A case study for efficient magma mixing: Constraints from early Paleozoic high Ba–Sr granitoids in the North Qinling Orogen, Central China

Abstract

Magma mixing is the key to revealing deep geodynamic processes. High Ba–Sr granitoids (Ba > 500 ppm; Sr > 300 ppm) are initially defined as products that reflect the entrainment of mantle sources into the melting zone, which could provide constraints for magma mixing. Here, we collected a suite of spatiotemporally correlated high Ba–Sr granitoids (Ba = 831–1136 ppm; Sr = 991–1306 ppm) from the North Qinling Orogen, Central China, to constrain their petrogenesis based on whole-rock elemental and isotopic compositions, as well as zircon U–Pb, Lu–Hf, and O isotopes. High Ba–Sr granitoids in the North Qinling Orogen mainly formed during the early Paleozoic from ca. 450 to 420 Ma and exposed to a nearly east–west trend. In the Erlangping unit, they exhibited variable SiO2 and elevated Mg number (Mg#, 42–55) and Cr (39–77 ppm) and Ni (13–54 ppm) contents. Simple simulations of trace elements indicated fractional crystallization of clinopyroxene, garnet, hornblende, and accessory minerals. They have depleted Sr–Nd–Hf and low O isotopes (4.89 ‰–5.48 ‰), and isotopic modeling constrains a depleted mantle, which was metasomatized by the oceanic sediments and altered oceanic crust-derived fluid. Spatially adjacent high Ba–Sr granitoids in the Kuanping unit have more “crustal/enriched” geochemical and isotopic characteristics, but distinctly higher total Mg# (53–54) and Fe2O3 (3.77–4.90 wt%) and Ni (16.8–24.4 ppm) contents than typical crustal-derived melts, coupled with the occurrence of mafic microgranular enclaves (MMEs). The simulation of the fractional crystallization of coeval gabbroic intrusions well reproduced the trace element compositions of the MMEs. There was some complementarity in the composition of the simulated melt and neighboring crustal-derived melts (e.g., Nb, Ta, Zr, Hf, and heavy rare earth elements), supporting the mixing origin of the high Ba–Sr granitoids. Isotopic modeling constrained the mixing of mantle-derived melt and lower crust-derived melt to form high Ba–Sr granitoids. Although the host granitoids and MMEs shared an overlapping isotopic scope, the fractional crystallization process and initiation of local superheating of the felsic magmas provided the possibility for efficient magma mixing.