A Coupling Between Volcanic And Plutonic Rocks And Magma Plumbing Systems At Convergent Plate Margins: A Case Study Of Triassic Igneous Rocks At The Eastern Section Of The East Kunlun Orogen

It is still debated whether there were contemporaneous and genetically related plutons underlying the volcano,and how the massive and eruptive melt in the crust originated,accumulated,migrated,and eventually erupted.Understanding these issues is not only key to studying eruption mechanisms and improving volcano monitoring and forecasting capabilities but is also essential for constructing magma plumbing system and deepening understanding of the vertical growth and evolution of continental crust.A deep magma reservoir is a must for an eruption,where eruptive material is produced.The study of fossil magma reservoirs（plutons）will provide new clues to understand the coupling relationship between volcanic and plutonic rocks,the magmatic process within the reservoir,the driving mechanism,and the style of eruption.The East Kunlun Orogen lies in a typically convergent plate margin.Its eastern portion develops a titled mid-uppercrust Triassic volcanic-plutonic profile and is an excellent area for the study of the abovementioned scientific questions.In this study,we investigated the geochronology,mineral geochemistry,and petrochemistry of the Dehailonggang volcanic-plutonic complex,Kaketeer intermediate-basic pluton,and Nageng intermediate-acid volcanics,and established their genetic coupling relations by combining them with thermodynamic simulations.We analyzed the conditions of the generation and accumulation of preerupting high-silica melt in the crust and put forward the prerequisites and mechanism of triggering the migration and eruption of volcanic melt by comparing the petrogenesis and physicochemical conditions of the Jialuhe composite complex and the Dehailongang volcanic-plutonic complex with the thermodynamic simulation.The Early Triassic（250 Ma）homogenous volcanic-plutonic complex is developed in the Dehailongang area,in which plutonic and volcanic rocks are in contact with a fault– the former ones are tonalites,and the latter ones are dacites and rhyolites.Feldspar glomerocrysts develop in volcanic rocks,and tonalite has a typical cumulative structure,showing that non-zoned euhedral feldspars are tightly clustered to form cumulates and melt migration channels composed of fine felsic minerals develop in the gaps between cumulates.The trace elements of zircon in Dehailonggang rhyolite and tonalite highly overlap,and the trace elements of dacite zircon and tonalite zircon show a trend of continuous evolution.Similarly,Dehailonggang volcanic rocks have low An values of plagioclase（An1⁴）,similar to the lower limit of An values of tonalite plagioclase（An2³0）.This phenomenon not only suggests that the magmatism from tonalite to volcanic rocks in Dehailonggang may have been controlled by the differentiation of plagioclase,apatite,and zircon,but also indicates that some of the zircons and plagioclase in Dehailonggang volcanic rocks were inherited from tonalite and then underwent some degree of evolution.The similarity of mineral assemblages is also reflected in the highly consistent plagioclase CSD curves between the Dehailonggang tonalite and the volcanic rock.This CSD feature not only indicates that the accumulation process occurred in the magma reservoir,but also suggests that the crystallization dynamics of these plagioclases are similar,that is,the crystallization environments are similar,and that they may have come from the same magma reservoir.The geochemical properties of Dehailonggang tonalite and volcanic rocks are complementary.We use the mass-balance method to calculate the crystallinity（38⁶2 %）of the original mush and uncover its chemical composition.It was found that Dehailonggang tonalite is formed from crystal cargos dominated by plagioclase and alkaline feldspar after the crystal-melt phase separation of the original mush,while volcanic rock represents the extracted highly evolved melt.We also use the rhyolite-MELTS isobaric differentiation simulation to study the magma unmixing.When the system evolves to 38⁶2 % crystallinity,the solid phase,dominated by feldspar,or the mixed solid-melt phase,coincides with the Dehaolonggang tonalite and the extracted melt phase represents the volcanic rock.Therefore,a genetic coupling relationship between the Dehailonggang tonalite and volcanic rocks has been established.The Late Triassic Kaketeer intermediate-basic pluton has the same age（228 Ma）and crust-mantle mixed source as the adjacent Nageng intermediate-acid volcanic rocks.The volcanic rocks consist of andesite and dacite with plagioclase glomerocrysts.Intermediate-mafic rocks consist of olivine-gabbro,gabbro,and gabbro-diorite with distinct cumulative structures,magmatic differentiation structures,and magma mixing structures in the outcrops.Bulk-rock geochemistry shows a clear trend of fractional crystallization in plutonic rocks,which is controlled by the differentiation of olivine,pyroxene,plagioclase,and other minerals.We identified the plagioclase autocrysts and antecrysts in all the rocks.The rims of plagioclase autocrysts have been found to record the interaction between three magmas through in situ studies of major elements,trace elements,and 87Sr/86 Sr isotopes.The unique geochemical characteristics of cores from plagioclase antecrysts suggest that they are from other different magmas.These features all reveal the