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A Geochemical Approach to Mantle and Crustal Dynamics in Central Anatolia
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Gall, Helen Deborah
Doctor of Philosophy
Date of Defense:
November 13, 2018
Tanya Furman, Dissertation Advisor
Tanya Furman, Committee Chair
Maureen Feineman, Committee Member
Kevin Patrick Furlong, Committee Member
Scott P McDonald, Outside Member
This dissertation provides new geochemical analyses of Quaternary mafic and intermediate products erupted in south-Central Anatolia. We explore the petrogenesis of primitive mafic cinder cone products erupted from the Hasandağ Cinder Cone Province (HCCP) and the adjacent Karapınar Volcanic Field (KVF) as well as intermediate compositions (basaltic andesites, andesites and dacites) erupted contemporaneously from the Hasandağ stratovolcano. I begin by using whole rock geochemical analyses alongside Sr-Nd-Hf-Pb isotopic data to explore the mechanisms of mantle melting that generated the HCCP and KVF lavas and compare their petrogenesis to those observed in other Anatolian volcanic provinces. In doing so, I document changes in magma chemistry that reflect the complex and evolving tectonic regime, build upon previously proposed mechanisms of lithospheric removal, and clarify the evolution of the mantle beneath Central Anatolia in order to better understand the complex regional geodynamic history. Major and trace element geochemistry indicate melting of the spinel-bearing lithosphere to produce individual small mafic magma batches of heterogeneous melts and further suggest contribution from metasomatic phases. Isotopic analyses require contributions from the metasomatized Anatolian subcontinental lithosphere along with input from a spatially heterogeneous sublithospheric mantle. The mafic lavas record an increase in degree of melting with depth accompanied by gradual reduction in the contribution from metasomatic phases, which is indicative of lithospheric drip melting. This geochemical signature is also observed in Western Anatolian late Miocene and Quaternary mafic lavas, but is not exhibited in the Quaternary East Anatolian lavas or the Sivas Basin basalts and basanites in north-Central Anatolia. Our findings demonstrate a complex interplay between lithospheric and asthenospheric source domains and suggest drip melting was initiated after delamination of the subducted NeoTethys slab, resulting in upwelling asthenosphere and destabilization of the remaining lithosphere which melted to produce the observed mafic volcanism at the HCCP and KVF. Next, I examine the crustal dynamics of Central Anatolia by assessing the genesis of intermediate compositions erupted during the most recent episodes of volcanism at the Hasandağ stratovolcano (Neo- and Mesovolcano, <1 Ma). My research is motivated by questions about the physical state of magma reservoirs and the processes that operate within them to produce eruptible bodies of intermediate magma. I implement whole rock major and trace element geochemistry, Sr isotopic analyses, petrographic observations and detailed zoning transects of mineral compositional data to evaluate the contributions from fractional crystallization, magma mixing and mafic recharge processes in a shallow crustal chamber, ultimately establishing a petrogenetic model for the abundant intermediate lavas (i.e., basaltic andesites, andesites and dacites). I find that the Hasandağ plumbing structure exhibits a homogenous and shallow (~4 km) rhyodacitic magma chamber with a highly viscous crystalline framework composed of abundant An40 plagioclase crystals. This framework is fractured and remobilized through periodic recharge by mafic magma resulting in heterogeneous crystal populations, distinct textures and numerous mineral disequilibrium features. Similar observations at the Erciyes stratovolcano just 150 km north of Hasandağ suggest magma mixing is regionally ubiquitous in Central Anatolian stratovolcanoes. I complement our observations of magma mixing via kinetic diffusion modeling of MgO in plagioclase in order to evaluate mixing to eruption timescales. I employ a forward in time finite difference scheme to assess the diffusional relaxation of magnesium across the core-rim boundaries of plagioclase crystals indicative of interaction with a mafic recharge magma. I find that the intermediate samples exhibit a bimodal distribution where half of the results are best fit by hour to day timescale calculations and the other half indicate mixing to eruption takes place on the order of weeks to months. I interpret these results to indicate mixing at very shallow depths, possibly within the conduit and during eruption, or at greater depths within the magma reservoir, respectively. Individual thin sections often document both short and long timescales, suggesting these crystals were gathered together in the same magma batch prior to eruption. I end my dissertation with a discussion on undergraduate idea development with regard to the paradigm of Plate Tectonics. I build upon the learning progression previously developed for K-12 students and add hypothetical levels to two of the three progress variables which document new ideas expressed by the geoscience majors. Interviews with 13 upper-level undergraduate students enrolled within a newly developed plate tectonics course both pre- and post-instruction focus our efforts on the implementation of complex systems analysis, a teaching methodology that is now stressed by the geoscience community for the development of successful geoscientists. My findings reveal the importance of expert terminology (e.g., lithosphere and asthenosphere) to the construction of sophisticated explanatory models for the complex subsystems of plate tectonics. I develop an upper anchor that focuses on the oceanic plate cycle as opposed to phenomena which result from plate motion to encourage the implementation of expert terminology via the explanation of partial melting. I urge college-level instructors to consult this upper anchor and the additional levels of the hypothetical learning progression to develop curriculum which requires model building and complex systems analysis to best prepare geoscience undergraduates for diverse career paths.
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