Regional and tectonic implications of parallel Caledonian and Permo-Carboniferous lamprophyre dyke swarms from Lismore, Ardgour

1987 ◽  
Vol 77 (4) ◽  
pp. 279-288 ◽  
Author(s):  
M. A. Morrison ◽  
G. L. Hendry ◽  
P. T. Leat
Keyword(s):  
Alkali Basalt ◽  
Geochemical Data ◽  
Regional Models ◽  
Calc Alkaline ◽  
Secondary Alteration ◽  
Early Devonian ◽  
Tectonic Implications ◽  
Dyke Swarms

ABSTRACTGeochemical data are presented for 166 minor intrusions collected across the axis of the Ardgour swarm in the Isle of Lismore. The intrusions can be divided into: an alkali basalt-camptonite-monchiquite group resembling other Scottish Permo-Carboniferous dykes; a group of calc-alkaline (shoshonitic) lamprophyres, diorites and porphyrites with affinities to the late Silurian-early Devonian appinite suite of Scotland; and Tertiary dolerites. The different groups cannot be unambiguously distinguished in the field and secondary alteration precludes petrographic division in many cases. The data indicate that Caledonian and Permo-Carboniferous lamprophyres have probably been confused in previous accounts of dyke distributions in the region. In Lismore the two groups have identical azimuths but the Caledonian intrusions appear to have a greater aggregate volume. The implications for tectonic and regional models of the area are discussed.

Early Cretaceous crust–mantle interaction linked to rollback of the Palaeo-Pacific flat-subducting slab: constraints from the intermediate–felsic volcanic rocks of the northern Great Xing’an Range, NE China

2021 ◽  
pp. 1-22
Author(s):  
Jia-Hao Jing ◽  
Hao Yang ◽  
Wen-Chun Ge ◽  
Yu Dong ◽  
Zheng Ji ◽  
...  
Keyword(s):  
Early Cretaceous ◽  
Volcanic Rocks ◽  
Geochemical Data ◽  
Calc Alkaline ◽  
Ne China ◽  
Asthenospheric Mantle ◽  
High K ◽  
Wide Range ◽  
Great Xing’An Range ◽  
Subducting Slab

Abstract Late Mesozoic igneous rocks are important for deciphering the Mesozoic tectonic setting of NE China. In this paper, we present whole-rock geochemical data, zircon U–Pb ages and Lu–Hf isotope data for Early Cretaceous volcanic rocks from the Tulihe area of the northern Great Xing’an Range (GXR), with the aim of evaluating the petrogenesis and genetic relationships of these rocks, inferring crust–mantle interactions and better constraining extension-related geodynamic processes in the GXR. Zircon U–Pb ages indicate that the rhyolites and trachytic volcanic rocks formed during late Early Cretaceous time (c. 130–126 Ma). Geochemically, the highly fractionated I-type rhyolites exhibit high-K calc-alkaline, metaluminous to weakly peraluminous characteristics. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depleted in high-field-strength elements (HFSEs), with their magmatic zircons ϵHf(t) values ranging from +4.1 to +9.0. These features suggest that the rhyolites were derived from the partial melting of a dominantly juvenile, K-rich basaltic lower crust. The trachytic volcanic rocks are high-K calc-alkaline series and exhibit metaluminous characteristics. They have a wide range of zircon ϵHf(t) values (−17.8 to +12.9), indicating that these trachytic volcanic rocks originated from a dominantly lithospheric-mantle source with the involvement of asthenospheric mantle materials, and subsequently underwent extensive assimilation and fractional crystallization processes. Combining our results and the spatiotemporal migration of the late Early Cretaceous magmatic events, we propose that intense Early Cretaceous crust–mantle interaction took place within the northern GXR, and possibly the whole of NE China, and that it was related to the upwelling of asthenospheric mantle induced by rollback of the Palaeo-Pacific flat-subducting slab.

scholarly journals Geochemical features and formation conditions of Early-Devonian cherty-argillaceous shales and the underlying basalts in the Ishkildino section (eastern slope of the Southern Urals)

LITOSFERA ◽  
2019 ◽  
pp. 30-47
Author(s):  
A. M. Fazliakhmetov
Keyword(s):  
Volcanic Rocks ◽  
Southern Urals ◽  
Geochemical Data ◽  
Upper Ordovician ◽  
The Southern Urals ◽  
Geochemical Composition ◽  
Early Devonian ◽  
Ocean Ridges ◽  
Clay Shales ◽  
Conodont Zone

Research subject.The West Magnitogorsk zone of the Southern Urals in the vicinity of the Ishkildino village features a subaerially exposed basaltic sequence superposed by cherts and siliceous-clay shales. The basalts and the overlying shales are assumed to have formed during the Ordovician and Silurian (?)–Early Devonian (up to the conodont zone excavates inclusive) periods, respectively. The aim of this research was to reconstruct, using geochemical data, the conditions under which the rocks present in this geological location were formed.Materials and methods. Five samples of the basalts (XRD and ICP-MS methods), 27 samples of the siliceous-clay shales and 10 samples of the cherts (XRD and ICP-AES methods) were analyzed.Results.According to the ratio of SiO2, Na2O and K2O, the volcanic rocks from the lower part of the section are represented by basalts and trachybasalts. Their geochemical composition corresponds to the N-MORB and is established to be similar to that of the basalts in the Polyakovskaya formation (the Middle–Upper Ordovician). In terms of main elements, the shales under study consist of quartz and illite with a slight admixture of organic matter, goethite, quartzfeldspar fragments, etc. The degree of the sedimentary material weathering according to the CIA, CIW and ICV index values is shown to be moderate. The values of Strakhov’s and Boström’s moduli correspond to sediments without the admixture of underwater hydrothermal vent products. The values of Cr/Al, V/Al and Zr/Al correspond to those characteristic of deposits in deep-water zones remote from the coasts of passive and active continental margins, basalt islands and areas adjacent to mid-ocean ridges. For most samples, the values of Ni/Co, V/Cr, Mo/Mn are typical of deposits formed under oxidative conditions. However, several samples from the upper part of the section, which is comparable to the kitabicus and excavatus conodont zones, demonstrate the Ni/Co, V/Cr, and Mo/Mn values corresponding to deposits formed under reducing atmospheres. An assumption is made that the existence of these deposits can be associated with the Bazal Zlichov event.Conclusion.The investigated pre-Emsian shales have shown no signs of volcanic activity in the adjacent areas. The studied deposits are established to correspond to the central part of the Ural Paleoocean.

Exemples de mélange de magmas en contexte plutonique: les enclaves des tonalites–granodiorites du massif de Bono (Sardaigne septentrionale)

1989 ◽  
Vol 26 (6) ◽  
pp. 1264-1281 ◽  
Author(s):  
C. Cocirta ◽  
J. B. Orsini ◽  
C. Coulon
Keyword(s):  
Geochemical Data ◽  
Mixing Process ◽  
Calc Alkaline ◽  
Specific Group ◽  
Mineralogical Analysis ◽  
Magmatic Origin ◽  
Interaction Mechanisms ◽  
Compositional Variations ◽  
End Members ◽  
Host Rocks

