Lithic Inclusions in the Taupo Pumice Formation

<p>The Taupo Pumice Formation is a product of the Taupo eruption of about 1800a, and consists of three phreatomagmatic ash deposits, two plinian pumice deposits and a major low-aspect ratio and low grade (unwelded) ignimbrite which covered most part of the central North Island of New Zealand. The vent area for the eruption is located at Horomatangi Reefs in Lake Taupo. Lithics in the phreatoplinian ash deposits are negligible in quantity, but the plinian pumice deposits contain 5-10% lithics by volume in most near-vent sections. Lithics in the plinian pumice deposits are dominantly banded and spherulitic rhyolite with minor welded tuff, dacite and andesite. The ground layer which forms the base of the ignimbrite unit consists of dominantly lithics and crystals and is formed by the gravitational sedimentation of the 'heavies' from the strongly fluidized head of the pyroclastic flow. Lithic blocks in the ground layer are dominantly banded and spherulitic phenocryst-poor rhyolite, welded tuff with minor dacite and andesite. Near-vent exposures of the ground layer contain boulders upto 2 m in diameter. Friable blocks of hydrothermally altered rhyolite, welded tuff and lake sediments are found fractured but are preserved intact after transportation. This shows that the fluid/pyroclastic particle mixture provided enough support to carry such blocks upto a distance of 10 km from the vent. The rhyolite blocks are subdivided into hypersthene rhyolite, hypersthene-hornblende rhyolite and biotite-bearing rhyolite on the basis of the dominant ferromagnesian phenocryst assamblage. Hypersthene is the dominant ferromagnesian phenocryst in most of the rhyolite blocks in the ground layer and forms the major ferromagnesian crystal of the Taupo Sub-group tephra. The rhyolite blocks have similar whole rock chemistry to the Taupo Sub-group tephra and are probably derived from lava extrusions associated with the tephra eruptions from the Taupo Volcanic Centre in the last 10 ka. Older rhyolite domes and flows in the area are probably represented by the intensely hydrothermally altered rhyolite blocks in the ground layer. The dacite blocks contain hypersthene and augite as a major ferromagnesian phenocryst. Whole rock major and trace element analyses shows that the dacite blocks are distinct from the Tauhara dacites and from the dacites of Tongariro Volcanic Centre. The occurrence of dacite inclusions in significant quantity in the Taupo Pumice Formation indicates the presence of other dacite flows near the vent area. Four types of andesite blocks; hornblende andesite, plagioclase-pyroxene andesite, pyroxene andesite and olivine andesite occur as lithic blocks in the ground layer. The andesites are petrographically distinct from those encountered in deep drillholes at Wairakei (Waiora Valley Andesites), and are different from the Rolles Peak andesite in having lower Sr content. The andesite blocks show similar major and trace element content to those from the Tongariro Volcanic Centre. The roundness of the andesite blocks indicates that the blocks were transported as alluvium or lahars in to the lake basin before being incorporated into the pyroclastic flow. Two types of welded ignimbrite blocks are described. The lithic-crystal rich ignimbrite is correlated with a post-Whakamaru Group Ignimbrite (ca. 100 ka ignimbrite erupted from Taupo Volcanic Centre) which crops out to the north of Lake Taupo. The crystal rich ignimbrite is tentatively correlated with the Whakamaru Group Ignimbrites. The lake sediment boulders, pumiceous mudstone and siltstone in the ground layer probably correlate to the Huka Group sediments or younger Holocene sediments in the lake basin. A comparative mineral chemistry study of the lithic blocks was done. A change in chemistry of individual mineral species was found to accompany the variation in wholerock major element constituents in the different types of lithics. The large quantity of lithic blocks in the ground layer suggests extensive vent widening at the begining of the ignimbrite eruption. A simple model of flaring and collapse of the vent area caused by the down ward movement of the fragmentation surface is presented to explain the origin of the lithic blocks in the ground layer. The lithics in the Taupo Pumice Formation are therfore produced by the disruption of the country rock around the vent during the explosion and primary xenoliths from depths of magma generation were not found. Stratigraphic relations suggest that the most important depth of incorporation of lithics is within the post-Whakamaru Group Ignimbrite volcanics and volcaniclastic sedimentary units.</p>

