Petrogenesis and tectonic setting of two types of diabase in the Tongdao area, Jiangnan Orogenic Belt

Objective The Tongdao district is located in the southwest part of the Jiangnan Orogenic Belt, where the mafic rocks are widely distributed, but their age, genesis, and tectonic setting are not effectively restricted. This research focuses on diabases from the Longcheng−Changjie and Xiadong mafic rock groups in the Tongdao area, aiming to clarify their ages, orgins and diagenetic process. thereby providing constraints for revealing the regional tectonic evolution of the Jiangnan Orogenic Belt.

Methods Systematic geochronological, mineralogical, wholerock major and trace element, and Sr−Nd−Pb isotopic analyses were conducted on the two types of diabase in the study area. Combined with the test data, a comprehensive discussion was carried out on their diagenetic mechanism and tectonic setting.

Results The mafic dikes in both rock groups are dominated by finegrained diabase,with pyroxene, plagioclase, hornblende as the main minerals, and zircon, apatite as accessory minerals. The apatite LA−ICP−MS U−Pb results reveal that the Longcheng−Changjie diabases formed in Middle Ordovician (465.5±8.7 Ma, MSWD=0.8), while the zircon SIMS U−Pb dating indicates that the Xiadong diabases formed in the Early Jurassic (190.9±1.4 Ma, MSWD=0.2). The diabase of Longcheng−Changjie Group is subalkaline basalts with high Mg^# value (60~76), enriched in LILE (Cs, Ba, U, Pb) and LREE, depleted in HFSE (Nb, Ta). The diabase of Xiadong Group is characterized by high SiO₂(50.4%~50.9%), TiO₂ and CaO, also belonging to subalkaline basalt., which is enriched in LILE (Cs, Rb, Ba), slightly enriched in LREE, and have a depleted of Nb. The Sr−Nd−Pb isotope compositions and trace element geochemistry indicate that the diabase of the Longcheng−Changjie Group was formed by partial melting (25%~35%) in the spinelgarnet stable region of the lithosphere mantle source region enriched by subduction fluid replacement, and the diabases of Xiadong Group originate from metasomatic lithospheric mantle modified by asthenosphere new material, and is the product of 20%~30% partial melting of spinel dipyroperidotite. Both types of diabase show low degrees of crustal contamination.The fractional crystallization of plagioclase, diopside, olivine, and hornblende took place during the Longcheng−Changjie Group basaltic magma evolution process, while the fractional crystallization of the diabase in the Xiadong Group is weak, the fractional crystallization of augite occurs in the shallow part of the mantle.

Conclusions Based on the results of this study and previous studies, it is concluded that the diabase of Longcheng−Changjie Group formed in the subduction environment of the active continental margin of the Middle Ordovician, while the diabase of Xiadong Group formed in the Early Jurassic intraplate environment. The age and tectonic environment of the two groups confirm that the Jiangnan orogenic belt underwent subduction of the interoceanic arc under the influence of the Kwangsian event in the early Paleozoic, and formed basic magma with subduction properties. The Paleo−Pacific plate subducted to the Jiangnan orogenic zone before the Jurassic period, and then began to fracture and sink at about 190 Ma. The disturbed upflowing asthenosphere new material reacted with the lithospheric mantle and induced melting, which led to largescale Mesozoic basic magmatic activity in South China.