【摘 要】
：
Many studies focus on mineralization of huge magnetite ore deposits and petrogenesis of their large-volume host rocks. However, magma generation of those small-scale intrusions with enrich- ment of magnetite is poorly reported and paid attention to. We he
【机 构】
：
State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,W
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Many studies focus on mineralization of huge magnetite ore deposits and petrogenesis of their large-volume host rocks. However, magma generation of those small-scale intrusions with enrich- ment of magnetite is poorly reported and paid attention to. We here carry out an integrated study of magnetite chemistry, U-Pb zircon dating, geochemistry, and Sr-Nd-Hf isotopes for the magnetite-rich intermediate-mafic rocks from the Helishan pluton in the southwestern Alxa Block, Northwest China. This, together with several previously reported magnetite/iron-rich intrusions nearby, is capable of pro- viding some constraints on magma generation of magnetite/iron-rich intrusive rocks. The Helishan plu- ton, dated at ca. 290 Ma, consists of hornblende gabbro, diorite, and quartz monzodiorite with～3％- 5％ magnetite in all the lithologies. Study on magnetite chemistry manifests a magmatic origin for them. All the lithologies display high TFeO/MgO ratio (1.71-1.89), weakly fractionated REE patterns ((La/Yb)N = 1.82-10.17), enrichment of Rb, Sr, and Pb, and depletion of high field strength elements. They have (87Sr/86Sr)i values of 0.7052 to 0.7058,εNd(t) values of+0.03 to+0.64, and zirconεHf(t) values of +6.5 to +12.0. We propose that they were derived from partial melting of iron-rich metasomatized lithospheric mantle. The systematic variations of Sr/Y ratios and Nd-Hf isotopic compositions with time for the Paleozoic igneous rocks at the southwestern Alxa Block indicate ever existence of thinning and rebirth of lithospheric mantle. This geodynamic process could be the potential mechanism to give rise to the iron-rich signature of the reborn mantle sources of the Helishan pluton. For intermediate- mafic intrusions at subduction zones, they are unlikely to form considerable magnetite ore deposits since their modest magmatic flux and early fractional crystallization of magnetite at a high oxygen fu- gacity and H2O condition.
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