archean detrital zircons, but the age of this group is no older than 2300 Ma. A facies and age analogue of the Don Group is the Temryuk Formation of the Central Azov Group of the Ukrainian shield. In the Paleoproterozoic, the eastern margin of Sarmatia was likely a continental arc, which was nearly coeval with the island arc–backarc basin system of the Losevo Group.
The results of long-term geochronological and isotope-geochemical studies of mesothermal gold deposits of orogenic type in the world’s largest Lena metallogenic province are summarized, and the evaluation of existing conceptions of their genesis is given. On the basis of geochronological data the presence of three ore-forming events manifested in the region in the Paleozoic time is proved. The early Late Ordovician-Early Silurian (450-430 Ma) event is associated with large-scale vein-embedded gold-sulfide mineralization in Neoproterozoic carbonaceous terrigenous-carbonate rocks of the Baikal-Patom fold belt (BPB). In terms of time, ore formation coincides with the development of metamorphic processes in the region. The reactivation of hydrothermal action in the BPP, which led to the formation of vein gold-quartz mineralization, occurred in the Middle Carboniferous (340-330 Ma) and was produced by post-collisional granitoid magmatism. The most recent ore-forming event occurred in the Early Permian (290-280 Ma) and was manifested exclusively in the Precambrian structures of the Baikal-Muya fold belt (BMP). It was synchronous with the development of intraplate alkaline and subalkaline magmatism in the region. Isotopic-geochemical data (87Sr/86Sr, 143Nd/144Nd, Pb-Pb, and δ34S) indicate that the mesothermal ore-forming systems of the Northern Transbaikalia were heterogeneous in their isotopic characteristics, which distinguishes them from ore-magmatic (intrusion related type) systems. Comparison of Sr, Nd and Pb isotopic composition of ores and rocks of the region, including magmatic rocks of the same age as gold mineralization, proves the leading role of the Precambrian continental crust in the supply of mineral-forming components to the hydrothermal systems of gold deposits. The contribution of the magmatic source of matter in the genesis of orogenic gold mineralization was limited and is established only for the Early Permian ore-forming systems of the BMP, for which the input of metals from alkaline melts of basite composition is assumed.
Large granitoid provinces of Central and North-East Asia (Angara–Vitim, Khangai, Kalba-Narym, and Kolyma) can be divided into areal and linear types, which differ significantly in the area and volume of granitoids in their composition. It is assumed that these differences are caused by the structure of pregranitic basement and the degree of thermal impact on the lower and middle continental crust. An important factor in the formation of granitoid provinces is a mantle mafic magmatism, the estimated scale of which correlates with the volumetric and areal characteristics of the granitoid provinces. The role of mafic magmatism is an additional input of heat from the fluids into the melting region of crustal protoliths, as well as a material contribution through various mechanisms of magma mixing. Mixing at a deep level is the most efficient, resulting in the formation of significant volumes of increased basicity silicic magmas. The petrogenetic role of contrasting magmas mixing at the mesoabyssal crustal level, as well as at hypabyssal conditions is not great, but mingling dikes formed in this process serve as a key argument in justifying the simultaneous formation of mafic and granitoid magmatism. Granitoids of Silicic Large Igneous Provinces (SLIPs) are characterized by a heterogeneous isotopic composition generally corresponding to the parameters of the continental crust. The extremely high heterogeneity of spatially conjugate granitoids is caused by mixing of silicic magmas formed through the melting of a few isotopically contrasting sources, including mixing with magmas of mantle origin. The mafic rocks ascribed to the granitoid provinces correspond to the isotopic composition of the enriched mantle (Angara–Vitim batholith) or indicate a significant crustal contribution (Khangai area). The metallogeny of SLIPs is determined by the degree of erosional section and the crustal protolith type, the metamorphic grade of which largely determines the initial fluid content of silicic magmas. The melting of high-grade ancient crustal protoliths produces relatively “dry” silicic melts, the melting of low-grade crustal sources leads to the formation of “aqueous” melts, the differentiation of which ends with pegmatite formation with rare metal mineralization. The formation of non-subduction SLIPs is associated with the mantle plume impact (in the form of simultaneous basaltic magmatism) on the heated crust of young orogenic regions, where tectonic processes were completed no more than a few tens of Ma.
Sediment samples from the Mugheb River in Bamenda were analyzed by inductively coupled plasma-optical emission spectrometry to determine their major, trace, and rare earth element contents, so as to ascertain their provenance, paleoweathering, and tectonic setting. The enrichment of Fe2O3 suggests these are derived from hematization of basaltic rocks. Significant SiO2 content recalculated to an anhydrous basis and adjusted to 100% (SiO2(adj)) indicates the abundance of quartz and kaolinite in sediment samples. Enrichment of transition elements relative to the reference values indicates mafic source rocks. The rare earth element patterns indicate negative Eu (Eu/Eu* = 0.54–0.82) and positive Ce (Ce/Ce* ~ = 1.04–1.67) anomalies, suggesting that they were derived by fractionated mafic rocks and, to a little extent, by fractionated felsic rocks. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) indicate intense weathering in the source area in a hot humid climate reflected by the removal of labile cations relative to stable residual constituents, which corroborate with the climate of the Cameroon. The ratios of SiO2(adj)/Al2O3, K2O/Al2O3, and K2O/Na2O and the index of compositional variability (ICVn) indicate compositionally mature sediments in which sediment samples experienced recycling and noticeable effect of sorting and reworking. The river sediments are deposited in oxic conditions within the riverine environment. The sediments were sourced dominantly from mafic and little contributions from felsic sources, and represent mature recycled detritus. Tectonic discrimination diagrams suggest that Mugheb River sediments were derived from rock types, which represent a passive continental margin, which is consistent with several tectonic history models of the Cameroon Volcanic Line and the Pan-African fold belt.
