Differentiation of Continental Crust: insights from the Variscan Orogeny and the Global Array of Terrigenous Sediments
Restricted (Penn State Only)
- Author:
- Connop, Charlotte
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 11, 2024
- Committee Members:
- Donald Fisher, Program Head/Chair
Andrew Smye, Chair & Dissertation Advisor
John Mauro, Outside Unit Member
Mark Caddick, Special Member
Tanya Furman, Outside Field Member
Jesse Reimink, Major Field Member - Keywords:
- metamorphic petrology
Variscan Orogeny
partial melting
metamorphism
s-type granites - Abstract:
- The stabilization of the continental crust over geologic timescales is reliant upon the stratification of the crust into a felsic (Si-, Al-, K-rich) upper crust, enriched in heat producing elements (HPEs; U, Th and K) and a more mafic (Fe-, Mg- rich) lower crust with lower concentrations of HPEs. This chemical stratification of the crust is deeply influenced by metamorphism and partial melting of the middle-to-lower continental crust. These processes cannot occur unless temperatures are raised above the steady-state continental geotherm – a process that is energy intensive, particularly in the middle crust due to elevated heat fluxes imposed by surface temperatures. Contrary to this view of a global mafic lower crust, felsic, HPE-rich metasedimentary rocks are commonly found in exhumed metamorphic terranes and xenoliths from the deep crust. The presence of these sedimentary rocks within the deep crust alters the thermal profile and rheology of the continental crust, potentially leading to destabilization. However, the mechanisms by which sedimentary rocks are entrained into the deep crust and the thermal consequences of shifts in sedimentary rock composition over geologic remain enigmatic. In this dissertation, I investigate the mechanisms responsible for the addition of sedimentary rocks to the deep crust, the mechanisms and timescales of metamorphism and partial melting of these sedimentary rocks, and the control their composition has on melt genesis. In Chapter 2, I present a petrochronologic investigation of the Trois Seigneurs Massif, French Pyrenees, an exposed middle crust section that underwent metamorphism and partial melting at the end of the Variscan Orogeny (310-290 Ma). I used a combination of thermobarometric methods to show that the crust had a two-part thermal structure, with the shallow crust having high temperature gradients (50-60 °C/km) while the deep crust had shallow temperature gradients (10-12 °C/km). Monazite and zircon U-Th/Pb geochronology indicate that peak metamorphic temperatures were reached at 305 Ma throughout the section. A one-dimensional thermal model shows that this unique thermal structure can be achieved on timescales of 10 Myr or less through the advection of heat from the deep to shallow crust via melt migration. Chapter 3 focuses on a different Variscan basement massif, the Ivrea-Verbano Zone, Italian Alps – an archetypal lower continental crust section comprised of metasedimentary rocks. In this chapter, I use garnet Lu-Hf geochronology and garnet major element (Fe, Mg, Mn, and Ca) compositional zoning to show that tectonic accretion was the most probable mechanism of sediment addition to the lower continental crust. The garnet Lu-Hf dates span a nearly 50 Myr period from 311 to 265 Ma, with the oldest dates recorded at the shallowest structural levels, demonstrating the protracted timescales over which metamorphism and partial melting occur. Chapter 4 uses a large database of global terrigenous sedimentary rock compositions to investigate the control sediment compositions have on melt production during continental mountain building. A statistical assessment of 39,381 samples shows that variations in SiO2 and Al2O3 dominate the geochemical variability within sedimentary rock compositions. Phase equilibria calculations of a reduced set of end-member compositions show that sedimentary rocks with higher Al2O3 and K2O contents produce the largest volumes of melt at pressure-temperature conditions relevant to Phanerozoic mountain belts due to stabilization of high modal abundances of muscovite and biotite. One-dimensional thermal models of thickened continental crust indicate that shales are the most important sedimentary lithology for the genesis of sediment-derived granites due to their high internal heat content and high fertility. Additionally, I discuss how changes in surface processes brought about by the rise of land plants changed the potential magnitude of radiogenic heat production within continental mountain belts.