In calc-alkaline orogenic plutons, the dark xenoliths and their host rocks must be considered the expression of partial mixing of magma.Three associations of this type have been investigated and are illustrated by the Bono pluton (northern Sardinia)— a composite pluton including three intrusives of different nature (tonalitic to granodioritic) and containing a very large number of basaltic xenoliths of magmatic origin. Detailed mineralogical analysis of the two end members in each association, coupled with geochemical data, has determined the major petrogenetic mechanisms intervening in the mixing process in a plutonic setting: temperature equilibration, mechanical exchanges of crystals, chemical exchanges, etc. The most important result of this article, however, is to show that each intrusion is related to a specific group of xenoliths that is characterized by constant FeOt/MgO. The latter reflects the different composition of basaltic components, and it is concluded that each intrusive event is associated with a unique mixing episode. As in volcanic settings, the mixing process may have initiated the intrusion.The extreme compositional variations in the magmatic xenoliths, recognized in several series of orogenic plutons, is explained here by different initial basaltic end members and by variation in the intensity of the interaction mechanisms. [Journal Translation]

Release of Uranium from Uraninite in Granites Through Alteration: Implications for the Source of Granite-Related Uranium Ores

2021 ◽  
Author(s):  
Long Zhang ◽  
Zhenyu Chen ◽  
Fangyue Wang ◽  
Noel C. White ◽  
Taofa Zhou
Keyword(s):  
Uranium Concentration ◽  
Geochemical Data ◽  
Uranium Deposits ◽  
Bulk Rock ◽  
Calc Alkaline ◽  
Alkaline Granite ◽  
Compositional Evolution ◽  
High K ◽  
Backscattered Electron ◽  
Uranium Sources

Abstract Uraninite is the main contributor to the bulk-rock uranium concentration in many U-rich granites and is the most important uranium source for granite-related uranium deposits. However, detailed textural and compositional evolution of magmatic uraninite in granites during alteration and associated uranium mobilization have not been well documented. In this study, textures and geochemistry of uraninites from the Zhuguangshan batholith (South China) were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The geochemical data indicate that the Longhuashan and Youdong plutons are peraluminous leucogranite, the Changjiang pluton is highly fractionated high-K calc-alkaline granite, and the Jiufeng pluton belongs to a high-K calc-alkaline association. Uraninites from the Longhuashan and Youdong granites have lower concentrations of ThO2 (0.9–4.0 wt %) and rare earth elements (REE)2O3 (0.1–1.0 wt %) than those from the Changjiang and Jiufeng granites (ThO2 = 4.4–7.6 wt %, REE2O3 = 0.7–5.1 wt %). Uraninites observed in the Longhuashan, Youdong, Changjiang, and Jiufeng granites yielded chemical ages of 223 ± 3, 222 ± 2, 157 ± 1, and 161 ± 2 Ma, respectively. The samples (including altered and unaltered) collected from the Longhuashan, Youdong, and Changjiang granites are characterized by highly variable whole-rock U concentrations of 6.9 to 44.7 ppm and Th/U ratios of 0.9 to 7.0, consistent with crystallization of uraninite in these granites being followed by uranium leaching during alteration. Alteration of uraninite, indicated by altered domains developing microcracks and appearing darker in backscattered electron (BSE) images compared to unaltered domains, results in the incorporation of Si and Ca and mobilization of U. In contrast, the least altered samples of the unmineralized Jiufeng granite have low U concentrations (5.3–16.4 ppm) and high ΣREE/U (13.6–49.4) and Th/U ratios (2.1–5.6), which inhibit crystallization of uraninite, as its crystallization occurs when the U concentration is high enough to exceed the substitution capacity of other U-bearing minerals. These results indicate that the Longhuashan, Youdong, and Changjiang granites were favorable uranium sources for the formation of uranium deposits in this area. This study highlights the potential of uraninite alteration and geochemistry to assist in deciphering uranium sources and enrichment processes of granite-related uranium deposits.

scholarly journals A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U–Pb geochronology of Permian magmatism

2020 ◽  
Vol 113 (1) ◽  
Author(s):  
Michel Ballèvre ◽  
Audrey Camonin ◽  
Paola Manzotti ◽  
Marc Poujol
Keyword(s):  
Early Stage ◽  
Western Alps ◽  
Geochemical Data ◽  
Alkaline Magmatism ◽  
Low Grade ◽  
Calc Alkaline ◽  
Alpine Orogeny ◽  
History Of ◽  
External Part ◽  
The One