Banded Iron Formations (BIFs) interbedded with schists characterize the Mbarga prospect in the Ntem Complex at the northwest edge of the Congo Craton. This study presents new whole-rock geochemical, Sr-Nd, and zircon U-Pb isotopic data for the BIFs and schists to constrain the timing and geodynamic setting of the deposit. The abundances of SiO2 (52.81 to 79.14 wt%) and Na2O+K2O (4.24 to 8.54 wt%) in the schists indicate andesitic, dacitic, to rhyolitic protoliths. Trace element signatures, such as high Ba and depleted Nb-Ta concentrations, suggest a volcanic arc affinity. A well-defined U-Pb zircon age of 2890 ± 4 Ma implies a Mesoarchaean protolith age, while an imprecise Rb-Sr whole-rock age of ca. 2.65 Ga is consistent with known tectonothermal events (∼2.75 and 2.65 Ga) in the Ntem Complex. Initial εNd(2.89) values of + 0.8 to + 2.0 for the schists indicate an unevolved, mantle-like source for the protoliths. The BIFs show partial to extensive alterations of magnetite to hematite-martite and are of the Algoma type. They are characterized by high Fe2O3 (∼54.06 wt%) and SiO2 (∼45.40 wt%) but low Al2O3 (∼0.14 wt%), TiO2 (∼0.1 wt%), Zr (∼4.92 ppm), Th (∼0.11 ppm), and REE-Y contents. Rare earth patterns marked by LREE depletion, positive Eu anomalies (∼2), mild Ce depletion (Ce/Ce* 0.67 to 1.16), and super-chondritic Y/Ho ratios (∼34) suggest formation under anoxic to suboxic Archaean marine conditions, possibly involving mixing of Archaean seawater with minor (0.1–1 %) contributions from medium- to high-T hydrothermal fluids. Sparse 2951 ± 24 Ma zircons, presumably of detrital origin, establish a depositional link to the associated schists, redefining the age of BIF deposition within the Ntem Complex to ca. 2.95–2.89 Ga. However, whole-rock Sm-Nd isotope data for five BIF samples define a scattered array with an imprecise slope equivalent to an age near 1004 ± 78 Ma, which may reflect a previously unrecognized recrystallization event in the BIFs. The initial εNd of this array (−11.1 ± 2.0) suggests a crustal source. The mineralogical, geochemical, and isotopic datasets reconcile the Mbarga BIF prospect with arc magmatism in the Late Archaean, suggesting their formation in a back-arc basin setting.
The stream sediment samples collected from the Bat Xat district in Vietnam have been studied to establish their geochemical characteristics, provenance, weathering intensity, and climate. This comprehensive investigation included major, trace, and rare earth elements (REEs). The sediments were classified as greywacke, lithicarenite, and arkose, denoting compositionally immature deposits. The sediments exhibited moderate chemical weathering in the source domain, with a CIA average value of 67.01, PIA of 73.58, and ICV of 1.05. This suggests a depositional environment influenced by humid climatic conditions attributed to the attenuation of the Indian summer monsoon. The trace element and REE geochemistry of the Bat Xat sediments symbolize their characteristic signatures, with REE patterns mirroring those of their origins. The initial source materials are primarily associated with felsic rocks, with a smaller contribution from sedimentary and metamorphosed sedimentary rocks in the study area. Geochemical characteristics are large fluctuations in Eu/Eu* values and high (Gd/Yb)N ratios. We assume that Bat Xat sediments originated from the exhumated Archean crystalline basement or mixed rocks of the Archean component.
The geochemical composition of Ocean Island Basalts (OIBs) from the Azores reflects the spatial distribution, shape, and temporal evolution of small-scale geochemical heterogeneities within their mantle plume source. Here, we investigate the time-related evolution of volcanism at São Jorge Island, Central Azores. New field observations, a magnetic survey, 40Ar/39Ar and 14C ages and geochemical data indicate that the fissural volcanic activity at São Jorge produced at least four main mafic volcanic complexes (V. C.). The oldest V. C., São João, produced the thickest lava piles at ca. 1.3 Ma in the eastern part of the island. After a period of quiescence, the Serra do Topo V. C. was produced at ca. 0.8-0.5 Ma in the central part of the island. The Rosais V. C. was emplaced between ca. 0.4 and 0.1 Ma on the entire island. Finally, the Holocene Manadas V.C. volcanism became active in the western part of the island and includes three historic eruptions (1580, 1808 and 1964 CE).
Magmas were formed at low melting degrees from a peridotitic mantle with possible minor contributions of recycled components. Olivine compositions and whole-rock trace element ratios discard a significant contribution from pyroxenitic source rocks. Melting temperatures (ca. 1420-1480 °C) were slightly higher than those of the ambient upper mantle. The four V.C. are characterized by distinct geochemical compositions in terms of incompatible trace elements and Sr-Nd-Pb isotopic ratios. The oldest V.C., São João, is characterized by Pb isotopic compositions (e.g., markedly negative Δ7/4 and 8/4 values) plotting well below the Northern Hemisphere Reference Line (NHRL). The Upper Pleistocene (Rosais V.C.) lavas from the north-western cliffs have compositions similar to enriched mantle (EM) end-member basalts (e.g., high 207Pb/204Pb at moderate 206Pb/204Pb; high Ba/Nb), which are rare among northern hemisphere OIBs. Finally, high 206Pb/204Pb (up to 20), reflecting contribution from a HIMU-type component characterizes the Holocene Manadas lavas and is occasionally found in lavas from other volcanic complexes from 1.3 to 0.1 Ma. These findings indicate that magmas from São Jorge and the nearby Central Azores islands were sourced from a strongly heterogeneous mantle plume, which displayed localized filaments of heterogeneous material that were rapidly exhausted (in ca. 0.2 Ma). The dominant component at São Jorge and in the Central Azores in general appears to be the HIMU-type endmember, which instead is not significant in the Eastern Azores. Possibly, the Central and Eastern Azores were produced by distinct branches of the Azores mantle plume, as would also be consistent with previous tomographic studies.
Tungsten enrichment during the formation of giant W deposits is thought to be related to magmatic and hydrothermal processes. However, the mechanisms of W enrichment and their role in controlling ore formation remain unclear. Zircon is a ubiquitous accessory mineral that can provide a record of the physicochemical conditions during mineralization. Dahutang in South China is a giant W deposit (1.89 Mt WO3 at 0.18%) associated with the late Mesozoic granites. In this study, we report new zircon morphological, geochronological, and chemical data for the most evolved Li-mica albite granite in the Dahutang deposit, in order to determine the processes of W enrichment. We classified the zircons into three types based on their appearance and composition. Type-IA and -IB zircons (ca. 145 Ma) successively crystallized from metasedimentary-derived magmas (δ¹⁸O = 8.9 ± 0.3‰) at 786–732 °C. Type-II zircons formed by interaction between volatile-rich melts and Type-I zircons at 669 ± 39 °C. Type-III zircons formed by autometasomatism of earlier Type-I and -II zircons, which involved exsolved hydrosilicate fluids. Our numerical model shows that the granitic melts have undergone > 95% fractional crystallization and experienced metasomatism by hydrosilicate fluids, during which the rare-metals (W, Nb, and Ta) were extensively enriched. Furthermore, we compiled data for ten W deposits across South China to investigate the key factors controlling the formation of giant W deposits. The strong correlation (R² = 0.79) between WO3 tonnage and zircon Hf content indicates that an extensive and multi-stage evolution may be the key factor controlling the formation of giant W deposits.
