title: The metamorphic sheet and underlying volcanic rocks beneath the Semail ophiolite in the northern Oman mountains of Arabia
Creators: name: Searle, Michael Paul
Publisher: name: The Open University
Year: 1980

description: 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.<br><br> 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 <sup>Y</sup>/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 (to MORB) 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.<br><br> 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 distribution coefficient K<sub>D</sub> for coexisting garnet and clinopyroxene suggests a temperature range of 670<sup>o</sup> to 750<sup>o</sup>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.<br><br> 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 Cretaceous. 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.