HIGH PRECISION U-PB GEOCHRONOLOGY OF THE FANGSHAN PLUTON, BEIJING, CHINA: IMPLICATION FOR THE INCREMENTAL GROWTH OF ZONED PLUTONS

Numerous petrology, geochemistry and geochronology studies show clear evidence that magmatic plumbing systems grow incrementally. It is often less clear to what degree this incremental growth played in the formation of large, normally zoned plutons. The Fangshan Pluton, located in the Mesozoic arc, North China Craton, is a natural laboratory in which to test the incremental growth of zoned plutons because of its outstanding exposure and long history of study by Chinese geologists.

The Fangshan pluton has a circular surface shape with an outer fine-grained quartz-diorite and inner granodiorite, which has some internal gradations. Model variations in mineralogy and a change in K-feldspar phenocrysts sizes can be recognized in the granodiorite unit, but there are no sharp, mappable contacts. One sample from each unit has been analyzed at the Berkeley Geochronology Center using the CA-TIMS method applied to zircons to approximate implacement ages of the two pulses. Sample F046 from the outer unit has a weighted mean ²⁰⁶Pb/²³⁸U age of 131.3 +/-0.4 Ma from five out of the six analyses (one antecrystic age is at 143.5 Ma). Zircon ages in sample F031 from the center unit are dispersed along concordia with ages between 132 and 145 Ma, suggesting a mix between antecrysts and the implacement age. Calculated zircon saturation temperatures are 784°C and 760°C for the inner and outer unit, respectively. These data indicate that the exposed portion of this pluton consists of at least two distinct phases and probably intruded through slightly older phases at depth.

The new U-Pb ages and recently published U-Pb SHRIMP ages all indicate that the Fangshan pluton grew incrementally in at least two pulses. The possible existence of older pulses not exposed. A more detailed study using CL imaging and trace element distribution in individual zircons is expected to verify the incremental growth model of this pluton.