北秦岭碾道奥陶纪碱性正长岩的厘定及其对早古生代构造演化的制约

Confirmation of the Ordovician alkali syenite in Niandao:implications for the Early Paleozoic tectonic evolution of North Qinling

通过详细的野外地质调查,结合岩相学、岩石地球化学、锆石U-Pb测年等研究,首次在北秦岭造山带内乡县碾道一带厘定出一套奥陶纪碱性正长岩体。该岩体现以构造岩块的形式赋存于朱阳关-夏馆断裂带内,但保留早期与秦岭岩群的侵入接触关系,岩石组合由角闪正长岩和霓辉正长岩构成。岩体低硅(SiO_(2)=52.09%~67.48%),富碱(K_(2)O+Na 2O=5.82%~11.75%),贫镁(MgO=0.09%~1.99%),高铝(Al_(2)O_(3)=11.87%~17.81%),富集轻稀土元素、Rb和K,亏损Nb、Ta、P和Ti,显示出A_(2)型花岗岩的特点。岩石地球化学特征指示,岩体可能源于受俯冲组分改造的富集型地幔的部分熔融,形成背景与岛弧环境有关。获得角闪正长岩LA-ICP-MS结晶锆石U-Pb年龄为457.4±1.8 Ma,形成时代为晚奥陶世。综合区域研究成果认为,碾道碱性正长岩体可能代表了北秦岭早古生代约450 Ma抬升折返过程中构造负反转体制下的初始岩浆记录,其侵位结晶年龄则代表了构造负反转发生的初始年限。

Through detailed field geological surveys,combined with systematic studies of petrography,geochemical and zircon U-Pb chronology,the Ordovician alkali syenite is determined in Niandao area of Neixiang County,which was first discovered in North Qinling.The Niandao alkali syenite was involved in Zhuyangguan—Xiaguan fault belt and transformed into tectonic massif,however,the early intrusive contact with Qinling Complex was preserved,and intrusive rocks were mainly composed of hornblende syenite and aegirine-augite syenit.Samples are characterized by relatively low silicon(SiO_(2)=52.09%~67.48%),total alkali(K_(2)O+Na 2O=5.82%~11.75%),abundant aluminum(Al_(2)O_(3)=11.87%~17.81%),depleted magnesium(MgO=0.09%~1.99%),enrichment of LREE,Rb and K,depletion of Nb,Ta,P and Ti,indicate that the intrusion is aluminous A-type granite and further classified to A_(2) type.The geochemical characteristics show that this pluton may be dominantly derived from partial melting of an enriched mantle modified by subduction-related components,and the magma produced was subjected to minor amounts of crustal contamination during its ascending process,and its formation might be related to an island arc environment.Zircon LA-ICP-MS U-Pb dating of hornblende syenite gave 206 Pb/238 U weighted mean ages of 457.4±1.8 Ma,showing that the Niandao alkali syenite emplaced in Late Ordovician.Combined with the regional geological data,the Niandao alkali syenite ought to be initial magmatism record of negative structure inversion of North Qinling that experienced in the Paleozoic stage of exhumation at ca.450 Ma,and its formation age should represent the initial time of negative structure inversion.