微陆块属性及过程:我国东北地区岩石圈地幔性质差异之根本

Nature and processes of microcontinents: Primary derivation for the heterogeneity of lithospheric mantle beneath northeastern China

我国东北地区位于中亚造山带的东段,由多个微陆块俯冲拼合所组成,是研究壳幔相互作用的理想地区。但该区岩石圈地幔性质,包括壳幔属性是否解耦、是否有古老地幔残留、导致地幔性质差异的机制是什么,都还不清楚。东北地区多地出露新生代玄武岩并有橄榄岩捕虏体,如松嫩-张广才岭地块的阿巴嘎、五大连池和蛟河,兴安地块的哈拉哈、诺敏和科洛,以及与华北拼合褶皱带处的双辽、伊通和汪清等。其中阿巴嘎、哈拉哈、诺敏、科洛和蛟河的橄榄岩含有Mg#≥91.5的橄榄石,最大Re亏损年龄是2.1~1.9Ga,富集Sr、Nd同位素组成且变化范围大,反映交代作用强烈,这些特征与大陆岩石圈地幔有明显的相似性;而拼合带处的双辽、伊通、汪清及松嫩-张广才岭地块中的五大连池地幔包体橄榄石,Re亏损年龄总体为中元古代,Sr、Nd同位素组成变化范围小,交代作用弱,与大洋岩石圈地幔亲缘性更明显。这些差别总体反映出地块内部与地块边缘相比,有岩石圈地幔形成时代相对较老、亏损程度较高、地幔交代作用较强的特点。但也有例外的情况,如五大连池与科洛相比,更远离贺根山-黑河断裂带,但地幔属性更饱满,反映地块内部深部作用过程可能更强烈。因此,我们认为东北岩石圈地幔性质差异的根本原因与微陆块初始属性和后来的俯冲拼合及软流圈-岩石圈相互作用等有关。

Northeast China, located in the eastern part of the Central Asian Orogenic Belt and formed by subduction and amalgamation of several micro-continents, is an ideal area for studying the interaction between crust and mantle. However, the nature of the lithospheric mantle beneath the area, including the origin of the ancient mantle relics, decoupling of crust and underlying mantle, and mechanism for mantle heterogeneity, are still under dispute. The Cenozoic basalts, which contain peridotite xenoliths, are widely distributed in areas such as Abaga, Wudalianchi and Jiaohe in Songnen-Zhangguangcailing block, Halaha, Nuomin and Keluo in Xing＇an block, and Shuangliao, Yitong and Wangqing in suture zone between Northeast China and North China. The peridotites from Abaga, Halaha, Nuomin, Keluo and Jiaohe contain olivines with Mg# ≥ 91.5, and have maximum Re depletion ages ranging from 2.1Ga to 1.9Ga and a large range of Sr and Nd isotopic compositions, which reflects that they have experienced strong metasomatism. These features are similar with those in the continental lithospheric mantle. However, the xenoliths from Shuangliao, Yitong, Wangqing in the suture zone and Wudalianchi in Songnen-Zhangguangcailing blocks are characterized by olivine-Mg#〈91.5, Re depletion ages of Mesoproterozoic and a small variation range of Sr and Nd isotopic compositions, suggesting weak metasomatism. All these features display an affinity with the oceanic lithospheric mantle. These differences suggest that compared with the edge of microcontinents, the lithospheric mantle beneath the interior of microcontinents are characterized by older formational age, higher degree of melt depletion and stronger metasomatism. Nonetheless, there are some exceptions. For example, Wudalianchi is further away from the Hegenshan-Heihe fault belt than Keluo, where the lithospheric mantle underlies show fertile characteristic, possibly implying strong deep process in the interior of this microcontinent. We therefore conclude that the origin of mantle heterogeneity beneath Northeast China was resulted from the primary nature of micro-continents and their subsequent subduction and amalgamation and interactions between asthenosphere and lithosphere.