Comparative analysis of the earlier obtained and new geological data, age, mineral, petrological, and geochemical compositions of plutons of calc-alkalic granitoids and rare-metal Li–F granites shows their evolution in a wide time interval during the formation of Mesozoic areas of granitoid magmatism. Mineralogical and geochemical analysis of the evolution of plutons of palingenetic calc-alkalic granitoids (Baga Hentiyn (MZ1) and Ikh Narotiin = Hiid (MZ2)) and intrusions of rare-metal Li–F granites of Central and Eastern Mongolia revealed their petrological and geochemical differences. The closure of the Mongol–Okhotsk Basin with the formation of large plutons of calc-alkaliс granitoids, obviously related to collisional processes, did not cause significant enrichment of the late granite phases with lithophile and ore elements. Within the peripheral zones of еру MZ1 and MZ2 magmatic areas, mineralization is often associated with Mongolian multiphase plutons and small intrusions of rare-metal Li–F granites. The rare-metal granites are characterized by a decrease in indicative K/Rb, Nb/Ta, and Zr/Hf values and a regular increase in F, Li, Rb, Cs, Sn, W, Be, Ta, and Nb contents during the evolution of Li–F magmas. Igneous and, particularly, metasomatic rocks in most intrusions of ore-bearing rare-metal Li–F granites are characterized by significant variations in Sn and W contents. At the magmatic stage, the pegmatoid varieties of amazonite–albite granites and pegmatites of the zonal Baga Gazriin (MZ1) and Barun Tsogto (MZ2) plutons are significantly enriched in both Sn and W. Maximum Sn and W enrichment has been established in greisenized granites and zoned greisen bodies (zwitters), which is due to the percolation of ore-bearing solutions into the upper horizons and the mineralization of ore elements in the late phases of intrusions and in metasomatites. The wide variations in the age (321–126 Ma) and trace element and isotope compositions of Mongolian rare-metal Li–F granites within various zones of large magmatic areas suggest the influence of mantle plume sources on the composition of rare-metal granitic magmas and on their ore potential in intermediate chambers in the continental crust.
The paper presents newly obtained and summarizes preexisting data of long-term geochronological and isotope studies of orogenic mesothermal gold deposits in the world’s largest Lena metallogenic province and reviews interpretations of their genesis. Geochronologic data indicate that the gold mineralization was formed during three ore-forming Paleozoic events. The early Late Ordovician–Early Silurian (450–430 Ma) event produced the abundant veinlet—disseminated gold–sulfide mineralization in Neoproterozoic carbonaceous terrigenous–carbonate rocks of the Baikal–Patom foldbelt (BPB). The mineralization was formed simultaneously with regional metamorphic processes. The rejuvenation of hydrothermal activity in the BPB resulted in gold-bearing quartz veins, which was produced in the Middle Carboniferous (340–330 Ma) in relation to postcollisional granitoid magmatism. The latest ore-forming event occurred in the Early Permian (290–280 Ma) and affected exclusively in Precambrian structures of the Baikal-Muya foldbelt (BMB). It was synchronous with the development of intraplate alkaline and subalkaline magmatism in the region. Newly obtained and preexisting isotopic-geochemical ( ⁸⁷ Sr/ ⁸⁶ Sr, ¹⁴³ Nd/ ¹⁴⁴ Nd, Pb–Pb, and δ ³⁴ S) data indicate that the mesothermal ore-forming systems of northern Transbaikalia were heterogeneous in their isotopic characteristics, which distinguishes them from the ore–magmatic (intrusion-related type) systems. Comparison of the Sr, Nd, and Pb isotopic composition of the ores and rocks, including magmatic rocks coeval with the gold mineralization, indicates that the Precambrian continental crust was the dominant source of mineral-forming components for the hydrothermal systems of the gold deposits. The contribution of the magmatic source to the genesis of the orogenic gold mineralization was limited and is identifiable only for the Early Permian ore-forming systems of the BMB, for which the input of metals from alkaline mafic melts was suggested.
The Southeast Asian Tin Province comprises western, central, and eastern belts and hosts significant granite-related Sn deposits. The genetic links between granites and Sn mineralization are still unclear. Most Sn deposits are in Thailand’s western and central belts, but their origin remains poorly elucidated due to the absence of direct dating of mineralization. Herein in-situ U-Pb age data of wolframite and cassiterite grains from nine representative Sn deposits in Thailand are obtained, which fall into two stages. Triassic deposits (224–210 Ma) are found in the Central belt, with Cretaceous deposits (78–67 Ma) in the Central belt being younger than those in the Western belt (84–74 Ma). However, ore-bearing granites, spanning two periods (227–205 Ma and 85–69 Ma), occur in central and western belts. Some Triassic ore-bearing granites exhibit significantly older ages than ore-forming ages. Newly identified ore-forming granites contain zircon grains with relatively low εHf(t) values (−29.5 to + 4.1; average = − 11.8), indicative of an origin from supracrustal sediments from the Sibumasu block. In contrast, barren granites have ages from 303–224 Ma and higher zircon εHf(t) values (−9.9 to + 13.2; average = +1.2), which suggests that they were derived from the juvenile mafic crust. Even after experiencing hydrothermal fluids exsolution, some low-fractionated ore-forming granites (D.I. < 90) still maintain remarkably high tin contents. Both ore-forming and barren granites crystallized under reducing conditions. Our study highlights the importance of Sn-rich sources of parental magmas in forming Sn deposits. The metasediment-rich basement of the Sibumasu block distributing along the continental margin is likely the Sn-rich source. These sources played a crucial role in forming two stages of tin deposits in distinct tectonic settings, that are syn-collisional crustal thickening in Paleo-Tethys and post-collisional extension-related settings in Neo-Tethys.
