北秦岭早古生代陆壳增生——来自太平镇地区弧岩浆岩的年代学、地球化学和Sr-Nd-Hf同位素证据

Early Paleozoic continental crust accretion in the North Qinling:Evidences from geochronology, geochemistry and Sr-Nd-Hf isotopes of arc magmatic rocks in Taipingzhen area

二郎坪单元是位于北秦岭构造带中部的一个年轻地体,发育了大量的花岗岩类,是研究北秦岭早古生代大陆地壳增生的理想场所。本文对北秦岭构造带太平镇北英云闪长岩-奥长花岗岩体和蛮子营黑云母二长花岗岩体开展了岩石学、年代学、地球化学及Sr-Nd-Hf同位素研究。LA-ICP-MS锆石U-Pb测年表明,太平镇北奥长花岗岩和蛮子营黑云母二长花岗岩的形成时代分别为468.8±2.8Ma和462.2±1.9Ma。太平镇北岩体为高硅(71.79%-78.66%)、富钠贫钾(K_(2)O/Na_(2)O=0.27-0.77)的低钾拉斑-钙碱性系列岩石;蛮子营岩体为高硅(72.20%-74.90%)、富钾(K_(2)O/Na_(2)O=0.97-1.36)的高钾钙碱性岩石。两者轻重稀土分异均较明显,均具有富集Rb、Ba、Th、U、K等大离子亲石元素,而亏损Nb、Ta、P、Ti等高场强元素的特征。太平镇北岩体和蛮子营岩体具有类似的锆石εHf(t)值(奥长花岗岩8.2-12.7;黑云母二长花岗岩8.9-13.2)、全岩(87Sr/86Sr)i(奥长花岗岩0.704038-0.705221;二长花岗岩0.703876-0.705371)和全岩εNd(t)值(奥长花岗岩1.49-2.03;黑云母二长花岗岩1.68-1.92)。研究表明,太平镇北岩体岩浆源区为玄武质弧下地壳,岩浆结晶分异作用形成英云闪长岩和奥长花岗岩;蛮子营岩体岩浆为早期形成的英云闪长岩部分熔融形成。太平镇北岩体和蛮子营岩体均形成于洋内弧的构造环境,从弧玄武岩到富钠英云闪长岩、奥长花岗岩再到富钾的二长花岗岩,代表了地幔物质经过多阶段岩浆演化形成富硅富钾长英质地壳的过程。综上,认为洋内弧的形成和岩浆演化是北秦岭大陆地壳增生的重要方式之一。

The Erlangping unit provides an ideal place to study the continental crust accretion since it is a juvenile terrane in the North Qinling tectonic belt where numerous granitic rocks widely developed. To achieve this target, we conducted integrated investigation of in situ zircon U-Pb dating, whole-rock geochemical, as well as Sr-Nd-Hf isotopic compositions for the north Taipingzhen tonalite-trondhjemite pluton and Manziying monzogranite pluton from the Erlangping unit. LA-ICP-MS zircon U-Pb dating indicated that the north Taipingzhen pluton and Manziying pluton were formed at 468.8±2.8 Ma and 462.2±1.9 Ma, respectively. The north Taipingzhen pluton has high SiO_(2) contents(71.79%-78.66%), and rich in sodium but poor in potassium(K_(2)O/Na_(2)O=0.27-0.77), and it belongs to the calcium-alkaline rock or low potassium tholeiitic rock sereies. The Manziying pluton has high SiO2contents(72.20%-74.90%), rich in potassium(K2O/Na_(2)O=0.97-1.36), and it belongs to the high potassium tholeiitic rock series. Both of the two plutons are enriched in large ion lithophile elements(Rb, Ba, Th, U, K) and LREE and depleted in high field strength elements(Nb, Ta, P, Ti) and HREE. Furthermore, the north Taipingzhen and Manziying plutons all have low(87Sr/86Sr)iratios(0.704038-0.705221 and 0.703876-0.705371, respectively), positive εNd(t)(1.49-2.03 and 1.68-1.92, respectively) and positive εHf(t)(8.2-12.7 and 8.9-13.2, respectively). The analysis of the data showed that the source area of the north Taipingzhen pluton is from the juvenile basaltic lower crust, and the tonalite and trondhjemite were formed during the magma crystallization differentiation process. The Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the north Taipingzhen pluton. Both of the north Taipingzhen and Manziying plutons were formed in an oceanic arc tectonic environment. The formation of sodic tonalite and trondhjemite and high silicon and potassium monzogranite in oceanic arc reflects the process of the formation of felsic crust through multi-stage magmatic evolution of mantle materials, and thus highlight the significance of oceanic arc for the continental crust accretion.