华北中部造山带南缘洛宁东部太华变质杂岩SIMS锆石U-Pb年龄及其地质意义

太华变质杂岩出露于华北克拉通中部造山带南缘,整体呈近EW向展布。洛宁地区的太华变质杂岩岩性多样,保留了至少3个阶段的变质矿物组合。作为前人研究的补充,报道了该地区东部含榴斜长角闪片麻岩和长英质片麻岩的SIMS锆石U-Pb定年结果,证实该区太华变质杂岩至少保留了3期地质事件的年龄。其中两期岩浆年龄(2.32~2.33 Ga和2.18 Ga)表明,太华变质杂岩与华北中部造山带中—北段各杂岩一样,共同经历了华北克拉通的地壳再造过程;另一期变质年龄(1.94~1.95 Ga)表明,太华变质杂岩的变质作用开始时间比该造山带中—北段大部分杂岩区早了约0.1 Ga,变质作用延续时间也长了约0.1 Ga。这再次说明华北中部造山带古元古代末的构造 变质事件是一个漫长且复杂的过程。

SIMS U-Pb Ages of Zircon and Tectonic Significance of Taihua Metamorphic Complex in the Eastern Luoning,the Southern Terminal of Trans-North China Orogen

Located in the southern terminal of the Palaeoproterozoic Trans-North China Orogen (TNCO), garnet-bearing felsic and amphibolitic gneisses in the eastern Luoning terrane within the nearly EW-striking Taihua metamorphic complex, preserve three generations of metamorphic mineral assemblages, i.e., the prograde, peak and retrograde metamorphic stages. The prograde assemblages (M1) comprise of hornblende+plagioclase+quartz inclusions preserved in the garnet porphyroblasts. The metamorphic peak assemblages (M2) are represented by the garnet porphyroblasts as well as the matrix minerals (plagioclase+hornblende+ quartz+zircon±biotite±K-feldspar). The retrograde assemblages (M3) are the fine grained “white-eye socket” symplectitic assemblages formed between the relict garnet porphyroblasts and the adjacent matrix minerals, consisting of plagioclase+hornblende+quartz. In order to complement our previous research, SIMS U-Pb dating results of zircons separated from the felsic and amphibolitic gneisses are reported here, which show that (a) the Luoning complex records two magmatic episodes (2.32-2.33 Ga and 2.18 Ga), suggesting that the southernmost terminal of the TNCO is also involved in the crust reworking; and (b) the peak metamorphic event took place at 1.94-1.95 Ga in the Luoning complex, implying that the Precambrian metamorphism of the Luoning Taihua complex not only occurs earlier (~100 Ma) than, but also lasts longer than that of most of the other metamorphic terranes within the middle and northern TNCO. Combined with the literature data concerning the different discrete terranes within the TNCO, it is therefore certified that the tectono-metamorphic event of the Palaeoproterozoic TNCO is a long and complex history.