Electron Microprobe (Ultrachron) Geochronology of Monazite from the Homestake Iron Formation, Lead, the Black Hills, South Dakota, U.S.A. PDF Download

Author: Lindsay E. Chasten
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ISBN:
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Languages : en
Pages : 194

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Book Description
Monazite dating of rocks in the Black Hills area has led to a better understanding of the timing of tectonic events and episodes of mineral growth. However, detailed geochronologic studies have not been done on rocks from the Homestake Iron Formation. The objective of this study was to constrain the age of mineralization by analyzing monazite in rocks of the Homestake Formation, and to relate these ages to textures and episodes of mineral growth. Thin sections were investigated that represent a range of metamorphic conditions. Monazite was identified optically and using a scanning electron microscope, then U-Th-Pb dated with the Ultrachron. X-ray chemical maps were used to identify potential domains for which chemical ages were calculated. An allanite to monazite reaction was trapped in garnet that displayed yttrium zonation, indicating exhaustion of the allanite REE source during garnet growth. The reaction yields an age of 1757 ± 30 Ma, consistent with regional D2 related to the Trans-Hudson Orogen (THO). several regional tectonic events are represented by monazite ages in this study. A ~1850 Ma age of one monazite grain may be attributed to early southern THO activity, such as collission between the Wyoming craton and Dakota block. Regional D1 and D2 ages are well-represented. The ~1775 Ma ages are associated with D1 Yavapai island-arc collision to the south of the Black Hills. Some ~1670 Ma ages may have formed during far-field Mazatzal deformation to the south of the Yavapai terrane. The ~1715 Ma ages associated with intrusion of the Harney Peak Granite are prevalent in the Black Hills are considered a minimum age for mineralization. These ages have not yet been found in the Homestake Formation. This may be due to lower temperatures during mineralization that did not intersect a stability range of monazite, which experimentally forms at low temperatures and at a higher amphibolite facies grade. Several ~1300 and ~1200 Ma ages occur in lower-grade rocks and may have formed in the low-temperature stability range of monazite during the slow cooling period of the Black Hills.