Tittle: Tracing magma mixing and crystal–melt segregation in the genesis of syenite with mafic enclaves: Evidence from in situ zircon Hf–O and apatite Sr–Nd isotopes
Abstract: Alkaline rocks contain essential information on post-collisional/intraplate extensional magmatic processes within the continental lithosphere. The Erhulai syenitic pluton in the Liaodong Peninsula, North China Craton, consists of quartz syenite, mafic enclaves, and porphyritic K-feldspar granite. Field observations and zircon U– Pb dating reveal that the quartz syenite and the hosted mafic enclaves were coeval (with identical emplacement ages of 129–126 Ma), and slightly older than the porphyritic K-feldspar granite (ca. 123 Ma). The mafic enclaves have relatively low SiO2 and high MgO contents (up to 3.0%), and lack Ba or Eu anomalies. Whole-rock analyses of the mafic enclaves and accessory minerals in the enclaves show relatively low initial 87Sr/86Sr ratios (0.7056 to 0.7078) and variable εNd(t) (−11.2 to −2.0) and εHf(t) (−15.8 to −5.5) values. Large proportion of zircons from the mafic enclaves possess mantle-like δ18O values. These geochemical features suggest an enriched mantle source, with the addition of crustal material. The quartz syenites have relatively high SiO2 and low CaO and MgO contents, as well as significantly positive K, Ba, and Eu anomalies. All the quartz syenites show variable (87Sr/86Sr)i ratios (0.7073 to 0.7081) and εNd(t) values (−10.4 to −8.6) for the whole-rocks and apatites, εHf(t) values of −12.4 to −3.6 for the zircons, and zircon δ18O values of +4.4‰ to +7.0‰ (mainly greater than +5.3‰). All these features can be isotopically produced by the partial melting of ancient crustal material, with the involvement of mantle-derived magma. Crystal fractionation process, such as K-feldspar crystallization and accumulation, played an important role in the formation of the quartz syenite. The porphyritic K-feldspar granites possess relatively evolved geochemical compositions with high silica contents (SiO2 N 69 wt%) and remarkable negative Ba and Eu anomalies, which are chemically complementary to the quartz syenite. A crystal mush model was employed to establish the genetic link between the porphyritic K-feldspar granite and the quartz sy- enite, with the granite representing the interstitial melt extracted from the crystal mush and the quartz syenite representing the residual accumulation of crystals. The variable isotopic compositions of the quartz syenite and the porphyritic granite were mainly the result of extensive interaction of isotopes between mantle- and crust- derived magmas during magma mixing.
In situ analyses of the Sr, Nd, Hf, and O isotopes of accessory minerals, coupled with field observations and whole- rock geochemical data for the Erhulai syenitic pluton, indicate a complex multi-stage magmatic process involving magma mixing, crystal–melt segregation, and wall-rock assimilation. Our study also shows that in situ isotopic analyses of accessory minerals provide unique information on, and are powerful geochemical tracers of, mantle–crust interactions and magmatic processes.
Citation; Sun J., Zhang J., Yang J., Yang Y., Chen S., (2019). Tracing magma mixing and crystal–melt segregation in the genesis of syenite with mafic enclaves: Evidence from in situ zircon Hf–O and apatite Sr–Nd isotopes. Lithos, 334-335, 42-57.
