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Formation Time, Magma Source and Mineralization in the Zhaojinggou Nb-Ta Deposit, Inner Mongolia: Evidence from Columbite-Group Minerals U-Pb Dating, Rock and Mineral Geochemistry of the Amazonite Granitic Pegmatite
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End members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas and the interlayer-cation-deficient micas. The determination of the crys-tallochemical formula for different available chemical data is outlined, and a system of modifiers and suffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill-defined materials and a list of names formerly or erroneously used for micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names ("Commission") of the International Mineralogical Association (IMA). The definitions and recommendations presented were approved by the Commission.
In their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluids which can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of granitic melts, but the behavior of both elements at the magmatic-hydrothermal transition remains unclear. Using a compilation of whole-rock geochemical data available in the literature, we demonstrate that fractional crystallization alone is not sufficient to explain the distribution of Nb-Ta in most peraluminous granites. However, we notice that most of the granitic samples displaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decrease of the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization and sub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ∼5 fingerprints the magmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ∼5 appears to be a good marker to discriminate mineralized from barren peraluminous granites.
The Sapucaia pegmatite is located in the well-known Eastern Brazilian Pegmatitic Province (EBPP), Minas Gerais, Brazil. Detailed mapping of the pegmatite body revealed five zones, showing distinct mineral assemblages: a border zone (BZ), an external wall-zone (EWZ), an internal wall-zone (IWZ), an intermediate zone (IZ), and a quartz core (Q). Phosphate masses found in the pegmatite contains two different assemblages: assemblage I is formed under oxidizing conditions, and assemblage II is formed under less strongly oxidizing conditions. In both assemblages, triphylite is the only primary phosphate, and several secondary species are produced by its alteration. In association I, three main transformation stages are observed: (i) the progressive oxidation of triphylite accompanied by the leaching of Li leads to the successive crystallization of ferrisicklerite and heterosite; (ii) a hydration stage transforms triphylite into hureaulite, barbosalite, and tavorite, and (iii) a final stage, corresponding to meteoric alteration, the latest highly hydrated phosphate and oxide species. In assemblage I, ferrisicklerite is replaced by minerals like jahnsite s.l. and frondelite s.l. during the hydration stage, and in both assemblages, a second generation of Mn-rich triphylite (triphylite II) is observed. Two unusual petrographic textures were also observed, showing reactions between triphylite and albite to form montebrasite and garnet rims. These textures formed during the albitization stage, and correspond to the reactions triphylite + albite -> ferrisicklerite + montebrasite + quartz, and triphylite + albite -> almandine + quartz, respectively. Finally, the geochemical trends in phosphates and silicates indicate that Sapucaia is a weakly fractionated and geochemically primitive LCT-type pegmatite. The degree of differentiation of the pegmatite increases from the border to the core, as shown by a decrease of Fe/(Fe + Mn) in olivine-type phosphates, which decreases from 0.75 in the IWZ, to 0.71 in the IZ. Muscovite and tourmaline chemical compositions also show evidence of increasing degree of differentiation; for example, the Ga and Nb contents of muscovite increases from 57 and 49 ppm in the IWZ, to 86 and 80 ppm in the IZ, respectively.
The A-type granitoids can be divided into two chemical groups. The first group (A1) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A2) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A1 group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A2 group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.
The Early Cretaceous Hohhot metamorphic core complex (mcc) of the Daqing Shan (Mtns.) of central Inner Mongolia is among the best exposed and most spectacular of the spatially isolated mcc’s that developed within the northern edge of the North China “craton”. All of these mcc’s were formed within the basement of a Late Paleozoic Andean-style arc and across older Mesozoic fold-and-thrust belts of variable age and tectonic vergence. The master Hohhot detachment fault roots southwards within the southern margin of the Daqing Shan for an along-strike distance of at least 120 km. Its geometry in the range to the north is complicated by interference patterns between (1) primary, large-scale NW-SE-trending convex and concave fault corrugations and (2) secondary ENE-WSW-trending antiforms and synforms that folded the detachment in its late kinematic history. As in the Whipple Mtns. of California, the Hohhot master detachment is not of the Wernicke (1981) simple rooted type; instead, it was spawned from a mid-crustal shear zone, the top of which is preserved as a mylonitic front within Carboniferous metasedimentary rocks in its exhumed lower plate. 40Ar–39Ar dating of siliceous volcanic rocks in basal sections of now isolated supradetachment basins suggest that crustal extension began at ca. 127 Ma, although lower-plate mylonitic rocks were not exposed to erosion until after ca. 119 Ma. Essentially synchronous cooling of hornblende, biotite, and muscovite in footwall mylonitic gneisses indicates very rapid exhumation and at ca. 122–120 Ma. Contrary to several recent reports, the master detachment clearly cuts across and dismembers older, north-directed thrust sheets of the Daqing Shan foreland fold-and-thrust belt. Folded and thrust-faulted basalts within its foredeep strata are as young as 132.6 ± 2.4 Ma, thus defining within 5–6 Ma the regional tectonic transition between crustal contraction and profound crustal extension.
