Alkali Enrichment Research Articles

On the Pan-African transition of the Arabian–Nubian Shield from compression to extension: The post-collision Dokhan volcanic suite of Kid-Malhak region, Sinai, Egypt

The Pan-African Kid-Malhak Dokhan volcanic suite (609 ± 12 Ma) is exposed in the northernmost part of the Arabian–Nubian Shield. The suite consists of non-metamorphosed varicolored alternating succession of porphyritic lava flows of commonly felsic composition (rhyolite–dacite) interlayered with compositionally equivalent pyroclastic beds (dominantly ignimbrites). These Dokhan volcanics are quite evolved (SiO 2 ≈ 65–77 wt.%), with strong high-K calc-alkaline affinity and are characterized by relative enrichment in total alkalis, Ba, Y, Zr and total REEs, depletion in Sr, and a LREE-enriched REE patterns with significant negative Eu anomalies. The Kid-Malhak Dokhan lavas display geochemical characteristics of both orogenic arc-type and anorogenic within-plate environments, suggesting eruption in a transitional “post-collisional tectonic setting. The ages of emplacement of the Dokhan volcanics in Egypt including that of Kid-Malhak region (580–620 Ma) coincide with end of the documented collision between the juvenile Arabian–Nubian crust and Saharan Metacraton and the subsequent extensional collapse event. This post-collision transition from compression to extension is explained by the extensional collapse following continental collision, which was controlled mainly by lithospheric delamination and slab breakoff (passive rifting). Various trace element characteristics discussed herein have indicated that the studied Dokhan magma was highly likely generated from crustal sources and that assimilation–fractional crystallization (AFC) and crustal contamination have played a major role and are most probably superimposed on fractional crystallization during the magmatic evolution of Kid-Malhak Dokhan volcanic suite. The eruption of the high-K calc-alkaline post-collisional Dokhan volcanics in Egypt defines a tectono-magmatic transition between the older calc-alkaline arc-related and the subsequent alkaline magmatism in the northern part of the Arabian–Nubian Shield.

Differentiation and crystallization conditions of basalts from the Kerguelen large igneous province: an experimental study

Phase relations of basalts from the Kerguelen large igneous province have been investigated experimentally to understand the effect of temperature, fO2, and fugacity of volatiles (e.g., H2O and CO2) on the differentiation path of LIP basalts. The starting rock samples were a tholeiitic basalt from the Northern Kerguelen Plateau (ODP Leg 183 Site 1140) and mildly alkalic basalt evolved from the Kerguelen Archipelago (Mt. Crozier on the Courbet Peninsula), representing different differentiation stages of basalts related to the Kerguelen mantle plume. The influence of temperature, water and oxygen fugacity on phase stability and composition was investigated at 500 MPa and all experiments were fluid-saturated. Crystallization experiments were performed at temperatures between 900 and 1,160°C under oxidizing (log fO2 ~ ΔQFM + 4) and reducing conditions (log fO2 ~ QFM) in an internally heated gas-pressure vessel equipped with a rapid quench device and a Pt-Membrane for monitoring the fH2. In all experiments, a significant influence of the fO2 on the composition and stability of the Mg/Fe-bearing mineral phases could be observed. Under reducing conditions, the residual melts follow a tholeiitic differentiation trend. In contrast, melts have high Mg# [Mg2+/(Mg2+ + Fe2+)] and follow a calk-alkalic differentiation trend at oxidizing conditions. The comparison of the natural phenocryst assemblages with the experimental products allows us to constrain the differentiation and pre-eruptive conditions of these magmas. The pre-eruptive temperature of the alkalic basalt was about 950–1,050°C. The water content of the melt was below 2.5 wt% H2O and strongly oxidizing conditions (log fO2 ~ ΔQFM + 2) were prevailing in the magma chamber prior to eruption. The temperature of the tholeiitic melt was above 1,060°C, with a water content below 2 wt% H2O and a log fO2 ~ ΔQFM + 1. Early fractionation of clinopyroxene is a crucial step resulting in the generation of silica-poor and alkali-rich residual melts (e.g., alkali basalt). The enrichment of alkalis in residual melts is enhanced at high fO2 and low aH2O.

How unique is the Udachnaya-East kimberlite? Comparison with kimberlites from the Slave Craton (Canada) and SW Greenland

