A metasomatized 18O-rich veined lithospheric mantle source for ultrapotassic magmas
A metasomatized 18O-rich veined lithospheric mantle source for ultrapotassic magmas
复制标题
DOI：
10.1016/j.lithos.2020.105964
复制
发表时间：
2021-02
期刊：
影响因子：
3.5
通讯作者：
Wenbo Fan;Neng Jiang;Jun-Cheng Hu;Danqing Liu;Lang Zhao;Tiejun Li
中科院分区：
文献类型：
--
作者：
Wenbo Fan;Neng Jiang;Jun-Cheng Hu;Danqing Liu;Lang Zhao;Tiejun Li
虽然超钾质岩石通常被认为是来自交代地幔源，其特征性的高δ 18 O值是否代表一个主要的或次要的特征，以及地幔交代作用是局部的还是普遍的仍然是悬而未决的问题。华北克拉通北方古老的亏损岩石圈地幔在晚古生代被俯冲沉积物中的物质局部交代，形成了富含不相容元素的含金云母辉石岩脉，其δ 18 O显著升高（9.9-12.3‰），高87 Sr/86 Sr（0.7091 - 0.7164），极低ε Nd（−25.2-− 18.2）。随后的交代脉熔融加上亏损橄榄岩寄主产生了早中生代超钾质岩石，同位素组成富集。该设想适用于以高δ 18 O值和高演化Sr单键Nd同位素组成为特征的阿尔卑斯-喜马拉雅期超钾质岩石。有人认为，交代的富18 O源只能作为地幔橄榄岩中的脉。极端的化学和同位素变化的超钾质岩石可以反映那些交代脉和/或可变比例的静脉材料和橄榄岩主机在部分熔融过程中参与。因此，不能直接利用幔源岩石的同位素组成来推断岩石圈地幔的性质。对于其他类型的幔源岩，如玄武岩、高镁安山岩和太古代sanukitoids，也可以推断出交代脉状地幔源。
While ultrapotassic rocks are generally considered to be derived from a metasomatized mantle source, whether their characteristically high δ18O values represent a primary or a secondary feature and whether the mantle metasomatism is localized or pervasive remain unresolved questions. Here we report that the old, depleted lithospheric mantle in the northern North China craton was locally metasomatized during the late Paleozoic by agents derived from subducted sediments, resulting in incompatible-element-rich phlogopite-bearing pyroxenite veins with anomalously elevated δ18O (9.9–12.3‰), high87Sr/86Sr from 0.7091 to 0.7164, and extremely low εNdbetween −25.2 and − 18.2. Subsequent melting of the metasomatic veins plus depleted peridotite host produced the early Mesozoic ultrapotassic rocks with variably enriched isotopic compositions. The proposed scenario can be applied to the Alpine-Himalayan ultrapotassic rocks that are characterized by elevated δ18O values and highly evolved Srsingle bondNd isotopic compositions. It is argued that the metasomatized18O-rich source can only be present as veins in the mantle peridotites. The extreme chemical and isotopic variations of the ultrapotassic rocks could reflect those of the metasomatic veins and/or variable proportions of vein material and the peridotite host involved during partial melting. Therefore, the isotopic compositions of mantle-derived rocks may not be directly utilized to infer the nature of the lithospheric mantle. A metasomatized veined mantle source may also be inferred for other types of mantle-derived rocks such as basalts, high-Mg andesites and the Archean sanukitoids.