Igneous Rock Associations 21. The Early Permian Panjal Traps of the Western Himalaya

The Early Permian (290 Ma) Panjal Traps are the largest contiguous outcropping of volcanic rocks associated with the Himalayan Magmatic Province (HMP). The eruptions of HMP-related lava were contemporaneous with the initial break-up of Pangea. The Panjal Traps are primarily basalt but volumetrically minor intermediate and felsic volcanic rocks also occur. The basaltic rocks range in composition from continental tholeiite to ocean-floor basalt and nearly all have experienced, to varying extent, crustal contamination. Uncontaminated basaltic rocks have Sr–Nd isotopes similar to a chondritic source (ISr = 0.7043 to 0.7073; eNd(t) = 0 ± 1), whereas the remaining basaltic rocks have a wide range of Nd (eNd(t) = –6.1 to +4.3) and Sr (ISr = 0.7051 to 0.7185) isotopic values. The calculated primary melt compositions of basalt are picritic and their mantle potential temperatures (TP ≤ 1450°C) are similar to ambient mantle rather than anomalously hot mantle. The silicic volcanic rocks were likely derived by partial melting of the crust whereas the andesitic rocks were derived by mixing between crustal and mantle melts. The Traps erupted within a continental rift setting that developed into a shallow sea. Sustained rifting created a nascent ocean basin that led to sea-floor spreading and the rifting of microcontinents from Gondwana to form the ribbon-like continent Cimmeria and the Neotethys Ocean.RÉSUMÉLes Panjal Traps du début Permien (290 Ma) constituent le plus grand affleurement contigu de roches volcaniques associées à la province magmatique de himalayienne (HMP). Les éruptions de lave de type HMP étaient contemporaines de la rupture initiale de la Pangée. Les Panjal Traps sont essentiellement des basaltes, mais on y trouve aussi des roches volcaniques intermédiaires et felsiques en quantités mineures. La composition de ces roches basaltiques varie de tholéiite continentale à basalte de plancher océanique, et presque toutes ont subi, à des degrés divers, une contamination de matériaux crustaux. Les roches basaltiques non contaminées ont des contenus isotopiques Sr–Nd similaires à une source chondritique (Isr = 0,7043 à 0,7073; eNd (t) = 0 ± 1), alors que les roches basaltiques autres montrent une large gamme de valeurs isotopiques en Nd (eNd (t) = –6,1 à +4,3) et Sr (Isr = de 0,7051 à 0,7185). Les compositions de fusion primaire calculées des basaltes sont picritiques et leurs températures potentielles mantelliques (TP de ≤ 1450°C) sont similaires à la température ambiante du manteau plutôt que celle d’un manteau anormalement chaud. Les roches volcaniques siliciques dérivent probablement de la fusion partielle de la croûte alors que les roches andésitiques proviennent du mélange entre des matériaux de fusion crustaux et mantelliques. Les Traps ont fait irruption dans un contexte de rift continental qui s’est développé dans une mer peu profonde. Un rifting soutenu a créé un début de bassin océanique lequel conduit à une expansion du fond océanique et au rifting de microcontinents tirés du Gondwana pour former le continent rubané de Cimméria et l'océan Néotéthys

