Middle Paleoproterozoic tectonic setting of Khondalite Belt: Evidence from ~2.2Ga charnockite in Zhuozi

Abstract

麻粒岩-紫苏花岗岩杂岩在华北克拉通孔兹岩带内分布较广但研究程度较低，本文对孔兹岩带东部卓资地区大什字紫苏碱长花岗岩进行了详细的岩相学、地球化学和地质年代学研究，探讨了其形成时代、岩石成因及构造背景，为孔兹岩带构造演化及孔兹岩系沉积环境提供约束。SHRIMP锆石U-Pb测年结果表明大什字紫苏碱长花岗岩形成于古元古代中期（~2.2Ga），并经历了古元古代晚期构造热事件及~1.89Ga变质作用叠加改造。岩石具有铁质、钙碱性至碱钙性、准铝质至弱过铝质特征，具有较高SiO₂、Na₂O+K₂O、Ga、Zr含量及FeO^T/（FeO^T+MgO）值，低Al₂O₃、CaO、MgO、Sr、Cr含量及较高的全岩锆石饱和温度（835~887℃，平均860℃），与A型花岗岩指标相近。样品呈现平坦的右倾型稀土元素配分模式和弱的负Eu异常，富集K、Rb、Ba、Zr、Hf，亏损Sr、Nb、Ta、P、Ti。综合研究表明大什字紫苏碱长花岗岩是在弧后伸展背景下幔源岩浆上涌导致英云闪长质、花岗闪长质地壳岩石部分熔融形成，熔体可能经历了斜长石、含钛矿物相及磷灰石分离结晶作用。孔兹岩带东部在古元古代中期（~2.2Ga）可能处于弧后伸展背景，孔兹岩带是否存在古元古代中期陆内裂解事件有待进一步研究。;The granulites-charnockites complex is widely distributed in the Khondalite Belt, North China Craton. Despite its significance, relatively little attention is paid to its further research. This paper makes a detailed study on the petrography, geochemistry, and geochronology of the Dashizi charnockite in Zhuozi of eastern Khondalite Belt, and reveals its formation age, petrogenesis, and tectonic setting, which provides constraints for the tectonic evolution of the Khondalite Belt and sedimentary environment of the khondalite series. SHRIMP zircon U-Pb dating shows that Dashizi charnockite formed in middle Paleoproterozoic (~2.2Ga), and was subjected to late Paleoproterozoic tectono-thermal events and~1.89Ga metamorphism. The samples are ferroan, calc-alkalic to alkali-calcic, and metaluminous to slightly peraluminous, they exhibit high SiO₂, Na₂O+K₂O, Ga, Zr contents and FeO^T/(FeO^T+MgO) value, low Al₂O₃, CaO, MgO, Sr, Cr contents, and relatively high whole-rock zircon saturation temperature (835~887℃, average 860℃), which is similar to A-type granites. They show flat right dipping REE patterns and slightly negative Eu anomalies, enriching K, Rb, Ba, Zr and Hf, depleting Sr, Nb, Ta, P, and Ti. The Dashizi charnockite was formed by partial melting of tonalitic and granodioritic crustal rocks triggered by upwelling of mantle-derived magma under the back-arc extension setting, and the magma may experience fractional crystallization of plagioclase, Ti-bearing mineral and apatite. The eastern segment of the Khondalite Belt may be in a back-arc extensional tectonic setting in the middle Paleoproterozoic (~2.2Ga), whether there was a middle Paleoproterozoic intracontinental rift event in the Khondalite Belt remains to be further studied.