Some Studies of New Zealand Quaternary Pyroclastic Rocks

<p>The development of volcanic "ash" studies in New Zealand can be traced through three broad periods (Jeune 1970). During the late 19th century the extensive pumice deposits surrounding Lake Taupo received considerable comment (Crawford 1875, Smith 1876 and Cussen 1887). Thomas (1887) recognised a covering of younger andesitic ash from Mts. Tongariro and Ruapehu overlying the pumice from Taupo and in his 1888 report on the eruption of Mt. Tarawera in 1886, Thomas provided a valuable description of the eruption and the deposits resulting from it. Tephra deposits received only cursory attention during the following years until soil surveys initiated as part of the research effort into bush sickness demonstrated a relationship between incidence of the disease and soil derived from tephra (Aston 1926). Extended soil surveys followed (Granger 1929, 1931, 1937, Taylor 1930, 1933, Grange and Taylor 1931, 1932) during the course of which many important soil forming tephras were named, described and mapped. On the basis of minerals studies, contributions were recognised from four recently active volcanic centres; Taupo, Rotorua, Tongariro National Park and Mt. Egmont.</p>

Some Studies of New Zealand Quaternary Pyroclastic Rocks

<p>The development of volcanic "ash" studies in New Zealand can be traced through three broad periods (Jeune 1970). During the late 19th century the extensive pumice deposits surrounding Lake Taupo received considerable comment (Crawford 1875, Smith 1876 and Cussen 1887). Thomas (1887) recognised a covering of younger andesitic ash from Mts. Tongariro and Ruapehu overlying the pumice from Taupo and in his 1888 report on the eruption of Mt. Tarawera in 1886, Thomas provided a valuable description of the eruption and the deposits resulting from it. Tephra deposits received only cursory attention during the following years until soil surveys initiated as part of the research effort into bush sickness demonstrated a relationship between incidence of the disease and soil derived from tephra (Aston 1926). Extended soil surveys followed (Granger 1929, 1931, 1937, Taylor 1930, 1933, Grange and Taylor 1931, 1932) during the course of which many important soil forming tephras were named, described and mapped. On the basis of minerals studies, contributions were recognised from four recently active volcanic centres; Taupo, Rotorua, Tongariro National Park and Mt. Egmont.</p>

Effusive Badi Silicic Volcano (Central Afar, Ethiopian Rift); Sparse Evidence for Pyroclastic Rocks

We report field observation, textural description (thin section and scanning electron microscope (SEM)) and mineral chemistry (backscattered electron imaging and dispersive X-ray analysis) for rhyolitic obsidian lavas from previously under described effusive Badi volcano, central Afar within the Ethiopian rift. These rhyolitic obsidian lavas are compositionally homogeneous and contain well developed flow bands. Textural analysis is undertaken to understand the formation of flow band, and to draw inferences on the mechanism of emplacement of this silicic volcano. Flow band arises from variable vesicularity (i.e., alternating domains of vesicular, light glass and non-vesicular, brown glass). Such textural heterogeneities have been developed during distinct cooling and degassing of the melt in the conduit.

The zircon proxy in tephrostratigraphy and magma evolution studies. Fingerprinting Miocene silicic pyroclastic rocks in the Pannonian Basin and its surroundins.

