Geochronological and geochemical constraints on the origin of the Mesozoic granitoids in the Fanchang volcanic basin, the Middle-Lower Yangtze Metallogenic Belt

Abstract

The Fanchang volcanic basin is located between the ore-cluster districts of Tongling and Ningwu，the Middle-Lower Yangtze Metallogenic Belt (MLYMB), regionally. This study extends a set of methods，i.e., petrographic observations, chemical compositions of intrusive rocks and LA-ICP-MS analysis of magmatic zircons, zircon LA-ICP-MS U-Pb geochronology and whole-rock Sr-Nd isotope analysis, on several intrusive rocks, such as Baimashan, Xiangxingdi, Zhuhouling, Chichong, Suishan, Puyingshan and Zishanling. In detail, in this area, monzonite, quartz diorite, granite and syenite are characterized by a series of high-K calc-alkaline and shoshonite, with a positive anomaly in Rb, Th, U, Ce, Pb, Nd, Sm, Gd and a negative anomaly in Ba, Nb, Ta, Sr, P, Ti, plus LREE enrichments and a flattened right-dip of HREE. Moreover, the diagenesis ages of these intrusions mainly overlap the 128–121 Ma time, all but two exceptions, including Baimashan quartz dioritic porphyrite (138.5 ± 4.9 Ma) and Puyingshan diorite (132.1 ± 3.8 Ma). Interestingly, the inherited zircon ages in the Baimashan intrusion are polyphased, primarily in three intervals, 2211.1–2675.9 Ma, 1862.0–2057.1 Ma and 661.1–990.7 Ma, respectively, indicating diachronic magmatic sources and possible existence of the late-Archean and Proterozoic basements. In addition, crystallization temperatures of these intrusive rocks in the Fanchang volcanic basin significantly increase from 802 °C to 931 °C, while the values of (Ce⁴⁺/Ce³⁺)_zircon decrease from 192.34 to 24.84, throughout the period from 132 Ma to 125 Ma, probably due to the enhanced magma assimilation process with the reductive carbonaceous wall rocks. Furthermore, A-type granites in this area, as defined in this paper, would have been derived from the partial melting of the residues of the early I-type granite source area. Values of (^S7Sr/⁸⁶Sr)_i array between 0.70718 and 0.72642 (a.v. 0.70970) and εNd(t) between −14.0 and 2.2 (a.v. −7.1). In brief, the intrusive rocks in Fanchang volcanic basin is considered to have characteristics of both enriched mantle and lower crust and a strong assimilation process had occurred.