Petrogenesis and mineralization on numerous Mesozoic composite granites in South China is the subject of an ongoing debate. This paper reports biotite and muscovite single-grain mineral Rb-Sr ages and geochemical results for the Weishan granite, a typical Mesozoic composite granitic pluton in South China that includes Xinpu highly-evolved granite, Tangshi biotite monzogranite and Xiangzikou two-mica granite, probing into multistage magmatisms and implications for the petrogenetic and metallogenic processes. The results show that (1) biotite that crystallized at 227.0 +/- 13Ma (MSWD = 1.6) from the Xinpu highly-evolved granite is low in Al, Mg, Rb and ACNK ratios, but high in Fe and Fe/(Fe + Mg) ratios; (2) biotite that crystallized at 221.9 +/- 5.8Ma (MSWD = 3.8) from the Weishan's main body, the Tangshi biotite monzogranite, is high in Mg and Sr, but low Fe/(Fe + Mg) and Rb/Sr ratios; and (3) biotite from the Xiangzikou two-mica granite, formed at 210.1 +/- 3.3Ma (MSWD = 0.21) that were determined by single-grain muscovite Rb-Sr isochron age, is low Sr and high in Al, Rb, ACNK and Rb/Sr ratio; and (4) muscovite from the Tangshi biotite monzogranite is formed during the hydrothermal process, and is characterized by very low concentrations for most major and trace elements and high Fe/(Fe + Mg), Cs/Nb, Rb/Sr ratios, whereas muscovite from the Xiangzikou two-mica granite is formed by metasomatisms of biotite and K-feldspar etc., with higher Al, Ti and Na. The magma temperature and oxygen fugacity drop from the Tangshi biotite monzogranite to the Xiangzikou two-mica granite, finally to the Xinpu highly-evolved granite, while the magma for the Xiangzikou two-mica granite was involved in more pelitic sediments. Muscovite from the Xiangzikou two-mica granite is secondary in origin, and thus cannot be used as a pressure indicator to designate another magma source. The clear fractionation trends of biotite and muscovite compositions claim kindred among multistage magmas for the Weishan composite granite, which were mainly derived from a single psammite magma chamber, and intruded in batches under different tectonic regimes after extensive fractional crystallization and assimilation. Both of the Xinpu highly-evolved granite and the Xiangzikou two-mica granite are potential sources for uranium mineralization, but the latter shows a better perspective because of intense hydrothermal alternation.
Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≃ Rb ≃ (≃ Tl) ≃ Ba(≃ W) > Th > U ≃ Nb = Ta ≃ K > La > Ce ≃ Pb > Pr(≃ Mo) ≃ Sr > P ≃ Nd (> F) > Zr = Hf ≃ Sm > Eu ≃ Sn (≃ Sb) ≃ Ti > Dy ≃ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the istopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. -from Authors
The behaviour of niobium and tantalum in magmatic processes has been investigated by conducting MnNb2O6 and MnTa2O6 solubility experiments in nominally dry to water-saturated peralkaline (aluminium saturation index, A.S.I. 0.64) to peraluminous
(A.S.I. 1.22) granitic melts at 800 to 1035 °C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the
concurrence of the solubility products from dissolution, crystallization, Mn-doped and Nb- or Ta-doped experiments at the
same pressure and temperature. The solubility products of MnNb2O6 (Ksp
Nb) and MnTa2O6 (Ksp
Ta) at 800 °C and 2 kbar both increase dramatically with alkali contents in water-saturated peralkaline melts. They range from
1.2 × 10−4 and 2.6 × 10−4 mol2/kg2, respectively, in subaluminous melt (A.S.I. 1.02) to 202 × 10−4 and 255 × 10−4 mol2/kg2, respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explained by five
non-bridging oxygens being required for each excess atom of Nb5+ or Ta5+ that is dissolved into the melt. The Ksp
Nb and Ksp
Ta also increase weakly with Al content in peraluminous melts, ranging up to 1.7 × 10−4 and 4.6 × 10−4 mol2/kg2, respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subaluminous and peraluminous melts (A.S.I.
1.02 and 1.22) are strongly temperature dependent, increasing by a factor of 10 to 20 from 800 to 1035 °C. By contrast columbite-tantalite
solubility in the peralkaline composition (A.S.I. 0.64) is only weakly temperature dependent, increasing by a factor of less
than 3 over the same temperature range. Similarly, Ksp
Nb and Ksp
Ta increase by more than two orders of magnitude with the first 3 wt% H2O added to the A.S.I. 1.02 and 1.22 compositions, whereas there is no detectable change in solubility for the A.S.I. 0.64
composition over the same range of water contents. Solubilities are only slightly dependent on pressure over the range 800
to 5000 bars. The data for water-saturated sub- and peraluminous granites have been extrapolated to 600 °C, conditions at
which pegmatites and highly evolved granites may crystallize. Using a melt concentration of 0.05 wt% MnO, 70 to 100 ppm Nb
or 500 to 1400 ppm Ta are required for manganocolumbite and manganotantalite saturation, respectively. The solubility data
are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt partition coefficients
increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions. It is demonstrated that
the γNb2O5/γTa2O5 activity coefficient ratio in the melt phase depends on melt composition. This ratio is estimated to decrease by a factor
of 4 to 5 from andesitic to peraluminous granitic melt compositions. Accordingly, all the relevant accessory phases in subaluminous
to peraluminous granites are predicted to incorporate Nb preferentially over Ta. This explains the enrichment of Ta over Nb
observed in highly fractionated granitic rocks, and in the continental crust in general.
New analyses of 131 samples of A-type (alkaline or anorogenic) granites substantiate previously recognized chemical features, namely high SiO2, Na2O+K2O, Fe/Mg, Ga/Al, Zr, Nb, Ga, Y and Ce, and low CaO and Sr. Good discrimination can be obtained between A-type granites and most orogenic granites (M-, I and S-types) on plots employing Ga/Al, various major element ratios and Y, Ce, Nb and Zr. These discrimination diagrams are thought to be relatively insensitive to moderate degrees of alteration. A-type granites generally do not exhibit evidence of being strongly differentiated, and within individual suites can show a transition from strongly alkaline varieties toward subalkaline compositions. Highly fractionated, felsic I- and S-type granites can have Ga/Al ratios and some major and trace element values which overlap those of typical A-type granites.A-type granites probably result mainly from partial melting of F and/or Cl enriched dry, granulitic residue remaining in the lower crust after extraction of an orogenic granite. Such melts are only moderately and locally modified by metasomatism or crystal fractionation. A-type melts occurred world-wide throughout geological time in a variety of tectonic settings and do not necessarily indicate an anorogenic or rifting environment.
Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an FeNi core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at ∼ 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an FeNi alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence (∼ 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk of ∼ 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth (∼ 14%) and the core (∼ 86%). Chondritic meteorites display a range of ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.
Strongly peraluminous (SP) granites have formed as a result of post-collisional processes in various orogens. In `high-pressure' collisions such as the European Alps and Himalayas, post-collisional exhumation of overthickened crust (>50 km), heated by radiogenic decay of K, U and Th during syn-collisional thickening, produced small- to moderate-volume, cool (<875°C) SP granite melts with high Al2O3/TiO2 ratios. In `high-temperature' collisions such as the Hercynides and Lachlan Fold Belt (LFB), there was less syn-collisional crustal thickening (≤50 km). Crustal anatexis was related to post-collisional lithospheric delamination and upwelling of hot asthenosphere, forming large-volume, hot (≥875°C) SP granite melts with low Al2O3/TiO2 ratios. Both clay-rich, plagioclase-poor (<5%) pelitic rocks and clay-poor, plagioclase-rich (>25%) psammitic rocks have been partially melted in high-pressure and high-temperature collisional orogens, with the pelite-derived SP granites tending to have lower CaO/Na2O ratios (<0.3) than their psammite-derived counterparts. The predominance of pelite-derived SP granites in the Himalayas and psammite-derived SP granites in the LFB suggests that mature continental platforms made up more of the accreted crust in the Himalayan collision than in the LFB.
The Separation Lake area is host to the most important rare-element pegmatites in Ontario, Canada. They include the Big Whopper and Big Mack petalite pegmatite systems which potentially represent the world's second largest lithium deposit of this type. The pegmatites occur in two distinct clusters adjacent to the Separation Rapids pluton which is thought to be the source of the rare-elements. Beryl-type and complex-, petalite-subtype pegmatites are the most common and a few pegmatites have characteristics similar to the lepidolite-subtype. This study reveals that columbite-tantalite in the pegmatites has an extremely wide range of composition from primitive ferrocolumbite to evolved, almost end-member manganotantalite. Evidence is provided that melt evolution resulted in increased fluorine activity (as seen in microlite compositions) and that in situ fractionation of magma within individual pegmatites often led to the crystallization of rare-element-enriched, Li mica-fluorapatite-cleavelandite pods. Zonation patterns seen in backscattered electron images show primary compositions of columbite-tantalite were modified by secondary processes related to extreme fractionation and involving the late stage development of albitic units in individual pegmatites. This alteration led to recrystallization of columbite-tantalite and produced compositions with lower Ta contents, but with little change in Mn content.
The Zhaojinggou rare-metal deposit is located in the Wulashan-Daqingshan region, central north margin of the North China Craton. Nb-Ta mineralizations are mainly hosted in the biotite alkali-feldspar granite (BAG) and albite granite (AG), with subordinate mineralization in the alkali-feldspar aplite dyke (AFA), the amazonite granitic pegmatite dyke (AGP), and the greisen (black + white colored greisen). In this paper, we present a systematic work of geochronology (CGM U-Pb combined with cassiterite U-Pb, monazite U-Th-Pb, and muscovite ⁴⁰Ar-³⁹Ar dating), whole-rock geochemistry, and Sr-Nd isotopes to understanding the timing of mineralization, petrogenesis, geodynamic setting, and ore-forming processes. Our geochronological data reveal two episodic Nb-Ta mineralization events in the Zhaojinggou deposit during the Early Cretaceous. The first and major ore-forming episode occurred at ∼130 Ma, forming a sequence of ore-bearing rocks of the BAG-AG-greisen-AFA. The second but subordinate ore-forming episode occurred at ∼114 Ma, forming the ore-bearing AGP. The whole-rock geochemistry indicates the granitic rocks (the BAG-AG-AFA and AGP) from the two episodes are highly evolved with analogous trace element and REE patterns, and have A-type affinity. The whole-rock εNd(t) values of these granitic rocks are relative similar, ranging from -9.6 to -7.6, with corresponding T2DM ages of 1490-1713 Ma. These features suggest that these A-type granites from the two discrete episodes are derived from the same magma reservoir by high-temperature partial melting of the Middle Proterozoic basement in an infracrustal extensional environment. These A-type granites experienced extremely fractional crystallization during emplacement. The whole-rock REE tetrad effects, element ratios (Nb/Ta, Zr/Hf and K/Rb), and Nb-Ta fractionation modeling indicate that both magmatic and hydrothermal processes are involved to form the two episodic Nb-Ta mineralizations. High degree of factional crystallization and magmatic-hydrothermal processes after high-temperature anataxis are crucial to form the Zhaojinggou rare-metal deposit. Moreover, these highly-evolved A-type granites (the BAG-AG-AFA and AGP) formed during the Early Cretaceous (at ∼130 Ma and ∼114 Ma respectively) are the important targets for rare-metals prospecting in the central north margin of the NCC.