The origin of alkali carbonates and chlorides in the groundmass of unaltered Udachnaya-East kimberlites in Siberia is still controversial. Contrary to existing dogma that the Udachnaya-East kimberlite was either contaminated by the crustal sediments or platform brines, magmatic origin of the groundmass assemblage has been proposed on the basis of melt immiscibility textures, melt inclusion studies, and strontium and neon isotope compositions. We further tested the idea of alkali- and chlorine enrichment of the kimberlite parental melt by studying olivine-hosted melt inclusions and secondary serpentine in kimberlites from the Slave Craton, Canada (Gahcho Kué, Jericho, Aaron and Leslie pipes) and southern West Greenland (Majuagaa dyke). Host olivine phenocrysts closely resemble groundmass olivine from the Udachnaya-East kimberlite in morphology, compositions (high-Fo, low-Ca), complex zoning with cores of varying shapes and compositions and rims of constant Fo. Melt inclusions in olivine consist of several translucent and opaque daughter phases and vapour bubble(s). The daughter crystals studied in unexposed inclusions by laser Raman spectroscopy and in carefully exposed inclusions by WDS-EDS are represented by Na-K chlorides, calcite, dolomite, magnesite, Ca-Na, Ca-Na-K and Ca-Mg-Ba carbonates, bradleyite Na 3 Mg(CO 3)(PO 4), K-bearing nahpoite Na 2(HPO 4), apatite, phlogopite and tetraferriphlogopite, unidentified sulphates, Fe sulphides, djerfisherite, pyrochlore (Na,Ca) 2Nb 2O 6(OH,F), monticellite, Cr-spinel and Fe-Ti oxides. High abundances of Na, K (e.g., (Na + K)/Ca = 0.15-0.85) and incompatible trace elements in the melt inclusions are confirmed by LA-ICPMS analysis of individual inclusions. Heating experiments show that melting of daughter minerals starts and completes at low temperatures (~ 100 °C and 600 °C, respectively), further reinforcing the similarity with the Udachnaya-East kimberlite. Serpentine minerals replacing olivine in some of the studied kimberlites demonstrate elevated abundances of chlorine (up to 3–4 wt.%), especially in the early generation. Despite heterogeneous distribution of chlorine such abundances are significantly higher than in the serpentine in abyssal and ophiolitic peridotites (< 0.5 wt.%). The groundmass of most kimberlites, including those studied here and altered kimberlites from the Udachnaya pipe, contain no alkali carbonates and chlorides and have low Na 2O (< 0.2 wt.%). We believe that alteration disturbs original melt compositions, with the alkaline elements and chlorine being mostly affected. However, the compositions of melt inclusions and serpentine are indicative of the chemical signature of a parental kimberlite melt. It appears that enrichment in alkalies and chlorine, as seen in unaltered Udachnaya-East kimberlites, is shared by other kimberlites, and thus can be assigned to deep mantle origin.

Late Paleozoic underplating in North Xinjiang: Evidence from shoshonites and adakites

Shoshonitic series volcanic rocks (SSVR) and adakites are widely distributed in the Permian terrestrial volcanic strata of the Yishijilike–Awulale range of west Tianshan, north Xinjiang, China. Isotopic dating yields Permian ages of 280–250 Ma. The SSVR include absarokite, shoshonite and banakite which are characterized by enrichment of alkalis, particularly in K, combined with lower Ti, higher Al (A/NKC = 0.70–0.99, metaluminous) and Fe 2O 3 > FeO. The SSVR that are rich in LILE with high REE contents and Eu/Eu⁎ range from 0.59 to 1.30. They are rich in LREE ((La/Yb) N 2.15–11.97) and depleted in Nb, Ta and Ti (TNT negative anomalies). The adakites are metaluminous to weakly peraluminous (A/NKC = 0.85-1.16) and belong to the high-SiO 2 type of adakite (HSA, SiO 2 = 62%–71%). They are characterized by lower ΣREE with strong LREE enrichment ((La/Yb) N 13–35). Pronounced positive Eu anomalies (Eu/Eu⁎ = 1.02–1.27), very low Yb contents and distinct TNT-negative anomalies are evident. The SSVR have ε Nd(t) (+ 1.28 to + 4.92) and ( 87Sr/ 86Sr) i (0.7041–0.7057) that are similar to adakites in the regions which are characterized by ε Nd(t) = 0.95 to + 5.69 and ( 87Sr/ 86Sr) i = 0.7050–0.7053. Trace element, REE and Sr/Nd isotopic compositions suggest that both SSVR and adakites possess similar source regions associated with underplated mantle-derived basaltic materials. Lithosphere extension driven by magmatic underplating was responsible for the generation of both the SSVR and adakites. This magmatism serves as a petrological indicator of underplating during the Permian. Obviously thickened crust (62–52 km), a complex Moho discontinuity, high heat flow (~ 100 mw·m − 2 ), widespread contemporary alkali-rich granites, basic dike swarms (K–Ar ages of 187–271 Ma, Ar–Ar ages of 174–270 Ma and Rb–Sr ages of 255 ± 28 Ma; ε Nd(t) + 1.84 to + 10.1; ( 87Sr/ 86Sr) i 0.7035 and 0.7065), and basic granulites (SHRIMP U–Pb age of 268–279 ± 5.6 Ma) provide additional evidences for the underplating event in this area during Permian.

Geochemistry of Permian bimodal volcanic rocks from central Inner Mongolia, North China: Implication for tectonic setting and Phanerozoic continental growth in Central Asian Orogenic Belt

Zircon U–Pb ages, geochemical and Sr–Nd–Pb isotopic data are presented for the late Paleozoic volcanic rocks from Xilinhot in central Inner Mongolia, North China. The Xilinhot volcanic rocks show a bimodal distribution in composition, with dominant rhyolite and dacite, subordinate basalt and local andesite. New SHRIMP and LA-ICPMS zircon U–Pb isotopic data constrain their magmatic emplacement at ca. 280 Ma. The mafic samples are characterized by high abundances in Th, U and Pb, slightly enriched LREE patterns and low HFS/LREE ratios. These features, together with their OIB-like isotopic signature ( 87Sr/ 86Sr ( t) ≈ 0.704 to 0.705, ε Nd( t) ≈ + 6.87 to + 7.90, ( 206Pb/ 204Pb) i ≈ 18.30 to 19.06), indicate that they were likely derived from high percentage partial melting of a subduction-related metasomatized asthenospheric mantle in the spinel facies field at depths shallower than 60–70 km. The felsic rocks show an A-type affinity, with typical enrichment in alkalis, Ga, Zr, Nb, Ce, Y, and depletion in Sr, P, Eu, and Ti, and fractionated REE patterns with very strong negative Eu anomalies. Combined with their moderately positive ε Nd( t) values (+ 1.85–+ 3.60), they are inferred to be generated by the mixing between partial melts of a newly underplated lower crustal source and an ancient felsic crustal source. The typical magma progression starting with asthenospheric melts then proceeding to lithospheric melts indicates that Early Permian magmatism in central Inner Mongolia occurred in an extensional setting, most probably related to the post-collisional delamination. Therefore, the occurrence of this unique bimodal magmatism not only demonstrates that the Solonker zone experienced final amalgamation of arc-related terranes by the late-Carboniferous, but also indicates that significant vertical continental growth of crustal continuum through repeated magmatic underplating occurred in the continental interior during post-collisional extension.

