No description available.

V-Th-Pb fractionation trends, and the associated variations in Pb-isotopes, provide important constraints on the nature, and age, of a wide range of geological processes. This thesis presents three detailed case studies from a carbonatite intrusive complex, lower crustal granulite xenoliths, and young volcanic rocks which represent a range of processes that fractionate V, Th and Pb. New major, trace element and Sr, Nd and Pb isotope results are presented for carbonatites and pyroxenites from the 130 Ma old Jacupiranga complex in southern Brazil. The data preclude simple models in which the Sr-Nd and Sr-Pb isotope arrays are the result of crustal contamination processes, or liquid immiscibility between the carbonatites and magmas similar to those from which the pyroxenites crystallised. The initial Sr, Nd and Pb isotope ratios in the Jacupiranga complex are similar to those in the Parana high-Ti basalts and to the oceanic basalts of the Walvis Ridge and Tristan da Cunha, and were apparently inherited from the mantle source regions associated with incipient magmatism of the Tristan da Cunha hotspot and the opening of the South Atlantic. Most carbonatites have high V/Pb and low Rb/Sr ratios and infiltration and/ or metasomatism by such melts is one process that may be responsible for the negative correlation of U/Pb and Rb/Sr inferred for the source of certain oceanic basalts such as Tristan da Cunha. Correlations between trace and major elements in mostly mafic granulite xenoliths from southern Africa probably result from magmatic processes such as fractional crystallisation and crystal accumulation. From Pb and Nd isotope systematics it can be inferred that the northern Lesotho xenoliths are Proterozoic in age, whereas Cape province xenoliths appear to have been fonned in the Archaean and to have had their Nd isotopes reset in the Proterozoic. Most Pb isotope ratios from the granulites xenoliths are unradiogenic relative to the Geochron, which indicates relatively low V /Pb ratios in the later parts of their histories. The correlation between Pb isotope ratios and K₂O/A1₂O₃ in the granulites from Markt is considered to reflect magmatic control of the V /Pb fractionation. The Archaean, and some of the younger mantle derived rocks on the Kaapvaal craton, plot on similar Pb isotope trends. It is argued that segments of the crust and uppennost mantle beneath southern Africa stabilised together in the Archaean, and that both were remobilised in subsequent magmatic and orogenic events. Volcanic rocks in the Northland-Auckland peninsula, New Zealand, range from silica-undersaturated basanites to tholeiites and andesites. Although all the volcanic fields are in intraplate settings, two types of mantle source enrichment can be recognised. One is subduction-related and characterised by high Sr and low Nd isotope ratios and negative Nb anomalies. The other type has geochemical features characteristic of intraplate enriched mantle sources with Sr and Nd isotope ratios and Nb/Ba, Ba/La, similar to HIMV OIB, which has been attributed to the deep recycling of oceanic crust. The differences in Pb isotopes between the two mantle sources, i.e. higher /lB/4 and 7/4, but lower ²⁰⁶Pb/²⁰⁴Pb in the subduction-related material than in the inferred HIMU source, indicate that V/Pb in shallow recycled crustal material tend to be lower than in the deeper recycled material. These case studies confirm that (i) metasomatism by small degree melts can have significant effects on mantle trace element characteristics, (ii) the lower crust is a relatively low 11 environment and crystal accumulation may be an important mechanism in the fractionation of Th and U from Pb, cUld (iii) incorporation of recycled crustal material at different levels can result in different V-Th-Pb fractionation and so, with time, different Pb isotope ratios.

Micrometer-thin rims of hydrothermally altered zircon preserve significant geological information regarding the timing and nature of fluid infiltration. The research presented in this thesis details an investigation of the isotopic and geochemical composition zircon rims from deformed Archean meta-sedimentary rocks proximal to the world class Musselwhite gold deposit. A continuous ablation Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb technique is proposed in conjunction with Secondary Ion Mass Spectrometry (SIMS) U-Pb and LA-ICP-MS REE depth profile methods. These techniques are capable of identifying and analyzing isotopic and chemical modification of zircon rims that are <5 μm thick by ablating directly into the surface of unpolished crystals. The age of analyzed rims is >100 m.y younger than crystal interiors and corresponds to periods of regional magmatism and hydrothermal activity. The measured thickness of rims is variable across single grains and between grain suites suggesting that they do not form as a uniform mantle around the crystal interior. Instead the interacting fluids exploit pre-existing structural weaknesses caused by sedimentary transport and the α-decay of uranium. A novel LA-ICP-MS single element grain 2-dimension mapping technique for unpolished zircon demonstrates preferential element mobility along fractures and in isolated patches. Pressure-temperature experiments were conducted on a zircon reference material under lower greenschist facies conditions to better understand the low temperature incorporation of REEs into the zircon structure. LA-ICP-MS element mapping of unpolished grains reveals that zircon are chemically modified under these conditions, as characterized by an increase in Ce and Y concentration (up to an order of magnitude enrichment) in CePO4-bearing and YPO4-bearing experiments, respectively, when compared with the concentrations of unaltered primary grains. The integration of these micro-analytical techniques on unpolished zircon can provide insight into the timing of low- to moderate-temperature history of supracrustal rocks which would remain otherwise undefined.

