Petrology And Metamorphism Of Khondalites From The Jining Complex In The North China Craton

The basement of North China Craton (NCC) can be divided into the Eastern and Western Blocks, and the Trans-North China Orogen on the basis of petrological association, structure, metamorphic processes and isotopic ages. The khondalite series from Jining Complex lie in the centre of Western Block of NCC. We studied petrology, mineral chemistry and metamorphic P–T path of khondalite series from Jining Complex, and then obtained following conclusions.1. The predominant rocks of khondalite series from Jining Complex are biotite-sillimanite-garnet gneiss and quartzo-feldspathic gneiss. Sillimanites can obviously be divided into two types, one is fibrous minerals included within garnet grains, and the other is tabular megacryst scattered in matrix.2. There is approximately 56-62.5 mol% of almandite and 31.7-41.5 mol% of pyrope in garnet with low CaO (0.65-1.76 mol%) and MnO (0.20-0.47 mol%); high-magnesium in biotite: Mg/(Mg+Fe) varies from 0.76 to 0.88, followed by TiO₂ (4.3 wt. %), with subordinate Al₂O₃ (14.33 wt.%); most K-feldspar grew in the form of perthite and we can also easily observe microclines which have tartan twinning; plagioclase is mainly oligoclase (An=20-30) and andesine (An=30-40) takes second place. The ratio of Mg/(Mg+Fe²⁺+Fe³⁺) in the analyzed cordierite ranges from 0.80 to 0.83 (Table 1). The analytical totals are all less than 100 wt.%, indicating the presence of volatile components, such as CO₂ and H₂O.3. Typical mineral associations of prepeak, peak and afterpeak metamorphism were investigated, and THERMOCALC program with the database of Holland & Powell (1998) was used to calculate P-T conditions of metamorphic events. It is suggested that the temperature and pressure of metamorphism prepeak, peak and afterpeak are 550～600℃,4.90～6.12kbar,825～859℃,7.7～8.3kbar,580～639℃,3.9～4.9kbar. A clockwise P-T path which reflected continent- continent collision was gained.4. Jining area is in the east end of khondalite series of the Western Block, and lies in the intersection of the Western Block and the Trans-North China Orogen. Combining its special locality with investigated clockwise P-T paths, the three-fold tectonic subdivision model is stand to reason. It is that NCC is composed of the Eastern and Western Blocks along the Trans-North China Orogen. And as a result of collision between the Eastern and Western Blocks, finally combined to the whole basement of NCC.