U–Pb and Hf isotopic study of zircons of the Helanshan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton

Abstract

The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, calc-silicate rock and marble, together called the Khondalite Series rocks. LA-Q-ICP-MS U–Pb geochronology and LA-MC-ICP-MS Lu–Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U–Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85–2.53Ga) and the other in the Palaeoproterozoic (2.2–2.0Ga), suggesting that the sedimentary protoliths of the Khondalite Series rocks in the Helanshan Complex must have been deposited at some time after 2.0Ga. The Hf analyses show that the Archaean detrital zircons possess negative εHf(t) values from −7.64 to −0.15 with depleted mantle model ages ranging from 3.34 to 3.10Ga. This implies the existence of a Paleo-Mesoarchaean (3.34–3.10Ga) crust in the Western Block, which underwent a recycling event in the period 2.85–2.53Ga. Nearly all Palaeoproterozoic (2.2–2.0Ga) detrital zircons possess positive εHf(t) values (0.86–9.38) with depleted mantle model ages of 2.45–2.15Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~1.95Ga and the other at ~1.87Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~1.86Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex.