东秦岭商丹带北侧新元古代埃达克质花岗岩及其地质意义

Geochemical characteristics and geological significance of Neoproterozoic adakitic granitoids on the north side of the Shangdan zone in the East Qinling

东秦岭商丹构造带北侧的商南花岗岩体和吊庄花岗岩体属于钙碱性系列,具有岛弧花岗岩特征。这些岩石富Na2O(3.54%～5.52%)、贫K2O(0.75%～3.46%)、Na2O/K2O比值为1.09～6.16,SiO2含量>56%(61.84%～74.57%),Al2O3含量变化于13.41%～19.01%,多数≥15%,MgO含量均小于3%(0.65%～2.41%),Mg#为43.87～66.12,低Y(<18×10-6,为1.90×10-6～18.4×10-6)和Yb(<1.9×10-6为0.27×10-6～2.27×10-6),Sr含量高(350×10-6～830×10-6),多数大于400×10-6,Sr/Y比值在25.29～216.71之间,大于20～40;LREE富集,HREE亏损,多数La/Yb比值>20(47.60～110.93),(La/Yb)N>20(32.09～99.13),不显Eu异常或有弱的Eu正异常,大离子亲石元素(Rb、Ba、Th、Sr)相对富集,高场强元素(如Nb、Ta、Zr、Hf、Ti)相对亏损。同位素组成表明,花岗岩体具有相对低的143Nd/144Nd比值(0.512127～0.512134),与商南松树沟蛇绿岩的143Nd/144Nd比值(0.512651～0.512917)比较接近,εNd(t)为正值(+1.23～+2.51),暗示源岩以N-MORB为主。这些地球化学特征与洋壳俯冲MORB的部分熔融形成的埃达克质岩相似,表明这些晋宁期的花岗岩类可能属于埃达克质岩。研究表明。

The Shangnan granitoid and Diaozhuang granitoid on the north side of the Shangdan fault zone in the East Qinling belong to the calc-alkaline series and have the characteristics of island-arc granitoids. These rocks have higher Na2O(3.54%～5.52%) and lower K2O(0.75%～3.46%), Na2O/ K2O = 1.09～6.16,SiO2>56%(61.84%～74.57 %),Al2O3≥15%(13.41%～19.01%),MgO<3%(0.65%～2.41%) and Mg#=43.87～66.12. They are notably depleted in Y with Y<18×10-6, ranging from 1.90×10-6 to 18.4×10-6, and Yb with Yb<1.9×10-6, ranging from 0.27×10-6 to 2.27×10-6, and enriched in Sr with Sr>400×10-6,ranging from 350×10-6 to 830×10-6. Sr/Y ratios vary from 25.29 to 216.71,being >20～40. The granitoid rocks are enrich in LREE and depleted in HREE with most La/Yb>20 (47.60～110.93) and (La/Yb)N>20(32.09～99.13),and exhibit no Eu anomaly or only weak positive Eu anomaly. The large ion lithophilous elements(Rb, Ba, Th and Sr) are relatively enriched and high field strenth elements(Nb, Ta, Zr, Hf and Ti)are relatively depleted. The isotopic composition indicates that the 143Nd/144Nd ratios(0.512127～0.512134)of the granitoids are very similar to those(0.512651～0.512917) of the Songshugou ophiolite in the Shangnan area and that their εNd(t=889Ma) values are positive(+1.23 to +2.51), implying that the source rocks of these granitoids are dominated by MORB. These geochemical characteristics are similar to those of adakitic rocks formed by partial melting of MORB related to subduction of oceanic crust, suggesting that these Neoproterozoic granitoids in this study area might be adakitic rocks. The adakitic granitoids in the Shangdan area may be the result of the northword subduction of the ancient ocean basin (ancient Qinling ocean), represented by the Songshugou ophiolite and the Heihe N-MORB type basalt, along the Shangdan tectonic zone during the Neoproterozoic. This slab subduction of oceanic crust led to the partial melting of MORB rocks and the formation of the adakitic granitoids in the Shangdan area.