REE geochemistry of eclogites and associated rocks from Sauviat-sur-Vige, Massif Central, France

Representative samples of metagabbros, eclogites and a garnet peridotite from the Massif de Sauviat-sur-Vige (West Massif Central) have been analyzed for major and trace elements (including REE). The LREE enriched patterns are distinct from those of modern MORB but resemble those of continental or ocean island tholeiites. An intraplate tectoning setting is thus inferred. Geochemical modeling using REE and major element contents suggests that all the rock types might be related genetically by fractional crystallization, and, in some instances, by mixing of cumulates and differentiated liquid as in the case of the garnet peridotite. P, T estimates from microprobe analyses of coexisting garnet-clinopyroxene pairs yield about 820°C, and P > 15 kb for the eclogite facies metamorphism.     

The Greenland Ice Core Chronology 2005, 15–42 ka. Part 1: constructing the time scale

The Greenland Ice Core Chronology 2005, GICC05, is extended back to 42 ka b2k (before 2000 AD), i.e. to the end of Greenland Stadial 11. The chronology is based on independent multi-parameter counting of annual layers using comprehensive high-resolution measurements available from the North Greenland Ice Core Project, NGRIP. These are measurements of visual stratigraphy, conductivity of the solid ice, electrolytical melt water conductivity and the concentration of Na⁺, Ca²⁺, SO₄²⁻, NO₃⁻, NH₄⁺. An uncertainty estimate of the time scale is obtained from identification of ‘uncertain’ annual layers, which are counted as 0.5±0.5 years. The sum of the uncertain annual layers, the so-called maximum counting error of the presented chronology ranges from 4% in the warm interstadial periods to 7% in the cold stadials. The annual accumulation rates of the stadials and interstadials are on average one-third and half of the present day values, respectively, and the onset of the Greenland Interstadials 2, 3, and 8, based on 20 year averaged δ¹⁸O values, are determined as 23,340, 27,780, and 38,220 yr b2k in GICC05.     

中国北方造山带地壳形成时间的Nd同位素制约

Nd同位素模式年龄研究表明，中国北方造山带的地壳具有幕式生长特点，主体地壳形成小于1000Ma。结合中亚造山带其它地区的研究结果，可以认为地质历史上的新元古代-显生宙也是地壳生长的重要时期。     

Age of the Archean Talga-Talga Subgroup, Pilbara Block, Western Australia, and early evolution of the mantle: new SmNd isotopic evidence

Archean komatiites, high-Mg basalts and tholeiites from the North Star Basalt and the Mount Ada Basalt formations of the Talga-Talga Subgroup, Warrawoona Group, Pilbara Block, Western Australia, define a linear correlation on the normal¹⁴³Nd/¹⁴⁴Nd vs.¹⁴⁷Sm/¹⁴⁴Nd isochron plot. The data give an age of 3712 ± 98 Ma and initialε_Nd(T) of +1.64 ± 0.40. The 3712 ± 98 Ma date is consistent with the regional stratigraphic sequence and available age data and the SmNd linear array may be interpreted as an isochron giving the eruption age of the Talga-Talga Subgroup. An alternative interpretation is that the isochron represents a mixing line giving a pre-volcanism age for the Subgroup. Consideration of geochemical and isotopic data indicates that the true eruptive age of the Talga-Talga Subgroup is possibly closer to about 3500 Ma. Regardless of the age interpretation, the new Nd isotopic data support an existence of ancient LREE-depleted reservoirs in the early Archean mantle, and further suggest that source regions for the Pilbara volcanic rocks were isotopically heterogeneous, withε_Nd(T) values ranging from at least 0 to +4.0.     

Multi-chronometric ages and origin of Archaean tonalitic gneisses in Finnish Lapland: A case for long crustal residence time

