MAGMA MIXING AND MINGLING PROCESSES IN THE TIEMANN SHUT-INS, ST. FRANCOIS MOUNTAINS, MISSOURI: CONSTRAINTS FROM GEOCHEMICAL VARIATIONS WITHIN MAFIC ENCLAVES

The formation of continental crust is still a topic of debate. Its average composition is similar to andesite, a rock type that is thought to be largely the product of magma mixing and mingling between basaltic and granitic magma. In the 1.5 billion-year old St. Francois Mountains of SE Missouri, an excellent exposure of magma mixing/mingling is preserved in the Tiemann Shutins area where a granitic pluton hosts decimeter- to meter-scale mafic enclaves. Previous studies (Lowell and Young, 1999) present enclave SiO₂ values ranging from 59.1 to 70.0 weight %. With only one analysis per enclave, it is unclear whether the variability in the measurements is due to varying sampling location within each enclave from a population of heterogeneous enclaves or variable composition between more-or-less homogenous enclaves. Is each enclave homogeneous or are there variations within each enclave? Are these variations primary (magmatic) or secondary (subsolidus or hydrothermal)? How much variation exists within each enclave and do the enclaves all have similar variations? Constraining the magnitudes and spatial variations of enclave compositions may lead to a better understanding of the processes involved in magma mixing and mingling that may generate continental crust.

We analyzed the variation of several elements (Si, Fe, Al, and Ti) within several enclaves with an Olympus Delta portable X-ray spectrometer (pXRF). The pXRF allows one to determine, non-destructively, the spatial variation of certain chemical elements within each enclave. Results show that the variation in SiO₂ occurs within single enclaves (not just between enclaves) and occurs over a wider range of SiO₂ (53-70%) than previously shown. Although more detailed analyses are forthcoming, present results do not show any obvious concentric chemical zoning, or other simple patterns in chemical variation, suggesting a possible more random, streaky compositional pattern in the enclaves.

These results support a two-stage magma mixing/mingling model where mixing occurs at the boundary of basaltic and granitic magma chambers and then the hybrid, variably mixed magma subsequently mingles with the granitic magma and potentially undergoes further chemical exchange.