Normative Quartz Research Articles

Petrochemistry and Tectonic Setting of the Phroa-Wiang Pa Pao Granitic Rocks, Chiang Mai, and Chiang Rai Provinces, Central Granitic Belt of Thailand

The Phrao-Wiang Pha Phao granitic rocks are crop out along Highway Number 1150 (Phrao-Wiang Pa Pao) in Chiang Mai and Chiang Rai provinces. The geologic investigation as field observation, lithology, petrography, geochemistry, and tectonic interpretation applied for this study. The least-altered 19 rock samples were collected and performed in stained rock slabs for modal analysis and standard thin sections for petrographic analysis. Based on the Quartz-Alkali Feldspar-Plagioclase diagram for plutonic rocks, the rock samples in the Phrao area are granodiorite, and one is biotite-monzogranite, while most of the rock samples in the Wiang Pa Pao area are biotite-monzogranite. The petrographic investigation for rock samples indicated that granitic samples have medium to coarse-grained and show porphyritic texture. The orthoclase phenocrysts are sitting in the groundmass and are composed of quartz, plagioclase, and orthoclase feldspar, with a minor amount of biotite, muscovite, apatite, zircon, titanite, and opaque minerals. The granitic samples can be classified as granodiorite to monzogranite, which is high-K calc-alkalic affinity and peraluminous S-type based on geochemistry. Based on the normative mineralogy calculation, the rock samples were composed mainly of normative quartz, plagioclase, and orthoclase, with minor corundum, hypersthene, magnetite, ilmenite, apatite, zircon, and chromite. The S-type granitoid supported by normative corundum and Na2O/K2O ratios less than 0.49 at Na2O ranging from 1.53 - 2.35 wt %. Their tectonic setting of formation appears to be a post-orogenic environment (post-collisional granite) of the Sibumasu and the Indochina blocks. The Phrao Phrao-Wiang Pha Phao granitic rocks are a part of the Central Granitic Belt and are related to tin-tungsten deposits with rare earth elements. HIGHLIGHTS Tectonic setting of the Phroa-Wiang Pa Pao Granitic Rocks, Thailand. Petrography and modal analysis The geochemistry analysis by XRF and CIPW normative calculation. The Central Granitic Belt in Thailand. GRAPHICAL ABSTRACT

Transition from tholeiitic to alkali basalts via interaction between decarbonated eclogite-derived melts and peridotite

Intraplate basalts generally show a geochemical continuum from alkali to tholeiitic basalts. However, the genetic link between these two types of rocks has remained controversial. The Early Jurassic Karamay basalts in the West Junggar terrane, southern Central Asian Orogenic Belt (CAOB), erupted to form a small-volume outcrop in the stable continental intraplate region. The basalts are characterized by aphyric textures without any visible phenocrysts. Thus, they are different from the ubiquitous porphyritic-textured intraplate basalts and have a composition close to that of the mantle-derived primary melt. In contrast to chemically and petrographically well-defined alkali and tholeiitic basalts, the Karamay basalts exhibit transitional compositions spanning from alkali (with normative olivine+nepheline and normative olivine+hypersthene) to tholeiitic (with normative quartz+hypersthene), providing an important case to address the geochemical continuum of intraplate basalts. Similar to the alkali basalts in eastern China, the Karamay basalts have isotopic imprints of sedimentary carbonates, i.e., significantly lighter Mg (δ26Mg = −0.54‰ to −0.34‰) and heavier Zn (δ66Zn = 0.36–0.46‰) isotopes than the normal mantle. However, they display initial ((87Sr/86Sr)t ratios of 0.7047–0.7051, positive εNd(t) values (3.3–4.2) and positive anomalies of Nb-Ta-Ti-Zr-Hf, which are not expected in the case of incorporation of recycled carbonates. This contradiction can be reconciled by considering a decarbonation reaction between carbonates (i.e., dolomite and magnesite) and co-existing eclogite in the subducted oceanic slab at pressure >5 GPa, leaving light Mg and heavy Zn isotope signatures in the stagnant eclogite residue in the deep mantle. Combining the geochemical compositions of our samples with the geologic evidence, and considering the previous results of melt-peridotite reaction experiments, we conclude that the Karamay basalts might have originated from the interaction of silica-rich tholeiitic melt derived from the recycled decarbonated eclogite with fertile peridotite during its ascent. Our study highlights that intraplate alkali basalts, especially silica-rich ones (e.g., with SiO2>45 wt.%), can be transformed from tholeiitic melts through reaction with peridotite mantle, and demonstrates that deeply recycled oceanic crust stagnated in mantle can serve as a main source for alkaline lavas.

