Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

Five-Stage Model of the Palaeozoic Crustal Evolution in Xinjiang

The great majority of the Palaeozoic orogenjc belts of Central Asia are of the intercontinental type, whose evolution always follows a five-stage model, i.e. the basal continental crust-extensional transitional crust-oceanic crust-convergent transitional crust-new continental crust model. The stage for the extensional transitional crust is a pretty long, independent and inevitable phase. The dismembering mechanism of the basal continental crust becoming an extensional continental crust is delineated by the simple shear model put forward by Wernike (1981). The continental margins on the sides of a gently dipping detachment zone and moving along it are asymmetric: one side is of the nonmagmatic type and the other of the magmatic type with a typical bimodal volcanic formation. In the latter case, however, they were often confused with island arcs. This paper discusses the five-stage process of the crustal evolution of some typical orogenic belts in Xinjiang.     

A 5000 year record of intertidal peat stratigraphy and sea level change from northwest Alaska

Coastal environments of northwest Alaska preserve a detailed record of sea level change during the past 5000 ¹⁴C yr. Rapid burial of intertidal marshes by storm overwash processes, a short open water period, and the arctic maritime climate contribute to the preservation of marine and eolian facies. Eustatic and storm-controlled changes in sea level can be identified from interbedded sequences of marsh peat and coastal flood deposits on barrier islands and estuaries along northwest Seward Peninsula. Mean eustatic sea level has risen about 1.5 m during the last 5000 years, at an average of ca. 0.028 cm yr⁻¹, based on the depth and age relationship of a suite of 23 peat horizons sampled from a 140 km-long reach of coast. Nearshore erosion and sedimentation are controlled by secular variations in wave climate. Peaks in tidal amplitude and storminess correlate with transgressive sedimentary regimes and occurred during the periods 3300–1700, 1200–900, and since about 400 ¹⁴C yr BP. Short-term fluctuations in relative sea level are superimposed on the long-term regional eustatic trend, and record the coastal response to variable rates of sea-level change during the late Holocene.     

Depositional environments of the Hart coal zone (Paleocene), Willow Bunch Coalfield, southern Saskatchewan, Canada from petrographic, palynological, paleobotanical, mineral and trace element studies

Coal petrology, palynology, paleobotany and mineralogy of the Hart coal indicate deposition under wet, warm-temperate to subtropical climatic conditions in low-lying backswamps with fluvial channels and locally ponded areas. The coal is dominated by mixed xylitic/attrital lithotypes and by huminite macerals with secondary inertinite macerals and minor liptinite macerals. Good correlation exists between lithotypes and maceral composition. Local and vertical variations in proportions of huminites and inertinites reflect frequent fluctuations in water levels, periodic flooding, desiccation and burning of the peat. Swamps were dominated by Glyptostrobus-Taxodium forest with Betula-Myrica-Alnus communities and, locally, Laevigatosporites, which are the dominant contributors to the xylite-rich lithotypes. Attrital lithotypes with abundant Pandanus, Typha and Azolla are consistent with wetter areas of a fluvial environment, including ponds and channels. Trace elements Cr, Cu, Mo, Ni, Si, Ti, U, Se, V, W, K and Th, typically associated with synergetic minerals kaolinite, calcite and quartz, may have a volcanic source. High concentrations of Na, Ba and Ca found in organic complexes are of secondary origin and probably originate in deep source brines rather than marine surface waters.     

Strontium-calcium ratios in Cenozoic planktonic foraminifera

The $\text{Sr}\text{Ca}$ ratio and other parameters have been measured in fossil planktonic foraminifera from the Atlantic and Pacific Ocean basins in order to evaluate the $\text{Sr}\text{Ca}$ ratio of seawater during the last 75 million years. Results on well-preserved samples indicate that the ratio has increased to its present value by 10–15% during the Cenozoic, and that minima occurred between 55-45 Ma and 10-5 Ma, when the ratio was 15–25% less than at present. The long-term increase may reflect either decreasing deposition of aragonite with a high $\text{Sr}\text{Ca}$ ratio in shallow seas, or decreasing seafloor spreading rates and consequently decreasing hydrothermal supply of Ca during the Cenozoic. Other geologic evidence suggests that the Eocene minimum (near 50 Ma) may have resulted from increased aragonite sedimentation, while the Late Miocene minimum (between 10-5 Ma) may have been caused by an increased rate of seawater-basalt exchange when seafloor spreading rates increased on the East Pacific Rise near 10 Ma.     