Abstract The Briançonnais Domain (Western Alps) represented the thinned continental margin facing the Piemonte-Liguria Ocean, later shortened during the Alpine orogeny. In the external part of the External Briançonnais Domain (Zone Houillère), the Palaeozoic basement displays microdioritic intrusions into Carboniferous sediments and andesitic volcanics resting on top of the Carboniferous sediments. These magmatic rocks are analysed at two well-known localities (Guil volcanics and Combarine sill). Geochemical data show that the two occurrences belong to the same calc-alkaline association. LA-ICP-MS U–Pb ages have been obtained for the Guil volcanics (zircon: 291.3 ± 2.0 Ma and apatite: 287.5 ± 2.6 Ma), and the Combarine sill (zircon: 295.9 ± 2.6 Ma and apatite: 288.0 ± 4.5 Ma). These ages show that the calc-alkaline magmatism is of Early Permian age. During Alpine orogeny, a low-grade metamorphism, best recorded by lawsonite-bearing veins in the Guil andesites, took place at about 0.4 GPa, 350 °C in the External Briançonnais and Alpine metamorphism was not able to reset the U–Pb system in apatite. The Late Palaeozoic history of the Zone Houillère is identical to the one recorded in the Pinerolo Unit, located further East in the Dora-Maira Massif, and having experienced a garnet-blueschist metamorphism during the Alpine orogeny. The comparison of these two units allows for a better understanding of the link between the Palaeozoic basements, mostly subducted during the Alpine convergence, and their Mesozoic covers, generally detached at an early stage of the convergence history.

scholarly journals Widespread Permian granite magmatism in Lower Austroalpine units: significance for Permian rifting in the Eastern Alps

2020 ◽  
Vol 113 (1) ◽  
Author(s):  
Sihua Yuan ◽  
Franz Neubauer ◽  
Yongjiang Liu ◽  
Johann Genser ◽  
Boran Liu ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
Eastern Alps ◽  
Geochemical Data ◽  
Calc Alkaline ◽  
Three Samples ◽  
High K ◽  
Icp Ms ◽  
Granite Magmatism ◽  
Austroalpine Unit ◽  
Granite Suite

Abstract The Grobgneis complex, located in the eastern Austroalpine unit of the Eastern Alps, exposes large volumes of pre-Alpine porphyric metagranites, sometimes associated with small gabbroic bodies. To better understand tectonic setting of the metagranites, we carried out detailed geochronological and geochemical investigations on the major part of the porphyric metagranites. LA–ICP–MS zircon U–Pb dating of three metagranites sampled from the Grobgneis complex provides the first reliable evidence for large volumes of Permian plutonism within the pre-Alpine basement of the Lower Austroalpine units. Concordant zircons from three samples yield ages at 272.2 ± 1.2 Ma, 268.6 ± 2.3 Ma and 267.6 ± 2.9 Ma interpreted to date the emplacement of the granite suite. In combination with published ages for other Permian Alpine magmatic bodies, the new U–Pb ages provide evidence of a temporally restricted period of plutonism (“Grobgneis”) in the Raabalpen basement Complex during the Middle Permian. Comparing the investigated basement with that of the West Carpathian basement, we argue that widespread Permian granite magmatism occurred in the Lower Austroalpine units. They belong to the high-K calc-alkaline to shoshonitic S-type series on the base of geochemical data. Zircon Hf isotopic compositions of the Grobgneis metagranites show εHf(t) values of − 4.37 to − 0.6, with TDM2 model ages of 1.31–1.55 Ga, indicating that their protoliths were derived by the recycling of older continental crust. We suggest that the Permian granitic and gabbroic rocks are considered as rifted-related rocks in the Lower Austroalpine units and are contemporaneous with cover sediments.

