Early Neoarchean oceanic crust in the North China Craton: Evidence from geology, geochemistry and geochronology of greenstone belts in western Shandong

摘要

Western Shandong is the only area in the North China Craton where abundant supracrustal rocks of the early and late Neoarchean are developed. This study presents an integrated geological investigations, whole-rock geochemistry and SHRIMP U-Pb zircon dating investigation of the supracrustal rocks at Yanlingguang, one of the most typical areas of Neoarchean supracrustal rocks in western Shandong. The original Yanlingguan 'formation' is now subdivided into three belts: the Northeastern, Central and Southwestern belts, which are all in tectonic contact. The Northeastern and Southwestern belts are mainly composed of meta-komatiitic and basaltic rocks. Four granitic dykes intruding these belts have magmatic zircon ages of 2731-2530 Ma, supporting the conclusion that the supracrustal rocks were formed in the early Neoarchean. Meta-basalt with pillow structures has been identified in several locations, with the largest being similar to 35 m in thickness. The meta-komatiitic rocks are Alundepleted komatiites and have Sigma REE and (La/Yb)(n) of 2.30-26.21 ppm and 0.29-2.35, respectively, whereas the meta-basaltic rocks have Sigma REE and (La/Yb)(n) of 16.45-53.26 ppm and 0.67-1.78, respectively, and epsilon(Nd) (t) values of 3.36-4.76, being similar to N-MORB. Eleven samples of meta-sedimentary rocks (including metasandstones-siltstones and meta-conglomerates) from the Central Belt have zircon U-Pb ages extending from 2760 to 2520 Ma, providing evidence for the first time that they were deposited during the late Neoarchean, rather than the early Neoarchean as previously thought. The meta-sandstones-siltstones show large variations in major element contents, but have low Sigma REE (49.17-102.30 ppm) and (La/Yb)(n) (1.53-7.00) ratios, with negligible to weakly negative or positive Eu anomalies (Eu/Eu* = 0.60-1.17), and record epsilon(Nd)(t) values of 0.96-3.21. Combined with regional geological investigations, we conclude that the early Neoarchean supracrustal rocks were formed in an oceanic environment. It is also suggested that the sandstones-siltstones of the late Neoarchean were derived from the early Neoarchean basement, and that the conglomerates were derived from two separate source regions; one of early, and the other of late Neoarchean age. The identification of basal conglomerates is consistent with the arc model established in early studies. We suggest that the criteria employed in this study can be used for determining whether or not an Archean greenstone belt is of oceanic affinity. (c) 2020 Elsevier B.V. All rights reserved.