The newly discovered Zhaojinggou TaNb deposit in the northern margin of the North China Craton contains ore reserves of more than 38 Mt. at an average grade of 0.013 wt% Ta2O5 and 0.012 wt% Nb2O5. The TaNb mineralization is mainly hosted in a peraluminous, albite granite body, which is about 400 m long and 20 to 100 m wide, with a surface exposure of ~0.05 km². Granitic pegmatites and greisen veins that range from several centimeters to meters in width, occur in the margin of the granite body, and wolframite-quartz veins intrude the wall rocks up to tens of meters away from the granite contact. LA-ICP-MS UPb dating for columbite of the granite yields the lower intercept ages between 131.8 ± 1.0 Ma and 134.6 ± 2.6 Ma on the Tera-Wasserburg concordia diagram, indicating that the mineralization occurred in the early Cretaceous, coeval with the peak stage of lithospheric thinning and destruction of the North China Craton. The columbite grains in the granite are commonly euhedral, and are surrounded or penetrated by irregular tantalite patches/rims containing Ta2O5 as high as 59.5 wt%. The Ta concentrations within single columbite crystals commonly increase slightly from the center outwards, consistent with a systematic decrease of Nb/Ta caused by crystallization of columbite from the silicic magma. Mica in the granite shows disordered bright and dark domains in their BSE images; bright domains are F-rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in Fe, Li, Rb, Nb and Ta, whereas the dark domains are F-poor muscovite crystallized from later hydrothermal fluids that were relatively enriched in Al, Mn, Ba, Sn and W. It is thus likely that the tantalite patches/rims around the columbite grains crystallized from a Ta- and F-rich hydrosilicate melt in a magmatic-hydrothermal transitional stage. The wolframite-quartz veins adjacent to the granite body likely formed from a W-rich hydrothermal fluid.
We studied newly found high Nb–Ta alkaline rhyolites in the northern volcanic belt of the Great Xing’an Range, China. The LA–ICP–MS U–Pb weighted mean age is 114.07 ± 0.55 Ma, indicating that the rocks formed during the late Early Cretaceous and were the product of the late eruption of a Mesozoic volcano. The major element contents are characterized by high Si, rich K, low Fe, and poor Ca and Mg. In the total alkaline–silicon diagram, the sample points are in the alkaline rhyolite region. Meanwhile, rare earth elements show obvious Ce/Ce* positive anomalies and Eu/Eu* negative anomalies. In addition, trace elements are characterized by high Nb, Ta, and Yb, and low Sr. The two-stage Nd isotopic model age T 2DM of the depleted mantle is between 799–813 Ma, indicating that the diagenetic material originated from the depleted mantle or partial melting of newly formed young crustal materials. The source rocks melted at a relative shallow depth (<30 km), under lower pressure (<0.5 Gpa) and high oxygen fugacity; moreover, the residues in the source region were Ca-rich mafic plagioclase + amphibole + orthopyroxene. In the Nb–Y–3Ga and Nb–Y–Ce diagrams, the sample points are in the A1 type region. It can be concluded that the mantle-derived basaltic magma underplated and supplied the heat sources for partial melting of the metamorphic crustal rocks in an intraplate extensional tectonic environment related to a rift, mantle plume, and hot spot.
The Zhaojinggou Nb-Ta deposit is a newly explored large-scale deposit in Wuchuan County, Inner Mongolia. The Nb-Ta orebody is mainly hosted in the amazonite-bearing albite granite, and partly hosted in the granite aplite, greisen and amazonite granite pegmatite. In this paper, we use monazite and zircon LA-MC-ICP-MS U-Pb and biotite ⁴⁰ Ar- ³⁹ Ar dating methods to constrain the mineralization age. Monazite LA-MC-ICP-MS U-Pb geochronological dating of four samples from granites yields ages of 124±2 Ma (MSWD=2.9, n=27), 124±3 Ma (MSWD=2.0, n=46), 121±1 Ma (MSWD=3.3, n=43) and 124±2 Ma (MSWD=3.2, n=12). One zircon sample from albite granite yields age of 125±1 Ma (MSWD=1.6, n=18). Meanwhile, biotite ⁴⁰ Ar- ³⁹ Ar dating obtains the plateau age of 133.84±0.79 Ma (MSWD=3.31), isochron age of 134.55±0.79 Ma (MSWD=1.93) and reverse isochron age of 134.58±0.80 Ma (MSWD=1.99). Therefore, all these ages constrain that the ore-forming period of Zhaojinggou Nb-Ta deposit is Early Cretaceous. In conclusion, the Zhaojinggou deposit resulted from the tectonic-magmatic activities of extensional setting, and the Yanshanian Period is the main Nb-Ta metallogenic age in Inner Mongolia. © 2019, Editorial Department of Earth Science. All right reserved.
In rare-metal granites, niobium and tantalum are generally hosted by Nb-Ta oxides. However, in SE China, the Nb-specialized Huangshan granites are a unique occurrence in which Nb is essentially hosted by Li-Fe micas. The Huangshan granites are part of the Early Cretaceous (Late Yanshanian) Lingshan granite complex and belong to the A-type granite series, with two facies differing by their mica compositions: medium-grained "protolithionite" granite and medium-grained lithian (lithium-rich) annite granite. The granites are characterized by elevated whole-rock Nb contents (average 144 ppm in "protolithionite" granite and 158 ppm in annite granite), quite low Ta contents (average 9 and 4 ppm, respectively), leading to very high Nb/Ta ratios (average 15.3 and 31.2). Niobium is mainly hosted in the micas, with an average Nb content of 1347 ppm in the lithian annite and 884 ppm in the "protolithionite," which is the highest ever reported in granitic mica. With an estimated endowment of ~80 kt Nb, the Huangshan granites represent a new style of potential Nb resource. Contrasting with the great rarity of columbite, there is abundant Hf-rich zircon, Y-rich fluorite, and Th-rich fluocerite included in the Huangshan micas. Such accessory minerals being typical of alkaline rhyolitic magmas and niobium enrichment in the Huangshan granites results from A-type melt. The extreme Nb enrichment in the micas results from the highly compatible behavior of Nb in this melt, combined with the high magma temperature (estimated at 790-800 °C) and possibly enhanced magma oxidation.
