Geological And Geochemical Characteristics Of Anatectic Xenoliths And The Model Of Lithosphere In Xingyang, Henan

Anatectic Xenoliths are the realistic samples of the deep lithosphere. Because of being quickly carried to the earth's surface, these xenoliths can be quite good preserved with original texture, tectonics, temperature and pressure of the lithosphere's rocks. These xenoliths are some precious samples for building the composition and texture model of the lithosphere and understanding the condition of physical chemistry and the deep geological processes. The anatectic Xenoliths in Xingyang, Henan not only include the crust-derived and mantle-derived xenoliths, but also locate in north edge of northern Qinling Orogenic Belt's eastern, which close to the famous Dabie ultra-high pressure metamorphic belt from 100 km south-east. These xenoliths can provide some valuable information for understanding the composition, texture and evolution of the lithosphere in northern Qinling Orogenic Belt's eastern, besides building the completed model of lithosphere in the area. On the bases of geological and geochemical studies of the anatectic xenoliths of basic pyroclastic rocks in Xingyang, Henian, the paper has established the texture and composition model of the lithosphere. Compared the model with other area's lithosphere models, the paper has indicated the characteristics of this lithosphere model, showed the correlation of temperature and pressure of eclogites and granulites on statistic analysis, and discussed water's characteristics in transition and pressure gradients in lithosphere. Achievements gained by this study are as follows: 1 The article has announced a series data of rock chemistry, mineral chemistry and isotope ages of xenoliths in east Qinling orogenic belt. 2 The paper has established the lithosphere model in the area with xenoliths. Because of no xenoliths of the upper crust, from the rocks of the earth's surface and the studies of others, the paper has concluded that the upper crust is composed by amphibolites, granites and grabbos which belong to amphibolite facies and greenschist facies. The main composition of the medium crust is amphibolites and garnet amphibolites, which belong to amphibolites facies. These rocks have low temperatures (460~570 ℃) and low pressures(0.6~0.64GPa). Their averages of VP and density are respectively 6.96 m/s (the main extension of variation is 6.82~7.05 km/s) and 2.97 g/cm~3 (the main extension of variation is 2.84~3.04g/cm~3). The geothermal gradient of the medium crust is 22~26℃/km. The rocks of the low crust include mafic granulites as well as a little of medium granulites and acidic granulite etc, which belong to granulite facies. In the low crust the variations of temperatures, pressures, geothermal gradient, VP and densities are large. Temperature and pressure of these rocks are respectively 724~901℃and 0.82~1.10GPa. Averages of VP and density in this part are respectively 7.20 m/s (the main extension of variation is 7.03~7.32 m/s) and 3.16 g/cm3 (the main extension of variation is 2.95~3.41g/cm3). The geothermal gradient of the low crust is 20~31℃/km. In the transition, the main rocks are metamorphic gabbros, olivine rock and harzburgite as well as a little of garnet-websterite and eclogites. The temperatures and pressures in the upper of upper mantle respectively are 650~754℃and 1.08~1.21GPa, the geothermal gradient is 16~20℃/km, VP is 7.41~8.21km/s and the density is 3.17~3.65g/cm3. The temperature in the bottom of transition closes to 1050~1100℃. From this paper's data and the other's studies, the paper has reasoned that the upper mantle is composed by lherzolites, and the temperature in the upper mantle is more than 1050℃. 