Geochemical character of serpentinites associated with high‐ to ultrahigh‐pressure metamorphic rocks in the Alps, Cuba, and the Himalayas: Recycling of elements in subduction zones

Serpentinites associated with eclogitic rocks were examined from three areas: the Alps, Cuba, and the Himalayas. Most serpentinites have low Al/Si and high concentrations of Ir‐type platinum group elements (PGE) in bulk rock compositions, indicating that they are hydrated mantle peridotites. A few samples contain high Al/Si and low concentrations of Ir‐type PGE, suggesting that they are ultramafic cumulates. Among the hydrated mantle peridotites, we identified two groups, primarily on the basis of Al/Si and Mg/Si ratios: forearc mantle serpentinites and hydrated abyssal peridotites. Forearc serpentinites occur in the Himalayas and along a major deformation zone in Cuba. All serpentinites in the Alps and most serpentinites in Cuba are hydrated abyssal peridotites. Himalayan serpentinites have low Al/Si and high Mg/Si ratios in bulk rock compositions, and high Cr in spinel; they were serpentinized by fluids released from the subducted Indian continent and enriched in fluid‐mobile elements, and show high 87Sr/86Sr, up to 0.730, similar to the values of rocks of the subducted margin of the Indian continent. Although Himalayan serpentinites have a similar refractory geochemical signature as the Mariana forearc serpentinites, the former contain markedly high concentrations of fluid‐mobile elements and high 87Sr/86Sr compared to the latter that were hydrated by subducted Pacific Ocean crust. The data indicate that the enrichment of fluid‐mobile elements in forearc serpentinites depends on the composition of subducted slabs. Alpine serpentinites and most Cuban serpentinites show moderate Al/Si similar to abyssal peridotites. Hydration of peridotites near the seafloor is supported by micro‐Raman spectra of earlier formed lizardite, high δ34S (+11 to +17‰) of sulphides, and elevated 87Sr/86Sr, ranging from 0.7037 to 0.7095. The data support the contribution of S and Sr from seawater and sediments. These serpentinites are not highly enriched in fluid‐mobile elements because serpentinization occurred at a high water/rock ratio. Alkali elements are conspicuously unenriched in all serpentinites. This lack of alkali enrichment is explained by slab retention of alkalis. This is also consistent with the observation of relatively low alkali concentrations in volcanic front magmas, since partial melting related to the volcanic fronts is triggered by dehydration of serpentinites.

Magma evolution in the upper part (Stage IV) of the Fongen-Hyllingen Layered Intrusion, central Norway

The upper part (Stage IV) of the Fongen-Hyllingen intrusion, southeast of Trondheim, Norway, comprises a ∼ 1800 m thick sequence of cumulates showing a strong normal fractionation trend from olivine gabbros at the bottom through diorites to quartz-bearing syenites at the roof. Mineral compositions show extreme ranges: olivine Fo74-0; Ca-rich pyroxene Mg#79-0; plagioclase An60-0. The cumulate sequence has been divided into five intervals based on cumulus assemblages. The summation method has been used to calculate successive magma compositions for these intervals based on whole-rock major element chemical analyses of 49 samples. The most primitive calculated magma composition has 51.7% SiO2, 5.0% MgO, 5.3% total alkalis and a Mg-number of 47.5; it lies near the base of the basaltic trachyandesite field in the Total Alkali Silica (TAS) diagram, very close to the dividing line between tholeiitic and alkaline suites. This magma evolved through the trachyandesite field into the trachyte field. The most evolved magma composition has ∼ 11.4% alkalies; alkali enrichment is not due to assimilation of metapelites. There was very little iron-enrichment during fractionation and magma density decreased continuously, enabling repeated influxes of new magma to pond and to elevate the resident melt, facilitating magma stratification. The parental magma may have resided in a lower crustal chamber where it differentiated before repeatedly supplying the overlying Fongen-Hyllingen chamber.

Granitization of Paleoproterozoic high-pressure metagabbro-norites of the Belomorian Group in Gorelyi Island, Kandalaksha Bay area, Baltic Shield

