Petrology and chemistry of basal lherzolites above the metamorphic sole from Wadi Sarami central Oman ophiolite

摘要

We studied basal lherzolites that are exposed along the metamorphic sole at the base of the central Oman ophiolite (Wadi Sarami). We recognized two types of lherzolites (Types Ⅰ and Ⅱ) based on field occurrences, textures, and mineral compositions. Type Ⅰ lherzolites are massive and transition into harzburgites, whereas Type Ⅱ lherzolites are strongly foliated with mylonitic to porphyroclastic textures. Type Ⅱ lherzolites only crop out within a direct contact with the amphibolitic sole to few meters above this sole and are overlain and/or surrounded by Type Ⅰ. The clinopyroxenes [Mg# = Mg/(Mg + Fe) = 0.89-0.94] of Type Ⅱ lherzolites show higher contents of A1_2O_3 (4.5-7.3 wt%), Na_2O (0.5-1.2 wt%), Cr_2O_3 (0.6-1.4 wt%), and TiO_2 (0.2-0.4 wt%) than those of Type Ⅰ lherzolites. Positive correlations among the A1_2O_3, Na_2O, and TiO_2 contents of clinopyroxenes show a pronounced residual trend from Type Ⅰ lherzolites to depleted harzburgite, giving rise to chemical heterogeneities at the base of the mantle section. Clinopyroxenes in lherzolites and harzburgites show compositional trends that are similar to those in abyssal peridotites from normal ridge segments. Olivines (Fo_(89.4)-Fo_(91. 5)) show a residual character of the Sarami peridotites. Primary spinels show a wide range of Cr# [= Cr/(Cr + Al) from 0.04 to 0.53] and low Y_(Fe) [Fe~(3+)(Cr + Al + Fe~(3+)), ＜0.046], similar to spinels in abyssal peridotites. The wide range of spinel Cr# is a result of a wide range of partial-melting degrees, which are up to 10% for lherzolites and ~ 10-25% for harzburgites. The Type Ⅱ lherzolites, which occur near the paleo-fracture zone located to the east of Wadi Sarami, represent a remnant of asthenospheric materials trapped at the base of oceanic lithosphere mantle (Type Ⅰ) during detachment and obduction. The Type Ⅰ lherzolites experienced high-degree partial melting, resulting in the formation of harzburgites at the refractory end. The modal and compositional variations of Sarami pyroxenes and spinels indicate intrinsic mantle heterogeneity of Oman ophiolite formed as residues at an oceanic spreading center.