Geochemical characteristics and ~(40)Ar-~(39)Ar ages of the amphibolites and gabbros in Tarlang area:implications for tectonic evolution of the Chinese Altai.
Geochemical characteristics and ~(40)Ar-~(39)Ar ages of the amphibolites and gabbros in Tarlang area:implications for tectonic evolution of the Chinese Altai.
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发表时间：
2007
影响因子：
1.8
通讯作者：
K. Cai;C. Yuan;M. Sun;W. Xiao;Hl Chen;X. Long;Yj Zhao;Jl Li
中科院分区：
文献类型：
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作者：
K. Cai;C. Yuan;M. Sun;W. Xiao;Hl Chen;X. Long;Yj Zhao;Jl Li
中国阿尔泰塔朗地区产有角闪岩和辉长岩。角闪岩是镁铁质岩浆岩(SiO_2=45.42%~52.96%)的变质产物，Al_2O_3(13.14%~17.92%)、Fe_2O_3~T(9.08%~16.88%)和TiO_2(0.82%~3.23%)含量较高。角闪岩样品的稀土配分较平坦，没有明显的稀土分馏(La/Yb_N=1.53~1.79)。这些样品具有轻微的亏损，具有轻微的Ti-Nb-Ta负异常辉长岩样品(SiO_2=46.96%~49.86%)具有中等的TiO_2(0.38%~1.89%)和较低的K_2O含量(K_2O=0.34%~1.03%)，属于亚碱性岩石(Na_2OK_2O)。它们的Al_2O_3含量较高(15.0%~20.81%)，CaO含量较高(10.13%~11.60%)，镁含量中等(7.3%~8.3%)。辉长岩样品具有两种类型的稀土配分模式，一种是稀土元素轻微分馏(La/Yb_N=1.10~1.67)，Eu异常较弱(δEu=1.03~1.10)；另一种是LREE富集型(La/Yb_N=5.97~6.39)，Eu异常明显(δEu=1.31~1.44)，可能反映斜长石在岩浆房中的聚集。辉长岩样品的~(40)Ar/~(39)Ar谱简单平坦(坪年龄266.9±4.2 Ma；等时线年龄为(261±23)Ma，表明其K-Ar同位素体系没有受到后续热事件的严重干扰，可能反映了辉长岩的侵入时间，但角闪岩的~(40)Ar_(39)Ar分析产生了马鞍状光谱。角闪岩和辉长岩在年龄和地球化学上的差异表明它们可能代表不同的基性岩浆。角闪岩原岩可能形成于俯冲带附近的海山，随后形成于阿尔泰山脉。辉长岩具有俯冲作用的特征，与中国阿拉泰的伸展运动同龄，可能反映了二叠纪的构造转换。
Amphibolitic and gabbroic rocks occur in Tarlang area in Chinese Altay.Amphibolites are metamorphosed products of mafic magmatie rocks(SiO_2=45.42%～52.96%)and contain high Al_2O_3(13.14%～17.92%),Fe_2O_3~T(9.08%～16.88%)and TiO_2(0.82%～3.23%)contents.The amphibolite samples show relatively flat REE patterns,without significant fractionations of REEs(La/Yb_N=1.53～1.79).These samples are depleted in HFSEs slightly,with slightly Ti-Nb-Ta negative anomalies,which suggest a subtle relation to subduction-related environment.The gabbroic samples(SiO_2=46.96%～49.86%)have intermediate TiO_2(0.38%～1.89%)and relatively low K_2O contents(K_2O=0.34%～1.03%),belonging to subalkali rocks(Na_2OK_2O). They have relatively high Al_2O_3(15.0%～20.81%),CaO(10.13%～11.60%)and moderate MgO(7.3%～8.3%)contents. The gabbroic samples show two types of REE patterns.One is characterized by slight fractionation of REEs(La/Yb_N=1.10～1.67) and weak Eu anomalies(δEu=1.03～1.10),while the other is enriched in LREEs(La/Yb_N=5.97～6.39),with significantly positive Eu anomalies(δEu=1.31～1.44),which may reflect accumulation of plagioclase in the magma chamber.These gabbroic rocks all exhibit Ti-Nb-Ta troughs and Pb-Sr spikes,also implying a subduction-related origin.The~(40)Ar/~(39)Ar spectrum of the gabbroic sample is simple and flat(plateau age of 266.9±4.2Ma;isochron age of 261±23Ma),indicating that its K-Ar isotopic system was not seriously disturbed by subsequent thermal events and may reflect the intrusion time of the gabbro.However,~(40)Ar_~(39)Ar analysis of the amphibolite give rise to a saddle-like spectrum,and its youngest plateau(297.3±6.1Ma)probably reflects the last metamorphic event.The difference on age and geochemistry between the amphibolitic and gabbroic rocks implies that they probably represent different mafic magma.The protolith of amphibolites probably formed in a seamount near a subduction zone and subsequently accreated to the Altai.The gabbro exhibits subduction-releted characteristics and was coeval with transtensional movement in the Chinese Altai, probably reflecting a tectonic transition in the Permian.