The process of granitization was studied in a small massif of high-pressure Paleoproterozoic (2.45–2.36 Ga) metagabbro-norites (coronites) of the Belomorian Group at contacts with Bt-Hbl-Kfs-Pl-Qtz gneiss-granites. The granitization process occurred at the culmination of Svecofennian metamorphism, at T = 660–700°C and P = 9.5–10.5 kbar. The process affected the marginal portion (15–20 m thick) of the massif and proceeded under the effect of silicic-alkaline H2O-Cl-CO2 fluids related to the gneiss-granites and was of infiltration nature. The metagabbroids became enriched in Na, K, Si, Cl, CO2, and H2O and depleted in Ca, Mg, Fe, and Cr, which were removed outside the reaction zone. The debasified metabasites underwent melting during the final stages of the process. The changes in the bulk-rock compositions became more intense closer to the contact with the gneiss-granite, as can be clearly seen in the variation diagrams. Before their transformations, the metagabbroids contained magmatic minerals: pigeonite, pigeonite-augite, high-Ti biotite, and labradorite-bytownite, which are partly replaced by metamorphic minerals (sometimes in the form of reaction coronas): garnet, clinopyroxene, orthopyroxene, magnesian hornblende, anthophyllite, biotite, and andesine. According to the changes in the phase relations and the composition of the rocks, the contact-reaction interval can be subdivided into the following four zones: (I) weakly amphibolized metagabbro-norite; (II) apogabbro Hbl-Pl±Scp ± Qtz plagioamphibolites with rims, and then with pseudomorphs, after magmatic and corona minerals; (III) K-feldspathized Hbl-Bt-Pl-Kfs ± Scp-Qtz apogabbro amphibolites; and (IV) gneiss-granites with skialiths of bleached Bt-Hbl-Pl-Kfs ± Scp-Qtz amphibolites. Zone I contains newly formed embryonic Hbl, Opx, and Ath reaction rims around magmatic and coronitic minerals. In zone II, Pig, Pig-Aug, and Grt are completely replaced by hornblende and anthophyllite, and Lbr is replaced by andesine and scapolite with a calcite admixture. In zone III, Hbl is further replaced by biotite; andesine is replaced by oligoclase, potassic feldspar, and quartz; and Ath and Opx are decomposed. Biotitization becomes more intense in zone IV, where Hbl and Bt are partly replaced by feldspars and quartz, which mark the maximum of the debasification and enrichment in alkalis of the metagabbro-norites. The paper presents descriptions of reaction textures and mineral reactions that mark the transitions between the zones and considers the stability conditions of Ath and Opx in zones I and II and the causes of the disappearance of these minerals from zones III and IV. Equilibria of the Pl-Scp pair are discussed. The variations in the compositions of minerals from zone I through zone IV are systematic and display clearly pronounced tendencies. The hornblende becomes progressively more ferrous (its iron mole fraction increases from 0.31 to 0.69), along with an increase in the concentrations of alkalis and the K/Na ratio. The biotite also becomes more ferrous (from 0.28 to 0.61) and depleted in Ti in comparison with magmatic biotite. The anorthite content in plagioclases decreases from 60–80 to 20–22%. The content of the meionite end member in the scapolite varies from 30 to 50% but depends not on the zone but on the CO2 and Cl concentrations in the fluid. The distribution of the Me and An end members between Scp and Pl is equilibrium. Calculations indicate that the fluid in the most strongly transformed rear zone IV was a highly concentrated carbon dioxide-chloride brine rich in alkalis and silica. The transformations of the gabbroids in zones I–III was of metasomatic character. Zone IV is marked by the onset of the fragmentation of the amphibolized and bleached metagabbronorites, their transformation into skialiths submerged into gneiss-granite, and their further debasification and enrichment in alkalis. This process ends with the progressively more intense melting and dissolution of the nebular remnants of the metabasites in the anatectic granite migma or magma. The aureole described in this publication makes it possible to trace all granitization stages in metagabbroids in compliance with the known model proposed by D.S. Korzhinskii.

Slagging and fouling characteristics of seam 32/33, Panian coalfield, Semirara Island, Philippines

Twenty samples of seam 32/33, the main seam of Panian coalfield in Semirara Island, Antique Province, Philippines, were collected from a borehole drilled at the northeastern edge of the coalfield. The samples were analyzed to characterize the coal geochemistry of the seam and understand why the coals of Semirara Island exhibit a high tendency for slagging and fouling despite its low average ash content. Analysis of the slagging and fouling characteristics of this seam is important because it supplies five electric power utilities and several cement plants in the Philippines. Proximate analyses and vitrinite reflectance measurements designate the rank of the seam as sub-bituminous C, based on ASTM coal classification. H/C versus O/C ratios indicate that the seam is made up largely of huminite, denoting early stages of coalification. Chemical analysis of the ash reveals high contents of Na, Mg, Fe, Ca, Ba and Sr. The strongly negative correlation of these elements with the ash content indicates an organic affinity of the chemical elements of the seam. Owing to enrichment in alkali and alkali-earth elements, slagging and fouling indices indicate that the seam has medium to high propensity for slagging and a severe tendency for fouling. The detrimental characteristics of coal feedstock from Panian mine is mitigated by modifications to the boiler design and operational conditions and by blending with coals imported from Indonesia, China and Australia.

Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China

A late Mesozoic belt of volcanic–intrusive complexes occurs in SE China. Volcanic activity at Xiangshan in the NW of the belt took place mainly in the Late Jurassic (158–135Ma). The volcanic rocks from the Xiangshan volcanic complex include rhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyritic lavas, and associated subvolcanic rocks. Mineral assemblages in these magmatic rocks include K-feldspar, plagioclase, quartz, Fe-rich biotite and minor amphibole, orthopyroxene and almandine. Mineral geothermometry indicates a high crystallization temperature (>850 C) for the Xiangshan magmas. The volcanic rocks are generally peraluminous; SiO2 contents are between 65 4% and 76 8% and the samples have high alkalis, rare earth elements (REE), high field strength elements and Ga contents and high Ga/Al ratios, but are depleted in Ba, Sr and transition metals. Trace element geochemistry and Sr–Nd–O isotope systematics imply that the Xiangshan magmas were probably derived from partial melting of Middle Proterozoic metamorphic lower-crustal rocks that had been dehydrated during an earlier thermal event. These features suggest an A-type affinity. Quenched mafic enclaves, hosted by the subvolcanic rocks, consist mainly of alkali feldspar, plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometry calculations indicate that the primary magmas that chilled to form the quenched enclaves had anomalously high temperatures (>1200 C). The quenched enclaves have boninitic affinities; for example, intermediate SiO2 contents, high MgO and low TiO2 contents, high Mg-numbers and high concentrations of Sc, Ni, Co and V. However, they also have shoshonitic characteristics, e.g. enrichment in alkalis, high K2O contents with high K2O/Na2O ratios, high light REE and large ion lithophile element contents, low initial eNd values ( 4 2) and high initial Sr/Sr ratios (0 7081). We suggest a phlogopite-bearing spinel harzburgitic lithospheric mantle source for these high-Mg potassic magmas. Underplating of such anomalously high-temperature magmas could have induced granulite-facies lower-crustal rocks to partially melt and generate the Xiangshan A-type volcanic suite. A back-arc extensional setting, related to subduction of the Palaeo-Pacific plate, is favoured to explain the petrogenesis of the Xiangshan volcanic complex and quenched enclaves.

