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Abstract Newly obtained radiolarian biostratigraphic age combined with previous isotopic age of the Troodos ophiolite shows that the ophiolite becomes systematically younger from east to west: Turonian, early Campanian, and late Campanian. The youngest late Campanian part of the ophiolite is directly covered by the volcaniclastic sediment derived from an active island arc, whereas the older part is covered by pelagic radiolarite. These facts constitute evidence that the Troodos ophiolite was probably emplaced during the subduction of an active spreading ridge. 相似文献
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A compilation of available palaeomagnetic data from the Troodos (Cyprus) and Baër–Bassit (Syria) ophiolitic terranes of the eastern Mediterranean Tethyan orogenic belt is presented. The ophiolites represent fragments of oceanic lithosphere generated at a Neotethyan spreading axis in the Late Cretaceous, although debate continues over the tectonic setting of this spreading axis and its position within the eastern Mediterranean palaeogeography. Two types of model reconstructions have been proposed: Type 1—the ophiolites formed in a southerly Neotethyan basin by spreading above an oceanic subduction zone. The Baër–Bassit ophiolite was then emplaced a relatively short distance (tens of kilometers) southwards on to the Arabian continental margin, leaving the Troodos ophiolite isolated in an intra-oceanic setting to the west; and Type 2—the ophiolites formed in a northerly Neotethyan basin by spreading at a ‘normal’ oceanic ridge, with subsequent large-scale thrusting (hundreds of kilometers) to the south of emplaced ophiolites over microcontinental fragments to reach their present positions. Palaeomagnetic determination of the palaeolatitude of the Neotethyan spreading axis is, therefore, of considerable interest.Previous palaeomagnetic analyses have demonstrated the presence of significant, and in some cases extreme, relative tectonic rotations of a variety of origins in both ophiolites. To allow palaeomagnetic data from these rotated units to be combined, an inclination-only formulation of the palaeomagnetic tilt test is employed. This provides unequivocal evidence that both ophiolites retain pre-deformational remanent magnetizations, which are interpreted as original ocean-floor magnetizations acquired close to the time of crustal formation in the Late Cretaceous. The mean inclinations of 37.0±2.6° for the Troodos terrane and 41.1±3.4° for the Baër–Bassit terrane indicate respective palaeolatitudes for the spreading axes of 20.6°N±1.8° and 23.6°N±2.5°, consistent with a Late Cretaceous position between the Arabian and Eurasian margins. These data, together with a well-defined palaeolatitude of 25.5°N±4.5° for the eastern Pontides previously reported in the literature, provide constraints which must be incorporated in any successful tectonic reconstruction of the eastern Mediterranean Tethys. The implications of these constraints for Type 1 and 2 models are discussed using a series of plate tectonic cross-sections constructed along a line extending northwards from the Arabian continental margin. In the absence of palaeomagnetic data from Late Cretaceous rocks of the eastern Taurides, however, it is presently impossible to use these palaeolatitudinal constraints to resolve the root zone debate on a purely palaeomagnetic basis. Solutions which satisfy the constraints may be found for both types of model reconstruction. Additional, published field-based geological considerations, however, strongly support models in which the Troodos and Baër–Bassit (and other southerly) ophiolites were generated in a southern Neotethyan basin, rather than those involving generation in a northerly basin and subsequent large-scale thrust displacement to the south. 相似文献
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Graham J. Borradaile Mike Stupavsky Dawn-Ann Metsaranta 《Pure and Applied Geophysics》2008,165(7):1411-1433
For remanence-bearing minerals (RBM) such as magnetite-titanomagnetite, susceptibility to induced magnetism (M) measured in alternating fields (H
AC
) is field-dependent. However, for fields ≤ 400 A/m, measured in an AC induction coil instrument (at 19,100 Hz), susceptibility
k
0
= M/H
AC
is sufficiently linear to provide a reproducible rock (or mineral) magnetic characteristic and its anisotropy may be related
to arrangements of minerals in rock, or for single mineral grains to their crystalline or shape anisotropy. For any remanence-bearing
mineral at higher fields k
HF
(= M/H
AC
) is not constant and the term susceptibility is not normally used. This study bridges the responses between traditional low-field susceptibility measurements and those
due to high applied fields, for example when studying hysteresis or saturation magnetization of RBM. Where |k
HF
| is measured in alternating fields that peak significantly above 400 A/m the M(H
AC
) relation is forced to follow a hysteresis loop in which |k
HF
| > k
0
for small |H
AC
| and where |k
HF
| decreases to zero for very large fields that achieve saturation magnetization. Hysteresis nonlinearity is due to remanence
acquired with one field direction requiring a reverse field for its cancellation. We investigate the transition from initial,
traditional “low-field” susceptibility (k
0
) measurements at 60 A/m, through 24 different fields from 400 A/m to 40,000 A/m (for very high k
0
to 80,000 A/m). This reveals M(H
AC
) dependence beyond from conventional k
0
through the range of hysteresis behavior in fields equal to and exceeding that required to achieve saturation magnetization
(M
S
). We show k
HF
increases with peak H
AC
until the peak field is slightly less than saturation magnetization in natural rock samples rich in magnetite (TM0 = Fe3O4) and TM60 (Fe2.4Ti0.6O4). All sample suites predominantly contain multidomain grains with subordinate pseudo-single domain and single-domain grains.
