--> ABSTRACT: Weathering, Diagenesis, Recycling, and Cumulative Maturity as Principal Determinants of the Distribution of Varieties of Detrital Garnets and Zircons, by Emanuela Molinaroli, Abhijit Basu; #91020 (1995).
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Weathering, Diagenesis, Recycling, and Cumulative Maturity as Principal Determinants of the Distribution of Varieties of Detrital Garnets and Zircons

Emanuela Molinaroli, Abhijit Basu

Do chemical compositions, colors, and shapes of detrital garnets and zircons reflect their ultimate provenance or do they reflect modifications imposed by weathering, diagenesis, recycling, and cumulative maturity? In SW Montana we sampled separately (1) Oligocene Renova Formation sandstones derived from (a) Precambrian metamorphic rocks and (b) Cretaceous igneous rocks, (2) Miocene Six Mile Creek Formation sandstones (same sources plus recycled material mainly from the Renova), and (3) Holocene stream sands that derive from all above.

Polished grain mounts of about 600 detrital garnets and zircons in the size range of 100-300 microns were used to determine chemical compositions (electron microprobe), shapes (three classes for zircon), and color (five classes for garnets). Discriminant Function Analysis (DFA) of properties of garnets and zircons fails to identify igneous vs. metamorphic provenance (unacceptable Previous HitcanonicalNext Hit correlation with low apparent success). In contrast, separate DFA of detrital garnets and zircons identify formations (Renova, Six Mile Creek, Holocene) very successfully (Previous HitcanonicalTop correlation >0.99, success >96%).

We conclude that many provenance-sensitive properties of detrital garnets and zircons are modified substantially by processes of weathering (climate has changed significantly since Oligocene in this area), diagenesis, and recycling, which also create new characteristics that reflect cumulative maturity. Therefore, provenance analysis is better done with samples of similar cumulative maturity.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995