Characterising early Precambrian collisional orogens: A metamorphic perspective from the Lüliang Complex and the Trans-North China Orogen

Abstract

The Trans-North China Orogen (TNCO) is a typical Paleoproterozoic continental collisional orogen, which witnessed the amalgamation of the Eastern and Western blocks to form the North China Craton (NCC) and is comparable with many other early Precambrian collisional orogens in the world. As an important part of the TNCO, the Lüliang Complex comprises a series of greenschist to granulite facies metamorphosed rocks and is an ideal target for decoding the tectonic evolution of the TNCO and revealing the typical metamorphic pattern of early Precambrian collisional orogens. Garnet amphibolite samples were collected from the northwestern part of the Lüliang Complex to determine the metamorphic characteristic of the region. An integrated investigation of petrography, mineral chemistry and phase equilibria modelling suggests that the rocks have recorded clockwise P–T paths with thermal peaks of 8.5–9.5 kbar and 800–860 °C. Zircon U-Pb dating of a representative sample yields a metamorphic age of 1901 ± 17 Ma. The zircon grains show left-inclined REE patterns with HREE enrichment and low crystallization temperatures of 614–695 °C, indicating that the obtained metamorphic age should represent the retrograde stage of metamorphism. Combined with the metamorphic characteristics of other regions in the Lüliang Complex, it can be inferred that the Lüliang Complex has experienced medium-pressure facies series metamorphism, with greenschist to amphibolite facies in the middle and amphibolite to granulite facies in the northwest and southeast. Such a metamorphic framework is similar with that in the Hengshan-Wutai-Fuping area as well as other complexes in the TNCO, indicating that the metamorphic pattern of the orogen is characterized by a fan shape, with low- to medium-grade rocks in the middle and medium- to high-grade rocks on the flanks. Synthesized P–T–t data from the TNCO reflect that there exists a protracted (>100 Ma) orogenic evolution in the central NCC, including a main crustal thickening at 1.97–1.93 (~1.95) Ga followed by uplifting-cooling from 1.92 to 1.85 (1.80) Ga. Comparison of tectono-metamorphic evolution between the TNCO and other cases in the world indicates that progressive metamorphism and long-lived P–T–t evolution may be two common characteristics for early Precambrian collisional orogens.