Provenance Analyses of Sedimentary Strata in the Mesozoic Rift Basins in Connecticut Using Detrital Zircon Geochronology

Jacqueline Giblin & Melissa Luna (2016 GSC Grant Awardees)
Faculty Sponsor: Michael Wizevich
GSC Newsletter Article Project Summary

​The purpose of this project was to utilize uranium-lead (U-Pb) dating of detrital zircon grains to determine the provenance of the sediment infill of the Mesozoic Hartford and Pomperaug basins. The source areas for much of the sedimentary fill of the Hartford and Pomperaug rift basins is not well constrained due to discontinuous outcrop, complex paleocurrent patterns and complicated tectonic history of the source areas. Scientists have proposed sources from the east, west, and both the east and west; there is little agreement on a definitive source or sources of material in the basin. Zircon is a durable mineral found in nearly all sedimentary rocks and can be dated by Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) analysis of U-Pb in the detrital zircon grains.  The age of a detrital zircon provides the date of the source rock in which it formed. Dates of zircons and potential source rocks can be matched and thus provide the areas that they came from. 

 

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Figure 1: Map of Hartford and Pomperaug basins contain sample locations (stars) and paleocurrent data from Hubert et al. (1992).

rock crusher, sieved to separate the grains between​To get a broad representation of the basin fill within the Hartford basin, samples were taken from the Triassic New Haven Arkose (NH-15-1; SE basin- North Haven and NH-15-1; NW basin- Simsbury), the Jurassic East Berlin Fm (EB-15-2; center basin- Berlin), and the Jurassic Portland Fm (P-15-1; NE basin- Manchester and P-15-2; SE basin- Durham). Two samples of the Triassic South Britain Fm (SBF-2; Pierce Hollow and SBF-4; Rattlesnake Members) were collected in the SW Pomperaug basin (Figure 1). The samples were pulverized with a hydraulic  63 and 500mm in diameter,

treated to remove iron coatings from the grains, and then density separated using lithium metatungstate and diiodomethane heavy liquids in separatory funnels. The dense minerals were tapped off, rinsed, dried and magnetically separated using a Frantz magnetic separator.  ​
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Zircons were then handpicked from the non-magnetic portion of the sample.  Approximately 100 zircon grains were picked per sample and sent to the LaserChron Center (University of Arizona) for U-Pb age-dating on a LA-ICPMS. Several samples were “double dated,” where age-dates were taken on the core and rims of the same zircon. SEM cathodoluminescence (CL) was utilized to distinguish rims and cores of the zircon (Figure 2).

​In addition to the overwhelmingly complex geology surrounding the Mesozoic basins, multiple source areas, metamorphic alterations and recycling of zircons make pinpointing provenance a challenge. Overall, the samples contain diverse populations of zircons, primarily reflecting source areas affected by one or more orogeny: Alleghanian (270-320 Ma), Acadian (340-400 Ma), Taconic (420-490 Ma), Peri-Gondwanan (550-640 Ma), and Grenville (980-1320 Ma). Examinations of the results of this study show several notable features:

• Grenville age zircons are found in all but the North Haven sample, with a significant number found in Rattlesnake (53%), Pierce Hollow (14%), East Berlin (18%) and Manchester (11%) samples.

Figure 2: Detailed CL images of detrital zircons showing laser ablation pits, oscillatory zoning and metamorphic overgrowths of various ages.

• Grenville age zircons are found in all but the North Haven sample, with a significant number found in Rattlesnake (53%), Pierce Hollow (14%), East Berlin (18%) and Manchester (11%) samples.
• CL imaging revealed that some zircons have distinct core and alteration rim components. Age dates taken on the core and rims of the same zircon, reveal that some Grenville zircons have Acadian rims, but others are entirely Grenville age. We interpret the Grenville zircons in samples with significant amounts to reflect a western or northwestern source.
• In the Hartford basin, samples from the southeast basin near the Eastern Border Fault (EBF) contain 70-80% of combined Taconic and peri-Gondwanan zircons reflecting proximity to the Bronson Hill Arc and Avalon terranes.
• Surprisingly, there are few Alleghanian zircons, considering the proximity to the southeastern New England terranes that were affected by the event.  This indicates a local eastern source area. 
• On the western side of the basin, the Simsbury sample is dominated (87%) by Acadian ages and is the only sample to contain less than 10% Taconic zircons (1.7%), suggesting a dominantly western source. 
• The Berlin and Manchester (although taken from near the EBF) samples contain distributions that suggest substantial sediment from both east and west source areas.
• Pomperaug basin samples also contain subpopulations that reflect eastern and western source areas; most notably the significant number of Grenville zircons from the Rattlesnake Member. In the Pomperaug basin the Pierce Hollow sample contains 16% Alleghanian zircons. These almost certainly came from an eastern source, but, most likely not from afar; as local sources of Alleghanian zircons are nearby.

​The funding provided by GSC enabled us to analyze two samples at the University of Arizona LaserChron Center, which contributed to the wide representation of locations throughout the basins. The dates of this study provide a new data set and perspective to the ever complex and debated provenance of the CT Mesozoic basins. 

Figure 3: Cathodoluminescence (CL) images of typical Mesozoic detrital zircons.