Geochemistry, zircon geochronology, and isotopic systematics of the Zhanbuzhale granites in the East Kunlun, Qinghai Province, northwestern China: implications for the tectonic setting

2020 ◽  
Vol 57 (2) ◽  
pp. 275-291
Author(s):  
Hao-Ran Li ◽  
Ye Qian ◽  
Feng-Yue Sun ◽  
Liang Li
Keyword(s):  
Tectonic Setting ◽  
Alkali Feldspar ◽  
Northwestern China ◽  
Geochemical Data ◽  
Calc Alkaline ◽  
Qinghai Province ◽  
East Kunlun ◽  
Initial Stage ◽  
High K ◽  
Tethys Ocean

The Zhanbuzhale region, in the Eastern Kunlun Orogen of northwestern China, is characterized by large volumes of Phanerozoic granitoid rocks and is an ideal region for investigating the tectonic evolution of the Paleo-Tethys system. However, the exact timing of the final closure of the Paleo-Tethys Ocean and initial continental collision remains controversial because of a lack of precise geochronological and detailed geochemical data. In this paper, we report new zircon U–Pb ages and mineralogical, petrographic, and geochemical data for samples of Middle Triassic granodiorite and alkali feldspar granite from the Zhanbuzhale region. The zircon U–Pb ages indicate that the granodiorite and alkali feldspar granite formed at 239 and 236 Ma, respectively. The granodiorites are high-K calc-alkaline, metaluminous, high Sr content, high Sr/Y ratios, low Y content, and show adakite-like affinities. The alkali feldspar granites display high SiO2, extremely low MgO, and low Zr+Nb+Ce+Y contents as well as low Fe2O3t/MgO ratios, showing metaluminous to peraluminous and high-K calc-alkaline features. Geochemical and petrological characteristics of the alkali feldspar granites suggest that they are highly fractionated I-type granites. The granodiorites and alkali feldspar granites have zircon εHf(t) values ranging from –2.26 to –0.18, and from –2.17 to +2.18, respectively. Together with regional geological data, we propose that the Triassic (approximately 239–236 Ma) granitoids were generated during the later stages of northward subduction of the Paleo-Tethys oceanic plate, and that the initial stage of collision between the East Kunlun and the Bayan Har–Songpan Ganzi terrane occurred at approximately 236–227 Ma.

Occurrence and significance of Silurian K-bentonite beds at Arisaig, Nova Scotia, eastern Canada

1997 ◽  
Vol 34 (12) ◽  
pp. 1630-1643 ◽  
Author(s):  
Stig M. Bergström ◽  
Warren D. Huff ◽  
Dennis R. Kolata ◽  
Michael J. Melchin
Keyword(s):  
Nova Scotia ◽  
North America ◽  
Geochemical Data ◽  
Northern Coast ◽  
Calc Alkaline ◽  
The South ◽  
Eastern Canada ◽  
Critical Information ◽  
Plate Margin ◽  
Alkaline Magmas

The most extensive succession of K-bentonite beds known in the Silurian of North America occurs at Arisaig on the northern coast of Nova Scotia. At least 40 ash beds are present in the Llandoverian Ross Brook Formation and at least four in the early Ludlovian McAdam Brook Formation. Most of the beds are thin (< 5 cm), but one bed (the Smith Brook K-bentonite bed) in the late Llandoverian crenulata Zone and another (the McAdam Brook K-bentonite bed) in the early Ludlovian nilssoni Zone each reach a thickness of 20 cm. New graptolite collections provide critical information on the biostratigraphic position of the K-bentonite beds in the Ross Brook Formation. Geochemical data show that the Arisaig ash beds represent calc-alkaline magmas from plate margin, subduction-related volcanic vents. Differences in K-bentonite stratigraphic distribution, combined with paleogeographic considerations, suggest that the volcanoes were located much farther to the south in the Iapetus than the source volcanoes of the British–Baltoscandian Llandoverian K-bentonites.

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