大别—苏鲁超高压地体中面理化含榴花岗岩的成因研究

大别—苏鲁超高压地体中的面理化花岗岩因为常含石榴石而被简称为含榴花岗岩, 其岩石类型主要为二长花岗岩、花岗岩和微斜长石花岗岩, 岩石具有花岗结构和片麻状构造.详细的野外地质研究表明, 超高压片麻岩作为超高压榴辉岩的围岩与含榴花岗岩呈渐变过渡关系, 或在含榴花岗岩中呈与面理平行的残留条带, 体现超高压片麻岩通过构造置换和部分熔融向含榴花岗岩转化.含榴花岗岩在常量元素的总体组成上, w(SiO₂)为71.73%~79.15%;A/CNK为0.83~1.09, 平均0.98, 为准铝质; w(K₂O+Na₂O)为6.15%~9.00%, w(K₂O)/w(Na₂O)为0.16~1.54(绝大多数集中在0.9~1.1), 具有相对弱富钠-弱富钾特征.从标准矿物组成上看, 大别含榴花岗岩主要相当于奥长花岗岩; 山东含榴花岗岩主要相当于花岗岩; 东海含榴花岗岩主要相当于钾质花岗岩.在微量元素特征上, 含榴花岗岩的∑REE、∑LREE明显富集, δEu具有明显的负异常, 在原始地幔标准化蛛网图上, 相对亏损Nb、Ta、P、Zr、Ti等高场强元素及大离子亲石元素Sr, 富集Ba、La、Nd、Y、K等大离子亲石元素, 结合其贫w(Al)(平均11.6%)富w(Ga)(> 17×10-6)、(Fe/Mg)_M(1.087~20.330)等特征, 表明其地球化学特征相当于非造山的A型花岗岩.结合前人超高压变质作用和构造演化等研究成果, 可以推断含榴花岗岩是超高压地体折返到中下地壳, 在底侵、构造体制转换等因素作用下, 由高压片麻岩的部分熔融形成的.含榴花岗岩在大别—苏鲁不同区域上的规律变化, 表明东海含榴花岗岩的出露相对于大别更低位. 

Genesis of Garnet-Bearing Granites in UHP Terrane of Dabie-Sulu Orogenic Belt,Central China

Dabie-Sulu UHP terrane, central China, extends from western Tongbai Mountains through the Dabie Mountains to Sulu. The garnet-bearing granites account for more than 85% in volume in the UHP units, with a mineral assemblage of Or+Ab+Q+Bi+Mc+Grt+Zr+Ep+Ae+Hb. In some profiles where larger eclogute bodies occurred, there is a gradual transition from eclogite →amphibolite (frequently with symplecites of plagioclase+ amphibole) →garnet-bearing gneisses→garnet-bearing granite. The gneisses occur as relics or bands of several centimeters to several meters in the garnet-bearing granites. The bands are parallel to the foliation of garnet-bearing granites. All these spatial distributions result from the partial melting and transformation of structural regime. The garnet-bearing granites have w(SiO₂)=71.73%-79.15%; A/CNK=0.83-1.09; w(K₂O+Na₂O)=6.15%-9.00%, w(K₂O)/w(Na₂O)=0.16-1.54. They are in the range of weakly peralkaline to weakly peraluminous series. In trace elements, the garnet-bearing granites have w(Zr)/w(Hf)=20.40~48.99 (averaging 35.13), w(Nb)/w(Ta)=6.93-22.28 (averaging 13.24), w(Zr)/w (Nb)=7.42-226.82 (averaging 37.34), and w(∑REE) =56.61×10-6-795.58×10-6. All samples of the garnet-bearing granites show obvious Eu negative (anomaly). While in primitive mantle-normalized spider diagram, the garnet-bearing granites are relatively rich in K, Ba, Pb, Nd, Y and La and relatively depleted in Nb, Ta, Sr, Ti, Zr and P. Combined with another characteristics such as high SiO₂, (K₂O+Na₂O), Ga and low Al₂O₃, CaO, MgO, the garnet-bearing foliated granites are similar to A-type granites. From Dabie to Sulu, the content of SiO₂, (K₂O+Na₂O), ∑REE, Ga and the ratios of w(K₂O)/w(Na₂O), (Fe/Mg)_M and w(Zr)/w(Hf) increase gradually; the content of Al₂O₃, CaO, MgO, Sr and the ratio of w(Nb)/w(Ta) decrease gradually. The emplacement level for garnet-bearing granites in Sulu is lower than that in Dabie.