Granite G1 Research Articles

Highly fractionated S-type granites from the giant Dahutang tungsten deposit in Jiangnan Orogen, Southeast China: geochronology, petrogenesis and their relationship with W-mineralization

Abstract The Dahutang deposit is a newly discovered tungsten deposit, which is within the largest ones in the world with an estimated WO3 reserve of 2 million tones. W-mineralization is considered to be related with the Late Mesozoic granites in the district. However, the precise emplacement ages, sources of these granites, and their relationship with mineralization are not well understood. In this study, four mineralization-related granite bodies (G1 to G4) were identified in the Dahutang mining area, including the porphyritic-like two-mica granite (G1) (the size of its phenocrysts is from 0.2 × 0.2 cm2 to 0.7 × 0.3 cm2), middle- to fine-grained muscovite granite (G2), porphyritic two-mica granite (G3) (the size of its phenocrysts is from 0.3 × 0.2 cm2 to 0.9 × 0.6 cm2), and fine-grained two-mica granite (G4). LA-ICP-MS U–Pb dating of zircon grains from these four granite bodies yields emplacement ages of 144.0 ± 0.6 Ma, 133.7 ± 0.5 Ma, 130.3 ± 1.1 Ma and 130.7 ± 1.1 Ma, respectively. Granites contain quartz, K-feldspar and plagioclase as the principal phases, accompanied by muscovite, minor biotite and accessory minerals. Geochemically, the granites are strongly peraluminous, have high contents of alkalis, high Ga/Al ratios, enrichment in LILEs (such as Rb) and depletion in HFSEs (such as Zr, Nb, Ti). The granites formed at relatively low temperatures (679 °C to 760 °C) according to zircon saturation temperatures. Geochemical fractionation trends recorded by whole rocks and minerals permit to distinguish and model the two fractional crystallization series G1 and G2–G4. Fractional crystallization of orthoclase and albite in G1 and G4, and orthoclase and oligoclase in G2 and G3 was the principal process of magmatic differentiation that controlled Rb, Sr and Ba concentrations, whereas rare earth elements were fractionated by accessory minerals, such as apatite, zircon and monazite. The geochemical data suggest that the rocks are highly fractionated S-type granites. The granites show bulk rock eNd(t) values in the range of − 9.37 to − 5.92 and zircon eHf(t) values from − 8.44 to − 2.13, with late Mesoproterozoic TDMC ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these highly fractionated S-type granites G1 and G2–G4 were originated from two episodes of partial melting of different protoliths which have analogous components of metamorphosed pelitic rocks from the Neoproterozoic Shuangqiaoshan Group, which are enriched in tungsten. Extreme fractional crystallization resulted in further enrichment of tungsten in the evolved granitic magma. The new presented data together with previously published data suggest that the Dahutang granitic complex was likely to be formed during lithospheric thinning and asthenospheric upwelling process in Eastern China.

A unique sequential melting mechanism for the generation of anatectic granitic rocks from the Penafiel area, northern Portugal

Granitic rocks derived by sequential partial melting are very uncommon occurrences worldwide with only three known cases in Portugal (Tourém, Guarda–Sabugal and Penafiel) and one in Argentina (Achiras complex). The suite documented in this paper comprises two of the three main phases of a batholith in the Penafiel area. These Variscan peraluminous two-mica granite and granodiorites formed at 310 and 305Ma during and near the end of the Variscan D3 deformation and have the characteristics of S-type granites. None of the metamorphic country rocks have an adequate Sr isotopic composition to be a protolith. It is argued that progressive isotopic re-equilibration of crustal material, probably a metagreywacke for G1 and G3 with also some metapelitic contribution for G2, during granulization of the lower Variscan crust explains the difference in isotopic composition between outcropping granitic rocks and metamorphic country rocks. Biotite>muscovite granite G1 and biotite>muscovite granodiorite G3 have similar (87Sr/86Sr)i and εNdt values and were derived by sequential partial melting of the same source material. This is indicated by coherent trends of the whole rock major and trace element compositions, the compositions of feldspar, biotite, muscovite and tourmaline, the presence and type of xenocrystic mineral phases and xenoliths, and whole rock oxygen isotope compositions. This mechanism was the result of post-thickening Variscan extension and mantle upwelling. Another member of the same batholith is a biotite≈muscovite granodiorite (G2), with an intermediate age of 307Ma, but with geochemical characteristics suggesting that it corresponds to a chemically distinct pulse of magma, although likely stemming from a comparable, though more metapelitic, lower crustal source.

