Geochronology And Geochemistry Of Amphibolites From Erdaowa Group In The Khondalite Belt,North China Craton And Their Geological Implications

The Khondalite Belt is a near east-west tectonic belt between the Yinshan blocks in the north and the Ordos blocks in the sorth of the Western Block of the North China Craton.A currently popular model regards the Khondalite Belt in the North China Craton as a typical continent-continent collisional belt along which the Yinshan and Ordos blocks amalgamated to form the Western Block at?1.95 Ga.Although this model is fully supported by the geological data of high-grade granulites rocks from the belt,it remains unknown whether it is consistent with the low-to medium-grade units in the belt.The Khondalite Belt consists of both high-grade Al-rich gneiss(politic granulite),quartzites,felsic paragneisses,calc-silicate rocks and marbles,together called the"Khondalite Series" in the Chinese literature.The LA-ICP-MS and SHRIMP U-Pb zircon dating results show that the sedimentary protolithos of high-grade Khondalite Series were deposited in the period of 2.3-2.0 Ga and metamorphosed at 1.95-1.92 Ga.One of such low-to medium-grade units from the Khondalite Belt is the Paleoproterozoic Erdaowa Group that consists of low-to medium schists,quartzites,amphibolitic rocks,calc-silicate rocks,marbles and minor meta-conglomerates,by studying the geochemistry and geochronology of the amphibole,it can provide important insights into understanding the tectonic setting and timing of collision of the Yinshan and Ordos blocks along the Khondalite Belt.The Erdaowa amphibolitic protoliths rocks can be divided into two groups of which Group 1 is assigned to tholeiitic basalts,whereas Group 2 belongs to basaltic andesites,but they both experienced the fractional crystallization of the primary magma that was derived from the partial melting of the lithospheric mantle with minor crustal contamination.Such partial melting of the lithospheric mantle most likely occurred in a subduction zone.The two groups of amphibolitic are both relatively enriched large ion LILE elements(Rb,Ba,Sm and K)and relatively deficient high field strength elements(HFSEs),such as Th,U,Nb,Ta and Ti,suggesting that their igneous protoliths have affinities to continental marginal arcs.Metamorphic zircons from two amphibolitic rock samples yield 207Pb/206 Pb ages of 1941 ±14 Ma(MSWD=0.87,n=22)and 1910±9 Ma(MSWD=0.60,n=22),similar to those metamorphic ages obtained for high-grade pelitic granulites from the Khondalite Belt.This confirms that the Yinshan-Ordos collision occurred at 1.95-1.90 Ga,and provides direct evidence for the involvement of low-and medium-grade rocks during the collision.