Seismic velocities, anisotropy, hysteresis and poisson's ratio of ultrahigh pressure (UHP) metamorphic rocks

Elasticity -- Vp/Vs and Poisson's ratio -- Mixture rules -- Seismic anisotropy and shear-wave splitting -- Effect of chemical composition and metamorphic facies on seismic properties of rocks -- The sulu ultrahigh pressure (UHP) metamorphic terrane (China) and the Chinese continental scientific drilling (CCSD) project -- The Sulu utrahigh pressure metamorphic terrane -- The Chinese continental scientific drilling (CCDS) project -- Lithostructural profile of the CCSD-main borehole -- Calibration of crustal seismic structure for the upper crust of the Sulu terrame -- Age data of CCSD core and surface samples -- Garnet peridotite -- Rheology during formation and exhumation of the orogenic deep root -- Subduction/exhumation/uplift history of the Sulu UHP rocks -- Exhumation structure of the Sulu orogenic deep root -- P-wave velocities, anisotropy and hysteresis in ultrahigh-pressure metamorphic rocks as a function of confining pressure -- Samples -- Experimental techniques -- Experimental results and discussion -- Poisson's ratios of crystalline rocks as a function of hydrostatic confining pressure -- Composition and tectonic evelution of the Chinese continental crust constrained by Poisson's ratio -- Causes of Poisson's ratio variations -- Poisson's ratios of continental China -- Correlations between compressional and shear wave velocities and corresponding Poisson's ratios for some common rocks and sulfide ores -- Data for statistic analysis -- Linear relationship between Vp and Vs -- Scaling factor Rs/p -- Correlation of Poisson's ration with Vs, Vp, G and E -- Further work : seismic properties of rocks from Yunkai Mountains-the northern continental margin of the South China Sea

A reassessment of the stress and natural fracture orientations from analysis of image logs in the Chinese Continental Scientific Drilling Program borehole at Donghai county, Jiangsu province, China

The Chinese Continental Scientific Drilling (CCSD) project has drilled a 5100 m deep research borehole in the Sulu ultra-high pressure (UHP) metamorphic belt, eastern China. The UHP metamorphic belt is thought to be a product of continent-continent collision and has consequently experienced very intensive structural deformation. Based on a more complete well log data set of CCSD borehole, we can have a more detailed and reliable study on the structure features of borehole wall and other rock physical properties than a previous study published in 2009. Abundant data related to borehole breakouts (BOs), drilling induced tensile fractures (DITFs) and natural fractures were collected from the image logs. The BO and DITF data indicate that the average direction of the maximum horizontal principal stress (SH) of the CCSD borehole site is about 79.2° which is consistent with the convergent direction (E-W) of the Pacific Sea Plate with respect to the Eurasian Plate. Analysis of DITFs indicated that in the case of the CCSD borehole, axial drilling induced tensile fractures (ADITFs) occur occasionally in the upper section (0–2300 m) of the borehole with low dipping angle (0–10°), while transverse drilling induced tensile fractures (TDITFs) occur significantly in the lower section (3800–5000 m) with high dipping angle (10–30°). The natural fracture distribution at depth in the metamorphic rocks of the CCSD borehole indicates that (1) the failure strength of rocks and borehole depth are two factors that affects natural fracture frequency, (2) most of the dip azimuth of natural fractures is consistent with the dip azimuth of foliations observed in the core, (3) the development of most of the natural fractures probably was dominated by the development of foliations, and both the natural fractures and foliations developed in response to the subduction and exhumation of the Sulu terrene

Timing and kinematics of Mesozoic-Cenozoic mountain building and lithospheric thinning in the eastern North China: Constraints from geochronology and thermochronology

The Sulu UHP rocks and the Jiaobei region exhumed to temperatures below 180 ± 20 °C at 180–160 Ma and 260–160 Ma, respectively. The genesis for the 160 Ma granites requires the participation of external water. These observations can best be explained by the crustal detachment model. Two episodes of exhumation, first at the Early Cretaceous and again at 65–40 Ma, indicate an episodic lithospheric thinning in the eastern North China block

Titanium in phengite: a geobarometer for high temperature eclogites

Phengite chemistry has been investigated in experiments on a natural SiO2-TiO2-saturated greywacke and a natural SiO2-TiO2-Al2SiO5-saturated pelite, at 1.5-8.0GPa and 800-1,050°C. High Ti-contents (0.3-3.7 wt %), Ti-enrichment with temperature, and a strong inverse correlation of Ti-content with pressure are the important features of both experimental series. The changes in composition with pressure result from the Tschermak substitution (Si+R2+=AlIV+AlVI) coupled with the substitution: AlVI+Si=Ti+AlIV. The latter exchange is best described using the end-member Ti-phengite (KMgTi[Si3Al]O10(OH)2, TiP). In the rutile-quartz/coesite saturated experiments, the aluminoceladonite component increases with pressure while the muscovite, paragonite and Ti-phengite components decrease. A thermodynamic model combining data obtained in this and previous experimental studies are derived to use the equilibrium MgCel+Rt=TiP+Cs/Qz as a thermobarometer in felsic and basic rocks. Phengite, rutile and quartz/coesite are common phases in HT-(U)HP metamorphic rocks, and are often preserved from regression by entrapment in zircon or garnet, thus providing an opportunity to determine the T-P conditions of crystallization of these rocks. Two applications on natural examples (Sulu belt and Kokchetav massif) are presented and discussed. This study demonstrates that Ti is a significant constituent of phengites that could have significant effects on phase relationships and melting rates with decreasing P or increasing T in the continental crus

Application of physical properties measurements to lithological prediction and constrained inversion of potential field data, Victoria Property, Sudbury, Canada.

In recent years the number of near-surface deposits has decreased significantly; consequently, exploration companies are transitioning from surface-based exploration to subsurface exploration. Geophysical methods are an important tool to explore below the surface. The physical property data are numerical data derived from geophysical measurements that can be analyzed to extract patterns to illustrate how these measurements vary in different geological units. Having knowledge of links between physical properties and geology is potentially useful to obtain more precise understanding of subsurface geology. Firstly, down-hole density, gamma radioactivity, and magnetic susceptibility measurements in five drillholes at the Victoria property, Sudbury, Ontario were analyzed to identify a meaningful pattern of variations in physical property measurements. The measurements grouped into distinct clusters identified by the fuzzy k-means algorithm, which are termed ‘physical log units’. There was a meaningful spatial and statistical correlation between these physical log units and lithological units (or groups of lithological units), as classified by the geologist. The existence of these relationships suggests that it might be possible to train a classifier to produce an inferred function quantifying this link, which can be used to predict lithological units and physical units based on physical property data. A neural network was trained from the lithological information from one hole, and was applied on a new hole with 64% of the rock types being correctly classified when compared with those logged by geologists. This misclassification can occur as a result of overlap between physical properties of rock types. However, the predictive accuracy in the training process rose to 95% when the network was trained to classify the physical log units (which group together the units with overlapping properties). Secondly, lithological prediction based on down-hole physical property measurements was extended from the borehole to three-dimensional space at the Victoria property. Density and magnetic susceptibility models were produced by geologically constrained inversion of gravity and magnetic field data, and a neural network was trained to predict lithological units from the two physical properties measured in seven holes. Then, the trained network was applied on the 3D distribution of the two physical properties derived from the inversion models to produce a 3D litho-prediction model. The lithologies used were simplified to remove potential ambiguities due to overlap of physical properties. The 3D model obtained was consistent with the geophysical data and resulted in a more holistic understanding of the subsurface lithology. Finally, to extract more information from geophysical logs, the density and gamma-ray response logs were analyzed to detect boundaries between lithological units. A derivative method was successfully applied on the down-hole logs, and picked the boundaries between rock types identified by geologists as well as additional information describing variation of physical properties within and between layers not identified by the geologist.Doctor of Philosophy (PhD) in Mineral Deposits and Precambrian Geolog

Petrology and Geochronology of Metamorphic Zircon

Zircon is unusually well suited for investigating metamorphic processes because it is readily analyzed for U‐Pb ages, it harbors diverse mineral inclusions, and its chemistry can be linked to metamorphic parageneses and P‐T paths. Metamorphic zircon chemistry and ages are relevant only at the sub‐grain micron scale, and consequently many analytical methods, such as depth profiling, have been developed to exploit such spatially resolute infor­mation. Here we review how metamorphic zircon grows, and how its chemistry and inclusion assemblages may be used to link the age of a zircon domain to its metamorphic P‐T condition. Domain‐specific ages and inclusion assemblages from ultrahigh‐pressure (UHP) zircons constrain rates of subduction and exhumation. Textures and chemistry of zircon and garnet from high‐ and ultrahigh temperature (UHT) rocks reveal petrogenetic implications of deep crustal heating, melting, and melt crystallization. Trace elements, inclusion assemblages, and oxygen isotopes in zircon show that dehydration reactions may catalyze zircon growth during subduction. Future research should include identifying natural systems that constrain diffusion rates, determining crystal‐chemical controls on trace element uptake in zircon and garnet for understanding how rare earth budgets and patterns change during metamorphism, and identifying underlying principles that govern the dissolution and reprecipitation of zircon during metamorphism

