The Dongcaohe ophiolite, located at the south of the North Qilian subduction complexes, is a tectonic block with an exposed area of about 3 km×6 km. It consists of an intrusive section overlain by an extrusive section. The lower part of the intrusive section consists of cyclic layers of cumulate dunites, troctolites, anorthosites, anorthositic gabbros, and gabbros with small discordant dunite and troctolite bodies. This layered sequence grades upward to isotropic gabbros and gabbronorites, which are overlain by the extrusive sequence of diabasic sheeted dikes and basaltic lavas. The overall mineral crystallization sequence was olivine±Cr-spinel, plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. The Cr-spinel (Mg#: 42-66, Cr#: 41-57) in these layered cumulates and present-day abyssal peridotites have similar compositions. Also, the compositional variations of the plagioclase and clinopyroxene in the intrusive section reflect crystallization from melts compositionally similar to the present-day ocean basalts. Moreover, the rare earth element (REE) and multi-element distribution patterns of the intrusive and extrusive lithologies in the Dongcaohe ophiolite are consistent with crystallization of mid-ocean ridge basalts. The zircon grains separated from the gabbronorite have an SHRIMP average 206 Pb/ 238 U weighted age of 497 ± 7 Ma, which is considered as the tectonic emplacement age of the Dongcaohe ophiolite. The field occurrence, mineral and whole-rock compositions indicate that the Dongcaohe ophiolite represents a well-persevered oceanic crustal fragment composed of a complete oceanic crustal section of layered cumulates at bottom upgrading through isotropic cumulates to sheeted dikes and lava flows. ophiolite, North Qilian, oceanic crust, DongcaoheOphiolites are sections of the oceanic crust and subjacent upper mantle thrust up on continental and island arc margins [1,2] . Those in China usually occur as dismembered ophiolitic fragments. Well-preserved bottom-totop lithological sequence of peridotite-gabbro-sheeted dike-basalt that characterizes oceanic lithosphere is rare [2] . We, on the basis of lithological assemblages, petrographic textures, and geochemical characteristics, recently identified a section of entire oceanic crust at Dongcaohe on the south flank of the North Qilian orogenic belt and referred it to as the "Dongcaohe ophiolite".In this study, field occurrence, geochronology, mineral and bulk compositions were used to address the tectonic environments associated with the Dongcaohe ophiolite and their geological implications.
Two blueschist belts in the North Qilian Mountains occur in Middle Cambrian and Lower Ordovician strata and strike N30-35"W for about 500 km along the Caledonian fold belt on the south-west margin of the SineKorean plate. The styles of metamorphism and deformation are quite different in the two belts. The Middle Cambrian to Ordovician rocks in the high-grade belt are mainly blueschists and C-type eclogites in which six phases of lower and upper crustal deformation have been recognized. The rocks contain glaucophane, phengite, epidote, clinozoisite, chlorite, garnet, stilpnomelane, piedmontite, albite, titanite and quartz. The estimated P-T conditions of eclogites are 340 f 10" C, 8 f 1 kbar and, of blueschist, >380" C, 6-7 kbar. The Ordovician rocks in the low-grade belt are characterized by ductile to brittle deformation in the middle to upper crust. The low-grade blueschists contain glaucophane, lawsonite, pumpellyite, aragonite, albite and chlorite. The estimated P-T conditions are 150-250" C and 4-1 kbar.K-Ar and " A T /~' A~ geochronology on glaucophane and phengite from the high-grade blueschist belt suggest two stages of metamorphism at 460-440 and 400-380Ma, which may represent the times of subduction and orogeny. The subduction metamorphism of the northern low-grade blueschist belt took place approximately at the end of the Ordovician.
After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P‐T paths for the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P‐T paths from different methods indicate that all the high‐pressure rocks in the Qilian area recorded P‐T paths with clockwise loops starting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite facies or epidote‐amphibolite facies and ending up with the greenschist facies. The incremental Ar‐Ar dating shows that the plateau ages for the high‐pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for the subduction that resulted in the formation of these high‐pressure rocks in the Qilian area. It is proposed that the late‐stage decompressional and cooling P‐T paths with ends at the greenschist facies for these high‐pressure rocks probably reflect the uplift process which could occur after shifting the arc‐trench tectonic system to the system of continental orogenic belts. The retrograde paths for the high‐pressure rocks in the North Qilian tectonic belt are characterized by dramatic decompression with slight cooling, which suggests very rapid exhumation. Petrography supports that the mountain‐building for the Qilian mountain range could undergo a very fast process which caused rapid uplift and denudation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.