A red bole zircon record of cryptic silicic volcanism in the Deccan Traps, India

Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP.

Silicic magmas in the Michoacán-Guanajuato Volcanic Field: an overview about plumbing systems, crustal storage and genesis processes

The origin of silicic rocks in the Michoacán-Guanajuato volcanic field (MGVF) has been understudied since the volcanic field attracted the attention of researchers. Using geochemical, petrological and structural data from the literature, here we propose a model for the origin of silicic magmas. We found that all volcanic rocks known to date in the MGVF can be divided in 40 % andesite, 33 % basaltic andesite, 15 % basalt, 2 % trachybasalt to trachyandesite, and 10 % dacite-rhyolite. The structural systems that deformed the crust in the MGVF are NNW-SSE-oriented normal faults of the Taxco-San Miguel de Allende fault system, developed during the Oligocene, and the Morelia-Acambay fault system consisting of ENE-SSW to E-W sinistral strike-slip faults developed during the Oligocene-Miocene. In addition to bibliographic data, we present a gravimetric-magnetometric model to investigate the characteristics of the local basement where magmas acquire their final silicic composition, and a seismic tomography model to investigate the deep plumbing system that contribute to form the silicic rocks emplaced on the surface. The only report of assimilation experiments we found in the MGVF literature suggest that plagioclase and pyroxene are more easily digested than quartz by hotter magmas. The digestion of these mineral phases has a direct consequence on the generation of dacites and rhyolites. We propose that regardless of the genesis of andesitic melts, such intermediate magmas arrive to the upper-crust and are forced to evolve within local compression zones where they melt the local granitic basement and form crystal mushes. The compositional variability of silicic rocks in the MGVF is a consequence of the variable mixing between the intermediate magmas and the granitic partial melts.

The Petrology and Genesis of Silicic Magmas in the Kermadec Arc

<p>Recent work has shown that silicic volcanism can be abundant in intra-oceanic subduction settings, and is often associated with large explosive caldera-forming eruptions. Several major petrogenetic questions arise over the origin and eruption of large amounts of silicic magma at these relatively simple subduction settings. This study has investigated the geochemistry of pyroclasts collected from four volcanoes along the Kermadec arc, a young (<2 Myr) oceanic subduction zone in the southwest Pacific. Raoul, Macauley and a newly discovered volcano (here informally named 'New volcano') in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all erupted dacitic to rhyolitic pumice within the last 10 kyr. For Raoul, New volcano and Healy, whole rock major element compositions fall with a limited compositional range. In contrast, pumice dredged from around Macauley caldera covers a wide compositional range indicating that there have been multiple silicic eruptions, not just the Sandy Bay Tephra exposed on Macauley Island. Distinctive crystal populations in both pumice samples and plutonic xenoliths suggest that many of the crystals did not grow in the evolved magmas, but were mixed in from other sources including gabbros and tonalites. Such open system mixing is ubiquitous in magmas from the four Kermadec volcanoes studied here. Silicic magmas, co-eruptive mafic enclaves and previously erupted basalts show sub-parallel REE patterns, and crystal composition and zonation suggests that mafic and silicic magmas have a strong genetic affiliation. Examination of whole rock, glass and mineral chemistry reveals that evolved magmas can be generated at each volcano through 60-75% crystal fractionation of a basaltic parent. These findings are not consistent with silicic magma generation via crustal anatexis, as previously suggested for the Kermadec arc. Although crystallisation is the dominant process driving melt evolution in the Kermadec volcanoes, the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected by small scale chemical variations between eruption units. Larger scale chemical trends reflect the position of the volcanoes along the arc, which in turn may reflect structural changes in the subduction zone and variations in sediment influx.</p>

The Petrology and Genesis of Silicic Magmas in the Kermadec Arc

<p>Recent work has shown that silicic volcanism can be abundant in intra-oceanic subduction settings, and is often associated with large explosive caldera-forming eruptions. Several major petrogenetic questions arise over the origin and eruption of large amounts of silicic magma at these relatively simple subduction settings. This study has investigated the geochemistry of pyroclasts collected from four volcanoes along the Kermadec arc, a young (<2 Myr) oceanic subduction zone in the southwest Pacific. Raoul, Macauley and a newly discovered volcano (here informally named 'New volcano') in the northern Kermadec arc, and Healy volcano in the southern Kermadec arc have all erupted dacitic to rhyolitic pumice within the last 10 kyr. For Raoul, New volcano and Healy, whole rock major element compositions fall with a limited compositional range. In contrast, pumice dredged from around Macauley caldera covers a wide compositional range indicating that there have been multiple silicic eruptions, not just the Sandy Bay Tephra exposed on Macauley Island. Distinctive crystal populations in both pumice samples and plutonic xenoliths suggest that many of the crystals did not grow in the evolved magmas, but were mixed in from other sources including gabbros and tonalites. Such open system mixing is ubiquitous in magmas from the four Kermadec volcanoes studied here. Silicic magmas, co-eruptive mafic enclaves and previously erupted basalts show sub-parallel REE patterns, and crystal composition and zonation suggests that mafic and silicic magmas have a strong genetic affiliation. Examination of whole rock, glass and mineral chemistry reveals that evolved magmas can be generated at each volcano through 60-75% crystal fractionation of a basaltic parent. These findings are not consistent with silicic magma generation via crustal anatexis, as previously suggested for the Kermadec arc. Although crystallisation is the dominant process driving melt evolution in the Kermadec volcanoes, the magmatic systems are open to contributions from both newly arriving melts and wholly crystalline plutonic bodies. Such processes occur in variable proportions between magma batches, and are largely reflected by small scale chemical variations between eruption units. Larger scale chemical trends reflect the position of the volcanoes along the arc, which in turn may reflect structural changes in the subduction zone and variations in sediment influx.</p>

