Isotopic investigations from the Hercynian‐age fold belt between the Kazakhstan and Siberian cratons in the West Junggar region determine the timing of tectonic evolution and the petrogenesis of the granitic rocks of the region. Sphene from the leucogabbro phase of the Tangbale ophiolite melange, the oldest member of the ophiolite sequences in the fold belt, yields an isotopic Pb‐Pb age of 523.2±7.2 Ma. Zircon from a postcollision alkali granite yields slightly discordant isotopic U‐Pb ages that indicate magma crystallization at 321.4±6.7 Ma, dating it in the Lower Carboniferous period. Radiometric dating thus documents a time span of circa 200 Ma for igneous activity in the area. Petrogenetic studies were made to test whether Precambrian crustal rocks might underlie the Junggar sedimentary basin. Initial lead isotope ratios determined from potassium feldspars from five alkali granites show clear affinity with ratios from mid‐ocean ridge basalts in Pb isotope correlation diagrams. Sm‐Nd data from sphene and apatite from one of the granites yield an initial ε Nd (T)=+6.1. The granite sources are depleted mantle rocks of oceanic affinity that show no involvement of recycled aged granitic crustal rocks.
The Samail ophiolite is part of an elongate belt in the Middle East that forms an integral part of the Alpine mountain chains that make up the northern boundary of the Arabian‐African plate. The Samail ophiolite represents a portion of the Tethyan ocean crust formed at a spreading center of Middle Cretaceous age (Cenomanian). During the Cretaceous spreading of the Tethyan Sea, Gondwana Land continued its dispersal, and the Arabian‐African plate drifted northward about 10°. These events combined with the opposite rotation of Eurasia and Africa initiated the closing of the Tethyan during the Late Cretaceous. At the early stages of closure, downwarping of the Arabian continental margin combined with the compressional forces of closure from the Eurasian plate initiated obduction of the Tethyan oceanic crust along preexisting transform faults, and still hot oceanic crust was detached along oblique northeast dipping thrust faults. Amphibolites developed at the base of the detached hot peridotite as it was thrust southward over oceanic volcanic and sedimentary rocks. Plate configurations combined with palinspastic reconstructions show that subduction and attendant large‐scale island arc volcanism did not commence until after the Tethyan sea began to close and after the Samail ophiolite was emplaced southward across the Arabian continental margin. The Samail ophiolite nappe now rests upon a melange consisting mainly of pelagic sediments, volcanics, and detached fragments of the basal amphibolites which in turn rest on autochthonous shelf carbonates of the Arabian platform. Laterites and conglomerates with reworked laterites on the eroded upper surface of the ophiolite indicate a period of emergence prior to the deposition of shallow water Maestrichtian carbonates. Following emplacement (Eocene) of the Samail ophiolite, the Tethyan oceanic crust began northward subduction, and active arc volcanism started just north of the present Jaz Murian depression in Iran.
The Tonsina ultramafic and mafic assemblage, located in the northern Chugach Mountains along the Trans‐Alaska Crustal Transect (TACT), is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc. It is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the Peninsular terrane along the northern fringe of the Chugach Mountains in southern Alaska. These masses, the roots of a mid‐Jurassic island arc, have been uplifted along the north dipping Border Ranges fault system and crop out sporadically for 700 km from Kodiak Island to eastern Alaska. The Tonsina ultramafic‐mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite (1 km thick) overlain by a narrow zone of websterite and clinopyroxenite. Above these ultramafic rocks is a 5‐ to 6‐km thick layered gabbro that is garnet bearing at its base. Textural relationships and chemical trends throughout the series of dunite, pyroxenite, garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous, tholeiitic parent magma at deep levels, i.e., at the base of a mature island arc before being emplaced along the Border Ranges fault. The rocks were crystallized from a melt at pressures between 9.5 and 11 kbar (28‐ to 33‐km depth) and equilibrated at temperatures of 800° to 90O°C based on phase relationships in the cumulus assemblages, pyroxene thermobarometry, and olivine‐spinel thermometry. Transitional sequences from ultramafic to mafic rocks are characterized by increasingly AL‐rich pyroxenes and spinels (or garnet) and the absence of olivine + plagioclase assemblages. Deep‐level fractionation of Al‐poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of Al in the evolving magma. As crystallization continued, the phases recorded this alumina concentration and the resultant gabbros are Al‐rich. This observed trend supports the theory of high‐alumina basalt generation by fractional crystallization of olivine and pyroxene. Comparison of the Tonsina rocks with Aleutian lower crustal xenoliths as well as other exposures of arc‐related lower crustal magma chambers in Japan, Pakistan, and western North America indicates that this process is a common mechanism for island arc magma evolution at depth.
