云南大学王永彬等取得一项新突破。他们报道了对华北克拉通下大地幔楔开始的新认识。2024年7月2日出版的《地球化学学报》发表了这项成果。

据介绍，华北克拉通下大地幔楔(BMW)构造的起源至今仍有争议。团队人员对BMW之上胶东半岛晚中生代花岗岩与金矿床的成因的研究，可以为这一问题提供新的认识。

赵县—沙岭金矿区的二长花岗岩是在154-148Ma期间侵入的。这种I型花岗岩具有高钾钙碱性和金属铝的特征。二长花岗岩形成于中温(718-770°C)，石榴子石稳定场深处加厚的下地壳中。二长花岗岩中锆石Ce⁴⁺/Ce³⁺、Eu_N/Eu_N^*值较高，FeO^T/MgO比值较低，表明其形成于高氧环境。其变化的ε_Hf(t)值与T_DM2的比值为1.93-2.87Ga表明其起源于古地壳基底的熔融，并有地幔源物质的贡献。

花岗岩的LREEs和LILEs富集，HREEs和HFSEs亏损，微量元素构造辨析图和中等Sr/Y值表明，其在活动大陆边缘环境下与埃达克岩亲和。华北克拉通东部从S型花岗岩到I型花岗岩再到A型花岗岩的转变，表明构造环境由挤压型向伸展型转变。这种变化还表现在由平缓俯冲向陡俯冲的转变上。因此，二长花岗岩形成于晚侏罗世古太平洋板块俯冲角度变化引发的构造过渡环境中。这一事件可能标志着华北克拉通上方BMW的开始。

附：英文原文

Title: Fresh insights into the onset of big mantle wedge beneath the North China Craton

Author: Wang, Yingpeng, Wang, Xuance, Zhang, Wen, Yu, Xiaowei, Wang, Ligong, Wang, Jinhui, Zhu, Peigang, Wang, Yongbin

Issue&Volume: 2024-07-02

Abstract: The onset of the big mantle wedge (BMW) structure beneath the North China Craton remains debated. Research on the genesis of Late Mesozoic granites associated with gold deposits in the Jiaodong Peninsula above the BMW could provide fresh insights into this question. The monzogranite from the Zhaoxian-Shaling gold district was intruded during 154–148 Ma. This I-type granite has high-K calc-alkaline and metaluminous characteristics. The monzogranite formed at medium temperatures (718–770 °C) and was generated in a thickened lower crust at depths within the stability field of garnet. The monzogranite's high zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N^*values and low FeO^T/MgO ratios, suggest that it formed in a high oxygen environment. Its variable ε_Hf(t) values with T_DM2 of 1.93–2.87 Ga imply that it originated from the melting of ancient crust basement, with contributions from mantle-derived materials. The granite's enrichment in LREEs and LILEs, and depletion in HREEs and HFSEs, along with its trace element tectonic discrimination diagrams and medium Sr/Y, indicate an adakite affinity in an active continental margin setting. The transition from S-type granites to I-type granites and finally to A-type granites observed in the eastern part of North China Craton suggests a shift in the tectonic environment from compression to extension. This change is also reflected in the transition from flat subduction to steep subduction. Therefore, the monzogranite was formed in a tectonic transition setting triggered by a change in the subduction angle of the Paleo-Pacific Ocean slab during the Late Jurassic. This event may have marked the initiation of the BMW above the North China Craton.

DOI: 10.1007/s11631-024-00716-z

Source: https://link.springer.com/article/10.1007/s11631-024-00716-z