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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