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 FeO^T/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.

华北克拉通孔兹岩带中识别出两期新太古代晚期-古元古代早期以及古元古代晚期的含石榴子石淡色花岗岩，分别命名为包头含石榴子石淡色花岗岩和集宁-凉城含石榴子石淡色花岗岩。地球化学研究显示，含石榴子石淡色花岗岩具有较高的Al₂O₃含量与全铁（FeOᵀ）/MgO比值，且CaO含量以及K₂O/Na₂O比值变化范围较大。此外，该类岩石富集轻稀土元素与大离子亲石元素，亏损Nb、Ta、P及Ti。不过其Eu异常存在一定差异，表现为Eu正异常与Eu负异常两种模式。对包头含石榴子石淡色花岗岩和集宁-凉城含石榴子石淡色花岗岩开展的高灵敏度高分辨率离子探针（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，锆石εHf(t)值分别为-0.01~3.93和-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分析显示，包头变泥质片麻岩的形成年龄约为2.35~2.47 Ga，集宁-凉城变泥质片麻岩的形成年龄约为1.90~1.91 Ga，这代表了与深熔作用相关的构造变质事件年龄。结合地质与岩石学观测、地球化学及年代学数据，本研究得出以下认识：（1）含石榴子石淡色花岗岩由变泥质片麻岩深熔形成；（2）它们是淡色体、残留矿物与幔源物质的混合产物；（3）本次研究识别出两期含石榴子石淡色花岗岩；（4）两期含石榴子石淡色花岗岩分别对应新太古代晚期-古元古代早期以及古元古代晚期的构造热事件。