Type of Document Dissertation Author Das, Reshmi Author's Email Address email@example.com URN etd-11102006-132726 Title Geochemical and Geochronological Investigations in the Southern Appalachians, Southern Rocky Mountains and Deccan Traps. Degree Doctor of Philosophy Department Geological Sciences, Department Advisory Committee
Advisor Name Title A Leroy Odom Committee Chair Jeffrey Chanton Committee Co-Chair Vincent Salters Committee Member James Tull Committee Member Stephen Kish Committee Member Keywords
- Pumpkinvine Creek Formation
- Back Arc Lavas
- Isotopic Homogenization
- Hillabee Greenstone
- Kutch Volcanics
Date of Defense 2006-11-01 Availability unrestricted AbstractIn the southernmost Appalachians, bimodal volcanics of Pumpkinvine Creek Formation (PCF) and its proposed equivalent, the Hillabee Greenstone (HG) have indistinguishable ages (~460 Ma) and trace element-REE pattern similar to an arc/back-arc type setting. Eplison 143Nd values of felsic members of the HG and PCF indicate involvement of Grenville crust during petrogenesis. U-Pb dates (900-1500)Ma of detrital zircons in PCF meta-sandstone cluster around 1100Ma. Nd-model ages of the Ashland-Wedowee Supergroup metasediments range between 943-1439 Ma and cluster around 1000 Ma. Rb-Sr whole rock and U-Pb zircon dates of the Mulberry Rock Gneiss also demonstrate an Ordovician age (~460 Ma). It is concluded that the PCF-HG arc formed on the Laurentian continental margin on Ashland-Wedowee sediments during Ordovician and remained outboard of the continent until final closure during Alleghenian orogeny.
Geochronological investigations of the Tres Piedras Granite of northcentral New Mexico have revealed a sharp discordancy between Rb-Sr whole-rock and U-Pb zircon ages. Analyses of fifty individual zircons (most concordant) by LA-MS-ICPMS yield a ~1730 Ma magmatic crystallization age. Rb-Sr whole-rock isochron ages from separate localities are 1490+/-20 Ma and 1497+/-42 Ma. Sphene/whole-rock/biotite isochron ages and initial 87Sr/86Sr ratios from separate localities are indistinguishable from those of whole-rock isochrons. In both cases feldspar plots above the isochrons and appeared to be an open system as evidenced by 4% difference in 87Sr/86Sr in the core and rim of feldspar. Taken altogether, these data are interpreted to reflect a large (kilometer) scale redistribution and rehomogenization of strontium isotopes during an independently, well-documented metamorphic event in the region.
The geochemical character of the Kutch volcanics, northwest of Deccan Traps, India, have been investigated in order the magma's origin. Sr-Nd-Pb isotopic ratios and trace element patterns identifies three end members: Reunion plume-type alkali basalts, Mahabaleshwar-type alkali basalts and crustally contaminated tholeiites. The first type of alkali basalts that can be generated by very low degree of partial melting (1.6-1.8%) of Reunion plume like source at garnet stability field; the tholeiites can be explained by crustal contamination of Indian-MORB like magma. High 207Pb/204Pb (15.61-15.83) ratio of the tholeiites agrees well with the Pb-isotopes of local Archean crust.
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