Magma Generation of Magnetite-Rich Intermediate-Mafic Rocks and Its Mantle Processes in the Southwestern Alxa Block,NW China  

作　　者：Lu Tao Hongfei Zhang Jing Wu Xiaochun Zhou Liqi Zhang 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China [2]School of Earth Sciences,China University of Geosciences,Wuhan 430074,China [3]Gemmological Institute,China University of Geosciences,Wuhan 430074,China [4]Faculty of Earth Resources,China University of Geosciences,Wuhan 430074,China

出　　处：《Journal of Earth Science》2022年第1期161-176,共16页地球科学学刊（英文版）

基　　金：supported by the National Natural Science Foundation of China(No.41573021);the Most Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources(No.MSFGPMR201601-2)。

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 enrichment 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 providing some constraints on magma generation of magnetite/iron-rich intrusive rocks.The Helishan pluton,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 (^(87)Sr/^(86)Sr)_(i) values of 0.705 2 to 0.705 8,ε_(Nd)(t) values of+0.03 to+0.64,and zircon ε_(Hf)(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 intermediatemafic 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 fugacity and H_(2)O condition.

关 键 词：geochemishtry magnetite-rich intrusion mantle source PETROGENESIS lithospheric process Alxa Block Northwest China 

分 类 号：P588.11[天文地球—岩石学] P542[天文地球—地质学]