Caledonian Granites Research Articles

Two stages of gold mineralization in the Hebaoshan deposit: Evidence from the U-Pb dating and trace element geochemistry of rutile, monazite and apatite

The Hebaoshan gold deposit (41.5 t Au @ 3.5 g/t) is located in the southeastern region of the South China Block, central part of the Wuyishan metallogenic belt. The ore-hosting rocks in this area are predominantly Precambrian metasedimentary rocks and Caledonian granitic rocks. Two hydrothermal mineralization stages can be distinguished: a quartz-sericite-pyrite-native gold (stage I) and a chlorite-quartz-sericite-chalcopyrite-electrum (stage II), with hydrothermal monazite and rutile are firstly identified in separate stages. The complex geological history of the region has resulted in ongoing debates regarding the age of gold mineralization and the genesis of the major gold deposits in this area. In order to precisely constrain the mineralization age of the deposit and further establish a genetic model for the ore deposit, LA-ICP-MS U-Pb dating and trace element analysis on accessory minerals were conducted. Based on the textures mineral assemblages, and geochemical features of the accessory minerals, magmatic apatite, hydrothermal rutile, and both magmatic and hydrothermal monazite were identified. The U-Pb ages of magmatic apatite and monazite are determined to be 445.3 ± 15.80, 441.3 ± 15.10 Ma and 446.57 ± 1.03 Ma, respectively, suggesting that these ages represent the emplacement ages of the Caledonian intrusive rocks. The ages of hydrothermal monazite and hydrothermal rutile are determined to be 238.46 ± 2.01 Ma (single-mineral analysis), 238.46 ± 2.01 Ma (in-suit analysis) and 179.54 ± 7.28 Ma, respectively, suggesting that represent two mineralization events during the Late Triassic to early Jurassic in the Hebaoshan area. These data provide new constraints on the mineralization process in the Hebaoshan deposit and excludes the link between gold mineralization and the intrusion of the Caledonian granites. Regionally, It is speculated that the two mineralization events at Hebaoshan are respectively associated with intracontinental orogenic movements between the Yangtze Block and Cathaysia Block, the flat-slab subduction of the Paleo-Pacific Plate (stage I), and the subsequent extensional tectonics related to the collision between the Yangtze Block and Cathaysia Block (stage II).Our study indicates that the timing of multiple episodic mineralization can be constrained by analysis of accessory minerals, which provides a geological basis for better genetic model for the deposit and provide geological evidence for unraveling the relationships between magmatic activities and mineralization events in the region.

Uranium in groundwaters: Insights from the Leinster granite, SE Ireland

Uranium concentrations in groundwaters from >100 private wells in the Caledonian Leinster Granite Batholith of SE Ireland range from a few μg/l to >300 μg/l. Approximately 10% of the wells exhibit uranium concentrations that exceed the current WHO provisional guideline of 30 μg/l and a further 10% exceed the original WHO guideline value of 15 μg/l. New whole-rock geochemical analyses and laboratory batch leaching experiments confirm the granite as the ultimate source of the groundwater uranium, but carbonate-bearing glacial deposits that overlie the granite play an important role in facilitating the generation of soluble and mobile anionic uranyl carbonate complexes. Whilst the granite has average uranium concentrations broadly similar to other Irish and British Caledonian granites, it is enriched locally in uranium, with whole-rock values up to c. 30 mg/kg. Uraniferous zircon, apatite, monazite and associations with secondary Fe-bearing oxide minerals were detected in drillcore whole-rock samples that had the highest uranium concentrations, but uraninite was not detected. Broad positive correlations between groundwater uranium concentrations and bicarbonate alkalinity and electrical conductivity are found in both the sampled groundwaters and in laboratory leachates of the granite and surficial glacial deposits. Leachates from the granite had approximately an order of magnitude higher uranium concentrations compared with those from the overlying limestone-bearing glacial tills. (234U/238U) ratios for the groundwaters and the leachates always exceed unity, indicating preferential leaching of loosely bound radiogenic 234U from mineral surfaces or grain boundaries into the groundwaters, rather than wholescale dissolution of the primary uraniferous minerals in the granite. Strontium isotopes ratios in the groundwaters constrain the balance between limestone and granite-derived Sr, and by extension the sources of dissolved Ca. These data indicate that calcite dissolution is the main source of dissolved calcium, highlighting the role of weathering of overlying limestone-bearing glacial tills during recharge in determining the geochemistry of the groundwaters. Geochemical modelling indicates that in the oxidising groundwaters of the study area U6+ is dominant, in the form of UO2(CO3)3−4 at the high pH values (7.49–7.69) of waters from wells overlain by limestone-bearing till. By contrast, in a small number of wells overlain by granite-bearing till, modelling indicates that the dominant species is UO2(CO3)2−2 reflecting their lower pH (6.22–6.30). Overall, carbonate species play a major role in uranium solubility and mobility in these granite-hosted groundwaters.

