河北下堡基性岩墙地球化学特征及源区性质

Geochemical features and petrogenesis of Houcheng mafic dyke in Hebei: Evidence from major and trace elements

通过对河北后城基性岩墙进行系统研究，SiO2=47．07％--50．28％，（Na2O＋K2O）在2．83％-3．75％之间变化。具有明显的轻稀土元素富集（（La／Yb）N=4．40--4．86）和大离子亲石元素（Kb、Th和U等）富集现象，亏损高场强元素№和Ta。地球化学特征表明，岩墙产于板内拉张的构造环境，经过了橄榄石、单斜辉石、角闪石和黑云母等矿物的分离结晶作用。对La／Sm、Ta／La、Nb／La的分析表明，本区岩浆在上升过程中受到地壳物质的混染作用影响不明显，岩墙的源区与深俯冲作用引起的地幔交代作用有关。通过对微量元素及稀土元素比值的研究证明，基性岩墙源区为过渡型地幔源区，岩墙的母岩浆来自石榴子石二辉橄榄岩地幔，主要经历了石榴石＋尖晶石二辉橄榄岩部分熔融．熔融程度在10％-20％之间。岩浆的起源深度在75km左右。

Systemic studies of major elements and trace elements of the mafic dyke from Houcheng in Hebei Province indicate that SIO2=47.07%-50.28%, Na20＋K20 of the mafic dyke vary from 2.83% to 3.75%, LKEE（（La/Yb）=4.404.86）, with the enrichment of LILE（Rb, Th and U） and depletion of HFSE（Nb and Ta）. The geochemical features of the mafic dykes show that they were derived from an intraplate extemional setting and experienced fractional crystallization of olivine ＋pyroxene＋hornblende ＋biotite during the a. magmanc evolution. In addition, the La/Sm, Ta/La and Nb/La ratios suggest indistinct crustal contamination during the upward mi- gration of the magma. Based on the ratios of trace elements and KEE, the authors hold that the magma source belonged to transitional mantle. The parental magma of the maific dykes was formed in the garnet＋spinel mantle with 10%-20% partial melting; moreover, the magma came from about 75kin lithosphere mantle.