GEOCHEMISTRY OF COEVAL MESOZOIC PLUTONIC AND VOLCANIC SUITES IN HONG-KONG

Middle Jurassic to Lower Cretaceous intermediate to silicic plutonic a nd volcanic rocks of Hong Kong record a transition from calc-alkaline, through high-K calc-alkaline, to transitional shoshonitic composition s with time. Close spatial and temporal associations among comagmatic volcanic-plutonic pairs indicate that magmatism occurred in discrete e pisodes, mostly of less than one million years duration. Synchronous h igh-K calc-alkaline and transitional shoshonitic magmatic activity dur ing at least one pulse suggests a relatively rapid transition from a s ubduction-related to an extension-related tectonic setting. Geochemica l signatures indicate that the magmatic suites have a mantle origin wi th a decreasing crustal contribution from two distinct sources. The ea rliest mantle-derived magmas interacted strongly with a dominantly Arc haean 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 be tween the two dominant crustal sources. This interface marks a deep cr ustal discontinuity which promoted the passage of magmas to the surfac e.