Reduce or recycle? Revealing source to sink links through integrated zircon–feldspar provenance fingerprinting

AbstractThe geochemical character of detrital mineral grains carries information that can be used to track sediment generation and transport within the broader context of basin development and crustal evolution. Many provenance studies focus on single minerals, which, as a consequence of different source fertility, survivorship and sediment sample representativeness, generate bias and consequently potentially skew geological interpretations. While a range of approaches has been proposed to either quantify or mitigate some of these biases, a significant limitation associated with the most commonly employed provenance proxy – zircon – remains that the grains are remarkably robust and capable of surviving numerous cycles of erosion, deposition and uplift. Here, same‐sample integration of zircon and feldspar provenance information is used to propose a sediment recycling metric ‘R’. This metric quantifies the degree of relative recycling of distinctive source components through comparison of provenance proxies with similar source region fertilities but different survivorship potential. This approach is applied to a case study of several thousand new zircon U‐Pb ages and K‐feldspar Pb‐isotope ratios from 25 Carboniferous to Palaeogene clastic samples from the poorly studied Wollaston Forland–Clavering Ø region in hydrocarbon prospective north‐east Greenland. Concordant detrital zircon populations are dominated by Palaeoproterozoic (ca 1.9 to 1.8 Ga) grains (>60%), with more minor Palaeozoic, Neo‐Mesoproterozoic and Archean subpopulations. Feldspar grain Pb‐isotope ratios almost entirely (>>60%) correspond to Caledonian granites (ca 0.4 Ga). Zircon and feldspar provenance proxies appear decoupled as a consequence of: (i) limited resolvable growth of zircon within Caledonian granites, and more importantly; (ii) significant upgrading of recycled zircon components via rift flanking sedimentary basins. Collectively, the integration of detrital mineral proxies with disparate survivorship potentials through sediment cycles (labile first cycle feldspar versus potentially polycyclic zircon) is essential to facilitate more nuanced definition of sediment source regions, routing pathways and basin evolution.

Silurian S-type granite-related W-(Mo) mineralization in the Nanling Range, South China: A case study of the Pingtan W-(Mo) deposit

The Pingtan deposit in southwestern Hunan Province is a newly-discovered, large-sized W-(Mo) deposit. This deposit is situated in the northwestern part of the Miao’ershan granite batholith, western Nanling Range. The tungsten mineralization is mainly hosted in biotite monzogranite, and the alteration related to mineralization is controlled by a NE-trending fault zone. In this paper, we present new molybdenite Re–Os and zircon U–Pb ages, zircon Lu–Hf isotope and REE geochemical data, aiming to constrain the W (Mo) mineralization age and ore-hosting granite petrogenesis in the Pingtan W (Mo) deposit. Molybdenite Re–Os dating of eight ore samples yielded a weighted average model age of 427.0 ± 5.4 Ma (MSWD = 0.24). The LA-ICP-MS Zircon U-Pb analysis of two biotite monzogranite yielded 206Pb/238U ages of 430.8 ± 2.4 Ma and 431.0 ± 1.8 Ma, respectively, suggesting that the W-(Mo) mineralization is spatial-temporally associated with the biotite monzogranite. The ore-related granites have high contents of SiO2 and K2O, belong to weakly to strongly peraluminous (A/CNK = 1.00–1.30), and geochemically classified as S-type affinity. Molybdenite Re contents (2.262–7.297 μg/g), zircon εHf(t) (−10.34–−5.87) and 176Hf/177Hf (0.282241–0.282376) values of the biotite monzogranite suggest that the ore-forming materials are predominantly derived from the crust. This is the first identification of tungsten mineralization related to Silurian S-type granites Caledonian granites in the Miao'ershan granite intrusion, indicating Caledonian is an important mineralization period of W in the western part of Nanling Range, South China.