Lithic Inclusions in the Taupo Pumice Formation

<p>The Taupo Pumice Formation is a product of the Taupo eruption of about 1800a, and consists of three phreatomagmatic ash deposits, two plinian pumice deposits and a major low-aspect ratio and low grade (unwelded) ignimbrite which covered most part of the central North Island of New Zealand. The vent area for the eruption is located at Horomatangi Reefs in Lake Taupo. Lithics in the phreatoplinian ash deposits are negligible in quantity, but the plinian pumice deposits contain 5-10% lithics by volume in most near-vent sections. Lithics in the plinian pumice deposits are dominantly banded and spherulitic rhyolite with minor welded tuff, dacite and andesite. The ground layer which forms the base of the ignimbrite unit consists of dominantly lithics and crystals and is formed by the gravitational sedimentation of the 'heavies' from the strongly fluidized head of the pyroclastic flow. Lithic blocks in the ground layer are dominantly banded and spherulitic phenocryst-poor rhyolite, welded tuff with minor dacite and andesite. Near-vent exposures of the ground layer contain boulders upto 2 m in diameter. Friable blocks of hydrothermally altered rhyolite, welded tuff and lake sediments are found fractured but are preserved intact after transportation. This shows that the fluid/pyroclastic particle mixture provided enough support to carry such blocks upto a distance of 10 km from the vent. The rhyolite blocks are subdivided into hypersthene rhyolite, hypersthene-hornblende rhyolite and biotite-bearing rhyolite on the basis of the dominant ferromagnesian phenocryst assamblage. Hypersthene is the dominant ferromagnesian phenocryst in most of the rhyolite blocks in the ground layer and forms the major ferromagnesian crystal of the Taupo Sub-group tephra. The rhyolite blocks have similar whole rock chemistry to the Taupo Sub-group tephra and are probably derived from lava extrusions associated with the tephra eruptions from the Taupo Volcanic Centre in the last 10 ka. Older rhyolite domes and flows in the area are probably represented by the intensely hydrothermally altered rhyolite blocks in the ground layer. The dacite blocks contain hypersthene and augite as a major ferromagnesian phenocryst. Whole rock major and trace element analyses shows that the dacite blocks are distinct from the Tauhara dacites and from the dacites of Tongariro Volcanic Centre. The occurrence of dacite inclusions in significant quantity in the Taupo Pumice Formation indicates the presence of other dacite flows near the vent area. Four types of andesite blocks; hornblende andesite, plagioclase-pyroxene andesite, pyroxene andesite and olivine andesite occur as lithic blocks in the ground layer. The andesites are petrographically distinct from those encountered in deep drillholes at Wairakei (Waiora Valley Andesites), and are different from the Rolles Peak andesite in having lower Sr content. The andesite blocks show similar major and trace element content to those from the Tongariro Volcanic Centre. The roundness of the andesite blocks indicates that the blocks were transported as alluvium or lahars in to the lake basin before being incorporated into the pyroclastic flow. Two types of welded ignimbrite blocks are described. The lithic-crystal rich ignimbrite is correlated with a post-Whakamaru Group Ignimbrite (ca. 100 ka ignimbrite erupted from Taupo Volcanic Centre) which crops out to the north of Lake Taupo. The crystal rich ignimbrite is tentatively correlated with the Whakamaru Group Ignimbrites. The lake sediment boulders, pumiceous mudstone and siltstone in the ground layer probably correlate to the Huka Group sediments or younger Holocene sediments in the lake basin. A comparative mineral chemistry study of the lithic blocks was done. A change in chemistry of individual mineral species was found to accompany the variation in wholerock major element constituents in the different types of lithics. The large quantity of lithic blocks in the ground layer suggests extensive vent widening at the begining of the ignimbrite eruption. A simple model of flaring and collapse of the vent area caused by the down ward movement of the fragmentation surface is presented to explain the origin of the lithic blocks in the ground layer. The lithics in the Taupo Pumice Formation are therfore produced by the disruption of the country rock around the vent during the explosion and primary xenoliths from depths of magma generation were not found. Stratigraphic relations suggest that the most important depth of incorporation of lithics is within the post-Whakamaru Group Ignimbrite volcanics and volcaniclastic sedimentary units.</p>

