Author Keywords

Precambrian, isotope chemistry, Silver Mines Recreation area, diabase dikes, Eastern Granite and Rhyolite (EGR) province, Skrainka mafic group, Silver Mines mafic group, Butler Hill Granite, Slabtown Granite


Igneous rocks exposed in the St. Francois Mountains record the geologic history of volcanic activity and plutonic intrusions contributing to the growth and stabilization of the continental crust during the Precambrian. These igneous rocks also contain evidence of thermal overprinting both in the isotope chemistry and the crystalline structures. This study presents new isotopic, mineral and petrologic data to support the timing of dike emplacement and thermal overprinting in the host rock. The study area for this research is along the East Fork of the St. Francis River at the Silver Mines Recreation Area where diabase dikes intrude the surrounding granite. Samples of the Silvermines granite and diabase were collected for petrologic study and isotopic analyses. Thin section analyses revealed minerals and textures that suggest magmatic segregation as well as thermal variation in the diabase intrusions. The surrounding granite exhibits thermal alteration at the contact with the diabase. X-ray diffraction patterns support the thin section analyses and provide clarification on opaque minerals. An x-ray diffraction pattern for the granite also indicates orthoclase, supporting existing data sets for structural resetting of K-feldspar in both the volcanic and plutonic rocks in the area. Mineral separates were prepared for granite samples at the contact with the diabase and at a distance of 10 m from the contact. Gas ages were determined using Ar-Ar methodologies for the diabase groundmass as well as K-feldspar, biotite, and amphibole in the host granite. According to the Ar-Ar data collected, the total gas age for the diabase is 1167.1  7.9 Ma. Ar-Ar data from the granite is more complex indicating Precambrian timing with total gas ages of 904, 1066, 1119, and 1258 Ma. Based on this information, it is evident that a significant thermal event altered the Silvermines granite as early as 1258 Ma to as late as 904 Ma. This thermal event may be associated with the intrusion of the Silver Mines dike swarm and its magmatic source.