256 Recent Advances in Igneous Petrology and Geochemistry (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

GEOCHEMISTRY AND SR-ND ISOTOPIC COMPOSITIONS OF PERMIAN ULTRAMAFIC LAMPROPHYRES IN SOUTHERN ILLINOIS

Paper number: 256-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Ultramafic igneous intrusions in southern Illinois, exposed in coal mines, drill cores, and some surface locations, remain poorly understood. The Omaha Dome, Wildcat Hills, Cottage Grove, Will Scarlet, Williams, and Grant intrusions share similar geochemistry and can be classified as ultramafic lamprophyres. Major element compositions are typically 30-34 wt.% SiO2, 5-7% Al2O3, 12-14% FeOt, 16-19% MgO, 4-5% TiO2, 12-16% CaO, 0.1-0.7% Na2O, 1.2-2.6% K2O, and 0.4-1.3% P2O5. The Grant intrusion is an exception, with lower SiO2, Al2O3, FeOt, MgO, TiO2, and higher CaO, and may represent a mixture of lamprophyre and carbonatite. Typically these rocks are fine grained, with phlogopite (Al- and Ti-rich with core to rim trends to lower Al2O3 and TiO2), variably serpentinized olivine (~Fo88), diopside, perovskite, magnetite, apatite, and calcite. Small, strongly zoned garnets (andradite) seem to be associated with locations of secondary mineralization. The Omaha Dome intrusion is coarser grained with cumulate textures. The Grant intrusion differs in that it contains hornblende. These rocks have been called alnöites, but melilite has not been conclusively identified. Trace element patterns exhibit enrichment of LREE, strong REE fractionation, and relative depletions of K, Sr, Zr, and Hf, closely matching those of the mela-aillikites of Aillik Bay, Labrador. The Grant intrusion exhibits even greater REE enrichment and notable peaks at Nb, La, and Ce. The geochemical characteristics of the southern Illinois intrusions are consistent with near-primary melts from a metasomatized peridotite source containing phlogopite-rich veins. A Rb-Sr isochron age of ~230 Ma is in reasonable agreement with published ages of ~270 Ma, and highlights their coeval nature. These rocks exhibit a narrow range of initial 87Sr/86Sr (0.70301-0.70359), and initial εNd (3.7-5.1), suggesting a uniform mantle source close to Bulk Earth. There is no trend toward high 87Sr/86Sr to indicate crustal contamination. T-depleted mantle model ages range from 540-625 Ma, and might correlate with timing of enrichment of a lithospheric mantle source during the breakup of Rodinia. Future work will involve additional EMP analysis of minerals to gain insights into the origins of these intrusions and their relationship to the New Madrid Seismic Zone.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-287136

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.