| Summary: | 博士 === 國立成功大學 === 地球科學系碩博士班 === 95 === The Dongcaohe ophiolite, a tectonic block with an areal extent of 3 km×6 km, is a Proto-Tethyan ophiolite in the North Qilian Mountains and is associated with a volcanic belt and a mature backarc basin along the northern margin of the Qilian Block. It consists of an intrusive sequence and an extrusive sequence. The lower part of the intrusive sequence consists of modally cyclic layers of cumulate dunites, troctolites, anorthosites, anorthositic gabbros, and gabbros intruded by many small discordant dunite-troctolite layered bodies. This layered cumulates series grades upward into an isotropic gabbro series in the upper part, which consists of gabbros and gabbronorites. The intrusive sequence is overlain by the extrusive sequence of sheeted diabasic dikes and basaltic lavas. Order of mineral crystallization for the intrusive sequence is olivine±Cr-spinel, plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide minerals. The Cr-spinel with Mg# = 42-46 and Cr# = 41-57 from the layered cumulates are compositionally very similar to those from the present-day abyssal peridotite. The compositional variations of the plagioclase and clinopyroxene in the intrusive sequence reflect the crystallization of magmas compositionally analogous to the present-day oceanic crust.
Diopside, enstatite, pargasite, aspidolite, analcime, chlorite, serpentine, titanite, andradite, and grossular are found as mineral inclusions in chromian spinels in the troctolites and dunites of the layered cumulate series and discordant dunite-troctolite layered bodies. The mineral inclusions in the layered cumulate series are remarkably different in species and chemistry from those in the discordant dunite-troctolite layered bodies. Diopside was liquidus phase of the magmas and was enclosed by the growing chromian spinels to become mineral inclusions. Enstatite, pargasite and aspidolite were the closed-system reaction products of the trapped mineral phases and residual melts. 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 from olivine, pyroxene, or amphibole during post-magmatic hydrothermal alteration or subsequent sea-floor metamorphism of the ophiolite.
The mineral inclusions from the discordant dunite-troctolite layered bodies contain more Ti, Na, and H2O than those in dunite and troctolite from the layered cumulate series. Assimilation of the lower oceanic crust by the intruding olivine-spinel saturated magma from the upper mantle produced a modified magma rich in Si, Ti, Na, and H2O. Entrapment of the modified melt by the crystallizing Cr-spinel fromed the mineral iclusions in the Cr-spinels. This modified magma eventually solidified as the discordant olivine-troctolite layered bodies dispersed in the layered cumulate series.
The geochemical compositions of the basaltic lavas and diabasic rocks are very similar to those of the present-day mid-ocean ridge basalt. The basaltic lavas contain more incompatible elements (e.g., Th, U, Nb, Ta etc.) in concentration than the diabasic rocks. The modeled REE concentrations of the melts in equilibrium with the cumulate rocks are comparable with those of the basaltic lavas and diabasic rocks, implying that the basaltic lavas and diabasic rocks were solidified from the magma which was derived from the primary magma after the separation of the cumulate rocks. The geochemical characteristics of the basaltic and diabasic rocks show that the Dongcaohe ophiolite was originated in a tectonomagmatic environment of mid-ocean ridge of a major ocean or a mature backarc basin.
The zircon grains separated from the gabbronorite have an SHRIMP average 206Pb/238U weighted age of 497 ± 7 Ma, which may considered as the tectonic emplacement age or formation age of the Dongcaohe ophiolite. The field occurrence, mineral chemistry and whole-rock compositions indicate that the Dongcaohe ophiolite represents a well-preserved fragment of the Proto-Tethyan oceanic crust consisting of a complete oceanic crustal section with layered cumulates in the lower, isotropic rocks in the middle, and to sheeted dikes and lava flows in the upper part.
As manifested by the Dongcaohe ophiolite, the εNd(t) (t = 497 Ma) of the Proto-Tethyan oceanic asthenosphere was 4.0-6.1, which is interestingly more like the Indian type MORB than the Pacific type. Occurrence of the Dongcaohe ophiolite, Lolieygenshan volcanic belt, Menyuan backarc basin, Alaskan-type mafic-ultramafic intrusions, and arc-type granitoids on the Qilian Block suggest a southward subduction for the Paleo-North-Qilian oceanic lithosphere in the late-Cambrian to early Ordovician period.
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