Can Island Arc Tectonic Models be applied to Major Continental
Marginal Batholiths?
Charlotte M. Allen, Department of Geology, Key Centre for the
Geochemical Evolution and Metallogeny of Continents, The Australian
National University, Canberra ACT 0200, charlotte.allen@anu.edu.au
The lack of correspondence of the average compositions of continental
marginal batholiths and island arc volcanic rocks has been noted
for some time (cf Silver and Chappell, 1988; Table 1).
Batholiths are, on the whole, much more "continental"
with higher average contents of elements such as Si and incompatible
trace elements. I am interested in exploring the reasons for
the lack of correspondence, to learn about island arcs, and to
understand just what range of stress regimes can be categorised
as "island arc" or directly subduction-zone-related.
Is the major difference between batholith and island arc composition
simply the presence of thick continental crust that acts as a
physical barrier to mafic magmas, which therefore melts and adds
to batholith construction? Hildreth and Moorbath (1981) argued,
based on research in the central Andes, that crustal thickness
was the cause of observed systematic changes along the length
of the Quaternary continental marginal arc. They suggested that
increased crustal thickness leads to increased participation of
crustal partial melts in MASH processes. The product of these
interactions as sampled in Quaternary central volcanoes, however,
is still significantly more mafic than that of the Sierra Nevada
or Peninsular Ranges batholiths (Table 1). On the other hand
Ward (1995) suggests that major batholiths are the products of
a different tectonic setting not easily related to orderly chains
of volcanoes referred to as arcs. He argued that for the North
American Cordillera, major batholiths are much more easily related
to broad fields of disorganised, caldera-forming silicic magmatism
than to dominantly andesitic volcanism in places like the Cascades.
Furthermore, he compiled crystallisation ages that support the
idea that silicic magmatism and correlated batholith-building
events follow "arc" formation by 20-30 Ma in regular
tectonic cycles that can be encapsulated in three stages: 1)
rapid orthogonal subduction and arc generation, 2) clogging of
the subduction zone, slowed or stopped subduction, extension and
batholith formation, and 3) crustal thinning, mafic magmatism,
and chaotic plate boundary reorganisation. This model has several
appealing features if applied to the Carboniferous plutons of
the northern New England Fold Belt, Australia. Chappell and Stephens
(1988) argued that only several generations of melting ("remagmatisation")
can differentiate a basaltic parent to give the large volumes
of silicic plutons that average granodiorite in composition.
In their model, M-type plutons are directly related to mantle
derived magmas and these are found primarily in island arcs.
I-types, though, are reprocessed crust. It is the mechanism of
this reprocessing that interests me and whether M- and I-types
are truly related. A problem with these comparisons, that cannot
be ignored, is that sampling may have biased the averages, not
the actual field collection but the obvious difference of level
of intrusion between batholithic and volcanic rocks (ala Bruce
et al., 1989).
REFERENCES
Bruce, R.M., Nelson, E.P & Weaver, S.G., 1989. Effects of
synchronous uplift and intrusion during magmatic arc construction.
Tectonophysics 161, 317-329.
Chappell, B.W. & Stephens, W.E., 1988. Origin of infracrustal
(I-type) granite magmas. Transactions of the Royal Society
of Edinburgh: Earth Sciences 79, 71-86.
Ewart, A., 1982. The mineralogy and petrology of Tertiary-Recent
orogenic volcanic rocks: with special reference to the andesitic-basaltic
compositional range. In Thorpe, R.S. (ed) Andesites, p.
25-87, Wiley.
Fierstein, J, Bruggman, P, Schwarz, et al. (1989). Chemical analyses
of rocks and sediments from central Chile. U.S.G.S. Open File
Report 89-78.
Hildreth, W. and Moorbath S. (1988). Crustal contributions to
arc magmatism in the Andes of central Chile. Contributions
to Mineralogy and Petrology, 98, 455-489.
Silver, L.T. and Chappell, B.W., 1988. The Peninsular Ranges
batholith: an insight into the evolution of the Cordilleran batholiths
of southwestern North America. Transactions of the Royal Society
of Edinburgh: Earth Sciences 79, 105-121.
Table 1 Average Compositions
SW Pacifica | wPRBb | ePRBb | Central
Chilec | |
number | 174 | 149 | 104 | |
SiO2 | 57.77 | 63.41 | 66.05 | 60.33 |
TiO2 | 0.73 | 0.64 | 0.65 | 0.77 |
Al2O3 | 16.32 | 15.70 | 16.23 | 16.89 |
FeO | 7.32 | 5.00 | 3.41 | 5.18 |
MnO | 0.14 | 0.09 | 0.06 | 0.15 |
MgO | 4.62 | 2.71 | 1.49 | 3.09 |
CaO | 7.75 | 5.70 | 4.41 | 5.45 |
Na2O | 3.20 | 3.45 | 3.82 | 4.07 |
K2O | 1.64 | 1.69 | 2.26 | 2.33 |
P2O5 | 0.23 | 0.10 | 0.15 | 0.22 |
total | 99.72 | 98.49 | 98.53 | 98.48 |
Cr | 130 | 67 | 24 | 50 |
Ba | 462 | 451 | 863 | 487 |
Rb | 40 | 49 | 73 | 77 |
Sr | 523 | 268 | 501 | 510 |
Y | 24 | 24 | 12 | 22 |
Th | nd | 6.0 | 8.6 | 9 |
U | nd | 1.4 | 1.7 | 3 |
La | 22 | 12 | 20 | 25 |
a Ewart (1982)
b PRB=Peninsular Ranges batholith, Chappell and Silver (1988)
c Fierstein et al (1989)
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