Pervasive assimilation of carbonate and silicate rocks in the Hortavær igneous complex, north-central Norway

The intrusive complex at Hortavær represents a magma transfer zone in which multiple pulses of gabbroic and dioritic magmas evolved along Fe- and alkali-enrichment trends. Extreme alkali enrichment resulted in nepheline-normative and sparse nepheline-bearing monzodioritic and monzonitic rocks. More evolved monzonitic and syenitic rocks are silica saturated and, in some cases, quartz bearing. Previous and current research recognized an abundance of clinopyroxene and other Ca-rich phases, such as scapolite, grossular-rich garnet, and igneous-textured calcite among the mafic and intermediate rocks. Even the most pyroxene-rich samples contain low Sc concentrations, which suggests early, intense fractionation of clinopyroxene. These features and the alkali enrichment are consistent with assimilation of carbonate-rich host rocks. Carbon isotope ratios of the igneous-textured calcite indicate an origin of the carbon from host rocks rich in calcite, consistent with assimilation. However, low ɛ Nd values (−3.4 to −10.2) and moderate initial 87Sr/ 86Sr values (0.7052 to 0.7099) indicate the need for assimilation of quartzofeldspathic rocks as well. Models of combined assimilation and fractional crystallization indicate that assimilation of simple end members, either carbonate or silicate, cannot explain the entire data set. Instead, variable proportions of carbonate and silicate materials were assimilated, with the most pronounced assimilation effects in the mafic rocks. The reasons for variable degrees of assimilation are, as yet, uncertain. It is possible that assimilation of calc-silicate rocks with variable carbonate/silicate proportions resulted in the range of observed compositions. However, the importance of carbonate assimilation in mafic rocks compared to felsic ones suggests that assimilation of carbonates was predominant at high temperature and/or mafic magma compositions and assimilation of silicates was predominant at lower temperature and/or felsic magma compositions. We suggest that the ability of the mafic magma to dissolve higher proportions of carbonate contaminants is the result of the magma's ability to form clinopyroxene as a product of assimilation. In any case, extensive carbonate assimilation was possible because CO 2 escaped from the system.

Geochemistry of the Cenozoic Potassic Volcanic Rocks in the West Kunlun Mountains and Constraints on Their Sources

Abstract The geochemical characteristics of the Cenozoic volcanic rocks from the north Pulu, east Pulu and Dahongliutan regions in the west Kunlun Mountains are somewhat similar as a whole. However, the volcanic rocks from the Dahongliutan region in the south belt are geochemically distinguished from those in the Pulu region; Nd, 207Pb/204Pb and 208Pb/204Pb. Their trace elements and isotopic data suggest that they were derived from lithospheric mantle, consisting of biotite‐ and hornblende‐bearing garnet lherzolite, which had undertaken metasomatism and enrichment. On the primitive mantle‐normalized patterns, they display remarkably negative Nb and Ta anomalies, indicating the presence of early‐stage subducted oceanic crust. The metasomatism and enrichment resulted from the fluid released from the crustal materials enclosed in the source region in response to the uplift of asthenospheric mantle. Based on the previous experiments it can be inferred that the thickness of the lithosphere ranges from 75 to 100 km prior to the generation of the magmas. However, the south belt differs from the north one by its thicker lithosphere and lower degree of partial melting. The different thickness of the lithosphere gives rise to corresponding variation of the degree of crustal contamination. The volcanic rocks in the south belt are much more influenced by crustal contamination. In view of the tectonic setting, the generation of potassic magmas is linked with the uplift of asthenosphere resulted from large‐scale thinning of the lithosphere after the collision of Indian and Eurasian plates, whereas the thinning of the lithosphere may result from delamination. The potassic magmas mainly resulted from partial melting of lithosphere mantle caused by the uplift of asthenosphere.

Serpentinite Subduction: Implications for Fluid Processes and Trace-Element Recycling

