Middle Jurassic collision of an exotic microcontinental fragment: implications for magmatism across the southeast China continental margin

Thrusting, folding and metamorphism of late Paleozoic to middle Mesozoic sedimentary rocks, together with high precision U–Pb zircon ages from Middle to Late Jurassic volcanic and granitic rocks, reveals evidence for a major deformation event in northwestern Hong Kong between 164 and 161 Ma. This episode can be linked with collision of an exotic microcontinental fragment along the southeast China continental margin determined from contrasting detrital zircon provenance histories of late Paleozoic to middle Mesozoic sedimentary rocks either side of a NE-trending suture zone through central Hong Kong. The suture zone is also reflected by isotopic heterogeneities and geophysical anomalies in the crustal basement. Detrital zircon provenance of Early to Middle Jurassic rocks from the accreted terrane have little in common with the pre-Middle Jurassic rocks from southeast China. Instead, the zircon age spectra of the accreted terrane show close affinities to sources along the northern margin of east Gondwana. These data provide indisputable evidence for Mesozoic terrane accretion along the southeast China continental margin. In addition, collision of the exotic terrane, accompanied by subduction roll-back, is considered to have hastened foundering of the postulated flat-slab beneath southeast China leading to a widespread igneous flare-up event at 160 Ma

Volcanic-plutonic connections in a tilted nested caldera complex in Hong Kong

Exceptional exposures of four, precisely-dated, Middle Jurassic to Early Cretaceous,silicic volcanic centers and their plutonic equivalents in Hong Kong have provided an excellent opportunity to examine close connections in time and space between magma chambers and their overlying calderas. Here, we describe a ~14 km crustal section through a collapsed caldera in southeastern Hong Kong where the intracaldera fill suggests that the magmatic discharge was of supereruption scale. The main subvolcanic components that link a magma chamber with surface are revealed by well-established field relationships, supplemented by high precision geochronology, whole-rock geochemistry, and geophysical data. Exposures and outcrop reveal kilometer-scale caldera subsidence and evidence of the simultaneous evacuation of hundreds of cubic kilometers of high-silica rhyolite magma through dike-like conduits from a shallow subcrustal reservoir. The resultant volcanotectonic depression, within which is preserved a single cooling unit of massively columnar-jointed densely welded tuff (High Island tuff), is interpreted to form part of a larger tilted Early Cretaceous nested caldera complex. The High Island eruption signaled the end of a 24 Myr-Early Cretaceous silicic magmatism in the Hong Kong region characterized by four discrete ignimbrite ‘flare-ups’

Exposure dating ( 10 Be, 26 Al) of natural terrain landslides in Hong Kong, China

We successfully apply exposure dating using cosmogenic nuclides to natural terrain landslides in Hong Kong. Forty-five samples from eight landslide sites were exposure dated using 10Be, and a subset of six samples was also dated using 26Al. The sites comprised four large, deep-seated landslides featuring well-preserved rock scarps and associated debris lobes; two sites of rock and boulder fall; and two sites where scarps only are preserved. All of the deep-seated landslides gave ages within the last 50,000 yr, and the largest landslide gave an age of ̃32,000 yr. The youngest (̃2000 yr) and oldest (̃57,000 yr) landslide events dated came from the two sites of rock and boulder fall. Exposure ages from the deep-seated landslide scarps generally gave the most internally consistent ages for the landslides. However, only in rare cases did the landslide scarp ages overlap with those of boulders in the associated debris. Generally, boulders in the debris appeared to contain significant inheritance of cosmogenic nuclides from previous exposure and so yielded ages greater than those from the scarps. Surface exposure ages of ̃285,000 yr from boulders in the debris of two deep-seated landslides provide minimum ages considered to represent the original rock surfaces. This study has shown that it is possible to measure exposure ages of surfaces associated with large landslides from 70,000 yr down to a few thousand years old, despite low cosmogenic isotope production rates in Hong Kong due to low latitude and low altitude

Tracking the evolution of Late Mesozoic arc-related magmatic systems in Hong Kong using in-situ U-Pb dating and trace element analyses in zircon

