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Geochemical Characteristics and Petrogenesis of the Two Types of Neoarchean Gneisses from the Jiaobei Terrane
详细信息   
摘要
Two types of Neoarchean gneisses are identified in the three deep exploration drills in the Jiaobei terrane,including the dominating TTG gneisses and the minor garnet biotite plagiogneiss being sandwiched between the former.As recorded,their protolithes both formed at ～ 2.5Ga.The TTG gneisses are rich in SiO 2 (53.95% ～ 75.56%),Al 2 O 3 (most > 15%,10.45% ～ 17.64%) and Na 2 O (1.51% ～ 5.94%) with high Na 2 O /K 2 O ratios (> 1) and low Fe 2 O 3 T (2.22% ～ 9.54%) and MgO (0.53% ～ 4.87%) contents.Their A /CNK ratios (1.18 ～ 1.75) indicate that they belong to high-Al TTG series.The TTG gneisses are rich in LILE (Rb,Ba and Sr) but deplete HFSE (Nb,Ta,Zr and Hf),characterized by high Sr contents (204 ×10-6 ～ 2906 × 10-6) and Sr/Y ratios (7.30 ～ 355),relatively high ∑REE contents (40.4 × 10-6 ～ 512 × 10-6),strongly fractionated REE patterns ((La/Yb) N=11.5 ～ 121) and positive or no Eu anomalies (δEu=0.79 ～ 1.89).These geochemical features request melting of plagioclase component and relicts of garnet,amphibole and some Ti-bearing minerals in the source,indicating that the TTG were formed by partial melting of hydrous basaltic crust at high pressure.The whole rock Nd and zircon Hf isotopic studies,in which the t DM is close to the formation age,indicate that the TTG may originate from a juvenile crust from a depleted mantle at 2.57 ～ 2.64Ga.In addition,their high Mg # numbers (40 ～ 67) and high Cr (147 × 10-6 ～ 371 × 10-6) and Ni contents (6.68 ×10-6 ～ 156 × 10-6) indicate reaction with mantle wedge.Combining features such as negative Nb,Ta anomalies and the juvenile source,the TTG in this study may have formed in island arc environment related to subduction instead of thickened lower crust.DF values and the K-A diagram show that the protolith of garnet biotite plagiogneiss is pelitic-silty sedimentary rocks.Their Cr /Zr ratios (0.90 ～ 1.99,1.62 on the average),Th /Sc ratios (0.30 on the average) and REE patterns are similar to the corresponding characteristics (1.44 and 0.4,respectively) of Archean sedimentary rocks,thus indicating their source is mainly composed of quartz-feldspathic material.Most samples are rich in LREE with weak negative or no Eu anomalies (δEu=0.69 ～ 0.92,0.84 on the average) and have relatively flat HREE patterns.The characteristic values of REE (176 × 10-6 on average) contents,(La /Yb) N (14.7 on average) and LREE/HREE ratios (3.3 on average) indicate that the protolith may probably have formed in island arc environment or active continental margin.Amphibolite facies metamorphism,which is indicated by the peak metamorphic temperature obtained by Grt-Bi thermometer (549 ～ 663℃) and biotite composition,may represent the imprint of retrogressive metamorphic stages of granulite-high amphibolite facies metamorphism at ～ 2.5Ga.The protolith of garnet biotite plagiogneiss and TTG are the products of the same age and similar tectonic settings (island arc environment or active continental margin),suggesting a subduction process in the area at that time.And the metamorphism at ～ 2.5Ga,which is slightly later than the TTG magma event,may be related to the collision process after subduction.Hence,a significant subductioncollision event may have occurred in the study area at ～ 2.5Ga,during which TTG and the protolith of garnet biotite plagiogneiss formed at the subduction stage and granulite-high amphibolite facies metamorphism was caused by the following collision process.

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