Petrology and Geochronology of 1.48 to 1.45 Ga Igneous Rocks in the St. Francois Mountains Terrane, Southeast Missouri

Professional Paper 1866
Prepared in cooperation with the Missouri Geological Survey
By: , and 

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Abstract

The igneous geology of the St. Francois Mountains terrane in southeast Missouri is dominated by the products of 1.48 to 1.45 billion year old volcanic and plutonic magmatism but also includes volumetrically minor, compositionally bimodal contributions added during plutonism between 1.34 and 1.27 billion years ago. The 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are bimodally distributed between volumetrically dominant felsic rocks and volumetrically minor rocks with mafic to intermediate compositions. All of these rocks are ferroan, which like most of their trace element abundances, suggests a genesis associated with farfield intraplate extensional tectonism and decompression-related magmatism. The diversity of compositions among 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane probably reflects mixtures of mantle-derived mafic inputs and low-degree partial melting of more evolved crustal protoliths. Newly determined ages define essentially continuous magmatism during the 30-million-year period between 1.48 and 1.45 billion years ago. The products of this magmatism are essentially coeval, whether intrusive or extrusive or having mafic, intermediate, or felsic compositions. In addition, the iron oxide-apatite (for example, Pea Ridge) and likely the iron oxide-copper gold (Boss) deposits in the St. Francois Mountains terrane have ages coincident with this magmatic episode. Spatial and temporal relations between 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane and the mineral deposits they host suggest the associated magmatic and mineralization processes are also genetically related.

Geochemical, petrographic, geochronologic, and terrane-wide physical characteristics of the 1.48 to 1.45 billion year old igneous rocks in the St. Francois Mountains terrane are consistent with an origin involving extension well inboard from the margin of the Laurentian craton, associated mantle upwelling, lower crustal melting in response to mantle-derived thermal inputs, and mixing of mantle- and juvenile lower crustal-derived melts. Significant major and trace element compositional dispersion characteristics of these rocks likely reflect midcrustal magma reservoir fractionation of their principal rock-forming minerals. The resultant magmas constitute a series of variably hybridized reservoirs, emplaced at upper levels in the crust, that form a series of plutonic and associated eruptive products.

Suggested Citation

du Bray, E.A., Aleinikoff, J.N., Day, W.C., Neymark, L.A., Burgess, S.D., 2021, Petrology and geochronology of 1.48 to 1.45 Ga igneous rocks in the St. Francois Mountains terrane, southeast Missouri: U.S. Geological Survey Professional Paper 1866, 88 p., https://doi.org/10.3133/pp1866.

ISSN: 2330-7102 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Nature of Mesoproterozoic Igneous Rocks in the St. Francois Mountains Terrane
  • Geochronology of 1.48 to 1.45 Ga Igneous Rocks in the St. Francois Mountains Terrane
  • Methods
  • Results
  • Petrographic Characteristics
  • Whole-Rock Geochemistry
  • Tectonic Setting During 1.48 to 1.45 Ga Magmatism in the St. Francois Mountains Terrane
  • Conclusions
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Petrology and geochronology of 1.48 to 1.45 Ga igneous rocks in the St. Francois Mountains terrane, southeast Missouri
Series title Professional Paper
Series number 1866
DOI 10.3133/pp1866
Year Published 2021
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geology, Geophysics, and Geochemistry Science Center
Description Report: viii, 88 p.; 2 Data Releases
Country United States
State Missouri
Other Geospatial St. Francois Mountains
Online Only (Y/N) Y
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