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桐柏围山城地区位于桐柏山北部,秦岭东西复杂构造带的东段南支。该区的地层系由下古生界歪头山组变质岩和大栗树组变质火山杂岩组成。另外在其北和北西端还有混合花岗岩和变斑花岗闪长岩。歪头山组变质岩主要由互层状的云母石英片岩、云母变粒岩、角闪变粒岩、斜长角闪片岩和大理岩组成。大栗树组变质火山杂岩则由已变成的斜长角闪片岩和磁铁角闪变粒岩组成。二者之中或其间还有黑云斜长片麻岩体分布。三者同时遭受区域变质作用,片理和片麻理与区域构造线方向一致。本区的变质作用有明显的渐进特征,以其出现的标志矿物,按变质作用递增的顺序,依次可以划分出:绢云母——白云母带、黑云母带、铁铝榴石带、十字石——兰晶石带和硅线石带等五个渐进变质带。据矿物共生组合特征和合成矿物温压数据推断本区变质作用的温度范围为350—600℃±,,压力约为5—7 Kb。地热梯度在15—20℃/公里,属于中压偏高的区域变质相系兰晶石——硅线石型变质作用。
Tongbai Wai Shancheng area is located in the northern part of Tongbai Mountain, the eastern branch of the eastern section of the Qinling Mountains. The strata of this area are composed of the metamorphic rocks of the Lower Tesshan Formation and the metamorphic volcanic complexes of the Chestnut Group in the Lower Paleozoic. In addition, there are mixed granitic and albite granodiorites in the north and north west. The Yuntoushan Formation metamorphic rocks are mainly composed of interlayered mica-quartz schist, mica granulite, hornblende, amphibolite and marble. The Chestnut Group metamorphic volcanic complex consists of altered plagioclase and magnetoclite. There is also a biotite plagiogneiss in or between the two. The three metamorphism at the same time regional, filming and piecemeal and regional tectonic line direction. Metamorphism of this area has obvious gradual characteristics, with its appearance of minerals, according to the order of metamorphism in order, can be divided into: sericite - muscovite belt, biotite belt, almandine belt, - Lan Jing stone belt and sillimanite five progressive metamorphic belt. According to the characteristics of mineral assemblages and the temperature and pressure data of synthetic minerals, the temperature range of metamorphism in this area is deduced from 350-600 ℃ ± ,, and the pressure is about 5-7 Kb. Geothermal gradient in the 15-20 ℃ / km, belong to the high-pressure region of the metamorphic facies blue spar - sillimanite metamorphism.