Paleoproterozoic crustal growth in the North China Craton: Evidence from the Lüliang Complex

Abstract

The building of continental blocks and their assembly into supercontinents are linked to episodic crustal growth through Earth history, where both vertical and horizontal accretions in convergent plate margins play a significant role. Here we investigate the Paleoproterozoic crustal growth in the North China Craton (NCC) from a traverse covering continent-trench-ocean section in the Lüliang Complex within the western margin of the Trans-North China Orogen, a major accretionary belt that sutures the Eastern and Western Blocks in the NCC. We present petrological, geochemical and zircon U–Pb and Lu–Hf data from a suite of samples including meta granitoids and gabbroic diorite that built the continental arc, and metabasalts, felsic tuff, mafic and pelitic schists, and garnet amphibolites that form part of an imbricated package of oceanic and continental material that was accreted onto the eastern margin of the Western Block in the NCC. The salient geochemical features of the magmatic suite such as high Th/Ta and La/Nb ratios, negative Nb–Ta anomalies, and high concentration of LILE as compared to HFSE suggest magma derivation from sub-continental mantle source which has undergone different degree of enrichment with crustal components or material derived from subduction-related older components. In tectonic discrimination diagrams, the granitoids show VAG affinity and all the rocks straddle the field between IAB and MORB reflecting subduction-related arc signature. The majority of zircon grains in all the rock types from the Lüliang Complex show typical magmatic crystallization textures and high Th/U values. Zircons from the meta-granitoids and diorite enclave yield upper intercept or 207Pb/206Pb mean ages in the range of 2250±20 to 2084±69Ma. Those in the mafic schist show a mean age of 2234±9Ma. In the felsic schist group, the mean ages are in the range of 2298±15 to 1933±11Ma. A minor older population of zircons with ages of 2992±28Ma and 2610±80Ma also occurs in these rocks. In the metabasalt, zircon 207Pb/206Pb mean ages are between 2391±40 and 2022±39Ma with rare magmatic xenocrysts showing Mesoarchean age of 3294±21Ma. Zircons in the felsic tuff show mean ages of 2155±25 to 1897±24Ma, with xenocryst populations yielding mean ages of 2528±31Ma and 2710±20Ma. The youngest zircons with 207Pb/206Pb mean age of ca. 1.9Ga from the felsic schist (metasediment) and felsic tuff (volcanic suite) suggest deposition within the trench of an active convergent margin at this time. In the garnet-bearing amphibolite, the zircons yield a limited age range of 1996±17Ma to 1984±12Ma. The Lu–Hf data on zircons from all samples show dominantly positive ɛHf(t) values. In the granitoid–diorite suite, the ɛHf(t) values range up to 7.6 with crustal residence ages (TDMC) of 2371–2729Ma. Zircons in the mafic and felsic schists also possess positive ɛHf(t) values (up to 9.6) with TDMC in the range of 2366–2798Ma. In the metabasalt, both positive and negative ɛHf(t) values are displayed (3.8 to −3.2) with TDMC in the range of 2495–2830Ma. Zircons in the felsic tuff also show prominent positive ɛHf(t) value (up to 7.4) and crustal residence ages range from 2274 to 2607Ma. In the garnet-bearing amphibolite, the ɛHf(t) values lie between 3.3 and 4.8 and TDMC are in the range of 2321–2413Ma. The majority of ɛHf(t) values from all the rock types in the present study are compatible with juvenile magma sources, with limited input from crustal components. Their TDMC values indicate that both Neoarchean and Paleoproterozoic components were involved in the source, attesting to a continental arc setting. The Hf data also suggest the possible presence of Mesoarchean vestiges among the dominantly Neoarchean–Paleoproterozoic basement. Our data demonstrate active plate tectonics and major crustal growth during the Paleoproterozoic history of the NCC, broadly coeval with the making of the global supercontinent Columbia, and provide insights into continent building through subduction–accretion–collision tectonics analogous to those in Phanerozoic orogenic belts.