Magmatic and post-magmatic evolution of post-collisional rare-metal bearing granite: The Neoproterozoic Homrit Akarem Granitic Intrusion, south Eastern Desert of Egypt, Arabian-Nubian Shield

The Homrit Akarem granitic intrusion (HAGI) outcrops near the western edge of the south Eastern Desert basement exposure in Egypt. It is a composite of two cogenetic intrusive bodies: an early albite granite phase shallowly emplaced at the apex of a magmatic cupola, and a later subjacent pink granite phase with a marginal zone of muscovite granite and better preservation of magmatic features. Mineral chemistry of primary biotite and garnet, together with whole-rock chemistry, identify the HAGI as a highly fractionated A-type peraluminous intrusion. The chemistry of F-dominant, Li-bearing, Fe3+-rich primary magmatic mica in the pink granite resembles that typically found in highly evolved Nb-Y-F pegmatites. The HAGI is the evolved product of a primary magma generated by partial melting of juvenile crust of the Arabian-Nubian Shield (ANS), emplaced along a regional strike-slip fault system that promoted its ascent. The main emplacement mechanism and evolutionary sequence of the HAGI was magmatic, although secondary minerals and textures resulting from hydrothermal fluid interactions are observed, especially at its margins. Primary columbite-(Mn) crystallized from melt and was partly replaced by secondary fluorcalciomicrolite. The high fluorine content of magmatic fluids exsolved from the intrusion is indicated by quartz-fluorite veins, greisenization, albitization, and F-bearing secondary oxide minerals. The magmatic derivation of this fluid is demonstrated by the F-dominant primary mica, a siderophyllite-polylithionite solid solution commonly known as zinnwaldite. The chemistry of zinnwaldite constrains the F/OH activity ratio and oxygen fugacity of its parental melt and thereby resolves the ambiguity between pressure and the effects of F in controlling the normative quartz content of rare-metal granites. The HAGI is less mineralized than the post-collisional rare-metal granites found further east in the south Eastern Desert, replicating a trend observed previously in the central Eastern Desert and suggesting that east-west zoning in rare metal enrichment is a persistent feature across latitudes at the western edge of the ANS.

Geochemistry of the highly evolved Sn-W-Mo-bearing Mount Douglas Granite, New Brunswick, Canada: Implications for origin and mineralization

The Late Devonian (368 ± 1 Ma) post-orogenic peraluminous Mount Douglas Granite, located in southwestern New Brunswick, Canada, forms the eastern part of the Saint George Batholith. The batholith was emplaced following the accretion of the Gander and Avalon zones of the northern Appalachians during the Neoacadian Orogeny. The Mount Douglas Granite is divided into three units, Dmd1, Dmd2, and Dmd3, that formed by progressive high degrees of fractional crystallization. The two most fractionated units, i.e., Dmd2 and Dmd3, are locally associated with greisen/sheeted veins-related endogranitic Sn-W-Mo mineralization. Whole-rock δ18O values of +6.0 to +7.3‰, high initial 87Sr/86Sr ratios of 0.70550 to 0.71665, slightly positive εNd(368 Ma) values (+0.3 to +1.1), and Pb isotopic data indicate that the granite was derived dominantly from partial melting of juvenile Avalonian crust, contaminated by supracrustal rocks.Petrochemical features support the idea of multiple periods of fractional crystallization at lower temperatures producing compositionally more evolved magmas. This is supported by estimated zircon saturation temperatures that decrease from Dmd1 (747–826 °C) → Dmd2 (733–817 °C) → Dmd3 (729–816 °C). The estimated crystallization pressure based on normative quartz and feldspar (albite + orthoclase) contents also decreases with increasing silica content, from 2.3 kbar (~7.7 km) in Dmd1 to 0.3 kbar (~1 km) in Dmd3.Whole-rock geochemical characteristics of this system show that unit Dmd3 is the most highly evolved unit; it has the highest SiO2 (avg. 76.4 wt.%.), LILE contents (e.g., Li, Rb, Cs), Rb/Sr (mean = 42), certain HFSE (Ta, Th, U) including Y (≤138 ppm), and REE, and has the most pronounced negative Eu anomalies (avg. Eu/Eu* = 0.08) and the lowest K/Rb (70–127), Nb/Ta (mean = 4.9), and Zr/Hf (mean = 23.5). Therefore, unit Dmd3 with the highest content of incompatible elements is the most prospective for Sn, W, Zn, Bi, and U mineralization, and then Dmd2.

