北秦岭灰池子花岗质复式岩体的源岩讨论——元素-同位素地球化学制约

SOURCE OF HUICHIZI GRANITOID COMPLEX PLUTON IN NORTHERN QINLING,CENTRAL CHINA: CONSTRAINED IN ELEMENT AND ISOTOPIC GEOCHEMISTRY

灰池子复式岩体是北秦岭造山带核部规模最大的花岗岩体 ,它由 3个深成岩体组成 .花岗岩具有高SiO2 (6 6 .86 %～ 73 .48% ) ,Al2 O3(14.44 %～ 16 .32 % )和Na2 O (4 .17%～4.78% ) ,低TiO2 (<0 .74% ) ,MgO(<1.18% )和K2 O(<3.33 % )的特征 .花岗岩轻稀土元素富集 (2 7.45 <w(La) N/w (Yb) N<72 .99) ,Eu为负—弱正异常 ,微量元素Y的质量分数为(3.70～ 9.77)× 10 -6,Yb为 (0 .30～ 1.14)× 10 -6,Sr为 (310～ 775 )× 10 -6,w (Sr) /w(Y)为47.87～ 114.36 ,大离子亲石元素 (Rb ,Ba ,Th ,Sr等 )相对富集 ,高场强元素 (Nb ,Ta ,Ti,Zr)相对亏损 .同位素组成比较均匀 ,0 .5 12 31<w(14 3Nd) /w(14 4Nd) <0 .5 12 38,0 .70 783<w(87Sr) /w(86Sr) <0 .70 880 ,0 .95Ga <tDM<1.0 9Ga ,- 0 .81<ε(Nd ,437Ma) <0 .93 ,且 18<ε(Sr,437Ma) <32 ,这些特征与Adakitic质岩相似 .笔者认为灰池子花岗岩是由深部下地壳新元古代玄武质岩石部分熔融形成的Adakitic质岩 。

Huichizi complex pluton, the largest granitoid pluton in the core of northern Qinling orogenic belt, is composed of three sequences. These granitoid rocks have higher SiO 2 (66.86%-73.48%), Al 2O 3 (14.44%-16.32%), abundant Na 2O (4.17%-4.78%), and lower TiO 2 (<0.74%), MgO(<1.18%) and K 2O (<3.33%) mass fraction, and more enriched in LREE(27.45< w (La) N/ w (Yb) N <72.99), and distinct negative and weak positive Eu anomaly. In these granitoid rocks, Y, Yb, Sr mass fractions and w (Sr)/ w (Y) ratio of trace elements vary respectively from 3.70×10 -6 to 9.77×10 -6 (<18×10 -6 ), 0.30×10 -6 to 1.14×10 -6 (<1.90×10 -6 ), 310×10 -6 to 775×10 -6 and 47.87 to 114.36. While those of Huichizi complex display multi element patterns with large positive ion lithophile elements (Rb, Ba, Th and Sr ) anomalies and high negative field strong elements (Nb, Ta, Ti and Zr) anomalies and indicate the geochemical characteristics of calc alkalic granites and volcanic arc granites. All the granitoid rocks exhibit a similar range of Nd and Sr isotopic ratios: 0.512 31 < w ( 143 Nd)/ w ( 144 Nd)<0.512 38), 0.707 83< w ( 87 Sr)/ w ( 86 Sr)<0.708 80, 0.95 Ga< t DM (depleted mantle model age)<1.09 Ga, -0.81< ε (Nd, 437 Ma)<0.93 and 18< ε (Sr, 437 Ma)<32, and show the source of Huichizi complex from basic rocks. These characteristics are similar to those of the Adakitic rocks. The authors think that the granitoid rocks of Huichizi complex may have been the result of the subduction of the southern intracontinent rift along the Danfeng Shangnan fault by a north dipping of the northern intracontinent rift (Erlangping rift) and that along the Zhuyangguan Xiaguanying fault by a south dipping under the Northern Qinling block during late Caledonian-early Hercynian period. This subduction of the intracontinent rifts leads to the partial melting of Late Proterozoic underplating basic rocks of lower crust and the formation of the granitoid rocks of Huichizi complex.