宁芜地区中生代富钾和富钠火山岩的源区特征:岩石学和地球化学证据  

Petrology of the Mesozoic magmatic rocks in Ningwu area:Insights from in-situ zircon Hf isotope and Nd-Sr isotopes,East China

作　　者：陈长健[1,2] 陈斌[2,3] 王志强[3] 

机构地区：[1]桂林理工大学地球科学学院,桂林541004 [2]北京大学地球与空间科学学院,北京100871 [3]合肥工业大学资源与环境工程学院,合肥230009

出　　处：《岩石学报》2017年第2期415-439,共25页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41530206);桂林理工大学科研启动费资助项目联合资助

摘　　要：宁芜中生代火山盆地广泛产出早白垩世中基性火山岩,钾质岩石主要为龙王山组英安岩、粗安岩,钠质火山岩主要为大王山组玄武粗安岩、粗安岩、英安岩以及钠质安山玢岩类。钠质辉石安山玢岩的锆石U-Pb年龄为128.9±1.9Ma,为早白垩世产物。稀土元素球粒陨石标准化配分曲线均表现为一致的平缓右倾型式,Eu负异常不明显,富集大离子亲石元素,亏损高场强元素,具Nb、Ta的负异常。钾质龙王山组火山岩总体Cs、Rb、Ba、Th、U等LILE含量较钠质大王山组火山岩和钠质安山玢岩高,并具有较高的Si、K,较低的Ca、Mg,高Rb/Sr,高Isr(0.70521~0.70857);钠质火山岩具有较低的Si、富Na、高Sr、低I_(sr)(0.70544~0.70768),Mg~#相对较高。钠质辉石闪长玢岩斑晶中有斜长石的矿物结构和成分不平衡现象,锆石Hf同位素特征为ε_(Hf)(t)=-6.2^+0.6,变化较大,指示岩浆混合过程。地幔源区可能是由中元古代俯冲板块释放出流体发生交代作用后形成的富集地幔,钾质火山岩地幔源区交代矿物可能以金云母为主,崆岭群TTG片麻岩为其地壳端元。而钠质火山岩地幔源区交代矿物可能以角闪石和单斜辉石为主,中元古上溪群变质基性岩或新生下地壳基性岩为其地壳端元。The Early Cretaceous magmatic rocks in Ningwu basin are composed of basic to intermediate volcanic and andesitic porphyry rocks. Based on detailed study of the zircon U-Pb age,petrological,mineralogical and geochemical data,the characteristics of the magma system,the petrogenesis of different rock types and the nature of the mantle source were discussed to provide new constraints on the origin and tectonic setting of the Mesozoic Ningwu basin. The zircons from pyroxene diorite porphyry yield a U-Pb age of 128. 9Ma. Longwangshan volcanic rocks are rich in K,Si and poor in Ca,Mg,whereas Dawangshan volcanic rocks and andesite porphyry are rich in Na. The volcanic rocks and andesitic porphyry are all enriched in LILE and have a slightly depletion of HFSE with negative Ti,Nb,Ta anomaly. Sodic Dawangshan volcanic rocks have slightly lower I_（Sr）（ 0. 70544 ~ 0. 70768）,higher Sr content and Mg~#. Phenocrysts of plagioclase in pyroxene diorite porphyry display disequilibrium and in-situ zircon Hf isotopes εHf（ t） =-6. 2 ~＋ 0. 6,indicating magma mixing process. I_（Sr）（ t） = 0. 70525 ~ 0. 70774,ε_（Nd）（ t） =-8. 2 ^-4. 1 of the Cretaceous volcanic and andesitic porphyry rocks in Ningwu. The K-rich and Na-rich rocks were generated by melting of enriched lithospheric mantle then the melts mixed with lower crust Kongling grey gneiss and metabasite respectively. The mantle source of the K-rich and Na-rich rocks contain rutile and clinopyroxene respectively. They may form in the stretching environment with lithospheric mantle upwelling and small extent of partial melting,then the partial melts quickly underplayed and heated the grey gneiss and amphibolite base,mixing with its melts.

关 键 词：火山岩 安山玢岩 地球化学 Nd-Sr-Hf同位素 宁芜 

分 类 号：P588.122[天文地球—岩石学] P597.3[天文地球—地质学]