阿尔泰塔尔浪地区斜长角闪岩和辉长岩的形成时代、地球化学特征和构造意义

Geochemical characteristics and ^(40)Ar-^(39)Ar ages of the amphibolites and gabbros in Tarlang area:implications for tectonic evolution of the Chinese Altai.

阿尔泰塔尔浪地区出露一些斜长角闪岩和辉长岩。其中,斜长角闪岩为基性岩浆岩变质的产物,SiO_2含量为45.42%～52.96%,具有较高的Al_2O_3(13.14%～17.92%)、Fe_2O_3~T(9.08%～16.88%)和TiO_2(0.82%～3.23%)含量。斜长角闪岩样品稀土元素曲线比较平坦,轻重稀土元素分馏不明显(La/Yb_N=1.53～1.79),无明显的Nb和Ta亏损。通过野外产状及地球化学特征分析,认为该斜长角闪岩形成于海山的构造环境。辉长岩SiO_2(46.96%～49.86%)含量稍低,TiO_2(0.38%～1.89%)变化较大,并富Al_2O_3(15.0%～20.81%)、CaO(10.13%～11.60%),具有中等至较高MgO含量(7.3%～8.3%)和相对较低的钾含量(K_2O=0.34%～1.03%),属亚碱性岩石系列(Na_2O>K_2O)。辉长岩呈现两种稀土元素组成特征。一种轻重稀土分馏不明显,配分曲线比较平坦(La/Yb_N=1.10～1.67),具有轻微的Eu正异常(δEu=1.03～1.10);另一种轻重稀土元素分馏较明显(La/Yb_N=5.97～6.39)并具有正的Eu异常(δEu=1.31～1.44),反映其形成过程中可能受到斜长石堆晶的影响。在微量元素蛛网图上,辉长岩具有明显的Ti、Nb和Ta负异常和Pb、Sr正异常,反映出明显的消减带岩浆特征。角闪石的^(40)Ar-^(39)Ar测年结果显示,辉长岩样品的坪年龄为266.9±6.1Ma,且年龄坪谱显示在其形成之后没有受到后期热事件的明显干扰,基本上反映了辉长岩的形成年龄。斜长角闪岩的^(40)Ar-^(39)Ar结果呈现出似马鞍型坪谱,其中最小的年龄坪(297.3±6.1Ma)可能反映了最近一期的变质事件,而且明显早于辉长岩的侵入。斜长角闪岩和辉长岩不同的形成时代和地球化学组成表明二者可能代表了两期不同的基性岩浆活动。斜长角闪岩所具有的板内和消减带的复合特征表明其原岩可能是洋壳在俯冲消减过程中增生的海山残片,而辉长岩所具有消减带特征显示在二叠纪时期阿尔秦地区仍存在受俯冲作用影响的地幔源区。阿尔泰二叠纪岩浆活动无论在岩浆属性还是规模上都与泥盆和石炭纪的岩浆活动存在巨大差异,表明这两个时期的构造背景存在较大不同,可能反映阿尔泰正处于某种构造转换机制的控制之下。

Amphibolitic and gabbroic rocks occur in Tarlang area in Chinese Altay. Amphibolites are metamorphosed products of mafic magmatic rocks （SiO2 = 45.42% - 52.96% ） and contain high Al2O3 （ 13. 14% - 17. 92% ）, Fe2O^T 3 （9.08% - 16. 88% ） and TiO2 （0. 82% -3. 23% ） contents. The amphibolite samples show relatively flat REE patterns, without significant fractionations of REEs （La/YbN = 1.53 - 1.79）. These samples are depleted in HFSEs slightly, w/th slightly Ti-Nb-Ta negative anomalies, which suggest a subtle relation to subduction-related environment. The gabbroic samples （ SiO2 = 46. 96% - 49. 86% ） have intermediate TiO2 （0. 38% - 1.89% ） and relatively low K2O contents （ K2O = 0. 34% - 1.03% ）, belonging to subalkali rocks （ Na2O 〉 K2O）. They have relatively high Al2O3（ 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/Ybn = 1.10 - 1.67） and weak Eu anomalies （SEu = 1.03 - 1.10）, while the other is enriched in LREEs （ La/YbN = 5.97 - 6. 39）, with significantly positive Eu anomalies （SEu = 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 Ahai. The gabbro exhibits subduction-releted characteristics and was coeval with transtensional movement in the Chinese Altai, probably reflecting a tectonic transition in the Permian.