Paleoproterozoic tectonic evolution of the Khondalite Belt in the North China Craton: Constraints from the geochronology and geochemistry of 1.9–2.3 Ga felsic and basic intrusive rocks in the Jining area

Abstract

Although the Paleoproterozoic Khondalite Belt is one of the most important delineation boundaries in the North China Craton, its tectonic evolution remains controversial. Here, we present new geochronological and geochemical data for Paleoproterozoic granodiorite gneiss, amphibolite, and gabbro and monzogranite dykes in the Jining area in order to constrain the tectonic evolution of the Khondalite Belt. Zircon U–Pb dating indicates that the granodiorite gneiss, amphibolite and monzogranite were emplaced at ca 2.3 Ga, 2.0 Ga and 1.9 Ga, respectively. The 2.3 Ga granodiorite gneisses have positive εNd(t) values (+0.6 to +1.3) and high zircon saturation temperatures (864–873 °C), and show geochemical characteristics of A2-type granites, probably indicating extension background related to subduction. The 2.0 Ga amphibolites have high Nb contents and their protoliths display geochemical features of Nb-enriched basalts. The low MgO, Cr, Ni, LREE-enriched patterns, and negative εNd(t) values (−5.5 to −6.3) with relatively old Nd model ages suggest that the protoliths were derived from a fertile mantle source metasomatized by slab-derived melts. The distinct geochemical characteristics suggest that the formation of the amphibolites were attributed to the upwelling asthenosphere triggered by the opening of slab window. Based on the U-Pb ages of zircons and the contact relationship in the outcrops, the intrusive age of the gabbros was constrained to be 2.0–1.9 Ga. These gabbro dykes have high Mg#, Cr, and Ni contents, and show E-MORB-like REE patterns with high contents of large ion lithophile elements and negative Nb-Ta anomalies, indicating a subducting tectonic setting. Their high Ba/Th and Rb/Y ratios, low Th/Ta and Nb/Y ratios imply a mantle source metasomatized by slab-derived fluids. The 1.9 Ga monzogranite dykes have high Sr and Sr/Y ratios, low Yb and HREE, similar to those of typical adakitic rocks. Their low MgO, Cr, Ni, Mg# and negative εNd(t) suggest that the dykes were generated by partial melting of thickened lower continental crust. These different rocks, therefore, indicate a convergent margin occurred along the Khondalite Belt before the collision between the Yinshan Block and the Ordos Block, followed by extension event at ca. 2.3 Ga and closed with a collision at ca. 2.0–1.9 Ga, and then changed to post-collisional tectonic setting with the adakitic monzogranite dykes formed at ca. 1.9 Ga.