The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.

The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The Neoarchaean Agnew-Wiluna greenstone belt (AWB) of the Kalgoorlie Terrane, within the Eastern Goldfields Superterrane (EGS) of the Yilgarn Craton, Western Australia, contains several world-class, komatiite-hosted, nickel-sulphide ore bodies. These are commonly associated with felsic volcanic successions, many of which are considered to have a tonalite-trondhjemite-dacite (TTD) affinity. The Cosmos greenstone sequence lies on the western edge of the AWB and this previously unstudied mineralised volcanic succession contrasts markedly in age, geochemistry, emplacement mechanisms and probable tectonic setting to that of the majority of the AWB and wider EGS. Detailed subsurface mapping has shown that the footwall to the Cosmos mineralised ultramafic sequence consists of an intricate succession of both fragmental and coherent extrusive lithologies, ranging from basaltic andesites through to rhyolites, plus later-formed felsic and basaltic intrusions. The occurrence of thick sequences of amygdaloidal intermediate lavas intercalated with extensive sequences of dacite lapilli tuff, coupled with the absence of marine sediments or hydrovolcanic products, indicates the succession was formed in a subaerial environment. Chemical composition of the non-ultramafic lithologies is typified by a high-K calc-alkaline to shoshonite signature, indicative of formation in a volcanic arc setting. Assimilation-fractional crystallisation modelling has shown that at least two compositionally distinct sources must be invoked to explain the observed basaltic andesite to rhyolite magma suite. High resolution U-Pb dating of several units within the succession underpins stratigraphic relationships established in the field and indicates that the emplacement of the Cosmos succession took place between ~2736 Ma and ~2653 Ma, making it significantly older and longer-lived than most other greenstone successions within the Kalgoorlie Terrane. Extrusive periodic volcanism spanned ~50 Myrs with three cycles of bimodal intermediate/felsic and ultramafic volcanism occurring between ~2736 Ma and ~2685 Ma. Periodic intrusive activity, related to the local granite plutonism, lasted for a further ~32 Myrs or until ~2653 Ma. The Cosmos succession either represents a separate, older terrane in its own right or it has an autochthonous relationship with the AWB but volcanism initiated much earlier in this region than currently considered. Dating of the Cosmos succession has demonstrated that high-resolution geochronology within individual greenstone successions can be achieved and provides more robust platforms for interpreting the evolution of ancient mineralised volcanic successions. The geochemical affinity of the Cosmos succession indicates a subduction zone was operating in the Kalgoorlie Terrane by ~2736 Ma, much earlier than considered in current regional geodynamic models. The Cosmos volcanic succession provides further evidence that plate tectonics was in operation during the Neoarchaean, contrary to some recently proposed tectonic models.

Field evidence from exposed plutonic rocks indicates that mafic-felsic magma interaction is an important process during the construction and evolution of magma chambers. The exhumed, ~140 Ma, Halfmoon Pluton of Stewart Island, New Zealand is characterized by a sequence of mingled mafic sheets and enclaves preserved within an intermediate-felsic host, and provides a unique opportunity to directly investigate the physico-chemical processes that operate within an arc setting. Interpretation of mingling structures and textures in the field, in combination with extensive petrographic, geochemical and geochronological data, allow for conclusions to be reached regarding the nature of the mafic-felsic magma interactions, and the physical, chemical and thermal processes responsible for the generation and evolution of the calc-alkaline magmas. Detailed documentation and interpretation of mafic-felsic magma mingling structures and textures reveal that the Halfmoon Pluton formed incrementally as the result of episodic replenishments of mafic magma emplaced onto the floor of an aggrading intermediate-felsic magma chamber. Physico-chemical processes identified include fractional crystallization and accumulation of a plagioclase – hornblende – apatite – zircon mineral assemblage, episodic replenishment by hot, wet basaltic magmas, magmatic flow and compaction. Early amphibole and apatite crystallization played an important role in the compositional diversity within the Halfmoon Pluton. Variations in the style of magma mingling preserved within the magmatic “stratigraphy” indicate that processes operating within the chamber varied in space and time. Variations in mineral zoning and composition within hornblende indicate that the Halfmoon Pluton crystallized within a magma in which melt composition fluctuated in response to repeated mafic magma replenishments, fractionation, crystal settling and convection. Mineral assemblages, chemical characteristics, isotopic data and geochronological evidence indicate that the amphibole-rich calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting, most probably an island-arc. Data are consistent with a model whereby ‘wet’ amphibole-rich basaltic magmas pond at the crust-mantle interface and episodically rise, inject and mingle with an overlying intermediate-felsic magma chamber that itself represents the fractionated product of the mantle melts.

Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of amphibole from the most primitive non-cumulate compositions controls the compositions of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal is accomplished by the intercumulus crystallization of large amphibole oikocrysts in primitive olivine hornblendite cumulates. Experimental hornblende compositions that crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show a good correlation between octahedral Al in hornblende and pressure, and provide a means of estimating crystallization pressures during differentiation of primitive arc basalt. Application of an empirical barometer derived from experimental amphibole data (P = Al(6)/0.056 – 0.143; r2 = 0.923) to natural hornblendes from this study suggests that crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende from the most primitive basalt compositions to generate basaltic andesite, and a further 48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende removal is more efficient at generating intermediate compositions than anhydrous gabbroic fractionating assemblages, which require up to 70% crystallization to reach basaltic andesite from similar starting compositions. There are no magmatic analogues to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal appears to be a key factor in producing bulk continental crust.