Bank conditions and erosion along selected reservoirs

This analysis was done as the preliminary step in an ongoing study of reservoir bank erosion processes that are active in the northern U.S. The objectives of this analysis were to observe and document bank characteristics, conditions, and changes along reservoirs with eroding banks, to estimate the amounts of historical bank recession and to discuss its possible causes. Aerial photographs were used to observe the historical bank changes and estimate bank recession. Site reconnaissance, discussions with on-site personnel, and published reports were used to evaluate possible relationships between the local erosion and bank conditions. As part of this analysis linear regressions were done to determine if the estimated recession rates correlate with selected bank and climatic conditions and with physical characteristics of the reservoirs. The regression results, however, were generally not useful because they suggested relationships that are contrary to field observations and published results. Dominant bank erosion processes were wind-wave erosion, capillary wave erosion during high-water periods, groundwater-induced sliding, freeze-thaw processes, rain splash and rainwash, and boat waves. However, because of the complexity of the interrelationships of these and many other bank erosion processes and the variability of the processes at and between sites, it would be necessary to make site-specific measurements and observations year-round to evaluate the processes that are active along a particular bank.     

Mesozoic and cenozoic sandstone-hosted copper deposits in south China

Mesozoic and Cenozoic sandstone-hosted copper deposits in South China are confined to the variegated rock formation of the faulted basins, the intermontane basins and piedmont basins around a copper-bearing old land. The variegated rock formation is generally of fluviolacustrine delta facies, shore facies and shore-cauce facies. The ore bodies are stratiform, occurring in the light-coloured beds of the variegated rock formation. The mineralization comprises chalcocite, bornite, chalcopyrite, native copper, covellite, pyrite and hematite, showing obvious zonation. The associated profitable elements in the ore are mainly silver, lead, zinc, molybdenum, selenium and uranium. The copper deposits are believed to have been formed through three processes of enrichment, i.e. weathering (copper-bearing weathered crust was formed as a result), deposition (source bed formed) and diagenesis (ore body formed). The variegated rocks with Cu, Ag, Pb, Zn, U, Mo, Cl and organic carbon anormalies and a great deal of pyrite, distributed in the faulted basins around the Cu-bearing old land, are recognized as the most favourable area for copper ore exploration.     

Bizarre geochemistry of komatiites from the Crixás greenstone belt,Brazil

Komatiite lava flows in the Crixás greenstone belt, Goiás, Brazil, have textures and volcanic structures typical of Archean komatiites, but are geochemically most unusual. The flows are porphyritic and massive, or layered with spinifex upper parts and olivine cumulate lower parts. MgO contents range from 18 to 40%. In such lavas, only olivine (and minor chromite) can have crystallized, but neither major nor trace elements fall on olivine control lines. In MgO variation diagrams, CaO and Sr fall on lines with slopes steeper than olivine control lines; SiO₂, FeO, Na₂O, K₂O and Y show little systematic variation; Zr shows a large variation that does not correlate with MgO; and Al₂O₃ decreases markedly with decreasing MgO. The aberrant behaviour is highlighted by the REE (rare earth elements) in spinifex and olivine cumulate layers from three flows: in the spinifex layers, chondrite-normalized REE patterns are hump-shaped with maxima at Nd or Sm ((La/Sm)_N=0.6, (Gd/Yb)_N=1.6–2.3), whereas cumulate zones in the same flows have steadily sloping patterns, with LREE enriched relative to HREE ((La/Sm)_N=1.3, (Gd/Yb)_N=1.4). Neither normal magmatic processes acting during emplacement of the komatiites, nor thermal erosion and wall-rock assimilation can explain these effects, and we speculate that elements commonly thought of as “immobile” (e.g. Al, Zr, REE) migrated during hydrothermal alteration or metamorphism. A Pb-Pb whole rock isochron gave an age of 2,728±140 Ma and selected Sm-Nd analyses an apparent isochron age of 2,825±98 Ma (ɛ_Nd≈0). The Pb-Pb age is believed to be the approximate time of emplacement. Interpretation of the Sm-Nd data is complicated by the evidence of mobility of REE.     

泥盆纪最早期牙形刺带化石在蒙古南戈壁幕使盖(Mushgai)地区尕屋(Gavuu)段的首次发现

在蒙古南戈壁的Bayan—Khoshuu Ruins剖面原归入到志留系Mandalovoo组尕屋(Gavuu)段的地层中发现了牙形刺Caudicriodus woschimidti woschmidti,这是泥盆纪最早期牙形刺带化石在蒙古的首次发现。尕屋(Gavuu)段的下部属志留系,但其上部,即采集牙形刺样品M-9～M-12的层位肯定属下泥盆统(下洛霍考夫阶),而不是下志留统。Mandalovoo组应当改为Mandalovoo群,它的三个段应提升为三个组。