Reconstructions of past oceanic redox conditions, based on the measurement of metal distributions and their
stable-isotope ratios in marine sediments, have now become more commonplace, providing new constraints on
past environmental change. Almost all of these records, however, are based on organic-rich black-shale and
carbonate sedimentary archives, which have formed primarily in low-latitude regions. This limitation leads to
incomplete geographic coverage that complicates the global-scale interpretation of the results. In this study, the
potential of oceanic ‘red beds’ and grey shales as new archives of past environmental conditions are explored to
complement the datasets derived from other lithologies and extend reconstruction efforts to mid- to high-latitude
regions. Our records originate from open-marine red and grey shales from two sedimentary sections spanning the
Late Cretaceous Oceanic Anoxic Event 2 (OAE 2) in New Zealand, formerly deposited at high southern latitudes
in the palaeo-Pacific Ocean at c. 94 Ma and where contemporaneous black-shale and/or carbonate successions do
not exist. Access to the seawater-derived, authigenic fraction of the bulk sediment is essential for reconstructing
past marine redox state at these mid- to high-latitude locations. Therefore, a series of leaching experiments using
reagents of progressively increasing concentration were conducted to determine the best method for isolating the
authigenic Fe- and Mn-(oxyhydr)oxide fraction from the red and grey shale lithologies. Chemostratigraphic re-
cords for the concentrations of a suite of redox-sensitive metals (Fe, Mn, Co, Ni, Mo, Cr, V and U), as well as the
U-isotope palaeo-redox tracer (238U/235U; reformulated as δ238U), were obtained. These records imply that
leaching with 6 M HCl at room temperature is the best of the tested methods for the selective extraction of the Fe-
and Mn-(oxyhydr)oxide phase, whilst minimising the contribution from older inherited detrital material that
potentially could confound the interpretation of the chemostratigraphic records. This method offers potential for
reliably constraining the authigenic distributions of redox-sensitive elements in red and grey shales, provided Fe-
and Mn-(oxyhydr)oxides are predominantly of authigenic origin and constitute at least 5 % of the mineralogy. By
contrast, for the investigation of the U-isotope system, a larger Fe- and Mn-(oxyhydr)oxide proportion in the
sediment for an ocean–atmosphere perturbation event recording more severe changes in ocean redox conditions
and bigger U-isotope shifts than observed for OAE 2 are both necessary to obtain reliable results.
Late Cainozoic alkali volcanic rocks are widespread in the western Ross Sea embayment area of Antarctica. Many of the volcanic rocks are found along the front of the Transantarctic Mountain range but some localities lie within the range as well. Ultramafic xenoliths occur in many of the alkali basalts and basanites of the group, and they appear typical of those found in similar alkali basaltic rocks elsewhere in the world. Although reconnaissance mapping of the distribution of the volcanic rocks is nearly complete, only a few localities have been examined in detail. Attention is drawn to the many occurrences of the xenoliths, existing data reviewed, and some recent discoveries are briefly described. -J.M.H.
Xenoliths of lower crustal and upper mantle rocks occur in a host alkali basalt at the La Olivina peridot mine, Mexico. The ultramafic rocks are of three compositional groups distinguished by clinopyroxene chemistry similar to those from San Carlos, Arizona, Kilbourne Hole, New Mexico, USA, and Xalapasco de La Joya, San Luis Potosi (Mexico). The mantle xenoliths are of three textural groups: allotriomorphic granular (igneous?), porphyroclastic (metamorphic?) and granoblastic (metamorphic?). The lower crust xenoliths are mainly pelitic gneisses of uniform mineral assemblage, and pyroxene-granulites, mostly plagioclase-bearing, but some with scapolite instead. These xenoliths resemble those from Kilbourne Hole, and have been dated at approx 1400 m.y., which agrees well with Kilbourne Hole. It seems likely that the Precambrian craton underlying Kilbourne Hole extends SE into Mexico at least as far as La Olivina.-R.E.S.
Representative xenoliths and their contact-metamorphic equivalents from the Wehr volcano, W Germany, are studied in thin-section and by chemical analyses of their major and trace elements. The xenoliths are of Devonian country-rocks, trachytic pumice, and greenschist-facies rocks. Regionally, metamorphic fragments, mica schists, mica quartzites, amphibolites, and calc-silicate quartzites occur, derived from the higher-grade metamorphic basement. Most of them are affected to varying degrees by contact metamorphism and changed to spotted mica schists and partially molten rocks. A hypothetical crustal model for the E Eifel is given on the basis of the xenoliths studied and available seismic data. A postulated phonolitic to trachytic magma chamber is positioned at a shallow crustal level. A significant transfer of heat and material between the Wehr magma chamber and the country rocks is indicated. -I.Kb.
The Rb, Sr contents and 87Sr/86Sr ratios were studied for mafic and ultramafic inclusions (four lherzolites, six gabbros and amphibolites) and their host basaltic rocks from Itinome-gata, Northeast Japan. These lherzolites are relatively depleted in both Rb and Sr as compared with similar inclusions from other regions in the world. It is noteworthy that three of four lherzolites show significantly higher 87Sr/86Sr ratios (0.7044-0.7053) than those of host basaltic rocks (0.7030-0.7033), suggesting an accidental origin for these inclusions. However, one lherzolite is isotopically similar to the host rocks and may be related to the recent magma genesis in this region. The mafic inclusions are characterized by relatively low Rb contents (0.3 ∼ 1.8ppm), and amphibolites show 87Sr/86Sr ratios (0.7030-0.7032) similar to that of host basaltic rocks, but other gabbroic rocks show higher 87Sr/86Sr ratios (0.7035∼0.7048). These results suggest vertical inhomogeneities of the uppermost part of the mantle and lower crust in the Itinome-gata region with respect to Sr isotopes.
We investigate the crustal structure of the Australian continent using the temporary broadband stations of the Skippy and Kimba projects and permanent broadband stations. We isolate near-receiver information, in the form of crustal P-to-S conversions, using the receiver function technique. Stacked receiver functions are inverted for S velocity structure using a Genetic Algorithm approach to Receiver Function Inversion (GARFI). From the resulting velocity models we are able to determine the Moho depth and to classify the width of the crust-mantle transition for 65 broadband stations. Using these results and 51 independent estimates of crustal thickness from refraction and reflection profiles, we present a new, improved, map of Moho depth for the Australian continent. The thinnest crust (25 km) occurs in the Archean Yilgarn Craton in Western Australia; the thickest crust (61 km) occurs in Proterozoic central Australia. The average crustal thickness is 38.8 km (standard deviation 6.2 km). Interpolation error estimates are made using kriging and fall into the range 2.5–7.0 km. We find generally good agreement between the depth to the seismologically defined Moho and xenolith-derived estimates of crustal thickness beneath northeastern Australia. However, beneath the Lachlan Fold Belt the estimates are not in agreement, and it is possible that the two techniques are mapping differing parts of a broad Moho transition zone. The Archean cratons of Western Australia appear to have remained largely stable since cratonization, reflected in only slight variation of Moho depth. The largely Proterozoic center of Australia shows relatively thicker crust overall as well as major Moho offsets. We see evidence of the margin of the contact between the Precambrian craton and the Tasman Orogen, referred to as the Tasman Line.