Field evidence from exposed plutonic rocks indicates that mafic-felsic magma interaction is an important process during the construction and evolution of magma chambers. The exhumed, ~140 Ma, Halfmoon Pluton of Stewart Island, New Zealand is characterized by a sequence of mingled mafic sheets and enclaves preserved within an intermediate-felsic host, and provides a unique opportunity to directly investigate the physico-chemical processes that operate within an arc setting. Interpretation of mingling structures and textures in the field, in combination with extensive petrographic, geochemical and geochronological data, allow for conclusions to be reached regarding the nature of the mafic-felsic magma interactions, and the physical, chemical and thermal processes responsible for the generation and evolution of the calc-alkaline magmas. Detailed documentation and interpretation of mafic-felsic magma mingling structures and textures reveal that the Halfmoon Pluton formed incrementally as the result of episodic replenishments of mafic magma emplaced onto the floor of an aggrading intermediate-felsic magma chamber. Physico-chemical processes identified include fractional crystallization and accumulation of a plagioclase – hornblende – apatite – zircon mineral assemblage, episodic replenishment by hot, wet basaltic magmas, magmatic flow and compaction. Early amphibole and apatite crystallization played an important role in the compositional diversity within the Halfmoon Pluton. Variations in the style of magma mingling preserved within the magmatic “stratigraphy” indicate that processes operating within the chamber varied in space and time. Variations in mineral zoning and composition within hornblende indicate that the Halfmoon Pluton crystallized within a magma in which melt composition fluctuated in response to repeated mafic magma replenishments, fractionation, crystal settling and convection. Mineral assemblages, chemical characteristics, isotopic data and geochronological evidence indicate that the amphibole-rich calc-alkaline Halfmoon Pluton was emplaced into a juvenile arc setting, most probably an island-arc. Data are consistent with a model whereby ‘wet’ amphibole-rich basaltic magmas pond at the crust-mantle interface and episodically rise, inject and mingle with an overlying intermediate-felsic magma chamber that itself represents the fractionated product of the mantle melts.

Exposed on Vancouver Island, British Columbia, the Jurassic Bonanza arc is believed to represent the southerly continuation of the Talkeetna arc. Small bodies of mafic and ultramafic cumulates within deeper plutonic levels of the arc constrain the fractionation pathways leading from high-MgO basalt to andesite-dacite compositions. The removal of amphibole from the most primitive non-cumulate compositions controls the compositions of mafic plutons and volcanics until the onset of plagioclase crystallization. This removal is accomplished by the intercumulus crystallization of large amphibole oikocrysts in primitive olivine hornblendite cumulates. Experimental hornblende compositions that crystallize from high-MgO basalts similar to primitive basalts from the Bonanza arc show a good correlation between octahedral Al in hornblende and pressure, and provide a means of estimating crystallization pressures during differentiation of primitive arc basalt. Application of an empirical barometer derived from experimental amphibole data (P = Al(6)/0.056 – 0.143; r2 = 0.923) to natural hornblendes from this study suggests that crystallization of primitive basalts took place at 470-880 MPa. Two-pyroxene thermometry gives a result of 1058 +/- 91 ºC for the only olivine hornblendite sample with both pyroxenes. Lever rule calculations require the removal of 30-45 % hornblende from the most primitive basalt compositions to generate basaltic andesite, and a further 48% crystallization of hornblende gabbro to generate dacitic compositions. Hornblende removal is more efficient at generating intermediate compositions than anhydrous gabbroic fractionating assemblages, which require up to 70% crystallization to reach basaltic andesite from similar starting compositions. There are no magmatic analogues to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will push the bulk arc composition to high-Mg# andesite. Garnet removal appears to be a key factor in producing bulk continental crust.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22266号 / 理博第4580号 / 新制||理||1658(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 田上 高広, 教授 山路 敦, 教授 生形 貴男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Subduction is a dynamic and time-dependent process which requires time-dependent models for its study. In addition, due to the very high pressures within the Earth's interior, an evaluation of the role of compressibility in subduction studies should be undertaken. However, most subduction studies have been conducted by using kinematic, steady-state, and/or incompressible mantle convection models; these simplifications may miss important elements of the subduction process. In this dissertation, I evaluate the effects of time-dependence and compressibility on the evolution of subduction by using 2-D Cartesian numerical models. The effect of compressibility on the thermal and flow structures of subduction zones is evaluated by using kinematically prescribed slab and steady-state models. The effect of compressibility is primarily expressed as an additional heat source created by viscous dissipation. The heat results in thinner thermal boundary layer on the subducting slab and increases slab temperatures. With that exception, the effect of compressibility is relatively small compared with, for example, the effect of the mantle rheology on the thermal and flow structures of the mantle wedge. Plate reconstruction models show that the convergence rate and age of the incoming plate to trench vary with time, which poses a problem for steady-state subduction models. Thus, I consider the time-dependent convergence rate and age of the incoming plate in the kinematic-dynamic subduction models in order to understand the localization of high-Mg# andesites in the western Aleutians. The results show that the localization of high-Mg# andesites is a consequence of the time-dependent convergence rate and slab age along the Aleutian arc. The influence of mantle and slab parameters as well as compressibility on the slab dynamics is evaluated by using 2-D dynamic subduction models. The results demonstrate that periodic slab buckling in the mantle results in periodic convergence rate and dip of the subducting slab; time-dependence is a natural expression of subduction. The effect of compressibility on the slab dynamics is not significant. The periodic convergence rate and dip of the subducting slab explain time-dependent seafloor spreading at the mid-ocean ridge, convergence rate of the oceanic plate at trench and arc-normal migration of arc volcanoes. / Ph. D.