Geochemistry of Neoarchean mafic volcanic rocks and late mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton: Implications for geodynamic setting
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- 作者：Hao Deng ; Timothy Kusky ; Ali Polat ; Lu Wang ; Junpeng Wang ; Songjie Wang
- 关键词：North China Craton ; Zanhuang Complex ; Archean mé ; lange ; Mafic volcanic rocks ; Epidosites ; Intra-oceanic arc
- 刊名：Lithos
- 出版年：2013
- 出版时间：15 August, 2013
- 年：2013
- 卷：175-176
- 期：Complete
- 页码：193-212
- 全文大小：8021 K
文摘
An Archean m¨¦lange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the m¨¦lange belt are intruded by a ca. 2.5 Ga granite pluton and cross-cut by undeformed 2.5 Ga pegmatites, which constrains the Archean formation age of the m¨¦lange. Some exotic mafic-ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the m¨¦lange 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) and pronounced negative Nb and Zr 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 ca. 2.5 Ga, resulting in the formation of the Archean m¨¦lange belt in the Zanhuang Complex. All of the units of the m¨¦lange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the m¨¦lange and surrounding gneisses and are intruded by younger ca. 2.5 Ga pegmatites. 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 precise 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. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5 Ga granitic and pegmatitic melts into the m¨¦lange and the mafic dikes. The mafic dikes 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.