粤北桃村坝花岗岩锆石U-Pb年代学、地球化学及成因研究

Zircon U-Pb Geochronology,Geochemistry,and Petrogenesis of the Taocunba Granite,Northern Guangdong Province

桃村坝花岗岩体位于粤北贵东复式花岗岩体中部。锆石U-Pb年龄为161.5±1.8 Ma(MSWD=1.7),属于燕山早期岩浆活动产物。该岩体具有稍低的硅、富铝、富碱、钾大于钠、贫钙镁和高FeO*/MgO等特征。富集Rb,Th,U而亏损Ba,Sr,Ti和Nb;LREE富集(LREE/HREE=7.39~16.4,(La/Yb)N=8.79~25.5),Eu亏损较为明显(δEu=0.44~0.59);Ga/Al比值较高(平均值为2.99),Zr+Nb+Ce+Y含量(平均为518×10-6)大于350×10-6,可归属于A2型花岗岩;εNd(t)值低,为-9.7^-8.95,Nd模式年龄为1.66~1.76 Ga;锆石的εHf(t)值为-20.0^-14.6,相应的Hf模式年龄为2.12~2.46 Ga。综合以上特征表明桃村坝花岗岩是在地壳伸展—减薄的构造背景下、由古元古代地壳组分部分熔融的方式形成。

The Taocunba granite is one of the granitic plutons located within the central part of the Guidong composite granite batholith in northern Guangdong province. LA-ICPMS zircon U-Pb dating yields an age of 161.5＋ 1.8 Ma（MSWD=I.7）, suggesting that it was formed in the Early Yanshanian period. Geochemically, the granites of the Taocunba pluton are peraluminous, and low in silicon, calcium and magnesium, and rich in alkalis with potassium content higher than sodium. They also show high FeO＊/MgO ratios. The trace elements are characterized by the enrichment in Rb, Th and U and the depletion in Ba, Sr, Ti and Nb. They also display enrichment in LREE （LREE/HREE=7.39-16.4, （La/Yb）N=8.79～25.5） and evident depletion in Eu （δEu=0.44～0.59）. Furthermore, the granites have high Ga/A1 ratios with a mean value of 2.99 and their Zr＋Nb＋Ce＋Y contents are over 350×10^（-6）, showing affinity with Artype granites. Isotopically, they have low εNd（t） values （-9.7～-8.95） with two-stage Nd model ages of 1.66-1.76 Ga and low εNf（t） values （-20.0～-14.6） with Hf model ages of 2.12～2.46 Ga. Taken together, these features indicate that the granites of the Taoeunba pluton were derived from partial melting of the Paleoproterozoic crustal rocks under an extensional setting.