The Depth And Rock Erosion Of Fang Shan Pluton In Beijing And Its Tectonic Significance

Fangshan pluton is composed of quartz monzodiorite zone and granodiorite zone,whose main mineral assemblage is plagioclase, alkali feldspar, quartz, biotite andamphibole, including a little sphene, pyroxene and magnetic-ferrotitanium mineral.The mineral assemblage is accordant with the application condition of theAl-in-hornblende geothermobarometer. Using the electron probe, the analyticalresults of the composition of symbiotic plagioclase and amphibole are: theplagioclase in the Fangshan pluton mainly is intermediate-acid plagioclase includingalbite to oligoclase. The An of plagioclase in different phase zone of the Fangshanpluton changes little, its tendency to decrease from margins to core is clear and theplagioclase of dioritic enclaves is slightly more basic than the plagioclase ofgranodiorite in the same phase zone; Amphibole is calc-amphiboles and therelationship between Al2O3and TiO2shows that the sources of the Fangshan plutonand the dioritic enclaves are crust mantle mixed origin, while the source of the quartzmonzodiorite and dioritic enclaves approached to the mantle, these indicate that theFangshan pluton and the dioritic enclaves have the characteristics of adakities ingeochemical composition.The paper estimates the emplacement depth of Fangshan Pluton and its uplift rateby using the Al-in-hornblende geothermobarometer, and reports the uplift andexhumation history of Yanshan fold belt since the Cretaceous.The results indicatethat the forming pressure of the quartz monzodiorite which is on the external (nearthe outer edge) of the pluton is2.93-4.03kbar, and the forming pressure in themargin phase of the granodiorite is2.45-2.66kbar, and the pressure in the transitionphase is2.62-3.33kbar, and the pressure in the middle phase is1.47-2.64kbar. Theemplacement depth corresponding to the forming pressure is: the emplacement depthof quartz monzodiorite is12.03km, and that of the granodiorite margin phase is9.04km, and that of the transition phase is10.32km, and that of the middle phase is8.29km. The forming tempertures in the three phases of granodiorite are722.26℃to710.47℃to704.33℃,And the tempertures tend to decrease from the margin of thepluton to its core, and that is silimar to the dioritic enclaves. It shows that the originof the dioritic enclaves is diffferent from the granodiorite, the forming depth of thedioritic enclaves in the three phase zone is respectively:13.98km、12.67km and 13.05km, which are deeper than those of the granodiorite.By analysing the isotope of the granodiorite, the mineral fission track dates and theclosure temperatures of the corresponding minerals in the Fangshan pluton, weconclude that the rate of uplift and exhumation for the Fangshan pluton is not stable.During130.7Ma to129.4Ma, the pluton cooled to300℃at the rate of346.2℃/Ma,which was a Thermodynamic equilibrium process between the Fangshanpluton and the wall rock. Since129.4Ma, the Fangshan pluton underwent anuplift-exhumation process which was slow and complex: It was a rapid uplift stage ofthe pluton in the early time during129Ma to102Ma. The temperture of the plutondecreased from300℃to180℃at the rate of4.5℃/Ma.Based on the geothermalgradient in the pluton, the estimating uplift rate of the Fangshan pluton was0.18mm/a. During102Ma to45Ma, the temperture of the pluton dropped from180℃to110℃at the rate of1.2℃/Ma and the uplift rate was0.05mm/a, which is muchsmaller compared with the early stage. Since45Ma, the temperature of plutondecreased to the present surface temperature at the rate of2.2℃/Ma, and the upliftrate of the Fangshan pluton was0.09mm/a, which changed little compared with theformer stage, which refelects it was a stable state in this interval for the Western Hills,Beijing.