Abstract

Middle Jurassic to Lower Cretaceous silicic volcanic rocks and granitoid plutons in Hong Kong form part of the broad, NE-trending Yanshanian tectonomagmatic belt, that extends through SE China. Pyroclastic deposits in Hong Kong are typically proximal and infill linear flared vents and ellipsoidal calderas. These pass down through elongate rift-like conduits into extensive multiple fault- and shear zone-hosted ('sheeted') dyke complexes and granitoid plutons. Dykes also emphasize the limits of calderas. The spatial distribution and geometry of the vents, calderas, dykes and plutons are structurally controlled during each of four temporally well-constrained periods of petrogenetically related volcanism and plutonism. E- and ENE-trending axes of extension, and transtension were dominant. NW-trending faults increased in importance with time, and their intersections with ENE-trending faults influenced the loci of volcanic centres. Nesting of the calderas confirms persistent reactivation of some faults. The loci of magmatism migrated over 25 Ma from NW to SE. Zones of silicic crust created during these magmatic episodes have a broadly tripartite crustal 'stratigraphy' (pluton, dyke, volcanic) reminiscent in very general terms of ophiolites. The zones were created during periods of unusually rapid extension, as the continental margin of SE China changed from an active margin, dominated by subduction, to a back-arc regime, and dextral strike-slip along ENE-trending faults gave way to sinistral strike-slip.

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