Multiple generations of garnet, zircon and titanite: Temporal constraints on Fe skarn mineralization in the Middle-Lower Yangtze River Valley Metallogenic Belt, eastern China

Accurate and precise age constraints of mineral deposits are critical in understanding ore genesis and related geological processes. With the development of laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS), in situ U-Pb dating of garnet, zircon and titanite has been widely used to define the timing of hydrothermal mineralization. However, these minerals can grow at different stages of magma emplacement, metamorphic reaction and hydrothermal alteration. Distinguishing multiple generations of mineral growth is critical to the correct interpretation of U-Pb ages. Based on petrological occurrence, major and trace elements, and isotopic data, two generations of hydrothermal garnet, two types of zircon (magmatic and detrital), and three types of titanite (magmatic, metamorphic and hydrothermal) were clearly distinguished in the Machang iron skarn deposit in the Middle-Lower Yangtze River Metallogenic Belt (MLYRMB). In-situ U-Pb dating on hydrothermal garnets with low-U contents (<2.5 ppm) from the skarn yields lower intercept 206Pb/238U ages of 132-126 Ma in the Tera-Wasserburg diagram. This age is in agreement with the magmatic zircon and titanite U-Pb ages of 134-125 Ma for the diorite, metamorphic titanite U-Pb ages of 128 +/- 1 Ma for the marble, and hydrothermal titanite U-Pb ages of 132-126 Ma for the skarn. The consistency among the garnet, zircon and titanite U-Pb ages confirms the reliability and accuracy of U-Pb dating of low-U garnets. The magmatic, metamorphic and hydrothermal ages of 135-125 Ma for the Machang Fe skarn deposit are contemporaneous with the widespread magmatic and metallogenic activities in the Early Cretaceous along the MLYRMB. Compiled with the whole rock compositions of diorite with adakitic affinities, it is suggested that the Fe skarn minerali-zations resulted from the rollback of the Pacific plate and the consequent asthenospheric upwelling in eastern China.