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Sevim Polat 《Marine Ecology》2002,23(2):115-126
Abstract. The monthly changes in chlorophyll a , phytoplankton abundance and nutrient concentrations at two stations, one at the inshore and the other at the deep waters of the northern part of İskenderun Bay, were investigated between 1994 – 1995. The vertical distribution of nutrients and phytoplankton biomass were also studied at the deep station. The concentrations of NO3 +NO2 -N, PO4 -P and SiO4 -Si of surface water at both stations were 0.31 – 1.63 µg-at · l-1 , 0.08 – 0.60 µg-at · l-1 and 0.50 – 2.7 µg-at · l-1 , respectively. The highest concentrations were measured at the inshore station and clear differences were found between the inshore and deep-water stations. Chlorophyll a concentrations ranged from 0.17 to 2.78 µg · l-1 and the highest value was measured in March. At the inshore station, which was affected by land run-off, phytoplankton abundance reached the highest value (21,308 cells · l-1 ) in October 1995, with a marked dominance of Pseudonitzschia pungens (20,200 cells · l-1 ). The nutrient and chlorophyll a concentrations at the inshore station were higher than those at the deep station. One reason for this is the land-based nutrient input into the coastal area here. In spite of these effects, the bay is not eutrophicated because of circulation events in the northeastern Mediterranean. 相似文献
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The North Anatolian Fault (NAF) extends for about 1500 km from Karliova to the east, to the Egean Sea in the west. The Marmara region, located near the western end of the NAF, is a tectonically active zone characterized by the transition between a strike slip stress regime and an extensional one in the Aegean Sea. Microseismic studies performed around the Marmara Sea in 1995 [Tectonophysics 316, 2000, 1], and just before the 1999 Izmit Earthquake Bull. Seism. Soc. Am. 92, 2002a, 361;J. Seismol. 6, 2002b, 287) permitted the analysis of the evolution of seismicity connected to this destructive earthquake and its sequels. Several observations indicate that the aftershock distribution fits well the EW orientation of the NAF, but the ruptures are not simple and linear as a first glance would suggest. Instead they are segmented in at least five pieces as shown by the slip variation and aftershock clusters, showing complexity at different scales (Bull. Seism. Soc. Am. 92, 2002a, 361). There is still a gap, across the northern border of the Marmara Sea that has not ruptured, and this is the only sector that did not break on the NAF since the 1939 great Erzincan earthquake. Will it rupture as a whole with a large magnitude earthquake, or by segments with smaller magnitude events? The Hurst analysis of the overall behaviour of the seismicity in the Marmara region since historical times shows that if a large earthquake occurs in the near future, it might break the complete gap. The Hurst character of the time variation of seismicity is persistent with H= 0.82. The aftershocks of the 1999 Izmit earthquake can be analyzed by using the Hurst method, showing an exceptionally high persistent memory. 相似文献
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Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics? 总被引：2，自引：0，他引：2
Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts.Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to 350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to 2.7 Ga: Kenorland or Ur 2.7–2.4 Ga; Columbia 1.6–1.4 Ga; Rodinia 1100–750 Ma; and Pangea 230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean. 相似文献
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Authors Index
Index to Volume 6 相似文献7.
