『Abstract
Petrologic and geochemical data confirm that mudstones and sandstones
of the Johnnie Formation were the initial siliciclastic deposits
laid along the Cordilleran Laurentian margin following the Neoproterozoic
break-up of Rodinia. Sedimentary rocks of the Johnnie Formation
have corrected CIA values between 63 and 83 (or higher), which
suggest moderate to intense weathering of crystalline source rocks
or recycling. Based on modeling the fresh source rocks likely
consisted of 90％ granodiorite and 10％ high-K granite. This conclusion
is based on petrographic observations, major element geochemistry,
and investigation of the REE: (LaCN/SmCN = 4.19±1.26, GdCN/YbCN = 1.34±0.38, Eu/Eu* = 0.63±0.09
and LaCN/YbCN = 9.55±2.27).
Feldspars are unevenly distributed in the finer grained sedimentary
rocks. Observed fluctuations in feldspar content throughout the
Johnnie Formation are interpreted as a result of abrasion and
hydrodynamic sorting, which concentrated feldspars in the finer
grained sediment. None of the mudstone samples, including those
collected just below and above the flat-pebble conglomerate in
the upper Johnnie Formation, show evidence of true cold weather
depositional conditions. Consequently, Johnnie Formation mudstone
geochemistry does not record evidence of an extreme paleoclimatic
environmental shift in the succession. Textural characteristics
of Johnnie Formation sandstones are consistent with quiescent
tectonic conditions characterized by low relief, and deposition
of Johnnie Formation strata in a passive-margin setting.
Keywords: Neoproterozoic; Johnnie Formation; Provenance; Paleoweathering』
1. Introduction
2. Stratigraphic setting
3. Sampling and analytical techniques
4. Petrography
4.1. Composition
4.2. Textures
5. Sediment composition evolution
5.1. K-metasomatism
5.1.1. K-feldspar composition
5.1.2. Assessing the effects of K-addition
5.2. Na-metasomatism
5.3. Paleoweathering
6. Provenance
6.1. Petrography
6.2. Trace- and rare earth elements
6.3. Detrital zircon ages
6.4. Sediment mixing model
7. Discussion
7.1. Feldspar enrichment in the Johnnie Formation
7.2. Implications for paleoclimate
7.3. Implications for tectonic setting
8. Conclusions
Acknowledgements
References