The metamorphic sheet and underlying volcanic rocks beneath the Semail ophiolite in the northern Oman mountains of Arabia
Detailed mapping and structural analysis along the base of the
Semail ophiolite thrust sheet in the northern Oman mountains has
revealed a distinct and separate thrust sheet between the underlying
marine sediments of the Hawasina Complex, and the overlying 12 km+
thick Semail ophiolite. This thrust sheet is termed the Haybi complex,
and consists of sub-ophiolitic metamorphic rocks and serpentinite,
alkaline and tholeiitic basalts (the Haybi volcanics), associated
with mountain-sized "Exotic" limestones and an Upper Cretaceous
sedimentary melange. The rocks of the Haybi complex are bounded by
major thrust planes, the Semail thrust above and the Haybi thrust
beneath, which truncate all schistosities, fold axes, imbricate
thrust and associated features.
The Haybi volcanics are mainly Triassic in age and form a
substrate to or enclose large, isolated blOCKS of Permian or Triassic
"Exotic" limestones, Although they have been considerably disrupted
and imbricated during south-westward emplacement, intact sequences
show that the lower part is composed mainly of alkaline pyroclastics
and lavas, including ankaramites, nephelinites and trachytes, whereas
the upper part is predominantly tholeiitic pillow lavas and breccias.
Late sills of alkali pyroxenite, wehrlite and kaersutite gabbro
intrude the tholeiitic volcanics in a few localities and have been
dated as Turonian (Upper Cretaceous). Geochemical studies, particularly
of "immobile" elements show that the lower volcanics and the late
sills are strongly alkaline with high Ti, p, Zr and Nb contents,
low Y/Nb ratios and steep LREE enriched rare earth patterns. They are
typical "within-plate" alkaline magmas characteristic of continental
rift zones and some ocean islands. Two types of tholeiites are
recognised: a relative ttoMORSl trace-element enriched "transitional"
type which probably farmed in a transitional within-plate tectonic
setting and a "depleted" type with the characteristics of islandarc
tholeiites. The uppermost Haybi arc lavas are probably Middle
Cretaceous in age (from radiolaria in interbedded cherts) and may
be related to the initiation of a subduction zone in the Cretaceous.
prior to ophiolite formation and emplacement.
Metamorphic rocks showing an inverted metamorphic zonation from
upper amphibolite facies immediately beneath the peridotite to
greenschist facies at lower levels, outcrop discontinuously along
the base of the Semail ophiolite thrust sheet. These metamorphic
rocks show polyphase deformation, mylonitic fabrics, and have been
disrupted. folded and imbricated, and in places form tectonic
inclusions in a serpentinite melange. In the more intact sequences,
garnet-clinopyroxene amphibolites, with rare hornblende and
clinopyroxene-bearing marbles and banded quartzites occur at the
higher levels whilst a wide range of meta-sedimentary and metabasaltic
rOCKS occur in the greenschist facies. "Immobile" element and REE
geochemistry supports field assumptions that the amphibolites were
derived mainly from Haybi "transitional" (or MORS-type) volcanics.
Protoliths of the meta-sediments include "Exotic" limestones, Mn-rich
cherts and argillaceous turbidites probably derived from the Hawasina
Complex. Low glaucophane content of amphiboles and low jadeite
content of clinopyroxenes suggest relatively low pressures of
crystallisation. The distrtbution coefficient KO for coexisting
o garnet and clinopyroxene suggests a temperature range of 670 to
7500 C, the upper limit of the amphibolite facies. Residual heat
from the recently-formed ophiolite provided the dominant heat
source for metamorphism during the Turonian-Cenomanian (Upper
Cretaceous) although frictional heating during thrusting could
have supplemented this.
Ophiolite emplacement in Oman is thought to be essentially a
two-stage process. During initial displacement, high-temperature
metamorphism occurred along the base of the fractured ophiolite
forming the metamorphic sheet and successive tectonic slices were
incorporated onto the base of the ophiolite, presumably by underthrusting.
The metamorphism probably occurred along a shallowdipping
subduction zone (dipping north-east) which was initiated
during the formation of the Haybi arc lavas in the Middle Cretac(~ous.
Final emplacement of the Semail ophiolite by gravity sliding or
spreading was facilitated by a thin decollement layer of basal
serpentinite along the Semail thrust plane which truncates all
underlying structures. Since continent-continent collision has not
occurred in this area of Tethys, the Oman ophiolite remains largely
undeformed unlike the ophiolites along the Zagros suture zone of
Iran or the Indus-Tsangpo suture zone in the Himalayas.