保山地块漕涧复式岩体晚白垩世花岗岩地球化学特征及锆石U-Pb年代学意义

滇西保山地块存在晚白垩世岩浆活动,其岩浆岩成因、源区属性及地球动力学背景尚不明确。对出露于保山地块北部漕涧复式花岗岩体晚白垩世花岗岩全岩地球化学及锆石U-Pb年龄进行了研究。1件样品LA-ICP-MS锆石U-Pb同位素测年结果为68.8±1.2Ma(MSWD=3.3,n=12),显示晚白垩世的年龄信息。全岩地球化学研究表明,漕涧复式花岗岩体晚白垩世花岗岩为高硅(SiO_2=69.72%～76.09%)、富钾(K_2O=4.63%～6.65%)、过铝质(A/CNK=1.06～1.16)的S型花岗岩,岩浆形成于陆壳中部泥质岩石的部分熔融,残留相为麻粒岩,残留相主要组成矿物为斜长石+角闪石+石榴子石。保山地块在晚白垩世—古近纪发育多期过铝质花岗岩,部分学者认为这些花岗岩可能为新特提斯洋板片东向俯冲过程中保山地块内陆增厚地壳熔融的产物,但新特提斯洋开始俯冲的时限存在争议,可能比该期花岗岩晚。研究认为,保山地块晚白垩世花岗岩可能是在印支运动形成的全球性潘加亚大陆多次裂解形成现今全球海域分布格局这一区域性的伸展背景下,深部热流(地幔物质)上升,同时有来自于地壳的含水流体的加入,引起了中部地壳物质的部分熔融。岩体的岩石地球化学特征是岩浆源区的反映,而非岩浆产出的大地构造背景的体现。

Geochemical characteristics of Late Cretaceous granites and zircon U-Pb chronological significance of the Caojian complex granites in the Baoshan block

There existed Late Cretaceous magmatic activity in the Baoshan block, western Yunnan Province. The origin of magmatic rocks, source area properties and geodynamic background are not clear. The whole rock geochemistry and zircon U-Pb geochronology of Late Cretaceous granites in the Caojian complex granite exposed to the northern Baoshan block were studied in this paper. A sample LA-ICP-MS zircon U-Pb isotope dating result is 68.8±1.2Ma (MSWD=3.3, n=12), indicating age information of Late Cretaceous. The whole rock geochemical study shows that the Late Cretaceous granite in the Caojian complex granite body is the S type granite of high silicon (SiO₂=69.72%~76.09%), rich potassium (K₂O=4.63%~6.65%) and peraluminous nature (A/CNK=1.06~1.16). The magma was formed from the partial melting of the argillaceous rocks in the central part of the continental crust with the residual facies being granulite, and the residual facies is mainly composed of plagioclase+hornblende+garnet. There existed many peraluminous granites in the Late Cretaceous to Paleogene period in the Baoshan massif. Some experts believe that these granites might have been formed by the thickened crust in the inland of the Baoshan massif during the eastward subduction of the new Tethys oceanic plate, but the time limit for the start of the subduction of the new Tethys Ocean is controversial, and it was probably formed later than the formation of this period of granite. It is believed that the Late Cretaceous granites in the Baoshan massif may be under the regional extension background of the global distribution pattern of the global pan Gaia formed by the Indosinian movement. The deep heat flow (mantle material) migrated upward with the addition of water-containing fluid from the crust. These factors caused partial melting of the central crust. The geochemical characteristics of the rock mass are the reflection of the magma source area rather than the tectonic background of magmatic production.