Petrological and geochemical data for a suite of mafic granulitic xenoliths in Quaternary alkali basalts from the northwestern part of the Arabian plate (NE-Jordan) indicate that these xenoliths represent gabbroic cumulates crystallized at lower crustal pressures. The xenolith mineral assemblages and geothermometry on coexisting minerals suggest equilibration conditions between 6 and 8 kbar and 720-880°C. The xenoliths are a part of a lower crustal gabbroic intrusive complex that underlies the Arabian shield, and may represent the mafic roots of a Pan-African arc complex of Arabia. -Author
Two contrasting suites of granulite xenoliths exhibiting the effects of crustal heating and uplift have been collected from basaltic volcanoes located in two of the widespread Cainozoic basaltic provinces associated with uplift in eastern Australia during the last 70 m.y. Their mineral assemblages and compositions indicate equilibration at mid to lower crustal depths at T too high for normal geothermal gradients. Differences in the mineral compositions of the two suites are interpreted as a function of different whole-rock compositions, T-P histories and tectonic setting. Details of whole-rock chemistry and textural and mineralogical characteristics of the xenoliths are reported and their mineral chemistry is discussed with particular reference to criteria for defining T and P conditions.-M.S.
Granulite fades xenoliths recovered from kimberlites in the Colorado-Wyoming State Line and Iron Mountain, Wyoming Districts are primarily two pyroxene-granulite, two pyroxene garnet granulite, and clinopyroxene garnet granulite. An igneous xenolith suite consisting primarily of gabbronorite appears to be gradational with the granulites. No known granulite facies rocks are exposed in this area and the entire nodule population is interpreted as lower crustal in origin.
The southern Sierra Nevada offers an oblique section through young Cordilleran-type batholith generated crust spanning surface to deep levels. Regional mapping and Pb/U zircon geochronology reveal structural continuity through this crustal section for volcanic, plutonic and metamorphic assemblages developed at ~100 Ma, making it one of the youngest sections in the world. Construction of a synthetic cross-section is well-constrained by the oblique section map pattern, abundant age data, and a published crustal structure section that was based on geophysical and lower crustal xenolith data from across the shallow levels of the batholith. The synthetic cross-section depicts the state of the sialic crustal section during its ~100 Ma petrogenesis. -from Author
A suite of lithic and single-crystal inclusions occur within nepheline-normative, alkali-rich basanites in the eastern Engle basin of the southern Rio Grande rift. The inclusion suite includes lherzolites, pyroxenites, two-pyroxene granulites, and megacrysts of clinopyroxene, olivine, anorthoclase, and spinel. No inclusions are cognate to their basalt hosts; rather they form a petrographic sequence derived from depth. A plot of equilibrium temperatures of these inclusions from coexisting pyroxene compositions versus estimated limits of pressure stability for the observed phase assemblages was fitted to published steady-state conductive heat flow curves for the southern Rio Grande rift to yield a best-fit geotherm of 30°C/km and a heat flow of 2.3 HFU. We speculate the following positions of each inclusion type at depth: granulite 29 to 31 km, pyroxenite 32 to 36 km, megacrysts of olivine, clinopyroxene and pleonaste 32 to 36 km, and lherzolite 36 to >42 km depth. The position of the anorthoclase megacrysts is unknown but thought to be <29 km depth. The Mohorovicic discontinuity corresponds to a marked density increase from granulites to pyroxenites. The presence of similar inclusions beneath the rift, and within a rift-associated graben of the Mogollon volcanic field suggests that a similar petrologic sequence of pyroxenite, megacrysts, and granulite may be a characteristic subcrustal feature of the Rio Grande rift and possibly of the Mogollon volcanic field. Due to the high heat flow, lower pressure phases will be most important in fractionation of the basalts generated within the southern Rio Grande rift.
Nodules of garnet granulites and “crustal” eclogites occur in many kimberlites around the edge of the Kaapvaal craton, but are apparently absent from pipes within the craton. In most Lesotho pipes this suite is dominated by basic cpx+plag+gnt±opx granulites, with smaller numbers of eclogites, garnet websterites and intermediate/acid granulites. Two-pyroxene granulites are common at Monastery Mine.
Mafic and ultramafic inclusions are abundant in potassic latite about 25 my in age that crops out on the southwest structural border of the Colorado Plateau. About 60% are eclogite composed of garnet (Py40Alm37Gross23) and clinopyroxene (5–30 mol.% jadeite) with minor amphibole, apatite, rutile, Fe-Ti oxides and rare altered clinozoisite. Phase layering is common and eclogites with contrasting gnt/cpx ratios occur in sharp contact. Layering in general appears to reflect a cumulus origin rather than metamorphic segregation or tectonic or intrusive juxtaposition. Thirty percent of the inclusions are dominated by pargasitic amphibole (up to 90 modal %) with minor diopside, garnet, phlogopite, apatite and oxides.
An unusual suite of ultramafic xenoliths from Tertiary latites in Chino Valley near Prescott, Arizona, is comprised of garnet pyroxenites and eclogites and represents the first occurrence of eclogite xenoliths in non-kimberlitic rocks on the Colorado plateau. The mineral assemblage of the garnet pyroxenites (calcic clinopyroxene, orthopyroxene, garnet, and Fe-Ti oxide) is the product of a complex subsolidus reequilibration of three aluminous pyroxenes involving exsolution of garnet and an Fe-Ti oxide, deformation, and recrystallization. Unmixing of sub-calcic clinopyroxene, originally coexisting with calcic clinopyroxene and orthopyroxene, resulted in a lamellar intergrowth of orthopyroxene and calcic clinopyroxene. Eclogites (omphacite, garnet) have reacted with the host magma and may show evidence of partial melting. The absence of metamorphic eclogite xenoliths in basaltic rocks in the southwestern United States does not prove that eclogites are absent at depth. Basaltic magmas were probably too hot to preserve the relatively low-temperature eclogite assemblages.
The Pikwitonei and Sachigo subprovinces of central Manitoba provide a cross-sectional view of the Superior province crust. In cross section, the upper to mid-level crust is composed of synformal greenstone belts surrounded by tonalitic gneisses, both of which are intruded by granitic plutons. This crustal structure persists downwards into the granulite facies, where keels of the greenstone belts can be found. To constrain models of the crust, heat production and thermal conductivity in 60 rocks from this terrain were measured using standard gamma -ray spectrometry and divided bar techniques. Large vertical and lateral heterogeneities in heat production in the upper crust are evident; heat production is high in granites and metasedimentary rocks, intermediate in tonalite gneisses, and low in the portions of greenstone belts dominated by mafic meta-igneous rocks. -J.M.H.