Serpentinites are important components of the oceanic lithosphere and are viewed as major water sources in subduction zones. Study of a set of ultramafic rocks equilibrated at increasing subduction depths (oceanic and high-pressure antigorite serpentinites, olivine-orthopyroxene rocks), shows that these rocks can act as carriers of Cl, B, Sr, Rb, Cs, and alkalis. Serpentinization of the oceanic mantle produces enrichment in Sr, Cl, B, and alkalis. The subduction path within the stability field of antigorite serpentine is accompanied recycling of oceanic Cl, B, Sr, and alkalis into variably saline fluids (4-8 wt% NaClequiv based on mass balance calculations; 10 to 50 wt% NaClequiv based on fluid inclusion analysis). Fluids are produced during a first (olivine-in) dehydration reaction releasing a maximum amount of 2 wt% H2O. The oxygen isotope compositions of the high-pressure antigorite serpentinites largely mirror those of the pre-subduction protoliths. This is interpreted as a lack of re-equilibration due to limited fluid production and mobility during high-pressure metamorphism Breakdown of the antigorite serpentinites to olivine-orthopyroxene rocks releases 6.5 wt% H2O with formation of a low salinity fluid (0.4-2 wt% NaClequiv based on mass balance calculations). The salt contents of subduction fluids appear to decrease with increasing depth: a "differentiation" in the Cl content of fluids can thus be expected, with shallower saline fluids being followed by deeper, less saline, solutions. Primary fluid inclusions in olivine-orthopyroxene rocks represent remnants of the antigorite-breakdown fluid. Their trace-element compositions are enriched in several LILE (Rb, Sr, Cs, Pb), Li, B, and alkalis and are depleted in HFSE; their trace-element patterns are similar to those of many present-day arc lavas. Oxygen isotope compositions of olivine-orthopyroxene rocks are rather homogeneous, and are suggestive of larger fluid mobility during dehydration. The antigorite breakdown reaction appears to produce a mobile fluid that can play a role in the metasomatism of sub-arc mantle. Serpentinized oceanic mantle thus represents a valuable candidate reservoir for fluids and incompatible elements: consequently, its role in the overall element cycle in subduction settings should be re-evaluated.

A New Geochemical Technique for Gold Exploration: Alkali Element Mobility Associated with Gold Mineralization in the West Australian Goldfields

This study tests the hypothesis that alkali element mobility can be used as a tool to monitor fluid flow associated with gold-bearing hydrothermal systems and as a geochemical vector for locating new gold deposits. This was achieved by analyzing 114 samples of two mafic rock types from four localities in the Archean Yilgarn craton of Western Australia; the giant Golden Mile deposit at Kalgoorlie, the smaller Victory mine at Kambalda, two drill holes that intersected subeconomic mineralization 8 km south of Kalgoorlie, and two drill holes well away from gold mineralization. The results show that Cs, Rb, and Ba are enriched in areas that are prospective for gold-bearing mineralization and depleted in areas that are not. The primitive mantle normalized ratio ((Cs + Rb)/Th) N is used as an alteration index to quantify and discriminate potentially mineralized, with ((Cs + Rb)/Th) N >5, from unmineralized areas, with ((Cs + Rb)/Th) N N >5 that is 10 to 100 times larger than the visible alteration halo. The alkali enrichment halos around the smaller Victory deposit and subeconomic mineralization south of Kalgoorlie are narrower, with lower ((Cs + Rb)/Th) N but are still over 10 times larger than the visible alteration halos adjacent to veins. These zones of high ((Cs + Rb)/Th) N around the mineralization show that the hydrothermal fluids penetrated tens of meters, and in the case of the Golden Mile, hundreds of meters into the wall rocks. Within this zone, ((Cs + Rb)/Th) N increases toward ore and provides a vector to mineralization.

Experimental constraints on the storage conditions of phonolites from the Kerguelen Archipelago

Phase relations of a phonolite (K1) and a tephri-phonolite (K2) from the Upper Miocene lavas in the Southeast Province of the Kerguelen Archipelago have been investigated in the P/T range 100–500 MPa and 700–900 °C at two fO2 conditions (~ NNO and ~ NNO+2.3) to clarify the differentiation and pre-eruptive conditions of these magmas. Crystallization experiments were performed in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) at various XH2O, under reducing (log fO2 ~ NNO) and oxidizing conditions (log fO2 ~ NNO+2.3). Under reducing conditions, the resulting phase assemblage for K1 was: titanomagnetite, nepheline, alkali feldspar, clinopyroxene and biotite; under oxidizing conditions, the assemblage was: magnetite, plagioclase, alkali feldspar, nepheline, titanite (minerals given in the order of appearance with decreasing T at 200 MPa for 4 wt% water in the melt). It is emphasized that an effect of fO2 on the phase stability of feldspars and feldspathoides was observed. Comparison of the natural and experimental phase assemblages shows that the pre-eruptive conditions for K1 must have been in the log fO2 range ΔNNO+1−ΔNNO+2, at pressures above 200–250 MPa. Assuming a temperature of 800 °C, the water content of the melt is constrained to be between 4 and 6 wt% H2O. The pre-eruptive fO2 conditions for the less evolved sample K2 are more oxidizing with log fO2 close to ΔNNO+2.3. The experimental results show that the enrichment of alkalis in residual melts during differentiation of tephri-phonolites is enhanced at high fO2.

Origin of oceanic phonolites by crystal fractionation and the problem of the Daly gap: an example from Rarotonga

Felsic alkalic rocks are a minor component of many ocean island volcanic suites, and include trachyte and phonolite as well as various types of alkaline and peralkaline rhyolite. However, there is considerable debate on the nature of their formation; for example, are they formed by partial melting of anomalous mantle or the final products of fractional crystallization of mafic magmas. The phonolites and foidal phonolites on Rarotonga were formed by low pressure crystal fractionation of two chemically distinct parental magmas. Low silica and high silica mafic magmas produced a basanite–foidal phonolite series and an alkali basalt–phonolite series, respectively. The foidal phonolite composition evolved from the low silica mafic magmas by approximately 60% fractionation of titanaugite + leucite + nepheline + magnetite + apatite. Fractionation continued with the crystallization of aegirine-augite + nepheline + kaersutite + magnetite + apatite. The phonolites formed from the alkali basalts by approximately 40% fractionation of kaersutite + titanaugite + Fe–Ti oxide + plagioclase + apatite and continued to evolve further by fractionation of anorthoclase + nepheline + aegerine-augite + Fe–Ti oxides. As the magmas fractionated in both suites, their overall viscosities (solid + liquid) increased until a point was reached whereby viscosity inhibited the eruption of magmas with compositions intermediate between the mafic rocks and the felsic rocks. However, the magmas continued to fractionate under static conditions with the residual fluid becoming foidal phonolitic in the low silica suite or phonolitic in the high silica suite. These phonolitic liquids, as a result of an increase in volatiles and enrichment of alkalis over aluminum, would actually have a lower viscosity than the intermediate liquids. This decrease in viscosity and the switch from a magma chamber being predominantly a liquid with suspended solids to a solid crystalline network with an interstitial liquid enabled phonolitic liquids to migrate, pool, and eventually erupt on the surface.