An Elemental and Isotopic Investigation of Quaternary Silicic Taupo Volcanic Zone Tephras from ODP Site 1123: Chronostratigraphic and Petrogenetic Applications

<p>This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.</p>

An Elemental and Isotopic Investigation of Quaternary Silicic Taupo Volcanic Zone Tephras from ODP Site 1123: Chronostratigraphic and Petrogenetic Applications

<p>This thesis presents a chemical and isotopic investigation of well-dated silicic tephra layers sourced from the Taupo Volcanic Zone (TVZ), central North Island, New Zealand, that were recovered from deep ocean sediment cores at Ocean Drilling Program Site 1123 (41 degrees 47.16' S, 171 degrees 29.94' W; 3290 m water depth), located approximately 1000 km east of the TVZ. The relative quiescence of the deep ocean sedimentary setting, the continuous supply of biogenic and terrigenous sediment and the favourable location of Site 1123 close to the main TVZ ash dispersal path have resulted in an extensive TVZ tephra record (70 Quaternary tephra layers preserved in 3 sediment cores) at Site 1123. This record extends and compliments the onshore record of silicic TVZ volcanism which has been obscured by erosion of non-consolidated volcanic material and burial of older units by younger volcanic deposits. The Site 1123 cores comprise an important paleo-oceanographic record for the Southwest Pacific Ocean and as a result of previous paleo-environmental studies, the Site 1123 tephras have been assigned orbitally tuned stable isotope ages that are more precise than is currently possible by any radiometric dating techniques. These features of the Site 1123 tephra record highlight its potential to be established as a type section for Quaternary tephrochronological studies in the New Zealand region. In addition, the continuous stratigraphy and precise age control of these tephras enables the Site 1123 record to be used as a petrogenetic archive to investigate changes in chemical and isotopic composition of these tephras that may be related to changes in the petrogenesis of TVZ silicic magmas during the last ~ 1.65 Ma. This thesis establishes major and trace element chemical 'fingerprints' for the Site 1123 tephras using traditional (electron probe microanalysis) and novel (laser ablation inductively coupled plasma mass spectrometry) in situ geochemical techniques. Trace element fingerprints are demonstrated to provide a more precise means of correlating and distinguishing between tephras with essentially identical major element chemistries. These fingerprints are used to refine the original Site 1123 composite stratigraphy and age model and identify a section of repeated sediments in the Site 1123 cores that have introduced a significant error into the original composite stratigraphy and age model for the interval ~1.1 to 1.4 Ma. Correlation of the tephra layers between the 3 sediment cores (1123A, B and C) establishes that ~37-38 individual tephra units are recorded with ages ranging from 1.655 Ma to 27.1 ka. Approximately 50% of the eruptive units and cumulative tephra thickness at the site were recorded during the first ~ 150 ka of silicic TVZ volcanism (1.65 to 1.50 Ma). The fragmentary onshore record does not preserve clear evidence for this early period of hyperactivity. Four broad silicic melt types are identified on the basis of chemistry and eruptive age. Trace element indices of fractional crystallisation suggests the origin of the four melt types is primarily due to differential degrees of fractional crystallisation of accessory zircon, hydrous mineral phases and Fe-Ti oxides. Sr-Nd-Pb isotopic compositions of 13 representative Site 1123 tephras cannot be generated using traditional models in which Torlesse meta-sedimentary rocks are the sole contaminant of mafic magmas. Instead the data support a model in which ascending TVZ basalts assimilate crustal rocks of both meta-greywacke terranes: firstly up to 15% of Waipapa crust is assimilated at depth, followed by assimilation of between 20 and 45% Torlesse crust at shallower levels. In this model the majority of Site 1123 tephras indicate a remarkably uniform amount of crust (~ 35%) with the most evolved sample requiring 45% crustal contribution. However, extensive fractional crystallisation (55-85%) is required to have accompanied crustal assimilation in order to drive the relatively low SiO2 compositions of these contaminated mafic magmas (SiO2 = 53-58 wt% after crustal contamination) to the high SiO2 rhyolite (74-78 wt%) compositions of the Site 1123 tephras. The large crustal contributions to TVZ silicic magmas (35-45%) implied by these data are high compared to large volume silicic magmas from different settings (e.g. Yemen-Ethiopia; Long Valley, USA), a feature that likely reflects the thin crust and high thermal flux into the continental crust beneath the TVZ.</p>

Time scales for pluton growth, magma chamber formation and super-eruptions

Generation of silicic magmas leads to emplacement of granite plutons, huge explosive volcanic eruptions and physical and chemical zoning of continental and arc crust1-7. While the time scales for silicic magma generation in the deep and middle crust are prolonged8 magma transfer into the upper crust followed by eruption is episodic and can be rapid9-12. Ages of inherited zircons and sanidines from four Miocene ignimbrites in the Central Andes indicate a gap of 4.6 Myr between the start of pluton emplacement and onset of super-eruptions, with a 1 Myr cyclicity. Here we show that inherited sanidine crystals were stored at temperatures <470oC prior to incorporation in the magma. Our observations are explained by silicic melt segregation in a middle crustal hot zone with episodic melt ascent from an unstable layer at the top of the zone with a time scale governed by the rheology of the upper crust. After thermal incubation of the growing batholith, large magma chambers formed in only a few thousand years or less by dyke transport from the hot zone melt layer. Instability and disruption of earlier plutonic rock occurred in a few decades or less just prior to or during super-eruptions.