Metallogenic type controlled by magma source and tectonic regime: Geochemical comparisons of Mesozoic magmatism between the Middle–Lower Yangtze River Belt and the Dabie Orogen, eastern China

Abstract

The Middle–Lower Yangtze River Belt (MLYRB) and the Dabie Orogen (DBO) are two contiguous tectonic units in which extensive late Mesozoic magmatism took place over similar time intervals. However, marked differences are observed in the geochemistry, lithology, and associated mineralization of the magmatic rocks of the two domains. Late Mesozoic magmatism in both regions can be divided into three stages (MLYRB: 148–133, 133–127, and 127–123 Ma; DBO: 143–130, 130–126, and 126–110 Ma). The MLYRB first-stage intrusions, which are closely genetically associated with Cu–Au polymetallic mineralization, are dominated by granodiorites characterized by higher Sr/Y ratios, arc-like trace-element distributions, moderately enriched Sr–Nd–Hf isotopic compositions, and higher radiogenic Pb isotopic values, indicating a mantle–crust mixed magma source. The mantle source was metasomatized by fluid and/or melt released from a subducted oceanic slab, and the crustal source was predominantly of Archean age, similar to late Archean to Paleoproterozoic basements in the MLYRB. However, the main metallogenic type in the DBO is Mo mineralization, whose ore-forming magmatic rocks are granites of the third stage of magmatism. The ore-barren magmatic rocks in the DBO are of the first and second stages of magmatism, with dominant rock types of adakitic granodiorite, andesite, and pyroxenitic or hornblenditic intrusions. In comparison, ore-forming rocks in the DBO show low Sr/Y ratios, less enriched Sr–Nd–Hf isotopic compositions, low radiogenic Pb isotopic values, and enriched light rare-earth element and large-ion lithophile element contents, which are geochemical signatures similar to those of the middle–upper crust represented by gneisses in the northern and central DBO. Separate tracing of the evolution of magmatic sources and deep processes of the two tectonic units during the three stages suggests that late Mesozoic magmatic activity in the MLYRB took place in a continental-arc geodynamic setting during the first stage, then transformed to a back-arc after the second stage. Northward flat subduction of the paleo-Pacific plate, followed by slab foundering and roll-back, provided not only the dynamic mechanism for the magmatism, but also a substantial material contribution. In contrast, late Mesozoic magmatic activity in the DBO, caused by an Indosinian continent–continent collisional orogen, occurred in a back-arc post-orogenic geodynamic setting, with orogenically thickened lithosphere preventing flat subduction. Subduction of the paleo-Pacific plate supplied the dynamic mechanism, but no material contribution to the magmatism of the DBO. A torn slab window formed along the present coordinates of the Yangtze River, corresponding to the Yangtze River fault, in the effect of sinistral shear of the Tan–Lu fault, a subordinate triggering dynamic setting for magmatism in the MLYRB. An integrated comparison of the two metallogenic belts suggests three key controls on metallogenic type: (1) Metal elemental abundance in the magma source was the first-order control on the minerogenetic series; (2) the physical–chemical conditions of the magma controlled the migration of metal elements from the source and incorporation into the metal-rich magma; and (3) the late Mesozoic tectonic regime exerted a broad influence on the generation of the different minerogenetic series.