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

Longevity of the Permian Emeishan mantle plume (SW China): 1 Ma, 8 Ma or 18 Ma?

After the formation of the ∼ 260 Ma Emeishan large igneous province, there were two volumetrically minor magmatic pulses at ∼ 252 Ma and ∼ 242 Ma, respectively. Alkaline mafic dykes intruding both 260 Ma and 252 Ma felsic plutons in the Panxi region, southwestern China, have compositions similar to the Emeishan flood basalts. One dyke is dated using the SHRIMP zircon U-Pb technique at 242 ± 2 Ma, ∼ 18 Ma younger than the start of Emeishan magmatism. The dykes have enriched light rare earth element patterns (La/YbN = 4.4-18.8) and trace element patterns similar to the Emeishan flood basalts and average ocean-island basalts. Some trace element ratios of the dykes (Zr/Nb = 3.8-8.2, La/Nb = 0.4-1.7, Ba/La = 7.5-25.6) are somewhat similar to EM1 source material, however, there are differences. Their εNd values (εNd = +2.6 and +2.7) and ISr( ISr = 0.704542 and 0.704554) ratios are indicative of a mantle source. Thus Emeishan magmatism may have lasted for almost 20 Ma after the initial eruption. However, geological evidence precludes the possibility that the post-260 Ma magmatic events were directly related to Emeishan magmatism, which began at and ended shortly after 260 Ma. The 252 Ma plutons and 242 Ma dykes represent volumetrically minor melting of the fossil Emeishan plume-head beneath the Yangtze crust. The 252 Ma magmatic event was likely caused by post-flood basalt extension of the Yangtze crust, whereas the 242 Ma event was caused by decompressional melting associated with the collision between the South China and North China blocks during the Middle Triassic. © 2008 Cambridge University Press.published_or_final_versio

Evidence of silicate immiscibility within flood basalts from the Central Atlantic Magmatic Province

Publisher's Version/PDFThe role silicate-liquid immiscibility plays in the formation of macro-scale, bimodal volcanic/plutonic igneous complexes, and Fe-Ti oxide deposits is debated as the rock compositions produced by immiscibility are similar to those produced by other petrological processes. Within the flows of the North Mountain basalt of the Central Atlantic Magmatic Province are centimeter-thick granophyre layers. The granophyre layers are a mixture of mafic (i.e., ilmenite, magnetite, ferroaugite, plagioclase, stilpnomelane, ferrorichterite) and felsic (i.e., sanidine, quartz) minerals and highly siliceous (>75 wt% SiO[subscript 2]) mesostases. Petrological modeling indicates that the siliceous mesostasis + sanidine + quartz [plus or minus] ferrorichterite represents a Si-rich silicate immiscible melt whereas the ferroaugite + plagioclase + stilpnomelane represent the Fe-rich silicate immiscible liquid. The identification of naturally occurring silicate-liquid immiscibility at scales greater than micron level is an important observation which may be useful in identifying volcanic and plutonic rocks which formed by macro-scale silicate-liquid immiscibility.The role silicate-liquid immiscibility plays in the formation of macro-scale, bimodal volcanic/plutonicigneous complexes, and Fe-Ti oxide deposits is debated as the rock compositions produced by immiscibilityare similar to those produced by other petrological processes. Within the flows of the North Mountain basaltof the Central Atlantic Magmatic Province are centimeter-thick granophyre layers. The granophyre layersare a mixture of mafic (i.e., ilmenite, magnetite, ferroaugite, plagioclase, stilpnomelane, ferrorichterite) andfelsic (i.e., sanidine, quartz) minerals and highly siliceous (&gt;75 wt% SiO2) mesostases. Petrologicalmodeling indicates that the siliceous mesostasis&nbsp;+ sanidine&nbsp;+ quartz&nbsp;&plusmn; ferrorichterite represents a Si-richsilicate immiscible melt whereas the ferroaugite&nbsp;+ plagioclase&nbsp;+ stilpnomelane represent the Fe-rich silicateimmiscible liquid. The identification of naturally occurring silicate-liquid immiscibility at scales greaterthan micron level is an important observation which may be useful in identifying volcanic and plutonicrocks which formed by macro-scale silicate-liquid immiscibility</p

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

Chemical heterogeneity of the Emeishan mantle plume: Evidence from highly siderophile element abundances in picrites

Highly magnesian lavas or picrites have the potential to preserve important information about the origin and thermochemical state of the mantle source(s) of large igneous provinces. We have conducted a comprehensive study of highly siderophile element (HSE) concentrations in picrites from the ca. 260 Ma Emeishan large igneous province. We show that HSE abundances in the Emeishan picrites are greater than those in mid-ocean ridge basalts (MORBs) and parental melts of Hawaiian picrites, but are similar to those in komatiites. The picrites have two types of C1-normalized HSE patterns: (a) type 1, as represented by the Muli picrites is similar to that of the primitive upper mantle; (b) type 2, as represented by the Dali picrites resembles East Greenland and Iceland picrites. Pt/Ir and Pd/Ir ratios in the type 2 picrites are higher than those in type 1 picrites. The primary melt compositions of the studied samples have been estimated by back-addition of equilibrium olivine. The calculated HSE abundances of the parental liquids of the Dali and Muli picrites are higher than those of the parental melts to Hawaiian picrites. Along with previously published isotopic data, our study provides further evidence for chemical heterogeneity of the Emeishan mantle plume