The Yichun Ta-Nb deposit, which is located in Jiangxi Province, China, can be divided into four lithological zones (from bottom upward): two-mica granite, muscovite granite, albite granite, and lepidolite-albite granite zones. It remains controversial whether these distinct vertical zones were formed through late magmatic-hydrothermal metasomatic alteration or fractional crystallization of magma. To investigate the evolution mechanism of rock- and ore-forming fluid in this deposit, we studied fluid and melt inclusions in quartz and lepidolite in these four granite zones. These fluid inclusions are mainly composed of H2O-NaCl, and have homogenization temperatures ranging from 160°C to 240°C, with densities between 0.86 and 0.94 g/cm³ and salinities between 0.5 and 6.5 wt% NaCl equivalent. Raman spectroscopic analyses showed that the daughter minerals contained in silicate melt inclusions are mainly quartz, lepidolite, albite, muscovite, microcline, topaz, and sassolite. From the lower to upper granite zones, the albite contents in silicate melt inclusions increase, while the muscovite contents decrease gradually until muscovite is substituted by lepidolite in the lepidolite-albite granite zone. Additionally, the calculated densities of the silicate melt inclusions exhibit decreasing trends from bottom upward. The total homogenization temperatures of silicate melt inclusions, which were observed under external pressures created in the sample chamber of a hydrothermal diamond-anvil cell, decreased from 860°C in the lower lithological zone to 776°C in the upper lithological zone, and the initial melting temperatures of solid phases were 570-710°C. The calculated initial H2O contents of granitic magma showed an increasing trend from the lower (∼2 wt％ in the two-mica granite zone) to the upper granitic zones (∼3 wt％ in the albite granite zone). All of these features illustrate that the vertical granite zones in the Yichun Ta-Nb deposit formed through the continuous fractional crystallization of the granitic magma. Additionally, the low H2O contents and gradual enrichment of incompatible elements (F, B, Li, Ta, Nb, etc.) in the residual granitic magma favored the formation of a granite-type Ta-Nb deposit.
Two types of greenstone belts occurring within Archeozoic and Proterozoic respectively have been identified in the Daqingshan - Wulashan district, which belongs to central part of Inner Mongolia autonomous area. The average Au contents of Archeozoic and Proterozoic greenstone are 2.8 × 10-9 - 32 × 10-9, 2.6 × 10-9 - 34 × 10-9 respectively. Compared with the other types of rocks, the greenstone is characterized with high Au content, and always occur the large or middle scale gold deposits. The gold deposits of the study area may be classified as five types: (1) stratoid veinlet-disseminated type, such as the Motianling gold deposit; (2) sedimetamorphic rocks stratabound type, such as the Maoduqing and Youlougou gold deposits; (3) K-feldspar-quartz vein type, such as the Wulashan gold deposit; (4) altered rock-quartz vein composite type, such as the Donghuofang gold deposit; (5) quartz vein type, such as the Houshihua gold deposit. The former two types belong to the Proterozoic greenstone type gold deposit, and the latter three types belong to the Archeozoic greenstone type gold deposit. According to the Xie Xuejin's theory of geochemical prospecting and the geological features of the study area, six geochemical block had been choosed as ore-finding prospective area: (1) the Xihezi - Xindigou - Hongpan Au geochemical block; (2) the Yinhao - Erdaowa - Shanggaotai Au geochemical block; (3) the Amawusu - Xishanwan - Bainaimiao Au-Cu geochemical block; (4) the Heiaobao - Saiwusu - Chagancilao Au geochemical block; (5) the Jinpen - Mamitu - Manzhouli Au-Cu geochemical block; (6) the Wulashan - Yushuwan Au geochemical block. It is estimated that the Au productivity reach to 67550t within the six geochemical blocks, which indicates that there is good metallogenetic material foundation and ore-finding prospective potential in the study area.
The columbite-group minerals from the Koktokay No. 3 granitic pegmatite, Altay, NW China were studied in the paper by using the electron microprobe analysis and back-scattered electron imaging. The results indicate that the columbite-group minerals in this district are mainly Mn-enriched, which are mainly manganocolumbite and manganotantalite. From the border zone to the core of the pegmatite, the ratio of Ta/(Nb+Ta) varies almost vertically with nearly constant ratio of Mn/(Fe+Mn). The chemical compositions of the columbite-group minerals from the earlier magmatic stage vary limitedly, while those from the later magma-hydrothermal transitional stage vary over a wide range. The back-scattered electron images show that the compositional zonation is not clear in columbite-group minerals from the magmatic stage (zones I ∼ IV), while those from the magma-hydrothermal transitional stage (zones V ∼ VII) exhibit a complex compositional zonation. Indeed, the zonation patterns vary from the progressive, via the oscillatory to the replacement structure during the consolidation of the granitic pegmatite. The study indicates that the composition variations of the columbite-group minerals from different zones of pegmatite and their compositional zonation are due to hydrothermalism during the crystallization of columbite-group minerals, and the zonation patterns are mainly controlled by the degree of hydrothermalism.
Western Sichuan is an important area of rare metal resources in China. Keeryin pegmatite type rare metal ore deposit is located at the center of the Songpan - Garze orogenic zone, western Sichuan. To study it can trace the evolution of the Songpan - Garze orogenic zone. In this paper, 40Ar/39Ar plateau age of 176.25±0.14 Ma of muscovite from the muscovite - microcline pegmatite in Genze intrusive rock and the age of 152.43± 0.60 Ma from Dangba lithium deposit are gotten. Considering the genesis of the granites and published K-Ar, Rb-Sr, U-Pb isotope ages, it is concluded that in the late of Indosinian epoch and the early of Yanshanian epoch, large scale magmations happened in Keeryin region, and lasted a long time. But rare metal mineralization took place in steady and close geologic environment in the late of magmations. These geological phenomena showed that the Songpan - Garze orogenic zone evolved into stable period until the beginning of Himalayan movement. The stable period is ready for ore-forming of many type deposit.
