Chemical Mohometry: Assessing Crustal Thickness of Ancient Orogens Using Geochemical and Isotopic Data

Area of Science:

• Geology and Geophysics
• Tectonics and Orogeny
• Igneous Petrology

Background:

• Convergent plate boundaries are crucial for continental crust formation and recycling.
• Understanding crustal thickness and paleoelevation evolution in ancient convergent margins is vital for orogenic system analysis.
• Igneous rock chemistry, particularly in supra-subduction zones, reflects Moho depths, offering insights into past crustal thickness.

Purpose of the Study:

• To quantify crustal thickness and paleoelevation evolution along ancient convergent margins.
• To evaluate and refine the use of chemical and isotopic parameters as quantitative proxies ('mohometers') for paleo-Moho depths.
• To reconstruct crustal thickness evolution using detrital zircons as archives.

Main Methods:

• Calibration of whole-rock chemical parameters as quantitative proxies ('mohometers') for paleo-Moho depths.
• Extension of the mohometer concept to detrital zircons using mineral-melt partition coefficients.
• Integration of chemical and geochronological data from detrital zircons for paleocrustal thickness reconstruction.
• Utilizing a MATLAB® app for computations and analysis of modern and ancient geological records.

Main Results:

• Combined usage of various chemical and isotopic parameters enhances paleocrustal thickness estimates.
• The mohometer technique, applied to detrital zircons, successfully reconstructs crustal thickness evolution.
• Mohometers are valuable for reconstructing orogenic paleoelevation due to isostatic equilibrium in arcs.
• Correlations between magma composition and crustal thickness often originate in the sub-arc mantle, with intracrustal differentiation also playing a role.

Conclusions:

• The developed mohometric techniques provide robust tools for reconstructing paleo-crustal thickness and paleoelevation.
• Detrital zircons serve as valuable archives for deciphering the history of crustal evolution in orogenic belts.
• Understanding the mantle and crustal influences on magma composition is key to accurate mohometric interpretations.