28 D18. Petrology, Igneous/Metamorphic

Monday, 26 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

MAGMA MINGLING IN THE SOUTHERN OKLAHOMA AULACOGEN: A STUDY OF THE COLD SPRINGS BRECCIA

Paper number: 28-7
Program: Technical Sessions
Time: 6:50 PM – 7:00 PM

Authors

Abstract

The Wichita Mts of SW Oklahoma are exposed igneous rocks of the Southern Oklahoma Aulacogen. Rifting during the Early Cambrian produced the compositionally bimodal suite of the Wichita Igneous province over a few million years. Where relationships between large mafic and felsic bodies can be observed, mafic anorthosites and hydrous gabbros underlie A-type granites and rhyolites. Mafic enclaves, drill cores, and U-Pb zircon ages for the Wichita large igneous province demonstrate that mafic and felsic magmas co-existed although the exact nature of their relationship has yet to be fully understood. The Cold Springs Breccia (CSB) is found in a few locations west of the main outcrops of larger bodies of anorthosite and granite. Interactions of coeval felsic and mafic liquids are recorded in the CSB, where mafic “pillows” ~30 cm in size are separated by granitic zones ~10 cm wide. The mafic phase is fine-grained with abundant plagioclase microlites; the felsic phase is hypidiomorphic and medium grained tonalite to granodiorite. Determining how the CSB formed will provide insights into the felsic and mafic association of the greater Wichita Igneous Province. At the type locality, margins of mafic pillows are crenulate to lobate and wispy protrusions of disaggregating clusters of crystals can be seen at microscopic scales suggesting mingling of viscous mafic blobs into granitic melt. Here it seems that pillows formed when mafic magma was injected into granitic magma and higher density mafic blobs settled to the bottom of the granitic magma body, trapping some granitic melt between individual pillows. Nearby outcrops show relationships signifying that the felsic phase intruded mafic magma. These disparate behaviors show a heterogeneity in the rheology of denser, more viscous mafic and buoyant, more liquid felsic parts of the intrusion at the time the breccia formed. Examining the CSB in the field, hand specimen, and microscopically gives us important evidence of magmatic behavior at different scales. Petrographic study shows no chilled margins or reaction zones though there is an increase in grain size in the adjacent granitoid and moderate alignment of plagioclase microlites along the margin of the mafic phase.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-359025

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