大别山北部磨子潭变质英云闪长岩内的暗色包体

Dark Inclusions in Mozitan Meta-Tonalite Pluton of Northern Dabie Mountain, Central China

大别山北部磨子潭附近新元古代英云闪长岩内包裹有许多暗色包体 ,其中可能保存有该区地壳早期演化的一些信息。对其中三个比较大的包体进行初步研究 ,发现它们的组成矿物主要为角闪石和长石 ,此外还含有少量绿泥石和不透明铁质矿物。这些包体可能遭受与晓天 -磨子潭断裂有关的比较强烈的后变质糜棱岩化 ,但没有同变质或后期的脉体。岩石化学分析结果表明 ,其SiO2 含量为 4 9.10 %～ 53.2 7% ,TiO2 为 0 .74 %～ 1.32 % ,Al2 O3 为 11.80 %～15.72 % ,Na2 O为 1.39%～ 3.58% ,K2 O为 4 .32 %～ 6.17% ,P2 O5为 0 .82 %～ 1.0 2 % ;稀土元素丰度为 332 .85× 10 - 6～ 4 4 0 .2 3× 10 - 6,配分曲线为没有明显Eu异常的右倾型 ;14 3Nd/ 14 4 Nd比值为 0 .511790～ 0 .512 185,εNd( 0 )为 - 8.9～ - 17.6;87Sr/ 86Sr比值为 0 .70 8732～ 0 .70 94 17,Sm -Nd亏损地幔模式年龄分别为 1854Ma、1899Ma和 1578Ma。没有迹象显示岩石经历了交代作用的改造 ,使得可以根据上述特征得出如下初步推论 :( 1)所研究包体的原岩是形成于前寒武纪陆内拉张背景的富碱质基性岩浆岩 ;( 2 )大别山地壳组成中可能包括了比该岩浆岩更古老的早前寒武纪地质体 。

There are abundant dark inclusions in the Neoproterozoic tonalite pluton near Mozitan town, northern Dabie Mountain, which probably contain some information on the early evolution of the crust in this area. Preliminary studies of three inclusions have revealed amphibole facies of the main stage of metamorphism with constituent minerals of feldspars, amphiboles, chlorites and opaque minerals. These inclusions underwent relatively intense mylonitization of post_metamorphism, probably associated with activities of Xiaotian-Mozitan faulted zone. No veins of syn_or post_metamorphism exist within and around the inclusions. Chemical analysis of three inclusion samples shows that major elements are characterized by SiO 2 content of 49.10%～53.27%, TiO 2 of 0.74%～1.32%, Al 2O 3 of 11.80%～15.72%, Na 2O of 1.39%～ 3.58%, K 2O of 4.32%～6.17%, P 2O 5 of 0.82%～1.02% and REE abundance of 332.85×10 -6 ～440.23×10 -6 , with obvious enrichment of LREE and no evident Eu anomaly for right_dipping REE patterns, Their 143 Nd/ 144 Nd ratios are 0.511790～0.512185, εNd(0) are equal to -8.9～-17.6 and 87 Sr/ 86 Sr are 0.708732～0.709417. Sm-Nd dating for these samples yields depleted mantle model ages of 1854 Ma, 1899 Ma and 1578 Ma. No evidence of metasomatism in these inclusions can reach the following deductions on the basis of above data: (1) primary protoliths of these inclusions may be alkaline_rich basic magmatic rocks which were probably formed in the extensional tectonic setting within continents, (2) Precambrian bodies much older than the rocks may exist in Dabie Mountain, and Dabie metamorphic complexes were possibly part of the early Precambrian block.