Timing of magmatism of the gneissic-granite plutons along north Qaidam margin and implications for Precambrian crustal accretions: Zircon U-Pb dating and Hf isotope evidences

Abstract

Laser abration microprobe-inductively coupled plasma mass spectrometer (LA-ICP-MS) technique was applied for zircon dating of the Quanjihe granite in the east of Xitieshan and Shaliuhe granite in north Dulan County in the Qaidam margin; Laser abration microprobe-multi-collector-inductively coupled plasma mass spectrometer (MC-ICP-MS) technique was used to analysis Hf isotope compositions of zircon from the Shaliuhe granite. For the Quanjihe granite, a Concordia age of 910 + 15/ - 17Ma was yielded for the magmatic zircons, 207Pb/ 206Pb age of 1351 Ma for the detrital zircon and 206Pb/ 238U age of ca. 520Ma ( 232Th/ 238U = 0.04) for the metamorphic zircon resulted from a metamorphic event prior to the HP UHP metamorphism. The zircon from the Shaliuhe granite often contains inherited detrital cores. Of the 21 measurements on 21 zircon grains, 10 measurements of magmatic zircon yielded Robust (Median) 206Pb/ 238 U age of 920 + 31/ - 17Ma, and the detrital zircon yielded 207Pb/ 206Pb age older than 1250Ma. Of the 30 measurements on 30 grains of zircon from the Shaliuhe granite, 27 measurements of magmatic zircon yielded 176Hf/ 177Hf = 0.282081 - 0.282222 with Robust (Median) average of 0.282178 + 0.000009/ -0.000015, Hf 1 = 0.28206 ∼ 0.28220 with average of 0.28215 +/ -0.00003, ε Hf = + 0.21 ∼ -4.70 with Robust (Median) average of -1.53 + 0.41/ -0.52, zircon Hf isotope crustal model age T DM C = 1.59 - 1.75Ma, Robust (Median) average of 1.68 + 0.03/ - 0.02Ma. The other two of the rest detrital zircons have 176Hf/ 177Hf ratio of 0.281892 and 0.281792, Hf 1 = 0.28186 ∼ 0.28178, ε Hf = -11.83 ∼ -14.82, T DM C = 2.18 ∼ 2.33Ga. It suggested that the magmatism for the gneissic granites on the North Qaidam margin should have a duration time from ∼950Ma to ∼910Ma, with peak age of ∼920Ma. The magma of Shaliuhe granite and others with the same S-type peraluminous characteristics was generated from highly recycled upper crustal material-derived basemental suppracrustal rocks beneath the corroded Shaliuhe Group. The age of these materials should be constrained to be 1.4Ga ∼ 1.7Ga and had recorded two crustal accretion events at ∼1.7Ga and ∼2.3Ga for their provenances.