How local crustal thermal properties influence the amount of denudation derived from low-temperature thermochronometry

ranslating amounts and rates of rock cooling derived from low-temperature thermochronometry into denudation requires assumptions about the local geothermal gradient. The temperature gradient in the crust depends on many factors, including basal heat flow, crustal heat production, and thermal conductivity. Consequently, geothermal gradients may be variable on time scales over which rock cooling is tracked by thermochronometry. Using one-dimensional numerical modeling of heat transfer in rocks of varying thermal characteristics, we show that the geothermal gradient of the eroded layer is the most important factor for accurate estimation of denudation amounts. Using a three-dimensional numerical model (Pecube), we demonstrate the impact of crustal heat production and thermal conductivity on estimates of total denudation derived from apatite fission track data from central west Britain. We show that the regional variation in cooling ages measured in Caledonian granites can be explained by geothermal gradient variation due to the presence of a heat-producing granite batholith and removal of insulating sedimentary rocks, and does not require variable denudation. Neglecting the blanketing effect leads to twofold overestimation of the amount of denudation. The occurrence of heat-producing basement that was once covered by a sedimentary blanket is common, in particular in the core of mountain belts. Accurate determination of the amount and rate of denudation from thermochronometric studies in these situations must take into account the composition of the eroded rocks.

Interrogating the provenance of large river systems: multi-proxy in situ analyses in the Millstone Grit, Yorkshire

Establishing the source(s) of sedimentary material is critical to many geological applications, but is complicated by the ability of some minerals to be recycled. To test the relative utility of current proxies for determining a unique provenance, new samples have been collected from the Namurian Millstone Grit Group of Yorkshire, England. Two K-feldspar 206 Pb/ 204 Pb isotope populations between 12.5 and 15.5 and c. 18.4 are consistent with Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age populations at c. 2700, 2000 – 1000 and 430 Ma reflect a mixture of Archaean basement, overlying Proterozoic sediments and intrusive Caledonian granites, and εHf values in zircons of all ages indicate crystallization from reworked crust. Garnet major element compositions are relatively rich in Fe and low in Ca, indicative of derivation from a granulitic or charnockitic source. Rutile Cr/Nb ratios indicate that source rocks were dominantly metapelitic, and Zr-in-rutile thermometry records two populations representing lower ( c. 650°C) and higher ( c. 800°C) metamorphic grade material. Combining these results with published monazite and muscovite data suggests overall derivation from the Greenland Caledonides, with additional contributions from NE Scotland and western Norway, highlighting the power of multi-proxy provenance work, especially in tectonically and geologically complicated regions. Supplementary material : Sample details, full analytical methods, data tables and references for compilation figures in the text are available at https://doi.org/10.6084/m9.figshare.c.3515457 .

A study on the Dushiling tungsten-copper deposit in the Miao’ershan-Yuechengling area, Northern Guangxi, China: Implications for variations in the mineralization of multi-aged composite granite plutons

The polymetallic Dushiling W-Cu deposit is a large, altered, skarn-type deposit, located in the northeastern part of the Miao’ershan-Yuechengling pluton, China. Two types of granite have been identified in the deposit: a medium-grained porphyritic biotite granite, and a medium- to fine-grained biotite granite. Both are spatially and temporally related to ore bodies, suggesting they may be the source of mineralization in the deposit. A medium- to fine-grained porphyritic biotite granite is exposed at the surface in the region of mineralization. U-Pb dating of zircons yielded magmatic ages of 423 Ma for the medium-grained porphyritic biotite granite and 421 Ma for the medium- to fine-grained porphyritic biotite granite, while a younger age (217 Ma) obtained for surface samples indicates later diagenesis. Thus, magmatism occurred during the Caledonian and Indosinian, respectively. The petrological and geochemical characteristics of the two Caledonian granites show that both are calc-alkaline and peraluminous. They are moderately enriched in Cs, Rb, U, and REE, and strongly depleted in Sr,Ba,P,and Ti; they show similar REE behavior, including negative Eu anomalies. These geochemical similarities suggest that the two granites were derived from the same source, although they were emplaced during different stages of the evolution of the magma. Furthermore, the granites are associated with mineralization, suggesting they were the source of mineralization in the Dushiling W-Cu deposit. Sm-Nd ages of scheelite from the Dushiling W-Cu deposit indicate that metallogenesis occurred at 417±35 Ma, while the two types of titanite, intergrown with scheelite, yield U-Pb ages of 423–425 Ma (in altered granite sample) and 218 Ma (in skarn sample). These ages place the main mineralization event in the late Caledonian, and later magmatic-hydrothermal activity occurred in the Indosinian. The ages obtained for the Dushiling W-Cu deposit in the western Nanling Range, northern Yuechengling, together with the occurrence and ages of the Niutangjie W deposit in southern Yuechengling, provide insight into the process of ore concentration during the Caledonian and Indosinian.

