江西崇义铁木里钨铁矿区内辉绿玢岩的年代学、地球化学特征及地质意义

Geochronology,Geochemistry and Geological Implications of Diabase Porphyrite in Tiemuli W-Fe Deposit,Chongyi County,Jiangxi Province

华南地区中生代基性岩浆岩以拉斑玄武岩系列为主,碱性系列少见.江西崇义铁木里地区发育由碱性辉绿玢岩和A型花岗岩组成的双峰式侵入岩组合,对其中的辉绿玢岩开展了矿物学、年代学和地球化学研究.辉绿玢岩的斑晶矿物以次透辉石为主,少量为透辉石和普通辉石;基质中的斜长石为中长石(Ab=39.11%~43.30%),角闪石为钛闪石.LA-ICP-MS锆石U-Pb定年结果表明辉绿玢岩的形成不早于136.6 Ma.岩石具有低SiO_(2)(41.73%~46.68%)、TiO_(2)(1.72%~1.94%),富TFeO(7.64%~9.24%)、Al_(2)O_(3)(15.70%~17.22%),全碱含量高(5.28%~6.60%)的特征,Mg^(#)值为0.51~0.54,属于碱性玄武岩系列.轻重稀土元素之间、轻稀土元素内部分馏强((La/Yb)N=17.58~22.28,(La/Sm)N=4.72~5.18),Eu异常不明显(δEu=0.84~0.99),显著富集大离子亲石元素,轻微富集高场强元素,过渡族元素含量较原始地幔偏低.Nd同位素表现为亏损特征(εNd=+3.45),(_(87)Sr/^(86)Sr)i=0.7075,(^(206)Pb/^(204)Pb)i=18.7696,(^(207)Pb/^(204)Pb)i=15.7337,(^(208)Pb/^(204)Pb)i=39.1100,显示OIB与EMII过渡特点.矿物成分、岩石地球化学、Nd同位素特征指示铁木里辉绿玢岩源自软流圈,部分熔融发生于石榴石稳定域,是软流圈地幔减压低程度熔融的产物,碱性玄武质岩浆与陆壳物质之间的同化混染作用较弱,岩浆演化过程中发生了单斜辉石、磷灰石和铁钛氧化物的分离结晶作用.华南内陆在白垩纪处于伸展拉张的应力状态,碱性玄武质岩石形成于岩石圈拉张和超壳深大断裂联合作用形成的板内伸展裂解(类裂谷)构造环境.

The Mesozoic basic igneous rock in South China is dominated by tholeiite series,while the alkali series is rare.Bimodal intrusive pluton composed of alkali diabase porphyrite and A-type granite occurs in Tiemuli area,Chongyi County,Jiangxi Province.Mineralogical,geochronological and geochemical analyses of the diabase porphyrite were carried out.The Tiemuli diabase porphyrite displays porphyritic texture.Phenocrysts are dominated by salite,with minor diopside and augite,while groundmass minerals comprise andesite (with Ab values of 39.11%-43.30%),kaersutite and clinopyroxene chemically equivalent to phenocrysts.The diabase porphyrite was emplaced after the granite (136.6 Ma) and belongs to alkali basalt series,characterized by low SiO2(41.73%-46.68%) and TiO2(1.72%-1.94%) contents,high TFeO (7.64%-9.24%),Al2O3(15.70%-17.22%)and alkali (5.28%-6.60%) contents,and moderate Mg#values (molar ratios of MgO/(MgO+TFeO),0.51-0.54).The fractionation between light rare earth elements (LREEs) and high rare earth elements (HREEs) and fractionation of LREEs are remarkable ((La/Yb)N=17.58-22.28,(La/Sm)N=4.72-5.18),while the Eu anormalies are negligible (δEu=0.84-0.99).All rocks are significantly enriched in large iron lithophile elements (LILEs) and are slightly enriched in high field strength elements(HFSEs),with lower contents of transition elements than primitive mantle.They haveεNd,(87Sr/86Sr)i,(206Pb/204Pb)i,(207Pb/204Pb)i and (208Pb/204Pb)ivalues of+3.45,0.707 5,18.769 9,15.733 7 and 39.110 0,respectively,displaying transitional features between OIB and EMII.Mineralogical,geochemical and Nd-Hf isotopic characteristics suggest that the Tiemuli diabase porphyrite was generated in asthenosphere and the partial melting took place in garnet stability field.The alkali basalt magma was derived from small degree of decompression melting of depleted asthenospheric mantle and contamination of crustal materials was minor.Fractional crystallization of clinopyroxene,apatite and Fe-Ti oxides took place during the basaltic magma evolution.Some enriched components from lithospheric mantle might be involved in.The Cretaceous alkali basaltic rocks developed interior of South China were formed in intracontinental extension (rift-like) regime related to regional lithospheric extension and deeppenetrating faults.