Mineral Equilibria in Metagabbros: Evidence for Polymetamorphic Evolution of the Asimah Window, Northern Oman Mountains, United Arab Emirates

Abstract

The greenschist metamorphic areas of Wadi Dibba — Dahir — Wadi al Fay and Asimah are dominated isoclinally folded metacherts with less abundant intercalated metabasites, metacarbonates, and some metamorphic gabbros.

The gabbros are slightly alkaline rocks of transitional to tholeiitic affinity that intruded the Hawasina-like pelagic sediments, forming small bodies and dykes. The primary igneous phases are kaersutite, clinopyroxene, biotite, plagioclase (An_60–80) and ilmeno-hematite.

Two metamorphic events are recognized in the region. M₁ includes a typical upper greenschist-to epidote amphibolite-facies paragenesis which in the gabbro is:

Mg-hornblende — albite — quartz — chlorite ₁ — epidote₁ (X_Ps 0.17-0.26) —sphene_l. M₂ overprints M₁, resulting in the lower greenschist-facies paragenesis:

actinolitic hornblendelactinolite — albite — chlorite ₂ — epidote₂ (X_Ps 0.12-0.15) — Sphene₂ (0.023-0.043 Fe³⁺ pfu) ± stilpnomelane.

M₂ amphiboles are characterized by lower edenite substitution ((Na^A+K), Al^IV ⇔[A], Si), minor Tschermak substitution ((Al^VI,Fe³⁺,Ti), Al^IV ⇔(Fe²⁺,Mg,Mn), Si), some richterite substitution (Na^A, Na^M4 ⇔[A], Ca^M4) and slightly higher X_Mg values compared to M₁ amphiboles. Chlorite in M2 is lower in Tschermak substitution (Al^VI, Al^IV ⇔(Fe²⁺,Mg,Mn), Si) and generally richer in X ²⁺_Fe

Mineral assemblages, phase compositions, trends of partitioning of (Fe²⁺ + Mn)/Mg and FeMg_-1, and Tschermak substitution in amphibole and chlorite suggest that the Asimah rocks passed first through medium P/medium T (P = 500 ± 50 MPa; T = 450 – 500°C) conditions, then through medium P/low T (P = 400 ± 100 MPa; T = 340 – 380°C) conditions during their metamorphic evolution.

that the overlying Semail nappe was 12–15 km thick. This agrees with a 16–20 km thickness at the time of detachment, as inferred by Hopson et al. (1981).