Late Paleoproterozoic tectono-thermal event in the northwestern North China Craton: Evidence from U-Pb dating and O-Hf isotopic compositions of zircons from metasedimentary rocks north of Hohhot City, Inner Mongolia, northern China

摘要

The Khondalite Belt is located in the northwestern North China Craton and contains late Paleoproterozoic metasedimentary rocks that generally underwent high-grade metamorphism. Only a few supracrustal sequences experienced low-grade metamorphism such as the Meidaizhao sequence in the Daqingshan area. The Erdaowa sequence north of Hohhot City, Inner Mongolia, is another late Paleoproterozoic supracrustal unit showing relatively low-grade metamorphism compared with the adjacent metapelitic khondalites. It is composed of metavolcanic as well as clastic metasedimentary rocks and marble. It was traditionally subdivided into the Halaqin and Hongshangou Formations. We undertook an integrated study of U-Pb zircon dating and O-Hf isotopic analysis, combined with whole -rock chemistry, on metasedimentary rocks of the Erdaowa sequence in order to understand the relationship between the low- and high-grade rocks and their tectono-thermal history in the Khondalite Belt. Cores and metamorphic domains of detrital zircons from a two-mica schist of the Halaqin Formation have Pb-207/Pb-206 ages of 2.0 and 1.86 Ga, delta(18) values of 6.69-9.45 parts per thousand and 6.68-12.15 parts per thousand, and epsilon(Hf)(t) values of + 7.7 to +16.2 and +3.3 to +20.9, respectively. A recrystallized core, fir-tree domain and altered domain of zircon from a Grt-St-Ky two-mica gneiss have Pb-207/Pb-206 ages of 2.0 Ga, 1.88 Ga and 1.86 Ga, delta(18) values of 12.10-12.75 parts per thousand, 11.57-12.65%o and 11.57-12.27 parts per thousand, and epsilon(Hf)(t) values of + 0.7 to + 5.4, 2.0 to +4.7 and 2.0 to +0.5, respectively. These data limit the time of deposition of the sedimentary rocks between 2.0 and 1.88 Ga and show a high maturity, similar to khondalites of the same sequence. Compared with previous studies, we infer that the Erdaowa sequence consists of rocks of different origins and metamorphic grades (including khondalites), which were brought up to the same crustal level as tectonic slices due to late Paleoproterozoic tectonic processes.