Tectonic controls on post-subduction granite genesis and emplacement: The late Caledonian suite of Britain and Ireland

Rates of magma emplacement commonly vary as a function of tectonic setting. The late Caledonian granites of Britain and Ireland are associated with closure of the Iapetus Ocean and were emplaced into a varying regime of transpression and transtension throughout the Silurian and into the early Devonian. Here we evaluate a new approach for examining how magma volumes vary as a function of tectonic setting. Available radiometric ages from the late Caledonian granites are used to calculate probability density functions (age spectra), with each pluton weighted by outcrop area as a proxy for its volume. These spectra confirm an absence of magmatic activity during Iapetus subduction between c. 455Ma and 425Ma and a dominance of post-subduction magmas between c. 425Ma and 380Ma. We review possible reasons why, despite the widespread outcrop of the late Caledonian granites, magmatism appears absent during Iapetus subduction. These include shallow angle subduction or extensive erosion and tectonic removal of the arc.In contrast to previous work, we find no strong difference in the age or major element chemistry of post-subduction granites across all terranes. We propose a common causal mechanism in which the down-going Iapetus oceanic slab peeled back and detached beneath the suture following final Iapetus closure. The lithospheric mantle was delaminated beneath the suture and for about 100km back beneath the Avalonian margin. While magma generation is largely a function of gravitationally driven lithosphere delamination, strike slip dominated kinematics in the overlying continental crust is what modulated granitic magma emplacement. Early Devonian (419–404Ma) transtension permitted large volumes of granite emplacement, whereas the subsequent Acadian (late Early Devonian, 404–394Ma) transpression reduced and eventually suppressed magma emplacement.

Geophysical investigations of the geology and structure of the Pangushan-Tieshanlong tungsten ore field, South Jiangxi, China—Evidence for site-selection of the 2000-m Nanling Scientific Drilling Project (SP-NLSD-2)

The Pangushan-Tieshanlong tungsten ore field, located in the center of southern Jiangxi Province, eastern Nanling region, contains many granitic plutons, both exposed and concealed, and hosts numerous tungsten-polymetallic ore deposits. Despite many previous geological studies, the deep geology and structure of the field are poorly known because few geophysical measurements have been undertaken. In this study, we carried out a variety of geophysical measurements, including gravity, magnetic field detection, magnetotelluric sounding (MT) and high-resolution seismic reflection to better outline the nature of the ore field and to aid in site selection for the 2000-m Nanling Scientific Drilling Project (SP-NLSD-2), to be carried out by the Chinese SinoProbe Program. Four granitic intrusives, the Tangcun, Pangushan, Huangsha and Bai’e plutons, crop out independently at the surface but were found to be parts of a large batholith at depth. The Tangcun composite pluton, which at the surface consists chiefly of Caledonian granite and minor Early Yanshanian intrusive material, was found to be chiefly Early Yanshanian granite concealed beneath the lower part of the Caledonian granite. The Bai’e pluton initially decreases in size with depth and then expands as it merges with the major batholith. The Pangushan and Huangsha plutons are domes on the inferred batholith. Four granitic magma channels and a considerable number of faults have been identified. Due to its favorable geological conditions, the top of the Pangushan pluton is predicted to be the best candidate for ore prospecting. It is basically a granite cupola with many cross-cutting and parallel faults. In addition, it is characterized by a high gravity anomaly, a high magnetic anomaly and locally a low electrical resistivity anomaly.

