辽东青城子矿集区白云金矿床煌斑岩的岩石成因：年代学、地球化学与Pb、Hf同位素约束

贾宏翔; 陈仁义; 庞振山; 林鲁军; 李安; 沙德铭; 薛建玲; 陈辉

doi:10.12029/gc20220516

辽东青城子矿集区白云金矿床煌斑岩的岩石成因：年代学、地球化学与Pb、Hf同位素约束

Lamprophyre origin of the Baiyun gold deposit in Qingchengzi ore concentration area, eastern Liaoning Province: Constraints of geochronology, geochemistry and Pb、Hf isotopes

摘要

摘要:
研究目的 白云金矿是辽东地区代表性的大型金矿床之一，通过厘清矿区内基性脉岩的形成时代、成因及其构造背景，有助于深化对区域构造演化过程及矿床成因的认识。
研究方法 本文基于矿床的野外地质特征，对与矿化有密切关系的煌斑岩开展了岩相学、年代学和地球化学研究。
研究结果 煌斑岩LA-ICP-MS锆石U-Pb年龄为（126.81±0.65）Ma，表明形成于早白垩世。岩石地球化学结果显示，煌斑岩属于碱性系列、钾玄质岩石，具有富镁（MgO=3.63%~4.07%，Mg^#=54~60）、富钾（K₂O/Na₂O=1.26~3.47）、富碱（K₂O+Na₂O=4.41%~5.23%）的特征；富集大离子亲石元素（Rb、Th、U）和轻稀土元素，亏损高场强元素（Nb、Ta、Ti）和重稀土元素，具有典型的与俯冲有关的弧岩浆岩特征。同位素地球化学结果显示，煌斑岩富集放射性Pb同位素；锆石¹⁷⁶Hf/¹⁷⁷Hf为0.282117~0.282471，具有负的ε_Hf(t)值（-20.80~-7.81），指示岩浆源区为受俯冲熔体/流体交代的富集岩石圈地幔。
结论结合区域大地构造背景，白垩纪时，由于古太平洋板块俯冲后撤产生伸展拉张环境，软流圈地幔物质的上涌可能促使EMII型富集地幔的部分熔融而形成基性岩浆，并沿郯庐深大断裂带及其次一级断裂上侵形成白云矿区煌斑岩。因此，本区早白垩世基性岩浆活动是对该时期华北克拉通东部岩石圈减薄的响应。

Abstract:
This paper is the result of the mineral exploration engineering.
Objective The Baiyun gold deposit, located in the eastern margin of the North China Craton, is one of the typical gold deposits in the Qingchengzi ore concentration area of Fengcheng City, Liaoning Province. By clarifying the formation age, genesis and tectonic background of the mafic dikes in the mining area, it is beneficial to deepen the understanding of the regional tectonic evolution process and the genesis of the deposit.
Methods Based on the field geological characteristics of the deposit, the lamprophyres closely related to mineralization, have been studied in terms of petrography, chronology and geochemistry in this paper.
Results LA-ICP-MS zircon U-Pb data reveals that the lamprophyre formed at (126.81±0.65) Ma, indicating that it developed in the Early Cretaceous. Petrogeochemical characteristics show that the lamprophyres belong to alkali series and potassium hypostatic rocks, which are rich in magnesium (MgO=3.63%-4.07%, Mg^# =54-60), potassium (K₂O/Na₂O=1.26-3.47) and alkali (K₂O + Na₂O= 4.41%-5.23%). The lamprophyres are enriched in large ion lithophile elements (Rb, Th, U) and light rare earth elements, but depleted in high field strength elements (Nb, Ta, Ti) and heavy rare earth elements, and exhibit typical arc magmatic features associated with subduction. Isotopic geochemical characteristics demonstrate that the lamprophyres have a high concentration of radioactive Pb isotopes. The initial ¹⁷⁶Hf/¹⁷⁷Hf value of zircon is 0.282117-0.282471, with negative ε_Hf(t) values (-20.80--7.81), suggesting that the magma source is the enriched lithospheric mantle influenced by subducted melt/fluid metasomatism.
Conclusions Integrated with the regional tectonic setting, the bottom of the lithospheric mantle was eroded by the heat of the asthenosphere mantle during the Cretaceous period due to the extensional environment caused by the subduction and rollback of the Paleo-Pacific plate, which resulted in partial melting and the formation of the Early Cretaceous mafic magma. The lamprophyres with arc characteristics in the Baiyun mining area may have formed along the Tanlu fault zone and its secondary faults. Therefore, the Early Cretaceous mafic magmatism in this region represents a response to the lithosphere thinning in the eastern North China Craton.

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