Dunite xenoliths and olivine xenocrysts in gabbro from Taihang Mountains: Characteristics of Mesozoic lithospheric mantle in Central China

Abstract

This article reports the petrography and mineral chemistry of dunite xenoliths and olivine xenocrysts entrained by the Early Cretaceous Xi’anli (西安里) hornblende (Hb)-gabbros from the southern Taihang (太行) Mountains, with the aim of constraining the nature of the Mesozoic lithospheric mantle in Central China. Rounded dunite xenoliths are 1–3 cm3 in size and display porphyroclastic, tabular, and protogranular textures. Chromite with Cr#=60–89 is common in the xenoliths. Olivine xenocrysts of 4–6 mm in size are also found in the Hb-gabbros. Orthopyroxene reaction rims are commonly observed around olivine xenocrysts or between dunite xenoliths and host rocks. The porphyroclastic olivines within dunite xenoliths and olivine xenocrysts have kink bands and Mg#=83–94. The Mg# of olivine cores and rims are 89–94 (average, 90) and 83–86 (average, 84.4), respectively. The CaO contents of all olivines from the xenoliths and xenocrysts are less than 0.1 wt.%, suggesting a lithospheric mantle origin. The Ca content (214 ppm–818 ppm) and Ti content (15 ppm–137 ppm) in the xenoliths and xenocrysts are similar to those of olivines from the dunite xenoliths, but are much higher than those of olivines from harzburgite and lherzolite xenoliths in the Fushan (符山) intrusion. This finding implies that the xenoliths and xenocrysts may have originated from harzburgites or lherzolites that were intensively modified by silica-rich melts. This result, combined with high Mg# (92–94) of olivine cores from the dunite xenoliths and xenocrysts, indicates that these olivine xenocrysts and dunite xenoliths could represent the residue of ancient (Archean or Paleoproterozoic) lithospheric mantle and might have experienced the same intensive modification by silica-rich melts as the host magma, resulting in enrichment in MgO and SiO2.