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

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 degrees C to 931 degrees 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 (Sr-S7/Sr-86)(i) array between 0.70718 and 0.72642 (a.v. 0.70970) and epsilon Nd(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. Copyright (C) 2021, Guangzhou Institute of Geochemistry. Production and hosting by Elsevier B.V.

Automated summary

The Fanchang volcanic basin hosts a variety of intrusive rocks with high-K calc-alkaline and shoshonite characteristics, with diagenesis ages mainly around 128-121 Ma. Some intrusions exhibit polyphased inherited zircon ages, indicating diachronic magmatic sources and the potential existence of late-Archean and Proterozoic basement rocks.