Amphibolites in the rare metal- and tin-bearing Bastar–Malkangiri pegmatite belt in Chhattisgarh and Odisha, India

Amphibolites and Abukuma-type low-grade and low-pressure meta-sedimentary rocks (MSR) host mineralised rare metal (Nb–Ta, Be and Li)- and Sn-bearing granite pegmatites and their parental, fertile S-type granites in the Bastar–Malkangiri Pegmatite Belt (BMPB) in the SE part of the Bastar craton in parts of Chhattisgarh and Odisha, India. Amphibolites, based on their field relationship (as sills in MSR), mineral assemblage and textural features, are broadly classified into three types: (i) tremolite-, (ii) hornblende–actinolite-, and (iii) hornblende-bearing amphibolites. Petrochemically, amphibolite types (i–iii) show a regular decrease in their Mg content and related trace elements such as Cr (81–1731 ppm), Ni (135–379 ppm) and Co (23–68 ppm) and, hence, are designated, respectively, as high-, intermediate- and low-Mg amphibolites. All the three types show slight enrichment in Si, Fe and P, reflected in their normative quartz, hypersthene and lesser diopside, magnetite, ilmenite and apatite, but no olivine. They are relatively enriched in incompatible elements such as Rb (16–526 ppm), Ba (15–538 ppm), Zr (56–147 ppm) and Nb (3–44 ppm), with relatively a higher concentration of these elements in those near to the contacts with pegmatites, thereby demonstrating some influence of the pegmatites on the chemistry of the host amphibolites. In spite of these, the three types of amphibolites show regular differentiation trends, reflected by their systematic decrease of Ni, Cr and Co with a decrease of Mg, and also show Fe-enrichment in their alkalies \((\hbox {Na}_2\hbox {O} + \hbox {K}_2\hbox {O})-(\hbox {FeO} + \hbox {Fe}_2\hbox {O}_3)-\hbox {MgO}\) (AFM) plot. The field, petrography and major–minor trace elements petrochemistry, including rare earth elements with slight negative to no Eu-anomaly, and their chemical patterns cumulatively point out that these amphibolites are ortho-type and sub-alkaline in nature, with quartz tholeiitic magmatic affinity, and the formation due to more of fractional crystallisation and less of 5–25% partial melting of an enriched primordial mantle magma (enriched mid-oceanic ridge basalt – E-MORB), emplaced at a deduced shallow depth of 10–25 km, with the main controlling phases being olivine, plagioclase, clinopyroxene and apatite.

Evaluation of the petrogenetic significance of melt inclusions in pegmatitic schorl-dravite from graphic tourmaline-quartz assemblages: Application of LA-ICP-QMS analyses and volume ratio calculations