k/k
0
increases by ≤ 5% for fields up to 2 kA/m. Above 4 kA/m k/ k
0
increases steeply and peaks, usually between 24 kA/m and 30 kA/m where all grains magnetic moments are activated by H
AC
since this exceeds the coercive force of most grains. For higher peak H
AC
, k/k
0
declines sharply as increased H
AC
values more effectively flip M with each field-direction switch, leading to the low gradient at distal portions of the hysteresis loop. For M0-TM60 bearing rocks, susceptibility peaks for fields ~12 kA/m and for magnetite rich rocks up to 24 kA/m. These values are approximately
10% of saturation magnetizations (M
S
) reported for the pure minerals from hysteresis DC field measurements. Both the field at peak k/k
0
and the peak k/k
0
value appear to be controlled by the dominant domain structure; multidomain behavior has larger k/k
0
peaks at lower H
AC
. Stacked k/k
0
versus H
AC
curves for each sample suite (n = 12 to n = 39) were successfully characterized at the 95% level by a polynomial fit that requires the cubic form k/k
0 = a + bH + cH
2 + dH
3. Thus, for most M-TM bearing rocks, susceptibility and anisotropy of susceptibility (AMS) measurements made on different
instruments would be sufficiently precise for most geological applications, if peak alternating fields are ≤700 A/m. 相似文献
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Chromite ores formation is still a debated topic and, in this study, detailed analyses of major, minor elements by EMPA and Rare Earth Elements (REE) by SIMS, were performed on silicate inclusions detected in chromite grains of chromitite pods, enveloping dunites and on clinopyroxene of lherzolitic host rock of well known Trodoos chromite ores. Results show the complexity of relationships among lithologies that reflects the subtlety of genetic events and of chromite ore occurrence.Analyses of textural and chemical features have led to a better understanding of the chromite ore genetic process, that is related to a supra-subduction geodynamic setting where partial melting processes were overlapped by metasomatic events.Metasomatism, that marks the general genetic context, is characterized by the presence of a hydrated boninitic melt that favors the precipitation of chromite ores. Chromite ores, typical of ophiolite complexes and usually enveloped by dunite are, in this case, characterized by the presence of two different types of dunites whose geochemical differences are reflected by olivine mineral chemistry and by different REE patterns of analyzed clinopyroxenes. Such geochemical marks, related to different magmatic and metasomatic events, could be a main key for location of fertile or barren dunites in terms of chromite ore occurrence in the field. 相似文献
6.
The lava section in the Troodos ophiolite, Cyprus, is chemically stratified and divided into a shallow lava sequence with low TiO2 content and a deeper lava sequence with high TiO2 content. We calculate the viscosity at magmatic temperature based on major element chemistry of lavas in Cyprus Crustal Study Project (CCSP) Holes CY-1 and 1A. We find that typical shallow low-Ti lavas have a magmatic viscosity that is two to three orders of magnitude lower than that of the deeper high-Ti lavas. This implies that, after eruption on-axis, Troodos low-Ti lavas would have been able to flow down the same slope faster and farther than high-Ti lavas. The calculated lava viscosity increases systematically from the lava-sediment interface to the bottom of the composite Hole CY-1/1A. This suggests that an efficient process of lava segregation by viscosity on the upper flanks of the paleo Troodos rise may have been responsible for the chemical stratification in the Troodos lava pile. Calculated magmatic temperature and molar Mg/(Mg+Fe), or Mg#, decrease systematically down-section, while SiO2 content increases. Correlation of Mg# in the lavas with Mg# in the underlying, lower crustal plutonic rocks sampled by CCSP Hole CY-4 shows that the shallow lavas came from a high-temperature, lower crustal magma reservoir which is now represented by high-Mg# pyroxenite cumulates, while the deeper lavas were erupted from a lower-temperature, mid-crustal reservoir which is now represented by gabbroic cumulates with lower Mg#. 相似文献
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