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Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Some of the more important constraints on crustal growth mechanisms are reviewed in relation to the trace element composition of orogenic igneous rocks, with specific reference to granitoids, and in relation to tectonothermal models of subduction zones. The wide range of magma types associated with subduction zones is difficult to explain by a single magma-generating mechanism. This variation can be accommodated if some lavas ( are tholeiites and boninites) are formed through wet melting of upwelling asthenosphere at the initiation of oceanic subduction, whereas others are generated from the mantle wedge during the more mature stage of ares, with either hornblende or phlogopite dominated assemblages, where induced convection in the mantle wedge enables source replenishment. Yet other products are generated through melting of the slab (±subducted sediment), but only where the subducting ocean crust is very young and/or warm.
The range and variation of trace-element patterns displayed by basic lavas is also prevalent in more siliceous compositions, implying that the mantle component in many granitoids may be much higher than is commonly assumed. As well as the traditional I-type, S-type and A-type granites with generally low-Ba and low-Sr characteristics, there is another important group of granitoids that has high-Ba and high-Sr concentrations, and that consistently has about 10 trace-element characteristics that are complementary in nature to the first groups. This type of granitoid is common in the Arehaean, rare in the Proterozoic, but is surprisingly abundant amongst post-Cretaceous to Recent orogenic volcanic and plutonic rocks. Trace element patterns may be inherited from mineralogically-controlled element fractionation occurring deep in the lithosphere; major element characteristics are more dependent on mineral-fluid control during magma generation.
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