Petrogenesis and tectonic setting of Neoarchean tonalitic-trondhjemitic-granodioritic gneisses in the Xiwulanbulang area of the Yinshan block, North China craton

Abstract

As one of the major Archean microcontinental blocks in the North China craton, the Yinshan block consists of the Guyang granite-greenstone belt and the Wuchuan high-grade complex, both of which preserve large volumes of Neoarchean tonalitic-trondhjemitic-granodioritic (TTG) gneisses. The petrogenesis and tectonic setting of these TTG gneisses are of great significance to understanding the Neoarchean crustal growth and evolution of the Yinshan block. Although extensive investigations have been carried out on the TTG gneisses in the Yinshan block, controversy remains regarding their petrogenesis and tectonic setting. In this study, we present new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the Neoarchean TTG gneisses in the Xiwulanbulang area from the Wuchuan high-grade complex. Laser ablation−inductively coupled plasma−mass spectrometry zircon U-Pb dating results from six TTG gneissic samples show that their igneous TTG plutons were emplaced in the period 2.61−2.48 Ga. The results from zircon Lu-Hf isotopes show that εHf(t) values (−3.2 to +7.5) and depleted mantle model ages (3270−2651 Ma) are like those of the oldest trondhjemite (ca. 2.7 Ga) exposed in the Xiwulanbulang area, indicating that it mainly resulted from partial melting of 2.7 Ga juvenile mafic crust. Geochemically, the Xiwulanbulang TTG gneisses contain high Sr (280−800 ppm) contents, low Y (1.05−9.54 ppm) and Yb (0.05−0.90 ppm) contents, and relatively high LaN/YbN (12.8−105) and Sr/Y (39.91−1392.6 ppm) ratios, with relative enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), showing the characteristics of adakitic rocks. In addition, their lower contents of MgO, Ni, Cr, and Mg# indicate that the Xiwulanbulang TTG rocks belong to adakite sourced from thickened continental lower crust. Also, the TTG gneisses show similar initial 87Sr/86Sr ratios (0.701659−0.702635) and εNd(t) values (−4.37 to + 3.55), with 206Pb/204Pb(t) = 13.910485−15.241164, 207Pb/204Pb(t) = 14.806723−15.259938, and 208Pb/204Pb(t) = 33.758621−35.592727, suggesting a depleted mantle source modified with some enriched components. Collectively, the Xiwulanbulang TTG rocks were most likely derived from the partial melting of thickened mafic crust between 2.61 and 2.48 Ga. Combining previous studies with data presented in this paper, we propose that the Yinshan block underwent two major igneous events during Neoarchean time, with the earlier one occurring ca. 2.7 Ga, forming a thickened mafic crust and minor TTG rocks, and the second igneous event at 2.61−2.48 Ga, resulting in the partial melting of the ca. 2.7 Ga continental lower (mafic) crust to form the TTG plutons. The geochemical data for the Xiwulanbulang TTG gneisses are inconsistent with a model for the partial melting of subducted slabs, but combined with other geologic considerations, a model in which the partial melting of a thickened continental lower (mafic) crust was heated by a mantle plume is favored for the formation of these Neoarchean TTG rocks in the Yinshan block.