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在大别山超高压变质带的东缘,广泛出露原岩为新元古代岩浆活动产物的片麻状变质花岗岩.对区内5个典型岩体(黄镇、大坝、石马、双河和三祖寺)进行了详细的年代学和地球化学研究,旨在确定这些岩石的原岩属性,并探讨它们对扬子板块北东缘新元古构造-岩浆作用的启示.这些岩石的组成矿物主要为石英、钠长石(更长石)、钾长石和少量角闪石、黑云母等,部分岩体还发育霓石或霓辉石等碱性铁镁矿物,由于经历超高压变质作用,岩石中还常见多硅白云母、绿帘石及少量蓝晶石、石榴石等变质成因矿物.锆石LA-ICP-MSU-Pb定年结果表明,区内片麻状变质花岗岩的原岩形成于新元古中期(770～780Ma),于早中生代时经历超高压变质作用,并在215Ma左右遭受高压榴辉岩相重结晶的改造.地球化学上,它们均具有高硅(SiO2%=70.23%～77.23%)、准铝-弱过铝(ASI=0.90～1.05)的特征,但不同岩体的地球化学组成仍存在差别,与三祖寺岩体相比,黄镇、大坝、石马和双河地区的片麻状变质花岗岩全碱含量(ALK=7.76%～9.45%)、铁镁比值[FeO*/(FeO*+MgO)=0.82～0.96]、高场强元素和Ga的含量均偏高,104×Ga/Al平均值高达3.07,结合岩石中常含霓辉石,以及锆石饱和温度较高(=816～918℃)等特点,表明它们的原岩应为过碱性A型花岗岩.三祖寺片麻状变质花岗岩的铁镁矿物主要为黑云母和由角闪石变质形成的绿帘石,化学成分上富钙贫钾,K2O/Na2O=0.42～0.54,其原岩应归属钙碱性花岗岩类(I型).这些片麻状变质花岗岩同时具有“弧”和“裂谷”双重岩浆活动的地球化学印记,其原岩为先前俯冲形成的弧来源地壳物质于伸展引张环境再造的产物,指示扬子北东缘在新元古中期应处于被动裂谷的初始阶段,而不应为地幔柱上涌引发的主动裂谷环境.
On the eastern edge of the UHP metamorphic belt in the Dabie Mountains, the gneissic metamorphic granites with protoliths as the product of the Neoproterozoic magmatism are widely exposed. Five typical rock masses (Huangzhen, Dam, Shima, Shuanghe and Tri Ancestral temples) conducted detailed geochronological and geochemical studies aimed at identifying the protolith properties of these rocks and exploring their implications for the Neoproterozoic tectono-magmatic activity along the northern margin of the Yangtze block. The compositional minerals of these rocks are mainly Quartz, albite (more feldspar), potassium feldspar and a small amount of amphibole, biotite, etc. Some rock also developed alkaline iron ore or niite and other alkaline iron and magnesium minerals, due to experiencing ultra-high pressure metamorphism, rock Phyllostachys, epidote, a small amount of kyanite, garnet and other metamorphic minerals are also common.The dating results of zircon LA-ICP-MSU-Pb show that the original rocks of gneissic metamorphic granites in the area formed in the new element The Mesozoic (770 ~ 780 Ma), experienced metamorphism during the early Mesozoic and was subjected to recrystallization of high-pressure eclogite at about 215 Ma.Geochemically, both of them have high silica (SiO2% = 70.23% ~ 77.23 %), Quasi-aluminum - weak over aluminum (ASI = 0.90 ~ 1.05) features, but different rocks (ALK = 7.76% ~ 9.45%) in the Huangzheng, Dam, Shima and Shuanghe areas, while the iron and magnesium (FeO * / (FeO * + MgO) = 0.82 ~ 0.96]. The contents of high-field elements and Ga are high, and the average of 104 × Ga / Al is as high as 3.07. The saturation temperature is higher (= 816 ~ 918 ℃) and other characteristics, indicating that their original rock should be over-alkaline type A granite.Anti-ancestral gneissic granitic iron and magnesium minerals are mainly biotite and amphibole metamorphism The formation of the epidote, the chemical composition of calcium and potassium, K2O / Na2O = 0.42 ~ 0.54, the original rock should be attributed to calc-alkaline granitoids (Type I) .These gneissic metamorphic granite at the same time with the “arc” And “Rift” double magmatic activity. The original rock is the product of the previously crushed arc source crustal material re-developed in the extension and extension environment, indicating that the northern margin of the Yangtze should be in the mid-Neoproterozoic period with passive split The initial stage of the valley should not be the active rifting environment triggered by upwelling of mantle plumes.