Genetic Mechanism and Metamorphic Evolution of Khondalite Series Within the Paleoproterozoic Mobile Belts, North China Craton

Abstract

The Paleoproterozoic khondalite series rocks of the North China Craton, consisting mainly of graphite-bearing Al-rich gneiss/granulite, garnet quartzite, felsic paragneiss, calc-silicate rock, and marble, contain important information about the composition, tectonic evolution, and growth history of the early continental crust, and provide important insights into the formation and evolution of the North China Craton. Integrated analysis of petrography, geochemistry, mineral compositions, metamorphic reaction textures and history, thermobarometry and pseudosection modeling, and geochronology of these khondalite series rocks from the Wulashan-Daqingshan Complex within the Khondalite Belt and Jiaobei Complex within the Jiao-Liao-Ji Belt, combined with previous metamorphic and geochronological data, indicated the following sedimentary, metamorphic, magmatic, and tectonic processes. (i) The sedimentary protoliths of the khondalite series rocks in the Khondalite Belt and Jiao-Liao-Ji Belt were likely deposited in an active continental margin in the late Paleoproterozoic, ranging from 2200 to 2000 Ma and from 1950 to 1930 Ma, respectively. (ii) During the period from 2000 to 1950 Ma, the protoliths of the khondalite series rocks from the Khondalite Belt and Jiao-Liao-Ji Belt experienced a crustal thickening event due to subduction of the Paleoproterozoic oceanic crust. As a result of tectonic burial and heat conduction, a greenschist facies to amphibolite facies prograde metamorphism occurred in the upper and middle crust region of the Khondalite Belt and Jiao-Liao-Ji Belt, forming the early mineral assemblage of garnet (core) and inclusion-type minerals (e.g., biotite, quartz, and plagioclase). (iii) During the period from 1950 to 1900 Ma, accompanied by cessation of oceanic crust subduction, the aggregation between the Paleoproterozoic arc and Archean continent and the collision between the Archean continents caused regional middle- and high-pressure granulite-facies metamorphism, as represented by the mineral assemblage of garnet and matrix-type minerals (e.g., kyanite/sillimanite, K-feldspar, plagioclase, biotite, quartz, Fe–Ti oxide, and liquid) in the pelitic granulites of the Khondalite Belt and Jiao-Liao-Ji Belt, occurring in a region of ca. 35–45 km of the thickened lower crust or arc root at ca. 1950–1900 Ma. (iv) During the period from 1900 to 1800 Ma, after the main collision and peak middle- and high-pressure granulite-facies metamorphism, the Paleoproterozoic khondalite series rocks experienced post-peak retrogressive metamorphism in the post-collisional exhumation and extension stage. The post-peak metamorphic and tectonic processes can be divided into two stages. During the early fast tectonic exhumation stage, the pelitic granulites of the Paleoproterozoic khondalite series rocks from the Khondalite Belt and Jiao-Liao-Ji Belt experienced slight heating or a nearly isothermal decompression, which resulted in overprinting of the regional middle- and low-pressure granulite-facies metamorphism, and the development of cordierite-bearing symplectite around the garnets. Meanwhile, emplacement of the ca. 1900–1850 mafic dyke/sill swarms and partial melting of the crustal rocks were caused by underplating of mantle-derived mafic magma in the early stage. After the temperature peak, the late retrogressive metamorphism was dominated by slow near-isobaric cooling processes, which resulted in overprinting of the regional amphibolite facies metamorphism and the development of biotite-bearing symplectites around garnets in the pelitic rocks. In summary, the Paleoproterozoic khondalite series rocks from the Khondalite Belt and Jiao-Liao-Ji Belt are characterized by clockwise P–T–t paths bearing isothermal decompression and isobaric cooling processes metamorphic processes, suggesting that they were involved in the rapid deposition-, subduction-, aggregation-, and collision-related tectonic processes in an active continental margin or continental arc setting during the period from 2200 to 1800 Ma.