This dissertation consists of three manuscripts that will be submitted for publication. All three of these examine various aspects of the evolution of the India-Asia suture zone in southern Tibet after the India-Asia collision. Continent-continent collision is one of the basic tectonic plate boundary types, has occurred repeatedly throughout geologic history, and represents one of the principle mechanisms responsible for the formation of high elevation plateaus and orogens. Uplift within these zones has also drastically changed the earth's climate and atmospheric circulation, and erosion from continental collision has resulted in some of the thickest accumulations of sediment in the world (Curray, 1991; Einsele et al., 1996). However, despite the global significance of continental collision, much of the fundamental geodynamic and geologic processes governing these events remain enigmatic. This is the result of several factors. First and foremost, intense deformation and uplift of rocks, often from mid crustal levels, over very short periods of time (Hodges and Silverberg, 1988; Seward and Burg, 2008; Zeitler et al., 2014) results in the erosive removal of much of the geologic record of a collision zone. Second, because the best modern example of continental collision is the Tibet-Himalayan system, the study of continental collision in general has been hampered by high elevations, remoteness, difficult working conditions, and political unrest. The work presented here represents a step toward better understanding the geology, geologic history, and geodynamic evolution of the Tibetan Plateau, the Himalaya, and the India-Asia collision. This has been accomplished through study of two of the post-collisional sedimentary basins which formed near or within the India-Asia suture zone. Appendix A addresses the structure, sedimentology, age, and provenance of the Liuqu Conglomerate. The key conclusions of this section are: 1) The Liuqu Conglomerate was deposited in north flowing, stream dominated alluvial fans. These were located situated in a wedge-top position within a system of north verging thrust faults likely associated with the Great Counter Thrust, and sediment was accommodated via burial beneath thrust structures. 2) The age of the Liuqu Conglomerate has been refined to ~20 Ma based on detrital zircon U-Pb and fission track dating, ⁴⁰Ar/³⁹Ar dating of biotite from a cross-cutting dike, re-analysis of previously published pollen data, regional structural considerations, and oxygen isotope composition of paleosol carbonates. 3) Sand-sized and finer-grained sediment eroded from the southern margin of Asia prior to collision was transported southwards across the Xigaze forearc basin, deposited within the subduction trench, and then accreted within the subduction complex mélange. After collision, this sediment was eroded from the mélange and shed northward into the India-Asia suture zone. Appendix B focuses on the abundant paleosols preserved within the Liuqu Conglomerate. This study uses major element geochemistry of these paleosols and stable isotope analyses of paleosol carbonates to constrain the degree and type of chemical weathering, and thus the paleoclimate and paleoelevation, of the Liuqu Conglomerate. The key conclusions of this paper are: 1) at ~20 Ma, the India-Asia suture zone experienced warm and wet conditions that promoted intense chemical weathering of soils exposed in the inactive portions of alluvial fans. Paleorainfall is estimated at ~1500 mm/yr, and weathering intensity was similar to soils formed in the Neogene Siwalik Group of India, Nepal, and Pakistan, which formed under wet, semitropical, and low elevation conditions. 2) The India-Asia suture zone experienced these conditions at ~20 Ma despite extensive deformation and crustal thickening which has been documented within the Tethyan Himalayan and Himalayan thrust belts. This crustal thickening should have resulted in the (surface) uplift of the entire India-Asia collision zone, and there is evidence that at least some portion of the Himalayan crest was at or near modern elevations by ~17 Ma. Our results require either that the Tethyan Himalaya and India-Asia suture zone were not uplifted despite as much as 40 million years of intense crustal shortening or that these regions attained high elevation prior to ~20 Ma, and then lost elevation around this time before being immediately re-uplifted. The viability of these two scenarios cannot be explicitly tested with the data presented in this chapter; however, based on the data presented in Appendix C, I strongly favor the second scenario. Appendix C focuses on the Kailas Formation, exposed ~20 km north of the Liuqu Conglomerate within the India-Asia suture zone. The Kailas Formation is exposed along ~1300 km of the India-Asia suture zone. For this study, I present new sedimentologic, provenance, and geochronologic data for the Kailas Formation. Key findings of this study are that 1) the Kailas Formation is younger in the center of the suture zone, near 90°E, and becomes progressively older to the west; preliminary data suggest that these rocks are older to the east as well, but additional age constraints are required. 2) The pattern of sedimentation documented for the Kailas Formation is nearly identical to the spatio-temporal pattern of adakitic and ultrapotassic rocks in southern Tibet. These rocks have been attributed to rollback and breakoff of the Indian continental slab. Sedimentation within the Kailas basin has also been attributed to rollback of the Indian slab (DeCelles et al. 2011), and this idea is corroborated by the agreement of the sedimentary and magmatic records. 3) This presents an interesting possibility for explaining the existence of low elevations within the India-Asia suture zone at ~20 Ma, as documented in Appendix B. High elevation topography produced by crustal shortening and thickening likely remained intact until slab rollback and breakoff started around 30 Ma and caused the India-Asia suture zone to experience large scale extension and subsidence. The Kailas Formation was deposited in the resulting basin, which opened first in the west, and propagated eastward. After slab breakoff occurred, contractional deformation would have resumed, and the area would have been quickly uplifted to its modern elevations.

