Geochronology and petrogenesis of Paleoproterozoic post-collisional quartz monzodiorites from the Helanshan Complex, North China Craton: implications for crust–mantle interaction

Abstract Insights into crust–mantle interaction in continental collisional orogens is key to understanding high- and ultrahigh-temperature (HT–UHT) metamorphism, continental crustal growth, and the tectonic evolution of such orogens. Here, we present whole-rock major- and trace-element and Nd isotope geochemistry, as well as zircon U–Pb geochronology and Hf isotope data, for quartz monzodiorites from the Helanshan Complex to constrain their petrogenesis and draw implications regarding crust–mantle interaction during formation of the Khondalite Belt in the North China Craton. LA–ICP–MS zircon U–Pb dating yields crystallization ages of 1938–1931 Ma for three samples, contemporaneous with spinel-bearing UHT pelitic granulites from the Helanshan Complex. The quartz monzodiorite samples have variable whole-rock compositions, with relatively low SiO2 (low to 51.41 wt.%) but high MgO (up to 5.05 wt.%) and Cr (up to 80.9 ppm) contents, as well as high Mg# values (up to 0.50), which suggest their derivation from the mantle. These samples exhibit a narrow range of eNd(t) values of −0.3 to +1.0, with no decrease as decreasing whole-rock MgO content, and variable zircon eHf(t) values of −2.4 to +5.8. In addition, Mg# values of hornblende and biotite, and Xan values of plagioclase decrease gradually with increasing whole-rock SiO2 content. All of these geochemical data indicate that the studied quartz monzodiorites were produced by fractional crystallization of mantle-derived magma, with assimilation of crustal material. On the basis of previously published data in the Khondalite Belt, the Helanshan quartz monzodiorites formed in the post-collisional setting, and record a tectonic transition from compression to extension during the period ca. 1.95–1.93 Ga. The emplacement of mantle-derived magma that formed the quartz monzodiorites through differentiation at ca. 1.93 Ga, was possibly responsible for the formation of ca. 1.93 Ga UHT rocks in the Helanshan Complex.