Igneous Rock Associations 16. The Late Permian Emeishan Large Igneous Province

The Late Permian Emeishan large igneous province (ELIP) covers ~0.3x106 km2 of the western margin of the Yangtze Block and Tibetan plateau of SW China with displaced, correlative units in northern Vietnam (Song Da zone). The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the Late Capitanian mass extinction. The flood basalts are the signature feature of the ELIP but there are also picritic and silicic volcanic rocks and layered mafic–ultramafic and silicic plutonic rocks exposed. The ELIP is divided into three zones (i.e. inner, middle and outer) which correspond to a decrease in crustal thickness from the inner to the outer zone. The eruptive age of the ELIP is ~260 Ma and is constrained by paleomagnetic observations to an interval of ≤ 3 m.y. The presence of picritic and basaltic volcanic rocks is evidence for a high temperature regime; however, it is uncertain if these magmas were derived from sub-continental lithospheric mantle or sub-lithospheric mantle (i.e. asthenosphere or mantle plume) sources or both. The range of Sr (ISr ≈ 0.7040 to 0.7132), Nd (eNd(T) ≈ -14 to +8), Pb (206Pb/204PbPb1 ≈ 17.9 to 20.6) and Os (gOs ≈ -5 to +11) isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to source origin but it is clear that some rocks were affected by crustal contamination. However, the identification of depleted isotope compositions suggests that there is a sub-lithospheric mantle component in the system. The ELIP is considered to be a mantle plume-derived large igneous province and may have contributed to ecosystem collapse during the latest Capitanian.SOMMAIRELa grande province ignée d’Emeishan de la fin du Permien (ELIP) s’étend sur environ 0,3 x 106 km2 à la marge ouest du bloc Yangtze et du plateau tibétain du sud-ouest de la Chine, avec des unités corrélatives déplacées dans le nord du Vietnam (zone de Song Da). L’ELIP est intéressant parce qu’il renferme de nombreux gisements de métaux de base de classe mondiale et qu’il est contemporain de l’extinction de masse de la fin du Capitanien. Les basaltes de plateau sont la signature géologique de l’ELIP, bien qu’on y rencontre aussi des roches volcaniques picritiques et siliciques ainsi que des formations stratifiées de roches mafiques à ultramafiques et plutoniques acides. L’ELIP est divisé en trois zones (interne, médiane et externe) correspondant à une diminution de l’épaisseur crustale de la zone interne vers la zone externe. L’éruption de l’ELIP date d’environ 260 Ma mais les observations paléomagnétiques limitent sa durée à ≤ 3 m.a. La présence de roches volcaniques picritiques et basaltiques indique un régime à haute température mais on ne sait pas si ces magmas proviennent de sources mantelliques lithosphériques sous-continentales ou sous-continentales mantelliques (c.-à-d. asthénosphère ou panache mantellique) ou des deux. La gamme des valeurs isotopiques Sr (ISr ≈ 0,7040 à 0,7132), Nd (eNd(T) ≈ –14 à +8), Pb (206Pb/204PbPb1 ≈ 17,9 à 20,6) et Os (gOs ≈ –5 à +11) des roches ultramafiques et mafiques ne permet pas de décider de l’origine de la source mais il est clair que certaines roches ont subis de contaminations crustales. Cependant l’existence de compositions isotopiques appauvries indique la présence dans le système d’une composante mantellique sous-lithosphérique. L’ELIP est considéré comme une grande province ignée dérivée d’un panache mantellique qui pourrait bien avoir contribué à l'effondrement de l’écosystème à la toute fin du Capitanien.

The Emeishan large igneous province: A synthesis

AbstractThe late Permian Emeishan large igneous province (ELIP) covers ∼0.3 × 106 km2 of the western margin of the Yangtze Block and Tibetan Plateau with displaced, correlative units in northern Vietnam (Song Da zone). The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the late Capitanian (∼260 Ma) mass extinction. The flood basalts are the signature feature of the ELIP but there are also ultramafic and silicic volcanic rocks and layered mafic-ultramafic and silicic plutonic rocks exposed. The ELIP is divided into three nearly concentric zones (i.e. inner, middle and outer) which correspond to progressively thicker crust from the inner to the outer zone. The eruptive age of the ELIP is constrained by geological, paleomagnetic and geochronological evidence to an interval of ≤3 Ma. The presence of picritic rocks and thick piles of flood basalts testifies to high temperature thermal regime however there is uncertainty as to whether these magmas were derived from the subcontinental lithospheric mantle or sub-lithospheric mantle (i.e. asthenosphere or mantle plume) sources or both. The range of Sr (ISr ≈ 0.7040–0.7132), Nd (ɛNd(t) ≈ −14 to +8), Pb (206Pb/204Pb1 ≈ 17.9–20.6) and Os (γOs ≈ −5 to +11) isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to ultimate source origin of the primitive rocks but it is clear that some rocks were affected by crustal contamination and the presence of near-depleted isotope compositions suggests that there is a sub-lithospheric mantle component in the system. The silicic rocks are derived by basaltic magmas/rocks through fractional crystallization or partial melting, crustal melting or by interactions between mafic and crustal melts. The formation of the Fe-Ti-V oxide-ore deposits is probably due to a combination of fractional crystallization of Ti-rich basalt and fluxing of CO2-rich fluids whereas the Ni-Cu-(PGE) deposits are related to crystallization and crustal contamination of mafic or ultramafic magmas with subsequent segregation of a sulphide-rich portion. The ELIP is considered to be a mantle plume-derived LIP however the primary evidence for such a model is less convincing (e.g. uplift and geochemistry) and is far more complicated than previously suggested but is likely to be derived from a relatively short-lived, plume-like upwelling of mantle-derived magmas. The emplacement of the ELIP may have adversely affected the short-term environmental conditions and contributed to the decline in biota during the late Capitanian