presence of complex magma mixing among the three plutonic rocks.While bulk-rock geochemistry indicates that olivine-gabbro,gabbro,and gabbro-diorite are governed by fractional crystallization,detailed mineral chemistry indicates that gabbro deviates from this fractional crystallization trend and follows magma unmixing with andesite and dacite.The in situ geochemical signatures of zircon and amphibole record this magma unmixing,showing that the mineral differentiation of amphibole,plagioclase,biotite,apatite,and titanite occurred from gabbro to volcanic rocks.Most importantly,An values,Sr content,and ⁸⁷Sr/⁸⁶Sr ratio of plagioclase antecryst cores from the andesite and dacite show the geochemical signature of gabbro,which indicates that the plagioclases from andesite and dacite are inherited from gabbro,indicating a genetic coupling between gabbro and volcanic rocks.In addition,it has been found that the physicochemical conditions of gabbro and volcanic rocks are very close to each other,in contrast to other rocks,which also suggests a genetic affinity between gabbro and volcanic rocks.Combined with the Dehailonggang volcanic-plutonic complex,we suggest that there is a genetic coupling relationship between volcanic rocks and plutonic rocks.Plutonic rocks represent the crystal-rich cumulates in the magma reservoirs,which may contain a small amount of melt,while volcanic rocks represent the highly evolved melt extracted after magma unmixing,which may contain a small amount of antecrysts from plutonic rocks.Early Triassic Jliauhe composite pluton is mainly biotite granodiorite and contains a large number of mafic microgranular enclaves（MMEs）.The MMEs in the northern and southern pluton are orientationally distributed,while the MMEs are clustered and disordered in the central,indicating that mafic magma was injected in the middle of the pluton,leading to large-scale convection in the magma reservoir.The amphibole aggregate band develops within the MME cluster in the central pluton,which consists of many euhedral zoned amphibole megacrysts.It was found that the major elements of the amphibole megacrysts change regularly from the core to the rim,and this trend is controlled by the substitution of the amphibole crystal lattice,which is closely related to changes in physicochemical conditions.After calculating the physicochemical conditions,we find that amphibole megacrysts grow in a melt with increasing water content.In situ core-to-rim trace elements and the chemical composition of the amphibole’s equilibrium melt indicate that the growth of the amphibole is strongly influenced by magma mixing,and the increasing water content in the melt is due to injected mafic magma.The hightemperature fluid causes the host magma to remelt,creating a water-rich environment that favors the coarsening of the original amphibole in the host magma,resulting in the formation of the amphibole aggregate band.This special geological phenomenon can be used not only as evidence to identify the thermal rejuvenation of mush in the field but also to highlight the contribution of magma mixing to the thermal rejuvenation of mush,which provides a dynamical basis for the subsequent formation of highly evolved melts.We ran rhyolite-MELTS isobaric differentiation models with different initial pressures and initial water contents.It is found that the evolution path is not seriously affected when the initial pressure of the system is maintained but the initial water content of the system is changed.As the mineral crystallizes,there is little change in the amount of SiO₂ that the system can reach.When the initial water content of the system is kept constant and the initial pressure of the system is varied,the SiO₂ content that can be reached by the system and the overall evolution path change significantly with the crystallization.The effects of these initial physicochemical conditions also affect the physical properties of the crystal mush.When the initial water content is kept low and the pressure is high,the density difference between the crystal and the melt in the mush is large and the viscosity of the melt is small,which is more conducive to the crystal-melt phase separation.When the initial water content of the system is changed and the pressure is maintained,the melt migration time does not change significantly,but when the water content is maintained and the pressure is increased,the melt migration time decreases significantly.All these results,compared to the water content of the system,suggest that pressure is the main constraint factor for the formation,migration,and even the eventual eruption of the pre-eruptive melt.This pressure-dominated mechanism suggests that mechanical compaction in the magma reservoir or tectonic stress compression is the trigger mechanism that controls the formation,migration,or eruption of pre-eruptive melt.Finally,based on all the studies and the regional data,the magma plumbing system of the East Kunlun Orogen during the Triassic oceanic-continental subduction and continental-continental collision has been established.We also believe that magma unmixing is one of the nonnegligible mechanisms for the generation of large-scale homogeneous granitic magma in the crust and the high-silica intrusions（such as granitic porphyry）in the shallow crust.It provides a new genetic model for the formation of granitic rocks at mid-upper crustal scales and a new mechanism for the vertical growth of the crust at the convergent margins.