Exposed cross sections of the continental crust are recognized on the basis of geophysical and geological data. In some cases, high-grade metamorphic portions of the sections can be traced into contemporary lower crust by geophysical methods. Geobarometric, structural and geophysical data, however, indicate that the deepest crustal levels are not exposed. Exposed cross sections can be emplaced by thrust faults in collisional or intracratonic settings, in transpressional uplifts, extensional regimes and impactogens. Continental crustal evolution, as portrayed by these sections, involves addition of magmas in continental magmatic arcs or island arcs and incorporation of supracrustal rocks into the lower crust. -from Authors
A new variant of the model estimation of the average chemical composition of the upper crust is discussed. The selected representative compositions used in the computations belong to the following three regional petrological associations: (1) the average composition of Precambrian shields and platform (70%), (2) granitoids of orogenic areas (20%), and (3) tonalitic series of batholiths in continental margins (10%). The calculated proportions are consistent with the tectonic systematics and the distribution of the main types of crustal rocks over a standard seismic profile of the crust to a depth of 20 km. In order to reveal the geochemistry of the upper crust, the calculations also involved data on the distribution of some trace elements: Rb, Sr, Ba, Zr, Nb, La, Th, U, Cr, and Ni. The calculated average composition of the upper crust corresponds to granodiorite and is similar to other known model estimates, in spite of the significant differences in the approaches used in these models. This similarity seems to be caused by general petrological regularities: a system of evolutionary trends common for the whole crust and the convergence of processes of crustal magmatism and metamorphism.
Late alkaline lamprophyre-intrusives from the Deccan Trap along the western Indian continental margin entrain rare granulite and pyroxenite xenoliths some of which contain kaersutite ± phlogopite ± apatite ± scapolite ± sulphides providing evidence orfluid induced metasomatism.
P-Testimates on carefully selected mafic granulite and pyroxenite xenoliths define a palaeogeotherm from 550°C, 5 kb to -830°C, 8.5 kb for the Mumbai region along the western continental margin. This elevated geotherm suggests advective heat transfer from magmas ponded near the crust mantle boundary.
Xenolith petrology coupled with seismic data suggests that the lower crust beneath Mumbai consists of mafic granulites under- and intra-plated by pyroxenites. Spinel peridotites interlayered with pyroxenites predominate below -20 km. The seismic Moho is located within the layered pile.
The uniformity of rare earth element patterns in sedimentary rocks is used to provide estimates for the composition of the Post-Archean upper crust. This approximates to granodiorite. The island-arc model for continental growth is used to provide a total crustal composition, from which the upper crust is derived by partial melting, leaving a depleted lower crust. The Archean sedimentary rock patterns reveal a different composition for the upper crust in that era. They closely resemble those of present- day island-arc suites, which the composition of the Archean crust is inferred to be less fractionated, with much less development of high-level granodiorites. Continental evolution appears to be episodic, rather than uniform, with a major period of production of differentiated upper crust at about 2.5 aeons, long recognized as the Archean-Protoerozoic boundary.-from Author
Studies of ecologite and other high-pressure assemblage xenoliths which form a large component of the lowermost crust of terrains such as southeastern Australia have shown that a considerable variety of trace element and isotopic characteristics are present. The anticipated positive Eu anomalies complementary to the persistent negative anomalies typical of the upper crust are generally absent. Overall rare earth element (REE) abundance patterns vary from strongly light REE-depleted to enriched relative to the heavy REE. 87Sr/86Sr and 143Nd/144Nd data for these xenoliths are also heterogeneous and defy selection of representative isotopic ratios for the lower crust in the absence of knowledge of the volumetric significance of individual samples. Thermobarometric calculations assuming equilibrium give temperatures in the range 575-1065 C and pressures of about 10-16kbar. from Authors
The first part of this review summarizes the results of crustal geophysical and geological mapping studies, which have provided both the broad framework and, more recently, some local detail for the continental crust. In the second part the role of crustal xenolith studies is outlined, and the occurrence of xenoliths is summarized by tectonic setting. In the course of this review, several generalizations about the lower crust are emphasized: 1) compositional inferences based on matching of mean seismic velocities and rock types are not unique: 2) the occurrence of lower crustal metasedimentary rocks in several xenolith suites and in exhumed lower crustal sections indicates that mechanisms exist for buryinh surface rocks to great depths, presumably in thrust zones: 3) the regional variability of the lower crust most probably correlates with tectonic setting and should temper extrapolations of lower crustal characterisitcs found in exposed lower crustal sections to other regions.-Authors
Mafic granulite xenoliths occur in basaltic host rocks of Recent to Jurassic age throughout eastern Australia. The granulites
were derived mainly from depths of 15–35 km, with some from up to 75 km depth. The crust-mantle boundary is at about 30 km
beneath eastern Australia, so that some mafic granulite xenoliths were entrained within the upper mantle. The protoliths of
the mafic granulites were derived from basaltic magmas but Sr and Nd isotopic data suggest at least three different petrogenetic
histories for groups of xenoliths from various regions. The first group formed by the simple one-stage process of intrusion
of basaltic magmas, mainly around the crust-mantle boundary and probably closely related to Jurassic-Tertiary volcanism. This
group shows no obvious geochemical interaction with crustal wall-rocks. The second group is also a result of basaltic magmatic
underplating, but shows evidence of mixing between basalts, with depleted isotopic signatures, and a crustal or enriched-mantle
component with 87Sr/86Sr > 0·705. The third group shows a Sr-Nd isotopic mixing relationship suggesting a more complex petrogenesis involving a
component of crustal material subducted into the upper mantle.
Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of ∼ 10-12 kbar and temperatures of ∼ 750-800°C. Approximately 25% have mafic compositions and bear hornblende + plagioclase + clinopyroxene + quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite±sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths-mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al2O3, and Al2O3/CaO, which correspond to high garnet abundance. The mineralogy and major-and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High 87Sr/86Sr and low 143Nd/144Nd, together with highly discordant zircons from a single sample with Pb/Pb ages of ∼ 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages> 4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg-number, and is distinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.