3075 Ma) Ivisaartoq greenstone belt, southern West Greenland: Evidence for seafloor hydrothermal alteration in supra-subduction oceanic crust Ali Polat Peter W.U. Appel Robert Frei Yuanming Pan Y&#x;ld&#x;r&#x;m Dilek Juan C. Ordez-Caldern Brian Fryer Julie A. Hollis Johann G. Raith 《Gondwana Research》2007,11(1-2):69
The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt in southern West Greenland includes variably deformed and metamorphosed pillow basalts, ultramafic flows (picrites), serpentinized ultramafic rocks, gabbros, sulphide-rich siliceous layers, and minor siliciclastic sedimentary rocks. Primary magmatic features such as concentric cooling-cracks and drainage cavities in pillows, volcanic breccia, ocelli interpreted as liquid immiscibility textures in pillows and gabbros, magmatic layering in gabbros, and clinopyroxene cumulates in ultramafic flows are well preserved in low-strain domains. The belt underwent at least two stages of calc-silicate metasomatic alteration and polyphase deformation between 2963 and 3075 Ma. The stage I metasomatic assemblage is composed predominantly of epidote (now mostly diopside) + quartz + plagioclase ± hornblende ± scapolite, and occurs mainly in pillow cores, pillow interstitials, and along pillow basalt-gabbro contacts. The origin of this metasomatic assemblage is attributed to seafloor hydrothermal alteration. On the basis of the common presence of epidote inclusions in diopside and the local occurrence of epidote-rich aggregates, the stage I metasomatic assemblage is interpreted as relict epidosite. The stage II metasomatic assemblage occurs as concordant discontinuous layered calc-silicate bodies to discordant calc-silicate veins commonly associated with shear zones. The stage II metasomatic assemblage consists mainly of diopside + garnet + amphibole + plagioclase + quartz ± vesuvianite ± scapolite ± epidote ± titanite ± calcite ± scheelite. Given that the second stage of metasomatism is closely associated with shear zones and replaced rocks with an early metamorphic fabric, its origin is attributed to regional dynamothermal metamorphism. The least altered pillow basalts, picrites, gabbros, and diorites are characterized by LREE-enriched, near-flat HREE, and HFSE (especially Nb)-depleted trace element patterns, indicating a subduction zone geochemical signature. Ultramafic pillows and cumulates display large positive initial εNd values of + 1.3 to + 5.0, consistent with a strongly depleted mantle source. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents Mesoarchean supra-subduction zone oceanic crust. 相似文献
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Anorthosite-bearing layered intrusions are unique to the Archaean rock record and are abundant in the Archaean craton of southern West Greenland and the Superior Province of Canada. These layered intrusions consist mainly of ultramafic rocks, gabbros, leucogabbros and anorthosites, and typically contain high-Ca (>An70) megacrystic (2–30 cm in diameter) plagioclase in anorthosite and leucogabbro units. They are spatially and temporally associated with basalt-dominated greenstone belts and are intruded by syn-to post-tectonic granitoid rocks. The layered intrusions, greenstone belts and granitoids all share the geochemical characteristics of Phanerozoic subduction zone magmas, suggesting that they formed mainly in a suprasubduction zone setting. Archaean anorthosite-bearing layered intrusions and spatially associated greenstone belts are interpreted to be fragments of oceanic crust, representing dismembered subduction-related ophiolites. We suggest that large degrees of partial melting (25–35%) in the hotter (1500–1600 °C) Archaean upper mantle beneath rifting arcs and backarc basins produced shallow, kilometre-scale hydrous magma chambers. Field observations suggest that megacrystic anorthosites were generated at the top of the magma chambers, or in sills, dykes and pods in the oceanic crust. The absence of high-Ca megacrystic anorthosites in post-Archaean layered intrusions and oceanic crust reflects the decline of mantle temperatures resulting from secular cooling of the Earth. 相似文献
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Mafic volcanic rocks in the 3.8–3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called ‘ocelli’. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO = 9–21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO2 = 62.9–72.0 wt.%; MgO = 0.60–3.50 wt.%; Ni = 58–143 ppm; Cr = 250–510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO2 = 46.6–50.6 wt.%; MgO = 8.70–12.30 wt.%; Ni = 119–175 ppm; Cr = 330–600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO2 = 54.2–60.7 wt.%; MgO = 3.95–7.72 wt.%; Ni = 127–158 ppm; Cr = 500–570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts.On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Smcn = 0.30–0.94; Gd/Ybcn = 1.03–1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Smcn = 1.30–2.60; Gd/Ybcn = 1.32–2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma–crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean. 相似文献
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中天山卡瓦布拉克地区侵入岩类广泛发育,并多处被中-基性岩脉穿插。LA-ICP-MS锆石U-Pb测年获得闪长岩和辉长岩脉年龄分别为296.1±2.5 Ma(MSWD=1.4),299.5±2 Ma(MSWD=1.07),均侵位于早二叠世。岩石学和岩石地球化学分析结果表明该区中-基性岩脉具有富集轻稀土元素(LREE)和大离子亲石元素(Rb,U,Th)、亏损高场强元素(Nb,Ta,Ti)的地球化学特征,SREE为69.19~234.62,LREE/HREE=3.97~11.18。闪长岩脉锆石(176Hf/177Hf)值为0.280 618~0.283 329,对应的εHf(t)值为-14.57^+25.06。研究数据表明,该区中-基性岩脉可能起源于亏损岩石圈地幔,原始岩浆在侵位过程中发生了分离结晶作用,同时还受到了地壳混染作用的影响。基于卡瓦布拉克地区中-基性岩脉的野外地质特征和地球化学分析结果,并结合区域地质资料,认为它们形成于南天山洋闭合之后的后碰撞伸展构造背景。 相似文献