The composition of melt inclusions in tourmaline was studied from graphic quartz-tourmaline intergrowth assemblages of common, muscovite – rare-element class pegmatites belonging to the lithium-caesium-tantalum family. This study applied laser ablation inductively coupled quadrupole mass spectrometry in combination with 2D and 3D microscopic volume calculation to analyse these unhomogenised melt inclusions. The determination of the analytical precision of the method is performed by repeated microscope-scanning and laser ablation analyses of tourmaline mineral inclusions in muscovite hosts for which the compositions have been independently measured by electron microprobe.The analysed melt inclusions are hosted in schorl-dravite series tourmaline from the early-Neoproterozoic pegmatite dykes of the Gatumba-Gitarama field in Rwanda. Multiple dykes have been studied, representing three regional pegmatite zones showing an increasing bulk regional degree of differentiation. The sampled graphic quartz-tourmaline assemblages are the only manifestation of an internal mineralogical zonation within these pegmatite dykes.The melt inclusions dominantly contain muscovite, quartz, dickite, feldspars, jeremejevite and childrenite minerals and variable amounts of Cs-rich, interstitial fluid. The major element composition of the ablated melt inclusions is variable but mainly plot on the low Ca-granitic compositional trend. The inclusions show, moreover, a significantly higher normative quartz content compared to bulk pegmatitic rock counterparts. The melt inclusions demonstrate a near-constant Rb concentration (100 up to 160 ppm) whereas Cs concentrations vary between 10 and 1000 ppm. Although Rb and Cs enrichment trends are accurate proxies to deduce pegmatite melt differentiation in general, both the Rb and Cs concentrations of the melt inclusions are remarkably unrelated to the regional degree of differentiation deduced from the constituent bulk rock and rock-forming minerals of the Gatumba-Gitarama pegmatites. A comparison of the alkali metal content of melt inclusions and cogenetic fluid inclusions with previously modelled bulk melt concentration indicates that this deviating Rb and Cs behaviour is caused by heterogeneous trapping of variable portions of the Cs-enriched aqueous fluid phase together with the residual melt and this during melt inclusion formation.The melt inclusions in tourmaline represent local and residual, hydrous boro-aluminosilicate melt compositions. The formation of graphic intergrowths inside the pegmatite dykes indicates rapid tourmaline growth rates causing in turn quartz saturation and nucleation in the surrounding residual melt. Fast tourmaline crystallisation contributed directly to the formation of these “melt pools” with distinct residual compositions. Our study shows that already for less-differentiated common pegmatites, melt inclusions from internal mineralogical zones represent closed melt subsystems that cannot be directly linked to the bulk system. These findings imply, moreover, that the use of tourmaline trace element compositions as an indirect geochemical proxy for the bulk melt composition of pegmatites and evolved granites, has to be considered with caution.

Precise microbeam dating defines three Archaean granitoid suites at the southwestern margin of the Kaapvaal Craton

Precise microbeam U-Pb zircon dates have been determined for 17 granitoid samples which crop out along the southwestern margin of the Kaapvaal Craton. In the Marydale High two main types of granitoid are distinguished mainly by their normative Quartz – Alkali Feldspar – Plagioclase mineral proportions. The Draghoender Granite type is generally tonalitic to trondhjemitic and some samples have very low heavy Rare Earth Elements, probably originating as melts of eclogitic protoliths. Their ages vary by ∼50 Ma from 2946 ± 9 Ma to 2892 ± 6 Ma (2σ), reflecting a long period of magmatism possibly due to subduction. The granodioritic to monzogranitic Skalkseput Granite type intrudes the Draghoender granite in places and has a coherent age of 2901 ± 14 Ma. These two granite types thus overlap in time and space. The more evolved Skalkseputs type could have been derived by melting of the Draghoender type during assembly of the Kaapvaal Craton by collision of the Kimberley and Witwatersrand Terranes.A third granite type is exemplified by the 2721 ± 6 Ma monzogranitic Steenkop Granite Gneiss which occurs south of Prieskapoort, and corresponds in age to a 2718 ± 8 Ma monzogranite dyke which intrudes a 2871 Ma Skalkseput monzogranite. The previous age of 2718 ± 8 Ma for the Skalkseput Granite probably also dated a Steenkop-type dyke. A basal conglomerate to the Zeekoebaard Formation of the Ventersdorp Supergroup has a single 2720 ± 4 Ma detrital zircon population.South of the Marydale High a narrow strip of Draghoender-type Granite extends to Franzenhof where it is exposed in a Dwyka glacial outwash surface and is dated at 2931 ± 9 Ma. The southernmost exposures of granite are windows in the Dwyka group about 130 km southeast of Marydale on both sides of the Doornberg Fault. The 2905 ± 7 Ma Maritzdam Granite which is basement to Ventersdorp Supergroup volcanic rocks at Kuip is monzogranitic and corresponds to the Skalkseput type. On the southwest side of the Doornberg Fault the 2907 ± 4 Ma Welgevonden Granite underlies Marydale Group volcanic rocks, showing that the Kaapvaal basement extends across the fault.

