Emplacement and evolution of zoned plutons: Multiproxy isotopic and geochemical evidence from the peraluminous Laojunshan leucogranite suite, southwestern China, and implications on the regional geodynamic and metallogenic history

Abstract

The petrogenesis and geodynamic setting of peraluminous leucogranites are important topics for reconstructing regional geodynamic and metallogenic models. Here we present the results of an integrated study of zircon U-Pb ages and Hf-O isotopic compositions, monazite U-Th-Pb-Nd isotopes, tourmaline boron isotope, and bulk-rock elemental and Sr-Nd isotope geochemistry of an unusual suite of leucogranite, the Laojunshan leucogranite (LJSL), from the southwestern margin of the Yangtze Block in southwest China. The LJSL samples are strongly peraluminous with their zircon d18O (8.8–10.2 ‰) and tourmaline d11B ( 14.32 to 12.75 ‰) values suggesting S-type origin. The LJSL rocks show high SiO2, Al2O3, K2O and Rb/Sr, low P2O5, CaO, FeOt , MgO, low REE abundances with moderate LREE enrichment and strong negative Eu anomalies, high (87Sr/86Sr)i (0.7127–0.7421) and low eNd(t) ( 10.36 to 12.67). These features suggest protracted fractional crystallization from S-type parental granitoid magmas with Ti-in-zircon thermometry giving a mean temperature of 760 C. The bulk-rock Sr-Nd, monazite Nd and zircon HfO isotope data suggest that the S-type magmas parental to the LJSL formed through partial melting involving muscovite-breakdown in the Paleoproterozoic middle-upper crustal metasedimentary rocks. Zircon and monazite U-(Th)-Pb dating reveals three intrusive phases for the LJSL (91–89 Ma, 87– 85 Ma and 83–82 Ma), correlating with the compression-to-extension tectonic regime transition in response to the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the Neotethys oceanic lithosphere. Our findings contribute to the understanding of Mesozoic plate reconstruction associated with the India-Asia collision in the Cenozoic.