Neoarchean intra-oceanic arc system in the Western Liaoning Province: implications for early Precambrian crustal evolution in the Eastern Block of the North China Craton

Abstract

Archean crust–mantle interaction processes provide keys to understanding crustal evolution in the Early Earth. The North China Craton (NCC) represents a natural laboratory to evaluate early Precambrian crustal evolution. The tectonic framework of the NCC is composed of two major crustal blocks, the Western Block (WB) and the Eastern Block (EB). The northwestern margin of the EB preserves voluminous metamorphosed supracrustal basaltic to andesitic rocks of ca. 2522–2640 Ma in the Fuxin greenstone belt within the Western Liaoning Province (WLP). These were intruded by the magmatic precursors of ~2495–2521 Ma granitoid gneisses,which were generally metamorphosed under ca. 2485 Ma regional metamorphism up to granulite facies, followed by ca. 2401–2450 Ma amphibolite facies retrogression. Geochemically, the metamorphosed volcanic rocks can be subdivided into five groups with affinities to MORBs, island arc tholeiites (IATs), calc-alkaline basalts (CABs), high-magnesium andesites (HMAs), and adakites. The petrogenesis and temporal–spatial distribution of these metamorphosed volcanic rocks and the intrusive granitoid gneisses in theWLP indicate that this basement terrane probably evolved as an intra-oceanic arc system, with major crustal growth at ca. 2.5–2.6 Ga. These rocks record complex crust–mantle evolution history as follows. Partial melting of the depleted to slightly enriched asthenospheric mantle at a spreading ridge occurred at ~2600–2640 Ma, generating both MORB-like rocks and juvenile oceanic lithospheric mantle sources. The mantle sources were gradually metasomatized by slab-derived fluids and melts during ~2506–2550Ma, and the partial melting of both the metasomatized lithosphericmantle and the descending oceanic slabs generated IATs, CABs, HMAs, and adakites (as well as tonalite–trondhjemite– granodiorite (TTG) gneisses of the high-magnesium group). Meanwhile, the coeval underplating of mantle-derived materials triggered the partial melting of the arc-root materials, yielding TTG gneisses of the low magnesium group. Finally, the accretion of the intra-oceanic arc terrane to the ancient nucleus of the EB at ca. 2490 Ma resulted in the regional granulite-facies metamorphism and crustal anatexis (resulting in the formation of potassium-rich granitoid rocks), possibly related to asthenospheric upwelling. Based on a comprehensive analysis of the basement terranes in the EB, a late Neoarchean (~2.5–2.6 Ga) intraoceanic arc system with relict MORB-like basaltic rocks bordering the northwestern margin of the continental nucleus (ca. 2.7–3.8 Ga) was established, extending from northern Liaoning Province, through western Liaoning Province, the Zunhua–Qinglong block of eastern Hebei Province, northern Hebei Province, and the Huai'an– Xuanhua complexes of northwestern Hebei Province, up to theWutai complex. To the northwest of the proposed “Wutai–Zunhua–Majuanzi” boundary, the intra-oceanic arc systemis characterized by the general lack of ancient crystalline basement (≥2.7 Ga), and late Neoarchean metamorphosed basaltic rocks showing geochemical affinities to MORBs–IATs–CABs represent the oldest rocks yet identified. Accordingly, the development of both plume-related komatiites and metamorphosed arc-related calc-alkaline volcanic rocks as well as granitoid gneisses within the interior of the EB to the east suggest early Neoarchean (ca. 2.7 Ga) plume–craton and plume–arc interactions as the major crust–mantle geodynamic processes. In contrast, late Neoarchean (ca. 2.5–2.6 Ga) slab–wedge interaction and arc–continent accretion processes are compatible with the intra-oceanic arc system as well as the asymmetric distribution of the basement rocks.