Juvenile granite in the Sanandaj–Sirjan Zone, NW Iran: Late Jurassic–Early Cretaceous arc–continent collision

The Sanandaj–Sirjan Zone (SSZ) of western Iran is characterized by numerous granitoids of mainly calc-alkaline affinities. Several leucogranite and monzonite bodies crop out in the eastern Sanandaj. Whole-rock Rb–Sr isochrons demonstrate that the Mobarak Abad monzonite (MAM) formed in two phases at 185 and 131 Ma. Low 87Sr/86Sr(i) (i represents initial) and high 143Nd/144Nd(i) ratios, resulting in positive ϵt Nd, imply that the source magma originated from a depleted mantle; large ion lithophile element (LILE) and light rare earth element (LREE) enrichments imply that slab fluid was involved in the evolution of the parent magma. Geochemical characteristics of the MAM rocks show an affinity with I- and A-type granites, and the positive values of ϵt Nd (+2 to +6), confirm that the MAM represents juvenile granite. Therefore, the MAM rocks are different from Himalayan, Hercynian, and Caledonian granites. Based on the geology of granitic host rocks that form the protoliths of metamorphic rocks, it is likely that the mafic part of the MAM formed in an island arc setting on Neo-Tethyan oceanic crust during Early to Middle Jurassic time. Subsequent collision of the island arc with the western part of the SSZ occurred in the Late Jurassic to Early Cretaceous. Metamorphism, accompanied by partial melting, occurred during collision. Finally, leucogranite magmas of the young Mobarak Abad dikes and the Suffi Abad body were generated in this collision zone. This new model suggests a Late Jurassic–Early Cretaceous arc–continental collision before final closing of the Neo-Tethys.

Geology and fluid evolution of the Wangfeng orogenic-type gold deposit, Western Tian Shan, China

The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO2–H2O inclusions (C-type), pure CO2 inclusions (PC-type), NaCl–H2O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO2–H2O fluid inclusions with salinities of 1.62 to 8.03wt.% NaCl equivalent, bulk densities of 0.73 to 0.89g/cm3, and homogenization temperatures of 256°C–390°C. Vapor bubbles are composed of CO2. The middle-stage quartz contains all four types of fluid inclusions, of which the CO2–H2O and NaCl–H2O types yield homogenization temperatures of 210°C–340°C and 230°C–300°C, respectively. The CO2–H2O fluid inclusions have salinities of 0.83 to 9.59wt.% NaCl equivalent and bulk densities of 0.77 to 0.95g/cm3, with vapor bubbles composed of CO2, CH4, and N2. Fluid inclusions in the late-stage quartz are NaCl–H2O solution with low salinities (0.35–3.87wt.% NaCl equivalent) and low homogenization temperatures (122°C–214°C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO2–H2O inclusions are 110–300MPa and 90–250MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO2. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit.

Fluid inclusions in Irish granite quartz: monitors of fluids trapped in the onshore Irish Massif

ABSTRACTFluid inclusion studies of granite quartz provide an opportunity to study fluid flow associated with igneous activity and post-emplacement fluid processes. This study presents new fluid inclusion data from the late Caledonian Donegal granites and Newry granodiorite, and the Tertiary Mourne Mountains granite in Ireland, which identify three distinct fluids. Aqueous-carbonic fluids (Type 1) have been recorded in late Caledonian granites with a significant mantle component (Newry granodiorite and the Ardara and Thorr granites in Donegal). These fluids represent late-magmatic fluids trapped at high temperatures (up to 575°C), and the ultimate source of these carbonic fluids is linked to sub-lithospheric processes during the Caledonian orogeny. The dominant fluid type (Type 2) in late Caledonian granites is a H2O+NaCl±KCl fluid which may be related to thermal convection cells around granite bodies and/or to regional scale influx of surface derived fluids at the end of the Caledonian orogeny. High salinity NaCl–CaCl2 fluids (Type 3) overprint quartz in the Ardara granite in Donegal, and in the Newry granodiorite, and are interpreted to represent basinal brines, sourced in overlying sedimentary basins, which circulated through the crystalline basement during a period of crustal extension (possibly during the Carboniferous or the Triassic). Fluid inclusion studies of the Tertiary Mourne Mountains granites have identified only Type 2 fluids related to thermal convection cells, consistent with stable isotope evidence, which indicates that this younger granite is unaffected by regional-scale fluid influxes.