The Geology and Lamongan Volcanic Rocks Case Study at Ranu Pakis, Klakah, Lumajang, East Java Province, Indonesia

Lamongan volcano is one of the unique volcanoes in the Sunda Volcano. This volcano has side eruption centers or on the slopes of the volcano. The morphology of parasitic eruptions in this volcanoes complex includes maars and boccas. There are about 64 parasitic eruption centers consisting of 37 volcanic cones (bocca) and 27 ranu (maar). The purpose of this research is to study the characteristics of lithology and petrogenesis of this volcano complex, especially in Ranu Pakis and surrounding areas. The analytical method used is to do geological mapping and petrographic analysis. The lithology found in this research area consists of magmatic and phreatomagmatic eruption deposits. Genetically this lithology includes pyroclastic flow, pyroclastic fall (scoria fall and phreatomagmatic scoria fall/accretionary lapili), tuff (phreatic) and basaltic lava. In some pyroclastic deposits, especially in maar there are fragments of accretionary lapilli, while in bocca there are basaltic lavas. Other fragments present in pyroclastic deposits are basalt scoria blocks and bombs embedded in the groundmass of volcanic ash. The results of petrographic analysis indicate that the volcanic rocks in the study area are calc alkaline affinity consisting of pyroxene andesite, basalt and pyroxene basalt lava. The pyroxene basalt lava is composed by plagioclase, clinopyroxene and little olivine embedded in the volcanic glass. Lavas are structured scoria and textured porphyritic, intersertal, trachytic, aphyric and pilotaxitic. Trachytic texture is found in the basalt fragments of pyroxene from the pyroclastic fall deposits in Ranu Pakis and Ranu Wurung. While pyroxene andesite lavas composed by plagioclase, clinopyroxene embedded in the volcanic glass. Lavas are structured scoria and textured porphyritic, intergranular, pilotaxitic and aphyric.

A reinvestigation of holotype wadalite from Tadano, Fukushima Prefecture, Japan

ABSTRACTA reinvestigation of the type specimen of wadalite, ideally Ca12Al10Si4O32Cl6, from Tadano, Fukushima Prefecture, Japan, reveals that it has well-defined chemical sector zoning. The mineral occurs in skarn xenoliths in two-pyroxene andesite and forms tris-tetrahedral crystals up to 1 mm in size. The mineral is cubic,I¯43d, witha= 12.009(2) Å,V= 1731.8(8) Å3andZ= 2. The zoning is divided into two sectors, {21¯1} and {211}, on the basis of back-scattered-electron images. The {21¯1} sector is more enriched in Fe3+, Si, Mg and Cl, and depleted in Al than the {211} sector. The empirical formulae yielded by the averaged compositions for the {21¯1} and {211} sectors are: Ca12.01(Al7.88$\hbox{Fe}_{{\rm 0}{\rm. 99}}^{{\rm 3 +}} $Si4.51Mg0.56Ti0.05)Σ13.99O32.22Cl5.55and Ca12.05(Al8.42$\hbox{Fe}_{{\rm 0}{\rm. 85}}^{{\rm 3 +}} $Si4.20Mg0.44Ti0.04)Σ13.95O32.19Cl5.38, respectively. Compositional variations in the wadalite grains suggest that the Si contents are controlled mainly by the substitutionT(Al,Fe3+) +W□ ↔TSi +W(Cl,F). The presence of sector zoning suggests that the wadalite grew during rapid non-equilibrium crystallization.

Note on the evolution of a Miocene composite volcano in an extensional setting, Zârand Basin (Apuseni Mts., Romania)

