EVIDENCE FOR MULTIPLE MECHANISMS OF CRUSTAL CONTAMINATION OF MAGMA FROM COMPOSITIONALLY ZONED PLUTONS AND ASSOCIATED ULTRAMAFIC INTRUSIONS OF THE ALASKA RANGE

Models of continental crustal magmagenesis commonly invoke the interac tion of mafic mantle-derived magma and continental crust to explain ge ochemical and petrologic characteristics of crustal volcanic and pluto nic rocks. This interaction and the specific mechanisms of crustal con tamination associated with it are poorly understood An excellent oppor tunity to study the progressive effects of crustal contamination is of fered by the composite plutons of the Alaska Range, a series of nine e arly Tertiary, multiply intruded, compositionally zoned (peridotite to granite) plutons. Large initial Sr and Nd isotopic contrasts between the crustal country rock and likely parental magmas allow evaluation o f the mechanisms and extents of crustal contamination that accompanied the crystallization of these ultramafic through granitic rocks. Three contamination processes are distinguished in these plutons. The most obvious of these is assimilation of crustal country rock concurrent wi th magmatic fractional crystallization (AFC), as indicated by a genera l trend toward crustal-like isotopic signatures with increasing differ entiation. Second, many ultramafic and mafic rocks have late-stage phe nocryst reaction and orthocumulate textures that suggest interaction w ith felsic melt. These rocks also have variable and enriched isotopic compositions that suggest that this felsic melt was isotopically enric hed and probably derived from crustal country rock. Partial melt from the flysch country rock may have reacted with and contaminated these p artly crystalline magmas following the precipitation and accumulation of the cumulus phenocrysts but before complete solidification of the m agma. This suggests that in magmatic mush (especially of ultramafic co mposition) crystallizing in continental crust, a second distinct proce ss of crustal contamination may be super-imposed on AFC or magma mixin g involving the main magma body. Finally, nearly all rocks, including mafic and ultramafic (Sr-87/Sr-86)(i) that are too high, and epsilon(T ) N-d that are too low, to represent the expected isotopic composition of typical depleted mantle. However, gabbro xenoliths with typical de pleted-mantle isotopic compositions are found in the plutons. This sit uation requires either an additional enriched mantle component to prov ide the parental magma for these plutons, or some mechanism of crustal contamination of the parent magma that did not cause significant crys tallization and differentiation of the magma to more felsic compositio ns. Thermodynamic modeling indicates that assimilation of alkali- and water-rich partial melt of the metapelite country rock by fractionatin g, near-liquidus basaltic magma could cause significant contamination while suppressing significant crystallization and differentiation.