Rock‐buffered versus fluid‐buffered geochemistry of structurally controlled, hydrothermal dolomite: Insights from the Sichuan Basin, China

Structurally controlled hydrothermal dolomitization, commonly driven by high‐pressure, high‐temperature fluid–rock reactions, offers insights into the tectonic evolution of sedimentary basins and the geochemical composition of their diagenetic fluids. Despite the numerous conceptual models that have been proposed to explain mechanisms of hydrothermal dolomitization, the behaviour of trace and rare earth elements in these reactions remains inadequately explored. To address such knowledge gaps, this study presents a detailed petrographical and geochemical analysis of the Longwangmiao Formation (Lower Cambrian) in the Sichuan Basin, China. Petrographical examinations revealed several generations of planar‐s, replacive dolomite (RD) and non‐planar, saddle dolomite (SD). RD1 (10 to 50 μm) and RD2 (50 to 120 μm) have similar geochemical signatures, which are indicative of a seawater‐derived diagenetic fluid in a shallow‐burial setting. These paragenetic stages denote the transition from an open, fluid‐buffered system during replacement dolomitization to a closed, rock‐buffered system during recrystallization. SD1 and SD2 are coarsely crystalline (0.3 to 5 mm), with high T h values (168 to 178°C), salinities and δ 18 O fluid values (4.9 to 7.4‰ VSMOW). SD1 and SD2 are also characterized by high 87 Sr/ 86 Sr and [Fe + Mn] values, but they have markedly different Eu/Eu* SN values (SD1 = 1.26 ± 0.22; SD2 = 0.68 ± 0.01). These results indicate that SD1 and SD2 precipitated in an intermediate‐ to deep‐burial setting from a mixture of seawater and hydrothermal fluids that had interacted with underlying siliciclastic or metamorphic basement rocks. These paragenetic stages precipitated in two different fluid‐buffered systems that likely correlate with distinct tectonic activities linked to the (i) Emeishan Large Igneous Province (Permian) and (ii) Longmenshan orogeny (Late Triassic). Our findings establish a genetic connection between basin‐scale tectonism and hydrothermal fluid flow that is recorded in the trace and rare earth element signature of dolomite, offering a framework to differentiate between rock‐buffered and fluid‐buffered geochemical signatures in structurally controlled settings worldwide.