Geochronology and Chemistry of the Ophiolite in the Southern Tianshan, Uzbekistan

Abstract

AbstractThe ophiolite in the southern Tianshan, Uzbekistan, is interrupted distributed along the northern border (NW‐trending) of the Nuratau Mountain. It's the suture zone between kyzylkum‐Alai and Kazakhstan paleo‐continents. In order to study the tectonic evolution of the Turkestan ocean during the Paleozoic, we present zircon U‐Pb ages, major element, trace element data for rocks from the ophiolite in the Nuratau segment. The ophiolite comprises the rock blocks of serpentinite, altered basalt, allalinite, altered diabase, pillowed basalt, limestone, etc and the matrix of quartz schist, forming the grid structure. The matrix suffers strong deformation and deterioration, while the mylonitization occurred in the western outcrop. New LA‐ICP‐MS zircon U‐Pb ages were determined for six magmatic rock blocks and two quartz schist matrix and these ages confirm the presence of Late Devonian‐Early Permian (ca. 411 to 294 Ma). Five zircon grains from greenschist facies metamorphic basic rock yielded a group minimum weighted mean age of 226 Ma, interpreted as the result of Late Triassic thermal event. The apex of zircon ages for 2 quartz schist is about 450 Ma (Late Ordovician). That indicates sedimentary material from the Late Ordovician. The basalt (SiO2=48.15‐49.93%) are Na‐enriched (average Na2O=2.32‐4.02%), with high HREE/LREE ratios (average=4.93), Weak positive anomalies (average Eu*/Eu=1.02) and convex‐type mantle‐normalized immobile trace elements patterns, which are similar to the geochemical characteristics of Alkaline Ocean island basalt (OIB) and subalkaline Oceanic plateau basalt (OPB). This ophiolite is characterized by Mantle (serpentinite) and Ocean crust (altered basalt, allalinite, pillowed basalt), formed in Early Devonian at the latest. During the evolution of the Turkestan ocean, the growth of OIB and OPB, Ordovician provided a large amount of sedimentary material. The earliest start time of the ocean closed and paleo‐continents assembly is the Early Permian. Afterward, a Late Triassic thermal event occurred on a region scale and is recorded by metamorphic zircon.