柴北缘元古宙鹰峰环斑花岗岩及其共生岩石的地球化学特征、成因及地质意义

The Geochemistry Features,Origin of the Yingfeng Rapakivi Granite and Its Associated Rocks in North Qaidam Basin and the Geological Significance

中元古代鹰峰岩体的主体是环斑花岗岩,与其共生的岩石有石英闪长岩—奥长花岗岩和辉绿岩。环斑花岗岩高碱(Na2O+K2O=8.49%～9.39%)、富钾(K2O/Na2O=1.12～1.43),铝近饱和,高铁镁比值[(ΣFeO)/MgO=4.91～7.19];富Rb、Ba、Ga、Th、Zr、Nb、Ta,贫Cr、Ni、V;高ΣREE(392.24×10-6～594.76×10-6),稀土元素强分异[(La/Lu)N=12.67～17.09],弱铕负异常(δEu=0.58～0.78),显示碱性花岗岩的特征,与密云环斑花岗岩相似。石英闪长岩—奥长花岗岩具钙碱性系列岩石的特征;与环斑花岗岩相比,其Rb、Ba、Ga、Nb、Ta、Th、Hf、Zr低,而Ni、Cr、V高;ΣREE较低(ΣREE=77.04×10-6～129.85×10-6),轻重稀土分异明显,但(La/Lu)N的比值较小(11.62～14.06),铕异常更弱(δEu=0.69～0.93)。辉绿岩具低碱、高ΣFeO的特征,属拉斑玄武质,与洋中脊拉斑玄武岩相比,K2O等不相容元素高,具大陆拉斑玄武质的特征。辉绿岩的ISr(1776Ma)为0.7066,εNd(1776Ma)为+3.6,环斑花岗岩的ISr(1776Ma)为0.7181,εNd(1776Ma)为-5.5,显示辉绿岩起源于年轻的地幔,花岗质岩浆主要源自古老的地壳。综合分析显示,这些侵入岩形成于伸展背景,是北半球中元古代非造山环斑花岗岩的成员之一,在加里东期卷入到柴北缘造山带的古老地壳中。这在世界上提供了一个古老克拉通及环斑花岗岩卷入古生代造山带的一个实例。

The Mesoproterozoic Yingfeng intrusions are predominately composed by rapakivi granite. Diabase and quartz diorite--oligosite granite are also associated with it. The rapakivi granite exhibit the features of high value of alkali （Na2O＋K2O= 8. 49% ~9.39%） and potassium （K2O/Na2O= 1.12~ 1.43）, almost Al-saturated, high （∑FeO）/MgO ratio （∑FeO）/MgO= 4. 91 ~ 7.19） ; the enrichment of Rb, Ba, Ga, Th, Zr, Nb, Ta and poor in Cr, Ni, V; high content of EREE （392.24× 10^- 6 ~594.76× 10^- 6）, remarkable differentiation of REE （ （La/Lu） N = 12.67 ~ 17.09 ） and weakly negative Eu anomaly （δEu = 0. 58~0.78）, showing the characteristics of alkali granite, and being similar to the Miyun rapakivi granite, Beijing, China . The quartz diorite--oligosite granite shows low value of Rb, Ba, Ga, Nb, Ta, Th, Hf, Zr and high value of Ni, Cr, V; low value of （ EREE= 77. 04 ×10^- 6 ~129. 85 ×10^- 6）, obvious differentiation of LREE and HREE, but low value of （La/Lu）N （11.62~ 14.06）, more weaker negative Eu anomaly （δEu=0. 69~0. 93）. The Isr（1776 Ma） value of oligosite granite is 0. 6924 and the εnd （1776Ma） value is very high （＋9.7）, indicating its protolith could be the lower crust which strongly suffered anatexis and poor in early period. The Isc（1776 Ma） value of diabase is 0. 7066 and the εNd（1776Ma） value is ＋3. 6, showing the magma originated from modified mantle. The Isr（1776 Ma） value of rapakivi granite is 0. 7181 and the εNd（1776Ma） value is --5. 5, showing it mainly originated from ancient crust. These analysis results indicate that the diabase originated from tholeiitic magma, and the granite was genesis from ancient crust. This study suggests that these intrusions formed in an extension environment and the Yingfeng rapakivi granite is one member of anorogenic rapakivi granite of the North Hemisphere in Proterozoic, however, the intrusions were involved in ancient crust of North Qaidam basin orogenic belt in Caledonian period.