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

Abstract： 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.