The role of fractional crystallization in the genesis of early syn-D3, tin-mineralized Variscan two-mica granites from the Carrazeda de Ansiães area, northern Portugal

A multiphase granitic complex (G1–G6) in the Carrazeda de Ansiães area, Central Iberian Zone, was emplaced during syn-kinematic stages of the Variscan orogeny, between 329.9±0.8Ma and 318.9±1.9Ma, as determined by ID-TIMS U–Pb on zircon and monazite. The complex intruded the metasedimentary Douro Group in a NW trending antiform formed during the D1 and D3 deformation phases, but was itself affected by ductile and brittle deformation. The granites have equal or higher amounts of muscovite than biotite, and contain surmicaceous enclaves and more rarely xenoliths, “schlieren” and microgranular enclaves. They are S-type, mainly alkaline-calcic and peraluminous with ASI ranging between 1.21 and 1.45 and normative corundum of 2.63–4.77%. They are enriched in LREE relatively to HREE, with negative Eu anomalies, (87Sr/86Sr)i=0.7097–0.7159, εNd=−6.2 to −9.1, and δ18O=10.96–11.81‰. Geochemical and isotopic characteristics show that granites G1, G2, G4 and G5 correspond to distinct pulses of magma formed by partial melting of heterogeneous metasedimentary material. Geochemical fractionation trends recorded by whole rocks and minerals permit to distinguish and model two fractional crystallization series G2–G3 and G5–G6, the latter also affected by crustal assimilation during fractionation. Granites G1, G2, G3 and G6 have ≥ 20ppm Sn and G5 16ppm Sn; they are also associated with mineralized Sn- and W-bearing quartz veins. Fractional crystallization was the responsible process for the increase in Sn content in granites from the series and their micas.

Sequential melting and fractional crystallization: Granites from Guarda-Sabugal area, central Portugal

Seven distinct phases of Variscan two-mica granite are recognized in the Guarda-Sabugal area. They intruded the Cambrian schist-metagraywacke complex, crystallized in the middle crust, and are syn- to late-D3 (309.2 ± 1.8 Ma), late-D3 (304–300 Ma) and late- to post-D3 (299 ± 3 Ma; ID-TIMS ages on zircon and monazite). Two of the granites, G2 and G5, are close in age and have similar Sr, Nd and O isotope characteristics but contrasting whole rock and mineral features and formed by sequential increasing degree of partial melting of a common metasedimentary protolith. During sequential melting Ti, total Fe, Mg, Ca, Zr, Zn, Sr, Ba and REE contents and (La/Yb) N increase and Si and Rb contents decrease, plagioclase becomes richer in anorthite and biotite and muscovite richer in Ti and Mg. Each of these granites evolved subsequently by fractional crystallization of quartz, K-feldspar, plagioclase, biotite and ilmenite, defining separate series G2–G3–G7 and G5–G6 containing late Sn-bearing differentiates. Two other granites G1 and G4 represent distinct pulses of magma with individual fractionation trends for major and trace elements and distinct ( 87Sr/ 86Sr) 300, ɛNd 300 and δ 18O values.

