D. Abbott | Geology Reports

USGS2012

The structural and geochemical evolution of the continental crust: Support for the oceanic plateau model of continental growth

The problem of the origin of the continental crust can be resolved into two fundamental questions: (1) the location and mechanisms of initial mantle extraction of the primitive crust and (2) the processes by which this primitive crust is converted into the continental crust that presently exists. We know that Archean continental crust is compositionally distinct from younger continental crust. Archean magmatism was dominantly bimodal, mafic thoeleiitic plus dacitic, heavy rare earth element depleted, in contrast to the dominantly unimodal, roughly andesitic calc‐alkaline magmatism on younger crust [ Taylor and McLennan , 1985; Condie , 1989]. The problem is whether these compositional differences are primarily due to different mechanisms of crustal extraction from the mantle or to different mechanisms of differentiation and alteration of newly formed continental crust.

Reviews of Geophysics↗

USGS2000

Quantifying precambrian crustal extraction: The root is the answer

We use two different methods to estimate the total amount of continental crust that was extracted by the end of the Archean and the Proterozoic. The first method uses the sum of the seismic thickness of the crust, the eroded thickness of the crust, and the trapped melt within the lithospheric root to estimate the total crustal volume. This summation method yields an average equivalent thickness of Archean crust of 49±6 km and an average equivalent thickness of Proterozoic crust of 48± 9 km. Between 7 and 9% of this crust never reached the surface, but remained within the continental root as congealed, iron-rich komatiitic melt. The second method uses experimental models of melting, mantle xenolith compositions, and corrected lithospheric thickness to estimate the amount of crust extracted through time. This melt column method reveals that the average equivalent thickness of Archean crust was 65±6 km, and the average equivalent thickness of Early Proterozoic crust was 60±7 km. It is likely that some of this crust remained trapped within the lithospheric root. The discrepancy between the two estimates is attributed to uncertainties in estimates of the amount of trapped, congealed melt, overall crustal erosion, and crustal recycling. Overall, we find that between 29 and 45% of continental crust was extracted by the end of the Archean, most likely by 2.7 Ga. Between 51 and 79% of continental crust was extracted by the end of the Early Proterozoic, most likely by 1.8–2.0 Ga. Our results are most consistent with geochemical models that call upon moderate amounts of recycling of early extracted continental crust coupled with continuing crustal growth (e.g. McLennan, S.M., Taylor, S.R., 1982. Geochemical constraints on the growth of the continental crust. Journal of Geology, 90, 347–361; Veizer, J., Jansen, S.L., 1985. Basement and sedimentary recycling — 2: time dimension to global tectonics. Journal of Geology 93(6), 625–643). Trapped, congealed, iron-rich melt within the lithospheric root may represent some of the iron that is ‘missing’ from the lower crust. The lower crust within Archean cratons may also have an unexpectedly low iron content because it was extracted from more primitive, undepleted mantle.

Tectonophysics↗