The different sources and petrogenesis of Jurassic intrusive rocks in the southern Lhasa subterrane, Tibet: Evidence from the trace element compositions of zircon, apatite, and titanite

Abstract

ABSTRACT The Jurassic magmatism distributed in the southern Lhasa subterrane (SLT) has been recognized as a record of the early-stage evolution of the Neo-Tethys ocean; however, the magma compositions and detailed crystallization histories remain an enigma. Zircon, apatite, and titanite in igneous rocks can serve as effective petrogenetic indicators. In this paper, we present the whole-rock geochemical, zircon U Pb data, zircon Hf isotopic analyses, and major and trace elemental compositions of zircon, apatite, and titanite from Baiga granite porphyry (EGP), Shanba granodiorite (EGD), Pengcuolin granodiorite porphyry (MGP), and Dazhuqu monzonite (LM) rocks in the SLT to constrain their source and petrogenesis. The results from this study show that a combination of zircon Ti, Th/U, Hf, and rare earth element (REE) contents, apatite halogen, REE and Sr contents, and titanite REEs can be used to track the detailed magmatic crystallization histories. Higher Sr contents are found in apatite (SrAp) from EGP (ca. 192 Ma) and EGD (ca. 176 Ma) than those in their host rocks (SrWR), indicating that their magma genesis may have involved some mafic magma. The magma genesis of LM (ca. 159 Ma) may have involved more reduced mafic (or least evolved) magma, as evidenced by the higher Th/U but lower Yb/Gd ratios, higher zircon saturation and Ti-in-zircon temperatures in zircon, highest SrAp/SrWR ratios, and large positive Eu anomalies in titanite from LM. The lower Cl contents and higher (La/Sm)N values in apatite from MGP (ca. 172 Ma) are inherited from the low-Cl melts. The MGP rocks have higher zircon Ce4+/Ce3+ and EuN/EuN* ratios, a greater apatite SO3 content, and a higher titanite total REE content, which indicates that MGP has a higher oxidation state than do EGP, EGD, and LM. The MGP also exhibits a higher H2O content (>11 wt%) dissolved in the melt, greater Ba/La and Nb/Ta ratios in whole rock, and higher Nb/Ta ratios in zircon. However, MGP demonstrates lower zircon saturation and Ti-in-zircon temperatures, which suggest that MGP was generated by fluid-fluxed melting below 750 °C. The low Ti-in-zircon temperatures but high La/Yb and Sr/Y ratios of MGP indicate that a local thickened crust likely existed in the Middle Jurassic period in the SLT due to mafic magma underplating during this time.