Bontâu is a major eroded composite volcano filling the Miocene Zârand extensional basin, near the junction between the Codru-Moma and Highiş-Drocea Mountains, at the tectonic boundary between the South and North Apuseni Mountains. It is a quasi-symmetric structure (16–18 km in diameter) centered on an eroded vent area (9×4 km), buttressed to the south against Mesozoic ophiolites and sedimentary deposits of the South Apuseni Mountains. The volcano was built up in two sub-aerial phases (14–12.5 Ma and 11–10 Ma) from successive eruptions of andesite lava and pyroclastic rocks with a time-increasing volatile budget. The initial phase was dominated by emplacement of pyroxene andesite and resulted in scattered individual volcanic lava domes associated marginally with lava flows and/or pyroclastic block-and-ash flows. The second phase is characterized by amphibole-pyroxene andesite as a succession of pyroclastic eruptions (varying from strombolian to subplinian type) and extrusion of volcanic domes that resulted in the formation of a central vent area. Numerous debris flow deposits accumulated at the periphery of primary pyroclastic deposits. Several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes are known in the volcano vent complex area. Distal epiclastic deposits initially as gravity mass flows and then as alluvial volcaniclastic and terrestrial detritic and coal filled the basin around the volcano in its western and eastern part.Chemical analyses show that lavas are calc-alkaline andesites with SiO2 ranging from 56–61%. The petrographical differences between the two stages are an increase in amphibole content at the expense of two pyroxenes (augite and hypersthene) in the second stage of eruption; CaO and MgO contents decrease with increasing SiO2. In spite of a ∼4 Ma evolution, the compositions of calc-alkaline lavas suggest similar fractionation processes. The extensional setting favored two pulses of short-lived magma chamber processes.

Eleven million years of arc volcanism at the Aucanquilcha Volcanic Cluster, northern Chilean Andes: implications for the life span and emplacement of plutons

ABSTRACTThe arid climate of the Altiplano has preserved a volcanic history of ∼11 million years at the Aucanquilcha Volcanic Cluster (AVC), northern Chile, which is built on thick continental crust. The AVC has a systematic temporal, spatial, compositional and mineralogical development shared by other long-lived volcanic complexes, indicating a common pattern in continental magmatism with implications for the development of underlying plutonic complexes, that in turn create batholiths.The AVC is a ∼700-km2, Tertiary to Recent cluster of at least 19 volcanoes that have erupted andesite and dacite lavas (∼55 to 68 wt.% SiO2) and a small ash-flow tuff, totalling 327 ± 20 km3. Forty 40Ar/39Ar ages for the AVC range from 10·97 ± 0·35 to 0·24 ± 0·05 Ma and define three major 1·5 to 3 million-year pulses of volcanism followed by the present pulse expressed as Volcán Aucanquilcha. The first stage of activity (∼11–8 Ma, Alconcha Group) produced seven volcanoes and the 2-km3 Ujina ignimbrite and is a crudely bimodal suite of pyroxene andesite and dacite. After a possible two million year hiatus, the second stage of volcanism (∼6–4 Ma, Gordo Group) produced at least five volcanoes ranging from pyroxene andesite to dacite. The third stage (∼4–2 Ma, Polan Group) represents a voluminous pulse of activity, with eruption of at least another five volcanoes, broadly distributed in the centre of the AVC, and composed dominantly of biotite amphibole dacite; andesites at this stage occur as magmatic inclusions. The most recent activity (1 Ma to recent) is in the centre of the AVC at Volcán Aucanquilcha, a potentially active composite volcano made of biotite-amphibole dacite with andesite and dacite magmatic inclusions.These successive eruptive groups describe (1) a spatial pattern of volcanism from peripheral to central, (2) a corresponding change from compositionally diverse andesite-dacite volcanism to compositionally increasingly restricted and increasingly silicic dacite, (3) a change from early anhydrous mafic silicate assemblages (pyroxene dominant) to later biotite amphibole dacite, (4) an abrupt increase in eruption rate; and (5) the onset of pervasive hydrothermal alteration.The evolutionary succession of the 327-km3 AVC is similar to other long-lived intermediate volcanic complexes of very different volumes, e.g., eastern Nevada (thousands of km3, Gans et al. 1989; Grunder 1995), Yanacocha, Perú (tens of km3, Longo 2005), and the San Juan Volcanic System (tens of thousands of km3, Lipman 2007) and finds an analogue in the 10-m. y. history and incremental growth of the Cretaceous Tuolumne Intrusive Suite (Coleman et al. 2004; Glazner et al. 2004). The present authors interpret the AVC to reflect episodic sampling of the protracted and fitful development of an integrated and silicic middle to upper crustal magma reservoir over a period of at least 11 million years.