Pelitic xenoliths derived from amphibolite grade basement rocks occur within a Pleistocene, trachytic, pyroclastic unit of the Wehr volcano, East Eifel, West Germany: With increasing temperature and/or prolonged heating at high temperature, quartz-plagioclase and micaceous layers of the xenoliths have undergone melting to form buchites and thermal reconstitution by dehydration reactions, melting and crystallization to form restites respectively. The xenoliths provide detailed evidence of melting, high temperature decomposition of minerals, nucleation and growth of new phases and P-T-f o2 conditions of contact metamorphism of basement rocks by the Wehr magma.Melting begins at quartz-oligoclase (An 17·3Ab 82·3Or 0·4 -An 20·0Ab 78·1Or 1·9 ) grain boundaries in quartz-plagioclase rich layers and the amount of melting is controlled by H 2O and alkalis released during dehydroxylation/oxidation of associated micas. Initially, glass compositions are heterogeneous, but with increasing degrees of melting they become more homogeneous and are similar to S-type granitic minimum melts with SiO 2 between 71 and 77 wt. per cent; A/(CNK) ratios of 1·2-1·4; Na 2O < 2·95 and normative corundum contents of 1·9-4·0 per cent. Near micas plagioclase melts by preferential dissolution of the NaAlSi 3O 8 component accompanied by a simultaneous increase in CaAl 2Si 2O 8 (up to 20 mol. per cent An higher than the bulk plagioclase composition) at the melting edge. With increasing temperature the end product of fractional melting is the formation and persistence of refractory bytownite (An 78-80) in those xenoliths where extensive melting has taken place.Initial stage decomposition of muscovite involves dehydroxylation (H 2O and alkali loss). At higher temperatures muscovite breaks down to mullite, sillimanite, corundum, sanidine and a peraluminous melt. Mullite (40-43 mol. per cent SiO 2) and sillimanite (49 mol. per cent SiO 2) are Fe 2O 3 and TiO 2 rich (up to 6·1-0·84 and 3·6-0·24 wt. per cent respectively). Al-rich mullite (up to 77 wt. per cent Al 2O 3) occurs with corundum which has high Fe 2O 3 and TiO 2 (up to 6·9 and 2·1 wt. per cent respectively). Annealing at high temperatures and reducing conditions results in the exsolution of mullite from sillimanite and ilmenite from corundum. Glass resulting from the melting of muscovite in the presence of quartz is peraluminous (A/(CNK) = 1·3) with SiO 2 contents of 66-69 per cent and normative corundum of ∼4 per cent. Sanidine (An 1·9Ab 26·0Or 72·1 -An 1·3Ab 15·9Or 82·9 ) crystallized from the melt.Dehydroxylation and oxidation of biotite results in a decrease of K 2O from 8·6 to less than 1 wt. per cent and oxide totals (less H 2O + contents) from 96·5 to 88·6, exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg) ratio from 0·41 to 0·17. Partial melting of biotite in the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite, sanidine(An 2·0Ab 34·9Or63·1) and melt takes place at higher temperatures. Glass in the vicinity of melted biotite is pale brown and highly peraluminous (A/CNK = 2·1) with up to 6 wt. per cent MgO+FeO (total iroq) and up to 10 per cent normative corundum. Near liquidus biotite with higher Al 2O 3 and TiO 2 than partially melted biotite crystallized from the melt. Ti-rich biotites (up to 6 wt. per cent TiO 2) occur within the restite layers of thermally reconstituted xenoliths. Melting of Ti-rich biotite and sillimanite in contact with the siliceous melt of the buchite parts of xenoliths resulted in the formation of cordierite (100 Mg/(Mg+Fe+Mn) = 76·5-69·4), Al-Ti magnetite and sanidine, and development of cordierite/quartz intergrowths into the buchite melt. Growth of sanidine enclosed relic Ca-plagioclase to form patchy intergrowths in the restite layers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64-69), quartz, sillimanite, mullite, magnetite and ilmenite, crystallized from the peraluminous buchite melt.Green-brown spinels of the pleonaste-magnetite series have a wide compositional variation of (mol. per cent) FeAl 2O 4-66·6-45·0; MgAl 2O 4-53·0-18·7; Fe 3O 4-6·9-28·1; MnAl 2O 4-1·2-1·5; Fe 2TiO 4-0·6-6·2. Rims are generally enriched in the Fe 3O 4 component as a result of oxidation. Compositions of ilmenite and magnetite (single, homogeneous and composite grains) are highly variable and result from varying degrees of high temperature oxidation that is associated with dehydroxylation of micas and melting. Oxidation mainly results in increasing Fe 3+, Al and decreasing Ti 4+, Fe 2+ in ilmenite, and increasing Fe 2+, Ti 4+ and decreasing Fe 3+ in associated magnetite. A higher degree of oxidation is reached with exsolution of rutile from ilmenite and formation of titanhematite and with exsolution of pleonaste from magnetite. Ti-Al rich magnetite (5·1-7·5 and 8·5-13·5 wt. per cent respectively) and ilmenite crystallized from melts in buchitic parts of the xenoliths.Chemical and mineralogic evidence indicates that even with extensive melting the primary compositions of individual layers in the xenoliths remained unmodified. Apparently the xenoliths did not remain long enough at high temperatures for desilication and enrichment in Al 2O 3, TiO 2, FeO, Fe 2O 3, and MgO that results by removal of a 'granitic' melt, and/or by interaction with the magma, to occur.T °C-f o2 values calculated from unoxidized magnetite/ilmenite give temperatures ranging from 615-710°C for contact metamorphism and the beginning of melting, and between 873 and 1054°C for the crystallization of oxides and mullite/sillimanite from high temperature peraluminous melts. f o2 values of metamorphism and melting were between the Ni-NiO and Fe 2O 3-Fe 3O 4 buffer curves. The relative abundance of xenolith types, geophysical evidence and contact metamorphic mineralogy indicates that the xenoliths were derived from depths corresponding to between 2-3 kb P load = P fluid. The xenoliths were erupted during the latest phreatomagmatic eruption from the Wehr volcano which resulted in vesiculation of melts in partially molten xenoliths causing fragmentation and disorientation of solid restite layers.
Sulphur-rich scapolite is described as a primary constituent of several granulite inclusions occurring, along with eclogitic and rare ultrabasic rocks, in the breccia- and basalt-filled pipes of eastern Australia. The wide distribution of such inclusions in space and time indicates that granulites, some scapolite-bearing, form an important part of the base of the crust of this region.Two inclusions with the assemblage orthopyroxene-clinopyroxene-plagioclase-scapolite, two with the assemblage clinopyroxene-garnet-plagioclase-scapolite, and one with the assemblage orthopyroxene-clinopyroxene-plagioclase, have been analysed and chemical and optical data on the constituent minerals are given. Since it is impossible to adequately separate scapolite from primary and secondary plagioclase, partial analyses of these minerals have been made with an A.R.L. electron probe X-ray microanalyser.From our data and that obtained from the literature, we suggest that scapolites with more than 10 mol per cent sulphur in the anion group are confined to rocks of the granulite meta-morphic facies. Within the granulite facies there appears to be a progressive increase in the sulphur content of scapolite in equilibrium with such sulphur-rich minerals as pyrrhotite, with increasing metamorphic grade.
