Ancient xenocrystic zircon in young volcanic rocks of the southern Lesser Antilles island arc
Yamirka Rojas‐AgramonteIan S. WilliamsRichard ArculusAlfred KrönerAntonio García‐CascoConcepción LázaroStephan BuhreJean WongHelen GengCarlos Morales EcheverríaTeresa E. JeffriesHangqian XieRegina Mertz‐Kraus
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Keywords:
Island arc
Back-arc basin
Ultramafic rock
Felsic
Aeromagnetic survey
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Tholeiitic (TH) series rocks occur in almost all types of tectonic settings, whereas calc-alkalic (CA) series rocks occur characteristically in island arcs and continental regions. Abyssal tholeiites and associated gabbros in mid-oceanic ridges show much narrower ranges of contents and FeO* (total iron as FeO)/MgO ratios than volcanic rocks in oceanic islands and island arcs. Abyssal tholeiites and oceanic island volcanics tend to show higher contents than island arc volcanics. These are among the diagnostic features of volcanic rock series. In terms of volcanic rock series, the ophiolitic complexes may be classified into a number of distinct classes, three of which are discussed here. The ophiolitic complexes of Class I are characterized by the presence of volcanic rocks of both CA and TH series. A typical example is the Troodos massif in Cyprus, which was formed probably in an island arc. The ophiolitic complexes of Class II are characterized by the presence of TH series volcanics. Some of them were formed probably in island arcs and others in mid-oceanic ridges. The ophiolitic complexes of Class III are characterized by the presence of TH and alkalic series volcanics. Examples are ophiolites in high-pressure (glaucophanitic) metamorphic terranes. The volcanic rocks in such ophiolites show a marked chemical resemblance to the volcanics in some hot spot islands as well as to some volcanic rocks on stable continents. A few more classes may exist.
Island arc
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Abyssal zone
Ultramafic rock
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Permian deposits widely developing in the southern Kitakami mountains have been divided into the Sakamotosawa series, the Kanokura series and the Toyama subsystem in ascending order. Volcanic rocks and their associated pyroclastic sediments are rather rare in the Permain deposits of those mountains, however, there exist thin liparitic or dacitic tuffs and “schalstein” in the Sakamotosawa series ranging from the Pseudoschwagerina to the lower half of the Parafusulina zone in age. Except for the Sakamotosawa series, volcanics and their pyroclastics have never been found in the Permian systems in that area The Kanokura series is further diyided into, in ascending order, the Kattisawa and Iwaisaki stages. Lepidolina multiseptata horizon of the Iwaisakistage may be possibly correlative to the middle to upper Neoschwagerina zone, while the Lepidolina gigantea horizon of the same stage may be nearly equivalent to the Yabeina globosa zone. Outside the Kitakami mountains, a large amount of volcanic rocks and their associated pyroclastic rocks are commonly found in Lower Permian deposits of Japan. These rocks are mostly composed of “schalstein” or diabase, but acidic tuffs are also found in the Sakamotosawa series of the Kitakami mountains, upper and lower formations of the so-called Shirafune limestone in the Hida massif and the Yamanba limestone in the Sakawa basin, Shikoku island. It is worthy of note that Upper Permian deposits seem to lack volcanic and pyroclastic rocks in Japan.—authors' English summary.
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Abstract During the Late Paleozoic, West Junggar (Xinjiang, NW China) experienced a shift in tectonic setting from compression to extension. Ha'erjiao is an important area for investigating collisional structures, post–collisional structures, and magmatic activities. Based on the petrological and geochemical characteristics of pyroclastic and other volcanic rocks in the Permian Kalagang Formation from the borehole ZKH1205 in the Jimunai Basin, the main types of source rock for the pyroclastic rocks deposited in the basin are identified and their implications for the Early Permian tectonic setting examined. The abundance of basalt and andesite lithic fragments in the pyroclastic rocks, together with the REE characteristics and the contents of transition and high field strength elements show that the source rocks were chiefly intermediate–basic volcanic rocks. High ICV values, low CIA values, low Rb/Sr ratios, low Th/U ratios and the mineralogical features suggest weak chemical weathering of the source rocks; the geochemical patterns of the pyroclastic rocks might not only have been impacted by crustal contamination but also might be related to the nature of the magma from the source area. The geochemical properties of the pyroclastic rocks distinguish them from arc‐related ones, and such samples plot in the within‐plate basalt (WPB) field in some diagrams. This is consistent with the formation background of the Early Permian volcanic rocks in this region.
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The Neogene volcanic products of the Burda mountain range nearby Štúrovo belong to Burda Formation. At the base of the Burda Formation a succession of epiclastic volcanic rocks and pyroclastic rocks of andesites has developed. In the central part of the formation, the volcanic products associated with the activity of submarine volcanism of the Badenian age developed. Submarine extrusive volcanic domes of andesites are typical. In the upper part of the Burda Formation, pyroclastic and epiclastic facies of andesites were formed. Deposits of pyroclastic flows and redeposited pyroclastics are characterized by the presence of relics of petrified tree trunks, indicating transport from emergent forest-covered slopes from the higher levels of the volcanic edifice of the Börzsöny Mountains in todayʼs Hungary. This part of the Burda volcanics represents a transitional volcanic zone with the Börzsöny stratovolcano.
Andesites
Neogene
Stratovolcano
Pyroclastic fall
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Abstract Volcanic clastic rocks comprise various rocks of clastic texture, formed as a result of volcanic activity. Classification of these rocks should be based, in the first place, on petrographical characteristics, which are possible to determine macro- and microscopically. In the suggested classification volcanic clastic rocks are subdivided into four groups, according to the composition of cementing mass and to the relative contents of pyroclastic and sedimentary material, namely: lava breccias, pyroclastic rocks (welded tuffs, tuffs, volcanic breccias), essentially pyroclastic rocks (tuffites) and pyroclastic-sedimentary (tuffogenic-sedimentary) rocks. Further subdivision of the rocks depends on their granulometric composition, chemico-petrographical composition and also on homogeneity or heterogeneity of the clastic material. In some cases there are also taken into account the relative contents of vitric, crystal and lithic pyroclastic material and the degree of abrasion and sorting of the fragments. Research International
Breccia
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Ultramafic rock
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Island arc
Continental Margin
Pacific Plate
Volcanic arc
Discontinuity (linguistics)
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