Petrogenesis of an ultrahigh‐pressure garnet‐bearing ultramafic body from Maowu, Dabie Mountains, east‐central China

Abstract

The Maowu garnet‐bearing ultramafic body (∼ 250 × > 50 m2) in the Dabie ultrahigh‐pressure (UHP) terrane has several distinct petrologic characteristics: (i) most rocks are layers of garnet‐bearing ultramafics including orthopyroxenite and clinopyroxenite with minor harzburgite and omphacite‐rich layers; these compositional layers range in thickness from 5 cm to 1.6 m; (ii) rutile is ubiquitous and is most abundant in clinopyroxenite (up to 1–2 vol%); (iii) monazite is common as inclusions in silicates and as a matrix phase; (iv) exsolved plates of magnetite occur in olivine (Fo93), and monazite in apatite; (v) chromite occurs as fine‐grained inclusions in enstatite and clinohumite; (vi) hydrous phases including talc, clinochlore and amphibole are common as inclusions in coarse‐grained garnet; and (vii) major silicates are high in Mg/(Mg + Fe) values. Most of the ultramafic rocks are high in rare earth elements (REE), P, Cr and TiO2, and are significantly different from Mg–Cr or Fe–Ti garnet peridotites which are common in the western Alps, Western Gneiss Region, and the Bohemian Massif. Well‐foliated orthopyroxenite is composed of Grt (Prp60–74) + En (En92–96; 0.05–0.14 wt% Al2O3) + Chl (2–3 wt% Cr2O3) ± clinohumite ± magnesite ± Di (Di94–97) + chromite. Omphacitic rocks contain mainly elongated omphacite (> 90 vol%) with rare garnet (Prp45Alm41Gr13), rutile, apatite, and monazite. Omphacites (Jd63–67Ac6–12Aug20–31) display pronounced compositional zoning and contain inclusions of coesite relics and quartz pseudomorphs after coesite. Minor retrograde phases include talc/tremolite after Px, Chl after Grt, Cpx (Jd49Ac13Aug38) and Ab after omphacite. Garnet‐bearing ultramafic and omphacitic rocks have been subjected to UHP metamorphism at pressures of ∼ 35–50 kbar and 750 ± 50 °C under extremely low XCO2 conditions (< 0.001); a minor amphibolite facies overprint took place at P < 15 kbar and 650 °C. The protoliths may be Proterozoic ultramafic crustal cumulates that were subjected to metasomatism prior to Triassic subduction to mantle depths of more than 100 km during the collision of the Sino‐Korean and Yangtze Cratons.