Ages, geochemistry and metamorphism of the neoarchean basement in Shandong province : implications for evolution of the eastern block, North China Craton

Abstract

The Archean basement rocks in Shandong Province are important components of the Eastern Block in the North China Craton, consisting predominantly of granitoid gneisses with minor mafic and felsic supracrustal rocks and charnockites. They are exposed, from west to east, in the Luxi Granite-Greenstone Terrane, Yishui Terrane and Jiaodong Terrane, of which the low-grade Luxi Granite-Greenstone Terrane has been well studied, whereas little work has been done on the medium- to high-grade Yishui and Jiaodong Terranes. Controversies have long surrounded the timing of crustal growth and tectonic setting of these two terranes. This project is designed to resolve these issues by integrating field investigations, petrography, geochronology, geochemistry and metamorphism of the Yishui and Jiaodong Terranes.

New zircon U-Pb data from the major lithologies have revealed that the Jiaodong Terrane experienced multi-stage magmatism at ~2.9 Ga, ~2.7 Ga and 2.6-2.5 Ga followed by two metamorphic events at ~2.50 Ga and 1.9-1.8 Ga, whereas the Yishui Terrane underwent single magmatism at 2.57-2.53 Ga followed by a single metamorphic event at ~2.50 Ga. Zircon Hf isotopic data have revealed that the Jiaodong Terrane underwent juvenile crustal growth with significant ancient crustal material additions at 3.4-3.1 and 2.8-2.7 Ga, while the Yishui Terrane experienced juvenile crustal growth with significant addition of crustal material at 2.8-2.7 Ga. Both the Jiaodong and Yishui Terranes experienced crustal reworking with minor juvenile additions at 2.6-2.5 Ga.

Whole-rock compositions and Nd isotopes from the Jiaodong granitoid gneisses suggest that their protoliths are typical Archean tonalitic-trondhjemitic-granodioritic (TTG) suite. They are high in SiO2, Al2O3, Na2O, Sr and Sr/Y ratios, but low in MgO, K2O, TiO2, Cr, Ni, Y and Mg#. They are generally enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements and high field strength elements, with slight Eu anomalies. Two groups of granitoid gneisses have been recognized: low- and high-HREE groups, of which the former was generated from partial melting of metabasaltic rocks leaving eclogite in the residue, whereas the latter was formed by partial melting of metabasaltic rocks leaving garnet-amphibolite in the residue. Whole-rock Nd isotopes reveal that the protoliths of Mesoarchean granitoid gneisses were derived mainly from juvenile sources, whereas the early Neoarchean granitoid gneisses were derived from juvenile sources with significant additions of crustal material and the late Neoarchean granitoid gneisses were mainly derived from continental crustal sources. These features indicate that the protoliths of the Jiaodong granitoid gneisses were possibly formed by partial melting of thickened lower crust related to underplating of mafic magmas.

Mafic granulites (~2.50 Ga) of the Yishui Terrane show three distinct mineral assemblages corresponding to the pre-peak, peak and post-peak metamorphic stages, with P-T conditions constrained at 660-730℃/<6.6 kbar, 800-820℃ /8.0-8.5 kbar and 686-710℃/7.6-8.6 kbar, respectively, by using THERMOCALC pseudosection modeling. The results define an anticlockwise P-T path involving near-isobaric cooling following the peak metamorphism, suggesting that the ~2.50 Ga metamorphism was most likely related to the intrusion and underplating of mantle-derived magmas.

Collectively, the results of this study suggest that the underplating of mantle-derived magmas was most likely related to a mantle plume, which is favored to account for the significant Neoarchean crustal growth and the large-scale metamorphism at the end of Neoarchean in Shandong Province as well as the whole Eastern Block in the North China Craton.