Search

Field relationships, mineralogy and petrology, whole-rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan-type. The mineral chemistry of the chromian spinels (Cr-spinels) and clinopyroxenes, and the variation trend of the whole-rock compositional plot in the (Na2O + K2O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass-resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H2O-rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform-type layered sequence, followed by diapiric re-emplacement. The occurrence of the Alaskan-type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.

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 206Pb/238U 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.

Diopside, enstatite, pargasite, aspidolite, analcime, chlorite, serpentine, titanite, andradite, and grossular are found as mineral inclusions in grains of chromian spinel in the troctolites and dunites of the layered cumulate series and discordant layered bodies, Dongcaohe ophiolite, in the North Qilian Mountains, Qinghai Province, northwestern China. The mineral inclusions in the layered cumulate series are remarkably different in species and composition from those in the discordant layered bodies. Diopside, the liquidus phase, was enclosed by the growing crystals of chromian spinel. Enstatite, pargasite and aspidolite were products of the closed-system reaction between the trapped mineral phases and trapped residual melt. Analcime crystallized or precipitated directly from the evolved entrapped melt or hydrothermal solution in the inclusions. Chlorite, serpentine, titanite, andradite, and grossular are of secondary or metamorphic origin and were produced by alteration of olivine, pyroxene, or amphibole during postmagmatic hydrothermal alteration or subsequent seafloor metamorphism of the ophiolite. Syntexis of the lower oceanic crust was involved in the formation of a modified magma, which solidified as the discordant layered bodies scattered within the layered cumulate series of the Dongcaohe ophiolite.