粤北下庄铀矿田黄陂-张光营辉绿岩脉的成因：元素地球化学和Nd-Sr-Pb-O同位素证据

The origin of the Huangpi-Zhangguangying diabase-dykes in the Xiazhuang uranium ore district of northern Guangdong province:Evidence from trace elements and Nd-Sr-Pb-O isotopes.

黄陂-张光营辉绿岩脉位于粤北贵东复式花岗岩岩体东部的下庄铀矿田内,形成于142．6±3．0Ma。岩石化学呈现拉斑玄武宕系列的特征,富集大离子亲石元素；在蛛网图上,Th表现出正异常,而Ba呈负异常,P轻度亏损,无Nb,Ta,Zr,Hf异常；轻、重稀土之间分馏不明显,基本无Eu异常。微量元素和稀土元素特征与板内玄武岩的相似；铅同位素具有典型的Dupal异常铅特征,ε_(Nd)(t)值为3．2～4．7,(^(87)Sr/^(86)Sr),为0．70446～0．70628；在Pb-Pb,Nd-Pd,Nd-Sr和Sr-Pb相关图解上,数据点都位于亏损地幔和富集地幔Ⅱ之间。地球化学特征表明,黄陂-张光营辉绿岩脉是在拉张的构造环境下由介于亏损地幔和富集地幔Ⅱ之间的、相对富集的地幔源区部分熔融形成的,而这种富集地幔可能是由俯冲带流体交代形成的。

The Huangpi-Zhangguangying diabase dykes are located in the Xiazhuang uranium ore district of northern Guangdong province and emplaced in the eastern part of Guidong composite granite pluton in the Late Jurassic period. The petrochemical features show that the diabase dykes belong to tholeiitic series which have high contents of TiO2 and Na2 O, higher solidification index（ S. I. ） and ratios of Na2 O/K2 O, and low contents of MgO and ratios of NaO2/TiO2. Comparing to MORB, they are enriched in large ion lithophile elements, such as Sr, K, Rb and Ba, and high field strength elements, such as Th, Ta, Nb and Zr, and are depleted in Y and Yb. In trace element spider diagram the diabase dykes show positive anomaly in Th, negative anomaly in Ba, weakly anomaly in P, and no anomaly in Nb, Ta, Zr and Hf. There is no fractionation between LREE and HREE, nor obvious Eu depletion. The ratios of Hf/Th and Ta/Hf of the diabase dykes are consistent with those of intraplate basalts. The discrimination diagrams of Zr-Zr/Y, TiO2-Y/Nb, ZroTi and Ti/100-Zr-Y x 3 for tectonic setting show that they were formed in an intraplate environment. The εNd （t） value and initial ^87Sr/^86Sr ratios of the diabase dykes are 3.2-4.7 and 0.70446-0.70628 respectively. They have δ^18 O value of 6.5‰- 0.67‰, similar to the mantle value, and typical Dupal anomaly Pb isotope characteristics. In diagrams of Pb-Pb, Nd-Pb, Nd-Sr and Sr-Pb isotopes, they plot between DMM and EMII, with Pb similar to EMII, Nd relatively close to MORB and Sr in between. The detailed geochemical features suggest that the Huangpi-Zhangguangying diabase dykes were derived from partial melting of a mantle source subjected to metasomatism by fluids originated from subduction zone, and hence enriched and characterized with features between DMM and EMIl, under the tectonic environment of crust extension in the late Jurassic period.