A survey is given of the dimensions and composition of the present continental crust. The abundances of immobile elements in sedimentary rocks are used to establish upper crustal composition. The present upper crustal composition is attributed largely to intracrustal differentiation resulting in the production of granites senso lato. Underplating of the crust by ponded basaltic magmas is probably a major source of heat for intracrustal differentiation. The contrast between the present upper crustal composition and that of the Archean upper crust is emphasized. The nature of the lower crust is examined in the light of evidence from granulites and xenoliths of lower crustal origin. It appears that the protoliths of most granulite facies exposures are more representative of upper or middle crust and that the lower crust has a much more basic composition than the exposed upper crust. There is growing consensus that the crust grows episodically, and it is concluded that at least 60% of the crust was emplaced by the late Archean (ca. 2.7 eons, or 2.7 Ga). There appears to be a relationship between episodes of continental growth and differentiation and supercontinental cycles, probably dating back at least to the late Archean. However, such cycles do not explain the contrast in crustal compositions between Archean and post-Archean. Mechanisms for deriving the crust from the mantle are considered, including the role of present-day plate tectonics and subduction zones. It is concluded that a somewhat different tectonic regime operated in the Archean and was responsible for the growth of much of the continental crust. Archean tonalites and trond-hjemites may have resulted from slab melting and/or from melting of the Archean mantle wedge but at low pressures and high temperatures analogous to modern boninites. In contrast, most andesites and subduction-related rocks, now the main contributors to crustal growth, are derived ultimately from the mantle wedge above subduction zones. The cause of the contrast between the processes responsible for Archean and post-Archean crustal growth is attributed to faster subduction of younger, hotter oceanic crust in the Archean (ultimately due to higher heat flow) compared with subduction of older, cooler oceanic crust in more recent times. A brief survey of the causes of continental breakup reveals that neither plume nor lithospheric stretching is a totally satisfactory explanation. Speculations are presented about crustal development before 4000 m.y. ago. The terrestrial continental crust appears to be unique compared with crusts on other planets and satellites in the solar system, ultimately a consequence of the abundant free water on the Earth.
Alkaline lamprophyre intrusives from the western Deccan Traps (Murud-Janjira, south of Bombay) host rare lithospheric xenoliths and megacrysts. The xenolith suite consists of clinopyroxenites and granulites which show eclogitic affinities. The former have transitional (porphyroclastic to equigranular) textures whereas the latter are porphyroclastic, xenomorphic to meta-igneous. The textural features provide evidence of ductile-brittle deformation. The protoliths of the pyroxenite and granulite xenoliths were formed as cumulates of alkaline and sub-alkaline magmas respectively. Mineral chemistry and geochemical data for the xenoliths bear testimony to the metasomatized nature of the deep crust. The xenolith data coupled with the geophysical evidence indicate that the lower crust beneath Murud-Janjira is dominated by mafic granulites and pyroxenites. The latter have under- and intra-plated the continental crust beneath the region.
Studies of high-pressure xenoliths suggest that mafic rocks are dominant
in the lower crust and abundant in the uppermost mantle, beneath most
continental areas. Xenolith data allow construction of geotherms and
stratigraphic profiles and provide lithological constraints and physical
parameters for realistic interpretations of geophysical data. In
continental regions of high heat flow, the Moho, as defined by seismic
refraction data, may lie deeper than the crust/mantle boundary. The
seismic differences recorded for cratonic vs. “hot” lower
crust/upper mantle sections may be explained by temperature variations
alone and do not require large lithological differences. *Present
address: Division of Mineral Physics and Mineralogy, CSIRO, P.O. Box
136, North Ryde, New South Wales 2113, Australia
This completely rewritten new edition begins with a historical
perspective of the place of the solar system in the universe. Evidence
from meteorites is used to describe how the planets were formed and the
giant planets are considered in the light of the discovery of new
extrasolar giants. Other chapters discuss satellites, comets, centaurs,
asteroids and why Pluto is not a planet. Explanations on why Earth and
Venus turned out so differently, and how Mars and Mercury are the
survivors of many similar bodies, are also discussed.
The thermal history of the lower crust and upper mantle of the Colorado Plateau region is reconstructed on the basis of Nd and Sr isotopes in minerals and whole rock xenoliths hosted by Tertiary minette and kimberlite. The mineral data (garnet and clinopyroxene) indicate that lower crustal granulite and amphibolite (equilibration depth ≈25 km; equilibration temperature ≈700°C) were last equilibrated on a mineral scale at 1345 ± 10 Ma; a Sm-Nd garnet-whole rock age for a granitoid xenolith is also 1345 ± 10 Ma. Whole rock data indicate that the crustal rocks were extracted from the mantle at ca. 1900 Ma. The mineral ages, which are 30-100 m.y. younger than crystallization ages of Proterozoic "anorogenic" granitoids from regions bordering the Colorado Plateau, are interpreted as cooling ages set following the crustal thermal maximum at 1380-1440 Ma. The granulites and amphibolites have remained at temperatures below ≈450°C since 1350 Ma. Two eclogite xenoliths (equilibration depth ≈45-60 km; equilibration temperature ≈600°C), which are inferred to be Precambrian as well, yield Sm-Nd garnet-clinopyroxene ages of 21.6 ± 1.2 and 21.0 ± 0.8 Ma. The eclogite mineral ages are probably the ages of the host Garnet Ridge and Moses Rock diatremes, and require that Nd isotopes were maintained in equilibrium right up to the time of entrainment. The isotopic data and the mineral textures suggest that the eclogites were undergoing active recrystallization at 21 Ma. The contrast in mineral ages between granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilibration, and therefore cannot be considered as evidence for a negative thermal gradient at depth. The Rb-Sr mineral data from the xenoliths give variable early Paleozoic and Proterozoic ages that cannot easily be assigned to geologic events.
The abundant quartzofeldspathic and mafic granulite facies xenoliths
which occur at Kilbourne Hole maar in New Mexico provide an opportunity
to study samples of the earth's deep crust which have been directly
sampled by and incorporated into ascending magmas during explosive
volcanic eruptions. Sillimanite-bearing garnet granulite, garnet
orthopyroxenite, two-pyroxene granulite, charnockite, and anorthosite
are the rock types represented in the deep crustal xenolith population.
Equilibration temperatures of 750°-1000°C and pressures of 6-9
kbar are derived from the observed chemistries and phase assemblages of
the garnet granulites. The chemistry of the more mafic charnockites and
two-pyroxene granulites suggests equilibration in a similar
temperature-pressure regime. Mineral equilibria in the suite of crustal
xenoliths have been applied to estimate the geothermal gradient at their
depth of origin prior to their incorporation into the eruptive process
which formed Kilbourne Hole maar. The geotherm thus defined has a slope
of about 30°/km at depths between 22 and 28 km. This slope agrees
with the value predicted from heat flow interpretations in the southern
Rio Grande Rift. The whole-rock chemistry and anhydrous nature of the
garnet granulites suggest further that they may represent residues from
the partial melting of a pelitic sediment and extraction of a water-rich
acidic magma, which probably crystallized in the intermediate crust. The
association of these rock types suggest that a granulite facies
metamorphic complex, perhaps related to minor anorthosite, is
representative of the lower crust in this region.