Estimation of crystallization pressure of granite intrusions

A numerical method is presented to estimate the crystallization pressure of granite intrusions based on two polynomial equations obtained by an analysis of the existing haplogranite ternary phase diagram and associated dataset. The results indicate that the pressure is correlated respectively with normative quartz (Qtz) content and with normative albite (Ab) plus orthoclase (Or) contents of granitic rocks as follows.(1)P=−0.2426×Qtz3+26.392×Qtz2−980.74×Qtz+12563(2)P=0.2426×Ab+Or3−46.397×Ab+Or2+2981.3×Ab+Or−64224R2=0.9943where P is pressure in MPa, and R denotes correlation coefficient. It is noted that the procedure of normalizing the sum of CIPW norm (quartz, albite, orthoclase) contents to 100% is required before using Eqs. (1) and (2). The difference in pressure calculations between these two equations is ≤16MPa for the range of normative quartz contents from 15 to 40wt%. An example of how to use these equations to estimate the crystallization pressure of a granite intrusion is also provided to show the validity and convenience of this method. The uncertainty of such pressure estimation is not well known, although it must fall into the uncertainty range of the existing experimental work on pressure constraints.The simplicity of this empirical method is appreciable, although its applicability to natural granitoids needs further test. More experimental work is required to constrain the effects of components, such as CaO, FeO, MgO, F, Cl, CO2, on the granite phase equilibria. These equations, however, can be used for estimating crystallization pressures of water-saturated and quartz-oversaturated granitic systems.

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.

Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 1: Principles, procedures, and evaluation of the method

Constraining the pressure of crystallization of magmas is an important but elusive task. In this work, we present a method to derive crystallization pressures for rocks that preserve glass compositions (either glass inclusions or matrix glass) representative of equilibration between melt, quartz, and 1 or 2 feldspars. The method relies on the well-known shift of the quartz–feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure. The critical realization for development of the method is the fact that melt, quartz and feldspars need to be in equilibrium at the liquidus for the melt composition. The method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite-MELTS over a range of pressures, and searching for the pressure at which the expected assemblage (quartz+1 feldspar or quartz+2 feldspars) is found at the liquidus. We evaluate errors resulting from uncertainties in glass composition using a series of Monte Carlo simulations for a quartz-hosted glass inclusion composition from the Bishop Tuff, which reveal errors on the order of 20–45 MPa for the quartz+2 feldspars constraint and on the order of 25–100 MPa for the quartz+1 feldspar constraint; we suggest actual errors are closer to the lower bounds of these ranges. We investigate the effect of fluid saturation in two ways: (1) By applying our procedure over a range of water contents for three glass compositions; we show that the effect of fluid saturation is more important at higher pressures (~300 MPa) than at lower pressures (~100 MPa), but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant H2O concentrations >3 wt% and (2) by performing the same type of pressure determinations with a preliminary version of rhyolite-MELTS that includes a H2O–CO2 mixed fluid phase; we use a range of H2O and CO2 concentrations for two compositions characteristic of early-erupted and late-erupted Bishop Tuff glass inclusions and demonstrate that calculated pressures are largely independent of CO2 concentration (for CO2 <1,000 ppm), at least for relatively high H2O contents, as expected in most natural magmas, such that CO2 concentration can be effectively neglected for application of our method. Finally, we demonstrate that pressures calculated using the rhyolite-MELTS geobarometer compare well with those resulting from H2O–CO2 glass inclusion barometry and Al-in-hornblende barometry for an array of natural systems for which data have been compiled from the literature; the agreement is best for quartz-hosted glass inclusions, while matrix glass yields systematically lower rhyolite-MELTS pressures, suggestive of melt evolution during eruptive decompression.

Brine-assisted anatexis: Experimental melting in the system haplogranite–H2O–NaCl–KCl at deep-crustal conditions

