Geochemical Characteristics And Genesis Of Qipan Rock Body In Mentougou District Of Beijing

Rock Body board exposed in Mentougou District board Imamura northwest of Beijing along the exposed area of about 7km2. Rock of the North West to the buried fault control, NW extension to irregular oval, its long axis to northwest, the rock slope to north east. Neoproterozoic intrusive rock board session Qingbaikouan Maling group to the next Middle Cambrian xuzhuang group formation, and cut off from the ground up of nine mountains trending syncline, early predecessors say it is the fall Jurassic of rock, the first of measured that latest SHRIMP zircon age is 132.9±0.66Ma (DENG Jin-fu, etc., 2004), so, based on field investigation and the testing of chronology to samples indoor ,of that: the board rocks of gabbros is the product of the Early Cretaceous.Samples of Mg# calculated the Mg#= 55-73, note board gabbro rock with the nature of the original magma. Silicon base rock from the board of the figure shows, K2O + Na2O content of 2.12% - 5.79%, average 3.67%, the board of rock litho logy of the samples for the basic series, are alkali rock type. In the AFM diagram, the sample point to vote in the near F(Fe2O + FeO) in the region, indicating that volcanic rocks in this area than the iron-rich (average Fe2O + FeO = 6.47), belong to the tholeiite series. In the area 29 through the use of chemical analysis data of rock, volcanic rock series of the district, rock composition and chemical composition analysis results in general have a poor rock mass board of silicon, iron, aluminum side features.The rock samples of the rock is relatively enriched in large ion lithophile elements (LILE). P negative anomalies that magma may have been contaminated by crustal material. Board gabbro rock Nb / U = 26.4, and close to the primitive mantle (Nb / U = 30). It is deduced that the board of Rock gabbro body nature of the original mantle source region.Rock Body board of the rock samples was more REE fractionation obvious, show that the strong LREE enrichment, poor HREE. [W (La) / w (Sm)] N> 1 is the P-type, LERR enrichment type, and mantle plume or abnormality.To analysis the electron microprobe analysis of pyroxene rock (EMPA) results, in this area has iron-rich pyroxene features (FeO 7.72% ~ 18.13% fall within the average 10.44%). Thus, the features of pyroxene in this area is a rich Fe.I have participated for the National Natural Science Foundation Key Project "in North China Yanshan deep lithosphere thinning process" field on the board rocks in a measured profile, the profile of layered rock board obviously, is divided into thirteen layer, on board by late rock identification of rock samples for testing and further instructions set Rock Body from the bottom may be layered structure.