SHRIMP zircon U-Pb geochronology and lithogeochemistry of Caledonian Granites from the Laojunshan area, southeastern Yunnan province, China: Implications for the collision between the Yangtze and Cathaysia blocks

At the border between China and Vietnam, a deformed and metamorphosed dome (the Laojunshan-Song Chay Dome) developed at the western prong of the “Caledonian” South China Granitic Fold Belt, north of the Red River fault zone. The main section of the dome, in China, is called the Laojunshan Metamorphic Core Complex. Systematic SHRIMP zircon U-Pb dating has been undertaken on these Caledonian Granites for the first time. The ages of the Nanlao Gneisses (DN4015B), and granites of the Tuantian (LJS-2), Laochengpo (LJS-3) and Nanjiahe (DN4058) Units, are 435.7 ± 6.4, 442.2 ± 8.3, 437.9 ± 9.5 and 418 ± 5.8 Ma respectively. These ages indicate an extended period of igneous activity (ca. 24 million years) that can be divided into two episodes (at ca. 440 and 420 Ma) representing the ages of syn-collisional and post-collisional/late-orogenic granitoid plutons respectively. Inherited zircon with 206Pb/238U ages of 710 ± 22, 1051 ± 24 and 1657 ± 48 Ma is also present in these samples. The Neoproterozoic zircon might record igneous activity related to the break-up of the Rodinia Supercontinent. Lithogeochemistry results show that most of these rocks are “fractioned” S-type granites related to the Caledonian syn-collisional orogeny. A few of them have affinities to I-type granites produced by partial melting of intermediate-mafic rocks in the upper crust. Based on the geochronology and geochemical data, the inferred provenance and tectonic setting of the granites, and research on geotectonics and regional geology, the granites are interpreted as the products of the Caledonian collision between the Cathaysia and Yangtze blocks, and the fault zones along Shizong-Mile-Luoping-Xingyi-Wangmo-Luodian are proposed to be the southwestern boundary between those two blocks.

Age and origin of the Årdal dike complex, SW Norway: False isochrons, incomplete mixing, and the origin of Caledonian granites in basement nappes

Granitic dikes intruding the upper thrust sheets of the Jotun Nappe Complex, southwest Norway, previously assigned a Sveconorwegian age based on Rb‐Sr data, are redated to 427 ± 1 Ma by U‐Pb. The erroneous Rb‐Sr age can be explained by mixing of magmas with different isotopic compositions. Similar geochemical signatures, evidence of Sr mixing, inherited Proterozoic zircons and synkinematic modes of emplacement are also seen in Silurian granites in the Lifjorden Complex as well as in the Lindås Nappe and the Totland unit of the Bergen Arc Complex. We propose that these granites are related and derived from anatexis of Rb‐poor sediments of Baltica affinity overridden by the nappes during Scandian thrusting. This interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement.

The chemical character of the Late Caledonian Donegal Granites, Ireland, with comments on their genesis

ABSTRACTThe Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470–460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407–402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all 'I' type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba–Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba–Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoffon Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite–lamprophyre suite.

Zircon U–Pb geochronology of gneissic rocks in the Yunkai massif and its implications on the Caledonian event in the South China Block

This paper presents new zircon U–Pb data and interpretations for the gneissic rocks in the Yunkai massif in order to constrain the timing and evolution of the Caledonian tectonothermal event in the South China Block (SCB). Magmatic and inherited zircons from the orthogneiss in the region, previously thought to be of Precambrian origin, yielded 206Pb/ 238U apparent ages of 421–441 Ma and 513–1343 Ma, respectively. Also a weighted mean 206Pb/ 238U age of 236.0 ± 3.1 Ma was obtained, interpreted as the metamorphic resetting age during the Indosinian tectonic event. Our analyses show that the paragneiss in the region, previously regarded as Proterozoic sedimentary rocks, contains detrital zircons of the Archaean to Paleozoic origin, of which the youngest zircons yielded the U–Pb age of ∼ 423 Ma. These data indicate that (1) the Proterozoic and Archaean components may exist beneath the Yunkai massif; (2) most of the metaigneous rocks are actually the Caledonian anatectic granites possibly overprinted by Indosinian (∼ 236 Ma) reactivation; (3) some paragneiss might have originally deposited during the Devonian time; and (4) a subsequently rapid uplifting took place after the emplacement of the Caledonian granites, revealed by the observation that the Devonian clastic strata uncomfortably overlie the Caledonian granites. In combination with other geochronological data and geological observations throughout the SCB, we propose that the Caledonian tectonothermal event around Silurian (∼ 450–400 Ma) might be a result from an intracontinental collision between the Yangtze and Cathaysian blocks in response to the subduction/collision between the North China block and SCB.