Volcanic and Tectonic Constraints on the Evolution of Venus

Surface geologic features form a detailed record of Venus’ evolution. Venus displays a profusion of volcanic and tectonics features, including both familiar and exotic forms. One challenge to assessing the role of these features in Venus’ evolution is that there are too few impact craters to permit age dates for specific features or regions. Similarly, without surface water, erosion is limited and cannot be used to evaluate age. These same observations indicate Venus has, on average, a very young surface (150–1000 Ma), with the most recent surface deformation and volcanism largely preserved on the surface except where covered by limited impact ejecta. In contrast, most geologic activity on Mars, the Moon, and Mercury occurred in the 1st billion years. Earth’s geologic processes are almost all a result of plate tectonics. Venus’ lacks such a network of connected, large scale plates, leaving the nature of Venus’ dominant geodynamic process up for debate. In this review article, we describe Venus’ key volcanic and tectonic features, models for their origin, and possible links to evolution. We also present current knowledge of the composition and thickness of the crust, lithospheric thickness, and heat flow given their critical role in shaping surface geology and interior evolution. Given Venus’ hot lithosphere, abundant activity and potential analogues of continents, roll-back subduction, and microplates, it may provide insights into early Earth, prior to the onset of true plate tectonics. We explore similarities and differences between Venus and the Proterozoic or Archean Earth. Finally, we describe the future measurements needed to advance our understanding of volcanism, tectonism, and the evolution of Venus

Derivation of intermediate to silicic magma from the basalt analyzed at the Vega 2 landing site, Venus.

Geochemical modeling using the basalt composition analyzed at the Vega 2 landing site indicates that intermediate to silicic liquids can be generated by fractional crystallization and equilibrium partial melting. Fractional crystallization modeling using variable pressures (0.01 GPa to 0.5 GPa) and relative oxidation states (FMQ 0 and FMQ -1) of either a wet (H2O = 0.5 wt%) or dry (H2O = 0 wt%) parental magma can yield silicic (SiO2 > 60 wt%) compositions that are similar to terrestrial ferroan rhyolite. Hydrous (H2O = 0.5 wt%) partial melting can yield intermediate (trachyandesite to andesite) to silicic (trachydacite) compositions at all pressures but requires relatively high temperatures (≥ 950°C) to generate the initial melt at intermediate to low pressure whereas at high pressure (0.5 GPa) the first melts will be generated at much lower temperatures (< 800°C). Anhydrous partial melt modeling yielded mafic (basaltic andesite) and alkaline compositions (trachybasalt) but the temperature required to produce the first liquid is very high (≥ 1130°C). Consequently, anhydrous partial melting is an unlikely process to generate derivative liquids. The modeling results indicate that, under certain conditions, the Vega 2 composition can generate silicic liquids that produce granitic and rhyolitic rocks. The implication is that silicic igneous rocks may form a small but important component of the northeast Aphrodite Terra

Igneous Rock Associations 28. Construction of a Venusian Greenstone Belt: A Petrological Perspective