The links between large-scale silicic volcanism and plutonism offer insights into the dynamics of crustal magmatic systems and growth of continental crust. In Hong Kong, voluminous silicic ignimbrites and linked plutons record a ∼26 Myr period of magmatism from ∼164 to 138 Ma. We present data from these linked volcanic-plutonic assemblages at the Lantau and High Island caldera complexes, with an emphasis on the ∼143-138 Ma activity from the latter. To track the evolution of these magmatic systems, U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) were carried out on zircons from 21 samples from both volcanic and plutonic samples. The SIMS age data sets divide into two groups across volcanic and plutonic origins: (1) seven samples with unimodal age spectra [five of which have the same mean value as the published Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) age from the same sample]; and (2) 14 samples yielding multiple age components. Age patterns from both groups suggest that the previously separated ∼143 Ma Repulse Bay (RBVG) and ∼141-140 Ma Kau Sai Chau volcanic groups (KSCVG) instead represent activities over a single ∼5 Myr period. Direct linkages previously proposed between some volcanic and plutonic units for this period (e.g., High Island Tuff, Kowloon Granite) are no longer supported, and magmatism represented by exposed plutons continued until 137.8 ± 0.8 Ma (Mount Butler Granite). Under CL imagery, a wide range of zircon textures identified in both volcanic and plutonic samples is indicative of complex processes, some of which are identified through trace element data coupled with textural characteristics. Overall, intra-grain (cores vs. rims; sector-zonation) and intra-sample variations in trace element abundances and ratios are larger than those between samples. Zircon chemistries in both volcanic and plutonic samples fall into two groups during the ∼5 Myr history of the High Island caldera magmatic system. One group (RBVG and "cold" granites) includes inherited grains back to 164 Ma and wider ranges in Hf, Y, total trivalent elements, Th and U concentrations and Th/U, Yb/Gd, and U/Yb ratios than the other (KSCVG and "hot" granites). Two possible evolutionary models of the High Island caldera magmatic system are: (1) the system randomly tapped a single crustal domain that fluctuated in temperature as a result of varying interactions of hotter melts, or (2) the volcanic and plutonic records reflect the interplay of two crustal domains with contrasting "low-" and "high-Temperature" characteristics. In Hong Kong, some plutonic bodies were comagmatic with large-scale volcanism, while others were emplaced at shallow crustal levels independently of volcanism, matching the current two end-member views of the volcanic-plutonic relationship.D.L.K.T. acknowledges support from a Victoria University Doctoral Scholarship. C.J.N.W. acknowledges support from the Royal Society of New Zealand (Cook Fellowship, Marsden Fund grant VUW0813) and Victoria University (University Research Fund). L.S.C. acknowledges support from the research funding of the Department of Earth Sciences, Hong Kong University

Thermotectonic history of SE China since the Late Mesozoic: insights from detailed thermochronological studies of Hong Kong

<p>The late Mesozoic Yanshanian volcanic arc affected an extensive region of SE China, but the conclusion of magmatism and later evolution are not fully understood. Widespread Yanshanian ignimbrites and their contemporaneous granites exposed in Hong Kong represent a microcosm of this magmatic arc. To constrain the post-magmatic thermal history of the region, we present zircon and apatite fission-track analyses from these rocks. Double dating using laser ablation inductively coupled plasma mass spectrometry U–Pb and fission-track techniques on detrital zircons from post-volcanic Cretaceous sediments is used to further constrain the tectonothermal evolution. The resulting dataset and thermal modelling suggest that the igneous rocks and Cretaceous sediments together experienced post-emplacement or post-depositional heating to >250 °C, subsequently cooling through 120–60 °C after <em>c</em>. 80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. We interpret the enhanced gradient to represent continuing Yanshanian magmatic activity until <em>c</em>. 100–80 Ma, much later than previously considered. Our data also indicate a long-term, slow cooling (<em>c</em>. 1 °C myr^–1) since the early Cenozoic, linked to <em>c</em>. 2–3 km of erosion-driven exhumation. The thermotectonic history of Hong Kong reflects the mid-Cretaceous transition of SE China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic. </p

Thermotectonic history of SE China since the Late Mesozoic: insights from detailed thermochronological studies of Hong Kong

The late Mesozoic Yanshanian volcanic arc affected an extensive region of SE China, but the conclusion of magmatism and later evolution are not fully understood. Widespread Yanshanian ignimbrites and their contemporaneous granites exposed in Hong Kong represent a microcosm of this magmatic arc. To constrain the post-magmatic thermal history of the region, we present zircon and apatite fission-track analyses from these rocks. Double dating using laser ablation inductively coupled plasma mass spectrometry U–Pb and fission-track techniques on detrital zircons from post-volcanic Cretaceous sediments is used to further constrain the tectonothermal evolution. The resulting dataset and thermal modelling suggest that the igneous rocks and Cretaceous sediments together experienced post-emplacement or post-depositional heating to >250 °C, subsequently cooling through 120–60 °C after c. 80 Ma. The heating reflects the combined effects of an enhanced geothermal gradient and burial. We interpret the enhanced gradient to represent continuing Yanshanian magmatic activity until c. 100–80 Ma, much later than previously considered. Our data also indicate a long-term, slow cooling (c. 1 °C myr–1) since the early Cenozoic, linked to c. 2–3 km of erosion-driven exhumation. The thermotectonic history of Hong Kong reflects the mid-Cretaceous transition of SE China from an active to a passive margin bordered by marginal basins that formed in the early Cenozoic