Xinping Wang

Nature of Three Episodes of Magmatism (2181 Ma, 2115 Ma and 1891 Ma) in the Liaohe Rift of North China: Implications for Tectonic Evolution

to subduction and their respective duration time are usually largely debated. Such case occurs for the Liaohe rift: whether it went through a long subduction from 2100 to 1850 Ma or there were two different events in different tectonic settings? In this study, we investigate two episodes of granites in the Liaohe rift: the 2180 Ma Mafeng monzogranite gneiss and 1890 Ma Qingchengzi trondhjemite gneiss, in addition to a ~2115 Ma mafic sill event. SIMS U-Pb dating on magmatic zircons from two granites yields concordant ages of 2181 ± 6 Ma (n=20, MSWD=4.3) and 1891± 10 Ma (n=8, MSWD=1.8), representively for the monzogranite and trondhjemite. The monzogranites, with high-K calc-alkaline and metaluminous to weak peralumious characteristics, have high Fe2O3 of 3.70-4.08 wt.%, TiO2 of 0.29-0.31 wt.%, K2O of 4.98-5.07 wt.% but low MgO of 0.09-0.12wt.%, Al2O3 of 12.09-12.44 wt.%, CaO of 0.55-0.87 wt.%, and are enriched in light rare earth elements (total: 213-346 ppm, La/YbN: 5.04-10.57) and elements that are compatible in plagioclase (i.e., Eu, Ba and Sr) but depleted in high field strength elements (i.e., Nb, Ta, P and Ti). They have high 10000×Ga/Al ratios of 3.19-3.61, Zr concentrations of 315-441ppm and Zr+Nb+Ce+Y concentrations of 472-656 ppm. These above features indicate that these monzogranites belong to A-type granites. The trondhjemites, with medium-K calc-alkaline and medium peralumious features, have relatively high Al2O3 of 15.51-15.6 wt.%, CaO of 1.80-2.32 wt.%, Na2O of 5.37-5.4 wt.% but relatively low Fe2O3 of 1.20-1.40 wt.%, TiO2 of 1.91-2.32 wt.%, K2O of 1.91-2.32 wt.%, and are enriched in light rare earth elements (total: 17.621.6 ppm, La/YbN: 19.14-20.43) and large ion lithospheric elements (i.e., Ba, K and Sr) but depleted in high field strength elements (i.e., Nb, Ta, P, Ti and Y). These characters, combined with high Sr/Y ratios of 327-413, could be comparable with adakites. In the meanwhile, the Liaohe Group and the 2115 Ma Haicheng mafic sills followed the 2180-2140 Ma aluminous A-type granites, suggesting a rift environment while the 1900 Ma regional metamorphism companied with the 1890-1850 Ma trondhjemite, granitic pegmatite, porphyritic granite and alkaline syenite, suggesting a subduction-orogeny process.

Tectonic Environments of the Yan-Liao Rift during Earth's Middle Age (1.7∼0.75 Ga): Evidence from Mafic Dyke Swarms in Eastern Hebei, North China

mafic dykes/sills have been investigated and revealed in the North China Craton (NCC). Researchers identified more than 20 episodes of Precambrian dyke swarms, which have significantly improved our understanding of the tectonic evolution of the NCC (Peng, 2015 and references therein). Three-stage Proterozoic mafic dyke swarms are identified in the periphery of the Yan-Liao rift in eastern Hebei Province, all of which are diabase and intruded the Archean basement with clearly chilled margins. Precise U–Pb SIMS analysis yields Pb/Pb weighted average ages of 1677±12 Ma (baddeleyite), 1236±7 Ma (baddeleyite), and 775±5 Ma (zircon) for three representative dykes, which are interpreted as their crystallization ages. These dykes are referred to as the Tujiagou (~1680 Ma; NW trending, >20 m wide for each), Maojiagou (~1230 Ma; 0o–10o trending, ~40 m wide), and Dingjiagou (~775 Ma; ~70o trending, 12–20 m wide) dyke swarms, respectively. All these dykes show typical ophitic texture and their mineral assemblages are mainly composed of plagioclase and clinopyroxene, with various degrees of epidotization, chloritization, and uralitization. Based on whole-rock chemistry of these dykes, little crustal contamination was supposed to be involved during their emplacement. They are tholeiitic (for the 1680 Ma and 775 Ma dykes) and alkaline (for the 1230 Ma dykes) in compositions. The Tujiagou dykes (1680 Ma) have low Mg# (44–46) and have experienced clinopyroxeneand plagioclasedominated fractional crystallization. They have (La/Yb)N of 5.3–7.7 and are depleted in some HFSEs, such as Zr and Hf, with the εNd(t) value of -1.9. The Maojiagou dykes (1230 Ma) show relatively higher Mg# (51–57) and have experienced plagioclase-dominated fractional crystallization with the crystallization of Fe-Ti oxides resisted. The dykes exhibit light REE enrichment ((La/Yb) N =6.48–12.8) and slight or no depletion in HFSEs. Both the 1680 Ma and 1230 Ma dykes exhibit similar εNd(t) values (-1.9 to 1.3). All these features indicate similar depleted mantle sources for both swarms. The Dingjiagou dykes (775 Ma) have Mg# of 47–60 and have experienced plagioclase-dominated fractionation. They also present significantly negative Ceanomalies and show medium light REE enrichment ((La/Yb)N =2.3–4.12) and deplete in HFSEs (such as Nb, Ta, Zr, etc.) with εNd(t) values of -7.2 to -5.1. We suggest that they could be originated from the metasomatized sub-continental lithospheric mantle. The 1680 Ma Tujiagou dykes are coeval and chemically similar to and could be equivalents of the Laiwu dykes in Luxi area (west Shandong province) (Li et al., 2015). The 1230 Ma Maojiagou dykes are similar to some other 1240 –1210 Ma mafic dykes/intrusions in several regions of the NCC. The 775 Ma Dingjiagou dykes are newly reported. These dyke swarms occurred in the adjacent areas of the Yan-Liao rift, and could represent a long-living (from ~1730 Ma to ~775 Ma) and stepwise opening rift in the northern parts of the craton. These dyke swarms could be potential geological records that were dismembered during the breakup of supercontinents in the Middle Age (1700700 Ma) and the markers for paleogeographic reconstruction. Specifically, the 1230 Ma dykes and the synchronous Sudbury dykes in Laurentia (Shellnutt and MacRae, 2011) could be parts of a large igneous province, and the NCC was neighboring the Laurentia till ~775 Ma. However, this hypothesis awaits more data, especially paleomagnetic study.