The Longwo and Baishigang plutons, locating in the eastern end of the EW-trending Fogang granite belt, are typical examples of granitic rocks formed by crust-mantle interaction in Nanling region. Zircon LA-ICP-MS U-Pb dating yields an age of (169.1 ± 2.5) Ma for the Longwo pluton and an age of (157.8 ± 2.3) Ma for the Baishigang pluton, indicating that both of them were formed in early Yanshanian. The major and trace element contents of biotites are measured by employing EMPA and LA-ICP-MS techniques, and their petrogenetic significances are discussed. The EMPA analyses of major elements demonstrate that biotites of the Longwo pluton are distinguished from that of the Baishigang pluton by higher abundances of TiO2, MgO, and lower concentrations of FeO, Al2O3 and volatile constituents (e.g., F and Cl), indicating that the Longwo pluton was likely to have formed under a relative high temperature and a more oxidated environment. Trace elements analyses by LA-ICP-MS technique illustrate that biotite is an important carrier of Rb, Ba, Nb, Ta, Sc, V, Cr, Co, Ni in granitic melts, but has less ability in hosting U, Th, Pb, Sr, Zr, Hf, Y, and thus the concentrations of these elements in biotite are much lower than that in the host rocks. Biotite in granitic rocks also displays quite low REE concentrations, suggesting that it is not the main mineral phase affecting the REE features of the host rocks. The mineral chemistry of biotite can be used as a powerful tool in identifying the differentiation degree of the host magmas and in evaluating the rock-forming physical-chemical conditions, and can also be employed in tracing the nature of the magma source. But in the latter aspect, other data (especially isotope compositions) should be coupled with in order to obtain more accurate information.
Hukeng granite intrusion locates in southeast limb of Wugongshan compound anticline, where developed large scale Hukeng tungsten deposit that is an important part of Wugongshan W-Cu-Bi-Mo poly-metallic metallogenic belt. Through precise LA-ICPMS U-Pb dating of zircon from Hukeng granite, the crystallization age of the intrusion was determined to be 151.6 ± 2.6Ma, corresponding to Late Jurassic era and ascribed to the first intruding event of granites in Mesozoic era in South China (164-153Ma). The formation temperature of granite magma was 676-695°C. Through the analysis of major and trace elements of Hukeng muscovite granites, they belonged to high-K calc-alkaline series and were thought to be metaluminous - weak peraluminous differentiation S-type granites. They should mainly derive from clay-enriched argillaceous rock source in upper crust and form under lithospheric extensional and thinning environment.
Late Mesozoic extension tectonics was quite significant on eastern Eurasia Continent and it was expressed by extensional domal structure such as the metamorphic core complexes, syntectonic plutons, and ductile detachment fault, even graben and half graben basins. According to our field observations, laboratory work and previous research, from north to south, five extension belts have been separated: Transbaikalia-Okhotsk belt, western part of North China Block, eastern part of North China Block, south margin of North China Block and Qinling-Dabie belt and South China Block. As a largest extension tectonic at the scale of crust in the world, these entire belts have NW-SE extensional direction and this extensional structure make the middle to lower curst rocks exhumed to the surface along the detachment normal fault. Geochronological work on these ductile detachment fault indicated a very narrow period around 130 ∼ 126Ma except the eastern part of North China Block, which have a width extensional period relatively. Lithosphere delamination could be considered as the geodynamic of this large scale extension tectonic. This geodynamic model could make us to understand the time, scale, and mechanism on the topic of North China Craton destruction from the view of structure analyses.
A combined compositional and optical spectrophotometric study of 24 biotite specimens from the granitic rocks of the Hepburn and Bishop intrusive suites of the early Proterozoic Wopmay orogen, Northwest Territories, shows that the chemical composition and the color of this mineral strongly reflect the tectonic origin of its host. The biotite quadrilateral effectively portrays the compositional trends of micas from continental-collision- and arc-related granites. -from Authors
Phase-equilibrium studies in the system FeO-Fe2O3—TiO2 permit determination of the temperature and oxygen fugacity of formation of coexisting pairs of titaniferous magnetite and
ilmenite in many rocks. Temperatures thus obtained are probably accurate to ±50°C. Temperatures indicated for most igneous
and metamorphic rocks for which data are available are generally consistent with temperatures inferred by other methods. Temperatures
for certain gabbroic rocks are too low for magmatic crystallization and probably reflect the migration of ilmenite from titaniferous
magnetite to form separate granules upon cooling.
The experimentally determined solubility of ilmenite in magnetite is much too small to account for most ilmeno-magnetites
by simple exsolution. Subsolidus oxidation of magnetite-ulvöspinelgSS to yield ilmenite-magnetite intergrowths has been experimentally verified and probably takes place during cooling of many
igneous and perhaps some metamorphic rocks. Oxidation at surface or hypabyssal conditions may produce metastable titanomaghemites.
In order of increasing intensity of oxidation, the following Fe—Ti oxide pairs occur in plutonic rocks: ulvöspinel-rich magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+ilmeniteSS, ulvöspinel-poor magnetiteSS+hematiteSS, hematiteSS+rutile.
