华北克拉通东部中生代高Mg闪长岩——对岩石圈减薄机制的制约

在华北克拉通东部鲁西—徐淮地区,存在一套辉石闪长岩-二长闪长岩-花岗闪长岩组成的adakitic岩石。锆石SHRI MP和LA-ICP-MS U-Pb定年结果表明它们形成于早白垩世(130~132Ma)。该类岩石具有较高的MgO含量(质量分数为1·46%~9·76%)、高的Mg#值(0·46~0·68)和高的Sr/Y值(主体介于30~52之间,个别高达410)。这些特征类似于由俯冲大洋板片部分熔融形成的adakitic岩石。然而,它们所表现出来的相对较高的87Sr/86Sr初始比值(0·7051~0·7077)和较低的εNd(t)值(-4·43~-15·92)则反映岩浆形成或演化过程中应有陆壳物质的参与。徐淮地区该类岩石中榴辉岩类捕虏体和石榴石捕虏晶的存在和鲁西辉石闪长岩中众多地幔橄榄岩捕虏体的发现,以及这些捕虏体中普遍发育富硅质交代作用,由此可以判定该类岩浆应起源于拆沉下部大陆地壳的部分熔融及其在上升过程中与地幔橄榄岩的反应,石榴石作为残留相。华北克拉通东部早白垩世adakitic岩石的存在以及榴辉岩类捕虏体的年代学表明,中生代早期曾存在一次重要的陆壳加厚过程,之后相继出现的加厚岩石圈的拆沉应是中生代岩石圈减薄的主导机制。

Mesozoic high-Mg diorites in eastern North China craton: constraints on the mechanism of lithospheric thinning

An igneous suite with pyroxene-diorite, monzodiorite and granodiorite association occurs in the western Shandong and Xu-Huai regions, in the eastern part of North China craton (NCC). SHRIMP and LA-ICP-MS zircon U-Pb dating indicate that the suite was formed in the early Cretaceous (130～132 Ma). These rocks have a high MgO content (1.46%～9.76%), high Mg~ # values (0.46～0.68), and high Sr/Y ratios (mostly ranging from 30 to 52, the highest being 410). These features are similar to adakitic rocks derived from partial melting of a subducted oceanic slab. However, their high initial ~ 87 Sr/~ 86 Sr (0.705 1～0.707 7) and low ε_ Nd (t) values (-4.43~-15.29) are inconsistent with an origin from slab melting and imply the involvement of crustal material in magma formation and evolution. Eclogite xenoliths and garnet residual crystals from the Xu-Huai area, and mantle dunite xenoliths overprinted by silica-rich metasomatism in western Shandong, suggest that the high-Mg adakitic magma could be derived from partial melting of delaminated lower continental crust and its subsequent interaction with mantle during its upward transport, leaving garnet as a residual phase. Early Cretaceous adakitic rocks in eastern NCC and the chronology of eclogite xenoliths imply that the continental crust thickening process took place in the early Mesozoic. The subsequent delamination of thickened lithosphere should be an important mechanism leading to the Mesozoic lithospheric thinning.