Saturday, December 26, 2015

Evidence of an Arc or Back Arc System From NeoArchean North China

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

Bai et al


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 DTTG 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 ɛHf(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.

LA–ICP–MS zircon U–Pb isotopic analyses reveal that the protoliths of the felsic paragneisses in the Northern Zunhua area were deposited in the Late Neoarchean with a maximum depositional age of 2517 ± 20 Ma. Most of the detrital zircon grains display apparent 207Pb/206Pb ages from 3432 ± 20 to 2502 ± 21 Ma, with negative ɛHf(t) values of −15.4 to −2.4 and calculated TDMC ages between 3995 and 3368 Ma, suggesting that they were formed from the recycling of Eo- to Paleoarchean crustal materials closely related to the southeastern ancient continental crust of the interior Eastern Block. These data together with previous investigations reveal that the basement rocks of Zunhua–Qinglong microblock formed in a Neoarchean arc–back-arc system with the subduction direction from present northwest to southeast.

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