No description available.

Thesis advisor: Ethan F. Baxter / Spiral garnets are well-documented metamorphic microstructures that have been observed in orogens throughout the world. The preferred orientation of spiral garnet axes has been proposed (Bell and Johnson, 1989) to record and preserve information about the timing, rate, and orientation of the tectonic-scale processes. Using the model of Be Spiral garnets are well-documented metamorphic microstructures that have been observed in orogens throughout the world. The preferred orientation of spiral garnet axes has been proposed (Bell and Johnson, 1989) to record and preserve information about the timing, rate, and orientation of the tectonic-scale processes. Using the model of Bell and Johnson (1989), Aerden et al. (2013) proposed a link between the preferred orientation of spiral garnets and changes in relative plate motion between Iberia and Africa. The goal of this thesis is to this relationship by absolutely dating, eight samples from the Betic-Rif arc with measurable spiral axis orientations were chosen for Sm-Nd garnet geochronology. Chapter one is a detailed literature review of prior work on the formation and interpretation of spiral garnets. In chapter two we present 11 bulk Sm-Nd garnet ages from eight samples, these ages range from 35.6 ± 2.8 to 13.62 ± 0.69 Ma. The results from the obtained bulk garnet ages reveal a more complex relationship between FIA orientations and plate motion that originally hypothesized in Aerden et al. (2013). Large-scale rigid block rotations that postdate garnet growth may have influenced the current orientation of FIA from the western Betic-Rif. In chapter three, zoned geochronology was conducted on a single sample from the Nevado-Filabride Complex. This study revealed spiral garnet formation occurring on a rapid timescale, just 〖0.45〗_(-0.32)^(+0.51) Myr. While other zoned garnet studies have shown similar rapid growth in subduction zone setting (Dragovic et al., 2012), this is the first such documentation of such rapid growth from a garnet hosting spiral inclusion trails in a regional metamorphic setting. We calculated strain rates considering different genetic models for the spiral inclusion trails either by garnet rotation in simple shear, or by episodic overgrowth of suborthogonal crenulation cleavages due to switching stress axes. In both cases a similar fast strain rate of ca. 10-13 s-1 was obtained, which is an order of magnitude faster than typical regional strain rates and faster than previous spiral garnet studies regardless of the method used to calculate strain-rate. / Thesis (MS) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

PhD - Science / This thesis presents the results of a broad geochronological investigation into the nature and evolution of the Central African Copperbelt, host of world class Cu-Co deposits in Zambia and the Democratic Republic of Congo. UPb SHRIMP dating of detrital, xenocrystic and magmatic zircons and metamorphic monazite as well as 40Ar-39Ar analyses on biotite, muscovite and microcline constrained the nature and the evolution of the basement, the deposition of the Katangan sedimentary sequence and the provenance of the sediments, and finally the different metamorphic episodes which affected the region. Regarding the pre-Katangan basement, U-Pb SHRIMP analyses of detrital and xenocrystic zircons revealed the first evidence of a cryptic c. 3.2-3.0 Ga Mesoarchaean terrane, named the Likasi terrane, in the basement of the Copperbelt. It was also discovered that the Lufubu schists, previously thought to be sedimentary in origin, are in fact intermediate metavolcanic rocks. These Lufubu schists, together with granitoids and gneisses from Zambia, yielded U-Pb SHRIMP ages between 2050 Ma and 1850 Ma and are interpreted as being related to the evolution of a large magmatic arc (or several magmatic arcs). These Paleoproterozoic terrains define the Lufubu Metamorphic Complex, which evolved together with the Bangweulu Block, the Ubendian Belt and the Tanzanian craton to collide with the Angola-Kasai craton to form the Kibaran Belt during the 1.4-1.0 Ga Kibaran Orogeny. Unconformably overlying the Lufubu Metamorphic Complex is the Muva Group, which is sedimentary in origin. A maximum U-Pb age of 1941 ± 40 Ma was found for its deposition in the Copperbelt area. Concerning the Katanga Supergroup, U-Pb SHRIMP analyses on detrital zircons showed that the sediments are mainly derived by erosion from the Paleaoproterozoic basement. 40Ar-39Ar analyses of detrital muscovites from the Biano Group, which forms the topmost unit of the Katanga Supergroup, yielded a maximum age of deposition of 573 ± 5. This implies a terminal Neoproterozoic and/or early Palaeozoic age for terminal Katangan deposition, and supports previous models for the deposition of the Biano Group in a foreland basin to the Lufilian Orogen . Finally, U-Pb SHRIMP analyses on monazites and 40Ar-39Ar analyses on biotite, muscovite and K-feldspar yielded ages at c. 590, c. 530, c. 512 and a range between 492 and 450 Ma. These ages correspond respectively to various events during and following the Pan-African Damaran-Lufilian- Zambezi orogeny, formed by collision of the Congo and Kalahari cratons, namely to a tectonic event coinciding with subduction-related eclogite facies metamorphism elsewhere in the Lufilian orogen; to the final stage of collision between the Kalahari and Congo cratons; to a wide-spread regional mineralising event; and finally to post-orogenic uplift and regional cooling.

