粤西大金山花岗岩体地球化学特征及岩石成因探讨

Geochemical features and petrogenesis of the Dajinshan granites in west Guangdong Province

大金山花岗岩体是一个与广东省大金山钨锡多金属矿床有关的隐伏岩体,岩性主要由中细粒黑云母花岗岩和似斑状黑云母花岗岩组成。大金山花岗岩体具有高硅、富碱、贫镁钙、准铝质的特点,属于高钾钙碱性系列;微量元素以富集Rb、Th、U而亏损Nb、Eu、Ti为特征;稀土元素含量较高,中细粒黑云母花岗岩和似斑状黑云母花岗岩稀土元素含量分别为203.36×10-6-248.42×10-6、243.76×10-6-255.08×10-6,似斑状黑云母花岗岩稀土元素含量略高,中细粒黑云母花岗岩略富集重稀土而似斑状黑云母花岗岩则富集轻稀土;两者均具有强烈的Eu负异常,δEu值分别为0.004-0.009、0.059-0.13。中细粒黑云母花岗岩和似斑状黑云母花岗岩的εHf(t)值分别为-2.05--8.64、-0.92--6.57;两阶段Hf模式年龄(tDM2)分别为1 277-1 692 Ma和1 204-1 556 Ma。综合区域地质背景和大金山花岗岩体的地球化学特征,认为大金山花岗岩体形成于地壳拉张-伸展的构造背景下,是中元古代地壳部分熔融的产物。

This paper aims at reporting the petrology and the geochemistry of the buried granite stock in Dajinshan area,which is composed mainly of medium to fine grained biotitic granitic rock and porphyritic biotic granitic rock.These granites are characterized by richness of silica and alkali and depletion of magnesium and calcium.In this study,all samples from Dajinshan are high-K calc-alkaline rocks,which are enriched in Rb,Th,U and depleted in Nb,Eu,Ti in the primitive mantle-normalized spidergrams.The REE values of medium to fine grained biotite granite and porphyritic biotite granite are 203.36×10-6-248.42×10-6 and 243.76×10-6-255.08×10-6 respectively.The medium to fine grained biotite granite body is slightly enriched in HREE whereas the porphyritic biotite granite is enriched in LREE.They all show prominent negative Eu anomalies with δEu values being 0.004-0.009 and 0.059-0.13.The εHf（t） values and two-stage Hf model ages of medium to fine grained biotite granite and porphyritic biotite granite are-2.05--8.64,-0.92--6.57 and 1277-1692Ma,1204-1556Ma,respectively.Based on regional geological and granite geochemical data,it can be concluded that the Dajinshan granites were derived from the partial melting of the Mesoproterozioc continental crust under a crustal extension geodynamic setting during the Late Yanshanian period.