Dehydration melting of amphibolite at 1.5 ​GPa and 800–950 ​°C: Implications for the Mesozoic potassium-rich adakite in the eastern North China Craton

Abstract

Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites. However, the lack of direct constraints from partial melting experiments at high pressures and temperatures fuels a debate over the origin of these rocks. In this work, we performed partial melting experiments at 1.5 ​GPa and 800–950 ​°C on amphibolite samples collected from the vicinity of the Mesozoic potassium-rich adakitic rocks in the Zhangjiakou area, northern margin of the North China Craton. The experimental melts range from granitic to granodioritic compositions, with SiO2 ​= ​56.4–72.6 ​wt.%, Al2O3 ​= ​16.1–19.3 ​wt.%, FeO∗ ​= ​2.4–9.6 ​wt.%, MgO ​= ​0.3–2.0 ​wt.%, CaO ​= ​0.6–3.8 ​wt.%, Na2O ​= ​4.7–5.3 ​wt.%, and K2O ​= ​2.6–3.9 ​wt.%, which are in the ranges of the surrounding Mesozoic potassium-rich adakitic rocks, except for the higher Al2O3 contents and the data point at 1.5 ​GPa and 800 ​°C. Trace element compositions of the melts measured by LA-ICP-MS are rich in Sr (849–1067 ​ppm) and light rare earth elements (LREEs) and poor in Y (<10.4 ​ppm) and Yb (<0.88 ​ppm), and have high Sr/Y (102–221) and (La/Yb)n (27–41) ratios and strongly fractionated rare earth element (REE) patterns, whereas no obvious negative Eu anomalies are observed. The geochemical characteristics show overall similarity to the Mesozoic potassium-rich adakitic rocks in the area, especially adakites with low Mg#, again except for the data point at 1.5 ​GPa and 800 ​°C. The results suggest that partial melting of amphibolite can produce potassium-rich adakitic rocks with low Mg# in the eastern North China Craton under the experimental conditions of 1.5 ​GPa and 850–950 ​°C. The experimental restites consist of hornblende (Hbl) ​+ ​plagioclase (Pl) ​+ ​garnet (Grt) ​± ​clinopyroxene (Cpx), a mineral assemblage significantly different from that of the nearby Hannuoba mafic granulite xenoliths which consist of Cpx ​+ ​orthopyroxene (Opx) ​+ ​Pl ​± ​Grt. Chemically, the experimental restites contain higher Al2O3 but lower MgO and CaO than the Hannuoba mafic granulite xenoliths. We therefore argue that the Hannuoba mafic granulite xenoliths cannot represent the direct products of partial melting of the experimental amphibolite.