GEOCHEMISTRY OF NEOARCHEAN MAFIC VOLCANIC ROCKS AND LATE MAFIC DIKES AND SILLS IN THE ZANHUANG COMPLEX, CENTRAL OROGENIC BELT, NORTH CHINA CRATON: IMPLICATIONS FOR GEODYNAMIC SETTING

An Archean mélange belt has been recognized in the Zanhuang complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a circa 2.5 Ga granite pluton and cross-cut by undeformed 2.5 Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic-ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) (La/Sm_cn=1.39-2.37, 1.89-2.76) and pronounced negative Nb (Nb/Nb*=0.29-0.72, 0.26-0.53) and Zr (Zr/Zr*=0.30-0.85, 0.58-0.82) anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc-forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5 Ga. The collision between the arc and the Eastern Block of the NCC happened at ~2.5 Ga, resulting in the formation of the Archean mélange belt in the Zanhuang complex, followed by intrusion of the granitic and pegmatitic melts of ~2.5 Ga.. All of the units of the mélange belt were then intruded by late mafic dikes and sills, which are present as bouninaged or lenticular shapes in the mélange and surrounding gneisses. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their age, an arc-polarity reversal event is proposed to have occurred following the arc-continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern block, forming the swarm of arc-affinity mafic dikes, which were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC.