Formation of late Archean high-δ18O diorites through partial melting of hydrated metabasalts

Abstract

Insight into the formation of Archean trondhjemite-tonalite-granodiorite (TTG) and sanukitoids is essential for understanding Archean crustal evolution and tectonic styles; however, their exact source and petrogenesis are still open to debate. Detailed chemical compositions of minerals, whole-rocks and whole-rock Sm-Nd isotopes, zircon U-Pb ages and Hf-O isotopes of the Zhulagou (ZLG) diorites and mafic enclaves from the Yinshan Block of the North China Craton are used to investigate the petrogenesis of the diorites and related geological processes. The ZLG diorites, which formed at ~2520 Ma, have moderate SiO2 (59.4-65.5wt %) and Mg# (49-52), high Al2O3 (15.6- 20.6wt %), Cr (90.4-438 ppm), Ni (15.0-95.9 ppm), Sr (436-882 ppm) and Ba (237-1206 ppm) contents, and also fractionated rare earth element patterns (REE; LaN/YbN=9.1-40.5) and depleted high field strength elements (HFSE) such as Nb, Ta and Ti, similar to Archean sanukitoids. The diorites may contain cumulus plagioclase and do not represent pristine melts, as indicated by their high Al2O3 contents and positive Eu and Sr anomalies. They have evolved whole-rock Nd isotope compositions [εNd(t)= 1.0-2.3, TDM=2.8-2.7 Ga], variable zircon εHf(t) (-1.6 to+6.0) and high zircon δ18O (~9.0±0.4‰, 2SD) values, indicating that the parental magma was most probably produced by partial melting of 2.8-2.7 Ga mafic crust. The high-δ18O character of the zircons was probably inherited from the magma source of hydrated metabasalts, which had elevated δ18O owing to previous low-temperature alteration, rather than resulting from granulite-facies metamorphism or wall-rock contamination. Coeval mafic enclaves (~2525 Ma) in the diorites are dominantly composed of clinopyroxene, amphibole and orthopyroxene. They have low SiO2 (46.5-50.3wt %) and high CaO (8.3-12.3wt %), Cr (647-1946 ppm) and Ni (197-280 ppm) contents, convex light REE patterns, and similar Nd isotope compositions to the host diorites. In addition, clinopyroxene and amphibole in the mafic enclaves have major and trace element compositions similar to those of the diorites. These observations collectively indicate that the mafic enclaves probably represent cumulates from the diorite magma. The diorites and enclaves experienced regional granulitefacies metamorphism at ~2500 Ma, soon after their emplacement. The late Archean large-scale crustal anatexis and high-grade metamorphism within the Yinshan Block of the NCC probably occurred at the root of a continental arc.