Geochemistry of coeval Mesozoic plutonic and volcanic suites in Hong Kong

Middle Jurassic to Lower Cretaceous intermediate to silicic plutonic and volcanic rocks of Hong Kong record a transition from calc-alkaline, through high-K calc-alkaline, to transitional shoshonitic compositions with time. Close spatial and temporal associations among comagmatic volcanic-plutonic pairs indicate that magmatism occurred in discrete episodes, mostly of less than one million years duration. Synchronous high-K calc-alkaline and transitional shoshonitic magmatic activity during at least one pulse suggests a relatively rapid transition from a subduction-related to an extension-related tectonic setting. Geochemical signatures indicate that the magmatic suites have a mantle origin with a decreasing crustal contribution from two distinct sources. The earliest mantle-derived magmas interacted strongly with a dominantly Archaean crustal protolith. Younger magmas show evidence for interaction with a dominantly Proterozoic crustal protolith. The strongest mantle influence is shown by magmas which were intruded along the boundary between the two dominant crustal sources. This interface marks a deep crustal discontinuity which promoted the passage of magmas to the surface.

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