华北西部贺兰山中段黄旗口花岗岩成因及地质意义

Geochemistry and Significance of Paleoproterozoic Granitoids from Huangqikou,Central Helanshan Area

华北克拉通孔兹岩带内发育类型复杂的各类岩石,是记录和反演华北西部早期块体拼合以及岩浆作用、变质演化的重要对象,而其中段发育的出露面积广阔、岩性复杂的黄旗口花岗岩体的研究还较为薄弱.在野外地质和岩相学研究基础上,主要报道了贺兰山中段黄旗口花岗岩的主、微量元素和Sr-Nd同位素组成,对岩体的源区特征、成因及地质意义进行了探讨.黄旗口复式花岗岩体主要由早期正长花岗岩、二长花岗岩和晚期英云闪长岩组成.两期岩体具有高K_2O(2.97%～6.71%)含量,A/CNK均大于1.1,但晚期侵入单元较早期岩石更贫硅、富铝.两期岩体都表现为轻稀土富集、Eu负异常的特点,且强烈富集K、Rb、Th等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素.岩石^(87)Sr/^(86)Sr初始比值变化范围较大,可能是后期改造的结果;ε_(Nd)(t)变化在+1.81～+4.90,对应的Nd同位素模式年龄TDM1为2.10～2.37Ga,TDM2为2.10～2.35Ga.这些特征表明黄旗口岩体为S型花岗岩,可能来自该区归属于孔兹岩的赵池沟组岩系的部分熔融,并可能有地幔岩浆的参与.结合区域变质、岩浆事件的综合研究成果,认为黄旗口不同期次的花岗岩具有造山带花岗岩的特征,分别形成于阴山地块和鄂尔多斯地块碰撞拼合以及造山后伸展的不同阶段.

Widespread rocks in the khondalite belt record magmatism,metamorphism and tectonic evolution of the North China craton(NCC).Whereas the further research on Huangqikou granitic plutons exposed in the central Helanshan area is needed to offer information about the genetic mechanisms and the dynamic backgroud.Based on the field geological and petrographic studies,major,trace elements and Sr-Nd isotopic compositions were carried out on the Huangqikou granitic plutons,so as to discuss their petrogenesis,source characteristics and geological significance.The early intruded Huangqikou granites consist of alkali granite and monzogranite,while the later intrusions are mainly tonalite.The Huangqikou granites display typical geochemical characteristics of S-type granite,such as high contents of K2 O(2.97%-6.71%),A/CNK>1.1.However,the later intrusions generally show lower SiO2 and higher Al2O3 relative to those early ones.All the studied granitic rocks show enriched LREE,and negative Eu anomalies.The samples generally show similar variation trend in the trace-element patterns,such as systematic enrichment of LILE(K,Rb,Th)and depletion of HFSE,e.g.Nb,Ta,P,Ti.All these geochemical features,combined with the majority of positiveεNd(t)(+1.81-+4.90)and Paleoproterozoic Nd model ages(TDM1=2.10-2.37 Ga,TDM2=2.10-2.35 Ga),suggest that the Huangqikou granites were mainly derived from the partial melting of the sedimentary rocks in the Zhaochigou Formation,but probably with minor mafic magma contributed from the mantle.Combined with previous regional metamorphic and magmatic studies,it is suggested that the early and later intruded granitic rocks in the Huangqikou region belong to orogenic granitoids,and were probably formed during the collision between the Ordos and Yinshan blocks and post-collisional extension setting,respectively.