Metamorphism of Neoarchean Basement in Shandong Province

Abstract

Mafic granulites from the Yishui Group of the Western Shandong Complex in the Eastern Block of the North China Craton occur as enclaves or boudins within Late Archean TTG gneisses, and are composed mainly of garnet, clinopyroxene, orthopyroxene, plagioclase, hornblende, and minor quartz, ilmenite, and magnetite. Petrographic examination has revealed three distinct metamorphic mineral assemblages: the pre-peak prograde assemblage (M1) of hornblende + plagioclase + quartz + ilmenite + magnetite occurring as inclusions within garnet and pyroxene grains, peak assemblage (M2) of orthopyroxene + clinopyroxene + plagioclase + garnet + hornblende + quartz + ilmenite + magnetite, and post-peak assemblage (M3) represented by garnet + quartz and garnet + ilmenite/magnetite symplectites. Pseudosection modeling using THERMOCALC in the NCFMASHTO model system for a representative sample constrains the P–T conditions of M1, M2, and M3 stages at 660–730 °C/<6.6 kbar, 800–820 °C/8.0–8.5 kbar, and 686–710 °C/7.6–8.6 kbar, respectively. The results of petrology and quantitative P–T pseudosection modeling define an anticlockwise P–T path involving near-isobaric cooling following the peak medium-pressure granulite-facies metamorphism, suggesting that the metamorphism of the Yishui Group was most likely related to the intrusion and underplating of mantle-derived magmas. Although the underplating of voluminous mantle-derived magmas leading to granulite-facies metamorphism with an anticlockwise P–T path involving isobaric cooling may occur in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift environments, a mantle-plume model is favored because this model can reasonably interpret many other geological features of Late Archean basement rocks from the Western Shandong Complex in the Eastern Block of the North China Craton as well as their anticlockwise P–T paths involving isobaric cooling. The relatively cooler mantle-plume head heated the crust initially, causing amphibolite-facies metamorphism (M1). Subsequently, the relatively hotter mantle-plume tail heated the crust, causing granulite-facies metamorphism (M2). Finally, a near-isobaric cooling process (M3) occurred when the mantle plume ceased to heat the crust.