New insights into granite genesis from isotopic and REE micro-analyses of zircons: The Scottish Caledonian Granites

New insights into granite genesis from isotopic and REE micro-analyses of zircons: The Scottish Caledonian Granites

New dates (415 Ma) for the Etive Dyke Swarm and the end of the Caledonian Orogeny in the SW Grampian Highlands of Scotland

New laser stepwise heating 40 Ar– 39 Ar dates (415 ± 1.8 and 414 ± 2 Ma) from the Etive Dyke Swarm in the SW Grampian Highlands of Scotland indicate that the dykes are 10–20 Ma older than previously assumed. The dykes constitute the final phase of the Late Caledonian Granites in the region, which themselves define the end of the Caledonian Orogeny, therefore these dates can be taken as the end of the orogeny in this area. The dates show a temporal link between the Etive magmatism and early precious metal mineralization at Tyndrum as well as emphasizing the NW to SE diachronous nature of the end of Caledonian magmatism across Scotland.

Grenvillian and Caledonian tectono-magmatic activity in northeasternmost Svalbard

Abstract The Nordaustlandet Terrane of NE Svalbard forms an exposed part of the Barentsia microcontinent. Augen gneisses, migmatites, granites and gabbros dominate the scattered outcrops along the northeastern coast of Nordaustlandet, and on the smaller islands to the north and east, as far as Kvitøya. These outcrops probably represent the deepest exposed crustal levels within the folded Caledonian basement of the Nordaustlandet Terrane. In the present study, a variety of rock types have been analysed by different U-Pb dating techniques (conventional, Pb-evaporation and ion microprobe) on zircon, titanite and monazite The major and trace element compositions and Sm-Nd isotope geochemistry of these rocks have also been investigated. The augen gneisses yield U-Pb ages of c. 950 Ma, indicating that they are deformed late Grenvillian granites, similar to the Grenville-age granites and augen gneisses of northwestern and central Nordaustlandet. Migmatites, grey granites, aplitic dykes and a syenite (boulder) yield U-Pb ages mainly falling in the 430–450 Ma range, slightly older than the 410–420 Ma late-tectonic Caledonian granites further west. Both the Grenvillian and Caledonian granites are of crustal anatectic origin, and the Caledonian granites and migmatites may have formed largely by remelting of Grenvillian crust. The ages of the mafic rocks are uncertain, but Sm-Nd data indicate a possible emplacement age of c. 700 Ma for two of the gabbros, suggesting that they may be the result of rift-related magmatism in connection with the opening of the Iapetus Ocean. A few enigmatic inherited zircons of similar late Neoproterozoic age found in younger granites may possibly be related to this event. No evidence for late Neoproterozoic orogenic activity, similar to that in the Timanides of northern Russia, is seen in eastern Svalbard. At this time, eastern Svalbard (Barentsia) was probably part of the Laurentian margin, and probably located far away from northern Baltica.

A Gravity Anomaly Map of the Irish Western Seaboard

This paper describes the results of two marine gravity surveys undertaken by the Dublin Institute for Advanced Studies and the Institut ffir Geophysik, University of Hamburg. The surveys cover a large part of the continental shelf to the west of Ireland and east of the Rockall and Porcupine basins between latitudes 51.2oN and 55.2°N. The two data sets have been combined with the land data of the Dublin Institute for Advanced Studies to produce a unified gravity anomaly map of the west of Ireland and its continental shelf. The offshore extension of various onshore anomalies, notably those due to Caledonian granites and Carboniferous basins, are delineated by the new map.