The first-melting temperature of haplogranite in the presence of H2O–NaCl–KCl solutions was determined experimentally at deep-crustal conditions of 6–14kbar and 700–900°C. Minimum melting occurs at fluid K/(K+Na) of 0.2–0.25. Melting temperature rises strongly with increasing salinity: at 10kbar, first melting occurs at 800°C with a H2O mole fraction (XH2O) of 0.62±0.02, similar to the salinity of fluid inclusions in minerals of some granites and migmatites. At 900°C and 10kbar, XH2O is 0.33±0.01 for first melting. All minimum melts are granitic over the entire P−T−XH2O range, with SiO2 melt concentrations of 74±2wt% on a H2O-free basis and Al2O3 and alkalis typical for granites. Normative quartz is near 30% for all liquids. The K/(K+Na) ratios of minimum melts increase strongly with decreasing H2O activity: at 10kbar and XH2O=1, K/(K+Na) in the melt is 0.25, whereas at XH2O=0.34 it is 0.55. This “brine trend” is similar to, but more pronounced than, the trend described by decreasing H2O activity in H2O–CO2 fluids, and it better explains the compositions of K-rich granites. Minimum-melting curves in the presence of brines of constant XH2O have strongly positive dP/dT slopes at P>2kbar, in contrast to vertical or negative dP/dT slopes for those in the presence of H2O–CO2 fluid. There is therefore a wide P–T range over which migrating low-H2O-activity brines may generate subsolidus, deep-crustal metasomatism in the form of K2O and silica enrichment. Moreover, once melting occurs, rising accumulations of granitic magma may be fluid saturated and even increase their melting capacity with decreasing depth because of the strong pressure dependence of H2O activity in salt solutions. Our results offer an explanation for mid-crust migmatization and granite production: rising hot brines may provoke rock melting at some threshold of decreasing depth in the range 15–20km. Because of their enhanced capacity for metasomatism, leading to eventual melting at granulite facies conditions of temperature, pressure and H2O activity, concentrated brines should be considered as possibly important agents in crustal evolution.

The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume

The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

Composition of Li-F granite melt and its evolution during the formation of the ore-bearing Orlovka massif in Eastern Transbaikalia

By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12–1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B2O3 contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO2 (by up to 6 wt %), Na2O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H2O. Two stages were distinguished on the basis of the behavior of trace elements and fluorine in the melts and rocks. The early stage, from the biotite granites of the parental massif to the microcline-albite granites with the pea-shaped quartz is characterized by a decrease in Si, Fe, Ca, Mg, Zr, Hf, and REE and accumulation of Al, Na, Li, Rb, F, Ta, and Nb, which correspond to the ongonite differentiation trend. During the second stage, amazonite-bearing rocks with Li micas and Ta mineralization (columbite-tantalite and microlite) were produced, and the melt was depleted in Al, Na, Li, F, Nb, and Ta. A considerable difference appeared between the compositions of rocks and aluminosilicate melts. There is a paradoxical discrepancy between the high contents of Li, Ta, and Nb in the rock (2289, 446, and 269 ppm, respectively) and the low contents of these elements in the melt from the amazonite granites (554 ppm Li, 23 ppm Ta, and 116 ppm Nb). This discrepancy could be related to quartz crystallization after the fractionation of Li-F micas, albite, topaz, columbite-tantalite, and microlite, which resulted in that the quartz trapped inclusions of already depleted melt. On the other hand, the dramatic depletion of the residual melt in Na, Al, Li, F, Ta, and Nb suggests the possible separation of a specific hydrosaline aluminofluoride melt, which accumulated Nb, Ta, and REE in some experimental systems. This suggestion was supported by the results of the investigation by melt and fluid inclusions in beryl from the pegmatoid bodies of Orlovka, which established the coexistence of two immiscible aluminosilicate melts and a CO2-rich supercritical aqueous fluid (Thomas et al., 2009). One of these melts closely corresponds to the volatile-rich hydrosaline and relatively subalkaline melt with high Li (2.09 wt %) and F contents (2.93 wt %). The Raman spectra of the coexisting fluid indicated the presence of columbite, which implies a Ta content in the fluid of approximately 6500 ppm.

Mechanisms of Crustal Anatexis: a Geochemical Study of Partially Melted Metapelitic Enclaves and Host Dacite, SE Spain

