天津蓟县盘山I型—A型复合花岗岩体——区域构造环境转变的记录?

I- and A-type composite granites of the Panshan pluton in the Jixian,Tianjin:a record of regional tectonic transformation?

晚三叠世末的盘山I型-A型复合岩体位于燕山陆内造山带东段,可划分为四个单元,从早到晚,依次为狼家峪单元、官庄单元、东罗庄单元和小盘山单元,前两个单元具有I型花岗岩特征,后两个单元为A型花岗岩。在岩石学、主量、微量、稀土元素和Sr-Nd同位素特征上,I型与A型花岗岩存在明显差异。I型花岗岩相对富碱(K_2O+Na_2O=8.15～9.50%)、铝(Al_2O_3变化于13.05%～15.49%)、钙(CaO介于1.22%～3.02%)、REE、Rb、Ba、Hf、Tb,相对亏损Nb、P、Ti、Zr和Th。A型花岗岩富Si、全碱、Th、Nd、Rb、Hf,相对亏损REE、Ba、Sr、P、Ti、Zr,并具有偏高的Rb/Sr、Ga/Al和明显的铕负异常。盘山复合岩体具有低的初始^(87)Sr/^(86)Sr比值(0.70234～0.70628),负的ε_(Nd)(t)值(-12.73～-15.70)。区域背景和地球化学综合分析表明,岩体定位于构造环境从挤压向伸展转折过程中,早期的I型花岗岩记录了挤压环境特征,晚期的A型花岗岩形成于相对伸展的构造环境。盘山I型-A型复合岩体的确定,表明燕山陆内造山带在晚三叠世末到早侏罗世经历过由挤压向伸展环境的转折。

The Late Triassic Panshan I- and A-type composite granites are located in the eastern segment of the Yanshan intracontinental orogenic belt. In accordance with their intruding order, from early to late, the composite granites can he divided into four units, the Langjiayu, Guanzhuang, Dongluozhuang and Xiaopanshan units. The Langjiayu and Guanzhuang units are characterized by I-type granite, but Dongluozhuang and Xiaopanshan units belong to A-type granite. Investigation resuits have shown that the features of petrology, major elements, trace elements, rare earth elements and Sr-Nd isotopic compositions between the I-type and A-type granites are quite different. Geochemistry of the I-type granites, is rich in alkalis （ K2O ＋ Na2O = 8.15% - 9.50% ）, Al （ Al2O3 = 13.05% -15.49%）, Ca （CaO = 1.22% -3.02%）, REE, Rb, Ba, Hf and Tb, poor in Nb, P, Ti, Zr and Th. The A-type granites are rich in Si, alkalis, Th, Nd, Rb, Hf, and depleted in REE, Ba, Sr, P, Ti and Zr, and have higher Rb/Sr and Ga/AI, and display obviously negative europium anomaly. The ^87Sr/^86Sr initial ratios of the rock from the Panshan composite granites ranging from 0. 70234 to 0. 70628 and the εNd （t） values of - 12.73 to - 15.70. Regional setting and geochemistry compositive analyse suggests that the Panshan pluton was emplaced in an environment of regional compressional shortening transforming to extension. Early I-type granites record the feature of regional compressional shortening environment, but late A-type granites record the feature of lithospheric extension and thinning. The Panshan I- and A-type composite granites are confirmed, showing that the Yanshan intracontinental orogenic belt in the end of Late Triassic to Early Jurassic went through environment transforming from regional compressional shortening to extension.