The St. Lawrence Platform is located along the northern shoreline of Lake Ontario, currently in an intra-cratonic setting exposing relatively flat-lying middle Ordovician sedimentary strata. The purpose of this study is to gain insight on recent brittle deformation events that have deformed the bedrock. Based on structural field observations, broadly trending E-W extensional joints are the youngest stress recorded in the bedrock. These joints are partially filled and sealed with calcite mineralization and were strategically sampled to gain insight on the source and timing of fluid flow. Trace element geochemistry and stable isotope (δ18O and δ13C) analysis on calcite mineralization indicate that their compositions are analogous to the host rock, thus, fluids originate from connate fluids that were released from pore space during deformation. In addition, U-Pb geochronology via LA-ICP-MS methods yield a model age of 101 ± 6 Ma (MSWD: 2.3). The date of calcite crystallization is contemporaneous with the establishment of North America’s modern compressive stress field, and is linked to a Cretaceous tectonic plate reorganization event that was global in scale. This study demonstrates that calcite veins can serve as a tool to date brittle deformation in limestone, which could have direct applications in hydrocarbon exploration, paleohydrology, and the consideration for locating sites to host deep geological repositories.

名古屋大学年代測定総合研究センターシンポジウム報告

The Tatric crystalline unit of the Western Carpathians in northern Slovakia displays an inverted metamorphic sequence where high-grade migmatite and orthogneiss units are overlying lower-grade mica schists. Enclosed within the migmatites are lenses of eclogite-bearing amphibolites. Conventional geothermobarometry coupled with isochemical modeling constrained P-T paths that exhibit contrasting metamorphic histories for rock units that are now heterogeneously interleaved. Relict eclogite facies assemblages with occasionally preserved omphacite record post-peak pressure conditions of 1.7-1.8 GPa followed by near isothermal decompression at ~750 °C leading to intensive re-equilibration of eclogites at high-pressure granulite facies conditions and development of diopside + plagioclase symplectitic textures. New ID-TIMS Sm-Nd dating of garnet separated from the omphacite-bearing eclogite yields a whole rock-garnet isochron age of 337 ± 10 Ma, with an epsilon Nd isotopic composition of +8.3. While major element profiles across the garnets display little variation, the trace element distribution shows a typical HREE enrichment profile and a slight core to rim disparity with LREE and MREE concentrations higher in the cores and higher HREE in the rims. Granulite-facies migmatites that host the eclogite boudins record lower pressure metamorphic conditions of 1.2 GPa at ~750 °C and a similar retrograde path. The lower-grade micaschists reached metamorphic conditions of 0.8 GPa at ~650 °C. Monazite U-Pb analysis from a migmatite surrounding the eclogite boudins yields one population of ca. 380 Ma age. Another migmatite away from the eclogite yields two populations monazite ages. A robust 340 ± 11 Ma monazite U-Pb age is indistinguishable from our garnet age and U-Pb SIMS age of zircons in the anatectic leucosome of the migmatite (347 ± 7 Ma). We interpret the ca. 340 Ma ages to represent the exhumation of the deep crustal root of the Variscan orogen into the middle crust coeval with anatexis. A younger monazite U-Pb age of 300 ± 16 Ma is consistent with 40Ar/39Ar thermochronology data of ca. 310 Ma that is likely indicative of the Late Carboniferous I-type magmatism and cooling in the Tatric block. Cooling rates calculated by garnet diffusion modeling yield estimates of ~30 °/Ma. This exhumation was likely tectonically forced by the action of a rigid indentor which prompted the weak lower crust to be heterogeneously extruded to mid-crustal levels at a time coeval with anatexis and subsequently extruded with mid-crustal material to the upper crust.