胶-辽-吉带辽东宽甸地区古元古代二长(正长)花岗质片麻岩的锆石U-Pb年代学、地球化学及成因

Zircon U-Pb chronology,geochemistry and their petrogenesis of Paleoproterozoic monzogranitic gneisses in Kuandian area, eastern Liaoning Province, Jiao-Liao-Ji Belt, North China Craton

古元古代二长(正长)花岗质片麻岩在胶-辽-吉构造带广泛出露,是构成胶-辽-吉带的最主要物质组成。2件中细粒二长花岗质片麻岩样品及1件正长花岗质片麻岩样品的LA-ICP-MS锆石U-Pb年代学分析表明,它们分别形成于2.17Ga和2.33Ga,代表两期花岗质岩浆事件,并经历了约1.89Ga的变质作用。这些花岗质岩石具高硅(SiO_2=71.68%~76.38%),富铁(Fe_2O_3+FeO=1.61%~4.35%)及全碱(K_2O+Na_2O=7.57%~9.3%),K_2O/Na_2O比值大于1,贫钙(CaO=0.2%~1.6%)、镁(MgO=0.05%~0.24%,Mg#=3.82~13.0)、磷(P_2O_5=0.01%~0.06%)及钛(TiO_2=0.12%~0.4%)的特征;它们具有较高的稀土元素总量(ΣREE=104×10^(-6)~440×10^(-6)),具有轻微的轻重稀土分异,(La/Yb)_N比值在2~11之间,具明显的铕负异常(Eu/Eu*=0.30~1.18,平均值为0.56),具有低Sr(大部分<100×10^(-6))、Rb(62.8×10^(-6)~187×10^(-6))、Cr及Ni含量,高的Zr(>250×10^(-6))及Yb(>2×10^(-6))含量,Zr+Nb+Ce+Y平均值为506×10^(-6),10000×Ga/Al值大于2.7;微量元素原始地幔标准化蛛网模式图显示,富集Rb、U、K、Zr、Hf等元素,亏损Nb、Ta、Sr及Ti等元素;它们属铁质、准铝质至弱过铝质、碱钙-钙碱性系列,全岩锆饱和温度大于800℃,属A2型花岗岩;结合胶-辽-吉带内古元古代花岗质片麻岩的全岩Nd及锆石Hf同位素研究结果,指示这些古元古代A型花岗岩可能形成于高温低压的伸展构造背景下,大陆物质(TTG岩石)的部分熔融。暗示胶-辽-吉带可能经历了古元古代早期多阶段的陆壳伸展拉张,并形成初始洋盆,然后再到洋-陆俯冲、陆(弧)-陆碰撞的演化过程。

Paleoproterozoic monzogranitic, syenogranitic gneisses are widely distributed in Jiao-Liao-Ji tectonic belt （JLJB）, and are the main component of the JLJB. The LA-ICP-MS zircons U-Pb isotopic analyses from two monzogranitie gneisses and one syenogranitic gneiss samples collected from the study area unraveled that they formed at - 2. 17Ga and - 2. 33Ga, respectively, and experienced metamorphism at -1.89Ga. They have high Si02 （71.68% - 76. 38% ）, Fe2O3 ＋ FeO （ 1.61% - 4. 35% ） and total alkali （K20 ＋ Na20 = 7.57% -9. 3% ） with K20/Na20 〉 1, and low CaO （0. 2% - 1.6% ）, MgO （0. 05% -0. 24%, Mg = 3.82 - 13.0）, P2O5 （0. 01% -0. 06% ） and TiO2 （0. 12% - 0. 4% ） contents; They have relatively high REEs contents （ ΣREE = 104 × 10^-6 ～440× 10^-6）, enrichment in light REE and depletion in heavy REE with （La/Yb）N ratios of 2 - 11, and strongly negative Eu anomalies （Eu/Eu ＊ = 0. 30 - 1.18, average of 0. 56）. These samples show low Sr content （mostly 〈 100 ×10^-6 ）, Rb（ 62. 8 × 10^-6 187×10^-6）,CrandNicontents,buthighZr （〉250×10^-6）, Yb （〉2×10^-6） andZr＋Nb＋Ce＋Y （average=506×10^-6） contents, and 10000 × Ga/A1 values of larger than 2. 7. In primitive mantle normalized trace element spider diagrams, they exhibit strong enrichments in Rb, U, K, Zr and Hf, but depletion in Nb, Ta, Sr and Ti. They are ferroan, alkali-calcic to calc-alkalic and metaluminous-weakly peraluminous, belong to A2-type granite with the zircon saturation temperature 〉 800℃. Combined with Nd and Hf isotope features of Paleoproterozoic granitoids in the JLJB, we proposed that magmatic precursors of the Paleoproterozoic monzogranitic and syenogranitic gneisses may originated from partial melting of continental crust （TTG rocks） under high temperature and low pressure extension setting, which further suggests that the JLJB might experience tectonic evolutionary history of multi-stage extensions, formation of initial ocean basin, ocean-continent subduction to continent-continent collision in Paleoproterozoic.