Petrology and geochemistry of the igneous rocks from Macao (South China) :tectono-magmatic implications

The territory of Macao is composed of several granitic intrusions belonging to one of the biggest granite provinces in the world, the Southeast China Magmatic Belt (SCMB), located in the southeast (SE) area of the Cathaysia Block. The SCMB is known for the occurrence of large volumes of Mesozoic magmatic rocks (over 90% are granitic rocks and equivalent volcanic rocks with minor basalts), occupying a total outcrop area of nearly 200,000 km². The geology of Macao (~30 km²) is dominated by granitic rocks displaying a wide range of textural, mineralogical and chemical features, making it an ideal region to study these rocks and the petrogenetic processes responsible for their diversity.

This study employed a wide range of research methodologies, including field studies, petrography, zircon geochronology, mineral chemistry, whole-rock elemental and isotopic geochemistry to determine the nature of the source, the petrogenetic processes and the tectonic regimes of the Mesozoic magmatism in this region. The data collected aims to provide new knowledge on the tectono-magmatic evolution of Macao, in particular, and of SE China, in general.

The results obtained from high-precision U–Pb zircon geochronology revealed different ages for Macao granites. Despite its relatively small area, the determined ages tightly constrain the Macao granitic magmatism to two periods: Macao Group I granites (MGI) ranging from 164.5 ± 0.6 to 162.9 ± 0.7 Ma, and Macao Group II granites (MGII) ranging from 156.6 ± 0.2 to 155.5 ± 0.8 Ma. Additionally, younger dacitic rocks were dated at 150.6 ± 0.6 Ma and <120 Ma. The existence of two proximal but distinct granitic pulses suggests that the Macao granitic suite was incrementally assembled.

The granitic rocks of Macao are mainly high-K calc-alkaline metaluminous to weakly peraluminous I-type granites with variable degrees of fractionation. Fractional crystallization played an important role in their evolution, though the fractionation paths differ for the highly fractionated facies of both groups. Significant differences in isotopic composition were also observed between MGI and MGII.

Isotope and major element compositions, together with model ages, strongly suggest that Macao granitic magmas were generated by partial melting of infracrustal medium-to-high K basaltic Paleo-Proterozoic to Mesoproterozoic protoliths. These were heated by, and mixed to some degree with, mantle-derived magmas. The Jurassic Macao granites are interpreted as being produced in an intraplate extensional setting related to the break-off and foundering of a previously flat-slab (Paleo-Pacific plate) beneath the Eurasian plate.

In addition to the Early Yanshanian (Jurassic) granitic magmatism, Macao was also affected, to a lesser degree, by Late Yanshanian (Cretaceous) magmatism, as evidenced by the younger ages obtained for the dacite dykes. This transition from granitic to dacitic magmatism likely corresponds to a change in the regional tectonic regime, which induced a significant change in the magma genesis processes. The geochemical features of the Macao dacite dykes are characteristic of arc-like subduction-related magmas, possibly indicating the reestablishment of a normal subduction system in this area of SE China.