Abstract
The Trans-North China Orogen (TNCO) is well-known as an early Precambrian orogenic belt in the North China Craton (NCC). Three meaningful groups of metamorphic ages have been reported in the metamorphic complexes throughout the TNCO, including c. 1.85 Ga, c. 1.95 Ga, and c. 2.5 Ga. The spatial distributions and relationships of these ages provide notable insight into the formation timing and tectonic evolution of the NCC basement. The Zuoquan and Zanhuang complexes are exposed in the south–central TNCO and are adjacent to the Eastern Block. In order to place new constraints on the timing of two phases of metamorphism that occurred in the complexes, combined U–Pb and rare earth element analyses were performed on zircons from different types of metamorphic rocks. Uranium–Pb zircon dating in this study shows that two groups of metamorphic ages of 1.88–1.85 Ga and 2.48–2.46 Ga were commonly recorded by metamorphic rocks in the Zanhuang and Zuoquan complexes, respectively. Our previous geochronological studies showed that metamorphic ages of c. 2.51 Ga and c. 1.90 Ga were locally recorded in the Zanhuang and Zuoquan complexes, respectively. These data indicate that metamorphic rocks in the two complexes underwent at least two phases of metamorphism, i.e., 2.51–2.46 Ga (Phase I) and 1.90–1.85 Ga (Phase II). In combination with previous studies regarding reaction microstructures, metamorphic pressure–temperature paths, and geochronology, the Phase II metamorphic ages are interpreted to be linked to the collision between the Western and Eastern Blocks along the TNCO between 1.97 Ga and 1.80 Ga, whereas the Phase I metamorphic ages, as a result of an earlier and extensive tectono-thermal event that occurred in the Eastern and Western Blocks of the NCC, were related to underplating of mantle-derived magma. It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and then were tectonically involved in the TNCO and experienced c. 1.90–1.85 Ga metamorphism. Metamorphic peaks occurred at different crustal levels in the orogen, resulting in distinct metamorphic ages and peak conditions preserved by metamorphic rocks in the two complexes.
Highlights
The Precambrian basement of the North China Craton (NCC) is considered to have formed through the amalgamation of several microblocks; there is disagreement regarding the timing of the main amalgamation [1,2,3,4,5,6,7,8,9]
It is inferred that the rocks with c. 2.51–2.46 Ga metamorphic ages in the two complexes formed in the Eastern Block and underwent regional metamorphism during that period, and were tectonically involved in the Trans-North China Orogen (TNCO) and experienced c. 1.90–1.85 Ga metamorphism
The majority of Archean metamorphic rocks from the WB and Eastern Block (EB) recorded counterclockwise P−T paths and c. 2.5 Ga metamorphic ages, which were thought to be related to the underplating of large amounts of mantle-derived magma, whereas metamorphic complexes exposed in the three orogenic belts showed clockwise P–T paths related to continental collisions [2,3,10]
Summary
The Precambrian basement of the North China Craton (NCC) is considered to have formed through the amalgamation of several microblocks; there is disagreement regarding the timing of the main amalgamation [1,2,3,4,5,6,7,8,9]. A model proposed by Zhao et al (1998, 2005, 2012) [2,3,10] attracted considerable attention due to it being the only study that considered metamorphic pressure–temperature (P−T) paths together with metamorphic geochronology. In their interpretation, the NCC basement was thought to have formed through three collisional stages and was divided into three major parts, namely, the Western. The second-stage collision occurred between the Longgang and Nangrim
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