The crustal evolution of Venus appears to be principally driven by intraplate processes that may be related to mantle upwelling as there is no physiographic (i.e. mid-ocean ridge, volcanic arc) evidence of Earth-like plate tectonics. Rocks with basaltic composition were identified at the Venera 9, 10, 13, and 14, and Vega 1 and 2 landing sites whereas the rock encountered at the Venera 8 landing site may be silicic. The Venera 14 rock is chemically indistinguishable from terrestrial olivine tholeiite but bears a strong resemblance to basalt from terrestrial Archean greenstone belts. Forward petrological modeling (i.e. fractional crystallization and partial melting) and primary melt composition calculations using the rock compositions of Venus can yield results indistinguishable from many volcanic (ultramafic, intermediate, silicic) and plutonic (tonalite, trondhjemite, granodiorite, anorthosite) rocks that typify Archean greenstone belts. Evidence of chemically precipitated (carbonate, evaporite, chert, banded-iron formation) and clastic (sandstone, shale) sedimentary rocks is scarce to absent, but their existence is dependent upon an ancient Venusian hydrosphere. Nevertheless, it appears that the volcanic–volcaniclastic–plutonic portion of terrestrial greenstone belts can be constructed from the known surface compositions of Venusian rocks and suggests that it is possible that Venus and Early Earth had parallel evolutionary tracks in the growth of proto-continental crust.L'évolution de la croûte de Vénus semble être principalement déterminée par des processus intraplaques qui peuvent être liés à des remontées mantelliques, car il n'y a aucune preuve physiographique d'une tectonique des plaques semblable à la Terre (c.-à-d. dorsale médio-océanique, arc volcanique). Des roches de composition basaltique ont été identifiées sur les sites d'atterrissage de Venera 9, 10, 13 et 14 et Vega 1 et 2 tandis que la roche rencontrée sur le site d'atterrissage de Venera 8 peut être silicique. La roche du site de Venera 14 est indiscernable de la tholéiite à olivine terrestre de par ses propriétés chimiques, mais ressemble fortement au basalte des ceintures de roches vertes archéennes terrestres. La modélisation pétrologique prospective (c.-à-d. cristallisation fractionnaire et fusion partielle) et les calculs de la composition de fusion primaire à partir des compositions des roches de Vénus peuvent donner des résultats indiscernables de nombreuses roches volcaniques (ultramafiques, intermédiaires, siliciques) et plutoniques (tonalite, trondhjemite, granodiorite, anorthosite) qui caractérisent les ceintures de roches vertes archéennes. Les preuves de roches sédimentaires précipitées chimiquement (carbonate, évaporite, chert, formation de fer rubané) et clastiques (grès, schiste) sont rares ou absentes, mais leur existence dépend d'une ancienne hydrosphère vénusienne. Néanmoins, il semble que la partie volcanique-volcanoclastique-plutonique des ceintures de roches vertes puisse être construite à partir des compositions de surface connues des roches vénusiennes et suggère qu'il est possible que Vénus et la Terre primitive aient eu des trajectoires évolutives parallèles de croissance de la croûte proto-continentale

Igneous Rock Associations 28. Construction of a Venusian Greenstone Belt: A Petrological Perspective