The Sn-rich Qiguling topaz rhyolite dike intrudes the Qitianling biotite granite of the Nanling Range in southern China; the granite hosts the large Furong Sn deposit. The rhyolite dike is typically peraluminous, volatile-enriched, and highly evolved. Whole-rock F and Sn concentrations attain 1.9 wt.% and 2700 ppm, respectively. The rhyolite consists of a fine-grained matrix formed by quartz, feldspar, mica and topaz, enclosing phenocrysts of quartz, feldspar and mica; it is locally crosscut by quartz veinlets. Lithium-bearing micas in both phenocrysts and the groundmass can be classified as primary zinnwaldite, “Mus-Ann” (intermediate member between annite and muscovite), and secondary Fe-rich muscovite. Topaz is present in the groundmass only; common fluorite occurs in the groundmass and also in a specific cassiterite, rutile and fluorite (Sn–Ti–F) assemblage. Cassiterite and rutile are the only Sn and Ti minerals; both cassiterite and Nb-rich rutile are commonly included in the phenocrysts. The Sn–Ti–F assemblage is pervasive, and contains spongy cassiterite in some cases; cassiterite also occurs in quartz veinlets which cut the groundmass. Electron microprobe and LA-ICP-MS compositions were used to study the magmatic and hydrothermal processes and the role of F in Sn mineralization. The presence of zinnwaldite and “Mus-Ann”, which are respectively representative of early and late mica crystallization during magma differentiation, also suggests a significant decrease in f(HF)/f(H2O) of the system. Cassiterite included in the zinnwaldite phenocrysts is suggested to have crystallized from the primary magma at high temperature. Within the Sn–Ti–F aggregates, rutile crystallized as the earliest mineral, followed by fluorite and cassiterite. Spongy cassiterite containing inclusions of the groundmass minerals indicate a low viscosity of the late fluid. The cassiterite in the quartz veinlets crystallized from low-temperature hydrothermal fluids, which possibly mixed with meteoric water. In general, cassiterite precipitated during both magmatic and hydrothermal stages, and over a range of temperatures. The original fluorine and tin enrichments, f(HF)/f(H2O) change in the residual magma, formation of Ca,Sn,F-rich immiscible fluid, decrease of the f(HF) during groundmass crystallization, and mixing of magma-derived fluids with low-saline meteoric water during the late hydrothermal stage, are all factors independently or together responsible for the Sn mineralization in the Qiguling rhyolite.
Late Mesozoic granitoid plutons of four distinct ages intrude the lower plate of the Hohhot metamorphic core complex along the northern margin of the North China craton. The plutons belong to two main groups: (1) Group I, deformed granitoids (148 and 140 Ma subgroups) with high Sr, LREE, and Na2O, low Y and Yb contents, high Sr/Y and La/Yb ratios, weak or no Eu anomalies, low Rb/Ba ratios, similar initial 87Sr/ 86Sr values (0.7064-0.7071) and low Mg# (<37 mostly, 100 × molar MgO/MgO + FeO t); (2) Group II, non-deformed granitoids (132 and 114 Ma subgroups) with low Sr, relatively low Na 2O, high Y and Yb contents, pronounced negative Eu anomalies, high Rb/Ba ratios, and initial 87Sr/ 86Sr values (0.7098-0.7161). The two groups share geochemical similarities in πNd(t) (-11.3 to -15.4) and TDM2 ages (1.85-2.18 thousand million years) as well as Hf isotopic ratios in zircons. Geochemical modelling (using the MELTS code) suggests that similar sources but different depths of magma generation produced the early, high-pressure low-Mg adakitic granitoids and late, low-pressure granitoids with A-type characteristics. The early granitoids likely represent a partially melted, deep-seated, thickened lower continental crust that involved a minor contribution from young materials, whereas the later group partially melted at shallower depths. This granitic magmatic evolution coincided with the tectonic transition from crustal contraction to extension.
Electron microprobe analyses of zoned columbite-tantalite crystals from the granitic pegmatites of the Eräjärvi area in Orivesi, southern Finland indicate wide compositional variation within the series FeNb 2 O 6 -FeTa 2 O 6 -MnNb 2 O 6 -MnTa 2 O 6 , especially in specimens from thin pegmatite dikes. Most crystals show gentle progressive zoning characterized by small-scale variations in the major elements. Where the compositional variation is large, backscattered electron images indicate oscillatory or patchy zoning or various replacement textures. The zones in oscillatory zoned crystals are usually 1-50 m in width, exceptions reaching 50-120 m. The wider zones often consist of a group of very narrow subzones of only slightly different composition. Zoning is due mainly to the compositional variation in Ta and Nb. In two of these crystals, the oscillations in Mn follow the strongest oscillations in Nb content. Patchy zoned crystals exhibit corroded cores of early columbite-tantalite, surrounded by later zones enriched in Ta. The mottled appearance of such crystals results from two or more successive replacements. Replacement tongues or network-like replacement textures are typical in the rims of some crystals. The zoning of the columbite-tantalite is related to the complex crystallization history of the pegmatite dikes. The main factors controlling oscillatory zoning are considered to be the growth dynamics of the crystals, the concentration and diffusion of the main elements, and the successive flows of the magma in an intrusion channel. The generation of a corrosive supercritical vapor phase at the end of magmatic crystallization caused resorption, patchy zoning and the replacement of the columbite-tantalite.
