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Christopher J.Spencer Peter A.Cawood Chris J.Hawkesworth Anthony R.Prave Nick M.W.Roberts Matthew S.A.Horstwood Martin J.Whitehouse EIMF 《地学前缘(英文版)》2015,6(3):357-372
Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of 'peaks' and 'troughs'.The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events,or preferential preservation of continental crust during global collisional orogenesis.The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks.Here these two end-member models(episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis) are assessed using U-Pb,Hf,and O analysis of detrital zircons from sedimentary successions deposited during the ~ 1.3-1.1Ca accretionary.~1.1-0.9 Ga collisional,and 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland.The pre-collisional,accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology.Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from~ 1500 to 1100 Ma.However,post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma,similar to that observed in modern North American river sediments.Zircons within the post-orogenic sedimentary successions have progressively lower εHf and higherδ~(18)O values from ~ 1800 to ~ 1200 Ma whereupon they have higher εHf and δ~(18) within the dominant1085-985 Ma age peak.Furthermore,the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material.The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Crenville Orogeny,which is a typical collisional orogenic belt.The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision.This temporal change in provenance highlights that the source(s),from which detrital zircons within syn-orogenic strata were derived,was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle.This may reflect some combination of tectonic burial,erosion,or possibly recycling into the mantle by tectonic erosion of the source(s).During continental collision,the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins.This tectonic isolation combined with sedimentaiy recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments.These results imply that zircon age peaks,which developed in conjunction with supercontinents,are the product of selective crustal preservation resulting from collisional orogenesis. 相似文献
914.
J.M. Hamblock C.L. Andronicos K.C. Miller C.G. Barnes M-H. Ren M.G. Averill E.Y. Anthony 《Tectonophysics》2007,442(1-4):14-48
A complete understanding of the processes of crustal growth and recycling in the earth remains elusive, in part because data on rock composition at depth is scarce. Seismic velocities can provide additional information about lithospheric composition and structure, however, the relationship between velocity and rock type is not unique. The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from reflection and refraction data in the area, offers an opportunity to place constraints on the composition of the crust and upper mantle from the surface to depths of 60 km. In this work, we calculate seismic velocities of crustal and mantle xenoliths using modal mineralogy, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are then combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle. Lower crustal xenoliths include garnet ± sillimanite granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust–mantle boundary is not present beneath the southern Rio Grande rift. Abundant mid-crustal felsic to mafic igneous xenoliths, however, suggest that plutonic rocks are common in the middle crust and were intraplated rather than underplated during the Cenozoic. Calculated velocities for garnet granulite are between 6.9 and 8.0 km/s, depending on garnet content. Granulites are strongly foliated and lineated and should be seismically anisotropic. These results suggest that velocities > 7.0 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion. Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with refraction and reflection studies that report Pn velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low Pn velocities compared to average uppermost mantle are the result of relatively high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed Pn velocities. 相似文献
915.
Geoffrey M. Morrison Sonia Yeh Anthony R. Eggert Christopher Yang James H. Nelson Jeffery B. Greenblatt Raphael Isaac Mark Z. Jacobson Josiah Johnston Daniel M. Kammen Ana Mileva Jack Moore David Roland-Holst Max Wei John P. Weyant James H. Williams Ray Williams Christina B. Zapata 《Climatic change》2015,131(4):545-557
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917.
Chris Pearson Tanya Lim Chris North George Bendo Luca Conversi Darren Dowell Matt Griffin Terry Jin Nicolas Laporte Andreas Papageorgiou Bernhard Schulz Dave Shupe Anthony J. Smith Kevin Xu 《Experimental Astronomy》2014,37(2):175-194
The different algorithms appropriate for point source photometry on data from the SPIRE instrument on-board the Herschel Space Observatory, within the Herschel Interactive Processing Environment (HIPE) are compared. Point source photometry of a large ensemble of standard calibration stars and dark sky observations is carried out using the 4 major methods within HIPE: SUSSEXtractor, DAOphot, the SPIRE Timeline Fitter and simple Aperture Photometry. Colour corrections and effective beam areas as a function of the assumed source spectral index are also included to produce a large number of photometric measurements per individual target, in each of the 3 SPIRE bands (250, 350, 500μm), to examine both the accuracy and repeatability of each of the 4 algorithms. It is concluded that for flux densities down to the level of 30mJy that the SPIRE Timeline Fitter is the method of choice. However, at least in the 250 and 350μm bands, all 4 methods provide photometric repeatability better than a few percent down to at approximately 100mJy. The DAOphot method appears in many cases to have a systematic offset of ~8 % in all SPIRE bands which may be indicative of a sub-optimal aperture correction. In general, aperture photometry is the least reliable method, i.e. largest scatter between observations, especially in the longest wavelength band. At the faintest fluxes, <30mJy, SUSSEXtractor or DAOphot provide a better alternative to the Timeline Fitter. 相似文献
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