PaleoProterozoic Tectonic Collision Revealed in Daqingshan Complex, Inner Mongolia

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex, North China Craton

Authors:

Cai et al

Abstract:

Medium-pressure pelitic granulites crop out in Daqingshan, Inner Mongolia, China, in the central segment of the Khondalite Belt, which is a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. The mineral assemblages and microtextures of representative pelitic granulites, including Sil–Grt–Bt gneisses, Crd–Grt–Bt gneisses, and Opx–Grt–Bt gneisses, allow a reconstruction of the metamorphic history of the area. The rocks contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, coupled with fine-grained inclusions of biotite + plagioclase + K-feldspar + quartz ± sillimanite ± ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle) + K-feldspar + plagioclase + biotite + quartz ± sillimanite ± orthopyroxene ± ilmenite ± magnetite ± rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet + biotite + cordierite + plagioclase + quartz ± K-feldspar ± sillimanite ± spinel ± ilmenite ± magnetite (M3) in the Crd–Grt–Bt gneisses. Retrograde cooling (M4) assemblages are represented by biotite + muscovite + sillimanite + quartz + plagioclase + K-feldspar ± ilmenite. Quantitative phase equilibria modeling in the system CaO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O, in combination with traditional thermobarometry, was applied to obtain P–T conditions of less than 780 °C and less than 9 kbar for M1, 840–880 °C and 9–11 kbar for M2, 800–870 °C and 5–7 kbar for M3, and less than 660 °C and 4.1–6.8 kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result, ties in with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved subduction or collision followed by exhumation and cooling. The clockwise P–T path reveals a collisional event between the Yinshan and Ordos blocks that amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.