科研之友
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摘要
Subduction-related shoshonitic and calc-alkaline plutons coexisted in the Paleoproterozoic Khondalite belt (North China Craton). They intruded at 1.97-1.96 Ga and were overprinted by 1.94-1.92 Ga high-temperature metamorphism. The mafic to intermediate (SiO2 = 51.8-55.5 wt%) Wudangzhao shoshonitic metamonzodiorite has high K2O (3.0-4.9 wt%) contents. In contrast, the mafic (SiO2 = 48.5-51.7 wt%) Yebaigou metagabbro is sodium-rich (Na2O/K2O > 2). In Harker diagrams, the two intrusions show different magmatic evolution trends for selected major and trace elements, suggesting that they belong to two distinct magma series. Both intrusions are enriched in light rare earth element and depleted in high field strength elements, such as Nb, Ta, and Zr, in the trace-element diagrams normalized to the primitive mantle. The low epsilon(Nd 1960 Ma) values (- 3.1 to + 0.6) of whole-rock samples, the low epsilon(Hf)(t) values (- 2.2 to + 0.9) of magmatic zircon samples, and the relatively high delta O-18 values (+ 5.9 to + 6.4 parts per thousand) of zircon crystals indicate that the mantle source of both intrusions had been metasomatized by fluids/melts derived from subducted continental crustal material. Partial melting of the amphibole and/or phlogopite-bearing mantle yielded the calc-alkaline melt with the chemical fingerprint characteristic for the Yebaigou metagabbro. Small-volume partial melting of a similar mantle source at greater depth produced shoshonitic melts that formed the Wudangzhao metamonzodiorites. The Wudangzhao metamonzodiorites and Yebaigou metagabbros represent part of a Paleoproterozoic continental magmatic arc on an Archean basement. This study shows that shoshonitic and calc-alkaline magmatism may exist coevally above ancient subduction zones and, thus, could provide some clues for distinguishing upper and lower plates in deeply eroded ancient collisional belts.
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单位南京大学; 中国科学院