Petrographic, Geochemical and Zircon Typology of Neoproterozoic Granitoids: Evidence of Continental Thickening in Saghro Arc (Eastern Anti-Atlas, Morocco)

Saghro is considered as a volcanic arc of the Neoproterozoic. The magmatic activity during this period is characterized by the emplacment of granitic plutons North-East from Bou-Azzer El Graara's suture zone. Sidi Flah-Bouskour buttonhole represents the near part of Saghro to this zone and Ougnat hills the far part. The buttonhole of Sidi Flah-Bouskour is cut by several successive granitic intrusions of Neoproterozoic. The earliest suite, composed of gabbro, quartz diorite and amphibole granodiorite constitute the Pan-African B1 phase related to pre- collision domain, and shows a calc-alkaline chemistry. The second phase includes the biotite granite of Bouskour which belong to the potassic calc-alkali suite emplaced during the Pan- African B2 phase in a syn-collisional environment. The granodioritic and granitic trends show separate evolution in CaO-Na,O-K,O diagram. Quartz diorite and amphibole granodiorite have moderate content of Sr (420-40ppm), Rb (133-88ppm), Cr (177-21ppm), Zr (131-67ppm) and V(470- 30ppm) and hight content of Ba (1156-368ppm). Garnet and biotite have low content of Sr (3lppm), Rb (104ppm), Cr (42ppm), V (93ppm), Zr (93ppm) and Ba (450ppm). REE normalised to MORB show that the Sidi Flah-Bouskour granitoids are enriched in LREE and LILE and relatively depleted in HFSE and Nb.The zircon populations of the quartz diorite and amphibole granodiorite show a strong frequency in the sub-type S17-18 with low @ index and high T index implying a temperature of 750 - 800°C. In addition to the T.E.T, the average points (0, T) placed in the (LA, 1.T) typological diagram permit to classify these rock into the hybrid granites domain which belong to the calc-alkali granodioritic affinity. The biotite granite shows zircon populations with dominance of the S19 sub-type implying high 0 index with apparition of the types P, D and J, and high T index. The average points (0, T) and the T.E.T. permit to classify the biotite granite among the hybrid granites implying a alkali-rich calc-alkali trend. Quartz diorite, amphibole granodiorite and garnet granite are emplaced in Ougnat hill, far separated from the suture zone. Petrographic, geochemical and zircon typology study of these rocks shows existence of two magmatic groups. The quartz diorite and the amphibole granodiorite represents a medium-K calc-alkaline trend and shows alkali enrichment (Bl). CaO-Na,O- KzO relationship shows that quartz diorite and granodiorite evolved separately from that of the garnet granite. Petrographic, mineralogical (zircon typology) and geochemical (overlap of garnet granite REE pattern on calculated crustal partial melt pattern) features of garnet granite are similar to those of crustal anatectic granites (B2). The occurrence of granodiorite very rich in biotite and mantled garnet grains identical to those in the crustal garnet granite indicates processes of assimilation and magma contamination during the emplacement of Ougnat granitoids. Biotite granite emplaced in Sidi Flah-Bouskour, adjacent to Saghro near the suture zone shows strong crustal participation and granitoids in Iknioun show high-K affinities. Away from the suture zone' in Ougnat' garnet granites of crustal origin occur. Isotopic data show variation of ENdi between biotite granite in Sidi Flah-Boukour (ENd= -1.8-2.4) and garnet granite in Ougnat (ENdi= -4.5). This is interpreted to indicate an increase in incorporation of crustal component while moving North-East. The contrast in isotopic data agree well with the spatial zonation observed in Saghro. This zonation might have evolved by crustal thickening toward the axis of Bou-Azzer El Graara's suture which is considered as a paleo-subduction zone during Neoproterozoic according to the model of Meen (1987).

Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: Part 2. Trace element characteristics of slab-derived fluids

Spinel peridotite xenoliths recovered from the Tubaf and Edison volcanoes, south of Lihir Island in the Tabar–Lihir–Tanga–Feni island arc in Papua New Guinea, are predominantly fresh, refractory harzburgites. Many of the harzburgite xenoliths have cross-cutting vein networks and show evidence of modal metasomatism. These metasomatic veins contain a secondary mineral assemblage consisting of fibrous, radiating orthopyroxene and fine-grained Fe–Ni sulfide with minor olivine, clinopyroxene, phlogopite, amphibole and magnetite. Adjacent to the veins, primary clinopyroxene is cloudy while orthopyroxene exhibits replacement by secondary fibrous orthopyroxene, similar in habit to orthopyroxene occurring in the veins. The mineralogical and geochemical characteristics of the Tubaf mantle xenoliths are the product of two major processes: an early partial melting depletion event that was overprinted by oxidation and alkali enrichment related to percolation of slab-derived, hydrous melts. HREE and MREE concentrations in clinopyroxene from the least metasomatised harzburgites indicate that they are the residues from a 15% to 25% partial melting event, consistent with formation in a MOR setting. The secondary vein assemblages show strong enrichment in the LILE (primarily Sr, Ba, Rb, Th, U and Pb) and the REE (primarily La, Ce, Nd, Sm, Eu and Gd), while the HFSE (Nb, Ta, Zr, Hf, and Ti) are neither enriched nor depleted. The mineral precipitates in the vein assemblages have high LREE/HFSE and LILE/HFSE, and reflect the relative solubility of these elements in hydrous melts. These trace element characteristics are similar to those of the Tabar–Lihir–Tanga–Feni arc lavas, and display the commonly observed HFSE depletion of arc magmatism. These findings support the hypothesis that this so-called “arc signature” is primarily dependent on the relative solubility of elements in slab-derived, hydrous melts, and the enrichment of these soluble elements in metasomatised mantle regions that are prone to preferential partial melting.

