Geochemistry of coeval Mesozoic plutonic and volcanic suites in Hong Kong

Journal of the Geological Society, Nov 1997 by Sewell, R J, Campbell, S D G

Abstract: Middle Jurassic to Lower Cretaceous intermediate to silicic plutonic and volcanic rocks of Hong Kong record a transition from caic-alkaline, through high-K 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.

Keywords: Hong Kong, Mesozoic, calc-alkaline composition, magmatism, tectonics.

High precision U-Pb zircon age-dating of volcanic and plutonic rocks in Hong Kong (herein the area of the Hong Kong Special Administrative Region) has placed tight constraints on the timing of Jurassic to Cretaceous magmatic events in a small area ( 1050 km^sup 2^) of the SE China igneous province (Davis et al. this volume). The data reveal four discrete episodes of magmatism, generally of less than one million years duration, separated by longer periods of volcanic and plutonic quiescence. Such precise dating, together with detailed field, petrographic, geochemical and isotopic data (Sewell et al. 1992; Darbyshire & Sewell in press; Campbell & Sewell this volume), provides an opportunity to examine the close interrelationships between volcanism, plutonism, and tectonism. This paper describes the compositional variation through time of Hong Kong volcanic and plutonic suites and discusses aspects of their petrogenesis and tectonic setting.

Geological setting

Destructive plate magmatism along the Eurasian continental margin during the Mesozoic Yanshanian Orogeny culminated in voluminous intermediate to silicic eruptions and the emplacement of numerous, high-level granitoid plutons (Jahn et al. 1976; Wang et al. 1984; Wang 1986; Jahn et al. 1990; Zhou & Lou 1990). These Jurassic to Cretaceous volcanic and plutonic rocks crop out over a broad (400 km) belt along the coastal region of SE China (Fig. 1) and record a complex tectonomagmatic history.

The coastal region of SE China is dominated by several large-scale NE-trending fault zones (Zhang 1983) (Fig. 1) the most prominent of which is the Lianhuashan Fault Zone (LFZ, Chen 1987; Huang & Zhang 1990) extending for over 200 km through the maritime provinces of eastern Guangdong, Fujian and Zhejiang (Campbell & Sewell this volume, Darbyshire & Sewell in press). In eastern Guangdong, the Lianhuashan Fault Zone comprises a 30 km wide zone bounded to the north by the Shenzhen Fault and to the south by the Haifeng Fault (Ding & Guo 1989) (Fig. 1). Numerous volcanic centres and plutonic assemblages within this zone (BGMRGP 1988), including those of Hong Kong (Campbell & Sewell this volume), suggest that the Lianhuashan Fault Zone was a major locus of magmatic activity throughout the Late Mesozoic. This is corroborated by Sr and Nd isotope studies of the granitoid rocks (Sewell et al. 1992: Darbyshire & Sewell in press), geophysical studies (Fletcher et al. this volume) and zircon inheritance studies (Davis et aL this volume), which collectively endorse the existence of a deep crustal boundary through Hong Kong (Sewell 1992) separating dominantly late Archaean crust in the NW from Proterozoic crust in the SE.

Approximately 85% of the area of Hong Kong is formed from Mesozoic volcanic-plutonic assemblages. The bulk of these rocks are of Mid-Jurassic to Early Cretaceous age (Davis et al. this volume) and are divided into two volcanic groups (Sewell et al. 1993) and two granitoid suites (Sewell et al. 1992) (Fig. 2). During the course of this study it has been necessary to introduce a refinement of this suite and group nomenclature in order to discuss the close compositional and temporal relationships among the lithological units. Accordingly, the granitoids are grouped in terms of magmatic pulses where a 'Pulse' is defined as a magmatic event in which one or more single intrusive units (plutons) with similar bulk chemistry and mineralogy were emplaced at the same time. Similarly, the volcanic units are grouped in terms of phases where a 'Phase' is the volcanic equivalent of a 'Pulse'. A period (or episode) of magmatic activity may comprise one or more pulses or phases.

Volcanic and plutonic associations