Abstract
Abstract The 290-282 Ma ophiolitic Liuyuan Complex in northwestern China preserves a section of oceanic crust formed in a back-arc basin during the final stages of the evolution of the Central Asian Orogenic Belt. We present results of detailed field work, petrography, whole rock, and mineral chemistry of the plutonic rocks of the Liuyuan Complex. These include: troctolite, melatroctolite, olivine gabbro, podiform olivine gabbro, podiform hornblende gabbro and intrusive hornblende gabbro. Major element data, petrography, low incompatible trace element concentrations, and positive Eu-Sr anomalies suggest that basal troctolite and melatroctolite are accumulations of olivine + plagioclase primocrysts, with varying proportions of interstitial, ophitic, and poikilitic clinopyroxene forming from trapped melt. Olivine gabbro dominates the plutonic crust between the troctolites and sheeted dykes. Troctolites and olivine gabbros have weak paleo-horizontal foliations and show systematic upward cryptic mineral-chemical trends of decreasing clinopyroxene Mg#, plagioclase An, and olivine Fo and NiO contents. Trace element modelling suggests low trapped melt fractions in the cumulates (5-15%) and implies clinopyroxene was a minor cumulus phase in the olivine gabbros. The cryptic mineral-chemical evolution and textures imply most troctolites and olivine gabbros formed as sequential accumulations from an evolving melt. Melatroctolite layers (~5m thick) in troctolite may represent fossil replenishment events. Immediately below the sheeted dyke unit, there are meter-scale pods of cumulate olivine gabbro containing abundant hornblende (25-45%), as well as decimeter- to meter-scale pods of quasi-liquid olivine gabbro with compositions similar to overlying dykes and lavas. Isolated intra-plutonic dykes of hornblende gabbro also have basaltic compositions. In the podiform hornblende gabbro, high-temperature magnesio-hornblende likely formed as a reaction product between clinopyroxene and a hydrous pore melt. Trace element inversion models indicate that these diverse plutonic rock units formed from a tholeiitic parental melt cogenetic with the overlying basalts. The Liuyuan ophiolitic complex represents a coherent slice of oceanic crust formed in a back-arc basin.
Published Version
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