High field strength element systematics and Lu-Hf & Sm-Nd garnet geochronology of orogenic eclogites

Concerning the Bulk Silicate Earth (BSE), the depleted mantle and the continental crust are thought to balance the budget of refractory and lithophile elements, resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) Niob and Tantal appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion. The two HFSE Zr and Hf on the other hand seem not to be fractionated between the silicate reservoirs. They show more or less chondritic Zr/Hf ratios. In this study a series of orogenic eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question if eclogites could form a hidden reservoir to account for the mass imbalance of the BSE. The results show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show no fractionation of Nb/Ta ratios and no enrichment of Nb compared to e.g. MOR-basalts, the likely precursor of these rocks. With an average Nb/Ta ratio of 14.9 these eclogites could not balance the differences between BSE and chondrite. Additionally, with an average Nb/Ta ≈ MORB they also cannot balance the small differences in the Nb/Ta of the crust and the mantle. LA-ICPMS analyses of rutiles in these eclogites reveal a zonation of Nb/Ta ratios in this mineral, with rutile cores having higher Nb/Ta than rutile rims. As a consequence, Laser Ablation data of rutiles have to be evaluated carefully and cannot necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.Die kontinentale Kruste und der Mantel sind die beiden wichtigsten geochemischen Reservoire der Silikaterde (Bulk Silicate Earth, BSE). Dementsprechend verhalten sie sich bezüglich ihrer Gehalte an refraktären lithophilen Elementen relativ zu Chondrit in der Regel komplementär. Dies gilt z.B. für viele Seltene Erden (REE) und Sr-Nd Isotope (HOFMANN, 1988; HOFMANN, 1997). Die High Field Strength Elemente (HFSE) Zr, Nb, Hf und Ta werden weitgehend ebenfalls als Spurenelemente betrachtet, die in chondritischer Häufigkeit in der Silikaterde vertreten sind und ebensolche Elementverhältnisse aufzeigen. Es zeigt sich allerdings, dass sowohl die Nb/Ta Verhältnisse der kontinentalen Kruste und des verarmten Mantels, als auch die gesamte Silikaterde, subchondritisch sind, ein Phänomen, das als "Nb-Paradox" bekannt ist (BARTH et al., 2000; KAMBER and COLLERSON, 2000; RUDNICK et al., 2000; MÄNKER et al., 2003; RAPP et al., 2003). Nb und Ta sind zwei HFSE mit sehr ähnlichen geochemischen Eigenschaften und wurden lange Zeit als "geochemische Zwillinge" betrachtet, die sich während der Entwicklung des Kruste-Mantel Systems identisch verhalten. Erste Ideen um die Massenbilanz der Silikaterde auszugleichen legen ein superchondritisches Reservoir nahe, welches im Mantel zurückgehalten wird, und somit zu der heutigen Fraktionierung der Nb/Ta Verhältnisse zwischen BSE und dem chondritischen Wert geführt hat. Ein solches Reservoir kann in Form subduzierter ozeanischer Kruste vorliegen, deren Restit als Eklogit in den Mantel absinkt (MCDONOUGH, 1991; RUDNICK et al., 2000). Diese Betrachtungen basieren auf einem chondritischen Nb/Ta Verhältnis von 17.6, welches aus Analysen von Chondriten (Allende und Murchison) und Eukriten (Vesta) hergeleitet wurde (WEYER et al., 2002). MÄNKER et al. (2003) dagegen haben zur Berechnung des chondritischen Nb/Ta einen deutlich größeren Datensatz zugrundegelegt und kommen auf ein höheres Verhältnis von 19.9. Es wurden zudem von diesen Autoren einige kohlige Chondrite der CV-Gruppe (z.B. Allende) aussortiert, da diese eine ungewöhnliche Anreicherung von Nb-verarmten Einschlüssen zeigten. Dieser neue und höhere Wert für das - IV - chondritische Nb/Ta Verhältnis wird in dieser Arbeit verwendet, da er diesen durch den größeren Datensatz besser repräsentiert. ..