Aspects of the Tectono-magmatic Evolution of Late Mesozoic Silicic Magmatic Systems in Hong Kong

<p>Hong Kong represents a microcosm of the magmatic and tectonic processes that are related to formation of the Southeast China Magmatic Belt (SCMB, ~1,300 km long by 400 km wide). The SCMB is dominated by extensive Mesozoic (Yanshanian Orogeny) igneous rocks, which form part of an extensive, long-lived circum-Pacific igneous province. In Hong Kong, large silicic ignimbrites, produced from several calderas identified through geological mapping, together with their sub-volcanic plutons record a ~26-Myr period of magmatic activities from ~164 to 138 Ma. This work studies these volcanic-plutonic assemblages with the associated Lantau and High Island caldera complexes, with an emphasis on the ~143-138 Ma period from the latter complex. This study uses multiple techniques, including field studies, zircon geochronology and trace element analyses, and zircon and apatite low-temperature thermochronology, to gain new insights into the Mesozoic tectono-magmatic history in this region. Field studies demonstrate that the High Island caldera complex (with its main collapse at 140.9±0.4 Ma in association with the High Island Tuff) is structurally more complex than previously suggested and represents a long-lived, large (320 km²) feature. The volcanic strata exposed in the eastern part of the caldera are inferred to have been tilted during syneruptive, asymmetric collapse of the caldera floor, whereas those in other parts have been affected by block faulting but not overall tilting. Two ignimbrites (e.g. Long Harbour: 141.4±1.0 Ma) exposed within the caldera outline are now interpreted to have accumulated in local volcano-tectonic basins, confined by faults that were later exploited by dyke intrusions. Field observations offer important constraints on the ages of volcanic and plutonic units, which have been tested by zircon U-Pb dating in this study. The field evidence also negates a previous interpretation that there was an overall tilting of the High Island caldera complex. U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) techniques have been carried out on zircons separated from 21 samples, chosen from both volcanic and plutonic samples within the Lantau and High Island Caldera complexes. The SIMS age datasets reveal two groups: (1) seven samples with unimodal age spectra; and (2) fourteen samples yielding multiple age components. Five samples in group 1 yield mean ages indistinguishable from their previously published ID-TIMS ages, demonstrating that the SIMS techniques have generated results fully in agreement with the ID-TIMS methods, although with overall less precision. Of the two other samples, one is slightly younger than the published ID-TIMS age, and the other has no previous age determination. Thirteen samples in group 2 are interpreted to have crystallisation/eruption ages that are younger (although often within 2.s.d. uncertainties) than their corresponding ID-TIMS values. The remaining sample from this group has no previous age determination. The overall age patterns from both groups suggest that, instead of separate phases of activity at ~143 and 141-140 Ma as previously inferred, magmatic and volcanic activities were continuous (within age analytical uncertainties) over a ~5 Myr period. Direct linkages between several plutonic and volcanic units in this period of activity (e.g. High Island Tuff and the Kowloon Granite) are no longer supported by the age data, and magmatic activity represented by exposed plutons continued until 137.8±0.8 Ma, as with the Mount Butler Granite. Under CL imagery, a wide variety of zircon textures is evident, indicative of complex processes that operated in the magmatic systems. Zircon trace element data coupled with textural characteristics enable identification of some common petrogenetic processes. Overall, the intra-grain (cores-rims, sector-zoned zircons) and intra-sample variations in trace element abundance and elemental ratios are more significant than the differences between individual samples. Zircon chemistries in samples from both the volcanic and plutonic records indicate that there are two groups of volcanic-plutonic products through the history of the High Island Caldera magmatic system. Two evolutionary models are proposed here to explain these two groups. In the first model, the magmatic system comprises a single domain that fluctuated in temperature through varying inputs of hotter melts (and was randomly tapped). In the second model the intrusive and extrusive products represent interplay of two magmatic domains in the crust, with contrasting characteristics. Zircon and apatite fission track analyses have been carried out on several of the rocks dated by U-Pb methods (either SIMS or TIMS), together with a selection of other Mesozoic igneous rocks and post-magmatic Cretaceous and Eocene sediments to cover the geographic area of Hong Kong. The fission-track dataset and associated thermal modelling show that the igneous rocks and Cretaceous sediments (but not the Eocene sediments) together experienced post-emplacement or post-depositional heating to >250 ºC, subsequently cooling through 120-60 ºC after ~80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. The enhanced geothermal gradient is interpreted to represent continuing Yanshanian magmatic activity at depth, without any documented surface eruption products, until ~100-80 Ma. The data also indicate a long-term, slow cooling (~1 ºC/Myr) since the early Cenozoic, linked to ~2-3 km of erosion-driven exhumation. The thermo-tectonic history of Hong Kong reflects the mid-Cretaceous transition of southeast China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. The inferred cessation of magmatism at depth below Hong Kong at ~100-80 Ma is broadly coincident with the cessation of plutonic activity in many other circum-Pacific magmatic provinces related to reorganisation of Pacific Plate motion.</p>