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

Abstract

Convergent plate boundaries are key sites for continental crustal formation and recycling. Quantifying the evolution of crustal thickness and paleoelevation along ancient convergent margins represents a major goal in orogenic system analyses. Chemical and in some cases isotopic compositions of igneous rocks formed in modern supra‐subduction arcs and collisional belts are sensitive to Moho depths at the location of magmatism, implying that igneous suites from fossil orogens carry information about crustal thickness from the time they formed. Several whole‐rock chemical parameters correlate with crustal thickness, some of which were calibrated to serve as “mohometers,” that is, quantitative proxies of paleo‐Moho depths. Based on mineral‐melt partition coefficients, this concept has been extended to detrital zircons, such that combined chemical and geochronological information extracted from these minerals allows us to reconstruct the crustal thickness evolution using the detrital archive. We discuss here the mohometric potential of a variety of chemical and isotopic parameters and show that their combined usage improves paleocrustal thickness estimates. Using a MATLAB® app developed for the underlying computations, we present examples from the modern and the deeper time geologic record to illustrate the promises and pitfalls of the technique. Since arcs are in isostatic equilibrium, mohometers are useful in reconstructing orogenic paleoelevation as well. Our analysis suggests that many global‐scale correlations between magma composition and crustal thickness used in mohometry originate in the sub‐arc mantle; additional effects resulting from intracrustal igneous differentiation depend on the compatible or incompatible behavior of the involved parameters.

Partial melting of amphibolitic lower crust and subsequent melt-crystal separation for generation of the Early Eocene magmatism in eastern Himalaya

“…Magmatic rocks are regarded as a "probe" for exploring the deep Earth, and carry crucial information for crustal evolution, plate movement and tectonic events (Mo et al, 2007;Zhu et al, 2009a;Mo, 2011;Hou et al, 2015;Li et al, 2020). It is noteworthy that significant breakthroughs have been made in qualitative and even quantitative estimation for crustal thickness in ancient orogens using some geochemical indices of intermediate-felsic magmatic rocks or accessory minerals therein (Profeta et al, 2015;Hu et al, 2017;Tang et al, 2020;Luffi and Ducea, 2022). For example, magmatic rocks with high Sr/Y and (La/Yb) N ratios, i.e., adakites or adakitic rocks (Martin, 1986;Defant and Drummond, 1990), are typically considered as prototypical products of high pressure condition, thus indicating an abnormally thick crust (>50 km) (Xu et al, 2002;Chung et al, 2003;Dai et al, 2020a;2020b;Zeng et al, 2020).…”

How and How Much Did Western Central Tibet Raise by India–Asia Collision?

“…(2021) to reconstruct the paleo‐crustal thickness of collisional orogen remains to be tested as it was calibrated against the whole‐rock (La/Yb) N mohometer of Profeta et al. (2015) that was built on the subduction‐related magmatism (Luffi & Ducea, 2022). However, the zircon Eu mohometer is tentatively used here given that the Paleogene‐Miocene paleo‐crustal thickness of southern Tibet is reconstructed well by this mineral‐scale approach (Tang et al., 2021).…”

“…Since the whole-rock (La/Yb) N values are generally more sensitive to the paleo-crustal thickness over Sr/Y values, especially in collisional orogen (e.g., Hu et al, 2017;Lipman, 2021;Zhu et al, 2017), and the western central Tibet became a (post)collisional setting since late Early Cretaceous (Kapp & DeCelles, 2019;Kapp et al, 2003), we use the whole-rock (La/Yb) N mohometer of Hu et al (2017) to estimate the paleo-crustal thickness through the late Early Cretaceous (<120 Ma) to Eocene. The availability of the zircon Eu mohometer of Tang et al (2021) to reconstruct the paleo-crustal thickness of collisional orogen remains to be tested as it was calibrated against the whole-rock (La/Yb) N mohometer of Profeta et al (2015) that was built on the subduction-related magmatism (Luffi & Ducea, 2022). However, the zircon Eu mohometer is tentatively used here given that the Paleogene-Miocene paleo-crustal thickness of southern Tibet is reconstructed well by this mineral-scale approach (Tang et al, 2021).…”

“…Geophysical investigations provide information only on present‐day lithospheric architecture. Whole‐rock Sr/Y and (La/Yb) N ratios and zircon Eu anomalies [Eu/Eu* = Eu N /(Sm N × Gd N ) 1/2 ; N‐chondrite normalized] of intermediate‐felsic rocks have recently been calibrated as quantitative proxies of paleo‐Moho depths (mohometer), based on pressure‐dependent nature of the mineral assemblages—plagioclase sequesters Sr and Eu at low pressure and depth, versus garnet and amphibole absorb Y and Yb at greater pressure (Luffi & Ducea, 2022 for reviews). These calibrations, together with geochronological and petrogenetic studies, allow examination of changes in the thickness, elevation, and constitution of paleo‐crust through time, such as the southern Tibet (Q. Xu et al., 2020; Sundell et al., 2021; Tang et al., 2021; W. Wang et al., 2022; Zhu et al., 2017) and the Cordilleran orogenic belt (Chapman et al., 2015, 2020; Lipman, 2021).…”

Supplemental Material: Pyroxenite melting at subduction zones

“…Reported major element oxide contents were then renormalized to a dry sum of 100 wt.%. Finally, to eliminate samples with anomalously alkalic geochemistry that are not representative of arc magmas, we discarded compositions which at a given SiO2 content have total alkali contents (Na2O + K2O) that are more than 1.5 wt.% greater than the subalkaline limit (Luffi and Ducea, 2022).…”