Petrogenesis and zircon LA-ICP-MS U–Pb dating of newly discovered Mesoarchean gneisses on the northern margin of the North China Craton

Geological mapping and zircon U–Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) dating has identified a Mesoarchean (2857 ± 17 Ma) geological unit in the Luanjiajie area of the northern margin of the North China Craton, within the northern part of Liaoning Province, China. This unit is dominated by tonalitic and trondhjemite gneisses that form part of a typical tonalite–trondhjemite–granodiorite (TTG) rock assemblage. These Mesoarchean gneisses are enriched in Na and depleted in K, yield K2O/Na2O ratios of 0.34–0.50, have Rittmann index (σ) values of 1.54–3.04, and are calc-alkaline. They have EuN/EuN* values of 0.77–1.20 (average of 1.03), indicating that these samples have negligible Eu anomalies, and yield high LaN/YbN values (4.92–23.12). These characteristics indicate that these Mesoarchean gneisses have fractionated rare earth element (REE) compositions that are enriched in the light REE (LREE) and depleted in the heavy REE (HREE), with steeply dipping chondrite-normalized REE patterns. These gneisses are also enriched in Rb, Th, K, Zr, and Hf, and are relatively depleted in Ta, Nb, P, and Ti. In summary, the magma that formed these tonalitic and trondhjemite gneisses was most likely derived from the partial melting of lower-crustal basaltic rocks during subduction. The timing of formation (2.85 Ga) of the Luanjiajie tonalite and trondhjemite gneisses probably represents the timing of initiation of plate tectonics within the LongGang Block during a SE-directed subduction event. The presence of inherited zircons with ages of >3.0 Ga within the Luanjiajie gneisses suggests that this area may contain as yet undiscovered rocks that formed before 3.0 Ga.

通过地质填图与锆石U-Pb激光剥蚀-电感耦合等离子体质谱（LA-ICP-MS）定年研究，在中国辽宁省北部华北克拉通（North China Craton）北缘的栾家街地区，识别出一个形成于中太古宙的地质单元，其年龄为2857±17 Ma。该单元以英云闪长质-奥长花岗质片麻岩为主，属于典型的英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）岩石组合。这套中太古宙片麻岩富钠贫钾，K₂O/Na₂O比值介于0.34~0.50之间，里特曼指数（σ）为1.54~3.04，归属于钙碱性岩石系列。其球粒陨石标准化后的EuN/EuN*值介于0.77~1.20（平均值为1.03），指示样品铕异常极微弱；同时LaN/YbN比值介于4.92~23.12之间，表明该套片麻岩的稀土元素（REE）配分具显著分馏特征：轻稀土元素（LREE）富集、重稀土元素（HREE）亏损，球粒陨石标准化稀土元素配分曲线呈陡倾右倾样式。该套片麻岩同时富集Rb、Th、K、Zr、Hf，相对亏损Ta、Nb、P、Ti。综上，形成该英云闪长质-奥长花岗质片麻岩的岩浆，最可能源自俯冲过程中下地壳玄武岩的部分熔融。栾家街英云闪长质-奥长花岗质片麻岩的形成时代（2.85 Ga），或代表龙岗地块（LongGang Block）在东南向俯冲事件中板块构造启动的时间。栾家街片麻岩中产出年龄大于3.0 Ga的继承锆石，暗示该区域可能存在尚未被发现的3.0 Ga之前形成的岩石。