Petrogenesis of Migmatites in Maine, USA: Possible Source of Peraluminous Leucogranite in Plutons?   总被引：4，自引：4，他引：4  

SOLAR  GARY S.; BROWN  MICHAEL 《Journal of Petrology》2001,42(4):789-823

In Maine, Siluro-Devonian turbidites were metamorphosed underhigh-T–low-P facies series conditions during deformationwithin a Devonian crustal-scale shear zone system, defined bykilometer-scale straight belts of apparent flattening strainthat anastomose around lozenges of apparent constrictional strain.At upper amphibolite facies grade, metapelites are partiallymelted, the onset of which is recorded by a migmatite front.The resulting migmatites are stromatic or heterogeneous, andsmaller-volume granites form sheets or cylinders according tothe structural zone in which they occur, suggesting that migmatitesand granites record syntectonic melt flow through the deformingcrust. Common leucogranite of the nearby coeval Phillips pluton,which was emplaced syntectonically, was sourced from crustalrocks with geochemical characteristics similar to those of thehost Siluro-Devonian succession. Migmatites have melt-depletedcompositions relative to metapelites. Leucosomes are peraluminousand represent the cumulate products of fractional crystallizationand variable loss of evolved fractionated liquid. Among theheterogeneous migmatites are schlieric granites, the geochemistryof which suggests melt accumulation before fractional crystallizationand loss of the evolved liquid. Smaller-volume granites areperaluminous with a range of chemistries that reflect variableentrainment of residual plagioclase and biotite, accumulationof products of fractional crystallization and loss of most ofthe evolved liquid. Common leucogranite of the Phillips plutonand larger granites in the migmatites have compositions thatsuggest crystallization of evolved liquids derived by fractionalcrystallization of primary muscovite dehydration melts. We inferthat the leucogranite represents the crystallized fugitive liquidfrom a migmatite source similar to that exposed nearby. Watertransported through the shear zone system dissolved in meltwas exsolved at the wet solidus to cause retrogression in sub-solidusrocks and retrograde muscovite growth in migmatites. KEY WORDS: anatexis of pelite; Maine; migmatite; peraluminous granite; plutons  相似文献   

A Re-Evaluation of Sulfur Budgets, Lifetimes, and Scavenging Ratios for Eastern North America   总被引：1，自引：0，他引：1  

GARY S. WOJCIK  JULIUS S. CHANG 《Journal of Atmospheric Chemistry》1997,26(2):109-145

Lifetimes, scavenging ratios, andbudgets describe the cycling of atmosphericconstituents and are often used in formulating airpollution control strategies. Most previous studiesof sulfur lifetimes, budgets, and scavenging ratioshave been based on limited observational data or datafrom highly simplified models. The Regional AcidDeposition Model (RADM2.61) shows some skill inpredicting atmospheric mixing ratios of acidicmaterials and other related trace constituents andacid deposition patterns in North America, and so,analysis of its established, theoretical, databaseserves as a counterpoint to previous studies of sulfurbudgets, lifetimes, and scavenging ratios. The annualbudget shows that the net transport (outflow minusinflow) of sulfur compounds out of eastern NorthAmerica is equal to the total deposition within thedomain. Of the total deposition, 63% is from wetdeposition and 37% is from dry deposition. Theannual average lifetime of sulfur dioxide (38 hours),estimated by the turnover time, is limited by aqueousconversion, while that for sulfate aerosols (54 hours)is limited by their removal in precipitation. Theannual average lifetime of sulfur in this domain isslightly more than three days. Episodic lifetimes andbudgets, based on particular synoptic situations, showlarge variations around the annual values. Episodicprecipitation scavenging ratios exhibit similarvariability and are used to offer explanations ofseveral potential biases found in the wet sulfurdeposition amounts as predicted by the EMEP sulfurtransport model and other published results.  相似文献   

Floodplain sedimentology of the Squamish River, British Columbia: relevance of element analysis   总被引：2，自引：0，他引：2  

