Multiple 3.8-3.1Ga tectono-magmatic events in a newly discovered area of ancient rocks (the Shengousi Complex), Anshan, North China Craton
Near Anshan city in the North China Craton, the oldest rocks in Asia (⩾3300 Ma) have been thoroughly documented at two localities (Baijiafen and Dongshan). In this paper we report the full geological context for more ancient rocks from a third Anshan locality – the polyphase migmatite Shengousi Complex. SHRIMP U–Pb zircon dating indicates a protracted tectono-magmatic history for the Shengousi Complex: The oldest recognised component is banded trondhjemitic gneiss (3773 ± 6 Ma), which is veined by strongly deformed granitic pegmatite. These occur with a second generation of trondhjemitic rocks (3454 ± 8 and 3448 ± 9 Ma). The next generation of plutonic rocks is a composite suite of iron-enriched mafic dykes (3332 ± 6 and 3331 ± 8 Ma) with broadly coeval felsic veins (3311 ± 4 Ma). Finally there was intrusion of monzogranite (3129 ± 6 Ma). Strong deformation has generally brought these 3773–3129 Ma plutonic phases into close concordance to form banded rocks. However, locally cross-cutting relationships are preserved in small lower strain domains, that give a relative chronology agreeing with the absolute zircon U–Pb chronology. All of this complex history is recorded in a single <50 m wide outcrop.
Some of the <3600 Ma rocks of the Shengousi Complex contain 3780–3730 and 3660–3600 Ma inherited zircon xenocrysts. The stronger prominence of 3660–3600 Ma components distinguishes the Shengousi Complex from the Baijiafen and Dongshan complexes, where components of this age are rarer. One possibility is that at 3660–3600 Ma, the Shengousi Complex was at a deeper crustal level than the Baijiafen and Dongshan complexes, and underwent migmatisation at that time.
The ∼3450 Ma igneous phases discovered in the Shengousi Complex are new ages for igneous rocks in the Anshan area. The Precambrian geology of the Anshan area is thus marked by polyphase Eoarchaean orthogneisses, the newly recognised ∼3450 Ma phases, widespread 3360–3300 magmatic activity, found in association with Mesoarchaean plutonic rocks and the Neoarchaean Anshan Group and Palaeoproterozoic Liaohe Group metasedimentary rocks. This shows that the geology of the Anshan area has a strong similarity with the Narryer Gneiss Complex of Western Australia, which contains the polyphase Eoarchaean Meeberrie gneisses, the 3490–3440 Ma Eurada gneiss association, ∼3300 Ma granites and migmatisation, and is intercalated with younger Precambrian metasedimentary rocks of different ages containing Hadean detrital zircons. No other ancient gneiss complexes in the world show such a close match in their history with the Anshan area rocks. This means that the Anshan area is prospective for locating new occurrences of Hadean crustal components intercalated with younger sedimentary rocks.