Fundamental problems with models currently used to explain the genesis and evolution of continental crust were raised. These problems focus around the difficulty of generating the upper continental crust from a lower crustal or mantle protolith without leaving a very large, and so far undetected, volume of restite. No conclusive resolution to the problems was achieved

Arculus, R. J.

English

NASA Technical Reports Server

r 
I 28 
CRUSTAL GROWTH-SOME MAJOR PROBLEMS 
R. J. Arculus, Dept. of Geological Sciences, University of Michigan,MI48109 
I The genesis and evolution of the continental crust are first-order 
geologic problems which remain unresolved. We still face several difficulties 
with respect to obtaining fundamental data necessary for solving the problems 
in the form of the composition and structure of the deep crust, and the nature 
of the crust-upper mantle boundary. While it is clear from a number of 
different approaches that the upper continental crust is generally of granitoid 
composition, the processes of upper crust production from middle and lower 
crust are unfortunately still obscure given the difficulty of inverting the 
geochemical data obtained on minimum and near-minimum melt magmas to the bulk 
composition of the sources. Thus it seems that the hope of using the granite 
cycle as a geochemical probe of the middle and lower crustal source regions 
is not going to be fulfilled, and other methods need to be employed. This is 
not to deny the use of granite geochemistry as a constraint on the isotopic 
spectrum of the source components and time-integrated parent/daughter isotopic 
ratios of these components. 
for the nature of the lower continental crust are: 1) exposed "deep crustal" 
segments; 2)xenoliths brought up in explosive, fast-moving alkalic igneous 
rocks; and 31 geophysical evidence (seismic wave travel times, electrical 
conductivity, gravity and the density-composition relations inferred from 
these data) 
There has been considerable debate in the literature concerning the 
true nature and relevance to normal lower crust of uplifted slices of the 
continental crust that have been metamorphosed at high grades. For example, 
recent estimates of the P-T maxima reached by para- and orthogneisses of the 
Ontario (Canada) Grenville province are 10 - 12 kbar and approximately 8OO0C, 
which would allow these materials to have been subjected to the conditions 
prevailing at the base of the normal thickness of continental crust. 
Nevertheless, there is still a thickness of some 30 - 40 km of crust beneath 
the present-day exposed Grenville and the exposed gneisses would appear to 
have been "middle" crust in some overthickened sequence about 1.2 Ga ago. 
A plausible mechanism for a) subjecting supracrustals to lower crustal 
conditions; b) allowing these to be exhumed to the surface is a Himalayan- 
style continent-continent collision, erosion and rebound. The conclusion 
therefore is that the Grenville terrain itself is not a direct sample of normal 
lower continental crust. 
Studies of lower crustal xenolithic material have been increasing in 
number in recent years because of the difficulties encountered in other, large 
scale approaches. 
sample, possible misrepresentative sampling of the lower crust and non-survival 
of some facies in hostile host magmas, and the lack of dimensional relations 
with surrounding lithologies, some interesting conclusions have emerged. 
For example, it is fair to say that one of the major conclusions of xenolith 
studies based on materials from southern Africa, the Colorado Plateau, 
northern Mexico, the Massif Central and eastern Australia is that the lower 
crust is predominantly mafic in character with polygenetic origins but 
predominantly of assimilation-modified under- and intrapiated basaltic magmas 
and cumulates derived therefrom. 
that might be drawn from studies of Grenville-age outcrops and deep-seated 
The other major lines of evidence both direct and indirect that we have 
Although these studies are plagued by the minute size of 
As an eTample of the different conclusions 
https://ntrs.nasa.gov/search.jsp?R=19890012826 2020-03-20T02:32:55+00:00Z
I 
CRUSTAL GROWTH- SOME MAJOR PROBLEMS 
R. J. Arculus 
29 
xenoliths (entrained in Tertiary basalts) in Mexico, Ruiz et al. (1987)(1) 
have shown that the exposed granulites are on average of andesitic 
compostion ( 63 wt% SiOz), whereas the xenoliths are significantly more 
mafic ( average 53 wt% Si02). 
that the lower continental crust is everywhere mafic in character. Although 
petrologic and geophysical evidence for a mafic lower crust are satisfyingly 
in accord in the case of eastern Australia (2) (thermobarometry, crustal 
velocity structure, lack of Moho ), the same is not everywhere true. 
While increasingly there is recognition that the Moho is not necessarily a 
simple ultramafic/intermediate rock contact, the persistence of rapid changes 
in seismic wave velocity at the base of the crust is incompatible with the 
presence of thick sequences of mafic materials in granulite-eclogite facies. 
In more general terms, our models of crustal formation which depend 
heavily on the modern plate tectonic cycle are also in trouble, Despite 
occasional opinions expressed to the contrary, there is a consensus amongst 
petrologists actively engaged in the study of island arc magma genesis that 
the primary flux of material to the arc crust is basaltic in character. 
This flux may be expressed as intermediate to silicic eruptives in some arcs, 
expecially those constructed on pre-existing continental crust, but the 
point remains that modern-day crustal growth in arcs appears to involve 
basalt as the prime building block. 
block without the same volumetric lever that would exist in the case of an 
intermediate composition protolith. The major difficulties with this type 
of model are 1) the creation of an embarassingly large mafic-ultramafic 
restite that has not been recognized petrologically, but may not be 
distinguishable seismically from peridotite in the upper mantle; 2) the 
necessity of general disposal of this restite at least from the crust given 
the lack of evidence for its residence at the base of the crust, 
relevant to the problem of the major periods of Archean and Proterozoic 
crustal growth. For example, higher temperatures gnerally prevailing in 
subduction zones might have permitted the direct anatexis of hydrated ocean 
floor (solidus at about 800OC in the pressure range 5 - 25 kbar ) rather than 
the devolatilization inferred at present. 
respect to this model of  direct silicic magma production in the Archean: 
a) the products of wet melting of basalt are not like "calcalkalic" series; 
b) the pervasive invasion of peridotite overlying the subducted lithosphere 
by melts of elevated Si/Mg ratios should surely have raised this ratio in 
upper mantle peridotite above chondritic, whereas the observed ratio is less 
than chondritic and thereby a major constaaint on models of mantle formation. 
It is clear that major problems persist with our detailed understanding 
of continental crustal growth, and that uniformitarianism may not be a 
helpful concept in the resolution of the difficulties. 
There are however, difficulties in the way of accepting the hypothesis 
The granite extraction cycle can of course operate on this building 
It might be argued that present-day arc petrogenesis is not directly 
Two points should be made with 
References 
Ruia, J. et al. (1987) Contrib. Mineral. Petrol. (submitted). 
2. Jackson, I. and Arculus, R.J .  (1984) Tectonophysics, 185-197. 


Crustal growth: Some major problems

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server