Geochemistry of granitic aplite-pegmatite sills and petrogenetic links with granites, Guarda-Belmonte area, central Portugal

Granitic amblygonite-subtype and lepidolite-subtype, aplite-pegmatite sills intruded a biotite>muscovite granite (G1). Two other biotite>muscovite granites (G2 and G3) and a muscovite>biotite granite (G4) crop out in the area. Variation diagrams for major and trace elements of the Variscan rocks show fractionation trends for a) G1 and G4; b) G2, G3 and aplite-pegmatite sills. The two series are confirmed by the two trends defined by major elements of primary muscovite. The sills also contain Li-bearing muscovite, which has higher Mn, Li, F and paragonite contents and lower Al V1 content than primary muscovite from G2, G3 and sills. All sills have pure albite and P 2 O 5 content of K-feldspar and plagioclase increases in the series G2, G3 and sills. Beryl occurs in all sills, but lepidolite and a nearly pure petalite only occur in lepidolite-subtype sills, which are the most evolved sills. Primary topaz and amblygonite have a similar composition in all sills. Aplite-pegmatite sills contain cassiterite, which shows sequences of alternating darker and lighter zones. The former are richer in (Nb + Ta + Fe + Mn) than the latter. Manganocolumbite is common in all sills, but ferrocolumbite only appears in amblygonite-subtype sills and manganotantalite in lepidolite-subtype sills. The sills richest in Li contain reversely-zoned crystals with a homogeneous microlite core and a heterogeneous uranmicrolite rim. Least squares analysis of major elements shows that granite G3 and amblygonite-subtype and lepidolite-subtype aplite-pegmatite sills can be derived from granite G2 magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Modelling of trace elements shows good results for Sr, but magmatic fluids controlled the Rb and Ba contents of the aplite-pegmatite sills and probably also their Li, F, Sn and Ta contents and crystallization of lepidolite, cassiterite and Nb-Ta oxide mineral assemblage. Schorl from the lepidolite-subtype sills that cut granite G1 has higher Mg/(Mg + Fe) than schorl from metasomatised granite at sill walls and resulted from the mixing of magmatic fluids carrying B and some Fe with a meteoric fluid that has interacted with the host granite G1 and carried Fe and Mg. Schorl and dravite, respectively from metasomatised granite and micaschist at sill walls, were also formed from the mixing processes.

Geochemistry of granites and metasediments of the urban area of Vila Real (northern Portugal) and correlative radon risk

Radon concentration was evaluated in dwellings of the urban area of Vila Real (Northern Portugal). The area is mainly composed of Hercynian granites and Cambrian metasediments, and CR-39 passive detectors (n = 112) were used for the purpose. The results obtained in winter conditions suggest that the most productive geological unit is the Hercynian granite G1 (geometric mean of 364 Bq/m3), while Cambrian metasediments of the Douro Group show the lowest average indoor radon concentration (236 Bq/m3). The geological, geochemical and radiological data obtained suggest that the most effective control on the radon concentrations of the area is related with the uranium content of the rocks; indeed, the highest contents were observed in granite G1 (21 ppm) and the lowest in the metasediments (3 ppm). This is also confirmed by the results obtained for groundwater, where granites present the highest concentrations of dissolved radon (up to 938 Bq/l), uranium (5–18 ppb) and gross α activities (0.47–0.92 Bq/l). No important radiometric anomalies were found in relation with geological structures such as faults, veins and contacts, but a moderate increase of the uranium content can occur locally in such structures. Petrographic observations and SEM studies show that uranium is mainly contained within the rock in heavy accessory minerals (apatite, zircon, monazite, xenotime), which reduces radon emanation. Notwithstanding, due to the high U contents granites show a significant potential to induce indoor radon concentrations in dwellings in excess of the recommended value of 400 Bq/m3. Overall, we can conclude that the region of Vila Real presents a moderate to high radon risk in dwellings and groundwater.

