Crystal-melt separation of the Cretaceous volcanic-plutonic complex in SE China: High Sr/Y rocks generated by plagioclase accumulation

Abstract

High Sr/Y intermediate-silicic rocks have attracted much attention due to their close connections with porphyry Cu-Mo deposits and TTGs. However, the origin of high Sr/Y signatures in these intermediate-silicic rocks remains controversial. In SE China, the ∼100 Ma high Sr/Y intermediate-silicic plutons are always adjacent to the contemporaneous low Sr/Y silicic rocks (volcanic rocks or granites). For instance, the Dalai volcanic-plutonic complex comprises the high Sr/Y quartz monzodiorite and coeval low Sr/Y rhyodacitic tuff . The consistency in time (100–99 Ma), space, and source between the quartz monzodiorite and rhyodacitic tuff of the Dalai volcanic-plutonic complex indicate that they were derived from the same magma reservoir . Abundant microstructural evidence (interstitial texture, plagioclase synneusis, plagioclase preferred orientation, and ductile deformation) shown by photomicrographs and Tescan Integrated Mineral Analyzer (TIMA) phase mapping, together with complementary geochemical compositions suggest a crystal-melt separation process dominated by plagioclase accumulation in the shallow crustal magma reservoir. The extracted melts formed rhyodacitic tuff, while the quartz monzodiorite was the remaining cumulate crystals (dominated by plagioclase) plus ∼30% trapped pore melt. Compaction (both pervasive mechanical compaction and locally developed viscous compaction) is the main mechanism of the crystal-melt separation. Accumulated plagioclase significantly increased the Sr/Y ratio of the Dalai quartz monzodiorite, while the extracted rhyodacitic melt showed complementary low Sr/Y. The crystal-melt separation dominated by plagioclase accumulation also sheds light on the formation of many other ∼100 Ma high Sr/Y intermediate-silicic plutons and the coexisting low Sr/Y silicic rocks in SE China, and is even a significant mechanism for the generation of high Sr/Y intermediate-silicic plutons. Additionally, the possible Sr/Y increment caused by plagioclase accumulation should be taken into account when quantifying the crustal thickness by Sr/Y ratios. • Reporting a new Cretaceous (∼100 Ma) volcanic-plutonic complex in SE China. • The Dalai volcanic-plutonic complex was formed by crystal-melt separation. • Microstructural evidence shows that compaction drives crystal-melt separation. • Plagioclase accumulation generates high Sr/Y intermediate-silisic rocks. • Quantifying crustal thickness by the Sr/Y should consider plagioclase accumulation.