A Neoarchean arc-back-arc system in Eastern Hebei, North China Craton: Constraints from zircon U-Pb-Hf isotopes and geochemistry of dioritic-tonalitic-trondhjemitic-granodioritic (DTTG) gneisses and felsic paragneisses

摘要

Dioritic-tonalitic-trondhjemitic-granodioritic (DTTG) gneisses are widely exposed in the Zunhua-Qinglong microblock of Eastern Hebei, North China Craton (NCC). LA-ICP-MS zircon U-Pb isotopic dating for the DUG gneisses in the Qinglong area reveals that their magmatic precursors were emplaced between 2517 +/- 10 and 2505 +/- 23 Ma. Zircons of these DTTG gneisses exhibit positive epsilon(Hf)(t) values of 0.0 to +6.5, with adakite-like geochemical features represented by relatively high MgO, Mg# and (La/Yb)(N) ratios. Petrogenetic studies reveal that dioritic magmas were derived from depleted mantle peridotites that were strongly modified by slab-derived melts/fluids and that the tonalitic-trondhjemitic-granodioritic (TTG) magmas were formed by a high degree of fractional crystallization in the dioritic magmas with hornblende as the primary fractionated phase.