Petrogenesis and tectonic significance of Neoarchean (~2.6 Ga) alkaline ultrapotassic granitic gneisses from the southeastern margin of the North China Craton: Constraints from U-Pb dating, Hf isotope and petrogeochemistry
Petrogenesis and tectonic significance of Neoarchean (~2.6 Ga) alkaline ultrapotassic granitic gneisses from the southeastern margin of the North China Craton: Constraints from U-Pb dating, Hf isotope and petrogeochemistry
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华北克拉通东南缘新太古代（~2.6 Ga）碱性超钾花岗质片麻岩的岩石成因及构造意义：U-Pb定年、Hf同位素和岩石地球化学的制约
DOI：
10.1016/j.lithos.2021.106324
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Rolfo F.
中科院分区：
文献类型：
--
作者：
Su Hai-Yan;Yang Yang;Wang Cheng-Cheng;Liu Yi-Can;Groppo C.;Rolfo F.
出露于中国克拉通北缘东南缘的五和杂岩是北克拉通东部地块的重要组成部分。为了更好地了解本区前寒武纪地壳组成和演化，对马山WC中碱性超钾质花岗片麻岩进行了锆石年代学、Hf同位素、全岩地球化学和锶-钕-铅同位素等综合研究。根据地球化学和矿物学标准(特别参考锆石阴极发光图像)，这些片麻岩被认为是变质火成岩。富含碱性长石(60%以上)和石英(30%以上)，斜长石稀少。镁铁质矿物为钠基角闪石，如钠镁闪锌矿和镁铝榴石、霓石和稀有黑云母。次要成分为金红石、白云母、磷灰石和重晶石。常量元素地球化学特征为高SiO_2(69.85%~74.51%)和K2O+Na_2O(9.67%~12.17%)，高K_2O/Na_2O(7.41~22.53)，较低的MgO(0.37%~0.87%)和CaO(0.10%~0.23%)。微量元素地球化学显示，相对于造山花岗岩，Nb、Y、Ce、Ga和REE(稀土元素)明显亏损。这些特征表明，所研究片麻岩的岩浆原岩来自伸展背景，可能处于俯冲相关的裂谷环境中，属于超钾质硅质饱和碱性系列。~(206)Pb/~(204)Pb、~(207)Pb/~(204)Pb和~(208)Pb/~(204)Pb比值分别为16.6297~17.1877、15.4454~15.5066和36.6036~38.0304，ε的ND(T)值变化范围为−1.7~+9.1.两个样品的锆石207Pb/206Pb年龄分别为2615 Ma±14.4 Ma和2617 Ma±15.5 Ma，代表其前驱年龄。锆石火成核域呈振荡生长环带，εHf(T)值为正(+2.5t−+6.6h)。结合球粒陨石归一化和原始地幔归一化元素配分模式，表明花岗片麻岩的先驱体主要来源于俯冲变质的超钾正长岩母岩浆，可被认为是一组特殊的A型花岗岩，参与了一次重要的地壳生长和改造事件。这些岩石经历了麻粒岩相变质阶段，并伴有部分熔融，锆石变质域中的单斜辉石+金红石+钾长石+尖晶石+石英+磷灰石包裹体证明了这一点。变质阶段的形成还受到Σ稀土元素含量较低、Eu负异常和变质锆石温度(>；800°C)较高的支持。最终，所研究的岩石遭受了后来的~1.85t Ga变质叠加，可能与该区记录的古元古代碰撞造山作用有关。
The Wuhe complex (WC), exposed at the southeastern margin of the North China Craton (NCC), is an important constituent of the Eastern Block of the NCC. In order to better understand the Precambrian crustal composition and evolution of this region, a comprehensive investigation on zircon geochronology and Hf isotopes, as well as whole-rock geochemistry and Sr-Nd-Pb isotopes was conducted on alkaline ultrapotassic granitic gneisses from the WC at Mashan. These gneisses are considered as meta-igneous rocks based on geochemical and mineralogical criteria (with special reference to zircon cathodoluminescence images). They are characterized by modally abundant alkali feldspar (more than 60%) and quartz (more than 30%) while plagioclase is rare. Mafic minerals are sodic amphiboles such as arfvedsonite and eckermannite, aegirine and rare biotite. Minor constituents are rutile, muscovite, apatite and barite. Major elements geochemistry shows high SiO2(69.85%–74.51%) and K2O + Na2O (9.67%–12.17%) contents, high K2O/Na2O (7.41–22.53) ratios, and relatively low MgO (0.37%–0.87%) and CaO (0.10%–0.23%) contents. Trace elements geochemistry shows significant depletions of Nb, Y, Ce, Ga, and REE (rare earth elements) relative to anorogenic granites. These features suggest that the magmatic protoliths of the studied gneisses belong to ultrapotassic silica-saturated alkaline series from an extensional background, perhaps in a subduction-related rifting environment. As concerning isotope geochemistry, their206Pb/204Pb,207Pb/204Pb and208Pb/204Pb ratios are 16.6297–17.1877, 15.4454–15.5066, and 36.6036–38.0304, respectively, and εNd(t) values vary from −1.7 to +9.1. Two samples yielded zircon207Pb/206Pb ages of 2615 ± 4 Ma and 2617 ± 5 Ma, respectively, representing their precursor ages. The zircon igneous core domains exhibit oscillatory growth zoning with positive εHf(t) values (+2.5 − +6.6). These data, coupled with chondrite-normalized and primitive-mantle-normalized element patterns, suggest that the precursors of the studied granitic gneisses were mainly derived from a subduction-modified ultrapotassic syenitic parental magma, and may be considered as a particular group of A-type granites, involved in an important crustal growth and reworking event at ~2.6 Ga. These rocks experienced a granulite-facies metamorphic stage accompanied by partial melting, as testified by clinopyroxene + rutile + K-feldspar + spinel + quartz + apatite inclusions in zircon metamorphic domains which were dated at ~2.5 Ga. The occurrence of this metamorphic stage is also supported by the lower ΣREE contents, negative Eu anomalies, and high Ti-in-zircon temperatures (>800 °C) of metamorphic zircon mantles dated at ~2.5 Ga. Eventually, the studied rocks suffered a later ~1.85 Ga metamorphic overprinting, possibly related to the Paleoproterozoic collisional orogeny recorded in the region.