To shed light on the mechanisms of crustal anatexis, a detailed geochemical study has been conducted on minerals and glasses of quenched anatectic metapelitic enclaves and their host peraluminous dacites at El Hoyazo, SE Spain. Anatectic enclaves, composed of plagioclase þ biotite þ sillimanite þ garnet þ glass K-feldspar cordierite þ graphite, formed during the rapid heating and overstepped melting of a greenschist-facies metapelite, and finally equilibrated at 850 508C and 5^7 kbar. Glass appears as melt inclusions within all mineral phases and in the matrix of the enclaves, and has a major element composition similar to that of peraluminous leucogranites. Melt inclusions and matrix glasses have normative quartz^orthoclase^albite compositions that plot in the vicinity of H2O-undersaturated haplogranite eutectics. Melt inclusions show some compositional variability, with high Li, Cs and B, low Y, first row transition elements (FRTE) and rare earth elements (REE), and zircon and monazite saturation temperatures of 665^7508C.They are interpreted as melts produced by muscovitebreakdown melting reactions at the onset of the process of rapid melting and mostly under H2O-undersaturated conditions. Compared with melt inclusions, matrix glasses show less compositional variability, lower large ion lithophile element contents, higher Y, FRTE and REE, and higher zircon and monazite saturation temperatures ( 695^8158C).They are interpreted as former melts recording the onset of biotite dehydration-melting. Matrix glasses in the dacite are compositionally different from glasses in the enclaves, hence the genetic connection between metasedimentary enclaves and dacite is not as straightforward as previous petrographic and bulk major element data suggest; this opens the possibility for some alternative interpretation. This study shows the following: (1) melt inclusions provide a window of information into the prograde evolution of anatexis in the enclaves; (2) melting occurred for the most part under H2O-undersaturated conditions even if, because of the rapid heating, the protolith preserved most of the structurally bound H2O contained at greenschist facies up to the beginning of anatexis, such that the excess H2O maximized the amount of H2O-undersaturated melt generated during anatexis; (3) although a large proportion of accessory minerals are currently shielded within major mineral phases, they have progressively dissolved to a considerable extent into the melt phase along the prograde anatectic path, as indicated by the relative clustering of accessory mineral saturation temperatures and closeness of these temperatures to those of potential melting reactions; (4) the dacite magma was probably produced by coalescence of melts

Chemical composition and diamond potential of kimberlites having experienced secondary alteration: Nyurbinskaya Pipe, East Siberia

Hydrothermal metasomatism greatly enriched kimberlites of the Nyurbinskaya Pipe, East Siberia, in secondary quartz. We have calculated normative secondary quartz values (Q) for each of 1035 X-ray fluorescence analyses for rock-forming oxides. The results are correlated with 437 diamond test intervals. A description of full diamond fraction composition is provided for each test interval. Eleven groups are recognized on the base of Q distribution, from dolomitic rocks with negative Q values to substantially quartzose rocks. Diamond potentials and crystal weights decrease with increasing Q values. We conclude that diamonds are dissolved during hydrothermal metasomatism.

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

AbstractThe Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNdt (t = 2000 Ma) varies between −0.19 and +2.22 for the basic volcanic rock and between −2.85 and −4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.

Normative quartz as an indicator of the mass transfer intensity during the postmagmatic alteration of the Botuobinskaya pipe kimberlites (Yakutia)

The intensity of postmagmatic processes in the Botuobinskaya pipe kimberlites was estimated from the calculated content of normative secondary quartz (Q). Several simple algorithms are proposed to calculate the Q content from chemical analyses of kimberlites. Ten groups of altered kimberlites have been recognized from the Q contents. The contents of MgO, some trace elements, and LREE in the groups, the contents of Cr and Ca of crimson garnets, the diamond contents of kimberlites, and the average weight of diamonds decrease as the Q content increases. It is shown that the negative SiO 2–MgO correlation is the most effective indicator of the postmagmatic alteration of kimberlites. As the degree of their secondary alteration increases, the kimberlites transform into an assemblage of quartz and clay minerals enriched in some trace elements and almost completely lacking REE and diamonds.

The normative mineralogy of 10 soil profiles in Fennoscandia and north-western Russia

