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661.
Marco Maffione Stefano Pucci Leonardo Sagnotti Fabio Speranza 《International Journal of Earth Sciences》2012,101(3):849-861
Anisotropy of magnetic susceptibility (AMS) represents a valuable proxy able to detect subtle strain effects in very weakly
deformed sediments. In compressive tectonic settings, the magnetic lineation is commonly parallel to fold axes, thrust faults,
and local bedding strike, while in extensional regimes, it is perpendicular to normal faults and parallel to bedding dip directions.
The Altotiberina Fault (ATF) in the northern Apennines (Italy) is a Plio-Quaternary NNW–SSE low-angle normal fault; the sedimentary
basin (Tiber basin) at its hanging-wall is infilled with a syn-tectonic, sandy-clayey continental succession. We measured
the AMS of apparently undeformed sandy clays sampled at 12 sites within the Tiber basin. The anisotropy parameters suggest
that a primary sedimentary fabric has been overprinted by an incipient tectonic fabric. The magnetic lineation is well developed
at all sites, and at the sites from the western sector of the basin it is oriented sub-perpendicular to the trend of the ATF,
suggesting that it may be related to extensional strain. Conversely, the magnetic lineation of the sites from the eastern
sector has a prevailing N–S direction. The occurrence of triaxial to prolate AMS ellipsoids and sub-horizontal magnetic lineations
suggests that a maximum horizontal shortening along an E–W direction occurred at these sites. The presence of compressive
AMS features at the hanging-wall of the ATF can be explained by the presence of gently N–S-trending local folds (hardly visible
in the field) formed by either passive accommodation above an undulated fault plane, or rollover mechanism along antithetic
faults. The long-lasting debate on the extensional versus compressive Plio-Quaternary tectonics of the Apennines orogenic
belt should now be revised taking into account the importance of compressive structures related to local effects. 相似文献
662.
José?F.?MolinaEmail author Stefano?Poli H?kon?Austrheim Johannes?Glodny Anatolij?Rusin 《Contributions to Mineralogy and Petrology》2004,147(4):484-504
Metasomatic amphibole-eclogite sequences grew in selvages of quartz veins from the Marun-Keu complex (Polar Urals, Russia) during high-pressure metamorphism. Relicts of a pre-metasomatic eclogite-facies assemblage are present in the wallrock layers as irregular patches. Wallrock interstitial quartz trails lying at a high angle to reaction fronts provide evidence for grain-scale pore channelisation which may be produced by intergranular-fluid compositional gradients parallel to the quartz trails. Disequilibrium at vein-wallrock scale is inferred from wallrock mineral heterogeneity and from variable initial Sr isotope ratios in mineral separates. Mass-balance calculations between relicts and wallrock assemblages reveal chemical imbalances caused by open system-behaviour with two way mass-transfer. The vein-wallrock system registers a prograde history from 408–434 °C (relicts) to 526–668 °C (vein precipitates). Vein and metasomatic assemblages formed during a single fluid-rock interaction process, implying high heating rates (100 °C/Ma), which could result from heat advection by large-scale fluid circulation.Editorial responsibility: W. Schreyer 相似文献
663.
Andrea De Stefano Nathalie Lefebvre Maya Kopylova 《Contributions to Mineralogy and Petrology》2006,151(2):158-173
A suite of 80 macrodiamonds recovered from volcaniclastic breccia of Wawa (southern Ontario) was characterized on the basis
of morphology, nitrogen content and aggregation, cathodoluminescence (CL), and mineral inclusions. The host calc-alkaline
lamprophyric breccias were emplaced at 2.68–2.74 Ga, contemporaneously with voluminous bimodal volcanism of the Michipicoten
greenstone belt. The studied suite of diamonds differs from the vast majority of diamond suites found worldwide. First, the
suite is hosted by calc-alkaline lamprophyric volcanics rather than by kimberlite or lamproite. Second, the host volcanic
rock is amongst the oldest known diamondiferous rocks on Earth, and has experienced regional metamorphism and deformation.
Finally, most diamonds show yellow-orange-red CL and contain mineral inclusions not in equilibrium with each other or their
host diamond. The majority of the diamonds in the Wawa suite are colorless, weakly resorbed, octahedral single crystals and
aggregates. The diamonds contain 0–740 ppm N and show two modes of N aggregation at 0–30 and 60–95% B-centers suggesting mantle
storage at 1,100–1,170°C. Cathodoluminescence and FTIR spectroscopy shows that emission peaks present in orange CL stones
do not likely result from irradiation or single substitutional N, in contrast to other diamonds with red CL. The diamonds
contain primary inclusions of olivine (Fo92 and Fo89), omphacite, orthopyroxene (En93), pentlandite, albite, and An-rich plagioclase. These peridotitic and eclogitic minerals are commonly found within single
diamonds in a mixed paragenesis which also combines shallow and deep phases. This apparent disequilibrium can be explained
by effective small-scale mixing of subducted oceanic crust and mantle rocks in fast “cold” plumes ascending from the top of
the slabs in convergent margins. Alternatively, the diamonds could have formed in the pre-2.7–2.9 Ga cratonic mantle and experienced
subsequent alteration of syngenetic inclusions related to host magmatism and ensuing metamorphism. Neither orogenic nor cratonic
model of the diamond origin fully explains all of the observed characteristics of the diamonds and their host rocks.
Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
664.
Anastassia Yu. Borisova Rémi Freydier Mireille Polvé Stefano Salvi Frederic Candaudap Thierry Aigouy 《Geostandards and Geoanalytical Research》2008,32(2):209-229
Microscopic melt inclusions found in magmatic minerals are undoubtedly one of the most important sources of information on the chemical composition of melts. This paper reports on the successful application of near-infrared (NIR) femtosecond laser ablation (LA) - inductively coupled plasma-mass spectrometry to in situ determination of incompatible trace elements (Li, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Ta, Th, U) and ore metals (As, Mo, Pb) in individual melt inclusions hosted in quartz from the Mount Pinatubo dacites, Philippines. The determined elements cover a concentration range of five orders of magnitude. Femtosecond LA-ICP-MS analyses of twenty-eight individual melt inclusions demonstrate the efficiency of the microanalytical technique and suggests a spectacular homogeneity of the entrapped melt, at least with respect to the following incompatible trace elements: Rb, Sr, Nb, Cs, Ba, La, Ce, Pr, Nd, Pb, Th. The analytical precision (1s) for Na, Ca, Rb, Sr, Y, Nb, Ba and LREE ranged from 3 to 20%. Comparison of trace element concentrations in Mt. Pinatubo melt inclusions determined by femtosecond LA-ICP-MS with those of melt inclusions previously analysed by secondary ion mass spectrometry analysis (SIMS) and those of matrix glasses previously determined by nanosecond LA-ICP-MS showed an agreement typically within 30–40%. The homogeneity of trace element concentrations of the Mt. Pinatubo melt inclusions and the matrix glasses is consistent with the melt inclusion origin as homogeneous rhyolitic melt that was trapped in quartz phenocrysts at the final crystallisation stages of the host adakite (dacite) magma. 相似文献