Ancient sub-continental lithospheric mantle (SCLM) beneath the eastern part of the Central Asian Orogenic Belt (CAOB): Implications for crust–mantle decoupling
Ancient sub-continental lithospheric mantle (SCLM) beneath the eastern part of the Central Asian Orogenic Belt (CAOB): Implications for crust–mantle decoupling
复制标题
DOI：
10.1016/j.lithos.2011.07.022
复制
发表时间：
2011-10
期刊：
影响因子：
3.5
通讯作者：
Yanlong Zhang;Chuan-Zhou Liu;W. Ge;Fu-Yuan Wu;Zhuyin Chu
中科院分区：
文献类型：
--
作者：
Yanlong Zhang;Chuan-Zhou Liu;W. Ge;Fu-Yuan Wu;Zhuyin Chu
中国东北部五大连池、二克山和克罗(韦克)地区广泛分布着新生代喷发的钾质玄武岩，并含有丰富的地幔包体。本研究地幔包体选自科洛钾质熔岩，主要为尖晶石二辉橄榄岩和方辉橄榄岩，并有少量的硅辉石和辉长岩。在某些含金云母和金红石的克罗包体中，存在明显的模式交代作用。矿物组成表明，克罗包体代表不同程度熔融后的地幔残留物，即二辉橄榄岩为1-5%，方辉橄榄岩为3-11%。块状岩石和单斜辉石数据均显示，轻稀土元素(LREE)和其他一些不相容元素(如Ba、U)的含量较高。部分克罗包体中的单斜辉石具有放射性成因的锶同位素，但相对于东北中国其他地区的新生代地幔包体，具有非放射性成因的NdHf同位素。克罗地幔包体的187Os/188Os比值在0.11458~0.13194之间变化。尽管大多数克罗地幔包体都受到了Re添加的影响，但它们的Os同位素组成并没有发生明显的变化。难熔方辉橄榄岩的187Os/188Os比值给出了~1.9~2.1Ga的贫化年龄。这表明克罗地区的陆下岩石圈地幔形成于古元古代，比主要形成于新元古代以来的上覆地壳的年龄要早。因此，在时间上，科卢地区下的南海地壳与上覆地壳是脱钩的。克罗地区新生代SCLm不可能是从保存有古地幔区域的软流圈中新吸积而来的。我们认为，科洛地区之下的古地幔是外来的，是从其他地方侵位的。
Potassic basaltic lavas erupted during the Cenzoic are widespread in Wudalianchi, Erkeshan and Keluo (WEK) areas, northeast China, and contain abundant mantle xenoliths. Mantle xenoliths in this study are selected from the Keluo potassic lavas, and mainly comprise spinel lherzolites and harzburgites with minor dunites and wehrlites. Modal metasomatism is evident in some Keluo xenoliths from the presence of phlogopite and rutile. Mineral compositions indicate that the Keluo xenoliths represent mantle residues after variable degrees of melting, i.e., 1–5% for lherzolites and 3–11% for harzburgites, with subsequent refertilization. Both bulk-rock and clinopyroxene data show enrichment in light rare earth elements (LREE) and some other incompatible elements (e.g. Ba, U). Clinopyroxenes from some of the Keluo xenoliths have radiogenic Sr isotopes, but unradiogenic Nd–Hf isotopes relative to the Cenozoic mantle xenoliths from other localities of the NE China. The Keluo mantle xenoliths have variable187Os/188Os ratios ranging from 0.11458 to 0.13194. Although most Keluo mantle xenoliths have been affected by Re addition, their Os isotope compositions have not been significantly modified. The unradiogenic187Os/188Os ratios of the refractory harzburgites give Re depletion ages (TRD) of ~1.9–2.1Ga. This suggests that the sub-continental lithospheric mantle (SCLM) beneath the Keluo area was formed during the Paleoproterozoic, which is older than the age of the overlying crust that was mainly formed since the Neoproterozoic. Therefore, the SCLM beneath the Kelou area is temporally decoupled from the overlying crust. It is unlikely that the Cenozoic SCLM beneath the Keluo area was newly accreted from the asthenosphere, in which ancient mantle domains were preserved. We propose that the ancient mantle beneath the Keluo region is extraneous and has been emplaced from other locations.