Chemical stratification of cratonic lithosphere: constraints from the Northern Slave craton, Canada

We describe the mineralogical and chemical composition of the Northern Slave mantle as deduced from xenoliths of peridotite within the Jericho kimberlite, Northwest Territories. Our data set includes modal, major, trace and rare earth element compositions of bulk samples of spinel peridotite, low-T and high-T garnet peridotite and minor pyroxenite. Compared to primitive upper mantle, Jericho peridotite shows depletion in the major elements and enrichment in incompatible elements (except for HREE). The Slave mantle is also uniquely stratified. Older, depleted spinel peridotite extends to a depth of 80–100 km and is underlain by garnet peridotite which shows a gradual decrease in Mg# with depth to 200 km. The youngest layer of fertile garnet peridotite, enriched in clinopyroxene and garnet, is underlain by a pyroxenite-rich horizon at the base of the petrological lithosphere. The Northern Slave is further distinguished from the Kaapvaal and Siberian upper mantle by a marked vertical stratification in Mg#, lower abundances of orthopyroxene and higher abundances of clinopyroxene. In addition, a deeper layer of garnet peridotite below Jericho shows less depletion than low-T peridotite from other cratons. The Northern Slave peridotite results from a series of chemical events that include: (i) high-degree melting of pyrolite at P>3 Gpa for low-T peridotite and lower pressure melting for high-T peridotite, (ii) enrichment of low-T spinel peridotite in orthopyroxene, and (iii) pervasive metasomatic enrichment in alkali and LREE’s by kimberlite-related fluids. The chemical stratification described for two of the three lithospheric domains of the Slave craton makes this craton an exception among cratons with commonly unstratified lithospheres. The gradual increase in fertility with depth below the Slave craton is related to age stratification and may have formed by incremental downward growth of mantle lithosphere with time, and/or later re-fertilization of deeper mantle horizons.

Clinopyroxene geobarometry of magmatic rocks. Part 2. Structural geobarometers for basic to acid, tholeiitic and mildly alkaline magmatic systems

The crystal structures of 212 experimentally synthesized, igneous clinopyroxenes were modeled from electronprobe chemical data. The coexisting melts span a wide range of petrologically relevant, dry and hydrous compositions, characterized by variable enrichment in silica and alkalis. Experimental conditions pertain to Earth's crust and uppermost mantle (P = 0–24 kbar; garnet absent) and a variety of f O2 values (from CCO-buffered to air-buffered) and mineral assemblages (Cpx ± Opx ± Pig ± Ol ± Plag ± Spl ± Mt ± Amp ± Ilm). Unit-cell volume (Vcell) versus M1-polyhedron volume (VM1) relations were investigated over a range of pressures and temperatures using data derived from structure modeling and corrected for thermal expansivity and compressibility. The relationships between pressure and clinopyroxene structural parameters were found to be dependent on the nature of the coexisting melt. To reduce compositional effects, only clinopyroxenes belonging to mildly alkaline (MA) and tholeiitic (TH) series were considered. Pressure was modeled as a linear function of Vcell, VM1, and Mg/(Mg + Fe2+)Cpx ratio. A calibration based on the whole data set (MA+ TH) reproduced the experimental pressures within 1.4 kbar at the 1-σ level. The maximum residuals were 3.5 kbar and 3.9 kbar for MA- and TH-clinopyroxenes, respectively. Better statistics were obtained by considering MA- and TH-clinopyroxenes separately. A calibration based on the 69 MA-clinopyroxenes reproduced the experimental pressures within 1.1 kbar (1σ) and with a maximum residual of 2.7 kbar. A calibration based on the 143 TH-clinopyroxenes reproduced the experimental pressures within 1.0 kbar (1σ) and with a maximum residual of 3.4 kbar. When these geobarometers are applied to natural samples for which P is unknown, the correction for compressibility is necessarily made through a trial-and-error procedure. This expedient propagates an additional error that increases the above uncertainties and residuals by a factor of about 2. Applications to natural, igneous rocks for which the pressures of crystallization could be constrained based on experimental, petrological or geological evidence yielded pressure estimates that reproduced the expected values to within ca. 2 kbar. Compared to the MA-formulation, the TH-formulation appears to be less robust to variations in magma composition. When applied to high-pressure (>10 kbar) clinopyroxenes synthesized from very low Na (Na2O < 1.5%) melts, the latter geobarometer can underestimate P by as much as 6 kbar. Calculation of P through the present geobarometers requires clinopyroxene major-element composition and an independent, accurate estimate of crystallization T. Underestimating T by 20 °C propagates into a 1-kbar increase in calculated P. The proposed geobarometers are incorporated in the CpxBar software program, which is designed to retrieve the pressure of crystallization from a clinopyroxene chemical analysis.