The relationship and evolution of the intermediate–mafic volcanic-intrusive rocks in the Luzong volcanic basin, Lower Yangtze River Belt, eastern China

Abstract The relationship between volcanic rocks and intrusive rocks represents one of the key avenues for comprehending the behavior of the magma plumbing system. Until now, studies of the relationship between volcanic rocks and intrusive rocks have mainly focused on the high-silica magma system, with the melt extraction and crystal accumulation model commonly proposed to explain those geological features. Research pertaining to intermediate–mafic volcanic rocks and their associated intrusive rock is lacking. The Luzong Basin, located in the Lower Yangtze River Belt of eastern China, hosts a suite of rocks that include trachyandesite, trachyte, pyroxene monzonite, and quartz syenite. Here, we present a systematic geochronologic, petrologic, and geochemical study of the Longmenyuan trachyandesite, Zhuanqiao trachyte, Bajiatan pyroxene monzonite, and Huangmeijian quartz syenite to investigate the intermediate–mafic magma plumbing system. Zircon dating by laser ablation–inductively coupled plasma–mass spectrometry revealed formation ages of 132.4 ± 1.0 Ma (n = 20; MSWD [mean square of weighted deviates] = 1.7) for the Longmenyuan Formation, 131.4 ± 0.5 Ma (n = 50; MSWD = 2.8) for the Zhuanqiao Formation, 133.2 ± 1.0 Ma (n = 29; MSWD = 2.3) for the pyroxene monzonite at Bajiatan, and 130.4 ± 0.5 Ma (n = 40; MSWD = 3.3) for the quartz syenite at Huangmeijian. Thus, the volcanic rocks and intrusive rocks were formed in the Early Cretaceous, and their crystallization ages are indistinguishable within error. Geochemical analyses revealed comparable Nd-Hf isotope compositions among these units. Specifically, Longmenyuan and Zhuanqiao volcanic rocks have εNd(t) ranging from −13.0 to −3.6 and εHf(t) ranging from −17.7 to −5.8. The Bajiatan pluton has εNd(t) of −7.8 to −5.2 and εHf(t) of −12.7 to −3.7, while the Huangmeijian quartz syenite displays εNd(t) of −6.1 to −3.3 and εHf(t) of −16.6 to −7.0. These similarities in Nd-Hf isotope compositions suggest that these rocks may have been derived from a similar source. The clinopyroxene-enriched and amphibole-scarce magma plumbing system within the Luzong Basin indicates a relatively “dry” magma system. The clinopyroxene crystals in all of these magmas formed within a narrow temperature range of 980 °C to 1079 °C, but over a wide range of pressures. The clinopyroxenes in different volcanic and plutonic formations have distinct pressure ranges, for example, 0.4–4.7 kbar for the Longmenyuan Formation, 4.1–6.7 kbar for the Zhuanqiao Formation, 0.5–5.5 kbar for the Bajiatan pluton, and 0.9–8.6 kbar for the Huangmeijian pluton. Also, the clinopyroxenes in the Huangmeijian pluton are regarded as cumulate residues from the Bajiatan pluton. Such a wide range of pressures indicates that the clinopyroxenes crystallized within magma reservoirs spanning multiple depths, which enabled dynamic interactions among them. Geochemical modeling reveals no compositional complementarity between the Longmenyuan and Zhuanqiao volcanic rocks and the Bajiatan pluton, which suggests that the melt extraction and crystal accumulation model falls short in elucidating the dynamics of the intermediate–mafic magma plumbing system within the Luzong Basin. Interestingly, the Huangmeijian pluton exhibits a complementary relationship with the Bajiatan pluton, which hints at their origin from highly fractionated melt components sourced from the magma reservoir. The young and isotopically depleted Huangmeijian pluton may indicate a crystal mush reactivation triggered by the injection of hot, depleted mantle-derived magma in an extensional tectonic setting. Our study indicates that the crystal mush model maybe not be applicable to the intermediate–mafic magmatic system. The whole-rock chemical modeling and clinopyroxene geochemistry reported here provide valuable insights into the processes within the alkali-rich, intermediate–mafic magma chamber system.