&lt;p&gt;We used combined trace element and U-Pb isotopic data of zircon from dacitic to rhyolitic pyroclastic rocks and Si-rich ash-bearing deposits to assess their tephrostratigraphic potential. Data were collected using LA-ICP-MS analyses, a rapid and cost-effective method, to obtain simultaneously trace element contents and U-Pb ages of a large number of zircon grains. The rationale in using zircon crystals for characterizing tephra deposits is that zircon is a resistant mineral phase and is usually a late crystallizing mineral in highly evolved magmas. Therefore, they are assumed to be in equilibrium with the erupted melt phase represented by the volcanic glass. Knowing the zircon/melt partition coefficients, equilibrium melt composition can be calculated even in cases when the volcanic glass in the pyroclastic material has undergone severe post-depositional alteration.&lt;/p&gt;&lt;p&gt;We studied Miocene silicic pyroclastic deposits in a broad area including the Pannonian Basin (eastern-central Europe) and its surroundings to characterize and correlate the explosive volcanic products. In regional scale, these deposits are usually assigned as important stratigraphic key horizons within sedimentary successions and thus, they help to understand better the chronostratigraphic framework and palaeoenvironmental changes having affected the highly-dynamic Mediterranean-Paratethys system.&lt;/p&gt;&lt;p&gt;The early to middle Miocene silicic pyroclastic deposits within the Pannonian basin are estimated to be more than 4000 km&lt;sup&gt;3&lt;/sup&gt; in volume within 4 Myr, suggesting an important ignimbrite flare-up event. At least 4 main eruption units were distinguished and characterized, each could have regional (&gt;&gt;100 km) effects. We demonstrate here the power of multivariate discriminant analyses as well as machine learning techniques in distinguishing the main eruptive units and their correlation with unclassified distal deposits based on zircon trace element data. The machine learning algorithms were trained using our zircon database with trace elements as input parameters. Both the discriminant analysis and the machine learning methods gave reliable results, i.e. distinguished the main 4 pyroclastic units and found the link of the distal deposits to them. As a result, we provide a robust zircon-based fingerprint that can be used as a proxy in tephrostratigraphy.&lt;/p&gt;&lt;p&gt;Zircon trace element compositions indicate distinct silicic magmas resided partly coeval in the upper crust. Using trace element content of zircon and glasses from the same samples of crystal-poor ignimbrites, we determined zircon/melt partition coefficients. The obtained values of the 4 main units are very similar and comparable with published data for silicic volcanic systems. This suggests that zircon/melt partition coefficients in calc-alkaline silicic systems are not significantly influenced by melt composition at &gt;70 wt% SiO&lt;sub&gt;2&lt;/sub&gt;. These findings let us use these zircon/melt partition coefficients to calculate the equilibrium melt compositions for the pyroclastic occurrences even in case when no glass data were available. The zircon proxy approach can be limited by the non-existence of zircon in the rocks and also by the fact that no systematic compositional difference is found between eruption products, although the latter problem similarly stands for glass chemistry-based tephrostratigraphic studies.&lt;/p&gt;&lt;p&gt;This study was supported by the NKFIH FK-131869 project.&lt;/p&gt;

Age of volcanic-sedimentary complex from Сape Svyatoi Nos (Eastern Arctic)

Geological structure and age of the volcanogenic-sedimentary complex of the Cape Svyatoi Nos (Svyatonosskaya formation) are presented. The rocks of Cape Svyatoi Nos are located on the border of the Novosibirsk-Chukotka and Verkhoyansk–Kolyma fold belts, on the coast of the Laptev and East- Siberian Seas. Field studies indicate that the rocks belong to a single volcanogenic-sedimentary complex. The maximum thickness of individual sections reaches up 700 m. Coarse-grained pyroclastic rocks with rare lava flows prevail on the north (on the coast of Laptev Sea). The proportion and dimension of volcanics and pyroclastic rocks decrease in the south, terrigenous rocks appear. In the modern structure, the rocks are deformed.Zircons of several populations were separated from the flow of basalts. Two, the most representative zircon populations are characterized by close subconcordant ages. The structure and U-Pb ages of zircons from the first population suggest their formation during magmatic crystallization with a superimposed postmagmatic thermal event. Zircons of the second population have a xenomorphic appearance, which is typical of zircons formed at the late or postmagmatic phases. The weighted average age (MSWD = 3) of the first two populations is 149.3 ± 1.2 Ma (Tithonian age). It corresponds to the age of crystallization of basalts and the superimposed (close in time) postmagmatic thermal event.The third population of zircons is represented by two rounded grains with Archean U-Pb ages. It is assumed that these grains were trapped by magmatic melt from pre-Jurassic clastic rocks.Late Jurassic-Early Cretaceous radiolarians were identified from different horizons of tuff-terrigenous and terrigenous rocks. This is confirm the obtained U-Pb ages and the coeval of all the studied sections. The Titonian age of volcanic-sedimentary rocks allows us to classify them as suprasubduction complexes of the Late Jurassic - Early Cretaceous, widespread in the Verkhoyansk-Chukotka Mesozoids.