Mineralogy and low-grade metamorphism of early Paleozoic clastic rocks from eastern North Qilian Fold Belt

Illite and chlorite cyrstallinity indices, muscovite b₀ value and diagnostic mineralogy of Silurian and Ordovician slates and metasandstones from seven outcrop sections of eastern North Qilian Fold Belt, NW China were determined utilizing X-ray powder diffraction (XRD) and petrographic techniques to investigate metamorphic conditions associated with the Caledonian orogeny. The illite and chlorite crystallinity indices have a nearly 1:1 correlation regardless of lithology and fall in the range of high-grade anchizone and epizone. The b₀ value of potassium white mica spans from 9.01Å to 9.04 Å. The fabric-forming mineralogy of most samples is depicted by an assemblage of quartz + muscovite + chlorite + albite ± pyrophyllite ± paragonite ± calcite. Kaolinite, prehnite, epidote (in the Lower Silurain rocks), biotite (Lower Ordovician rocks), and an assmeblage of actinolite + epidote (possibly late Proterozoic rocks) are locally present. Intermediatemafic volcaniclastic rocks are locally intercalated with some of the Lower Ordovician clastic rocks and contain an assemblage of biotite + actinolite ( ± magnesiohornblende). The data collectively suggest that the Lower Paleozoic clastic rocks of eastern North Qilian Fold Belt were affected by an orogenic intermediate P/T metamorphism (presumably Caledonian) under low- to medium-grade greenschist facies conditions. The event reached its peak at ca. 300°C and ca. 3-4 kbar in the Lower Silurian rocks, locally approaching ca. 400°C in the Ordovician rocks. It is tentatively suggested that the differences in grade and age of deposition may be directly related to the peak of subduction of the previously suggested North Qilian oceanic plate during late Ordovician.