The crustal evolution of Venus appears to be principally driven by intraplate processes that may be related to mantle upwelling as there is no physiographic (i.e. mid-ocean ridge, volcanic arc) evidence of Earth-like plate tectonics. Rocks with basaltic composition were identified at the Venera 9, 10, 13, and 14, and Vega 1 and 2 landing sites whereas the rock encountered at the Venera 8 landing site may be silicic. The Venera 14 rock is chemically indistinguishable from terrestrial olivine tholeiite but bears a strong resemblance to basalt from terrestrial Archean greenstone belts. Forward petrological modeling (i.e. fractional crystallization and partial melting) and primary melt composition calculations using the rock compositions of Venus can yield results indistinguishable from many volcanic (ultramafic, intermediate, silicic) and plutonic (tonalite, trondhjemite, granodiorite, anorthosite) rocks that typify Archean greenstone belts. Evidence of chemically precipitated (carbonate, evaporite, chert, banded-iron formation) and clastic (sandstone, shale) sedimentary rocks is scarce to absent, but their existence is dependent upon an ancient Venusian hydrosphere. Nevertheless, it appears that the volcanic–volcaniclastic–plutonic portion of terrestrial greenstone belts can be constructed from the known surface compositions of Venusian rocks and suggests that it is possible that Venus and Early Earth had parallel evolutionary tracks in the growth of proto-continental crust.L'évolution de la croûte de Vénus semble être principalement déterminée par des processus intraplaques qui peuvent être liés à des remontées mantelliques, car il n'y a aucune preuve physiographique d'une tectonique des plaques semblable à la Terre (c.-à-d. dorsale médio-océanique, arc volcanique). Des roches de composition basaltique ont été identifiées sur les sites d'atterrissage de Venera 9, 10, 13 et 14 et Vega 1 et 2 tandis que la roche rencontrée sur le site d'atterrissage de Venera 8 peut être silicique. La roche du site de Venera 14 est indiscernable de la tholéiite à olivine terrestre de par ses propriétés chimiques, mais ressemble fortement au basalte des ceintures de roches vertes archéennes terrestres. La modélisation pétrologique prospective (c.-à-d. cristallisation fractionnaire et fusion partielle) et les calculs de la composition de fusion primaire à partir des compositions des roches de Vénus peuvent donner des résultats indiscernables de nombreuses roches volcaniques (ultramafiques, intermédiaires, siliciques) et plutoniques (tonalite, trondhjemite, granodiorite, anorthosite) qui caractérisent les ceintures de roches vertes archéennes. Les preuves de roches sédimentaires précipitées chimiquement (carbonate, évaporite, chert, formation de fer rubané) et clastiques (grès, schiste) sont rares ou absentes, mais leur existence dépend d'une ancienne hydrosphère vénusienne. Néanmoins, il semble que la partie volcanique-volcanoclastique-plutonique des ceintures de roches vertes puisse être construite à partir des compositions de surface connues des roches vénusiennes et suggère qu'il est possible que Vénus et la Terre primitive aient eu des trajectoires évolutives parallèles de croissance de la croûte proto-continentale

Haida Gwaii (British Columbia, Canada): a Phanerozoic analogue of a subduction-unrelated Archean greenstone belt

Published versionUnderstanding the formation and evolution of Precambrian greenstone belts is hampered by gaps in the rock record and the uncertainty of the tectonic regime that was operating at the time. Thus identifying a modern analogue of a Precambrian greenstone belt can be problematic. In this paper we present geological, geochemical and petrological evidence outlining the case for Haida Gwaii (British Columbia, Canada) as a modern example of a greenstone belt. Haida Gwaii is comprised of two rift-related volcano-sedimentary sequences. The older (Early Triassic) Karmutsen volcanic sequence consists of subaqueous ultramafic-mafic volcanic rocks that are capped by marine carbonate and siliciclastic rocks. The younger (Paleogene) Masset bimodal volcanic sequence consists of tholeiitic and calc-alkaline basalt along with calc-alkaline silicic volcanic and intrusive rocks that are capped by epiclastic sandstones. The Karmutsen and Masset volcanic rocks have indistinguishable Sr-Nd-Pb isotopes demonstrating they were derived from a similar mantle source. Some of the Masset calc-alkaline rocks are compositionally similar to magnesian andesites (SiO2 = 56–64 wt%; Mg# = 0.50–0.64) that are typical of subduction-related Archean greenstone belts. We show that the calc-alkaline signature observed in the bimodal sequence of the Masset Formation is likely due to fractional crystallization of a tholeiitic parental magma under relatively oxidizing (ΔFMQ + 0.7) conditions indicating that a calc-alkaline signature is not prima facie evidence of a subduction setting. Given the geological and geochemical evidence, Haida Gwaii represents one of the best analogues of a modern subduction-unrelated Archean greenstone belt