3 Compared with the lithosphere model in the craton, this lithosphere model has the characteristics below: (1)The crust has the characteristics of amalgamation texture . In the crust, the rocks belonged to several facies sequences occur in the crust. The variations of temperature and pressure with depths are nonlinear and non-uniform and the geothermal gradient has largely changed. There are the transitional rock combination from granulites to eclogites, which are some granites and metamorphic gabbros with no orthopyroxene. The transitional rocks differ from normal granulites with no orthopyroxene and high pressures, differ from high-pressures granulites with low P-T, and differ from normal eclogites with having feldspar and low pressures. The transitional rock combination also occurs in the crust of Haiyang, Shandong province. The ages of rocks in the crust are complicated and the strata distributions are non-Smith. The ages of xenoliths approach to the ages having released in east Qinling orogenic belt except two period new ages: 1847±9 ~ 1811±7Ma in metamorphic gabbros and 1723±15~1685±3 Ma in mafic granulites and medium granulites, (2)There are super critic fluid (SCF) with rich SiO2 in the transition: there are strong zoisizations in metamorphic gabbros xenoliths and talcizations in mantle-derived xenoliths, especially talcizations are different from serpentinizations in mantle rocks of the craton. These evidences show that there is the strong hydrothermalism in the bottom of crust and the upper of mantle. The paper has concluded that water in transition is SCF with rich SiO2. 4 Below are the chemical compositions characteristics of xenoliths and the low crust in Xinyang (1)Compared with the continental low crust in the world, the chemical compositions of the low crust in Xinyang are more basicity, less SiO2(50.83%), rich total FeO(12.32%) and rich TiO2(1.66%), approaching to the chemical compositions of low part of the low crust in the world. (2)The correlations between Al2O3,K2O,Na2O,REE,Co,Ni,Cr,Rb,Sr,Ba,Cs,Hf,Zr and MgO of xenoliths and host-rocks in Xinyang are exponent function. There is the same correlation in Hanroba xenoliths, but the correlation coefficient is less than ones in Xinyang.(3)In double logarithmic figure of La/Sm-La, crust-derived xenoliths are differ from mantle-derived xenoliths. In Xinyang crust-derived xenoliths and host-rocks, there are good power function correlation between La/Sm and La. There also is the same correlation in Hanroba xenoliths, but the correlation coefficient is less than ones in Xinyang. (4)In granulites xenoliths of Xingyang, there are a good logarithmic correlation between La/Sm and La(R2=0.9816), and a good power correlation between Sr/Ca and Ba/Ca(R2=0.9583). 5 From the statistics of temperatures and pressures of eclogites and granulites etc, the paper has given the P-T formulas are different in two series rocks , T=519.82 P0.4964(or P=0.0003 T1.3231),R2=0.6568 (eclogites) and T=801.35 P 0.4283(or P=2×10-5 T1.6547) (granulites etc). Under some hypotheses, the functions of T-h and P-h can be established, then the depths and geothermal gradients of rocks can be calculated. At different hypotheses, there are obviously different explanations to the geneses of eclogites and granulites. When two rocks are formed under the same pressure gradient (such as lithostatic pressure gradient 0.03GPa/km), the geothermal gradient of eclogites is 0.511 H0.0681 times (0.69~0.85 times when10~200 km) as much as ones of granulites. In other words, the conditions to form eclogite are the low geothermal gradients. Under a same geothermal gradient, the pressure gradient of eclogite is 12 T-0.3316 times(2.07~1.21 times when200~1000℃) as graniutes. The pressure gradients of high pressure eclogite is 2.93 T-0.0384 times(2.39~2.25 times when200~1000℃) as much as ones of granulites. In other words, the condition to form eclogites is that there is the environment of partial equal-volume in the lithosphere. Below are the main innovations in the paper: (1)The paper has discovered that in orogenic belt the crust has several geothermal gradients, the transitional rocks combination from granite to eclogite and non Smith characteristics of strata distributions . (2)The paper has discovered that in double logarithmic figure of La/Sm-La the crust-derived xenoliths and mantle-derived xenoliths have obviously distinction, and that there are good correlations between many elements and MgO, La/Sm and La, Sr/Ca and Ba/Ca. (3)The paper has got tow period new ages. (4)The paper has indicated that there is SCF with rich SiO2 in the transition. (5)From the statistics of temperatures and pressures of eclogites and granulites etc., the paper shows that the P-T formulas of two rock series are different and discusses the significance of these functions. (6)The paper has preliminary discussed the lithosphere's pressure gradient which is a key problem in establishing lithosphere's model.