Whole-Rock and Zircon Geochemistry of the Tieshan Pluton in Daye, Eastern China: Implication for Petrogenesis and Fe-Cu-Au-(Co) Mineralization
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Metabasic rocks form an important constituent of the Chýnov and Český Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia).The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase.Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K) A ~ 0.5 apfu.Garnet with clinopyroxene are subordinate and occur in a few samples only.No relics of previous greenschist-or granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history.The petrology indicates rather close correlation of the Chýnov and Český Krumlov units.The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks.However, there is a major difference in the oxidation state.Most Chýnov amphibolites have Fe 2 O 3 /FeO = 0.70-1.00and their protolith probably experienced an early incipient oxidation.Great deal of the parental basalts thus could have been effusive.The Český Krumlov amphibolites have Fe 2 O 3 /FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units.The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (ε 50 Nd 0 = +8.6 to +9.4; T D Nd M = 0.43-0.50Ga).This argues stoutly against Precambrian age of the Varied Group in south Bohemia.The composition of the remaining samples reflects contamination by upper continental crust (ε 50 Nd 0 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots).A much smaller group of amphibolites is characterised by steep REE patterns (La N /Yb N = 5.5-11) and high contents of HFSE (Nb, Ta, Zr and P).It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7-1.6)generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (ε 50 Nd 0 = +4.5 to +6.1; T D Nd M = 0.75-0.83Ga).Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlík orthogneiss show rather unradiogenic Nd isotopic composition (ε 50 Nd 0 = +0.1 and -3.6; T D Nd M = 1.34 and2.03Ga).This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times.The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Český Krumlov Varied Unit seems ruled out.Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin.The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.
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A group of slightly peraluminous Variscan plutons in Northern Portugal were selected from the study of zircon composition. The selected plutons are: the Vila Pouca de Aguiar and the Lavadores-Madalena plutons with I-type affinities and the Vieira do Minho pluton, an l-S transitional type. Zircon occurs as euhedral to subhedral crystals and exhibit finely concentric oscillatory magmatic zoning mainly related to variations of Hf, Y, U and Th concentrations. Most zircon crystals show the dominant “xenotime” substitution. The zircon crystals have Zr/Hf ratio in the range of 21 to 52, with no significant differences between the different granites. These values are in the same range of other peraluminous granites and are in accordance with a crustal signature of zircon. Moreover, the range of Zr/Hf valu es in zircon crystals overlaps with that of crustal sources and consequently to the potential protoliths proposed in the genesis of the Vieira do Minho and the Vila Pouca de Aguiar plutons, namely meta-igneous crustal sources at different levels. Although zircon from the Lavadores-Madalena pluton has a compositional range similar to the other plutons, an origin by hibridisation has been proposed. However, similar zircon chemistry between this pluton and Vila Pouca de Aguiar and Vieira do Minho plutons could also suggest a similar crustal source.
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Abstract In the Central Cameroon Shear Zone, several studies were focused on granitoids and very few on mafic rocks. Here we report the petrography, geochemistry and mineralogy of the Mfengou-Manki mafic rocks in order to constrain their petrogenesis and tectonic settings and the role of lithospheric and asthenospheric mantle sources in their genesis. The studied mafic rocks are subdivided into columnar jointed basalts and mafic dykes. Clinopyroxene thermobarometry indicates that the mafic dykes crystallized at a temperature of 1071 to 1193 °C and a pressure of 4 to 12 kbar while the columnar jointed basalts emplaced at a temperature of 1064 to 1152 °C and 2 to 13 kbar pressure. The mafic dykes and columnar jointed basalts present high La/Sm, Sm/Yb, Nb/Yb and Th/Yb ratios, indicating garnet to spinel transition zone mantle source. The multi-element diagram of the mafic dykes display enrichment in Nb, Ta, Pb and Ti and depletion in Th, U, Ce and Zr compared to that of the columnar jointed basalts (slight depletion in Nb and Ta and pronounced depletion in U, Pb and Zr and enrichment in Cs, Ba and Rb) indicating the little involvement of the sub-continental lithospheric mantle to the formation of the columnar jointed basalts. The Nb/La ratio > 1 for the mafic dykes and < 1 for the columnar jointed basalts also suggest the derivation of the mafic dykes from the asthenospheric mantle and the columnar jointed basalts from the mixed lithospheric-asthenospheric mantle due to the sub-continental lithospheric mantle delamination under the Central Cameroon Shear Zone.
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