Some aspects of Tertiary acid magmatism in NE Ireland

AbstractGeochemical data (including REE determinations) are presented for all five Mourne Mountains granites and three Northern Ireland rhyolites. These confirm (1) the extremely fractionated nature of some of the rocks (Sr and Ba < 10 ppm, Rb > 400 ppm, Eu/Eu* < 0.1, and K/Rb < 100), and (2) a major revision to the outcrops of the E. Mourne granites G1 and G2 in which much of the former is reclassified as G2. Combined petrographic and geochemical studies have also indicated that magmatic pulses were involved in the emplacement of Mourne intrusions G2 (Revised)-G5 inclusive.The N. Ireland Tertiary acid rocks exhibit general geochemical similarities to their analogues elsewhere in the British Tertiary Igneous Province (in which Sr is generally < 100 ppm and CeN/YbN generally < 8 with Eu/Eu* often < 0.6), but as a suite the Mourne granites are enriched in Rb and some other LIL elements relative to their N. Arran counterparts.The more fractionated acid magmas of NE Ireland are believed to have evolved from primitive granitic parent liquids by crystal fractionation at depth which involved major and accessory phases (including zircon and allanite). In the Mourne (and County Antrim) areas the primitive acid compositions lie at the ends of basaltic (tholeiitic) differentiation series, and in the Mourne central complex there is a complete geochemical sequence from basic rocks through intermediate members to primitive and ultimately highly evolved, subalkaline, granitic intrusions. It is concluded that the data are consistent with the Mourne granites and Northern Ireland rhyolites being essentially basaltic differentiates, although Sr isotope evidence indicates some (probably minor) crustal involvement.

The determination of selenium in sea water, silicates and marine organisms

Spectrophotometric procedures are described for the determination of selenium in sea water, silicates (especially marine sediments) and marine organisms. Coprecipitation with iron(III) hydroxide at pH 4–6 is used to concentrate selenium and to separate it from many of the commoner elements. Separation from iron and other cations is achieved by ion exchange. Selenium is determined photometrically with diaminobenzidine. Isotope dilution with selenium-75 is used to correct results for the small losses occurring during the analysis. Silicates can be decomposed without loss of selenium by means of a mixture of hydrofluoric and nitric acids. The method of Cummins et al., with sulphuric and perchloric acids in presence of molybdate ion, is highly satisfactory for the decomposition of bio-materials. For sea water, which contains ca. 0.4–0.5 <mg Se/l, a standard deviation of 0.03 μg/l was obtained. A silicate sediment and a sea weed containing ca. 1.5 μg Se/g and 0.8 μg Se/g respectively gave coefficients of variation of 8.0% and 4.7%. The U.S. Geological Survey standard granite G1 was found to contain 2.5 ± 0.1 μg Se/g.

VI.—The Geochemistry of the Rosses Granitic Ring Complex, Co. Donegal, Ireland

SynopsisThe members of the Rosses Granitic Ring Complex have been chemically analyzed. The data are presented as averages with 95 per cent confidence limits for individual elements. A limited amount of crystallization differentiation is demonstrated when the major units—the biotite microgranite sheets, the porphyry dykes, the ring granites and the grey and pink microgranite sheets—are compared with one another. This involves an increase in Si, K, a decrease in Ti, Al, Fe, Mg, Ca and P, but Na (and Mn) remain constant. The ring granites G1-G4 undergo very little differentiation and variations in their composition, except for Si, are ascribed mainly to heterogeneity.The origin of the magma which gave rise to the complex is due to the palingenesis of the Older Granodiorite under the influence of a Na, K, alumino-silicate influx. This magma was the source of the biotite microgranite sheets and, after differentiation, the granites G1 and G2, together with some of the grey and pink microgranite sheets. After a period of quiescence a second influx produced the similar magma which was the source of the porphyry dykes and, after differentiation, gave rise to the granites G3 and G4, the Newer Granite bosses and the late grey and pink microgranite sheets.

Spectrochemical determination of La and Sc in standard granite G1 by the method of addition

The determination of La and Sc in standard granite Gl was carried out by the method of addition. The confidence limits of the unknown concentration obtained with two sets of two additions were compared with the limits obtained with a single addition and were found to be of the same order of magnitude. In geological work, a single addition may often be sufficient for relatively rapid semiquantitative determinations.