Platinum-group element geochemistry of Cenozoic basalts from the North China Craton: Implications for mantle heterogeneity

Forty-two Cenozoic （mostly Miocene） basalt samples from Jining, Chifeng, Fansi, Xiyang, and Zuoquan areas of the North China Craton （the NCC basalts hereafter） were analyzed for platinum-group elements （PGE, including Os, It, Ru, Rh, Pt, and Pd）. Most of them are alkaline basalts and tholeiites and all of them display little crustal contamination. The total PGE contents of the NCC basalts vary from 0.1 to 0.9 ppb, much lower than those of the primitive mantle values of 23.5 ppb. Primitive man- tie-normalized PGE patterns of these basalts define positive slopes and Pd/Ir ratios vary from 1.2 to 25. In terms of both PGE contents and Pd/Ir ratios, they are quite similar to the mid-ocean ridge basalts. There are no obvious negative correlations be- tween PGE vs. MgO, Ni, and Cu in the NCC basalts, indicating that fractional crystallization of olivine, pyroxene, and/or sul- fides during magmatic process cannot be the controlling factor for the observed PGE variation. The observed Pd/Ir variations of the NCC basalts require involvement of non-chondritic heterogeneous mantle sources. Based on Sr-Nd-Pb-Hf isotopic sys- tematics and incompatible-element signatures, a mixing of partial melts from both asthenospheric peridotites and enclosed mantle eclogites at the top of asthenosphere was proposed for the origin of these NCC basalts. The lenses of eclogites are de- rived from upwelling of recycled continental crust during the westward subduction of the Pacific plate from the -600 km dis- continuity zone. The PGE geochemistry of these basalts provides independent evidence to support this conclusion and the ob- served Pd/Ir variations may reflect variations in proportions of tapped peridotitic and eclogitic melts.

42 新生代(主要中新世) 从北方中国 Craton 的 Jining， Chifeng， Fansi， Xiyang，和 Zuoquan 区域的暗岩样品(NCC 暗岩此后) 为铂组元素(PGE，包括的 O，红外， Ru， Rh，磅，和 Pd ) 被分析。他们中的大多数是碱的暗岩和 tholeiites，所有几乎不他们显示很少外壳的污染。NCC 暗岩的全部的 PGE 内容从 0.1 ～ 0.9 ppb 变化，原始披风的比那些低的大部分 23.5 珍视 ppb。这些暗岩的原始使披风正常化的 PGE 模式定义积极斜坡和 Pd/Ir 比率从 1.2 ～ 25 变化。以 PGE 内容和 Pd/Ir 比率，他们相当类似于海中央山脉暗岩。在 PGE 之间没有明显的否定关联，显示在 magmatic 期间的橄榄石，辉石，或硫化物的部分结晶化处理对在 NCC 暗岩的 MgO， Ni，和 Cu 不能是为观察 PGE 变化的控制因素。NCC 暗岩的观察 Pd/Ir 变化要求异构的披风采购的 non-chondritic 的参与。基于 Sr-Nd-Pb-Hf 同位素的 systematics 和不兼容元素的签名，一混合部分在岩流圈的顶从 asthenospheric 橄榄岩和围住的披风 eclogites 融化为这些 NCC 暗岩的起源被建议。eclogites 的透镜从再循环的大陆人外壳的 upwelling 被导出在期间向西从 600 km 断绝地区的和平的板的 subduction。这些暗岩的 PGE 地球化学提供独立证据支持这个结论和观察 Pd/Ir 变化可以在拍的 peridotitic 的比例反映变化， eclogitic 融化。