Petrogenesis and magmatic–hydrothermal evolution time limitation of Kelumute No. 112 pegmatite in Altay, Northwestern China: Evidence from zircon U[sbnd]Pb and Hf isotopes

Abstract

Granitic pegmatites and associated rare-metal deposits, unique features in the Altay orogen, are located in a key tectonic position of the southern Central Asian Orogenic Belt (CAOB) and document the tectonic evolution of the Paleo-Asian Ocean. The Kelumute No. 112 pegmatite, which intruded into the Jideke two-mica granites, hosts abundant rare-metal (e.g., Li, Be, Nb, Ta) ore deposits and ranks only second to the Koktokay No. 3 pegmatite in both size and reserves. To explore the evolution time limitation of magmatic, magmatic–hydrothermal and hydrothermal stages in the pegmatite magma system, the tectonic setting and the genetic relationship between pegmatites and granites, this study presents zircon UPb and Hf isotopic compositions of the Kelumute No. 112 pegmatite and the wall rocks (two-mica granite and biotite granite) as determined by LA-ICPMS and LA-MC-ICPMS. The weighted mean 206Pb/238U ages of the internal textural zones of the pegmatite, including zones I, II, III, V and VI, are 238.3±2.0Ma, 233.5±3.7Ma, 188.3±1.7Ma, 218.8±1.9Ma and 210.7±1.6Ma, respectively. Wall rocks of two-mica granite and biotite granite are dated at 445.6±4.3Ma and 455.6±5.4Ma, respectively. Zircons from the Kelumute No. 112 pegmatite have lower positive εHf(t) values (+0.03 to +2.35), with TDM model ages of 1112–1225Ma. Wall rocks show similar zircon εHf(t) values (−1.41 to +4.13) and TDM model ages (1172 to 1515Ma). However, three xenocrystic zircons from the granites are characterized by larger negative εHf(t) values (−5.85 to −9.83) and older TDM model ages (1839 to 2090Ma). Thus, the following conclusions can be drawn: 1) the Kelumute No. 112 pegmatite and its wall rocks (Jideke two-mica granite and biotite granite) have no genetic relationship, as indicated by the large gaps between their formation ages. However, they did originate from a common source composed of ancient crust and mantle-derived materials under two different tectonic settings; 2) the magmatic, magmatic–hydrothermal transition and hydrothermal stages of the No. 112 pegmatite magma lasted for ~5Ma, ~23Ma and ~22Ma, respectively; and 3) the No. 112 pegmatite magma was most likely formed in a post-collision tectonic setting, indicating that block amalgamation and collisional orogeny of the CAOB continued into the Triassic.