Anatexis of late Neoarchean-Early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite from the Khondalite Belt in the North China Craton

Two episodes of late Neoarchean-early Paleoproterozoic and late Paleoproterozoic garnet-bearing leucogranite, named the Baotou and Jining–Liangcheng garnet-bearing leucogranite are recognized from the Khondalite Belt in the North China Craton. Geochemical studies show that garnet-bearing leucogranite has high Al2O3 content and FeOT/MgO ratio, with a large variation in the CaO content and K2O/Na2O ratio. Additionally, the garnet-bearing leucogranite is enriched in light rare earth elements and large ion lithophile elements and depleted in Nb, Ta, P, and Ti. However, there are some variations in Eu anomalies, as indicated by the Eu-enriched and Eu-depleted patterns. The sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of the Baotou garnet-bearing leucogranite and Jining–Liangcheng garnet-bearing leucogranite show that their anatectic ages were 2.35–2.43 Ga and 1.90–1.92 Ga, respectively, with zircon εHf (t) values of −0.01–3.93 and −4.36–1.99, and two-stage model ages (TDM2) of 2.76–3.02 Ga and 2.57–2.83 Ga, respectively. U-Th-Pb SHRIMP analyses on zircon reveal ages of ca. 2.35–2.47 Ga for the Baotou metapelite gneiss and ca. 1.90–1.91 Ga for the Jining–Liangcheng metapelite gneiss, which represent the ages of tectonic metamorphic events related to the anatexis. Our results, combined with geological and petrographical observations, geochemistry, and geochronology, suggest that (1) Garnet-bearing leucogranites formed as a result of anatexis of metapelite gneisses; (2) They are mixtures of leucosomes, residual minerals and mantle-derived materials; (3) Two episodes of garnet-bearing leucogranites have been identified; (4) Both two episodes of garnet-bearing leucogranites corresponded to late Neoarchean-early Paleoproterozoic and late Palaeoproterozoic tectonothermal events, respectively.

华北克拉通（North China Craton）孔兹岩带（Khondalite Belt）中识别出两期含石榴子石淡色花岗岩（garnet-bearing leucogranite），分别命名为包头含石榴子石淡色花岗岩与集宁-凉城含石榴子石淡色花岗岩，其形成时代分别为新太古代晚期-古元古代早期与古元古代晚期。地球化学研究表明，含石榴子石淡色花岗岩具有较高的三氧化二铝（Al₂O₃）含量与全铁镁比（FeOT/MgO），其氧化钙（CaO）含量与钾钠比（K₂O/Na₂O）存在较大变化范围。此外，该类花岗岩富集轻稀土元素（light rare earth elements）与大离子亲石元素（large ion lithophile elements），却亏损铌（Nb）、钽（Ta）、磷（P）及钛（Ti）。不过，其铕异常（Eu anomaly）存在一定差异，表现为铕富集与铕亏损两种模式。对包头及集宁-凉城含石榴子石淡色花岗岩开展的敏感高分辨率离子探针（sensitive high-resolution ion microprobe，SHRIMP）锆石U-Pb定年结果显示，二者的深熔年龄分别为2.35~2.43 Ga与1.90~1.92 Ga；对应的锆石εHf(t)值分别为-0.01~3.93与-4.36~1.99，两阶段模式年龄（TDM2）分别为2.76~3.02 Ga与2.57~2.83 Ga。对锆石开展的U-Th-Pb SHRIMP分析结果显示，包头泥质片麻岩的形成年龄约为2.35~2.47 Ga，集宁-凉城泥质片麻岩的形成年龄约为1.90~1.91 Ga，该年龄代表了与深熔作用相关的构造变质事件的时限。结合野外地质与岩石学观察、地球化学数据以及年代学结果，本研究得出以下认识：① 含石榴子石淡色花岗岩由泥质片麻岩发生深熔作用形成；② 该类花岗岩为浅色体（leucosome）、残留矿物与幔源物质的混合产物；③ 研究区共识别出两期含石榴子石淡色花岗岩；④ 两期花岗岩分别对应新太古代晚期-古元古代早期与古元古代晚期的构造热事件。