Abstract The mineralogical composition of soil horizons in different soil types of different ages was estimated by applying the NORMA software, which was developed originally for calculating the normative mineralogical composition of young podsols. Ten soil profiles from six sites in NW Russia, two in Finland, and one in NE Norway were sampled in 1999 as a part of the pilot phase of a large geochemical mapping project. Total element concentrations were determined from the 4 extraction. Extractable concentrations for Al, Ca, Cr, Fe, K, Mn, Mg, Na, P, S, Ti, Zr, and Ba were determined by ICP-MS or ICP-AES after aqua regia (a 1:3 mixture of strong HCl and HNO 3 ) extraction. Total C was determined using a thermal conductivity detector from a sample burned in an O 2 stream. The NORMA software was used to calculate the percentage of normative soluble minerals pyrite, apatite, titanite, calcite, biotite, chlorite, weathered albite, hydrous Al-silicate, goethite and soluble residue. The percentages of non-soluble normative minerals rutile, hornblende, K-feldspar, albite, anorthite, tremolite, wollastonite, kaolinite, magnetite, zircon, quartz, carbon (graphite), and non-soluble residue were calculated after soluble minerals. The calculated mineralogical composition of C-horizon samples in each profile reflected the known geological composition of the bedrock from which the soil parent material was derived during geological processes. Secondary minerals including goethite and hydrous Al-silicates, were detected in upper soil horizons reflecting the development of soils. Rather than age, the local bedrock geology together with the mineralogical composition and chemical properties of the parent material proved to be the controlling factor in the formation of secondary minerals. The results showed that the NORMA method can be used in defining the mineralogy of soil horizons in a large variety of soil types.

Highly silicic glasses in peridotite xenoliths from Avacha volcano, Kamchatka arc; implications for melting and metasomatism within the sub-arc mantle

Silicate glasses in peridotite xenoliths from Avacha volcano have high SiO 2 (up to 72 wt.%) and highly SiO 2-oversaturated characteristics; normative quartz content is up to 50 wt.%. The glasses represent secondary melts solidified after interaction with mantle peridotite, i.e. crystallization of secondary orthopyroxene at the expense of olivine. We identified two kinds of silicate glasses in Avacha peridotites; one is higher in K 2O and enriched in Rb, Ba, U, and Pb than the other. The glasses show basically similar chemical characteristics to the host basaltic andesite to andesite of the Avacha volcano. These chemical characteristics are inherited from slab-derived fluids/melts, which metasomatize the mantle wedge and induce partial melting. The differences of chemical features among the Avacha glasses are attributed to chemical difference of the slab-derived fluids/melts, possibly due to the difference of sediments/basalt ratio of the relevant slab. The low-degree partial melt of peridotite assisted by these fluids/melts, is primarily SiO 2-oversaturated, and can conduct silicate metasomatism, evolving through interaction with surrounding mantle peridotite, i.e. formation of orthopyroxene at the expense of olivine. Highly silicic glasses, also reported from peridotite xenoliths from oceanic hotspots and continental rift zones, mostly result from assimilation of orthopyroxene by SiO 2-undersaturated melts, which crystallize clinopyroxene and olivine. The glasses also show similar trace-element patterns to their host alkali basaltic magmas, as in the case of arc glasses/calc-alkali magmas. If the glasses in peridotite xenoliths are of silicate metasomatism origin, they are similar in chemistry to host magmas. Reaction between carbonatite melts and peridotites shows the same petrographical feature as that of SiO 2-undersaturated silicate melts with peridotites. The glasses originated from carbonatite metasomatism, however, exhibit clearly different trace-element patterns from their host alkali basaltic magmas.

Transitional tholeiitic basalts in the Tertiary Bana volcano–plutonic complex, Cameroon Line

The Bana transitional tholeiitic basalts occurring in a Tertiary volcano–plutonic complex of the Cameroon Line, Central Africa are plagioclase-bearing and olivine-free. K/Ar dating on separated plagioclases of the transitional tholeiitic basalts yields an Oligocene age of 30.1 ± 1.2 Ma. Their clinopyroxene compositions are marked by iron enrichment and calcium depletion in the Wo–En–Fs system. The whole-rock major element compositions are characterized by Mg# ∼ 36–48, normative quartz and hypersthene. The youngest alkali basalts from the same igneous complex have higher Mg# ∼ 56–66. These two groups of basalt have trace element characteristics of within-plate basalt with Zr/Nb ratios of 3.7–4.5 and 7.5–9.2 respectively, and different LILE/HFSE and LREE/HREE ratios. The overall trace element characteristics suggest that the transitional tholeiitic basalts of the Bana complex were derived by high degrees of partial melting in the upper mantle at shallow depths whereas younger alkali basalts in the complex were probably produced by a small degree of melting of the same source at slightly greater depths. The transitional tholeiitic character of these basalts suggests a significant lithospheric extension and mantle upwelling below the Cameroon Line in the Oligocene.