Geochemistry and geodynamic significance of the rare-earth mineralized Paleoproterozoic Longwangzhuang granite on the southern margin of the North China Craton

The Longwangzhuang granite pluton occurs on the southern margin of the North China Craton and consists mainly of biotite syenogranite with aegirine granite being locally distributed. The granites are characterized by high silicon and alkaline contents (SiO2=72.17%-76.82%, K2O+Na2O=8.28%-10.22%, K2O/Na2O≫1), AI (agpaitic index) =0.84-0.95, DI=95-97, ASI (aluminum saturation index)=0.96-1.13, and very high Fe* number (FeO*/(FeO*+Mg)=0.90-0.99), thus the granites are assigned to the metaluminous to weakly peraluminous, alkalic to calc-alkalic ferroan A-type granites. The granites are rich in large ion lithophile elements (LILE), especially high in REE concentrations (REE+Y=854×10-6-1572×10-6); whereas the enrichment of high strength field elements (Nb, Ta, Zr, Hf) is obviously less than that of LILEs, exhibiting mild depletions on trace element spider plots; and the rocks are significantly depleted in Ba, Sr, Ti, and Pb. The low εNd(t) values (-4.5- -7.2) and high model ages (2.3-2.5 Ga) of the granites as well as the low εHf(t) values (-1.11- -5.26) and high Hf model ages (THf1= 2.1-2.3 Ga, THf2=2.4-2.6 Ga) of zircons from the biotite syenogranite suggest that the granites were probably derived from an enriched mantle source. The zircons from the biotite syenogranite are mainly colorless transparent crystals exhibiting well-developed oscillatory zoning on the cathodoluminescence images with a LA-ICPMS zircon U-Pb age of 1602.1±6.6 Ma (MSWD=0.48). Petrochemical, trace elements, as well as Nd and Hf isotopic compositions of the rocks demonstrate that the granites were formed in a within-plate extensional tectonic regime possibly related to the breakup of the Columbia supercontinent. The granites were most likely formed through extreme fractional crystallization of alkali basaltic magma resulted from partial melting of the mantle, which was fertilized by recycling crustal rocks triggered by the delamination of lithospheric mantle and lower crust following the ~1.8 Ga collision and amalgamation of the North China Craton which is part of the Columbia supercontinent. However, contamination of neo-Archean to Paleoproterozoic crustal rocks during the ascent and emplacement of the magma could not be excluded. Being the youngest known anorogenic magmatism on the southern margin of the North China Craton related to Columbia breakup, it might represent the break off of the North China Craton from Columbia supercontinent at the end of Paleoproterozoic. © 2011 Science Press, Institute of Geochemistry, CAS and Springer Berlin Heidelberg.