The Tojottamanselkä gneisses of the Koitelainen region, northern Finland, have been dated by the Sm-Nd and the common Pb methods. The Sm-Nd data of seven samples from a small area (100 m × 100 m) define an isochron ofT=3.06±0.12 (2) Ga, with correspondingI _Nd=0.50848±9 (2), or _Nd(T)=–3.7±1.8. This age is in good agreement with the zircon U-Pb discordia age (3.1 Ga) reported by Kröner et al. (1981) and is interpreted as the time of magmatic emplacement. The distinctly negative _Nd(T) value is found for the first time for Archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long (200–500 Ma) crustal residence time. A few samples, on the other hand, possess _Nd(T) values close to zero, hence they are thought to be derived by partial melting of basaltic sources with nearchondritic REE distribution patterns.Common Pb isotopic data yield an isochron age of 2.64±0.24 (2) Ga which is in agreement, within error limit, with the published Rb-Sr isochron age of 2.73±0.24 Ga (Kröner et al. 1981). The age of ca. 2.7 Ga is interpreted as the time of regional metamorphism during which both Pb and Sr isotopes were rehomogenised.The tonalitic gneisses have highly fractionated REE patterns with (La/Yb)_N ratios varying from 9 to 43. Like most Archean gneisses of TTG composition (tonalite-trondhjemite-granodiorite), they could be derived by partial melting of crustal sources of basaltic to granodioritic compositions. Direct derivation by melting of mantle peridotites is excluded.The present geochemical study indicates that the Tojottamanselkä gneisses have had a very complex history that involved multi-stage development. Together with the published age data for the basement gneisses and greenstone belts of eastern central Finland (Vidal et al. 1980; Martin et al. 1983a), we conclude that the Archean crustal development in Finland started at least 3.5 Ga ago and passed through a series of magmatic and metamorphic events at 3.1, 2.85, 2.65 and 2.5 Ga before the final intrusions of K-rich granites about 2.4 Ga ago.     

Petrogenesis of the Maowu pyroxenite–eclogite body from the UHP metamorphic terrane of Dabieshan: chemical and isotopic constraints

The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial ⁸⁷Sr/⁸⁶Sr ratios (0.707–0.708) and inhomogeneous negative _Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ¹⁸O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial ⁸⁷Sr/⁸⁶Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.

Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level.     

Petrogenesis of A-type granites and origin of vertical zoning in the Katharina pluton, Gebel Mussa (Mt. Moses) area, Sinai, Egypt

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkali-feldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. These two varieties are chemically indistinguishable. Syenogranite, as compared with alkali-feldspar granites, is richer in Ca, Sr, K, Ba and contains less SiO₂, Rb, Y, Nb and U; Eu/Eu* values are 0.22–0.33 for syenogranite and 0.08–0.02 for alkali-feldspar granites. The δ¹⁸O (Qtz) is rather homogeneous throughout the pluton, 8.03–8.55‰. The δ¹⁸O (Afs) values in the syenogranite are appreciably lower relative to those in the alkali–feldspar granites: 7.59–8.75‰ vs. 8.31–9.12‰. A Rb–Sr isochron (n = 9) yields an age of 593 ± 16 Ma for the Katharina Ring Complex (granite pluton and ring dikes).

The alkali–feldspar granites were generated mainly by fractional crystallization of syenogranite magma. The model for residual melt extraction and accumulation is based on the estimated extent of crystallization ( 50 wt.%), which approximates the rigid percolation threshold for silicic melts. The fluid-rich residual melt could be separated efficiently by its upward flow through the rigid clusters of crystal phase. Crystallization of the evolved melt started with formation of hypersolvus granite immediately under the roof. Fluid influx from the inner part of the pluton to its apical zone persisted and caused increase of P_H2O in the magma below the perthite granite zone. Owing to the presence of F and Ca in the melt, P_H2O of only slightly more than 1 kbar allows crystallization of subsolvus Ab–Afs granite. Abundance of turbid alkali feldspars and their ¹⁸O/¹⁶O enrichment suggest that crystallization of alkali-feldspar granites was followed by subsolvus fluid–rock interaction; the δ¹⁸O (Fsp) values point to magmatic origin of fluids.

The stable and radiogenic isotope data [δ¹⁸O (Zrn) = 5.82 ± 0.06‰, I_Sr = 0.7022 ± 0.0064, ε_Nd (T) values are + 3.6 and + 3.9] indicate that the granite magma was generated from a ‘juvenile’ source, which is typical of the rocks making up most of the Arabian–Nubian shield.     

Solving complex fisheries management problems: What the EU can learn from the Nordic experiences of reduction of discards

A crucial issue for the new EU common fisheries policy is how to solve the discard problem. Through a study of the institutional set up and the arrangements for solving the discard problem in Denmark, the Faroe Islands, Iceland and Norway, the article identifies the discard problem as related to both natural, other material and to cultural conditions. Hence, solving the discard problem requires not only technical and regulatory instruments, but also arenas and structures that allow and facilitate processes of cultural change.