GARY J. BRIERLEY 《Sedimentology》1991,38(4):735-750

Element analysis of modern-day floodplains provides a framework for characterizing associations amongst depositional forms, the processes responsible for them and their local depositional environment. From interpretation of the spatial association of elements, mechanisms of floodplain evolution can be analysed. The Squamish River, in southwestern British Columbia, is a high-energy, gravel-based river, which exhibits a distinct downstream gradation in channel planform type. The floodplain sedimentology of this river is evaluated using an element approach. Five elements, defined on the basis of their morphological outline, position within sediment sequences and sedimentological character, describe the floodplain sedimentology: (i) top-stratum, (ii) chute channel; (iii) ridge; (iv) bar platform; (v) basal channel gravels. The sedimentological composition of each element is described. Each of these units relates directly to morphostratigraphic units which make up contemporary bars of the Squamish River. Associations among facies defined at the bedform scale, morphostratigraphic units on bar surfaces and elemental floodplain features are described and explained. The vertical stacking arrangement of elements is analysed in trenches (dug perpendicular to the main channel) and in bank exposures. Two elemental sedimentology models are proposed. In the first model, bar platform sands are discontinuous above basal channel gravels. Chute channel, ridge and proximal topstratum elements form thick sequences above. The second model is characterized by sequences in which distal top-stratum deposits are observed. In these instances, bar platform sands are better preserved beneath the distal top-stratum element, with proximal top-stratum elements above. The applicability of these models is determined primarily by position on the floodplain. Chute channel reworking of floodplain sediments and replacement by top-stratum elements is the dominant process marginal to contemporary bars. Sites in which channel avulsion has resulted in preservation of distal top-stratum deposits in the midsequence of the present-day channel banks determine the occurrence of the second model. Although channel planform style changes down-valley in the study reach from braided to meandering, these two models apply in each reach. It is concluded that processes operative at the element scale, rather than the channel planform scale, determine floodplain sedimentology.  相似文献   

Amalgamated accumulations resulting from climatic and eustatic changes, Akchar Erg, Mauritania   总被引：2，自引：0，他引：2  

GARY KOCUREK  KAREN G. HAVHOLM  MAX DEYNOUX  RONALD C. BLAKEY 《Sedimentology》1991,38(4):751-772

The Akchar Erg of the Sahara of western Mauritania shows a morphology and stratigraphy that can be recognized as the amalgamation of late Pleistocene and Holocene deposits that reflect eustatic and climatic events. Mapping, trenching, and dating by ¹⁴C methods and artefacts show that the prominent complex linear dunes (draas) of the Akchar Erg are actually composite features showing at least three constructional and two destructional phases. The constructional phases are represented by three convex-up layers: (i) a modern veneer moulded into superimposed crescentic dunes, which partially mantle the larger linear bedforms; (ii) a middle, partly root-turbated sand deposited sometime during the last 4000 years; and (iii) a core of linear dune sand formed during the last glacial period (13 000–20 000 yr BP), which today shows relict relief, intense root-turbation, and pedogenesis. These constructional phases are separated by super bounding surfaces that coincide with erg destructional phases. Surface 2 bounds the middle aeolian sand, and is marked by a lag surface of small granules. Surface 1 is a very prominent surface with an abundance of Neolithic artefacts, and represents stabilization of the linear dunes during the humid, interglacial period (4000–11 000 yr BP). Interdraa deposits originated during the interglacial period, and consist of continental lacustrine limestones and sandstones, humic sands deposited in marshes, and sabkhas on the coast. The sabkhas originated during interglacial highstand of sea-level when interdraa areas were marine embayments, and subsequently dried during regression. The draa and interdraa sequences, therefore, in spite of being adjacent facies, actually represent different events and were not formed simultaneously. The upwind sand-sheet margin of the Akchar Erg shows exposures of the middle and core aeolian sands (which were previously protected from deflation by vegetation) being progressively cannibilized in the current phase of erg construction, and revealing a crystalline basement rock. In this proximal area, conditions are not favourable for the incorporation of these aeolian accumulations into the stratigraphic record.  相似文献   

