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

摘要

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 Lidliang 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 Lilliang Complex show typical magmatic crystallization textures and high Th/U values. Zircons from the meta-granitoids and diorite enclave yield upper intercept or Pb-207/Pb-206 mean ages in the range of 2250 +/- 20 to 2084 +/- 69 Ma. Those in the mafic schist show a mean age of 2234 +/- 9 Ma. In the felsic schist group, the mean ages are in the range of 2298 +/- 15 to 1933 +/- 11 Ma. A minor older population of zircons with ages of 2992 +/- 28 Ma and 2610 +/- 80 Ma also occurs in these rocks. In the metabasalt, zircon Pb-207/Pb-206 mean ages are between 2391 +/- 40 and 2022 +/- 39 Ma with rare magmatic xenocrysts showing Mesoarchean age of 3294 +/- 21 Ma. Zircons in the felsic tuff show mean ages of 2155 +/- 25 to 1897 +/- 24 Ma, with xenocryst populations yielding mean ages of 2528 +/- 31 Ma and 2710 +/- 20 Ma. The youngest zircons with Pb-207/Pb-206 mean age of ca. 1.9 Ga 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 garnetbearing amphibolite, the zircons yield a limited age range of 1996 +/- 17 Ma to 1984 +/- 12 Ma. The Lu Hf data on zircons from all samples show dominantly positive epsilon Hf(t) values. In the granitoid diorite suite, the epsilon Hf(t) values range up to 7.6 with crustal residence ages (T-DM(C)) of 2371-2729 Ma. Zircons in the mafic and felsic schists also possess positive epsilon Hf(t) values (up to 9.6) with T-DM(C) in the range of 2366-2798 Ma. In the metabasalt, both positive and negative epsilon Hf(t) values are displayed (3.8 to 3.2) with T-DM(C) in the range of 2495-2830 Ma. Zircons in the felsic tuff also show prominent positive epsilon Hf(t) value (up to 7.4) and crustal residence ages range from 2274 to 2607 Ma.