Origin of the Alxa Block, western China: New evidence from zircon U–Pb geochronology and Hf isotopes of the Longshoushan Complex

Abstract

The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90Ga and ca. 1.85Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant 207Pb/206Pb ages between 2.2Ga and 2.0Ga, and a small number of metamorphic zircon rims provide a ca. 1.95Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0Ga to ca. 2.17Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (TDMC) ranging from 2.45 to 2.65Ga (peak at ca. 2.5Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered 176Hf/177Hf ratios, εHf(t) and TDMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with TDMC ranging from 2.76 to 3.04Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90Ga and ca. 1.85Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.