Communicating uncertainty in spatial databases   总被引：1，自引：0，他引：1  

GARY J HUNTER  MICHAEL F GOODCHILD 《Transactions in GIS》1996,1(1):13-24

As applications of spatial databases become more sophisticated and diverse, an issue of growing concern is the inability to communicate to users the uncertainty of products derived from their systems. This is important not only because users should be able to compare the quality of database outputs against the quality requirements of their tasks (in order to determine product suitability), but also to protect the integrity of past, present, and future decisions that may be made through the use of such information. While there has already been considerable research undertaken to develop models of spatial data error and uncertainty, there is an additional requirement for the results of these models to be effectively conveyed to users. This paper reviews advances now being made in communicating the uncertainty of spatial databases which may soon assist users in overcoming this critical barrier.  相似文献   

The role of mylonites in the uplift of an oblique lower crustal section, East Antarctica   总被引：1，自引：0，他引：1  

G. T. NICHOLS 《Journal of Metamorphic Geology》1995,13(2):223-238

Abstract Three generations of mylonites discovered in the northern Prince Charles Mountains (nPCM) are associated with episodes of crustal thickening and thinning. First-generation mylonites (MY1) are shallow thrusts which pre-date both folding and peak metamorphic conditions, and formed during early crustal thickening. Second-generation mylonites (MY2) are significant subvertical normal faults that formed at conditions of c. 5 kbar and 700° C, and throughout the nPCM consistently display NW-block uplift. It is argued that MY2 uplift was rapid in the north-west, produced exhumation of approximately 6–7 km, and caused re-equilibration of most nPCM assemblages at lower pressures. It is suggested that features of this terrane may be reconciled with a tectonic model involving simultaneous crustal thickening and lithospheric thinning; MY2 uplift may reflect isostatically induced uplift. In contrast, the adjacent east Lambert Glacier Region (eLGR) was unaffected by MY2 uplift and remained at lower crustal levels. P-T trajectories across this oblique terrane thus reflect a gradual transition in uplift rates: nPCM paths preserve mostly cooling after partial MY2 exhumation, while those in the eLGR are dominated by slower uplift which facilitated the retrograde growth of coronas and symplectites at amphibolite facies conditions. Amphibolite facies third-generation mylonites, MY3, post-date the preserved P-T segments and are low-angle normal faults which indicate consistent easterly transport across the entire terrane. It is proposed that they are related to tectonic collapse.  相似文献   

SNOW COVER CHARACTERIZATION USING MULTIBAND FMCW RADARS     

GARY KOH  NORBERT E. YANKIELUN  ANA I. BAPTISTA 《水文研究》1996,10(12):1609-1617

The use of radars to characterize the physical properties of a snow cover offers an attractive alternative to manual snow pit measurements. Radar techniques are non-invasive and have the potential to cover large areas of a snow-covered terrain. A promising radar technique for snow cover studies is the frequency modulated continuous wave (FMCW) radar. The use of a multiband radar approach for snow cover studies was investigated in order to fully exploit the capabilities of FMCW radars. FMCW radars operating at and near the C-, X- and K_a-bands were used to obtain radar profiles over a wide range of snow cover conditions. These frequency-dependent radar signatures were used to identify important snow cover features such as ice and depth hoar layers. Snow grain size information was also obtained from the frequency-dependent scattering losses that were observed in the snow cover. Several case studies of FMCW radar profiles are presented in order to demonstrate the advantages of a multiband radar approach for monitoring the spatial and temporal variability of snow cover properties and/or processes over an extended area.  相似文献   

An empirical model of aeolian dune lee-face airflow   总被引：12，自引：0，他引：12  

M. L. SWEET  GARY KOCUREK 《Sedimentology》1990,37(6):1023-1038

Airflow data, gathered over dunes ranging from 60-m tall complex-crescentic dunes to 2-m tall simplecrescentic dunes, were used to develop an empirical model of dune lee-face airflow for straight-crested dunes. The nature of lee-face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee-face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low-speed (0–3O% of crestal speed) back-eddy flow, typically occur leeward of steep-sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment. As each of the three types of lee-face flow is defined by a range of wind speeds and directions, the nature of lee-face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross-strata.  相似文献   

