The timing of calc-alkaline magmatism in parts of the Alpine–Himalayan collision zone and its relevance to the interpretation of Caledonian magmatism

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Abstract
Discussion of the timing of calc-alkaline Caledonian magmatism in Scotland has centred on its tectonic setting: the view that active subduction was responsible for the formation of 415–400 Ma magmatism is in conflict with the palaeogeographical and stratigraphical evidence. In order to shed light on the problem, magmatism in the continent–continent collision zone of modern times (Turkey, Iran and Tibet) is surveyed. The conclusion is reached that the final stage of calc-alkaline magmatism displays a time-lapse and possible space-shift after two continents have collided. From the evidence of magmatism, the timing of the major collision events in the Himalayas is thought to be diachronous, ranging from around or no later than 38–30 Ma for the central part, to later than 30 Ma for the eastern part. This view is supported by the available palaeogeographical and stratigraphical evidence. Evidence from this survey enables the Scottish data to be seen in a new context. By comparing modern collision zones with old ones, such as those in Scotland and the Appalachians, it is also concluded that collision-related magmatism is not volumetrically trivial and is diverse in composition.