USING PETROLOGIC AND GEOCHEMICAL EVIDENCE TO TEST INCREMENTAL GROWTH MODELS FOR THE FANGSHAN PLUTON, CHINA

Petrologic and geochemical evidence preserved in the frozen magma chamber are very useful in evaluating incremental growth models for zoned plutons. The Fangshan pluton, located in a Mesozoic magmatic arc of North China Craton, is an ideal place to test incremental growth models due to its good exposures and long history of study by Chinese geologists.

The Fangshan pluton consists of two mapable concentric units with the outer quartz diorite being partially replaced by the inner granodiorite unit. The outer unit is fine grained quartz-diorite with a published U-Pb age of 131.5 Ma, and the inner unit with a published U-Pb age of 130.5 Ma contains two phases of coarse grained granodiorite: 1) without K-feldspar phenocrysts and 2) with K-feldspar phenocrysts increasing in size from 0.5 to 2 cm towards the center. The contact between quartz-diorite and granodiorite is a mingling zone with felsic dikes intruded into the old unit. No sharp contacts were found in the granodiorite unit. Published major elements, trace elements and REE geochemistry show that within the granodiorite unit the magma generally changes from more mafic to more felsic but with 5 irregularities of the amount of SiO2 and 2 of the total REE (Cai et al., 2005). Enclaves are distributed all over the pluton. Published REE and isotope geochemistry tells us that these mafic enclaves were derived from the same source as the pluton (Yan et al., 1995).

The contact relationship and published ages indicate that 1) the granodiorite is younger than and intruded into the center of the old quartz diorite and 2) they were juxtaposed prior to crystallization. The irregularities of the geochemistry trend in the granodiorite indicate emplacement by several pulses very close to one another in time, which need to be tested by precise geochronology data. Additional geochemistry analyses, geochronology data and detailed microstructural study will provide a more robust test of incremental growth models for this pluton.