Late Younger Dryas and early Holocene palaeoenvironments in the Skagerrak,eastern North Atlantic: a multiproxy study     

DORTHE RENG ERBS‐HANSEN  KAREN LUISE KNUDSEN  ANTHONY CAVEDO GARY  EYSTEIN JANSEN  RICHARD GYLLENCREUTZ  VINCENT SCAO  KURT LAMBECK 《Boreas: An International Journal of Quaternary Research》2011,40(4):660-680

Erbs‐Hansen, D. R., Knudsen, K. L., Gary, A. C., Jansen, E., Gyllencreutz, R., Scao, V. & Lambeck, K. 2011: Late Younger Dryas and early Holocene palaeoenvironments in the Skagerrak, eastern North Atlantic: a multiproxy study. Boreas, 10.1111/j.1502‐3885.2011.00205.x. ISSN 0300‐9843 A high‐resolution study of palaeoenvironmental changes through the late Younger Dryas and early Holocene in the Skagerrak, the eastern North Atlantic, is based on multiproxy analyses of core MD99‐2286 combined with palaeowater depth modelling for the area. The late Younger Dryas was characterized by a cold ice‐distal benthic foraminiferal fauna. After the transition to the Preboreal (c. 11 650 cal. a BP) this fauna was replaced by a Cassidulina neoteretis‐dominated fauna, indicating the influence of chilled Atlantic Water at the sea floor. Persisting relatively cold bottom‐water conditions until c. 10 300 cal. a BP are presumably a result of an outflow of glacial meltwater from the Baltic area across south‐central Sweden, which led to a strong stratification of the water column at MD99‐2286, as also indicated by C. neoteretis. A short‐term peak in the C/N ratio at c. 10 200 cal. a BP is suggested to indicate input of terrestrial material, which may represent the drainage of an ice‐dammed lake in southern Norway, the Glomma event. After the last drainage route across south‐central Sweden closed, c. 10 300 cal. a BP, the meltwater influence diminished, and the Skagerrak resembled a fjord with a stable inflow of waters from the North Atlantic through the Norwegian Trench and a gradual increase in boreal species. Full interglacial conditions were established at the sea floor from c. 9250 cal. a BP. Subsequent warm stable conditions were interrupted by a short‐term cooling around 8300–8200 cal. a BP, representing the 8.2 ka event.  相似文献   

Along-strike variations in foreland basin evolution: possible evidence for continental collision along an irregular margin     

GARY G. Lash 《Basin Research》1988,1(2):71-83

Abstract The evolution of a passive margin to a foreland basin is generally assumed to entail early load-induced up warping of the stable continental platform followed by foreland subsidence. This relatively straightforward elastic response of the continental platform, however, may be complicated if the colliding passive margin is irregular in outline. In a tectonic scenario in which an irregular margin is migrating toward a trench (A-subduction), those areas of the margin which project seaward, the continental promontories, would be the first to 'feel' the approaching thrust terrane by flexing upward and eroding to form shelf unconformities. Those parts of the continental margin that are convex to the craton, the continental re-entrants, however, would remain subsiding depocentres unaffected by load-induced uplift at the promontories. Careful analysis of the geographic distribution of shelf unconformities in orogenic belts, then, may help to reveal the pre-deformation morphology of the passive continental margin. An example of this may be found in the early phases of Ordovician foreland basin development in the central Appalachian orogen. Here, the shelf unconformities are most pronounced (greatest erosional relief) at the inferred Virginia and New York continental promontories. An adjacent inferred continental re-entrant, the Pennsylvania re-entrant, is characterized by an uninterrupted Ordovician sequence suggesting that the area of the proto-North American platform, represented by this segment of the orogen, remained a depocentre during uplift in adjacent areas of the continental margin.  相似文献