Low-pressure peraluminous migmatites from the Peña Negra Complex (central Spain) stayed partially molten for 5–10 Ma, until the latest Hercynian deformations allowed the segregation of melt as discordant leucosome veins. Due to long residence within its source, the melt had time enough to equilibrate with residual phases other than accessories included within major, refractory minerals. We estimated crystal/melt partition coefficients as the concentration ratios between leucosome samples appearing to be pure melts and mesosome minerals, which were analyzed for trace elements with a laser probe coupled to an ICP mass spectrometer. Our data reveal that when biotite is stable Li, Rb, Cs, Tl, Sc, V, Cr, Ni, Nb and Ta become strongly compatible. The role of biotite in fractionating Y, Th, U and the REE is insignificant. Cordierite strongly fractionates Li and Be and also has some effect on the HREE and U. Garnet produces extreme fractionation of Sc, Y and the HREE. The REE partition coefficients for ferromagnesian silicates increase with the atomic number, this effect being progressively more important through biotite, cordierite and garnet. K-feldspar strongly fractionates Ba and Pb, but plays a secondary role to biotite for Rb and Cs. K-feldspar fractionation does not change the LREE/HREE ratio, but plagioclase fractionation produces a significant decrease in the LREE/HREE ratio. Both feldspars greatly fractionate Sr and Eu. Monazite fractionation produces a dramatic depletion in REE, Th and U, as well as a decrease in the LREE/HREE and Th/U ratios. Apatite also fractionates the REE although, in contrast with monazite, it increases the LREE/HREE ratio and does not affect the Th/U ratio. Zircon fractionation, like apatite and garnet, produces a strong depletion in the HREE and a concomitant increase in the LREE/HREE ratio. In contrast with monazite, zircon fractionation causes the Th/U ratio to increase.
The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructures of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous 40Ar/39Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directed, early tangential, high-temperature ductile shearing probably was initiated during ca. 148–140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major extensional ductile shearing along the detachment zones took place during ca. 140–132 Ma. Using these new U-Pb ages, combined with previously published 40Ar/39Ar cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).
The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe3+ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO2 was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO2 = 10−14to 10−15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO2 = 10−16to 10−18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO2 while fH2O2 remains relatively high.
The Berens River area is representative of a broad belt of Archean felsic plutonic rocks in the northwestern Superior Province of Ontario. The plutonic belt (Berens River subprovince) occupies the southern margin of the North Caribou terrane, a regional-scale enclave of pre-2.8-Ga plutonic and greenstone sequences. Plate-tectonic models maintain that the North Caribou terrane is an early continental mass against which other terranes (subprovinces) were rapidly accreted from 2.75 to 2.70 Ga. During accre-tion, a magmatic arc characterized by voluminous felsic intrusions developed on the southern margin of the old continent. In the Berens River area, five major groups of felsic plutonic rocks have hornblende-bearing assemblages suitable for application of amphibole + plagioclase thermometers and Al-in-hornblende barometers. Cross-cutting relations show that the biotite tonalite suite is oldest; it is cut consecutively by hornblende tonalite of the hornblende suite, hornblende granodiorite of the hornblende suite, the biotite granite suite and the sanukitoid suite. Average temperatures for the major plutonic suites range from 716 to 773°C. Temperature maps show irregular isotherms, with some low-T zones associated with faults in the hornblende tonalite, but not in hornblende granodiorite. The average temperature-corrected Al-in-hornblende pressures for the plutonic suites range from 4.4 to 2.3 kbar and decrease with age of the suites. The distribution of P measurements indicates that the Berens River area was uplifted significantly during magmatism, with uplift concentrated at the southern margin of the North Caribou continent.
Lanthanide tetrad effects are often observed in REE patterns of more highly evolved Variscan peraluminous granites of mid-eastern Germany (Central Erzgebirge, Western Erzgebirge, Fichtelgebirge, and Northern Oberpfalz). The degree of the tetrad effect (TE1,3) is estimated and plotted vs. K/Rb, Sr/Eu, Eu/Eu∗, Y/Ho, and Zr/Hf. The diagrams reveal that the tetrad effect develops parallel to granite evolution, and significant tetrad effects are strictly confined to highly differentiated samples. Mineral fractionation as a cause for the tetrad effect is not supported by a calculated Rayleigh fractionation, which also could not explain the fractionation trends of Sr/Eu and Eu/Eu∗. The strong decrease of Eu concentrations in highly evolved rocks suggests that Eu fractionates between the residual melt and a coexisting aqueous high-temperature fluid. Mineral fractionation as a reason for the tetrad effect is even more unlikely as REE patterns of accessory minerals display similar tetrad effects as the respective host rocks. The accessory minerals inherit the REE signature of the melt and do not contribute to the bulk-rock tetrad effect via mineral fractionation. These results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts: ionic radius and charge, which commonly control the element distribution between mineral and melt, are no longer the exclusive control. The tetrad effect and the highly fractionated trace element ratios of Y/Ho and Zr/Hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence. This distinct trace element behavior and the common features of magmatic-hydrothermal alteration suggest the increasing importance of an aqueous-like fluid system during the final stages of granite crystallization. The positive correlation of TE1,3 with bulk-rock fluorine contents hints at the importance of REE fluorine complexation in generating the tetrad effect. As the evolution of a REE pattern with tetrad effect (M-type) implies the removal of a respective mirroring REE pattern (W-type), the tetrad effect identifies open system conditions during granite crystallization.
Use of some petrologic diagrams applied to analyses of volcanic rocks is unnecessarily difficult due to lack of data for construction of discriminant lines between rock series. Coordinates are provided for sufficient points to enable accurate plotting of the boundary lines within seven diagrams, viz.: (1) TAS - total alkalies (Na2O+K2O) vs. SiO2; (2) K2O vs. SiO2; (3) AFM; (4) Jensen; (5) KTP - K2OTiO2P2O5; (6) vs. SiO2; and (7) vs. . Different versions of these boundaries are collated to indicate their variable position, and it is demonstrated that inter-laboratory analytical precision suffices to account for almost all of their spread on the TAS and K2O vs. SiO2 diagrams.