Multiple magmatic-hydrothermal processes recorded by titanite from the Early Cretaceous Liangwan pluton, Tongbai Orogen

Abstract

Titanite is a common accessory mineral in igneous rocks and hydrothermal deposits. Micro-texture and in situ chemical analyses on titanite are expected to reveal complex magmatic-hydrothermal processes. The Liangwan granitic pluton in the Tongbai Orogen is dominated by monzogranite with abundant mafic microgranular enclaves (MMEs). In this paper, an integrated study of LA-ICP-MS U Pb ages, textural, trace elemental and Nd isotopic compositions of titanites were carried out for the Liangwan pluton to decipher multiple magmatic-hydrothermal processes. U Pb dating results of zircon and titanite show that the Liangwan monzogranite and MMEs have similar crystallization ages of ca. 130 Ma. Combined with petrography and titanite geochemistry studies, MMEs are considered as magma mingling origin. According to the occurrence, texture, and chemical characteristics of titanite, four main types (Ttn-1 to Ttn-4) and several sub-types of titanite are identified in the Liangwan pluton. Among these types, Ttn-1, Ttn-2 g, and Ttn-4 g are from the monzogranites, while Ttn-2 m, Ttn-3, and Ttn-4 m mainly form in the MMEs. Ttn-1 grains are euhedral and show core-mantle texture. Their cores are characterized by high REEs, Nb, Ta, Th and Y contents, Th/U, Lu/Hf, and Y/Zr ratios, and low La/Ce, Nb/Ta rations, and relatively lower εNd (t) (−13.3 to −14.6) values, indicating that they are of magmatic origin and crystallized from the host monzogranites. However, Ttn-1's mantles and Ttn-3 grains show a reverse geochemical feature to those of Ttn-1's cores. Their relative higher εNd (t) (−12.1 to −13.9) values, suggest that they crystallized from a mafic magma. Both Ttn-2 g and Ttn-2 m contain abundant magnetite and ilmenite inclusions. They display geochemical characteristics between Ttn-1's cores and Ttn-3 but are closer to their host rocks, respectively. These features suggest that they are the product of the dissolution-reprecipitation of Ttn-1 in different magmas. Ttn-4 with few fluid inclusions, mostly existing as interstitial phases, is characterized by low REE, Y, HFSE, Th/U, LREEs( N )/HREEs( N ), and high Nb/Ta, indicating that they are hydrothermal origin that were derived from the residual melt. Textural and geochemical features of various types of titanites indicate that repeated recharges of mafic magmas happened at the syn - to late- fractional crystallization of felsic magmas. Nd isotopic data on titanite recorded various degrees of isotopic re-equilibration between the two end-member magmas during different pulses of mafic magma. Our study demonstrates that titanite is a reliable indicator to reveal complicated magmatic-hydrothermal processes. • Monzogranite and MMEs of the Liangwan pluton were both formed at ∼130 Ma. • MMEs in the Liangwan pluton were formed by magma mingling. • Titanite is a reliable indicator to reveal complicated magmatic-hydrothermal processes • Early Cretaceous granite could provide hydrothermal fluids for regional Au—Ag mineralization.