LA-(MC)-ICP-MS U-Th-Pb dating and Nd isotopes of allanite in NYF pegmatite from lesser qingling orogenic belt, central China

Abstract

U-Th-Pb dating and Sm-Nd isotopes of magmatic allanite from the Huangjiagou NYF pegmatites (lesser Qinling orogenic belt, China) were precisely measured and compared with zircon U-Pb dating and Lu-Hf isotopes to demonstrate the suitability and robustness of the U-Th-Pb and Sm-Nd system in allanite as a note-taker of formation age and magma sources for pegmatites. Allanite crystals from the Huangjiagou NYF pegmatites exhibit complex chemical zoning with a homogeneous primary internal domain, which has a typical magmatic origin with high La/Sm, Th/U, and low Eu/Eu*. The allanite has low La/Ce ratios (0.48–0.62) and thus is classified as allanite-(Ce). We used time-depended element fractionation and the changing of NAI with ablating different matrices to test the matrix differences between allanite and zircon standard 91500, and achieved excellent allanite U-Th-Pb date using zircon 91500 as external standard. Magmatic allanite-(Ce) yields a lower-intercept age of 1829 ± 23 Ma on the Tera-Wasserburg plot and a 208Pb/232Th weighted average age of 1840 ± 17 Ma, both of which are consistent with the zircon 207Pb/206Pb weighted mean age (1826 ± 11 Ma) within uncertainty, supporting the reliability and accuracy of non-matrix matched allanite U-Th-Pb dating. We used titanite standard (MKED1) to correct the time-drift in allanite Nd isotope analysis. And by integrating the zircon Lu-Hf isotopes and allanite Sm-Nd isotopes, together with field investigation and mineralogy, we suggest that the Huangjiagou NYF pegmatites belong to an abyssal class, formed in the late Paleoproterozoic, and likely related to the partial melting of the dehydrated Taihua Group (amphibolite and TTG granites) in the post-collisional tectonic setting between the Eastern and Western blocks along the trans-North China Orogen (TNCO). This study confirms the talent of allanite serving as a good indicator for the crystallization age and magma sources of NYF pegmatites using non-matrix matched U-Th-Pb dating and in situ Sm-Nd isotope analysis.